Balancing oxygen binding via bidirectional electronic effects in Ni-doped FeS2 for oxygen evolution reaction catalysis

The advancement of Fe-based oxygen evolution reaction (OER) catalysts lags behind their Ni-based counterparts, albeit their good theoretically predicted performance and low cost. Deeply understanding the catalytic mechanism would be necessary to advance the Fe-based catalysts. Currently, Fe-based non-metallic compounds, such as sulfides, phosphides, and selenides, have been widely explored as highly active OER precatalysts because of their tunable electronic structures and relatively high electrical conductivity. Nevertheless, these precatalysts usually undergo surface reconstruction into oxyhydroxides under OER conditions, making the real active phase different from the pristine compound and complicating the structure-performance relationship. Herein, taking the Ni-doped FeS 2 nanosphere catalyst as a platform, we discover a nuanced bidirectional electronic effect within the surface-reconstructed active phase, showcasing the vital roles of both residual FeS 2 and Ni dopants in optimizing catalytic performance. Structural characterizations confirm the successful incorporation of Ni dopants into the pyrite FeS 2 lattice. An amorphous carbon layer is coated on the sulphide nanospheres, which contributes to improving charge transfer efficiency and maintaining structural integrity during electrocatalysis. As a result, the obtained catalyst, typically denoted as Ni 0.2 -FeS 2 @C, achieves markedly enhanced OER activity in the alkaline electrolyte, requiring an overpotential of only 270 mV to reach 10 mA cm −2 , much lower than those of the undoped and uncoated counterparts (FeS 2 @C and FeS 2 ). Moreover, Ni 0.2 -FeS 2 @C exhibits negligible activity decay after 5000 cyclic voltammetry cycles and remains stable during 100 h of constant-potential operation, with Faradaic efficiency for oxygen production close to 100%, demonstrating good catalytic durability and selectivity. Spectroscopic analyses and density functional theory calculations reveal that surface reconstruction during OER leads to the formation of oxyhydroxide as reported elsewhere, but unique electronic interactions, which have not been observed earlier. On the one hand, the residual FeS 2 substrate upshifts the d -band center of the surface oxyhydroxide, strengthening the adsorption behavior of the crucial oxygenated intermediate. On the other hand, Ni dopants induce a downshift of the d -band center, which weakens over-binding inversely. The resulting bidirectional modulation optimizes the electronic structure of active sites and effectively balances the adsorption strength of OER intermediates, thereby reducing the OER energy barrier. Consequently, this work not only provides a viable strategy for the rational design of Fe-based OER catalysts but also deepens the mechanistic understanding of the real active phase of balanced electronic modulation in enhancing electrocatalytic activity. Surface reconstruction generates an oxyhydroxide active layer on Ni-doped FeS 2 for OER, where the FeS 2 substrate and Ni dopants oppositely regulate the d -band center to optimize oxygen-intermediate binding. Download: Download high-res image (295KB) Download: Download full-size image