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\begin{document}

\title{Recent progress in photocathode interface engineering for
photoelectrochemical CO2 reduction reaction to C1 and C2+ products}



\author[1]{Jae Hak Kim}%
\author[1]{Sung Hyun Hong}%
\author[2]{‪Sang Hyun  Ahn}%
\author[1]{Soo Young Kim}%
\affil[1]{Korea University}%
\affil[2]{Chung-Ang University}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Photoelectrochemical (PEC) systems harness light absorption to initiate
chemical reactions, while electrochemical reactions facilitate the
conversion of reactants into desired products, ensuring more efficient
and sustainable energy conversion in PECs. Central to optimizing the
performance of PECs was the pivotal role played by interface
engineering. This intricate process involved manipulating material
interfaces at the atomic or nanoscale to enhance charge transfer,
improve catalytic activity, and address limitations associated with bulk
materials. The careful tuning of factors such as band gap, surface
energy, crystallinity, defect characteristics, and structural attributes
through interface engineering led to superior catalytic efficiency.
Specifically, interface engineering significantly enhanced the
efficiency of semiconductor-based PECs. Engineers strategically designed
heterojunctions and manipulated catalyst surface properties to optimize
the separation and migration of photogenerated charge carriers,
minimizing recombination losses and improving performance overall. In
this review, we categorized our discussion into five sections focusing
on PECs and interface engineering, providing valuable insights into
recent research trends. Overall, the synergy between PECs and interface
engineering holds tremendous promise for advancing renewable energy
technologies and addressing environmental challenges by offering
innovative solutions for sustainable energy conversion and storage.%
\end{abstract}%



\sloppy


\textbf{Title}

Recent progress in photocathode interface engineering for
photoelectrochemical CO\textsubscript{2} reduction reaction to
C\textsubscript{1} and C\textsubscript{2+} products

\textbf{Jae Hak Kim\textsuperscript{1}, Sung Hyun
Hong\textsuperscript{1}, Sang Hyun Ahn\textsuperscript{2} and Soo Young
Kim\textsuperscript{1}}

\textsuperscript{1}Department of Materials Science and Engineering,
Korea University, Seoul, 02841, Republic of Korea

\textsuperscript{2} School of Chemical Engineering and Material Science,
Chung-Ang University, Seoul 06974, Republic of Korea

\textbf{Correspondence}

Sang Hyun Ahn, School of Chemical Engineering and Material Science,
Chung-Ang University, Seoul 06974, Republic of Korea

E-mail: \emph{shahn@cau.ac.kr}

Soo Young Kim, Department of Materials Science, Korea University, 145
Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea

E-mail:\emph{sooyoungkim@korea.ac.kr}

Jae Hak Kim, Sung Hyun Hong contributed equally to this work.

\textbf{Funding information}

National Research Foundation of Korea (NRF), funded by the Korean
government (2022M3H4A1A01012712, 2022M3H4A1A04096380)

\textbf{Abstract}

Photoelectrochemical (PEC) systems harness light absorption to initiate
chemical reactions, while electrochemical reactions facilitate the
conversion of reactants into desired products, ensuring more efficient
and sustainable energy conversion in PECs. Central to optimizing the
performance of PECs was the pivotal role played by interface
engineering. This intricate process involved manipulating material
interfaces at the atomic or nanoscale to enhance charge transfer,
improve catalytic activity, and address limitations associated with bulk
materials. The careful tuning of factors such as band gap, surface
energy, crystallinity, defect characteristics, and structural attributes
through interface engineering led to superior catalytic efficiency.
Specifically, interface engineering significantly enhanced the
efficiency of semiconductor-based PECs. Engineers strategically designed
heterojunctions and manipulated catalyst surface properties to optimize
the separation and migration of photogenerated charge carriers,
minimizing recombination losses and improving performance overall. In
this review, we categorized our discussion into five sections focusing
on PECs and interface engineering, providing valuable insights into
recent research trends. Overall, the synergy between PECs and interface
engineering holds tremendous promise for advancing renewable energy
technologies and addressing environmental challenges by offering
innovative solutions for sustainable energy conversion and storage.

\textbf{KEYWORDS}

Photoelectrochemical CO\textsubscript{2} reduction, interface
engineering, cocatalysts, junction engineering, nanostructure
engineering, defect engineering

\textbf{1 INTRODUCTION}

As industrialization increases worldwide, the number of artificially
generated greenhouse gases has risen, continuing to exacerbate climate
change. \textsuperscript{{[}1-3{]}} Efforts to reduce carbon dioxide
(CO\textsubscript{2}), the primary contributor to this issue, are
intensifying. Despite a temporary decline in global
CO\textsubscript{2}emissions due to the COVID-19 pandemic in 2020, 2021
saw record-high carbon emissions of approximately 36.1
GtCO\textsubscript{2}, a 6.3\% increase
year-on-year.\textsuperscript{{[}4{]}} The 2021 IPCC report states that
the remaining carbon budget to limit anthropogenic warming to 1.5 \selectlanguage{ngerman}°C and
2 °C above pre-industrial levels, starting from 2020, is 400
GtCO\textsubscript{2} and 1,150 GtCO\textsubscript{2},
respectively.\textsuperscript{{[}5{]}} Addressing climate change
urgently requires innovative solutions.\textsuperscript{{[}6{]}}

To meet this challenge, various technologies have been developed to
convert CO\textsubscript{2} into industrially useful chemicals,
utilizing methods such as electrochemical reduction
(ECR),\textsuperscript{{[}7-11{]}} photochemical reduction
(PCR),\textsuperscript{{[}12-15{]}} and photoelectrochemical (PEC)
reduction.\textsuperscript{{[}16{]}} However, significant obstacles
remain---CO\textsubscript{2} molecules are thermodynamically stable and
require considerable electrochemical overpotential for reduction.
Besides, in aqueous electrolysis systems, the ECR of
CO\textsubscript{2}competes with the hydrogen evolution reaction (HER),
necessitating the suppression of the latter to enhance the catalytic
activity of the CO\textsubscript{2} reduction reaction
(CO\textsubscript{2}RR). As depicted in Table 1, ECR involves a complex
reaction mechanism, producing a range of hydrocarbon compounds and
potential selectivity issues.\textsuperscript{{[}17,18{]}} Product
selectivity depends on the adsorption and activation energy of
intermediates on the catalyst's surface. For instance, HCOOH can be
selectively produced through the formation of \textsuperscript{*}OCHO,
while \textsuperscript{*}COOH is a key intermediate in the formation of
CO.\textsuperscript{{[}19{]}} The formed \textsuperscript{*}COOH
intermediate undergoes protonation to create a \textsuperscript{*}CO
intermediate, which then desorbs as CO. Besides HCOOH, the reaction
mechanism for other hydrocarbon products involves the
\textsuperscript{*}CO intermediate.\textsuperscript{{[}18,19{]}} Cu's
unique ability to support the formation of C\textsubscript{2+} products
via the two-electron reduction of CO\textsubscript{2} is due to its
distinctive properties: negative adsorption energy for the
\textsuperscript{*}CO intermediate and positive adsorption energy for
the \textsuperscript{*}H intermediate.\textsuperscript{{[}20{]}} The
\textsuperscript{*}CO intermediate is pivotal in CO\textsubscript{2}
ECR, impacting C--C coupling reactions and opening pathways for
C\textsubscript{2+} product formation.\textsuperscript{{[}21{]}}

Since the first PEC study achieved CO\textsubscript{2} reduction using a
p-GaAs photoelectrode in 1978, sustained efforts have been made to
convert photo-driven CO\textsubscript{2} into solar fuel across various
fields.\textsuperscript{{[}22{]}} PEC CO\textsubscript{2}RR has
advantages over ECR, including utilizing the photovoltage generated by
semiconductor photoelectrodes, reducing system complexity, and partially
offsetting the electrical energy required for
CO\textsubscript{2}reduction. Also, accelerated charge separation due to
bias-induced band bending enables higher production rates than PCR. PEC
cells typically comprise a photoactive semiconductor electrode
(photoelectrode), an electrolyte, and a counter electrode with a metal
electrode catalyst or a second photoelectrode. CO\textsubscript{2}
reduction occurs at the cathode, while water oxidation occurs at the
anode, with the photoelectrode as either or
both.\textsuperscript{{[}23-25{]}}

In PEC, each electrode can optimize catalyst efficiency based on factors
such as bandgap, surface energy, crystallinity, defect characteristics,
and structural attributes. Interface engineering is a promising approach
to overcoming the limitations observed in existing bulk materials. As
illustrated in Figure 1A, this review is categorized into five sections:
plasmonic noble metal cocatalysts, non-noble metal cocatalysts, junction
engineering, nanostructure engineering, and defect engineering---from an
interface perspective. These sections provide insights into recent
research trends. We also aim to compare materials generated by PEC,
categorizing them into C\textsubscript{1} and C\textsubscript{2+}, and
offer insights into future research directions. Through this article, we
aspire to contribute to advancing PEC technology grounded in interface
engineering, ultimately working toward achieving carbon neutrality.

\textbf{2 PRINCIPLES OF PEC CO\textsubscript{2}RR}

\textbf{2.1 Mechanism of PEC CO\textsubscript{2}RR}

PEC systems offer promising avenues for CO\textsubscript{2} reduction
using solar energy. In a fundamental approach, photocathodes utilizing
p-type semiconductor materials harness solar energy to generate
photovoltage, serving as the primary driving force to counter the
necessary potential for CO\textsubscript{2} reduction. A standard
three-electrode PEC CO\textsubscript{2}RR system consists of a p-type
semiconductor photocathode for CO\textsubscript{2} reduction, an n-type
semiconductor photoanode for the oxygen evolution reaction (OER), and a
reference electrode. Immersing a p-type semiconductor photocathode in a
CO\textsubscript{2}-saturated aqueous electrolyte creates a
semiconductor-electrolyte interface. The difference in Fermi levels
across this interface induces a built-in electric field, causing the
semiconductor's energy bands to bend downward. Upon light absorption,
electron-hole pairs (e\textsuperscript{--}/h\textsuperscript{+}pairs)
are generated by the promotion of an electron from the semiconductor's
valence band (VB) to the conduction band (CB), which then separate into
free carriers. Photogenerated electrons from the photoanode migrate to
the photocathode, while photogenerated holes transfer from the
photocathode to the photoanode. This organized movement of electrons and
holes involves the participation of electrons in the
CO\textsubscript{2}RR and holes in the OER at the interface between the
electrode and the electrolyte.\textsuperscript{{[}17,26{]}}A schematic
mechanism of PEC CO\textsubscript{2}RR is depicted in Figure 1B.

\textbf{2.2 Common parameters for measuring PEC performance}

2.2.1 Faradaic efficiency

Faradaic efficiency (FE) is crucial for evaluating selectivity towards
target products during the PEC reaction. It can be determined using the
following equation:

\begin{equation}
FE=\frac{\varepsilon\times n\times F\ }{Q}\times 100\%\nonumber \\
 \end{equation}

where \selectlanguage{greek}ε \selectlanguage{english}represents the number of electrons transferred during the
formation of the target product molecules, n is the quantity of the
target products, F is Faraday's constant, and Q is the total charge
passed. It is the proportion of the charge transferred to a specific
product relative to the total charge delivered during the PEC reaction.

2.2.2 Quantum efficiency

Quantum efficiency (QE) comprises both incident photon-to-current
efficiency (IPCE) and absorbed photon-to-current efficiency (APCE). They
can be determined using the following equations:

\begin{equation}
IPCE=\frac{\varepsilon\times\text{n\ }}{N}\times 100\%\nonumber \\
 \end{equation}\begin{equation}
APCE=\frac{\varepsilon\times\text{n\ }}{N\times A}\times 100\%\nonumber \\
 \end{equation}

where N is the total number of incident photons, and A is the light
absorption of the photoelectrode. IPCE assesses the efficiency of
converting incident photons into electrons transferred to the product at
a specific wavelength. APCE, on the other hand, measures the efficiency
of converting absorbed photons into electrons transferred to the
product.\textsuperscript{{[}27{]}}

2.2.3 Photocurrent density

Photocurrent density is a crucial parameter indicating the activity of
the PEC reaction. It is calculated by dividing the photocurrent, which
represents the flow of electric current through the photocathode, by the
photocathode's geometric area during the PEC reaction. It can be
categorized into total photocurrent density, representing the current
density associated with all products generated during the PEC reaction,
and partial photocurrent density, which corresponds to the current
density for a specific target product during the PEC reaction.

