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\begin{document}

\title{A measurement system for photoelectrochemical processes with high
quality irradiation, temperature control and automated gas analysis}



\author[1]{David Adner}%
\author[1]{Lorenz Pfordte}%
\author[1]{Milan Selle}%
\author[1]{Max Pohl}%
\author[1]{Marko Turek}%
\author[1]{Christian Hagendorf}%
\affil[1]{Fraunhofer-Center fur Silizium-Photovoltaik CSP}%


\vspace{-1em}



  
  \date{May 30, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
In this Application Note, a versatile and reliable measurement system
for photoelectrochemical investigations is described which aims to
assist scientists in obtaining reproducible photoelectrochemical data of
high quality, including the solar-to-hydrogen (STH) efficiency.
Specifically, it addresses the parameters irradiation quality, reaction
temperature and gas measurement. The setup is designed to exclude stray
light and uses a solar-grade mirror to reflect the light of a vertical
solar simulator on the electrochemical cell. The light quality in the
setup (\selectlanguage{ngerman}»AAA«, IEC 60904-9) is close to the classification of the solar
simulator itself. The temperature in the electrochemical cell is
controlled with an external Peltier element and can be kept constant in
the range of 20--45 °C. The influence of reaction temperature on the
photocurrent of a WO \textsubscript{3} photoanode is demonstrated. The
gaseous reaction products are analyzed with a mobile gas chromatograph,
using an automated measurement routine with discontinuous sampling from
the electrochemical cell. The system is applied to determine the Faraday
and STH efficiencies of a copper indium gallium selenide photocathode.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{Introduction}

Hydrogen is seen as a key element of a future carbon-free economy. The
International Energy Agency forecasts that the hydrogen demand will
increase from 90 Mt (2020) to 260 Mt by 2050 if the currently existing
government programs are implemented.{[}1{]} Yousif \emph{et al} .
calculated a demand of 568 Mt for a scenario which meets the goals of
the Paris Agreement (2015). {[}2{]}

Currently, hydrogen is produced primarily from fossil resources
(\emph{e.g} ., steam reforming, coal gasification). Carbon neutral
production is possible through water electrolysis with electricity from
renewable resources, which is expected to play a major role in the
coming years even through its present contribution to the global
hydrogen production is negligible.{[}1{]} However, scaling-up
electrolysis will require an corresponding expansion of the energy
infrastructure, which adds to the investment costs. For example, an
expansion of the power grid will be necessary in central Europe, because
wind and solar plants are placed decentral and are spatially separated
from large-scale electrolyzers. A solution to this problem could come
from approaches which combine energy uptake and hydrogen production in a
single system (\selectlanguage{ngerman}»artificial photosynthesis«) and offer a cost-effective
alternative for low production volumes and off-grid applications.{[}3{]}

Photoelectrochemical (PEC) water splitting is a method of artificial
photosynthesis, in which illuminated semiconductors (\emph{e.g.} ,
WO\textsubscript{3}, Fe\textsubscript{2}O\textsubscript{3}) in direct
contact with an aqueous electrolyte cause the splitting of water
molecules to hydrogen and oxygen without the need for traditional
photovoltaics or electrolyzers. Selected systems achieve high
efficiencies of up to 19\%, and a photoelectrochemical solar park with
an active area of 100 m\textsuperscript{2} was constructed and operated
for several months.{[}4, 5{]} However, semiconductors that offer a
balanced set of properties (efficiency, stability, cost and scalability)
are lacking.{[}6{]} Therefore, large parts of the current research still
focuses on material development on the laboratory scale. Among the
common measurements are methods for electrode characterization (e.g.,
voltammetry, stability measurements, impedance spectroscopy), and
methods for determining device metrics (e.g. the solar-to-hydrogen
efficiency, STH).{[}7{]}

It has been pointed out that measurements on similar materials in
different labs lead to different results, which was attributed to a lack
of accepted standards.{[}8{]} A number of recent reviews addresses this
issue, provides guidelines for measurement routines and gives
best-practice examples, e.g. for the solar-to-hydrogen (STH)
efficiency.{[}7,8,9,10{]} A second reason for insufficient comparability
is that photoelectrochemical measurements require special equipment that
is not common in chemical laboratories and is challenging to install and
operate (e.g., solar simulators, gas chromatographs). It has been shown
that standard measurement routines are prone to systematic errors
induced by the equipment (\emph{e.g} ., solar simulators), which can
result in an overestimation of the efficiency.{[}11{]}

In this Application Note, we present a versatile and reliable
measurement system for PEC water splitting which allows to obtain high
quality electrochemical data, including the STH efficiency. It addresses
three major issues which, to our experience, pose difficulties in many
labs and can hamper the accuracy and reproducibility of measurement
data: (1) the implementation of large area light sources with high
spectral quality, (2) the stabilization of the reaction temperature
under irradiation, and (3) the quantitative determination of hydrogen.

