Capturing a Transition Phase during Birnessite-to-Spinel Transition for Efficient Water Oxidation

The transition phase is a unique intermediate-state material with structural characteristics of two distinct phases, and the coexistence of diverse structural units endows it with intriguing chemical properties. However, preparing stable transition phase materials is a challenge. Here, a highly active transition phase is precisely captured from a birnessite-to-spinel transition via an ion-steric strategy. The intercalation of Ca²⁺ affects the thermodynamics of phase transition, hinders the migration of Co²⁺, and broadens the phase transition temperature window from 450-500 °C to 400-600 °C, enabling the capture of favorable transition phase material (Ca-TP-op) with short-range ordered spinel configuration and long-range disordered MO₆ (M = Mn, Co) octahedral framework structure at 500 °C. The novel structures within transition phases promote electron transfer, alter orbital hybridization, and activate lattice oxygen. Consequently, Ca-TP-op demonstrates efficient oxygen evolution reaction activity, surpassing both the composite and mechanical mixture of birnessite and spinel. The universality of the ion-steric strategy can be demonstrated by the intercalation of Na, K, Mg, Sr, and Ba ions. This work successfully pioneers a new transition phase catalytic material system that offers insights on the development of efficient functional catalysts.