2.2 Evaluation Criteria of Activity
The advanced/unadvanced performance of electrocatalyst is identified in
a reasonable evaluation system involving multiple descriptors. These
visualized crucial parameters are overpotential, Faradaic efficiency
(FE), current density, Tafel slope, electrochemical surface area (ECSA),
turnover frequency (TOF), electrochemical impedance spectroscopy (EIS),
and stability, which are not only related to the structural properties
of catalyst, but also are affected by the changed reaction conditions,
such as electrolyte, pressure, temperature. (1) Overpotential
(η). Except the inherent reaction electrode potential, the occurrence
of CO2 reduction requires an additional voltage to
overcome the kinetic energy barrier due to activation barriers and
resistance in the electrochemical system.
The difference value between actual
input potential and theoretical electrode potential is the
overpotential. The functions of catalyst are to reduce the activation
energy in reaction process and overpotential, so the overpotential or
the initial potential can be used to directly reflect the activity level
of catalyst. (2) FE. FE representing the percentage of the
electric quantity consumed to generate the target product in the total
electric quantity, reflects the selectivity of catalyst to generate a
certain product. The FE value of product can be calculated by the
formula [FEi = Qi/Q =
(n·F·P0·v·xi)/(jtotal·R·T)].[47,48]The closer to 100% FE is, the higher the catalyst selectivity will be.(3) Current density. In the ECR process, the measured current
density is divided into the total current density and partial
current density of the certain
product. Current density depending on the amount of catalyst, the number
of active sites and the mass transfer rate throughout the all reaction
system, is the most intuitive index of activity evaluation. The larger
the total and partial current densities are, the more likely the
industrial application will be. (4) Tafel slope. The Tafel slope
as an inherent property of catalyst can be acquired from the empirical
equation (η = a + b·log│j│),[49] which is
essential to explain the catalytic mechanism of the reaction, because
that it can be used for inferring chemical reaction path and reaction
velocity steps. In terms of kinetics, a smaller Tafel slope indicates
that the catalytic process has a faster reaction rate. (5) ECSA.ECSA, i.e. the effective area
involved in the electrochemical reaction, is usually used to normalize
the current and reaction kinetics. Since the amount and surface area of
the catalyst on the working electrode have an obvious influence on the
current density, it is extremely vital to evaluate the activity of
electrocatalyst. The most common strategy to measure ECSA is the
double-layer capacitance (Cdl ) via the equation
(ECSA = Cdl /Cs ·S).
Cs , S represent the specific capacitance and the
reaction area, respectively.[50] (6) TOF.The core definition of TOF is to estimate the intrinsic activity of
”active site”. Under a fixed potential, TOF value discloses the mole
numbers of product per unit time at each catalytic site. Usually, the
number of active sites in the catalyst is difficult to determine, so the
TOF value is not absolutely accurate, but that does not prevent it from
assessing the active level of the catalyst. (7) EIS. Charge
transfer resistance (Rct) obtained by fitting the
semicircle in the high frequency region from EIS can be applied to
describe the electron charge transfer rate.[51]The small Rct value can promote reaction rate. (8)
Stability. The stability refers to the change of activity and
selectivity of catalyst over time, which is a significant index to judge
whether it meets the requirements of practical application. It can be
evaluated by the chronoamperometry/chronopotentiometry (I-t/E-t) or
cyclic voltammetry (CV) tests.[51,52] In addition,
the change of material morphology, structure and performance after
long-term operation is also needed to be measured to judge the stability
of catalyst.