pH-dependent electrocatalysis for proton reduction by bis(2,2 ': 6 ',2 ''-terpyridine) cobalt(II) complex embedded in Nafion (R) membrane

Electrocatalysis for H+ reduction by a bis(2,2 ' :6 ' ,2 " -terpyridine)cob alt(II) complex ([Co(terpy)(2)(2+)])dispersed in a Nafion (R)) membrane coa ted on a basal-plane pyrolytic graphite (BPG) electrode was studied. It was found that the electrocatalysis for H+ reduction varies with the pH condit ions employed. Under acidic pH conditions (pH less than or similar to 4), t he electrocatalytic H+ reduction was induced by [Co(terpy)(2)(+)] (Co(I)), while under weakly acidic-neutral pH conditions (pH greater than or similar to 4), H+ reduction took place after forming [Co(terpy)(2)(+.)] Co(0)). Th e pH-dependent electrocatalysis was attributable to weak basicity of the el ectrogenerated Co(I): under low pH conditions (pH less than or similar to 4 ) Co(I) can coordinate H+ resulting in an intermediate (Co(I)-H). However, since the reduction potential of Co(II) is pH-independent above pH similar to 4, the H+ reduction took place by Co(0) via Co(0)-H. The electron transf er to generate catalytically active species at the polymer-coated electrode was also studied by potential-step chronoamperometry. Similar to the conce ntration dependence of the turnover number of the catalyst, the apparent di ffusion coefficient (D-app) of the electrons decreased with increasing the complex concentration. It is most probable that the overall kinetics in the electrocatalytic H+ reduction is dominated by the reduction of the complex . (C) 2001 Elsevier Science B.V. All rights reserved.