Mild Synthesis of Amorphous Multi‐Principal Element Phosphates via Hybrid Ionic Co‐Cross‐Linking to Unlock High‐Efficiency Oxygen Evolution Reaction

Multi‐principal element materials (MPEMs) are emerging as strong candidates for oxygen evolution reaction (OER) electrocatalysts due to their multi‐element synergy, structural robustness, and compositional versatility that can tune active sites. However, many reported MPEM syntheses involve harsh conditions (e.g., high temperature) that risk component instability or limited synthesis scalability, while conventional wet‐chemistry routes at lower temperatures often cause compositional nonuniformity, especially for ionic MPEMs. Here, we present a mild hybrid ionic co‐cross‐linking strategy that assembles diverse metal‐phosphate oligomers into amorphous multi‐principal element phosphates (MPEPs), scalable from ternary to septenary compositions. Total pair distribution functions‐based structure modeling revealed the molecular‐level spatial uniformity of metals with abundant oxygen vacancy. By increasing elemental diversity, the septenary MPEPs achieved an overpotential as low as 205 mV at 10 mA/cm 2 and a Tafel slope as low as 33 mV/dec in 1.0 M KOH and sustained 50 mA/cm 2 for 260 h. This composition‐general and mild route expands the accessible space of ionic MPEMs and provides a scalable platform for molecularly regulated active sites toward high‐efficiency OER.