Speelman Raiden, Marker Ezra J, Boamah Mavis D, Kupferberg Jacob, Bye Justin Z, Engelhard Mark, Zhao Yatong, Martinson Alex B F, Rosso Kevin M, Geiger Franz M
Department of Chemistry, Northwestern University, Evanston, IL, USA.
Pacific Northwest National Laboratory, Richland, WA, USA.
Nat Commun. 2025 Apr 15;16(1):3585. doi: 10.1038/s41467-025-58842-y.
Hematite photoanodes are promising for the oxygen evolution reaction, however, their high overpotential (0.5-0.6 V) for water oxidation and limited photocurrent make them economically unviable at present. The work needed to orient dipoles at an electrode surface may be an overlooked contribution to the overpotential, especially regarding dipoles of water, the electron source in the oxygen evolution reaction (OER). Here, we employ second harmonic amplitude and phase measurements to quantify the number of net-aligned Stern layer water molecules and the work associated with water flipping, on hematite, an earth abundant OER semiconductor associated with a high overpotential. At zero applied bias, the pH-dependent potentials for Stern layer water molecule flipping exhibit Nernstian behavior. At positive applied potentials and pH 13, approximately one to two monolayers of water molecules points the oxygen atoms towards the electrode, favorable for the OER. The work associated with water flipping matches the cohesive energy of liquid water (44 kJ mol) and the OER current density is highest. This current is negligible at pH 5, where the work approaches 100 kJ mol. Our findings suggest a causal relationship between the need for Stern layer water flipping and the OER overpotential, which may lead to developing strategies for decreasing the latter.
赤铁矿光阳极在析氧反应方面具有潜力，然而，其在水氧化过程中具有较高的过电位（0.5 - 0.6 V）且光电流有限，这使得它们目前在经济上不可行。在电极表面使偶极子定向所需的功可能是过电位中一个被忽视的因素，特别是对于水的偶极子而言，水是析氧反应（OER）中的电子源。在此，我们利用二次谐波振幅和相位测量来量化赤铁矿（一种储量丰富且与高过电位相关的OER半导体）表面净排列的斯特恩层水分子数量以及与水翻转相关的功。在零外加偏压下，斯特恩层水分子翻转的pH依赖电位呈现能斯特行为。在正外加电位且pH为13时，大约一到两个水分子单层将氧原子指向电极，这有利于OER。与水翻转相关的功与液态水的内聚能（44 kJ/mol）相匹配，且OER电流密度最高。在pH为5时，该电流可忽略不计，此时功接近100 kJ/mol。我们的研究结果表明斯特恩层水翻转的需求与OER过电位之间存在因果关系，这可能会促使开发降低后者的策略。
