Oxygen reduction performance measurements: Discrepancies against benchmarks

Oxygen electrocatalysis is crucial for renewable energy conversion and storage technologies. Specifically for proton exchange membrane fuel cells (PEMFCs), oxygen reduction reaction (ORR) is the primary reaction that determines performance and costs. The ORR activity and durability are commonly assessed using a rotating disk electrode (RDE). However, there are numerous inconsistencies in the RDE measurements among researchers when comparing newly developed ORR catalysts to state‐of‐the‐art ones. These inconsistencies in activity and durability evaluation and deviations from standard protocols result in irreproducible screening and benchmarking of electrocatalysts. Despite the fact that the US Department of Energy has established and regularly revises the standard protocols, many reported studies do not adhere to these guidelines. This perspective aims to draw attention to the discrepancies in ORR activity and stability measurements at RDE as primary screening and emphasizes implementing the mandatory standard for a meaningful comparison of ORR activity and durability. We intend to emphasize the use of available ORR test standards ubiquitously for more accurate comparisons and accelerate the development of fuel cell catalysts.


| OXYGEN REDUCTION EVALUATION AND STANDARD PROTOCOLS
Oxygen electrocatalysis is a key process in various sustainable energy conversion and storage technologies, including batteries and fuel cells. [1][2][3] Among the different energy conversion technologies, proton exchange membrane fuel cells (PEMFCs) are trending due to their highpower efficiency and zero carbon emissions. 4,5 The cathodic ORR needs a significant quantity of Pt catalyst to overcome the multi-electron sluggish kinetics, which raises the overall cost of fuel cells. 6,7 Despite tremendous advances in high-performance ORR electrocatalysts, their actual performance in fuel cells remains unsatisfactory, notably the low durability. [8][9][10][11] Platinum group metal (PGM)-based catalysts are still compulsory for the foreseeable large-scale application of PEMFCs due to the unsatisfactory performance of other PGM-free catalysts. 12,13 Nevertheless, several key issues concerning activity/durability testing must be clarified and standardized according to Department of Energy (DOE) standards (2025 targets, mass activity at 0.9 V iR -free 0.44 Amg Pt −1 , durability < 40% mass activity loss after 30,000 AST cycles at 0.6-0.95 V). 14,15 Achieving activity and durability standards specified by DOE is, therefore, a prerequisite for the successful validation of ORR performance. 16,17 Furthermore, rotating disk electrode (RDE) is a primary ORR activity assessment tool that may help building highperformance ORR catalysts for membrane electrode assembly (MEA) in PEMFCs. However, a huge gap exists in activity measurements at RDE and MEA evaluations due to significant differences in evaluation parameters, test conditions, and quantity of catalyst as well as the output current density. 18,19 Different studies have highlighted the existing gap in ORR electrocatalysts screenings at RDE and MEA and their testing protocols and parameters that differ at both testing conditions. [20][21][22] Typical parameters that differ both at RDE and MEA screening are electrode architecture, nature of the electrolyte, reactants, and product transport and operating conditions. For example, an ultra-thin catalyst layer with a tiny amount of catalyst (several tens of micrograms) is deposited over the nonporous conductive surface of a solid electrode in RDE screening, while in MEA, a porous catalyst layer with a higher amount of catalyst (several tens of milligrams) is used. Similarly, a liquid aqueous electrolyte is used at RDE, while MEA contains a solid electrolytic membrane known as a proton exchange membrane (PEM) for the better transport of reactants. Hence, reactants transport through liquid into the catalyst/electrode interface at RDE while it occurs through porous PEM at MEA. 23 Ambient working conditions are used at RDE, such as room temperature with standard pressure conditions. Mass transport is smooth at RDE, where a rotating electrode results in quick mass transport across the electrolyte to generate a tiny current (less than 10 mAcm −2 ). However, at MEA, due to higher current density requirements (More than 4 Acm −2 ), a thicker catalyst layer and solid electrolyte architecture result in significant mass transport resistance. RDE is a relatively simple setup that can be operated efficiently for multiple studies such as ORR activity, stability, and impedance analysis in a short time of merely a duration of minutes to a few hours with a simple and quick testing sample preparation. 24 However, the MEA instruments are costly, and a complicated setup that needs special operators to monitor the working parameters takes days to weeks for a single sample testing and complex catalyst layers preparation.
