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.