Advancing the Rigor and Reproducibility of Electrocatalyst Stability Benchmarking and Intrinsic Material Degradation Analysis for Water Oxidation

Abstract

The stability of electrocatalysts is a concern for nearly all materials; degradation can occur via dissolution, leaching, sintering, amorphization, or reduction/oxidation processes. Extreme pH or large applied potentials often exacerbate these effects, but scant fundamental understanding of these processes exists due to complex structural and nanoscale effects in electrocatalysis. Instead, "catalyst stability" is often reported using broad electrode performance metrics, such as measured activity over time. To advance the fundamental understanding and comparison of catalyst materials, we propose that it is necessary to establish improved benchmarking metrics that reflect intrinsic material dynamics and stabilities of catalysts, supports, and substrates as a function of testing parameters, to complement existing metrics that primarily capture the performance of the complete electrode. We consider many degradation processes of lab-scale aqueous media systems, as well as membrane electrode assemblies and proton exchange membrane water electrolyzers, and consider the relatability between the two systems. Herein, we summarize various approaches to standardizing or benchmarking electrocatalyst performance, consider their strengths and weaknesses, and provide an outlook for advancing the rigor, specificity, and reproducibility of these techniques.

Degradation Mechanism of Calcium Iridium Oxide for Oxygen Evolution Reaction in Acid

“…It is possible that the formation of a thin, porous iridium oxide layer at the surface of a catalyst particle acts as an anchor to avoid further metal dissolution. Considering that the calculation of the S-number varies with electrochemical testing conditions employed, such as potentiostatic vs potentiodynamic measurements, operating time and potential, and calculation methods used (dynamic vs steady-state), it is critical to compare S-numbers for materials that are tested under relevant reaction conditions to provide a more meaningful comparison of stability. The transient S-number of Ca 2 IrO 4 falls into a similar range with other iridates, such as Y 2 Ir 2 O 7 and Ba 2 PrIrO 6 , that were tested under a constant voltage at 1.60 V vs RHE for 2.5 h and 30 min, respectively.…”

“…The higher the number, the more stable the active center of the electrocatalyst. Here, we used two methods of evaluating S-numbers to compare the stability of catalyst phases that are formed at different stages of stability testing (Figure b) . The “steady-state” S-number is calculated by generating a linear trendline using the total OER charge passed and amount of Ir dissolved, such that it represents behavior of the system once it has reached a steady state under reaction conditions.…”

“…Here, we used two methods of evaluating S-numbers to compare the stability of catalyst phases that are formed at different stages of stability testing (Figure 4b). 34 The "steadystate" S-number is calculated by generating a linear trendline using the total OER charge passed and amount of Ir dissolved, such that it represents behavior of the system once it has reached a steady state under reaction conditions. In contrast, the "transient" S-number is calculated based on the cumulative charge and the amount of dissolved Ir at each individual time point such that it reflects dynamic behavior of the catalyst material, especially highlighting changes during early stages of stability testing.…”

Silver Compositing Boosts Water Electrolysis Activity and Durability of RuO2 in a Proton‐Exchange‐Membrane Water Electrolyzer

“…Due to the acidic environment and highly oxidizing operating conditions in PEMWEs, most of the electrocatalysts containing non-noble metal elements only exhibit poor catalytic activity and durability. [4,5] The state-ofthe-art OER electrocatalyst at current stage is still the highsurface-area IrO 2 . [6] However, the scarcity and high cost of Ir (4600 USD/Oz) significantly impede the large-scale application of PEMWEs.…”

Benchmarking Electrocatalyst Stability for Acidic Oxygen Evolution Reaction: The Crucial Role of Dissolved Ion Concentration

“…To address these limitations, researchers have developed improved metrics such as the stability number , and activity-stability factor, which normalize the amount of evolved oxygen to dissolved catalyst. Third, the stability number is not a “stable” metric since external factors can influence its measurement, as demonstrated by meticulous work on IrO x and a recent perspective . However, for RuO 2 , the impact of experimental methods on measuring the stability number is unknown, which makes it difficult to determine whether a reported stability is due to inherent catalyst properties or external factors.…”