2.2.4 Production rate (yield rate)

Production rate or yield rate is another critical parameter representing
the activity of photocathodes in PEC reactions. Gas products such as CO,
CH\textsubscript{4}, and C\textsubscript{2}H\textsubscript{4} are
quantified using gas chromatography (GC), while liquid products like
HCOOH, CH\textsubscript{3}OH, and C\textsubscript{2}H\textsubscript{5}OH
are detected using nuclear magnetic resonance (NMR) to determine
production yield. The production rate is then normalized by dividing it
by the geometric area of the photocathode.\textsuperscript{{[}26{]}}

2.2.5 Solar-to-fuel efficiency

Solar-to-fuel efficiency (STF) is the ratio of the chemical energy
stored in products to the overall energy input from sunlight
illumination. STF can be calculated using the following equation:

\begin{equation}
STF=\frac{G\times\text{n\ }}{P\times S\times t}\times 100\%\nonumber \\
 \end{equation}

where \(G\) is the standard Gibbs free energy of formation
for the product, P is the light illumination power density, S is the
geometric area of the photoelectrode, and t is the duration of the PEC
reaction. This parameter is crucial for evaluating the effectiveness of
PEC performance. Factors such as charge separation efficiency,
light-absorption intensity, surface area and morphology, and catalyst
activity can influence STF.\textsuperscript{{[}27{]}}

\textbf{3 INTERFACE ENGINEERING OF PHOTOCATHODES FOR PEC
CO\textsubscript{2}RR}

\textbf{3.1 Plasmonic noble metal cocatalysts}

Plasmonic noble metal cocatalysts incorporate noble metals like Ag and
Au, known for their abundance of free-mobility
electrons.\textsuperscript{{[}28{]}} Surface plasmon resonance in these
cocatalysts originates from the collective oscillations of
nanostructures and nanogaps under intense electromagnetic
radiation.\textsuperscript{{[}29-31{]}} Such resonance, characterized by
incident light interacting with the cocatalyst, enhances the
redistribution and conversion of light energy through the re-emission of
plasmon-induced light, non-radiative decay to excited carriers (hot
electrons and holes), and thermal effects over specific
timescales.\textsuperscript{{[}28{]}}

Regarding PEC reactions, the size effect of plasmonic materials becomes
significant, with plasmonic noble metal nanostructures improving light
absorption, catalytic activity, selectivity, and
efficiency.\textsuperscript{{[}32-36{]}} Hence, plasmonic noble metal
cocatalysts, by exciting surface plasmons, could harness broad-spectrum
sunlight, producing high-energy hot carriers that facilitate PEC
CO\textsubscript{2}RR.

For example, Liu et al. designed a
CuBi\textsubscript{2}O\textsubscript{4} inverse opal photocathode
modified with plasmonic Ag nanoparticles (Ag NPs) using a sacrificial
template method
(CuBi\textsubscript{2}O\textsubscript{4}IOs-Ag).\textsuperscript{{[}37{]}}
The 3D-ordered structure of CuBi\textsubscript{2}O\textsubscript{4}
inverse opal enabled higher mass transfer rates and light harvesting
efficiency. Furthermore, incorporating Ag NPs significantly enhanced the
surface charge distribution by forming an ohmic contact with
CuBi\textsubscript{2}O\textsubscript{4}. The
CuBi\textsubscript{2}O\textsubscript{4} IOs-Ag photocathodes showed
notable improvements in selectivity for CO production, achieving a
faradaic efficiency for CO (FE\textsubscript{CO}) of 92\% at 0.2
V\emph{vs.} reversible hydrogen electrode (RHE), which is 1.6 times
greater than that of the pristine
CuBi\textsubscript{2}O\textsubscript{4} thin film. Wang et al. prepared
n\textsuperscript{+}p\textsuperscript{--} Si coated with a
TiO\textsubscript{2} interlayer and coupled it with plasmonic Au NPs to
fabricate photocathodes
(Au/TiO\textsubscript{2}/n\textsuperscript{+}p\textsuperscript{--} Si)
for PEC CO\textsubscript{2}RR to CO.\textsuperscript{{[}38{]}} A
schematic illustration of the synthesis method is depicted in Figure 2A.
Initially, a TiO\textsubscript{2} layer was deposited on the
micro-pyramid Si surface using an ALD process, after which Au NPs were
fabricated on top of TiO\textsubscript{2} layer through an
electrodeposition method. The
Au/TiO\textsubscript{2}/n\textsuperscript{+}p\textsuperscript{--} Si
photocathodes exhibited an onset potential of +0.24 V \emph{vs.} RHE, a
maximum FE\textsubscript{CO} of 86\%, and a partial photocurrent density
for CO of -5.52 mA cm\textsuperscript{--2} at -0.8 V \emph{vs.} RHE, as
shown in Figure 2B-D. Additionally, these photocathodes demonstrated
superior long-term operational stability for CO production under
continuous illumination for 20 h, as shown in Figure 2E. Density
Functional Theory (DFT) calculations indicated that the synergistic
effect of Au NPs and TiO\textsubscript{2} enhanced
CO\textsubscript{2}adsorption and expedited the generation of the
\textsuperscript{*}COOH intermediate and \textsuperscript{*}CO
desorption from active sites. This research group further investigated
the localized surface plasmon resonance (LSPR) effect of Au on the
TiO\textsubscript{2} layer, which contributed to increased activity and
selectivity for CO production by utilizing hot electrons generated in Au
NPs. Bharath et al. designed photocathodes by integrating plasmonic Ag
NPs with TiO\textsubscript{2}/RGO (Ag-TiO\textsubscript{2}/RGO) via a
hydrothermal method followed by microwave
irradiation.\textsuperscript{{[}39{]}} In this composite, Ag NPs not
only absorbed visible light but also acted as efficient electron
scavengers, thus enhancing PEC performance for
CO\textsubscript{2}reduction. PEC measurements revealed that the
Ag-TiO\textsubscript{2}/RGO photocathodes achieved a notable total
photocurrent density of 23.5 mA cm\textsuperscript{--2} and exhibited
low resistance of 125  in a CO\textsubscript{2}-saturated 1.0M KOH
solution under UV-vis light illumination. Furthermore, the
Ag-TiO\textsubscript{2}/RGO photocathodes displayed a
CH\textsubscript{3}OH yield of 85 \selectlanguage{greek}μ\selectlanguage{english}mol
L\textsuperscript{--1}cm\textsuperscript{--2}, a QE of 20\%, and a
faradaic efficiency for CH\textsubscript{3}OH (FE\textsubscript{CH3OH})
of 60.5\% at an onset potential of -0.7 V \emph{vs.} Ag/AgCl. Bharath et
al. also fabricated photocathodes featuring plasmonic-Au and
RGO-incorporated \selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3} nanorods
(Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO) aiming for highly
selective CH\textsubscript{3}OH production.\textsuperscript{{[}40{]}}The
synergistic effects among the size-dependent properties of
\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}, the plasmonic nature of Au, and
the chemical interactions of Au, RGO, and
\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3} nanorods resulted in a higher
band gap for Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO (2.60 eV).
This band gap allowed the composite to absorb more intensely in the
high-energy range of the visible spectrum, enabling the efficient use of
photogenerated electrons and reducing
e\textsuperscript{--}/h\textsuperscript{+} pair recombination effects.
Consequently, the Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO
photocathodes exhibited an impressive photocurrent density of -31.5 mA
cm\textsuperscript{--2} and achieved a maximum CH\textsubscript{3}OH
yield of 43 \selectlanguage{greek}μ\selectlanguage{english}mol L\textsuperscript{--1} cm\textsuperscript{--2}, as
shown in Figure 2F,G. Additionally, the photocathodes attained QE and
FE\textsubscript{CH3OH} of 21.5\% and 91\%, respectively, at -0.6
V\emph{vs.} SCE in a CO\textsubscript{2}-saturated 0.1M KOH electrolyte
under illumination.

Li et al. developed a photocathode for PEC CO\textsubscript{2}RR by
dispersing Ag NPs onto a Cu-modified mesoporous TS-1 zeolite
(Ag/Cu-TS-1).\textsuperscript{{[}41{]}} The fabrication process involved
an ion exchange method followed by an in-situ photodeposition method,
resulting in the synthesis of Cu-TS-1 via ion exchange and the highly
dispersed Ag NPs onto Cu-TS-1 through in-situ photodeposition. The
Ag/Cu-TS-1 photocathode showed exceptional light absorption and
efficient separation of e\textsuperscript{--}/h\textsuperscript{+}pairs,
enhancing CO\textsubscript{2}RR. This improvement was attributable to
the heterostructure of Cu\textsubscript{2}O/CuO and the LSPR effect of
the Ag NPs. In PEC performance terms, the Ag/Cu-TS-1 photocathode
demonstrated conversion of CO\textsubscript{2} into
CH\textsubscript{3}OH and C\textsubscript{2}H\textsubscript{5}OH at
rates of 5.64 and 2.62 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2}h\textsuperscript{--1}, respectively, at -0.6 V
\emph{vs.} RHE in a CO\textsubscript{2}-saturated 0.1M
KHCO\textsubscript{3} electrolyte. Zhang et al. constructed a plasmonic
Ag-adorned Cu\textsubscript{2}O nanowire (Cu\textsubscript{2}O/Ag)
photocathode for PEC CO\textsubscript{2}RR targeting
C\textsubscript{2+}products.\textsuperscript{{[}42{]}} The LSPR effect
of Ag contributed to both enhanced rapid separation of
e\textsuperscript{--}/h\textsuperscript{+} pairs and improved surface
catalytic reactions for C\textsubscript{2+} product generation. To
substantiate the LSPR effect in the Cu\textsubscript{2}O/Ag
photocathode, the ultraviolet-visible (UV-vis) absorption spectrum was
used to confirm increased light absorption due to the presence of Ag
NPs. Additional in-situ attenuated total reflection infrared
spectroscopy (ATR-IR) results indicated that incorporating Ag NPs
improved the formation and adsorption of the
CH\textsubscript{3}O\textsuperscript{*} intermediate. PEC tests showed
the Cu\textsubscript{2}O/Ag photocathode achieved a faradaic efficiency
for CH\textsubscript{3}COOH (FE\textsubscript{CH3COOH}) of 47.7\%, with
a generation rate of 212.7 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2}h\textsuperscript{--1} at -0.7 V \emph{vs.} RHE
under illumination.

\textbf{3.2 Non-noble metal cocatalysts}

Due to the high structural stability of CO\textsubscript{2} molecules,
many semiconductors face challenges in selectively facilitating
CO\textsubscript{2} reduction. In such scenarios, cocatalysts play a
crucial role in minimizing the overpotential required for
CO\textsubscript{2} reduction and enhancing the
CO\textsubscript{2}reduction kinetics to improve overall selectivity.
Therefore, it is essential to integrate semiconductor materials with
appropriate cocatalysts to accelerate surface catalytic conversion and
enhance both PEC activity and selectivity. The electrical conductivity
of the cocatalyst, along with its adsorption and desorption capabilities
based on binding energies with relevant reaction species---especially
those with moderate binding affinities for key intermediates---can lead
to higher intrinsic CO\textsubscript{2} reduction
activity.\textsuperscript{{[}43-45{]}} This, in turn, facilitates the
promotion of CO\textsubscript{2} reduction at lower overpotentials,
thereby enhancing onset potentials.

