Ionomer-Free NiFe/NiFeO Bilayer Oxygen Evolution Reaction Electrocatalyst Prepared by a Magnetron Sputtering at Oblique Angle Bottom-Up Deposition Method

This manuscript reports on a Ni/Fe-based bilayer catalyst developed to boost the oxygen evolution reaction in anion exchange membrane water electrolyzers. The electrochemical behavior toward the oxygen evolution reaction of several NiFe/NiFeO metal-oxide bilayer catalysts, prepared by magnetron sputtering at oblique angle deposition (MS-OAD) on a flat stainless-steel substrate, was assessed in a three-electrode electrochemical cell in comparison with the behavior of both a metal NiFe and an oxide NiFeOx single-layer catalyst. The morphology and chemical nature of these catalysts, as prepared and after electrochemical usage, were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. A thorough electrochemical characterization of the different catalyst formulations revealed a higher efficiency for the bilayer catalysts, in terms of both activity and long-term stability, and provided some clues to account for this superior performance in terms of morphology and surface reactivity of each catalyst. As a proof of concept, the best-performing bilayer configuration was then deposited onto a stainless steel felt porous transport layer (PTL) substrate and tested as an ionomer-free anode electrode in a membrane electrode assembly (MEA). Results revealed that the MS-OAD catalysts performed well when deposited on PTLs and that, under this configuration, a bilayer catalyst anode is slightly more efficient than the NiFe single-layer catalyst. Additionally, the possibility of scaling up the MS-OAD procedure to large areas has been demonstrated by the preparation of the bilayer catalysts on a 64 cm² PTL and its successful integration and operation in a large prototype single cell.