\textbf{Materials and methods}

Materials: Experiments were carried out with a photoanode of sputtered
WO\textsubscript{3} on fluorine doped tin oxide (FTO)
(WO\textsubscript{3}/FTO/glass, \emph{d} \textsubscript{WO3} = 1 \selectlanguage{greek}µ\selectlanguage{english}m) or
with a sputtered Cu(Ga,Se)(S,Se)\textsubscript{2} (CIGS) photocathode
(CIGS/Mo/glass, \emph{d} \textsubscript{CIGS} = 0.25 \selectlanguage{greek}µ\selectlanguage{english}m). 1.0 M
HClO\textsubscript{4} and 0.1 M
H\textsubscript{2}SO\textsubscript{4}were used as electrolytes and were
purchased from Carl Roth (Rotipuran p.a.).

Methods: Illumination (spectrum AM1.5G) was provided by A+AA LED solar
simulator (SINUS 270, Wavelabs). Intensity calibration to 100 mW
cm\textsuperscript{--2} was carried out with a monocrystalline Si
reference solar cell (ReRa Solutions). The spectral distribution was
measured with a Zeiss MCS UV-NIR spectrometer. The spatial
non-uniformity was determined with a Homogeneity Tester (Fraunhofer
CSP). The temporal stability was recorded with a Si reference solar
cell.

The system for temperature control was constructed with a 65 W Peltier
element with two heat sinks, a 12 V peristaltic pump (Grothen), silicon
tubes (\emph{d} = 3 mm) and an electrolyte reservoir of 20 ml.

Gas measurements were carried out with a \selectlanguage{ngerman}»Mobile GC2« (ECH Elektrochemie
Halle), which was optimized by the manufacturer for the measurement of
H\textsubscript{2} and O\textsubscript{2} in photoelectrochemical water
splitting experiments. A sampling and analysis loop was implemented that
enables automated acquisition and evaluation of the Faraday and STH
efficiency.

Photoelectrochemical measurements were carried out in a commercial PEC
cell (PECC2, Zahner Elektrik). The working electrode had a fixed
diameter of 1.8 cm (\emph{A} = 2.54 cm\textsuperscript{2}). An
Ag\textbar{}AgCl electrode was used as reference electrode and a coiled
platinum wire as counter electrode. The Faraday and the solar to
hydrogen efficiencies were measured in a Porto cell {[}12{]} with a
platinum mesh as counter electrode. Bias was provided by a photovoltaic
module (5 cSi cells in series, \emph{A} = 6.0 cm\textsuperscript{2}),
mounted parallel to the absorber.

Current-voltage curves were recorded in the dark or under constant
illumination at a scan rate of 100 mV s\textsuperscript{--1} between 0.6
V\textsubscript{RHE} and 2.7 V\textsubscript{RHE} with a maximum current
of 5 mA. Before each measurement, the samples were equilibrated at 0.6
V\textsubscript{RHE} for 60 s. The potentials versus the reversible
hydrogen electrode (RHE) were calculated by

\emph{E} \textsubscript{RHE} = \emph{E} \textsubscript{AgAgCl} +\emph{E}
\textsuperscript{0}\textsubscript{AgAgCl} + 0.059 x pH

with \emph{E} \textsuperscript{0}\textsubscript{AgAgCl} = 0.209 V. Onset
potentials were determined as intercept of a tangent with the abscissa.