Noble metals, such as Ag and Au, remain efficient catalysts for
converting PEC energy into CO\textsubscript{2}. However, the use of
noble metal-based catalysts faces significant limitations due to their
high cost and limited availability, which constrain commercial
implementation.\textsuperscript{{[}27,46{]}} Consequently, extensive
research efforts have been directed toward developing alternative
catalysts for CO\textsubscript{2} reduction based on non-noble metal
materials, including transition metals. These materials, which encompass
metal oxides, metal NPs, metal-organic frameworks (MOFs), and metal
molecular catalysts, aim to provide low-cost, high-activity, and
long-term stability for CO\textsubscript{2} reduction, addressing the
challenges associated with noble metal
cocatalysts.\textsuperscript{{[}27, 47-49{]}}

Among these, cobalt molecular catalysts are widely used as cocatalysts
for the photocathode in PEC CO\textsubscript{2}RR. For instance, Shang
et al. prepared a p-type silicon photocathode incorporating a cobalt
phthalocyanine molecular catalyst immobilized on graphene oxide
(GO/CoPc).\textsuperscript{{[}50{]}} Initially, CoPc molecules were
immobilized on GO through ultrasonication in a DMF solution, taking
advantage of GO's conductive properties and capability to facilitate
electron transfer, as illustrated in Figure 3A. Subsequently, p-Si
wafers, coated with a protective TiO\textsubscript{2} layer
(Si-TiO\textsubscript{2}), were treated with a solution of
(3-aminopropyl)triethoxysilane (APTES), as depicted in Figure 3B.
Following this treatment, the Si-TiO\textsubscript{2}-APTES (STA)
substrate was immersed in a GO/CoPc aqueous dispersion, forming a
monolayer-like coating on the surface through electrostatic and hydrogen
bonding interactions between the amine groups of APTES and the
carboxylic acid groups of GO. The STA-GO/CoPc achieved a photocurrent
density of 0.7 mA cm-\textsuperscript{2}, while simultaneously attaining
a maximum FE\textsubscript{CO} of 86\% at -0.28 V \emph{vs.} RHE, as
observed in Figure 3C. Moreover, STA-GO/CoPc exhibited a remarkably low
onset potential of -0.36 V \emph{vs.} RHE, displaying a
FE\textsubscript{CH3OH} of 8\% at -0.62 V \emph{vs.} RHE. Roy et al.
constructed a hybrid photocathode structure comprising a cobalt
phthalocyanine catalyst with four phosphonic acid anchoring groups
(CoPcP) immobilized on mesoTiO\textsubscript{2}, which coated a p-Si
photocathode
(Si\textbar{}mesoTiO\textsubscript{2}\textbar{}CoPcP).\textsuperscript{{[}51{]}}Incorporating
the four phosphonic acid anchoring groups facilitated the immobilization
of the CoPcP catalyst on meso TiO\textsubscript{2}. To assess the impact
of the CoPcP catalyst on mesoTiO\textsubscript{2}, they synthesized a
mesoTiO\textsubscript{2}\textbar{}CoPcP hybrid electrode. The resulting
mesoTiO\textsubscript{2}\textbar{}CoPcP photocathode demonstrated highly
selective CO\textsubscript{2} to CO conversion, achieving a turnover
number for CO (TON\textsubscript{CO}) of 1949 \selectlanguage{ngerman}± 5 after 2 h of
controlled-potential electrolysis at a 550 mV overpotential in a 0.5M
KHCO\textsubscript{3} aqueous electrolyte. In addition, when combined
with a p-Si photocathode, the
Si\textbar{}mesoTiO\textsubscript{2}\textbar{}CoPcP exhibited a
TON\textsubscript{CO} of 939 ± 132 with 66\% CO selectivity under 0.5M
KHCO\textsubscript{3} conditions. Wen et al. prepared
Co\textsuperscript{II} (BrqPy) (BrqPy=4',4''-bis(4-bromophenyl)-2,2'
:6',2'' :6'',2'''-quaterpyridine) molecular catalysts with multiwalled
carbon nanotubes (CNT) on TiO\textsubscript{2}-protected p/n-Si
photocathodes
(Si\textbar{}TiO\textsubscript{2}\textbar{}CNT\textbar{}Co\textsuperscript{II}(BrqPy)),\textsuperscript{{[}52{]}}
as illustrated in Figure 3D. First, a p/n-Si wafer was covered with
TiO\textsubscript{2} through atomic-layer deposition (ALD). Then, a CNT
layer was drop-cast onto the Si\textbar{}TiO\textsubscript{2} layer.
Finally, a DMF solution containing Co\textsuperscript{II} (BrqPy)
molecular catalysts was drop-cast, and the catalysts were immobilized on
the CNT layer through \selectlanguage{greek}π--π \selectlanguage{english}stacking interactions. The hybrid
photocathodes, benefiting from the highly conductive nature of CNT,
achieved a remarkable photocurrent density of up to -1.4 mA
cm\textsuperscript{--2} at -0.11 V \emph{vs.}RHE, as seen in Figure 3E.
Furthermore, due to the exceptional activity and selectivity of the
cobalt molecular catalyst for CO\textsubscript{2}reduction, the total FE
and FE\textsubscript{CO} reached 99\% and 97\%, respectively, as
revealed in Figure 3F. These results surpassed other hybrid
photocathodes employing more complex configurations. Leung et al.
employed cobalt(II) bis(terpyridine) molecular catalysts (CotpyP) as
cocatalysts to construct a highly efficient photocathode for
CO\textsubscript{2} reduction.\textsuperscript{{[}53{]}} This hybrid
photocathode consisted of a p-type silicon photocathode coated with a
mesoporous TiO\textsubscript{2} layer with anchored CotpyP catalysts
(Si\textbar{}mesoTiO\textsubscript{2}\textbar{}CotpyP). The mesoporous
TiO\textsubscript{2} layer protected the Si, allowing for high loading
of CotpyP catalysts and ample surface contact with the electrolyte due
to its high surface area. Photoelectrochemical tests indicated CO and
formate (HCOO\textsuperscript{--}) production in aqueous acetonitrile
(MeCN) with 0.1M tetrabutylammonium tetrafluoroborate
(TBABF\textsubscript{4}) and pure CO\textsubscript{2}-saturated 0.1M
KHCO\textsubscript{3}. The turnover number for
CO\textsubscript{2}reduction reached 381 during a 24 h test in aqueous
MeCN.

Furthermore, nanowires (NWs), NPs, metal oxides, and MOFs, which consist
of non-noble metals, are also widely used as photocathodes in PEC
CO\textsubscript{2}RR. For example, Dong et al. designed a photocathode
comprising CuS-covered GaN NWs on silicon wafers (CuS/GaN/Si) to
efficiently convert H\textsubscript{2}S-containing
CO\textsubscript{2}mixture gas to HCOOH.\textsuperscript{{[}54{]}} The
fabrication involved thermally evaporating Cu NPs and plasma-assisted
molecular beam epitaxy (MBE) of GaN NWs on Si wafers. PEC experiments
were conducted in a 0.1M KHCO\textsubscript{3} solution purged with
CO\textsubscript{2} and H\textsubscript{2}S mixture gas under 1-sun
illumination, as shown in Figure 4A. Notably, Cu NPs spontaneously
transformed into CuS NPs during the PEC experiment. Compared to other
photocathodes of Cu/Si, CuS/Si, and Cu/GaN/Si, the CuS/GaN/Si
photocathode revealed an outperforming faradaic efficiency for HCOOH
(FE\textsubscript{HCOOH}) of 70.2\% at -1.0 V \emph{vs.} RHE and
achieved a maximum current density for HCOOH of 7.07 mA
cm\textsuperscript{--2}, as shown in Figure 4B,C. Zhou et al. fabricated
Bi NP cocatalysts supported on GaN NWs photocathodes for PEC
CO\textsubscript{2}RR to HCOOH.\textsuperscript{{[}55{]}} Focusing on
the electronic interaction between Bi NPs and GaN NWs grown on a planar
Si wafer (Bi/GaN/Si), the strong electronic interaction enhanced
CO\textsubscript{2} conversion due to electron sharing between Bi NPs
and GaN NWs. The Bi/GaN/Si photocathode demonstrated outstanding
selectivity for HCOOH, achieving a FE\textsubscript{HCOOH} of
\textasciitilde{}98\% at -0.3 V \emph{vs.} RHE, a high current density
of 10.3 mA cm\textsuperscript{--2} at -0.6 V \emph{vs.} RHE, and
sustained stable operation for 12 h under 1-sun illumination. This
underlines the critical role of electronic interactions between
photocathodes and cocatalysts in enhancing PEC performance. Dong et al.
showcased a novel nanoarchitecture with a Sn NP/GaN NW/Si photocathode
for aqueous PEC reduction of CO\textsubscript{2} to
HCOOH.\textsuperscript{{[}56{]}} Integrating defect-free GaN NWs grown
on a planar Si via MBE with electrodeposited Sn NPs, the photocathode's
PEC testing revealed exceptional performance, unprecedented TOF of 107
min\textsuperscript{--1}, a total current density of 17.5 mA
cm\textsuperscript{--2}, and a high FE\textsubscript{HCOOH} of 76.9\% at
a low potential of -0.53 V \emph{vs.} RHE under 1-sun illumination,
corresponding to a productivity of 201 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2}h\textsuperscript{--1}. The photocathode also
exhibited a high TON for HCOOH production, reaching 64,000 during stable
operation over 10 h. DFT calculations suggested that the synergistic
effects of covalent Ga-C bonding and ionic-like Sn-O bonding played a
crucial role in activating CO\textsubscript{2}, contributing to the
remarkable activity and selectivity for CO\textsubscript{2} reduction.
Deng et al. developed a Cu\textsubscript{2}O photocathode coated with
MOFs named Cu\textsubscript{3}(BTC)\textsubscript{2} (BTC =
benzene-1,3,5-tricarboxylate) for PEC CO\textsubscript{2}RR to
CO.\textsuperscript{{[}57{]}} The
Cu\textsubscript{3}(BTC)\textsubscript{2} coating played multiple roles
in the PEC CO\textsubscript{2}RR process, including preventing
photocorrosion of the Cu\textsubscript{2}O layer, facilitating electron
transfer, and providing catalytic active sites for
CO\textsubscript{2}reduction. PEC performance was measured in a
CO\textsubscript{2}-saturated MeCN containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF\textsubscript{6}). Results indicated that the
Cu\textsubscript{3}(BTC)\textsubscript{2}/Cu\textsubscript{2}O/ITO
photocathode reached a maximum FE\textsubscript{CO} of about 95\% at
applied potentials ranging from -1.77 to -1.97 V
\emph{vs.}ferrocene/ferrocenium (Fc/Fc\textsuperscript{+}), as displayed
in Figure 4D. Furthermore, the solar-to-CO (STC) efficiency of this
photocathode reached 0.83\% at -2.07 V \emph{vs.}
Fc/Fc\textsuperscript{+} under AM 1.5G illumination, as observed in
Figure 4E. Long-term chronoamperometry indicated that the photocathode
maintained a nearly constant current density under visible light and a
steady photocurrent density under chopped visible light, highlighting
its stability and performance under varying light conditions.

