A schematic of a sustainable energy economy as described by Hambourger and Moore in a Perspective on the work by Le Goff et al. Renewable energy sources, left, are converted to electricity and transported to end-users, right. Electrolyzers and fuel cells based on hydrogen provide one route for energy storage, which is critical for widespread implementation. The nickel-based catalyst reported by Le Goff et al. (is a step toward developing a practical hydrogen electrode based on Earth-abundant materials. The basic site (purple) assists in proton abstraction or donation coupled to the redox chemistry occurring at the nickel site (orange), during either the oxidation or the production of hydrogen. The dark arrows illustrate electron transfer pathways. Credit: Y. Greenman/Science. Click to enlarge.

The conceptual primary application target for the work is the development of unitized, regenerative fuel cells for storing renewable energy. The devices operate by storing energy in hydrogen generated via electrolysis of water driven by the renewable power and then releasing it as necessary through the reverse reaction. The devices are compact and have low internal resistance. However, because they currently rely on platinum for catalysis, they would be too expensive for widespread adoption.

The catalyst described by Le Goff et al. accomplished the interconversion by mimicking the activity of hydrogenases incorporating the more abundant metals iron and nickel. Hydrogenases bidirectionally catalyze interconversion between H₂ and a pair of protons and electrons (the same reaction taking place at electrodes in fuel cells and electrolyzers) as efficiently as platinum nanoparticles do and with remarkably high reaction rate, the authors note (1,500 to 9,000 s^–1 at pH 7 and 37 °C in water). However, they suffer from considerable oxygen sensitivity and their production in the active form cannot be scaled up to satisfy technological demand.

Because catalysis in hydrogenases requires only iron and nickel metal centers, the active sites are strong sources of inspiration for the design of synthetic catalysts that are more sustainable than platinum. In addition, bio-inspired synthetic catalysts are easy to prepare and can often be handled in air, which is key for the integration in a technological device. One of the most successful achievements of the biomimetic approach involves a family of bisdiphosphine nickel complexes that combine very well a nickel center in an electron-rich environment (as found in NiFe hydrogenases) with proton relays provided by a pendant base mimicking the putative azapropanedithiolato cofactor of FeFe hydrogenases.

...we have explored the prospect of assembling an electrode by binding these biomimetic nickel complexes to carbon nanotubes (CNTs). Among the advantages of CNTs for this purpose are their high surface areas (facilitating high catalyst loading), their high stability and electrical conductivity, and the availability of versatile and straightforward methods for grafting molecular complexes onto their surfaces.

—Le Goff et al.

Despite the present advance described by Le Goff et al., write Michael Hambourger from Appalachian State University and Thomas Moore from Arizona State University in a Perspective in the same issue of Science, challenges remain.

The catalyst reported by Le Goff et al. has turnover rates approximately one-tenth as high as those of related complexes previously reported, possibly because of the bulky functional groups used for immobilization. The current densities are two orders of magnitude lower than those achieved with a commercial platinum electrode. Nonetheless, given the ability to tune the catalytic activity of this family of nickel complexes, further optimization of the compound could result in a viable, non-noble metal catalyst for the hydrogen electrode of PEM electrolyzers and fuel cells.

Similar advances in the development of a reversible, efficient oxygen electrode could close a solar-to-fuel-to-work loop by allowing the inexpensive interconversion between renewable electricity and fuels. Such a system could be transformative, minimizing the environmental impacts of an energy-intensive first-world economy, while promoting third-world development by enabling lights, refrigeration, and telecommunications essential for education, health, and empowering populations to control their own destinies.

—Hambourger and Moore

Resources

• Alan Le Goff, Vincent Artero, Bruno Jousselme, Phong Dinh Tran, Nicolas Guillet, Romain Métayé, Aziz Fihri, Serge Palacin, Marc Fontecave (2009) Science Vol. 326. no. 5958, pp. 1384 - 1387 doi: 10.1126/science.1179773

• Michael Hambourger and Thomas A. Moore (2009) Nailing Down Nickel for Electrocatalysis. Science Vol. 326. no. 5958, pp. 1355 - 1356 doi: 10.1126/science.1183836