Coordination-Engineered Carboxylated MN₄-Macrocycles With Chloride-Modulated Proton Hopping for Efficient Hydrogen...

A pathway toward molecularly versatile, acid-stable, and chloride-tolerant catalysts for hydrogen evolution are designed via coordination engineering of π-conjugated frameworks. To elucidate the role of coordination environment, electronic delocalization, and electrolyte composition in the electrochemical hydrogen evolution reaction (HER), carboxyl-functionalized metal (M: Ni⁺²/Co⁺²) phthalocyanines were synthesized in both monomeric [mono-[CMPc]] and polymeric [poly-[CMPc]] forms. Among all the catalysts, poly-[CNiPc] demonstrated the lowest overpotential of -76 mV at -10 mA cm^-2 and a Tafel slope of 38 mV dec^-1 in 0.5 M H₂SO₄. The Ni²⁺/Ni¹⁺ redox pair and the conjugated macrocycle have a strong coupling and near-thermoneutral Ni-H adsorption, resulting in the increased activity. A volcano-type dependence was found when electrolyte was modulated with NaCl (0.1 to 0.5 M). The optimal performance was obtained with 0.1 M NaCl in 0.5 M H₂SO₄ (η₁₀ = -55 mV) electrolyte. Higher chloride levels resulted in the creation of Ni-Cl adducts and interfacial disruption, whereas chloride-induced double-layer compression, increased ionic conductivity, and Grotthuss-type proton hopping across hydrogen-bonded Cl^--H₂O and -COOH networks are the source of the improvement in HER activity. The catalyst maintained its Ni²⁺-N₄ coordination after 60 h of operation, confirmed by post stability characterization with moderate morphological roughening and reversible Na⁺/Cl^- incorporation. These results demonstrate the molecular-electrolyte co-design approach wherein halide-regulated proton transport and π-conjugated, carboxylated phthalocyanine frameworks work together to facilitate effective and long-lasting hydrogen evolution in acidic and chloride-containing media.