Wang et al. constructed a hybrid photocathode structure comprising CuO
adorned with asymmetric Cu-N sites (CuN\textsubscript{x}/CuO) for PEC
CO\textsubscript{2}RR.\textsuperscript{{[}58{]}} The
CuN\textsubscript{x}/CuO photocathode exhibited a faradaic efficiency
towards C\textsubscript{2} products (FE\textsubscript{C2}) of 15.2\%,
accompanied by a photocurrent density of -1.0 mA
cm\textsuperscript{--2}at 0.2 V \emph{vs.} RHE in
CO\textsubscript{2}-saturated 0.1M KHCO\textsubscript{3} under AM 1.5G
simulated sunlight. DFT calculations demonstrated that the adsorption of
OCCO\textsuperscript{*} and\textsuperscript{*}COCH\textsubscript{2}
intermediates on Cu-N sites, crucial for the formation of
C\textsubscript{2} products, was more favorable than on Cu-Cu sites, as
presented in Figure 4F,G. Theoretical calculations also indicated that
the CuN\textsubscript{x}/CuO photocathode had higher electron migration
efficiency than CuO due to the asymmetric d-p orbitals at Cu-N sites,
which lowered the energy barrier for C--C coupling. Roh et al.
fabricated Si NWs connected with a Cu NP ensemble to create
photocathodes (Cu NPs/Si NWs) for PEC CO\textsubscript{2}RR to
C\textsubscript{2}H\textsubscript{4}.\textsuperscript{{[}59{]}} Assessed
under 100 mW cm\textsuperscript{-2} air mass (AM) 1.5 simulated sunlight
in CO\textsubscript{2}-purged 0.1M KHCO\textsubscript{3}, the
photocathode achieved a selectivity for CO\textsubscript{2} reduction to
C\textsubscript{2}H\textsubscript{4} with a faradaic efficiency
(FE\textsubscript{C2H4}) of approximately 25\% and demonstrated activity
with partial current densities exceeding 2.5 mA cm\textsuperscript{-2}at
-0.50 V \emph{vs.} RHE. Moreover, the Cu NPs/Si NWs photocathode
exhibited long-term stability, maintaining PEC CO\textsubscript{2}RR
under 50 h of continuous bias and illumination. Kim et al. constructed a
metal-insulator-semiconductor (MIS) structure comprising
Cu/TiO\textsubscript{2}/p-Si photocathodes for PEC CO\textsubscript{2}RR
to multicarbon products.\textsuperscript{{[}60{]}} They also
investigated the effects of ionomer bilayer coatings, specifically
Nafion atop Sustainion, on the Cu surface of the
Cu/TiO\textsubscript{2}/p-Si MIS photocathodes. PEC testing indicated
that Cu/TiO\textsubscript{2}/p-Si photocathodes coated with Nafion on
top of Sustainion revealed partial current densities for
C\textsubscript{2}H\textsubscript{4} ranging from -0.9 mA
cm\textsuperscript{-2} to -2.3 mA cm\textsuperscript{--2} in
CO\textsubscript{2}-saturated 0.1M CsHCO\textsubscript{3} under wet-side
illumination, compared to the condition without ionomer bilayers on the
Cu surface. This underscores the enhancement of both activity and
selectivity for C\textsubscript{2}H\textsubscript{4} due to the Cu
cocatalyst and bilayer coatings.

\textbf{3.3 Junction engineering}

A heterojunction is typically defined as a structure composed of two or
more different semiconductors with a contacting interface, distinct band
energy levels, a matching crystal lattice, and similar thermal expansion
coefficients.\textsuperscript{{[}61{]}} Heterojunctions can be
classified into type-I, type-II (including p--n junctions), and Schottky
barrier junctions (metal-semiconductor). Semiconductor heterojunctions
often involve interfacing two semiconductor materials with differing
Fermi-level energies, creating a built-in electric field that promotes
the separation of electrons and holes upon light excitation.

In the case of type-I heterojunctions, two semiconductors with
overlapping band structures typically exhibit one semiconductor with a
more negative CB position and a more positive VB position than the other
semiconductor. Conversely, in type-II heterojunctions, the two
semiconductors have staggered band structures where electrons transition
from a more negative CB to a less negative CB, and holes move in the
opposite direction. In type-II heterojunctions, photogenerated electrons
and holes are efficiently separated, allowing for
e\textsuperscript{--}/h\textsuperscript{+} pairs to be excited using a
greater number of photons.\textsuperscript{{[}61{]}}

Schottky barrier junctions create band bending near the
semiconductor-metal catalyst interface, facilitating electron transfer
from the semiconductor to the metal. Nevertheless, this section focused
solely on semiconductor heterojunctions, as Schottky barrier junctions
overlap with concepts such as cocatalysts, and there is extensive prior
research in this domain.

For example, Quyang et al. prepared a photocathode tailored for PEC
CO\textsubscript{2}RR to HCOOH, consisting of Bi-modified 1D
ZnO/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3} nanotubes (1D Bi@ZFO
NTs).\textsuperscript{{[}62{]}} The formation of an n-n heterojunction
between the narrow bandgap of \selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}and
the wide bandgap of ZnO was instrumental in enhancing charge transfer,
establishing an internal electric field conducive to driving the
transfer of photoexcited charges, as illustrated in Figure 5A.
Additionally, depositing Bi onto the
ZnO/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3} heterojunction potentially
increased the carrier concentration at the electrode surface, thereby
enhancing the efficiency of photogenerated charge separation.
Consequently, the Bi@ZFO NTs photocathode indicated a low onset
potential of -0.53 V \emph{vs.} RHE, a low Tafel slope of 101.2 mV
dec\textsuperscript{--1}, and achieved a high FE\textsubscript{HCOOH} of
61.2\% at -0.65 V \emph{vs.} RHE, maintaining stability over 4 h under
visible light, as shown in Figure 5B,C. Jiang et al. reported a
photocathode incorporating CuO onto graphitic carbon nitride
(g-C\textsubscript{3}N\textsubscript{4}) supported on carbon paper
(CuO/g-C\textsubscript{3}N\textsubscript{4}/carbon paper) for PEC
CO\textsubscript{2}RR to
CH\textsubscript{3}OH.\textsuperscript{{[}63{]}} Establishing a type-I
heterojunction facilitated photogenerated electron transfer from the CB
of g-C\textsubscript{3}N\textsubscript{4} to the CB of CuO, which has a
relatively less negative CB energy level under illumination.
Concurrently, favorable hole transfer occurred from the VB of
g-C\textsubscript{3}N\textsubscript{4} to the VB of CuO, which held a
lower VB energy. As a result, the
CuO/g-C\textsubscript{3}N\textsubscript{4}/carbon paper photocathode
exhibited a high IPCE, an increased photocurrent response, and a
remarkable FE of 75\% and QE of 8.9\%, respectively, for
CH\textsubscript{3}OH production. Pan et al. presented a photocathode
consisting of a Cu catalyst decorated with flower-like
CeO\textsubscript{2} NPs and CuO NPs, functioning as n-type and p-type
semiconductors, respectively.\textsuperscript{{[}64{]}} The formation of
a p-n heterojunction with CeO\textsubscript{2} NPs/CuO NPs facilitated
the synergistic migration of photoexcited electrons and holes, leading
to exceptional PEC performance for CO\textsubscript{2} reduction. Thus,
the CeO\textsubscript{2} NPs/CuO NPs/Cu photocathode revealed a
CH\textsubscript{3}OH yield rate of 3.44 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2}h\textsuperscript{--1} and exhibited a high
FE\textsubscript{CH3OH} of approximately 60\% at -- 1.0 V \emph{vs.}
saturated calomel electrode (SCE) under visible light irradiation. Zheng
et al. utilized Zinc phthalocyanine (ZnPc) integrated with carbon
nitride nanosheets as a photocathode for PEC CO\textsubscript{2}RR to
CH\textsubscript{3}OH.\textsuperscript{{[}65{]}} The aligned energy
bands between ZnPc and carbon nitride facilitated electron transfer and
reduced recombination of e\textsuperscript{--}/h\textsuperscript{+}pair.
Simultaneous exposure to light and an external voltage ensured that
reductive electrons were generated not only through light excitation but
also by the external voltage. This augmented the transfer rate of
photogenerated electrons from the CB of carbon nitride to the LUMO of
ZnPc. Consequently, the ZnPc/carbon nitride photocathode exhibited a
predominant CH\textsubscript{3}OH product with a yield of 13 \selectlanguage{greek}μ\selectlanguage{english}mol
L\textsuperscript{--1} after 8 h at -1.0 V \emph{vs.} SCE. Tarek et al.
developed a heterostructured CdS-CuFe\textsubscript{2}O\textsubscript{4}
photocathode to convert CO\textsubscript{2} into
CH\textsubscript{3}OH.\textsuperscript{{[}66{]}} The
CdS--CuFe\textsubscript{2}O\textsubscript{4} photocathode exhibited a
higher IPCE of 12.09\% compared to
CuFe\textsubscript{2}O\textsubscript{4} with an IPCE of 7.28\% at 470
nm, illustrating effective visible light absorption during PEC
CO\textsubscript{2}RR. Within the
CdS--CuFe\textsubscript{2}O\textsubscript{4} heterojunction, the CB of
CdS served as the site for CO\textsubscript{2} reduction, capturing
photogenerated electrons originating from
CuFe\textsubscript{2}O\textsubscript{4}, while water oxidation occurred
at the VB of CuFe\textsubscript{2}O\textsubscript{4}. Accordingly, PEC
performance indicated that the
CdS--CuFe\textsubscript{2}O\textsubscript{4} photocathode revealed an
FE\textsubscript{CH3OH} of 72\% and a QE\textsubscript{CH3OH} of 16.9\%
and recorded a maximum CH\textsubscript{3}OH yield of 23.8 \selectlanguage{greek}μ\selectlanguage{english}mol
L\textsuperscript{--1} cm\textsuperscript{--2} in
CO\textsubscript{2}-saturated 0.1M NaHCO\textsubscript{3} electrolyte.

Xu et al. prepared a 2D heterojunction of
TiO\textsubscript{2}/Ti\textsubscript{3}CN MXene as a photocathode for
PEC CO\textsubscript{2}RR, synthesized using a simple hydrothermal
oxidation method.\textsuperscript{{[}67{]}} The 2D
TiO\textsubscript{2}/Ti\textsubscript{3}CN heterojunction, with its
large specific surface area, exceptional light absorption ability, and
abundant Ti\textsuperscript{3+} species, facilitated the efficient
generation and migration of
e\textsuperscript{--}/h\textsuperscript{+}pairs. To confirm the impact
of Ti\textsuperscript{3+}, electron paramagnetic resonance (EPR) spectra
were obtained, as depicted in Figure 5D. The EPR spectra demonstrated
widespread detection of Ti\textsuperscript{3+} species, proving
beneficial for trapping charge carriers and mitigating the recombination
of e\textsuperscript{--}/h\textsuperscript{+} pairs. Furthermore, DFT
calculations suggested that the 2D
TiO\textsubscript{2}/Ti\textsubscript{3}CN heterojunction could
spontaneously adsorb CO\textsubscript{2} molecules and stabilize key
intermediates crucial for HCOOH production, as shown in Figure 5E.
Consequently, the novel PEC system, composed of
Pd@TiO\textsubscript{2}/Ti\textsubscript{3}CN\textbar{}\textbar{}SCE\textbar{}\textbar{}BiVO\textsubscript{4},
effectively produced HCOO\textsuperscript{--}, CH\textsubscript{3}OH,
and C\textsubscript{2}H\textsubscript{5}OH with a remarkable formation
rate of 45.6 \selectlanguage{greek}μ\selectlanguage{english}M cm\textsuperscript{--2} h\textsuperscript{--1}, as
depicted in Figure 5F. Lu et al. developed a photocathode for PEC
CO\textsubscript{2}RR to C\textsubscript{2}H\textsubscript{5}OH,
comprising arrays of 0D/1D CuFeO\textsubscript{2}/CuO nanowire
heterojunction arrays synthesized through an in-situ
method.\textsuperscript{{[}68{]}} Due to the comparable energy band gaps
of CuO and CuFeO\textsubscript{2}, the photogenerated electrons
originating from the CB of CuFeO\textsubscript{2} migrated to the
surface of CuO, while photogenerated holes derived from the VB of CuO
moved to CuFeO\textsubscript{2}. This arrangement suppressed the
recombination of e\textsuperscript{--}/h\textsuperscript{+} pairs under
the built-in electric field of the heterojunction. The PEC performance
of the CuFeO\textsubscript{2}/CuO nanowire photocathode exhibited an
impressive faradaic efficiency for
C\textsubscript{2}H\textsubscript{5}OH (FE\textsubscript{C2H5OH}) of
66.73\% at -0.6 V \emph{vs.} Ag/AgCl. Zhang et al. designed a
CuFeO\textsubscript{2}/TNNTs photocathode, incorporating
high-temperature-durable n-type Nb-doped TiO\textsubscript{2} nanotube
arrays (TNNTs) and p-type CuFeO\textsubscript{2} for PEC
CO\textsubscript{2}RR.\textsuperscript{{[}69{]}} Initially, TNNTs were
synthesized through anodic oxidation on TiNb alloy sheets. Subsequently,
CuFeO\textsubscript{2}/TNNTs were constructed by coating precursor
solution on TNNTs, followed by annealing in an argon atmosphere. The
high heat stability of TNNTs preserved the well-maintained structure of
regular nanotube arrays. Additionally, TNNTs exhibited semiconductor
properties comparable to those of n-type TiO\textsubscript{2}, enabling
their integration with p-type CuFeO\textsubscript{2} to form a p-n
heterojunction. As a result, the CuFeO\textsubscript{2}/TNNTs
photocathode exhibited high light absorption and accelerated carrier
transport due to a suitable band gap and the presence of the p-n
heterojunction. In addition, the CuFeO\textsubscript{2}/TNNTs
photocathode demonstrated an outstanding photocurrent of 80 \selectlanguage{greek}μ\selectlanguage{english}A
cm\textsuperscript{--2}, resulting in
C\textsubscript{2}H\textsubscript{5}OH production at a rate of 3.3
\selectlanguage{greek}μ\selectlanguage{english}mol/5h\selectlanguage{ngerman}·cm\textsuperscript{--2}. Wang et al. presented a photocathode,
designated as BCW-X, which was achieved by depositing a
Bi\textsubscript{2}WO\textsubscript{6}/BiOCl heterojunction onto an
F-SnO\textsubscript{2} substrate through an in-situ hydrothermal
process.\textsuperscript{{[}70{]}} Notably, the exposed pristine (101)
crystal plane of BiOCl transformed into the (112) plane in the
heterojunction, facilitated by the excellent compatibility between the
(112) planes of BiOCl and the (113) planes of
Bi\textsubscript{2}WO\textsubscript{6}. Simultaneously, the
heterojunctions in BCW-X maintained a 2D layered structure, thereby
enhancing the efficiency of
e\textsuperscript{--}/h\textsuperscript{+}pair separation. PEC
experiments were conducted in a
BCW-6\textbar{}KHCO\textsubscript{3}\textbar{}BiVO\textsubscript{4} PEC
cell under illumination from an Xe lamp, with an external voltage
ranging from \selectlanguage{english}-0.6 to 1.1 V. The
BCW-6\textbar{}KHCO\textsubscript{3}\textbar{}BiVO\textsubscript{4}system
achieved a C\textsubscript{2}H\textsubscript{5}OH yield rate of 600 \selectlanguage{greek}μ\selectlanguage{english}mol
h\textsuperscript{--1} g\textsuperscript{--1} with an exceptional
selectivity of 80\% at -1.0 V.

