Coordination‐Engineered Carboxylated MN 4 ‐Macrocycles With Chloride‐Modulated Proton Hopping for Efficient Hydrogen Evolution

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 H 2 SO 4 . The Ni 2+ /Ni 1+ 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 H 2 SO 4 ( η 10 = −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 − –H 2 O and –COOH networks are the source of the improvement in HER activity. The catalyst maintained its Ni 2+ –N 4 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.