\textbf{Results and discussion}

The PEC measurement system (Figure 1) has been designed to work with a
standard vertical solar simulator. It offers room for PEC cells with
absorber sizes of up to 16x16 cm2, as well as for the required equipment
(e.g., for temperature control),

The system is built from a metal frame and six cardboard plates and
houses a solar-grade mirror and a sliding holder for the electrochemical
cell. The frame is made from commercial aluminum profiles and has feet
which are separately height-adjustable to allow installation on tables
or over edges. Five fixed and one removable cardboard plates are
attached to the frame. Black surfaces on the inside prevent light
reflection, which could otherwise result in an increased irradiation on
the photoelectrode. With a closed front side, the setup protects the
experimenter from irradiation. The light of a vertical solar simulator
enters the setup from above and is reflected by a tiltable solar-grade
mirror on the PEC cell. A sliding cell holder supports the positioning
of the cell with a high degree of reproducibility. It fixates the cell
on the platform and minimizes the danger of spilling chemicals. The
holder can be removed if large cells are investigated. The system can be
equipped with different PEC reactor types, a potentiostat and auxiliar
equipment for monitoring, imaging, or spectroscopic applications.

Table 1 Geometrical data of the PEC measurement system with data for
spatial non-uniformity according to IEC 60904-9\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
Platform dimensions (WHD) & 100 x 40 x 30
cm\textsuperscript{3}\tabularnewline
\midrule
\endhead
Mirror dimensions & 20 x 30 cm\textsuperscript{2}\tabularnewline
Light field (class A) & 10 x 10 cm\textsuperscript{2}\tabularnewline
Light field (class B) & 12 x 12 cm\textsuperscript{2}\tabularnewline
Light field (class C) & 16 x 16 cm\textsuperscript{2}\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 Photograph of the measurement system with equipment for gas
measurement (1 measurement system, 2 electrochemical cell, 3 solar
simulator, 4 potentiostat, 5 gas chromatograph, 6 pressure sensor)

\emph{Characterization and classification of irradiation}

Light accounts for the main part of the energy input in
photoelectrochemical measurements. It is common for photoelectrochemical
experiments to use the global standard spectrum AM1.5G (IEC 60904-3) and
to calibrate its intensity to 100 mW cm\textsuperscript{--2} with a
reference solar cell. The quality of a solar simulator is defined in IEC
60904-9 by three parameters which are spectral match, spatial
non-uniformity and temporal instability. Since the test setup contains a
mirror which possibly affects the irradiation quality, the parameters
were determined and compared with the characteristics of the solar
simulator.

The spectral distribution was measured with a spectroradiometer on four
points in the plane of the photoelectrode. The classification is based
on the cumulative intensity in six spectral ranges between 300 and 1200
nm. The measurements found that the deviation is maximal in the
predefined spectral range of 772--919 nm, where it is 14.8 \%. This
resulting classification of \selectlanguage{ngerman}»A« is lower than the classification of the
solar simulator (»A+«), which is probably due to a spectral dependence
of reflection of the mirror.

The spatial non-uniformity of the light is especially important for the
investigation of large photoelectrodes. It was be measured with an array
of intensity-calibrated cSi solar cells. The classification is based on
the ratio of the largest and smallest irradiance according to (\emph{I}
\textsubscript{max}--\emph{I} \textsubscript{min})/(\emph{I}
\textsubscript{max}+\emph{I} \textsubscript{min}). Depending on the
investigated area, classifications of »A«, »B« and »C« are obtained in
the investigated setup (Table 1). The degrading uniformity on larger
areas is a result of the limited mirror size.

Temporal instability of irradiance is defined by short term and the long
instability, of which only the latter is relevant for PEC measurements.
It can be determined from the light intensities before and after a PEC
experiment by (\emph{I} \textsubscript{max}--\emph{I}
\textsubscript{min})/(\emph{I} \textsubscript{max}+\emph{I}
\textsubscript{min}). A value of below 0.02 was found in the
investigated setup, which allows a classification of »A«.

It has to be noted that the use of Si solar cells for the measurement
light intensity, spatial non-uniformity and temporal instability is
legitimate only for absorbers with a similar band gap.{[}11{]} Since
many semiconductors for water splitting experiments have higher band
gaps (\emph{e.g} ., WO\textsubscript{3}, 2.8 eV) an uncertainty is
induced which will be especially prominent when solar simulators with
poor spectral fit are used.

\emph{Stabilization of the reaction temperature}

The rate \emph{k} of a chemical reaction depends on the reaction
temperature. For many chemical reactions an exponential relationship
of\emph{k} = \emph{A} exp{[}--\emph{E} \textsubscript{A}/(\emph{RT} ){]}
is found, whereas the processes within the semiconductor usually result
in a negative temperature coefficient. For the example of
WO\textsubscript{3} photoanodes it was measured that the photocurrent
increases by 64 \% in a temperature interval of 25 to 65 °C.{[}13{]}

Commercial photoelectrochemical cells are usually not equipped with
systems for temperature control (\emph{e.g} ., double jackets). We found
that the temperature in an idle cell under illumination with an LED
solar simulator rises by 3 °C in 10 min. For Xe arc lamp-based systems
the temperature increase is probably higher due to their stronger IR
emission. The changing temperature can limit the reliability of
measurement data, especially for long measurements under illumination
(\emph{e.g} ., voltammetry, impedance spectroscopy).