Wang et al. reported, for the first time, a
g-C\textsubscript{3}N\textsubscript{4}/ZnTe type-II heterojunction
photocathode for PEC CO\textsubscript{2}RR to
C\textsubscript{2}H\textsubscript{5}OH.\textsuperscript{{[}71{]}} This
heterojunction facilitated the efficient separation of photoexcited
e\textsuperscript{--}/h\textsuperscript{+} pairs and promoted electron
transfer from ZnTe to g-C\textsubscript{3}N\textsubscript{4}, driven by
the establishment of an interfacial internal electric field (IEF)
created between the two semiconductors. The
g-C\textsubscript{3}N\textsubscript{4}/ZnTe photocathode displayed a
remarkable C\textsubscript{2}H\textsubscript{5}OH production rate of
17.1 \selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2} h\textsuperscript{--1} at -1.1
V\emph{vs.} Ag/AgCl. Furthermore, DFT calculations suggested that the
collaboration between ZnTe, with high CO\textsubscript{2} adsorption
ability, and g-C\textsubscript{3}N\textsubscript{4}, rich in pyridinic
N, played a role as a CO-producing site, achieving the C--C coupling
process through the adsorption of CO with proton-coupled electron
transfer. Wang et al. utilized 3D C/N-doped heterojunctions of
Zn\textsubscript{x}:Co\textsubscript{y}@Cu as a photocathode for PEC
CO\textsubscript{2}RR to paraffin products.\textsuperscript{{[}72{]}}The
Zn\textsubscript{x}:Co\textsubscript{y}@Cu photocathode consisted of
p-type semiconductor Co\textsubscript{3}O\textsubscript{4} and n-type
semiconductor ZnO on Cu foam, forming heterojunctions with various
active sites that led to outstanding C--C coupling for paraffin product
generation. Upon exposure to light irradiation in a PEC cell, the
Zn\textsubscript{x}:Co\textsubscript{y}@Cu photocathode generated
photoexcited e\textsuperscript{--}/h\textsuperscript{+} pairs, which
were rapidly separated by the built-in electric field, leading to
enhanced mobilities of charge carriers, as illustrated in Figure 5G.
Electrons could migrate from the CB of p-type
Co\textsubscript{3}O\textsubscript{4} to the CB of n-type ZnO, while
holes were either trapped by electrons from the PEC cell circuit or
reacted with OH. Therefore, the high concentration of photoelectrons was
captured by protons on the surface, resulting in the formation of
abundant active hydrogen atoms capable of converting multiple
CO\textsubscript{2} molecules into paraffin products. The
Zn\textsubscript{x}:Co\textsubscript{y}@Cu photocathode demonstrated its
optimal PEC performance by achieving a paraffin yield rate of 325 \selectlanguage{greek}μ\selectlanguage{english}g
h\textsuperscript{--1} at -0.4V \emph{vs.} SCE, all while avoiding the
release of H\textsubscript{2}, as observed in Figure 5H.

\textbf{3.4 Nanostructure engineering}

Nanostructure engineering is an effective method for enhancing the
performance of photocathodes by manipulating the dimensions and
morphology of photocathode materials at the nanometer
scale.\textsuperscript{{[}73,74{]}} Notable examples of these
nanostructures include nanorods, nanowires, dendritic structures,
core/shell structures, as well as highly nanoporous and hollow
structures. The benefits of employing such nanostructures are manifold.
They facilitate enhanced light absorption through scattering and reduce
bulk recombination. Additionally, these structures increase the specific
surface area, leading to a corresponding increase in active
sites.\textsuperscript{{[}75{]}}

Furthermore, nanostructure engineering involves tuning the
crystallographic orientations. Semiconductor nanocrystals typically
exhibit anisotropic characteristics, composed of face-dependent
electronic structures, adsorption energy/reactive sites, and
photocorrosion resistance, attributable to different atomic
configurations and coordination on various crystal
facets.\textsuperscript{{[}76-81{]}} Overall, synthesizing
nanostructures with a high aspect ratio improves light harvesting and
charge separation in the semiconductor
bulk.\textsuperscript{{[}43{]}}Additionally, nanostructures that expose
selective facets, favorable for CO\textsubscript{2} reduction kinetics,
enhance charge separation at the surface.

For example, Liu et al. fabricated a photocathode for PEC
CO\textsubscript{2}RR to CO by combining InP nanopillar arrays with
Au-TiO\textsubscript{2} interfaces
(Au-TiO\textsubscript{2}/InP).\textsuperscript{{[}82{]}} In the
fabrication process, they used inductively coupled plasma reactive ion
etching (ICP-RIE) on Au/SiO\textsubscript{x}-masked InP wafers to
synthesize InP NPs. However, ICP-RIE led to plasma-induced surface
defects on the InP NPs, resulting in a reduced minority carrier lifetime
compared to planar InP. To remedy this, a dilute HCl solution was used
to remove the plasma-damaged layer, leading to fewer surface defects in
the InP NPs and improved minority carrier lifetime, as shown in Figure
6A. Thus, these treatments, aimed at eliminating surface defects,
contributed to an increased surface area, reduced light reflection, and
minimized carrier recombination losses, ultimately enhancing
light-harvesting efficiency. The PEC performance of the nanostructured
Au-TiO\textsubscript{2}/InP photocathodes demonstrated an onset
potential of +0.3 V \emph{vs} . RHE and an FE\textsubscript{CO} of
84.2\% at -0.11 V \emph{vs} . RHE in a CO\textsubscript{2}-purged 0.1M
KHCO\textsubscript{3} solution under simulated 1 sun illumination, as
illustrated in Figure 6B,C. Cai et al. constructed a core/shell
structured nanosheet array of porous ZnO@ZnSe photocathode on FTO
glass.\textsuperscript{{[}83{]}} The synthesis involved a
dissolution-recrystallization process, serving as a template for ZnSe
growth through an anion exchange reaction with a
Se\textsuperscript{2-}source after cultivating the ZnO Ns array on the
FTO glass coated with a ZnO seed layer. The optimal ZnSe loading
demonstrated that while higher loading increased light absorption
intensity and active site density, excessive ZnSe created additional
surface defect states. Consequently, ZnO-10@ZnSe-3, with an appropriate
loading amount, exhibited an onset potential of +0.39 V \emph{vs} . RHE
in a CO\textsubscript{2}-saturated 0.5M NaHCO\textsubscript{3}
electrolyte, a photocurrent of 1.17 mA cm\textsuperscript{--2} at +0.11
V \emph{vs} . RHE, and an FE\textsubscript{CO} of 52.9\% at -0.4V
\emph{vs} . RHE under visible light irradiation. Hu et al. developed a
photocathode for PEC CO\textsubscript{2} conversion into CO featuring
uniformly dispersed Au NPs with Au (111)/Au (200) boundaries on the p-Si
surface (b-Au\textsubscript{1}/Si).\textsuperscript{{[}84{]}} Initially,
small, consistently sized Au seeds were distributed on the p-Si surface
using a chemical deposition (CD) method. Subsequent electrodeposition
ensured continuous growth of the Au above the CD seeds, maintaining a
homogeneous distribution, as depicted in Figure 6D. DFT calculations
suggested that the presence of Au (111)/Au (200) boundaries
substantially decreased the energy barrier for forming
the\textsuperscript{*}COOH intermediate during
CO\textsubscript{2}reduction. As a result, the b-Au\textsubscript{1}/Si
photocathode achieved an impressive photocurrent density of -13.1 mA
cm\textsuperscript{--2} at -1.0V \emph{vs.} RHE with an
FE\textsubscript{CO} of 82.2\% at -0.4 V \emph{vs.} RHE, and it
maintained remarkable operational stability for over a week, as shown in
Figure 6E,F.

Mubarak et al. prepared a 3D nanoporous structured
TiO\textsubscript{2}NPs on a thin Ti foil photocathode for
CO\textsubscript{2} reduction to HCOOH; the process involved a chemical
treatment with H\textsubscript{2}O\textsubscript{2}, followed by
calcination at elevated temperatures ranging from 400 to 800
.\textsuperscript{{[}85{]}} The resulting 300 to 500 nm thick 3D
nanoporous layer on the Ti-foil surface displayed significant porosity.
This structure enhanced the photon conversion efficiency of
TiO\textsubscript{2} NPs, increasing photon absorption per unit surface
area and strengthening the electrochemical reaction capabilities due to
the large specific surface area. The photocathode produced HCOOH as the
primary product in PEC CO\textsubscript{2}RR after more than 25 h of
chronoamperometric electrolysis and achieved an
FE\textsubscript{HCOOH}of 64\% with an HCOOH yield of 165 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2}h\textsuperscript{--1} at -1.3 V \emph{vs} .
Ag/AgCl. Paul et al\emph{.}designed a photocathode using a
morphology-controlled synthesis of Ag NPs distributed onto
WO\textsubscript{3} nanorods (Ag/WO\textsubscript{3}-NR) for PEC
CO\textsubscript{2}RR to
HCOO\textsuperscript{--}.\textsuperscript{{[}86{]}} The one-pot
fabrication process employed cetyltrimethylammonium bromide (CTAB) as a
structure-directing agent for forming WO\textsubscript{3} nanorods.
Critical parameters such as CTAB concentration, reflux time, and
temperature were pivotal in determining the nanorod morphology. The
Ag/WO\textsubscript{3}-NR photocathode exhibited a significant current
density of 0.4 mA cm\textsuperscript{--2} for
HCOO\textsuperscript{--}production and achieved a rate of 31.7 mmol
h\textsuperscript{--1}.

