Transition from amorphous to crystalline in FeW oxides: Alterations in active center spin state to improve OER performance

Amorphous and crystalline catalysts both have their own advantages to reduce the overpotential of OER. However, there is limited research on the relationship between the OER catalytic performance and the crystallinity degree. Herein, this key issue was explored by preparing FeW oxides with varying degrees of crystallinity. DFT combined with practical experimental results indicate that the change in crystalline state not only affects the electronic interactions in Fe-W-O but also the M-O bond lengths, both influencing the spin state of FeW oxides. At the critical temperature of amorphous-to-crystalline transition, a low-spin state of Fe in FeWO-450 is found to optimize the adsorption of intermediates during OER. FeWO-450 exhibits even better performance than commercial noble metal catalysts and numerous catalysts reported in the literature. This work showcases the effect of crystallinity of metal oxides on electron spin and offer a guidance how to improve OER properties by altering the electron spin states.