Porphyrin-Based Electrochemical H2 Evolution: Role of Central Metal Ion on Overpotential and Catalytic Activity
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Various transition metal complexes of porphyrins, M-meso-tetrakis-(p-NH2phenyl)porphyrins [where M = Fe, Co, Ni, and Cu] have been synthesized and employed for electrochemical H2 evolution studies. The effects of nature of central metal ion in H2 evolution activity and overpotential were explored by conducting activity study in DMSO using organic acid and H2O as a proton source. In organic acid (trifluoroacetic acid, TFA), the catalysis occurs at a potential close to M2+/M1+ redox couple only for Co and Cu while at a more –ve potential (close to M1+/M0 redox event) for Fe and Ni with activity order of Co > Fe > Cu > Ni. At low acid concentration catalytic efficiency (C.E) of 85%, observed rate constant (kobs) of 240 s−1 and a current enhancement (Icat/Ip) of 29 are obtained for Co complex. In neutral aqueous solution, high activities were also observed for Co and Fe complexes than others. Based on our results, the redox potential of central metal ion and thermodynamic reduction potential of a proton source seem to play roles in tuning catalytic activity.
KeywordsMetallo porphyrins Elctrocatalysis Hydrogen evolution Overpotential
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