Beyond inert: how carbon supports tune the oxygen evolution activity of polyoxometalate-based single-atom catalysts

Phosphomolybdic acid (PMA)-based single-atom catalysts (SACs) are promising for the oxygen evolution reaction (OER), but the role of their ubiquitous carbon support has rarely been investigated. Using density functional theory (DFT), we systematically investigated the OER on TM₁/PMA (TM = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt; PMA = [PMo₁₂O₄₀]^3-) with no support, a pristine carbon support, a hydroxyl (-OH)-functionalized support, and a carboxyl (-COOH)-functionalized support. We found that the mere presence of carbon support strongly modulates catalytic activity, an effect that far outweighs that of its subsequent functionalization. Bader charge analysis reveals significant electron donation from the carbon support to the TM₁/PMA unit, markedly altering intermediate adsorption free energies, which in turn either improves or worsens OER performance by shifting the metals along the descriptor axis and slightly lowering the volcano apex. For example, the Ni₁/PMA@C achieves a low overpotential of 0.37 V, markedly lower than 0.70 V for bare Ni₁/PMA. This support-induced modulation carries a critical methodological implication: for single-metal-on-anion SAC systems such as PMA-based catalysts, the carbon support must be explicitly included in theoretical models to capture its essential charge transfer with SAC. The carbon support is far from an inert scaffold but is a key factor in designing high-efficiency PMA-based single-atom OER catalysts.