Core-Shell Structured Water Oxidation Anode for Minimum Platinum Group Metal Usage in Proton Exchange Membrane Water Electrolyzer

Abstract

The development of enhanced electrocatalysts and optimization of electrode structures are of great importance for the practical and widespread deployment of proton exchange membrane water electrolyzers (PEMWEs). In particular, the scarcity and high price of Ir are considered to be bottlenecks inhibiting the large-scale commercialization of PEMWEs. Even compared with Pt, a rare-earth metal with a low production rate of 150~200 tons per year, Ir availability is extremely limited, as only a few tons per year are produced [1]. Therefore, It is envisioned that the Ir loading on the anode should be lowered (e.g., to 0.05 mg/cm2) to meet the annual demand for the PEMWE system installation on the scale of ≈5 GW by 2040 [2].To reduce the usage of rare-earth metals in PEMWEs, a highly active water-oxidizing anode was fabricated by depositing a core–shell structured catalyst on a titanium porous transport layer (Ti PTL). Earth-abundant metal-based iron-nitride (Fe2N) nanostructure was deposited on a Ti PTL by sequential hydrothermal and nitridation reaction. The prepared Fe2N nanostructure was then passivated by thin, electrodeposited iridium-oxide films (EIROF). Fe2N@EIROF core-shell structured anodes exhibited substantially low ohmic and mass-transfer resistances, leading to a high current density of 4.50 A/cm2 (@1.9 Vcell). The unique Fe2N@EIROF core-shell structure enhanced PEMWE performance in terms of Ir mass activity by substituting inner Ir atoms with earth-abundant metal-based iron nitride. A high Ir mass activity of 103 A/mgIr (@1.9 Vcell) was achieved by reducing Ir loading amount to 0.064 mg/cm2 on the anode. The chemical instability of transition metal-based supports in a PEMWE operating condition was overcome by the passivation with Ir catalyst layers. As a result, the prepared core–shell-structured catalysts were stable under the PEMWE operating condition achieving high Ir mass activities.[1] K. Nose, T.H. Okabe, Chapter 2.10 - Platinum Group Metals Production, in: S. Seetharaman (Ed.) Treatise on Process Metallurgy, Elsevier, Boston, 2014, pp. 1071-1097,[2] C. Minke, M. Suermann, B. Bensmann, R. Hanke-Rauschenbach, Is iridium demand a potential bottleneck in the realization of large-scale PEM water electrolysis?, Int. J. Hydrogen Energy, 46 (2021) 23581-23590