Hydrogen Production DOI: 10.1002/anie.201000629 Electrochemical Hydrogen Production: Bridging Homogeneous and Heterogeneous Catalysis Marc T. M. Koper* and Elisabeth Bouwman coordination modes · electrochemistry · homogeneous catalysis · hydrogen · Sabatier principle The hydrogen evolution reaction (HER), 2 H + + 2e  !H 2 , is one of the most intensively studied and prototypical electro- chemical reactions. Detailed discussions on the kinetics of the HER and its reverse reaction, the hydrogen oxidation reaction (HOR), have a long history. [1] With the increased interest in finding new and cheaper materials for the catalytic production of fuels, for which hydrogen is a prominent future candidate, the HER has regained the attention of the chemistry community. Although with platinum an excellent catalyst for both HER and HOR is available, the cost and scarcity of this material is believed to potentially hamper large-scale (photo-)electrochemical production of hydrogen. Inspired by the efficacy of naturally occurring enzymes, that is, hydrogenases, that catalyze both the HER and HOR with an activity comparable to platinum, inorganic chemists have put considerable effort in recent years into the development of coordination complexes that are molecularly similar to the active sites of Fe-Fe and Fe-Ni hydrogenases and into the study of these compounds catalytic activity in the HER, mostly in non-aqueous solutions. [2] Simultaneous but largely independent efforts in heterogeneous electrocatalysis have recently also led to new metallic catalysts as well as to a better understanding of the origin of the overpotential in the HER/ HOR. [3] In a recent study, Le Goff et al. [4] report on a nickel phosphane compound that, when grafted onto carbon nano- tubes, shows an unprecedented activity for the HER in acetonitrile, and most significantly, also in water. Inspired by the active sites of the hydrogenases, initial research efforts for new HER catalysts have been focused on FeFe and NiFe compounds. Whereas numerous NiFe complexes have been synthesized, their activities in HER/HOR have either not been investigated, or they proved to be unstable or unreacti- ve. [2c] The use of FeFe complexes was more successful, and the first example of HER catalyzed by a dinuclear iron compound was reported by Rauchfuss and co-workers in 2001. [5] Since then, advances on the first model were achieved by substitu- tion of the ligands. Especially the introduction of proton- accepting basic sites resulted in major improvements. Other approaches deviating from the actual hydrogenase modeling proved to result in more-successful HER catalysts, the two most interesting examples being the nickel phosphane com- pound 1 [2a] and a cobalt glyoxime system (2) [2b] (Figure 1 a). Intriguingly, both of these systems have now been shown to be highly efficient in HER as immobilized electrocatalysts. Adsorption of the cobalt system onto a glassy carbon electrode also yields an active electrocatalyst for HER in aqueous solution requiring only low overpotentials. [6] Both the cobalt and the nickel compound yield immobilized electrocatalysts showing high Faradaic yields (80–95 %), comparable current densities (1–2 mA cm 2 ), and high stabil- ities with turnover numbers of (2–8)  10 4 per hour. However, Figure 1. a) Structures of the hydrogen-producing electrocatalysts dis- cussed herein. R 1 = Me; R 2 = Ph; R 3 = p-C 6 H 4 CH 2 C(O)O-phthalimide. b) Plot of the thermodynamic overpotential versus the binding energy of H*. The potential-determining steps differ on either side of the apex of the optimal catalyst. [*] Prof. Dr. M. T. M. Koper, Prof. Dr. E. Bouwman Leiden Institute of Chemistry, Leiden University PO Box 9502, 2300 RA Leiden (The Netherlands) Fax: (+ 31) 71-527-4451 E-mail: m.koper@chem.leidenuniv.nl Homepage: http://casc.lic.leidenuniv.nl A ngewandte Chemi e 3723 Angew. Chem. Int. Ed. 2010, 49, 3723 – 3725  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim