Stable and efficient oxygen evolution reaction (OER) catalysts for the oxidation of water to dioxygen in highly acidic media are currently limited to expensive noble metal (Ir and Ru) oxides since presently known OER catalysts made of inexpensive earth-abundant materials generally suffer anodic corrosion at low pH. In this study, we report that a mixed-polymorph film comprising maghemite and hematite, prepared using spray pyrolysis deposition followed by low-temperature annealing, showed a sustained OER rate (>24 h) corresponding to a current density of 10 mA cm⁻² at an initial overpotential of 650 mV, with a Tafel slope of only 56 mV dec⁻¹ and near-100% Faradaic efficiency in 0.5 M H₂SO₄ (pH 0.3). This performance is remarkable, since iron (III) oxide films comprising only maghemite were found to exhibit a comparable intrinsic activity, but considerably lower stability for OER, while films of pure hematite were OER-inactive. These results are explained by the differences in the polymorph crystal structures, which cause different electrical conductivity and surface interactions with water molecules and protons. Our findings not only reveal the potential of iron (III) oxide as acid-stable OER catalyst, but also highlight the important yet hitherto largely unexplored effect of crystal polymorphism on electrocatalytic OER performance.

由于目前已知的由廉价的地球资源丰富的材料制成的OER催化剂通常会遭受阳极腐蚀，因此在高酸性介质中将水氧化为双氧的稳定而高效的析氧反应（OER）催化剂目前仅限于昂贵的贵金属（Ir和Ru）氧化物。在低pH下。在这项研究中，我们报道了通过喷雾热解沉积然后进行低温退火制备的包含磁赤铁矿和赤铁矿的混合多晶型膜，显示出持续的OER速率（> 24小时），对应于10 mA cm ^-2的电流密度在初始过电势为650 mV时，Tafel斜率仅为56 mV dec ^-1，在0.5 MH ₂ SO _4中的法拉第效率接近100％（pH 0.3）。该性能是显着的，因为发现仅包含磁赤铁矿的氧化铁（III）膜具有可比的固有活性，但对OER的稳定性却低得多，而纯赤铁矿膜则对OER无活性。这些结果可以通过多晶型物晶体结构的差异来解释，这些差异会导致不同的电导率以及与水分子和质子的表面相互作用。我们的发现不仅揭示了氧化铁（III）作为酸稳定的OER催化剂的潜力，而且还强调了晶体多态性对电催化OER性能的重要但迄今未开发的影响。