(Invited) Multistep Electrocatalytic Processes in Fuel Cells: A Comprehensive Analysis
Authors: Gottesfeld Shimshon 
Published: December 2020

The onset potential of a multi-step electrocatalytic process and, the potential dependence of the faradaic resistance beyond the onset , are two key descriptors of such process. This paper offers a mechanistic perspective applicable to a range of technically significant, multi-step electrocatalytic processes, facilitating identification of the onset potential and projection of the potential dependence of the faradaic resistance.
A common pattern observed in mechanisms of electrocatalytic processes, involves an early ,overpotential driven step preceding the RDS, that sets up the surface composition required for onset of the RDS [1,2]. Such “pre-step” can involve exposure of surface sites by removal of blocking species and/or formation of adsorbed intermediates which serve as reactants in the RDS.
Recognizing that overpotential driven pre-steps involve “switching” of a surface redox system, the comprehensive expression for the rate of a multistep, cathodic electrocatalytic process, is projected to have the following general form [1,2] :

J(Ecath) = Cr
g Fk0 A

∗

× f(Ecath−E0
redox) ×10ˆ{[−
D
H#
act
/2.3RT]−[(Ecath−E◦
cell)/b]}

For a 1e surface redox system with the potential dependence of the ratio [ox]/[red] obeying the Nernst equation,
the explicit , full expression for J(Ecath), becomes:

J(Ecath)= Cr
g Fk0 A

∗

× 10ˆ[(F/2.3RT)(Ecath−E0
redox )+1]−1x

×10ˆ{[−
D
H#
act
/2.3RT]−[(Ecath−E◦
cell)/b]}

While Eq. (2) for J(E) presents a step forward in considering the overall effect of the overpotential on the rate of a multistep process , it projects the same signature in the J(E) characteristic for pre-steps generating RDS reactants and, for pre-steps re-exposing blocked surface sites. We show here that these two different functions of the pre-step can be distinguished from EIS measurements.
Low Tafel slopes are observed at low current densities for both the HOR and the ORR. In the case of the HOR, a sequence of two steps in series , taking place ,for example, along the H-V mechanism, results in two capacitive arcs in the Nyquist plot , as shown in Figure 1. In contrast, the EIS reported in the literature In the low current range of the ORR , exhibits only one capacitive arc in the frequency range 5kHz- 0.1Hz , corresponding to the rate determining charge transfer step in the ORR [ 3 ]. The low Tafel slope observed near the onset potential of the ORR has been ascribed to overpotential-driven removal of a surface blocking oxide formed by water discharge [ 4 ] , however, no spectral feature corresponding to this process could be detected in the EIS spectrum before EIS spectra for the ORR were recorded down to frequencies under 0.1Hz (Figure 2 ).
Figures 1 below shows the EIS spectrum for the HOR at Pt , with two capacitive loops corresponding to two charge transfer steps in series. Figure 2 below shows the EIS spectrum for the ORR , with a single capacitive loop between 5kHz and 0.1Hz [3] and, an inductive loop appearing at frequencies under 0.1Hz [5]. The difference in the form of the EIS spectra seen in Figures 1 and 2 , clearly suggests a different origin of the low Tafel slope in each case The inductive loop ( Figure 2) reflects a slow surface process which enhances the rate of electron transfer to the dioxygen molecule by increasing the population of active, metal surface sites [5]. Figure 3 provides another example of an EIS exhibiting an inductive loop at low frequency : this is an EIS recorded for hydrogen oxidation at Pt in the presence of COad [6]. The analogous patterns of the spectra in Figures 2 and 3, provide good support for the assignment of the inductive loop in the ORR EIS to slow removal of a site blocking species.

Acknowledgements: It is my great pleasure to participate in a symposium in Bob Savinell’s honor and I wish him many more years of activity in this exciting field!

References:

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Figure 1