Gurudayal et al. constructed a photocathode for PEC
CO\textsubscript{2}RR to C\textsubscript{2+} products, utilizing a
back-illuminated n-type Si absorber covered with an Ag-supported
dendritic Cu catalyst.\textsuperscript{{[}87{]}} The synthesis of the
Ag-supported dendritic Cu catalyst involved the evaporation of Ag,
followed by high-rate electrodeposition of Cu, resulting in a highly
porous structure with nanocactus-like morphology and featuring a
dendritic Cu structure on the pyramid-shaped Ag. The Ag-supported
dendritic Cu catalyst exhibited a high electrochemically active surface
area, enabling the photocathode to operate at a high current density.
Therefore, the photocathode produced C\textsubscript{2+} products,
including C\textsubscript{2}H\textsubscript{4},
C\textsubscript{2}H\textsubscript{5}OH, and
C\textsubscript{3}H\textsubscript{7}OH, in CO\textsubscript{2}-saturated
0.1M CsHCO\textsubscript{3} under simulated one sun illumination.
Additionally, the photocathode maintained over 60\% FE for hydrocarbon
and oxygenated products, primarily C\textsubscript{2}H\textsubscript{4},
C\textsubscript{2}H\textsubscript{5}OH, and
C\textsubscript{3}H\textsubscript{7}OH, for several days under simulated
diurnal illumination. Kempler et al. utilized high loadings of Cu
integrated onto Si microwire arrays
(n\textsuperscript{+}p\textsuperscript{--}Si \selectlanguage{greek}μ\selectlanguage{english}W/Cu) for PEC
CO\textsubscript{2}RR to
C\textsubscript{2}H\textsubscript{4}.\textsuperscript{{[}88{]}} The Si
microwire array structure could diminish trade-offs between catalyst
loading and light absorption intensity. A Si photocathode with Cu
electrodeposited onto the vertical sidewalls of high-aspect-ratio
microwires was designed to minimize parasitic absorption by the
catalyst, as revealed in Figure 6G, demonstrating a
\textbar{}J\textsubscript{ph}\textbar{} exceeding 25 mA
cm\textsuperscript{--2} before and after 48 h of PEC
CO\textsubscript{2}RR, resulting in the production of
C\textsubscript{2}H\textsubscript{4} at more positive potentials under
1-sun illumination. Thus, the
n\textsuperscript{+}p\textsuperscript{--}Si \selectlanguage{greek}μ\selectlanguage{english}W/Cu photocathode revealed
a maximum \textbar{}J\textsubscript{C2H4}\textbar{} of 2.1
\(\pm\) 0.2 mA cm\textsuperscript{--2} at -0.44 V \emph{vs} .
RHE and a maximum \textbar{}J\textsubscript{CH4}\textbar{} of 2.9
\(\pm\) 0.7 mA cm\textsuperscript{--2} at -0.62 V \emph{vs} .
RHE, as shown in Figure 6H,I.

\textbf{3.5 Defect engineering}

Defect engineering, beyond doping, involves native point defects such as
vacancies and interstitials occurring naturally during material
synthesis, influencing catalytic, electrical, and optical properties.
These defects are now recognized as a strategy to enhance photoactivity
in heterogeneous catalysis. Classified based on dimensional space,
defects include 0D (point defects), 1D (line defects), 2D (interface
defects), and 3D (bulk defects).\textsuperscript{{[}89-93{]}} Their
roles encompass both geometric and electronic effects, acting as
adsorption sites due to their high energy state and influencing
electronic structures. Defects, with their dynamic structures, improve
activation and diffusion in stable CO\textsubscript{2} reactions.
Additionally, they alter electronic structures to affect adsorption
energy, steering reaction pathways in processes like
CO\textsubscript{2}reduction. The controlled fabrication of desirable
defects is crucial for successful defect engineering, enhancing
CO\textsubscript{2}reduction kinetics, and minimizing
catalysis-recombination trade-offs without needing external catalytic
entities.\textsuperscript{{[}94-98{]}}

Dong et al. investigated grain boundary (GB) oxidation in Cu-Ag thin
films and its impact on the selectivity of CO and
CH\textsubscript{4}production.\textsuperscript{{[}99{]}} They developed
a photocathode incorporating a Cu-Ag thin-film cocatalyst and p-type Si
to enhance the efficiency of PEC CO\textsubscript{2}RR, as revealed in
Figure 7A,B. The electron beam evaporation method facilitated the direct
growth of Cu thin films on the substrate, with the grain size easily
controlled by adjusting the thickness through nucleation and growth
processes. It was observed that oxygen from the surrounding air
permeated the Cu thin film through gaps between the Ag islands, leading
to the oxidation of the Cu, particularly at the unstable GBs of
uncoordinated Cu atoms. Consequently, smaller Cu grains with a higher GB
density were prone to oxidation, which compromised the catalytic
activity of the Cu-Ag thin-film catalyst. In contrast, a relatively
thick Cu layer ([?]80 nm) with a larger grain size effectively prevented
oxidation, resulting in catalytic properties comparable to those of bulk
Cu-Ag catalysts. Optimizing the Cu (100 nm) - Ag (3 nm) thin-film
catalyst revealed a bifunctional characteristic. This catalyst could
selectively produce both CO (FE\textsubscript{CO} of 79.8\%) and
CH\textsubscript{4}(FE\textsubscript{CH4} of 59.3\%) at potentials of
-1.0 and -1.4 V\emph{vs.} RHE, respectively, as shown in Figure 7C,D.
Furthermore, introducing a novel PEC architecture comprising the
patterned Cu-Ag thin film, a SiO\textsubscript{2} passivation layer, and
a p-Si photocathode, significantly improved the selectivity of CO and
CH\textsubscript{4}under light illumination (100 mW
cm\textsuperscript{-2}). Cheng et al. explored the synthesis of CdS NPs
featuring controllable S-vacancies encapsulated within a Zeolitic
imidazolate framework-8 (ZIF-8) and utilized as a precursor for
nitrogen-doped porous carbon (NCP) through a two-step
process.\textsuperscript{{[}100{]}} Initially, CdS NPs were stabilized
with polyvinylpyrrolidone (PVP), followed by the deposition of a ZIF-8
shell onto their surface, resulting in a core-shell structure with the
CdS NPs as the core and ZIF-8 as the shell. The subsequent control of S
vacancies in the CdS and the pyridinic N content in the NCP was achieved
through pyrolysis at various temperatures. The CdS/NCP catalysts,
serving as cathodes with a TiO\textsubscript{2} nanotube array
photoanode, enhanced the conductivity and stability of TMS-based
catalysts and facilitated CO\textsubscript{2} reduction. Based on the
pyrolysis temperature, the tunable S vacancies in the CdS/NCP samples
led to enhanced selectivity towards CH\textsubscript{3}OH, which is
attributed to the synergistic impact of S-vacancies in CdS and the
pyridinic N content in nitrogen-doped porous carbon. The resulting
CdS/NCP material, with its porous structure, abundant S-vacancies, high
pyridinic N content, and enhanced conductivity, proved effective as a
cathode catalyst for CO\textsubscript{2} reduction. This synergistic
effect was precisely controlled by adjusting the temperature
(\emph{i.e.} , 300, 500, and 700 \selectlanguage{ngerman}°C) during the thermal treatment of the
hybrid material. Notably, the sample treated at 500 °C (CdS/NCP-500
catalyst) exhibited a high conversion rate (3052
nmol·h\textsuperscript{--1}·cm\textsuperscript{--2}) with a selectivity
of 77.3\% towards CH\textsubscript{3}OH. Tu et al. investigated the
mechanisms governing the impact of nitrogen vacancies on
photocatalysis.\textsuperscript{{[}101{]}} They synthesized
g-C\textsubscript{3}N\textsubscript{4} nanosheets with adjustable
nitrogen vacancies as photocatalysts for both
H\textsubscript{2}evolution and CO\textsubscript{2} photoreduction.
Combining experimental analyses with DFT calculations, the study found
that nitrogen vacancies in g-C\textsubscript{3}N\textsubscript{4}
triggered the formation of mid-gap states below the conduction band
edge, with the depth of these states increasing proportionally to the
number of nitrogen vacancies. The presence of nitrogen vacancies was
pivotal in exciting electrons into mid-gap states, enabling absorption
of longer-wavelength photons and minimizing recombination losses of
e\textsuperscript{--}/h\textsuperscript{+} pairs. The trapped electrons
were efficiently transferred to a Pt cocatalyst. Additionally, the
nitrogen vacancies induced a unique chemical environment on the surface,
leading to highly dispersed ultrasmall Pt NPs (1\selectlanguage{english}-2 nm) on
g-C\textsubscript{3}N\textsubscript{4}. The optimally doped
photocatalyst with the ideal nitrogen vacancy density showed markedly
improved photocatalytic activities, an 18-fold increase in
H\textsubscript{2} evolution, and a four-fold enhancement for
CO\textsubscript{2} reduction to CO. However, an excessive increase in
nitrogen vacancies resulted in diminished photocatalytic activity due to
the deeper mid-gap states acting as recombination sites for
photogenerated e\textsuperscript{--}/h\textsuperscript{+} pairs, thus
reducing overall efficiency.

Kan et al. ingeniously devised and crafted a
p-Si/n-ZnO\textsubscript{v}/p-Cu\emph{\textsubscript{x}} O
heterostructure, incorporating a ZnO\emph{\textsubscript{v}} -derived
Cu\emph{\textsubscript{x}} O defect level that exhibits a remarkable
capability for selective PEC CO\textsubscript{2}RR to
C\textsubscript{2}H\textsubscript{5}OH at low
biases.\textsuperscript{{[}102{]}} The p-n-p band alignment was pivotal
in confining and accumulating multiple electrons within the conduction
band of n-type ZnO\textsubscript{v}, facilitated by a built-in electric
field. Simultaneously, the shallow ZnO\textsubscript{v} defect level
(vE\textsubscript{ZnOv}) allowed electrons to escape from the confined
well and reach the Cu\textsubscript{x}O (E\textsubscript{Cu\emph{x} O},
0.05 V \emph{vs.} RHE). These tunneling defect energy levels on
Cu\textsubscript{x}O closely aligned with those required for the
CO\textsubscript{2} to C\textsubscript{2}H\textsubscript{5}OH reduction,
contributing to the heterostructure's exceptional selectivity for PEC
CO\textsubscript{2}RR towards C\textsubscript{2}H\textsubscript{5}OH at
low biases, accompanied by an outstanding FE. In contrast, control
samples, including p-Si/p-Cu\textsubscript{x}O and
p-Si/n-ZnO\textsubscript{v}, necessitated higher overpotentials to
overcome larger energy barriers, resulting in distinct
CO\textsubscript{2} reduction selectivity towards CH\textsubscript{4}and
HCOO\textsuperscript{--}, respectively. The transfer of photoelectrons
in the Si/ZnO\textsubscript{v}/Cu\textsubscript{x}O system was
facilitated by a built-in electric field of approximately 0.6 V through
a leaky tunnel formed in the defect levels of ZnO\textsubscript{v} and
Cu\textsubscript{x}O, allowing for a close matching of energy levels and
enabling the selective conversion of CO\textsubscript{2} into
C\textsubscript{2}H\textsubscript{5}OH. The optimized potential and
functional interface contributed to achieving an impressive FE exceeding
60\% for PEC reduction of CO\textsubscript{2} to
C\textsubscript{2}H\textsubscript{5}OH under 0 V \emph{vs.} RHE.

In addition to the prerequisites for efficient charge carrier transfer,
studies have indicated that defect engineering plays a crucial role in
regulating catalytic activity and CO\textsubscript{2} reduction
selectivity based on the surface state of the photocatalyst.
Nevertheless, the specific impact of sulfur vacancies on the transfer of
photogenerated e\textsuperscript{--}/h\textsuperscript{+} pairs and the
reaction mechanism during CO\textsubscript{2} reduction remains unclear.
Zhou et al. implemented a heat treatment strategy on
Cu\textsubscript{2}ZnSnS\textsubscript{4}/CdS (CZTS/CdS) photocathodes,
achieving simultaneous optimization of interface charge transfer and
surface S vacancy engineering.\textsuperscript{{[}103{]}} These
advancements significantly contributed to the enhanced overall PEC
CO\textsubscript{2}RR performance and manageable selectivity. Heat
treatment improved the CZTS/CdS heterojunction interface by promoting
elemental inter-diffusion between Cd in CdS and Cu/Zn in CZTS, as shown
in Figure 7E. This resulted in a more favorable p-n junction with an
enlarged built-in potential, prolonged carrier lifetime, and suppressed
charge recombination. Additionally, defects on the surface of CdS could
be modulated through heat treatment in different atmospheres. Heat
treatment in air replenished intrinsic S vacancies on the CZTS/CdS
surface with oxygen, enhancing CO\textsubscript{2} and CO adsorption
capability, as observed in Figure 7F, which leads to improved
CO\textsubscript{2} reduction activity and higher selectivity toward
CH\textsubscript{3}OH / C\textsubscript{2}H\textsubscript{5}OH.
Conversely, heat treatment in N\textsubscript{2} generated more S
vacancies on the surface, facilitating surficial CO desorption and
higher CO selectivity. By combining heterojunction design and
modification of catalyst surface properties through a simple heat
treatment strategy, this work established a new approach to designing
photocathodes for high-performance PEC CO\textsubscript{2}RR activity
with controlled selectivity. Table 2 lists the performance of
interface-engineered photocathodes for PEC CO\textsubscript{2}RR.

