Toward Understanding the Utilization of Oxygen Reduction Electrocatalysts under High Mass Transport Conditions and High Overpotentials

Abstract

There is currently a disconnect between the high electrocatalyst oxygen reduction reaction (ORR) performance measured ex situ, using the rotating disc electrode (RDE), and the in situ membrane electrode assembly (MEA) performance. The disconnect in the electrocatalyst performance raises questions both about the pitfalls of the RDE technique at extrapolating the performance to higher overpotentials and how to improve the in situ catalyst layer performance to meet ambitious fuel cell targets. This work aims to bridge the gap by measuring the ORR ex situ performance under high mass transport conditions, at high overpotentials, using the floating electrode (FE) technique. Here, we determine the performance of three Pt/C electrocatalysts using the FE in 1 M HClO4 and 1 M H2SO4 to show that the MEA activities measured at 80 °C, 150 kPag were substantially lower than the room temperature and pressure performance of the same catalyst in 1 M HClO4 using the RDE and FE methods and also lower than the FE in 1 M H2SO4, implying MEA limitations are not purely due to sulfonate adsorption from the Nafion. Finally, FE and MEA data was modeled which obtained j o values on the FE (oxide free conditions) which were 4–6× larger, at 11–26 μA cm–2, than those measured on the MEA. The difference is interpreted as due to better water removal in the FE system. This work shows that MEA catalyst layers are vastly underutilized, due to poor water transport, and current densities equivalent to 10–16 A cm–2 at 0.65 V for 400 μgPt cm–2 (25–40 A mg–1) are achievable, whereas the current mass activity of MEAs is <40% of this value at 25 and 80 °C, 150 kPag.

Which insights can gas diffusion electrode half-cell experiments give into activity trends and transport phenomena of membrane electrode assemblies?

“…Similar behaviour and conclusions were recently also presented by Jackson et al comparing MEA data of Pt/C with measurements carried out in a FE setup with ultrathin catalyst layers. 59 Interestingly, in this study this behaviour in the MEA mass-transport limitation regime is thus also observed for much thicker catalyst layers compared to the FE and close to industrial application. The study by Jackson et al generally has seen an improved performance of FE over MEA in the whole current regime, and does not show the behaviour seen in this study in the ohmic regime with a superior performance of MEA over GDE.…”

Benchmarking proton exchange membrane fuel cell cathode catalyst at high current density: A comparison between the rotating disk electrode, the gas diffusion electrode and differential cell

“…In the early 2000s, the group of Grenoble introduced the gas diffusion electrode (GDE) to benchmark PEMFC cathode catalyst [15,16], the idea relying on earlier works in the field of phosphoric acid fuel cell catalysis [17,18]. Then, the group of Kucernak at Imperial college, London, made several (mostly successful) attempts to unveil the kinetics of fast reactions in three-electrode cells using liquid electrolytes, with notably the so-called floating electrode setup [19][20][21][22]. However, the floating electrode has not been exported to other labs, yet, owing to difficulties in avoiding electrode flooding, related to the absence of convective flux at the vicinity of the active layer.…”

Challenges in Unravelling the Intrinsic Kinetics of Gas Reactions at Rotating Disk Electrodes by Koutecky–Levich Equation

“…In fact, compared to other techniques that applied to study the gaseous reactions, such as floating electrode, membrane electrode assemblies (MEA), and gas diffusion electrode (GDE), a major limitation of RDE studies (whether it is on smooth electrodes or on porous electrocatalysts layer) on ORR is that the kinetic information can only be obtained in the low overpotential region which is usually far away from the operational conditions in fuel cells (much higher current density which corresponds to much higher overpotential, compared to RDE). ,,,− However, the advantages of RDE compared to these techniques should be mentioned: 1) the mass of catalysts used for the test is usually relatively small; 2) the electrode is easy to fabricate, and the test or experiment is usually fast and easy to be handled. Both make RDE very suitable for quick screening of different nanocatalysts.…”