One-step electrochemical synthesis of earth-abundant multi-metal-ion–doped polyaniline films for OER electrocatalysis

Nitrogen-rich conducting polymers such as polyaniline offer a tunable scaffold for earth-abundant oxygen evolution reaction (OER) sites, but progress is slowed by two challenges: i) multi-metal incorporation is typically achieved by post-doping, giving poorly defined stoichiometry and coordination, and ii) trace Fe in alkaline electrolytes can dominate the apparent activity of Ni-based motifs. Here we report a one-step cyclic-voltammetry co-deposition strategy that electropolymerizes aniline while embedding Fe, Ni and Mn ions into polyaniline (PAni), enabling mono-, bi- and trimetallic films without post-doping. Films are grown on carbon paper to remove substrate-derived activity and isolate composition–structure–performance correlations. Element-specific X-ray absorption spectroscopy (XAS; Fe/Ni/Mn K-edges) confirms ion uptake and competitive incorporation during mixed-ion growth, with a pronounced tendency toward Ni uptake (fluorescence intensity up to ∼0.25 a.u. for PAni–Ni versus ∼0.02 and ∼0.01 a.u. for PAni–Mn and PAni–Fe, respectively), reflecting composition-dependent differences in the local metal coordination. Using 15N-labeled PAni, solid-state 15N NMR (CP-MAS and spin-echo) provides direct fingerprints of metal-nitrogen coupling beyond the sensitivity of extended X-ray absorption fine structure (EXAFS), distinguishing a distinct, strongly coupled N–Ni population from weaker Mn–N interactions and moderately coupled Fe–N environments. Crucially, OER testing in non-purified versus purified KOH decouples intrinsic activity from impurity-driven promotion: trace Fe increases the apparent activity of Ni-containing films, whereas only PAni–FeNiMn retains high activity under purified conditions, reaching an overpotential of 350 mV at 10 mA cm-2. Together, these results show that multi-ion co-deposition leads to a balanced coordination regime that survives rigorous impurity control, establishing PAni as a structurally tailorable, low-loading platform for enhanced OER catalysis.