\textbf{4 CONCLUSIONS AND PERSPECTIVES}

This review underscores the critical role of interface engineering in
optimizing the efficiency of PEC catalysts, providing valuable insights
into recent research trends and contributing to the global pursuit of
carbon neutrality. Interface engineering emerges as a pivotal strategy,
optimizing cocatalysts, functional engineering, and defect engineering
to overcome existing limitations. Noble metal cocatalysts, such as Ag
and Au, are seamlessly integrated with plasmonic materials to induce
surface plasmon resonance, thereby enhancing light absorption and
catalytic efficiency for photocatalytic CO\textsubscript{2} reduction.
This enhancement is evident in various studies utilizing diverse
plasmonic nanostructures and synthesis methods. When semiconductor
materials are integrated with various cocatalysts, including noble
metals, cobalt-based molecular catalysts, and non-noble metal materials,
they demonstrate promising advancements in enhancing efficiency,
selectivity, and stability for photocatalytic
CO\textsubscript{2}reduction. Semiconductor heterojunctions,
encompassing type-I, type-II, and Schottky barrier junctions, play a
pivotal role in enhancing PEC CO\textsubscript{2}RR by facilitating the
separation of photogenerated electrons and holes through distinct band
structures. Nanostructure engineering further augments photocathode
performance by manipulating materials on the nanometer scale, employing
structures such as nanorods, nanowires, and core/shell configurations.
Defect engineering, involving native point defects like vacancies and
interstitials, influences catalytic, electrical, and optical properties.
These defects, classified as 0D (point defects), 1D (line defects), 2D
(interface defects), and 3D (bulk defects), enhance photoactivity in
heterogeneous catalysis. Despite numerous research outcomes, interface
engineering still requires further studies, and researchers posit that
several topics will need more in-depth exploration.

* The PEC process based on interface engineering often involves complex
procedures and expensive equipment, necessitating the development of
more streamlined and cost-effective approaches.

* Extensive research is required to assess the long-term performance,
stability, and durability of catalysts used in
CO\textsubscript{2}reduction experiments, particularly in challenging
environmental conditions.

* Current investigations into PEC CO\textsubscript{2}RR predominantly
yield C\textsubscript{1} products; therefore, a critical need exists for
comprehensive studies focusing on C\textsubscript{2+} products, which
offer higher value-added potential.

* To facilitate the commercialization of catalysts, validating their
performance on a larger scale within the existing laboratory setting is
essential. This includes emphasizing creating and evaluating larger-area
catalysts when implementing interface engineering technology.

Researchers have made significant strides in addressing these
challenges. Notably, PEC co-catalysts have demonstrated potential in
improving PEC system performance by effectively capturing and
segregating photogenerated charge carriers while minimizing
recombination. However, the implementation of these co-catalysts still
encounters diverse challenges. Common issues associated with PEC systems
exist, along with strategies aimed at overcoming them. The mere
application of a co-catalyst onto the photoelectrode surface proves
insufficient for constructing a robust and efficient PEC cell.
Consequently, engineering the interface between co-catalysts and the
photoelectrode surface becomes imperative for facilitating the desired
reactions. Strategies to enhance PEC efficiency and stability involve
the incorporation of suitable interlayers, transport layers, and
electron-blocking layers. While co-catalysts play a pivotal role in
promoting desired PEC reactions, their inefficient utilization can
hinder overall performance. To address this concern, facile strategies
such as optimizing co-catalyst loading and integrating them into
composite materials need exploration. This approach aims to improve the
surface area and active sites of the co-catalysts. Additionally,
designing efficient configurations, such as core/shell structures with
multijunction components, represents an effective means of enhancing
co-catalyst utilization and overall efficiency. In summary, this review
paper explores current trends in PEC CO\textsubscript{2} reduction
research from the perspective of interface engineering. By identifying
these trends, the paper aims to make a significant contribution to the
broader field of carbon dioxide reduction research.

\textbf{ACKNOWLEDGEMENT}

This research was supported by the National Research Foundation of Korea
(NRF), funded by the Korean government (2022M3H4A1A01012712,
2022M3H4A1A04096380).

\textbf{CONFLICT OF INTEREST}

All authors declared that there are no conflicts of interest.

\textbf{ORCID}



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TABLE 1 Standard reduction potentials of the electrochemical
CO\textsubscript{2}RR in aqueous solution at pH7.\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
Products & Reactions & Standard reduction potential {[}V vs.
NHE{]}\tabularnewline
\midrule
\endhead
Carbon Monoxide (CO) & CO\textsubscript{2} + 2H\textsuperscript{+} +
2e\textsuperscript{--} - CO + H\textsubscript{2}O & -0.53\tabularnewline
Methane (CH\textsubscript{4}) & CO\textsubscript{2} +
8H\textsuperscript{+} + 8e\textsuperscript{--} - CH\textsubscript{4} +
2H\textsubscript{2}O & -0.24\tabularnewline
Formic acid (HCOOH) & CO\textsubscript{2} + 2H\textsuperscript{+} +
2e\textsuperscript{--} - HCOOH & -0.61\tabularnewline
Methanol (CH\textsubscript{3}OH) & CO\textsubscript{2} +
6H\textsuperscript{+} + 6e\textsuperscript{--} - CH\textsubscript{3}OH +
H\textsubscript{2}O & -0.38\tabularnewline
Ethylene (C\textsubscript{2}H\textsubscript{4}) & 2CO\textsubscript{2} +
12H\textsuperscript{+} + 12e\textsuperscript{--} -
C\textsubscript{2}H\textsubscript{4} + 4H\textsubscript{2}O &
0.06\tabularnewline
Ethanol (C\textsubscript{2}H\textsubscript{5}OH) & 2CO\textsubscript{2}
+ 12H\textsuperscript{+} + 12e\textsuperscript{--} -
C\textsubscript{2}H\textsubscript{5}OH + 3H\textsubscript{2}O &
0.08\tabularnewline
Ethane (C\textsubscript{2}H\textsubscript{6}) & 2CO\textsubscript{2} +
14H\textsuperscript{+} + 14e\textsuperscript{--} -
C\textsubscript{2}H\textsubscript{6} + 4H\textsubscript{2}O &
-0.27\tabularnewline
Acetic acid (CH\textsubscript{3}COOH) & 2CO\textsubscript{2} +
8H\textsuperscript{+} + 8e\textsuperscript{--} - CH\textsubscript{3}COOH
+ 2H\textsubscript{2}O & -0.30\tabularnewline
Isopropanol (C\textsubscript{3}H\textsubscript{7}OH) &
3CO\textsubscript{2} + 18H\textsuperscript{+} + 18e- -
C\textsubscript{3}H\textsubscript{7}OH + 5H\textsubscript{2}O &
--0.31\tabularnewline
\bottomrule
\end{longtable}

