Role of Fe in Enhancement of OER on Fe-Doped NiOOH Catalyst— Insights from Improved Electronic Structure Calculations

Low cost, high activity and high stability make Fe-doped NiOOH a good electrocatalyst for oxygen evolution reaction (OER) [1]. Although significant efforts have been dedicated to understand the enhanced electrocatalytic activity of this material [2,3,4,5], the complete information on the atomic scale mechanisms leading to the OER process improvement is still lacking. One of the reasons on the computational side is that the widely used density functional theory with Hubbard U correction for strongly correlated d electrons (DFT+U) is unable to predict correctly the electronic structure of NiOOH, especially its wide band gap [7], leading to contradictory results. In order to solve this problem, we use Wannier function to construct a more realistic projector for calculating the occupation of d-orbitals, severely overestimated by standard DFT+U approach [9]. With this approach, we obtained correct occupations of 3d orbitals for Ni and Fe, and the band gaps for Ni(OH)₂ and NiOOH that are consistent with experimental findings. With this improvements, we investigated the electronic structure spin of Fe in Fe-doped NiOOH. We show that Fe tends to be in low-spin state, which leads to significant decrease in the thermodynamic overpotential for OER. A spin transition from low-spin to high-spin state is predicted at Fe content of 25%, which contributes to the solubility limit of Fe in NiOOH [1]. We will discuss our computational findings against available experimental data, demonstrating important of joint computational and experimental investigation for understanding of catalytic performance of functional materials.<br/><br/>References:<br/>[1] Friebel et al., J. Am. Chem. Soc. <b>137</b>, 1305-1313 (2015)<br/>[2] Conesa, J. C., J. Phys. Chem. C <b>120</b>, 18999-19010 (2016)<br/>[3] Martirez, J. M. P., Carter, E. A., Chem. Mater. <b>30</b>, 5205-5219 (2018)<br/>[4] Tkalych, A. J. et al., Phys. Chem. Chem. Phys., <b>20</b>, 19525 (2018)<br/>[5] Rajan, A. G. et al., J. Am. Chem. Soc. <b>142</b>, 3600-3612 (2020)<br/>[6] Loschen, C. et al., Phys. Rev. B <b>75</b>, 035115 (2007)<br/>[7] Ratcliff E. L. et al., Chem. Mater. <b>23</b>, 4988-5000 (2011)<br/>[8] Tkalych A. J. et al., J. Phys. Chem. C <b>119</b>, 24315-24322 (2015)<br/>[9] Kowalski, P. et al., Front. Energy Res. <b>9</b>, 653542 (2021)