Abstract
Anion-exchange-membrane water electrolyzers (AEMWE) for hydrogen production have attracted interest because cost-effective Ni- and Fe-based catalysts can be used for the oxygen evolution reaction (OER). Although NiFe oxide/hydroxide-based catalysts have been extensively studied, the role of Fe and its chemical state during OER are not well understood, with inconsistent findings across different studies. In this work, we combined in situ57Fe Mössbauer (MS) and X-ray absorption spectroscopy (XAS) to investigate the chemical states of Fe and Ni and elucidate their synergy during the OER. A NiFe (8 : 1 molar ratio) aerogel catalyst with high surface area, nano crystallinity, and high performance in AEMWE was used. We show that both Fe and Ni are oxidized during anodic polarization, and the potential for the change of oxidation states correlates well with the onset of the OER. In situ MS shows that 80-90% of Fe3+ becomes tetravalent at OER potentials and remains so even after the potential is lowered below OER onset. Analysis of in situ XAS results suggests full Fe incorporation into Ni hydroxide. At OER potentials, lattice contraction indicates high oxidation states for both Ni and Fe. Upon returning to lower potentials, a portion of the Fe remains in its more oxidized form which corroborates the in situ MS findings. Results from this work affirm the importance of high-valent Ni and Fe in promoting the OER. Ni and Fe exhibit synergy during OER and the aerogel's unique nanomorphology leads to high OER activity.
| Original language | English |
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| Journal | EES Catalysis |
| DOIs | |
| State | Accepted/In press - 2025 |
Funding
This work was performed in the ElectroCat Consortium (ElectroCat 2.0), supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (technology managers David Peterson and McKenzie Hubert). This work was authored in part by Oak Ridge National Laboratory operated by UT-Battelle, LLC, under contract no. DE-AC05-00OR22725, by Argonne National Laboratory, a DOE Office of Science Laboratory managed by the UChicago Argonne, LLC under contract no. DE-AC-02-06CH11357, and by Los Alamos National Laboratory operated by Triad National Security, LLC under US DOE contract no. 89233218CNA000001. This research used resources of the National Synchrotron Light Source II; a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used resources of the Advanced Photon Source (APS); a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. MRCAT operations at the APS are supported by the Department of Energy and MRCAT member institutions. Electron microscopy analysis was supported by the Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( https://energy.gov/downloads/doe-public-access-plan ).