TABLE 2 Summary of the strategy for interface engineering of a
photocathode for PEC CO\textsubscript{2}RR\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Strategy & Photocathode materials & Electrolyte & Performance about
activity and selectivity & Ref.\tabularnewline
\midrule
\endhead
Plasmonic noble metal co-catalysts &
CuBi\textsubscript{2}O\textsubscript{4} IOs-Ag & 0.1M
KHCO\textsubscript{3} & FE\textsubscript{CO} of 92\% &
{[}37{]}\tabularnewline
& Au/TiO\textsubscript{2}/n\textsuperscript{+}p\textsuperscript{--} Si &
0.1M KHCO\textsubscript{3} & j\textsubscript{CO} of 5.52 mA
cm\textsuperscript{--2}, FE\textsubscript{CO} of 86\% &
{[}38{]}\tabularnewline
& Ag-TiO\textsubscript{2}/RGO & 1.0M KOH & j\textsubscript{total} of
23.5 mA cm\textsuperscript{--2}, FE\textsubscript{CH3OH} of 60.5\% &
{[}39{]}\tabularnewline
& Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO & 0.1M KOH &
j\textsubscript{total} of -31.5 mA cm\textsuperscript{--2},
FE\textsubscript{CH3OH} of 21.5\% & {[}40{]}\tabularnewline
& Ag/Cu-TS-1 & 0.1M KHCO\textsubscript{3} &
C\textsubscript{2}H\textsubscript{5}OH yield rate of 2.62 \selectlanguage{greek}μ\selectlanguage{english}mol
cm\textsuperscript{--2} h\textsuperscript{--1} & {[}41{]}\tabularnewline
& Cu\textsubscript{2}O/Ag & 0.1M KHCO\textsubscript{3} & yield rate of
212.7 \selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2} h\textsuperscript{--1},
FE\textsubscript{CH3COOH} of 47.7\% & {[}42{]}\tabularnewline
Non-noble metal co-catalysts & STA-GO/CoPc & 0.1M KHCO\textsubscript{3}
& j\textsubscript{total} of 0.7 mA cm\textsuperscript{--2},
FE\textsubscript{CO} of 86\%, FE\textsubscript{CH3OH} of 8\% &
{[}50{]}\tabularnewline
& Si\textbar{}mesoTiO\textsubscript{2}\textbar{}CoPcP & 0.5M
KHCO\textsubscript{3} & TONco of 939 \selectlanguage{ngerman}± 132, FE\textsubscript{CO} of 66\%
& {[}51{]}\tabularnewline
Junction Engineering Nanosturcture Engineering Defect Engineering &
Si\textbar{}TiO\textsubscript{2}\textbar{}CNT\textbar{}Co\textsuperscript{II}(BrqPy)
Si\textbar{}mesoTiO\textsubscript{2}\textbar{}CotpyP CuS/GaN/Si
Bi/GaN/Si Sn NP/GaN NW/Si
Cu\textsubscript{3}(BTC)\textsubscript{2}/Cu\textsubscript{2}O
CuN\textsubscript{x}/CuO Cu NPs/Si NWs Cu/TiO\textsubscript{2}/p-Si
Bi@ZFO NTs CuO/g-C\textsubscript{3}N\textsubscript{4}/carbon paper
CeO\textsubscript{2} NPs/CuO NPs/Cu ZnPc/carbon nitride
CdS-CuFe\textsubscript{2}O\textsubscript{4}
Pd@TiO\textsubscript{2}/Ti\textsubscript{3}CN CuFeO\textsubscript{2}/CuO
nanowire CuFeO\textsubscript{2}/TNNTs BCW-6
g-C\textsubscript{3}N\textsubscript{4}/ZnTe
Zn\textsubscript{x}:Co\textsubscript{y}@Cu Au-TiO\textsubscript{2}/InP
ZnO@ZnSe b-Au\textsubscript{1}/Si TO600 Ag/WO\textsubscript{3}-NR
Ag-supported dendritic Cu n\textsuperscript{+}p\textsuperscript{-}Si
\selectlanguage{greek}μ\selectlanguage{english}W/Cu Cu-Ag thin film CdS/NCP Pt on
g-C\textsubscript{3}N\textsubscript{4}
p-Si/n-ZnO\textsubscript{v}/p-Cu\textsubscript{x}O CZTS/CdS & 0.1M
KHCO\textsubscript{3} MeCN with 0.1M TBABF\textsubscript{4} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} MeCN with 0.1M TBAPF\textsubscript{6} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.1M
CsHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.1M
NaHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} 0.1M NaHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} Triethanolamine 0.1M NaHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.5M
NaHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 1M KOH 0.1M
KHCO\textsubscript{3} 0.1M CsHCO\textsubscript{3} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} 0.5M
KHCO\textsubscript{3} 0.5M Na\textsubscript{2}SO\textsubscript{4} 0.1M
KHCO\textsubscript{3} 0.1M KHCO\textsubscript{3} &
j\textsubscript{total} of -1.4 mA cm\textsuperscript{--2},
FE\textsubscript{CO} of 97\% TON of 381 j\textsubscript{HCOOH} of 7.07
mA cm\textsuperscript{--2}, FE\textsubscript{HCOOH} of 70.2\%
j\textsubscript{HCOOH} of 10.3 mA cm\textsuperscript{--2},
FE\textsubscript{HCOOH} of 98\% j\textsubscript{total} of 17.5 mA
cm\textsuperscript{--2}, FE\textsubscript{HCOOH} of 76.9\%
FE\textsubscript{CO} of about 95\% j\textsubscript{total} of -1.0 mA
cm\textsuperscript{--2}, FE\textsubscript{C2} of 15.2\%
j\textsubscript{C2H4} exceeding 2.5 mA cm\textsuperscript{--2},
FE\textsubscript{C2H4} of about 25\% j\textsubscript{C2H4} of -2.3 mA
cm\textsuperscript{--2} FE\textsubscript{HCOOH} of 61.2\%
FE\textsubscript{CH3OH} of 75\% CH\textsubscript{3}OH yield rate of 3.44
\selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2} h\textsuperscript{--1},
FE\textsubscript{CH3OH} of 60\% CH\textsubscript{3}OH yield of 13 \selectlanguage{greek}μ\selectlanguage{english}mol
L\textsuperscript{--1} CH\textsubscript{3}OH yield of 23.8 \selectlanguage{greek}μ\selectlanguage{english}mol
L\textsuperscript{--1} cm\textsuperscript{--2}, FE\textsubscript{CH3OH}
of 72\% yield rate of 45.6 \selectlanguage{greek}μ\selectlanguage{english}M cm\textsuperscript{--2}
h\textsuperscript{--1} FE\textsubscript{C2H5OH} of 66.73\%
j\textsubscript{total} of 80 \selectlanguage{greek}μ\selectlanguage{english}A cm\textsuperscript{--2},
C\textsubscript{2}H\textsubscript{5}OH yield rate of 3.3
\selectlanguage{greek}μ\selectlanguage{english}mol/5h\selectlanguage{ngerman}·cm\textsuperscript{--2} C\textsubscript{2}H\textsubscript{5}OH
yield rate of 600 \selectlanguage{greek}μ\selectlanguage{english}mol h\textsuperscript{--1} g\textsuperscript{--1},
FE\textsubscript{C2H5OH} of 80\% C\textsubscript{2}H\textsubscript{5}OH
yield rate of 17.1 \selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2} h\textsuperscript{--1}
paraffin yield rate of 325\selectlanguage{greek}μ\selectlanguage{english}g h\textsuperscript{--1} FE\textsubscript{CO}
of 84.2\% j\textsubscript{total} of 1.17 mA cm\textsuperscript{--2},
FE\textsubscript{CO} of 52.9\% j\textsubscript{total} of -13.1 mA
cm\textsuperscript{--2}, FE\textsubscript{CO} of 82.2\% HCOOH yield rate
of 165 \selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2} h\textsuperscript{--1},
FE\textsubscript{HCOOH} of 64\% j\textsubscript{HCOO--} of 0.4 mA
cm\textsuperscript{--2}, HCOO\textsuperscript{--} yield rate of 31.7
mmol h\textsuperscript{--1} FE\textsubscript{C2+} of 60\%
j\textsubscript{CH4} of 2.9 \(\pm\) 0.7 mA
cm\textsuperscript{--2}, j\textsubscript{C2H4} of 2.1 \(\pm\)
0.2 mA cm\textsuperscript{--2} FE\textsubscript{CO} of 79.8\%,
FE\textsubscript{CH4} of 59.3\% FE\textsubscript{CH3OH} of 77.3\% CO
yield rate of 6.21 \selectlanguage{greek}μ\selectlanguage{english}mol h\textsuperscript{--1} FE\textsubscript{C2H5OH}
of exceeding 60\% CO yield rate of 2.31 \selectlanguage{greek}μ\selectlanguage{english}mol cm\textsuperscript{--2}
h\textsuperscript{--1} & {[}52{]} {[}53{]} {[}54{]} {[}55{]} {[}56{]}
{[}57{]} {[}58{]} {[}59{]} {[}60{]} {[}62{]} {[}63{]} {[}64{]} {[}65{]}
{[}66{]} {[}67{]} {[}68{]} {[}69{]} {[}70{]} {[}71{]} {[}72{]} {[}82{]}
{[}83{]} {[}84{]} {[}85{]} {[}86{]} {[}87{]} {[}88{]} {[}99{]} {[}100{]}
{[}101{]} {[}102{]} {[}103{]}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

FIGURE 1 (A) Schematic of interface engineering strategies categorized
into five sections. (B) Schematic illustration of basic principles of
PEC CO\textsubscript{2}RR.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

FIGURE 2 (A) Schematic illustration of the fabrication process of an
Au/TiO\textsubscript{2}/n\textsuperscript{+}p\textsuperscript{--} Si
photocathode. (B) LSV curves, (C) FE\textsubscript{CO}, (D) partial
photocurrent density for CO, and (E) long-term operational stability
test of the
Au/TiO\textsubscript{2}/n\textsuperscript{+}p\textsuperscript{--} Si
photocathode. Reproduced with
permission.\textsuperscript{{[}38{]}}Copyright 2022, John Wiley and
Sons. (F) product yield for the
Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO photocathode at different
potentials (V \emph{vs.} RHE). (G) total photocurrent density of the
Au/\selectlanguage{greek}α-\selectlanguage{english}Fe\textsubscript{2}O\textsubscript{3}/RGO photocathode. Reproduced
with permission.\textsuperscript{{[}40{]}} Copyright 2021, Elsevier.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

FIGURE 3 (A) Schematic illustration of the preparation of GO/CoPC and
(B) STA-GO/CoPc photocathode. (C) photocurrent density with FE for
H\textsubscript{2} and CO of the STA-GO/CoPc photocathode. Reproduced
with permission.\textsuperscript{{[}50{]}} Copyright 2022, John Wiley
and Sons. (D) Illustration of the
Si\textbar{}TiO\textsubscript{2}\textbar{}CNT\textbar{}Co\textsuperscript{II}(BrqPy)
photocathode. (E) LSVs and (F) FE for H\textsubscript{2} and CO of the
Si\textbar{}TiO\textsubscript{2}\textbar{}CNT\textbar{}Co\textsuperscript{II}(BrqPy)
photocathode compared with other groups. Reproduced with
permission.\textsuperscript{{[}52{]}} Copyright 2022, John Wiley and
Sons.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

FIGURE 4 (A) Schematic illustration of the CuS/GaN/Si photocathode
synthesis and PEC experiments. (B) FE\textsubscript{HCOOH} and (C)
j\textsubscript{COOH} of the CuS/GaN/Si photocathode. Reproduced with
permission.\textsuperscript{{[}54{]}} Copyright 2021, \textbf{American
Chemical Society.} (D) FE\textsubscript{CO} and (E) STC efficiency of
the Cu\textsubscript{2}O and
Cu\textsubscript{3}(BTC)\textsubscript{2}/Cu\textsubscript{2}O/ITO
photocathode. Reproduced with
permission.\textsuperscript{{[}57{]}}Copyright 2019, \textbf{American
Chemical Society.} The calculated energy barrier for forming (F) EtOH
and (G) acetate for the CuO and CuN\textsubscript{x}/CuO photocathode.
Reproduced with permission.\textsuperscript{{[}58{]}} Copyright 2022,
Elsevier.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

FIGURE 5 (A) Proposed PEC CO\textsubscript{2}RR mechanism of 1D Bi@ZFO
NTs photocathode. (B) LSV curves and (C) FE of H\textsubscript{2},
CH\textsubscript{4}, CO, and HCOOH of the 1D Bi@ZFO NTs photocathode.
Reproduced with permission.\textsuperscript{{[}62{]}} Copyright
2022,\textbf{American Chemical Society.} (D) EPR spectra and (E)
calculated Gibbs free energy of forming HCOOH. (F) Various product yield
rates of the TiO\textsubscript{2}/Ti\textsubscript{3}CN MXene
photocathode. Reproduced with permission.\textsuperscript{{[}67{]}}
Copyright 2023, Elsevier. (G) Proposed mechanism of PEC
CO\textsubscript{2}RR to paraffin products and (H) paraffin yield rate
of Zn\textsubscript{x}:Co\textsubscript{y}@Cu photocathode. Reproduced
with permission.\textsuperscript{{[}72{]}} Copyright 2020, Elsevier.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

FIGURE 6 (A) TRPL spectra of samples. (B) Photocurrent density and (C)
FE\textsubscript{CO} and FE\textsubscript{H2} of the
Au-TiO\textsubscript{2}/InP photocathode. Reproduced with
permission.\textsuperscript{{[}82{]}} Copyright 2021, \textbf{American
Chemical Society.} (D) Schematic of the fabrication process, (E) LSV
curves, and (F) FE\textsubscript{CO} and FE\textsubscript{H2} of the
b-Au\textsubscript{1}/Si photocathode. Reproduced with
permission.\textsuperscript{{[}84{]}} Copyright 2023,
\textbf{Elsevier.}(G) Schematic illustration of the
n\textsuperscript{+}p\textsuperscript{--}Si \selectlanguage{greek}μ\selectlanguage{english}W/Cu photocathode. (H)
j\textsubscript{CH4} and (I) j\textsubscript{C2H4} of the
n\textsuperscript{+}p\textsuperscript{--}Si \selectlanguage{greek}μ\selectlanguage{english}W/Cu photocathode.
Reproduced with permission.\textsuperscript{{[}88{]}} Copyright 2020,

\textbf{American Chemical Society.}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

FIGURE 7 Schematic illustrations of Cu-Ag thin-film cocatalyst with (A)
small Cu grains and (B) large Cu grains. FE\textsubscript{total} from
(C) p-Si / Cu-Ag, (D) p-Si / SiO\textsubscript{2}-Cu-Ag. Reproduced with
permission.\textsuperscript{{[}99{]}} Copyright 2021, \textbf{American
Chemical Society.} (E) Schematic fabrication processes of CZTS/CdS by
different heat treatment conditions (Air or
N\textsubscript{2}atmospheres). (F) CO\textsubscript{2} reduction
products yield rate of CZTS, CZTS/CdS and CZTS/CdS (HA). Reproduced with
permission.\textsuperscript{{[}103{]}} Copyright 2021, John Wiley and
Sons.

AUTHOR BIOGRAPHIES\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Jae Hak Kim} is currently pursuing his M.S. at the Korea
University in the Department of Materials Science and Engineering. His
research interests include electrochemical carbon dioxide reduction
reactions over metal-organic frameworks and single-atom-based materials.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Sung Hyun Hong} is currently pursuing his Ph.D. at the Korea
University in the Department of Materials Science and Engineering. His
research interests include electrochemical carbon dioxide reduction
reactions over metal-organic frameworks and single-atom-based materials.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Sang Hyun Ahn} received his Ph.D. degree from School of Chemical
and Biological Engineering of Seoul National University in 2012. After
that, he joined Thomas Moffat's group as a postdoctoral researcher at
National Institute of Standard and Technology, USA. Currently, he is a
professor at School of Chemical Engineering and Material Science at
Chung-Ang University. His research focuses on electrochemical materials
and devices for energy conversion and storage, including water, carbon
dioxide, and ammonia electrolysis.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Soo Young Kim} is a professor in the Department of Materials
Science and Engineering at Korea University. He is interested in the
modulation of the properties of 2D materials, halide perovskites,
metal-organic frameworks, and multi-component single-atom catalysts and
their applications in various fields such as organic photovoltaics,
perovskite solar cells, light-emitting diodes, gas sensors, hydrogen
evolution reactions, and carbon dioxide reduction reactions.

\selectlanguage{english}
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\end{document}