In the developed system (Figure 2) for temperature control, the PEC cell
is connected to a closed loop of silicone tubes. A peristaltic pump,
which allows the use of acidic or alkaline electrolytes without
corrosion, circulates the electrolyte with a flow rate of 60 ml
min\textsuperscript{--1}. Heat exchange occurs through the silicon tubes
which are wound through the fins of an aluminum heat sink on a
voltage-controlled Peltier element. The temperature is measured with an
electronic sensor in an electrolyte reservoir situated between the cell
and the Peltier element. An automatic control of the Peltier element
with the sensor data is possible but has not yet been realized.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2 Block diagram of the system for temperature control

The system allows to carry out measurements at stable temperatures (\selectlanguage{ngerman}±1
°C) in the range of 20--45 °C (measured at room temperatures of 25--28
°C). The circulation induced minor noise in the measurement data
(approx. 1 \selectlanguage{greek}μ\selectlanguage{english}A), which could be eliminated by statistical methods
(\emph{e.g} ., a moving average).

The effect of the temperature on the reaction was investigated on the
example of a sputtered WO\textsubscript{3} photoanode (Figure 3). While
the onset potential under illumination is independent of temperature in
the investigated interval, the onset potential of the dark reaction
decreases slightly (Table 2). The photocurrent density increases by
approximately 15 \%, both at 1.23 V\textsubscript{RHE} and in the
plateau region. The results are close to published values.{[}13{]}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3 Temperature dependent photocurrent density of a
WO\textsubscript{3} photoanode (linear sweep voltammetry, solid lines
illuminated, dashed line not illuminated)

Table 2 Results from linear sweep voltammetry at 20 \selectlanguage{ngerman}°C and 45 °C\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
& 20 \selectlanguage{ngerman}°C & 45 \selectlanguage{ngerman}°C\tabularnewline
\midrule
\endhead
\emph{E}\textsubscript{onset} (light) & 0.85 V\textsubscript{RHE} & 0.85
V\textsubscript{RHE}\tabularnewline
\emph{E}\textsubscript{onset} (dark) & 2.81 V\textsubscript{RHE} & 2.69
V\textsubscript{RHE}\tabularnewline
\emph{j} (1.23 V\textsubscript{RHE}) {[}\selectlanguage{greek}µ\selectlanguage{english}A cm\textsuperscript{--2}{]} &
626 & 719\tabularnewline
\emph{j} (2.00 V\textsubscript{RHE}) {[}\selectlanguage{greek}µ\selectlanguage{english}A cm\textsuperscript{--2}{]} &
913 & 1060\tabularnewline
\bottomrule
\end{longtable}

The data allows to estimate the error of a measurement without
temperature control: During a measurement time of 10 min the temperature
increases by 3 \selectlanguage{ngerman}°C, which results in an error of 2.6 \% results in the
photocurrent. This could be acceptable for standard material screening
but should be avoided in high-quality measurements. It has to be noted
that the temperature effect can be different for other systems.

\emph{Automated GC measurement routine for STH determination}

The desired product of an electrochemical reaction is measured
quantitatively for the determination of the Faraday efficiency, which is
an important parameter of an electrode/electrolyte combination. Side
reactions (\emph{e.g} ., with the electrolyte) or subsequent reactions
(\emph{e.g} ., with cell components) result in a Faraday efficiency
below 100 \%. In photoelectrochemical water splitting, quantitative
determination of hydrogen and oxygen has been carried out with mass
spectrometers {[}14{]} or with electrochemical sensors {[}15{]}, but the
most common way is to measure with gas chromatographs. The integration
of gas chromatographs is a demanding task, as leakage can be a serious
problem. The accuracy is often limited by low gas amounts and by the
accuracy of supporting measuring devices (\emph{e.g.} , volume flow
controllers).

