Xi Lifei, Schwanke Christoph, Zhou Dong, Drevon Dorian, van de Krol Roel, Lange Kathrin M
Young Investigator Group Operando Characterization of Solar Fuel Materials (EE-NOC), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany.
Dalton Trans. 2017 Nov 21;46(45):15719-15726. doi: 10.1039/c7dt02647a.
Solar water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen gas. In this study, we demonstrate that the photoelectrochemical (PEC) performance of hematite photoanodes can be improved by modification with the oxygen evolution catalyst CoB. The current density at 1.23 V of the pristine hematite under one sun is 0.88 mA cm and it increases to 1.12 mA cm after CoB modification (∼27% improvement). The presence of a CoB cocatalayst layer is proposed to improve the oxygen evolution reaction (OER) kinetics and also to prevent electron-hole recombination at the surface via passivating surface defects as well as suppressing the tunneling of electrons from the hematite core, thus improving the photocurrents and resulting in a negative shift of photocurrent onset potentials. These effects of CoB modification are supported by experimental data obtained by performing electrochemical impedance spectroscopy (EIS), PEC and incident photon-to-current efficiency (IPCE) measurements. To investigate the electronic structure of the CoB cocatalyst deposited on hematite, XPS and in situ X-ray absorption spectroscopy (XAS) are employed. Co K-edge spectra at different potentials and light conditions are recorded. This makes the present work different from most of the previous studies. Using a quantitative analysis method, information on the mean oxidation state of Co in the CoB film under applied potential and illumination is revealed. We also compare different methods for determining the oxidation state from the edge position and find that the integral method and half height methods are most suitable. In summary, the present work underlines the improvement of the semiconductor/cocatalyst interface of oxygen evolving photoanodes and strengthens the importance of in situ XAS spectroscopy when studying catalysts. This study is the first report so far combining the studies of the PEC performance of a CoB modified hematite nanorod array photoanode and in situ XAS at the Co K-edge.
太阳能水分解是一种潜在的可扩展方法，可将太阳能以可再生氢气的形式储存起来。在本研究中，我们证明了通过用析氧催化剂CoB进行改性，可以提高赤铁矿光阳极的光电化学（PEC）性能。在一个太阳光照射下，原始赤铁矿在1.23 V时的电流密度为0.88 mA/cm²，CoB改性后增加到1.12 mA/cm²（提高了约27%）。提出CoB助催化剂层的存在可改善析氧反应（OER）动力学，并通过钝化表面缺陷以及抑制电子从赤铁矿核心的隧穿来防止表面的电子 - 空穴复合，从而提高光电流并导致光电流起始电位负移。通过进行电化学阻抗谱（EIS）、PEC和入射光子到电流效率（IPCE）测量获得的实验数据支持了CoB改性的这些效果。为了研究沉积在赤铁矿上的CoB助催化剂的电子结构，采用了X射线光电子能谱（XPS）和原位X射线吸收光谱（XAS）。记录了不同电位和光照条件下的Co K边光谱。这使得本工作与大多数先前的研究不同。使用定量分析方法，揭示了在施加电位和光照下CoB薄膜中Co的平均氧化态信息。我们还比较了从边缘位置确定氧化态的不同方法，发现积分法和半高法最合适。总之，本工作强调了析氧光阳极的半导体/助催化剂界面的改善，并强化了原位XAS光谱在研究催化剂时的重要性。本研究是迄今为止第一篇将CoB改性的赤铁矿纳米棒阵列光阳极的PEC性能研究与Co K边原位XAS相结合的报告。
