Goldsmith Zachary K, Harshan Aparna K, Gerken James B, Vörös Márton, Galli Giulia, Stahl Shannon S, Hammes-Schiffer Sharon
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706.
Proc Natl Acad Sci U S A. 2017 Mar 21;114(12):3050-3055. doi: 10.1073/pnas.1702081114. Epub 2017 Mar 6.
NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni to Ni, followed by oxidation to a mixed Ni state at a potential coincident with the onset of OER activity. Calculations on the 25% Fe-doped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixed-metal oxidation states in heterogeneous catalysts.
氢氧化氧镍材料是用于析氧反应（OER）的高活性电催化剂，OER是碳中性能量存储的一个重要过程。最近的光谱学和计算研究越来越支持铁是催化活性位点，但在相关铁的氧化还原状态方面存在分歧。混合周期密度泛函理论计算和光谱电化学实验相结合，阐明了纯镍和混合镍铁氢氧化氧薄膜电催化剂的电子结构和氧化还原热力学。纯镍催化剂的紫外/可见光吸光度取决于施加的电位，因为薄膜中的金属离子在OER活性开始之前就被氧化了。相比之下，在25%铁掺杂的催化剂中，直到OER活性开始，吸光度变化都可以忽略不计。质子耦合氧化还原电位和磁化强度的第一性原理计算表明，纯镍体系的特征是镍氧化为Ni，然后在与OER活性开始一致的电位下氧化为混合Ni状态。对25%铁掺杂体系的计算表明，在催化开始之前，催化剂是氧化还原惰性的，这与Fe和混合Ni氧化态的形成相吻合。计算表明，铁掺杂剂的引入改变了导带最小值的性质，从纯镍中的Ni-氧化物变为镍铁电催化剂中的主要Fe-氧化物。这些发现为镍和镍铁氢氧化氧电催化剂的电化学和光学性质提供了统一的实验和理论描述，并作为多相催化剂中混合金属氧化态计算表征的重要基准。
