The copper complex [(bztpen)Cu](BF4)2 (bztpen=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k(obs)) of over 10000 s(-1). A turnover frequency (TOF) of 7000 h(-1) cm(-2) and a Faradaic efficiency of 96% were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu](2+) in pH 2.5 buffer solution at -0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton-coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu](2+).

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The copper complex [(bztpen)Cu](BF 4 ) 2 (bztpen = N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k obs ) of over 10000 s À1 . A turnover frequency (TOF) of 7000 h À1 cm À2 and a Faradaic efficiency of 96 % were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu] 2+ in pH 2.5 buffer solution at À0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton-coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu] 2+ .The availability of electric energy directly generated from renewable energy sources, such as sunlight, wind, and tides, is often inconsistent with the temporal fluctuation and the geographic distribution of energy demand. One of the practical and promising technologies for the efficient storage of surplus electric energy is to use it for the electrolytic conversion of water into hydrogen, and the reverse hydrogen combustion reaction can regenerate energy when and where it is needed, for example, through the use of fuel cells. This energy storage and regeneration cycle is a clean process that does not release carbon dioxide or other pollutants. For this purpose, the key issue is to reduce the catalyst cost and enhance the energy conversion efficiency by developing catalysts that comprise only earth-abundant elements and display high activities with low overpotentials and good stabilities for electrolysis of water.