Zwaschka Gregor, Nahalka Igor, Marchioro Arianna, Tong Yujin, Roke Sylvie, Campen R Kramer
Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany.
Laboratory for Fundamental BioPhotonics, Institutes of Bioengineering (IBI) and Materials Science and Engineering (IMX), School of Engineering (STI), and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
ACS Catal. 2020 Jun 5;10(11):6084-6093. doi: 10.1021/acscatal.0c01177. Epub 2020 Apr 30.
Understanding the mechanism of the oxygen evolution reaction (OER), the oxidative half of electrolytic water splitting, has proven challenging. Perhaps the largest hurdle has been gaining experimental insight into the active site of the electrocatalyst used to facilitate this chemistry. Decades of study have clarified that a range of transition-metal oxides have particularly high catalytic activity for the OER. Unfortunately, for virtually all of these materials, metal oxidation and the OER occur at similar potentials. As a result, catalyst surface topography and electronic structure are expected to continuously evolve under reactive conditions. Gaining experimental insight into the OER mechanism on such materials thus requires a tool that allows spatially resolved characterization of the OER activity. In this study, we overcome this formidable experimental challenge using second harmonic microscopy and electrochemical methods to characterize the spatial heterogeneity of OER activity on polycrystalline Au working electrodes. At moderately anodic potentials, we find that the OER activity of the electrode is dominated by <1% of the surface area and that there are two types of active sites. The first is observed at potentials positive of the OER onset and is stable under potential cycling (and thus presumably extends multiple layers into the bulk gold electrode). The second occurs at potentials negative of the OER onset and is removed by potential cycling (suggesting that it involves a structural motif only 1-2 Au layers deep). This type of active site is most easily understood as the catalytically active species (hydrous oxide) in the so-called incipient hydrous oxide/adatom mediator model of electrocatalysis. Combining the ability we demonstrate here to characterize the spatial heterogeneity of OER activity with a systematic program of electrode surface structural modification offers the possibility of creating a generation of OER electrocatalysts with unusually high activity.
理解析氧反应（OER）——电解水分解的氧化半反应——的机理已被证明具有挑战性。或许最大的障碍在于对用于促进该化学反应的电催化剂活性位点获得实验性见解。数十年的研究已阐明，一系列过渡金属氧化物对析氧反应具有特别高的催化活性。不幸的是，对于几乎所有这些材料，金属氧化和析氧反应在相似的电位下发生。因此，预计催化剂表面形貌和电子结构在反应条件下会持续演变。要获得关于此类材料上析氧反应机理的实验性见解，就需要一种能够对析氧反应活性进行空间分辨表征的工具。在本研究中，我们使用二次谐波显微镜和电化学方法来表征多晶金工作电极上析氧反应活性的空间异质性，从而克服了这一巨大的实验挑战。在适度的阳极电位下，我们发现电极的析氧反应活性由<1%的表面积主导，并且存在两种类型的活性位点。第一种在析氧反应起始电位正方向的电位下观察到，并且在电位循环下稳定（因此大概延伸到体相金电极的多层中）。第二种出现在析氧反应起始电位负方向的电位下，并且通过电位循环被去除（表明它涉及仅1 - 2个金层深度的结构基序）。这种类型的活性位点最容易被理解为电催化所谓初始水合氧化物/吸附原子介导模型中的催化活性物种（水合氧化物）。将我们在此展示的表征析氧反应活性空间异质性的能力与系统的电极表面结构修饰程序相结合，为创造具有异常高活性的新一代析氧反应电催化剂提供了可能性。
