PH-dependent electrocatalysis for proton reduction by bis(2,2′:6′,2″-terpyridine) cobalt(II) complex embedded in Nafion ® membrane

Abstract

Electrocatalysis for H + reduction by a bis(2,2′:6′,2″-terpyridine) cobalt(II) complex ([Co(terpy) 2 2+]) dispersed in a Nafion ® membrane coated on a basal-plane pyrolytic graphite (BPG) electrode was studied. It was found that the electrocatalysis for H + reduction varies with the pH conditions employed. Under acidic pH conditions ( pH≲ 4 ), the electrocatalytic H + reduction was induced by [Co(terpy) 2 +] (Co(I)), while under weakly acidic–neutral pH conditions ( pH≳ 4 ), H + reduction took place after forming [Co(terpy) 2 + ] Co(0)). The pH-dependent electrocatalysis was attributable to weak basicity of the electrogenerated Co(I): under low pH conditions ( pH≲ 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∼4, the H + reduction took place by Co(0) via Co(0)–H. The electron transfer to generate catalytically active species at the polymer-coated electrode was also studied by potential-step chronoamperometry. Similar to the concentration dependence of the turnover number of the catalyst, the apparent diffusion 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.