Govind Rajan Ananth, Martirez John Mark P, Carter Emily A
Department of Chemical Engineering, Indian Institute of Science, Bengaluru, Karnataka 560012, India.
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA.
Phys Chem Chem Phys. 2024 May 22;26(20):14721-14733. doi: 10.1039/d4cp00315b.
Iron (Fe)-doped β-nickel oxyhydroxide (β-NiOOH) is a highly active, noble-metal-free electrocatalyst for the oxygen evolution reaction (OER), with the latter being the bottleneck in electrochemical water splitting for sustainable hydrogen production. The mechanisms underlying how the Fe dopant modulates this host material's water electro-oxidation activity are still not entirely clear. Here, we combine hybrid density functional theory (DFT) and Hubbard-corrected DFT to investigate the OER activity of the most thermodynamically favorable (and therefore, expected to be the majority) crystallographic facets of β-NiOOH, namely (0001) and (101̄0). By considering active sites involving both oxidation and reduction of the transition-metal active center during the redox cycle on these two different facets, we show that six-fold-lattice-coordinated Fe in β-NiOOH is redox inactive towards both oxidation and reduction while five-fold-lattice-coordinated Fe in β-NiOOH does exhibit redox activity. However, the determined redox activity of Fe (or lack of it) is not indicative of good (or bad) performance as a dopant on these two facets. Three of the four active sites investigated (oxo and hydroxo sites on (0001) and a hydrated site on (101̄0)) exhibit only a marginal (<0.1 V) decrease or increase in the thermodynamic overpotential upon doping with Fe. Only one of the redox-active sites investigated, the hydroxo site on (101̄0), exhibits a large attenuation in the thermodynamic overpotential upon doping (to ∼0.52 V from 0.86 V), although the doped overpotential is larger than that observed experimentally for Fe-doped NiOOH. Thus, although pure β-NiOOH facets containing four-, five-, or six-fold lattice-coordinated Ni sites have roughly equal OER activities, yielding similar OER onset potentials (shown in A. Govind Rajan, J. M. P. Martirez and E. A. Carter, , 2020, , 3600-3612), only those facets containing four-fold lattice-coordinated Fe (, as shown in J. M. P. Martirez and E. A. Carter, , 2019, , 693-705) would be active under analogous conditions for the Fe-doped material. It follows that, while undoped β-NiOOH demonstrates a roughly facet-independent oxygen evolution activity, the activity of Fe-doped β-NiOOH strongly depends on the crystallographic facet. Our study further motivates the investigation of strategies for the selective growth of facets with low iron coordination number to enhance the water splitting activity of Fe-doped β-NiOOH.
铁（Fe）掺杂的β-羟基氧化镍（β-NiOOH）是一种用于析氧反应（OER）的高活性、无贵金属的电催化剂，而析氧反应是可持续制氢的电化学水分解过程中的瓶颈。铁掺杂剂如何调节这种主体材料的水电氧化活性的潜在机制仍不完全清楚。在此，我们结合混合密度泛函理论（DFT）和哈伯德校正DFT，研究β-NiOOH最热力学有利（因此预计为多数）的晶体学面，即（0001）和（101̄0）的OER活性。通过考虑在这两个不同面上的氧化还原循环中涉及过渡金属活性中心氧化和还原的活性位点，我们表明β-NiOOH中六重晶格配位的Fe对氧化和还原均无氧化还原活性，而β-NiOOH中五重晶格配位的Fe确实表现出氧化还原活性。然而，所确定的Fe的氧化还原活性（或缺乏氧化还原活性）并不表明其作为这两个面上的掺杂剂的性能好坏。所研究的四个活性位点中的三个（（0001）面上的氧和羟基位点以及（101̄0）面上的水合位点）在掺杂Fe后，热力学过电位仅略有降低（<0.1 V）或升高。所研究的仅一个氧化还原活性位点，即（101̄0）面上的羟基位点，在掺杂后热力学过电位大幅降低（从0.86 V降至约0.52 V），尽管掺杂后的过电位高于实验观察到的Fe掺杂NiOOH的过电位。因此，尽管含有四重、五重或六重晶格配位Ni位点的纯β-NiOOH面具有大致相等的OER活性，产生相似的OER起始电位（如A. Govind Rajan、J. M. P. Martirez和E. A. Carter在2020年发表于[具体文献]，3600 - 3612页所示），但只有那些含有四重晶格配位Fe的面（如J. M. P. Martirez和E. A. Carter在2019年发表于[具体文献]，693 - 705页所示）在类似条件下对于Fe掺杂材料才具有活性。由此可见，虽然未掺杂的β-NiOOH表现出大致与面无关的析氧活性，但Fe掺杂的β-NiOOH的活性强烈依赖于晶体学面。我们的研究进一步推动了对选择性生长低铁配位数面的策略的研究，以提高Fe掺杂β-NiOOH的水分解活性。
