Crystallinity-Dependent Reverse Hydrogen Spillover Governs Electrochemical CO2 Reduction to Methane

The electrochemical CO 2 reduction reaction (CO 2 RR) is a proton−electron coupled transfer process in which active hydrogen (*H) plays a pivotal role in determining the reaction pathways, particularly for methane (CH 4 ), a representative deep hydrogenation product. However, how *H generation and migration govern CH 4 formation remains elusive, motivating mechanistic elucidation to enable rational regulation of reverse hydrogen spillover. Herein, we develop Cu-doped ZrO 2 catalysts and reveal that tuning the crystallinity of the ZrO 2 support enables control over the reverse hydrogen spillover process. Specifically, amorphous ZrO 2 functions as an efficient “hydrogen pump” by promoting *H generation and lowering the migration barrier toward adjacent Cu sites, thereby strengthening the reverse hydrogen spillover effect. This effect establishes an optimal *H coverage for deep hydrogenation of CO 2 , thereby reducing the energy barriers for key intermediates along the CH 4 formation pathway. Consequently, the Cu-doped amorphous ZrO 2 catalyst delivers a CH 4 Faradaic efficiency of 69.3% with a partial current density of −372.8 mA cm −2 , significantly surpassing that of its crystalline analogue. This work highlights that engineering the crystallinity of metal oxide supports provides an effective strategy to regulate reverse hydrogen spillover in electrocatalysis, offering valuable insights into controlling catalytic activity and product selectivity.