Electrocatalysis Beyond 2020: How to Tune the Preexponential Frequency Factor

Abstract

After a century of research on electrocatalytic reactions, a universal theory of electrocatalysis is still not established due to limited understanding of complex energy conversion processes at electrified electrode‐electrolyte interfaces. Most of the research efforts directed toward the acceleration of important electrocatalytic reactions (e. g. hydrogen evolution reaction) were in the direction of minimizing activation energy by tuning the adsorption energies of key intermediates. This kind of approach is well‐established and, importantly, in some cases it was valuable by predicting the design of electrocatalysts with advanced properties. However, in some very important research endeavors, advancement in performance of newly designed electrocatalysts could not be attributed to altered/minimized activation energy. Important to note is that modern electrocatalysis almost completely disregards influence of the preexponential factor on reaction rate. In this work, we open some important questions relevant for future of electrocatalysis and electrochemical energy conversion, with special focus on preexponential factor as major contributor to electrocatalytic reaction rate.

What Controls Activity Trends of Electrocatalytic Hydrogen Evolution Reaction?─Activation Energy Versus Frequency Factor

“…From the inverse value of the slope in Figure b, it seems that the reduction of activation energy by approximately 5.8 kJ mol –1 is coupled with a drop in the frequency factor by an order of magnitude. The slope of E a versus log A dependence is almost identical to the value we obtained in alkaline media, indicating some universal limitation to HER kinetics that was not revealed until now. Drawing from thermal catalysis, the compensation effect in electrocatalytic HER could be understood as an interplay between the adsorption energy (Δ G ad ), which is related linearly with activation energy via the BEP relation and the number of available active sites at an electrocatalyst surface (1 – θ) .…”

“…In contrast to our initial experiments that were done in alkaline electrolyte, experiments shown in this work were conducted in the acidic electrolyte ( i.e., 0.1 M HClO 4 ). The reason for this is that impurities in the form of various anions ( e.g., chlorides) desorb to a great extent as we approach from open circuit potential (OCP) toward the reversible potential for HER .…”

“…The increase of exchange current by an order of magnitude, in the case of metals that adsorb protons, is coupled with a reduction of Tafel slope by approximately 10 mV, while there was a coupled increase of the Tafel slope by approximately 30 mV in the case of metals that absorb protons. Importantly, if exchange currents and Tafel slopes follow the dependence shown in Figure b, then in the first approximation, it is sufficient to focus on electrocatalytic activity trends close-to-equilibrium by resolving intrinsic limitations of exchange current, which is quite the opposite to alkaline media, where it is necessarily to consider the behavior of HER also far-from-equilibrium …”

“…Taking into consideration that the scope of electrocatalysis is to establish a correlation(s) between properties of electrocatalytic materials and reaction rate, the conventional view that correlates the relative position of the metal d-band center (with respect to the Fermi level) with the exchange current density seems reasonable . However, exchange current is a very complex quantity dependent on the activation factor and frequency factor, each influenced by several parameters . So instead of linking one electrocatalytic property of a material with reaction rate (or exchange current), it would be necessary to make dissection of activation energy and especially important to make dissection of the pre-exponential frequency factor to obtain a more vivid picture of the structure and dynamics of the electrified interface ( i.e., interaction of electrode with electrolyte with/without applied overpotential) and to correlate single properties of electrocatalytic materials or interfaces to single parameters in the rate law as shown in (eq and/or eq ).…”

“…Importantly, if exchange currents and Tafel slopes follow the dependence shown in Figure 2b, then in the first approximation, it is sufficient to focus on electrocatalytic activity trends close-to-equilibrium by resolving intrinsic limitations of exchange current, which is quite the opposite to alkaline media, where it is necessarily to consider the behavior of HER also far-from-equilibrium. 31 2.3. Activation Energy Versus Frequency Factor.…”