The presented test setup aims to mitigate these problems by the use of a
commercial mobile GC which was optimized by the manufacturer for PEC
applications. The TC is equipped with an integrated Ar tank, a thermal
conductivity detector and two short columns to allow fast measurements.
It is connected to a Porto cell via metal gas transfer lines with
standard metric threads. A pressure sensor and a safety valve are
connected as well (Figure 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Figure 4 Block diagram of the system for gas measurement

The analysis is carried out by an automatic measurement routine. The
time-resolved data is sequentially acquired in series of cycles, each of
which consists of three steps (Figure 5):

The pressure in the electrochemical cell is set to 1.1 bar with Ar from
the integrated tank of the GC. In this step, the hydrogen concentration
drops.

With all valves closed, the water splitting reaction is started by
switching on the solar simulator. The episode may last for 300 s but can
be expanded if photoelectrodes with low efficiencies are investigated.
Pressure and hydrogen concentration in the cell increase linearly.

3. A valve in the GC is opened and allows the gas mixture in the
electrochemical cell to relax through the sampling valve. After a
certain volume flow, which correlates with the volume of the metal
tubes, the sample is taken. The GC measurement is started and runs
parallel to steps 1 and 2.

After the automatism is terminated by the user, the data of the GC, the
pressure sensor and the potentiostat are loaded into an Excel\selectlanguage{ngerman}® script
which calculates the molar amounts of hydrogen and oxygen, the Faraday
efficiencies and, if applicable, the STH efficiency. Leakage is detected
by a continuous pressure drop in the cell during an idle period and can
be corrected to a limited extend.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

FIGURE 5 Procedure of the automatic measurement routine with schematic
data for current, cell pressure and H\textsubscript{2} concentration.

The system was applied to investigate a photocathode of copper indium
gallium selenide in H\textsubscript{2}SO\textsubscript{4}, for which a
Faradaic efficiency of (95.4\selectlanguage{ngerman}±7.7) \% was determined (Figure 6). The STH
efficiency was calculated to be (1.9±0.1) \% for a system of a 25.0
cm\textsuperscript{2} photocathode and a 6.0
cm\textsuperscript{2}photovoltaic module. Both efficiencies agree with
previous literature values {[}16{]}.

A quantitative error analysis with consideration of the propagation of
uncertainty shows that the free gas volume of the electrochemical cell
is an important contribution to the overall error, for which an
individual measurement uncertainty of 2 mm\textsuperscript{3} results in
an absolute error of 0.9 \% in the Faraday efficiency. The determination
of the absorber area, variations in the reaction temperature and
non-ideal behavior of the gas/liquid system (\emph{e.g.} , trapped gas
bubbles) are significant contributions to the error. The GC measurement
itself has a high accuracy with an error of less than 0.01 \% but relies
on the quality of the calibration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 6 Measured vs. calculated molar amounts of H\textsubscript{2} for
a CIGS photocathode

\textbf{Conclusion}

The test setup allows to conduct various photoelectrochemical
measurements (e.g., STH determination) and can help to avoid common
problems with equipment. It allows the use of standard vertical solar
simulators by reflecting the light to the electrochemical cell with a
solar-grade mirror. The light in the plane of the photoelectrode had a
high spectral quality, a low spatial non-uniformity and a low temporal
instability, resulting in an AAA classification (IEC 60904-9). A system
for temperature control with an external Peltier element allows
measurements at constant temperature (\selectlanguage{ngerman}±1 °C) in the range of 20--45 °C.
It enables automated time resolved measurements with enhanced
metrological precision and the investigation of the temperature
dependency of photoelectrochemical reactions. Gas analysis is carried
out with a mobile GC, which is equipped with a thermal conductivity
sensor, two short columns and an internal gas supply. It makes use of an
automated measurement routine with discontinuous sampling from the
reactor cell. A software for data evaluation calculates Faraday and STH
efficiencies and allows the correction of leakage. The system was
applied to investigate a copper indium gallium selenide photocathode in
H\textsubscript{2}SO\textsubscript{4}, for which a Faradaic efficiency
of (95.4±7.7) \% was determined.

\textbf{Conflict of Interest Statement}

The authors declare no competing interests.

\textbf{Data Availability Statement}

CAD models, the software for GC data evaluation and data that support
the findings of this study are available from the corresponding author
upon reasonable request.

\textbf{ORCID}

David Adner, 0000-0002-5378-8642

Max Pohl, 0009-0002-0247-6806

Christian Hagendorf, 0000-0003-4402-6424

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