These working conditions and instrumental setup differences significantly impact the catalyst's final activity. For example, efficient mass transport of reactants to the active sites is an essential parameter for higher performance of the catalysts, which is significantly different at both the RDE and MEA evaluations. 25 Moreover, for a typical Pt/C catalyst, the degradation usually occurs due to Pt dissolution, carbon corrosion, particle growth, and ionomer poisoning. A recent study reported a comparative analysis of the halfcell and full-cell results of Pt/C, demonstrating that higher temperature leads to higher degradation at MEA, mainly due to enhanced carbon corrosion and increasing particle growth. 26 Similarly, the membrane degradation and flooding due to reactants gas humidification is also a critical factor for the activity degradation at MEA, while RDE with low oxygen solubility in electrolyte and being operated at ambient temperature does not suffer from such an issue.
Apart from RED and MEA gaps, numerous discrepancies in ORR activity and stability measurements at RDE procedures and testing conditions pose significant challenges in comparing the ORR performance of different catalysts. 21,27 Even though the standards have already been established and are regularly revised by the DOE, those are primarily designed for MEA testing. 20,28 Therefore, despite the RDE is a more reliable and useful tool, especially when appropriate MEA fabrication and testing are not warranted, there are considerable discrepancies in ORR measurements at RDE. 29,30 Figure 1 illustrates these inconsistencies in standard procedures and ORR stability data and mass activity calculations that result in uncertainties among the researchers.
For instance, the durability standards for Pt-based ORR electrocatalyst set by DOE are less than 40% decay in mass activity after 30,000 accelerated stress test (AST) cycles in a potential range of 0.6-0.95 V versus reversible hydrogen electrode (RHE). Yet many published works do not adhere to these protocols and report stability with a variable number of potential cycles under different voltage and scan rates. [31][32][33][34][35] As a result, it is hard to compare the durability if the protocols are not uniform. Hence, it is crucial to implement consistent test protocols with relevant operating conditions to enable a realistic comparison of newly established catalysts to state-of-theart ones. 24,28 This perspective overviews the existing discrepancies in activity and durability measurements of ORR catalysts at the RDE level and emphasizes the implementation of DOE standards for more clear understanding. The following section provides a brief description of numerous discrepancies existing in the reported literature on ORR activity and stability. We hope this discussion will play a dynamic role in the ubiquitous implementation of DOE standards for ORR performance and durability assessment.
The ORR mass activity of the catalyst, which is a key factor in the performance evaluation of Pt-based catalysts, is measured at a potential of 0.9 V versus RHE (mixed potential region of diffusion limiting current and kinetic current) (Figure 2A). 24 However, for some catalysts with more positive half-wave potential (E 1/2 ), the mass activity and specific activity data cannot guarantee the real kinetics of the catalyst, as the 0.9 V potential value falls in the diffusion current region. 37 Therefore, due to more positive E 1/2, some works consider mass activity at 0.95 V versus RHE, which is more reasonable, yet it deviates from the DOE protocols and consequences uncertainties compared with the reported literature. 38,39 Furthermore, in numerous papers, the mass activity is reported at different potential values (0.85, 0.9, 0.95 V vs. RHE). 40 36 For instance, the catalyst ink formulation, its composition, and thickness significantly affect the ORR current density. 44 Similarly, the scan direction (anodic vs. cathodic), scan rate (5-50 mV s −1 ), and the scan nature (linear sweep voltammetry [LSV] and staircase voltammetry [SCV]) considerably affect the ORR activity ( Figure 2B). 24 Detailed analysis on the impact of different parameters on the ORR activity highlights such discrepancies. 36 The kinetic current density (j k ) during the anodic sweep (from lower to higher potential) is found to be significantly larger than the cathodic sweep (from higher to lower potential). In addition, the LSV curve and corresponding kinetic current densities significantly vary at different potential values ( Figure 2C). These values increase even more when the scan rate changes (lower to higher). 45 Despite these significant impacts, there is still a discrepancy in ORR activity reported at RDE that leads to difficulties in benchmarking novel catalysts. Shinozaki et al. also demonstrated these discrepancies, where a huge difference can be observed in ORR activity upon changing the LSV scan direction. 25 They compared the collected data from the literature with their experimental results, which show significant diversity in results obtained with variable parameters.
Regarding durability assessments, DOE protocols state that the catalyst's durability should be assessed with 30,000 AST cycles in a potential range of 0.6-0.95 V versus RHE. 17 However, very few works report durability according to standards. At the same time, there are still considerable differences in scan rate and potential range for AST. Many research works report ORR durability F I G U R E 1 Graphical illustration demonstrating the inconsistencies in oxygen reduction reaction (ORR) activity and stability measurements among the researchers and deviations from standard procedures. These inconsistent practices result in erroneous, irreproducible, and unreliable electrocatalyst screening and benchmarking.
with a variable number of AST cycles ranging from 4000 to 100,000 cycles 46-49 scan rates (50-100 mV s −1 ). 34,50 and potential ranges (0.6-1.2 V vs. RHE). 38,51 For example, Yang et al. report that the fct-PtFeIr/C show excellent stability by retaining 89.9% mass activity and no change in ECSA after 10,000 AST cycles in a potential range of 0.6-1.1 V versus RHE at a scan rate of 100 mV s −1 ( Figure 3A). 34 Although this catalyst shows promising stability up to 10,000 AST cycles, it is not sufficient to validate the durability after 10,000 AST cycles only. Therefore, AST cycling test can be further extended to 30,000 AST cycles for a more reasonable durability evaluation under DOE requirements (30,000 AST cycles). Typically, during the first 5000-10,000 AST cycles, the activity sharply declines in the case when the unstable metal nanoparticles leach out. 52 In other cases, activity first decreases and then improves due to more stable and exposed active sites. 53,54 Even in our work, we reported a high mass activity PtCoNi@NCNTs catalyst, which demonstrated a reasonable MEA activity with high durability, yet it was screened for 10,000 AST RDE cycles only. Similarly, another catalyst with ultrafine jagged Pt nanowires possesses one of the highest ORR mass activities reported for Pt-based catalysts (13.6 A mg Pt −1 ).
Nevertheless, the durability of the catalyst is evaluated at merely 6000 AST cycles, which do not predict the practical durability of the catalyst ( Figure 3B). 35 Therefore, such a huge diversity in durability evaluations does not guarantee a realistic comparison of the catalysts, even with high mass activities. In a nutshell, the variations in electrocatalytic performance data show that it is hard to compare the activity of two catalysts with similar compositions if tested using different protocols. Despite the availability of standards for such evaluation, it appears to be no agreement in the electrochemical community on said procedures/parameters. Because of inconsistencies in experimental procedures, an invalid evaluation/comparison of ORR kinetic and durability parameters for ORR electrocatalysts is made, resulting in erroneous, F I G U R E 2 (A) Typical oxygen reduction reaction (ORR) polarization curve for Pt/C, Pt loading 10 mg Pt , Electrode area 0.2463 cm 2 (B) linear sweep voltammetry (LSV) and staircase voltammetry (SCV) polarization plots with different scanning directions. Reproduced with permission. 24 Copyright 2020, Elsevier (C) ORR kinetic activity based on different scan directions demonstrates a significant difference in kinetic current density extracted from cathodic and anodic scanning. Reproduced with permission. 36 Copyright © 2022, American Chemical Society irreproducible, and unreliable screening and benchmarking of electrocatalysts. These inconsistencies usually result from variable test protocols, different operating conditions, and the film preparation technique of catalysts. Therefore, standardization of the RDE test protocols, operating conditions, and catalysts' film preparation technique could improve the accurate first gate screening of newly developed catalysts compared to an established Pt/C activity.

| CONCLUDING REMARKS
This perspective aims to bring attention to the discrepancies in ORR activity and durability measurements at RDE. The inconsistent practices among the researchers outlined in this perspective show a significant difference in the ORR performances of reported catalysts. This is a fact that most of the highperformance ORR catalysts developed at RDE underperformed or could not be used at the MEA level. One of the primary reasons is inconsistency in screening processes, which raises concerns about the high activity and stability. As a result, the following suggestions are proposed to make a more reasonable comparison of ORR performance.
(i) RDE, a primary and quick screening technique for the ORR of newly developed catalysts, could foresee the potential applications of ORR catalysts in fuel cells. Therefore, standardized protocols should be followed in testing that provides reliable and reproducible results. (ii) Different parameters during RDE measurements and data extraction, such as the nature of the electrolyte, the local environment of the catalyst, and the governing mode of mass transport, directly determine the final ORR activity of the catalyst. Therefore, standard and universal protocols should be followed and clearly stated in the experimental section for a reasonable comparison of the ORR performance of catalysts. (iii) Although DOE protocols are primarily designed for MEA, these standards could be ubiquitously adopted in ORR measurements at RDE for universal analysis. (iv) Mass activity and specific activity should be defined precisely. For a more reasonable comparison, the scan rate and scan directions of LSV curves should be mentioned in the experimental details. At the same time, mass activity, specific activity, and ECSA calculations should be listed in supplementary information for clear readability.
(v) Complete AST cycles (30,000) should be performed with detailed experimental parameters according to DOE protocols for durability evaluations. (vi) Finally, for practical applications of the ORR catalysts more widespread use of MEA testing in the earlier stages of catalyst development is crucial. Yet, due to the large cost and time requirements, MEA testing is not viable in the early stages of catalyst development. Therefore, floating electrode and gas diffusion electrode approaches are feasible techniques due to their higher current density output at lower catalyst loadings, low cost compared to MEA setup, and optimal working conditions. While bridging the activity gap between RDE and MEA, these methods can maintain simplicity, ease, and controllability.