Biohybrid catalysts for solar hydrogen production

Hydrogen (H₂) and fuel-cell technology address both the need to mitigate greenhouse gas emissions and develop energy alternatives to fossil fuels.¹ Free H₂ is scarce, however, and cheap, efficient production technologies will be required for its wide-scale deployment.^1,2 Solar energy strikes the Earth every day in sufficient amounts to meet global power needs for 1 year.² Capturing it for conversion and storage as H₂ potentially represents a clean and renewable process.

A rich diversity of photosynthetic micro-organisms have been identified as having the capacity to manufacture H₂ as biological systems.³ Several species are able to directly couple photosynthetic water oxidation with accelerated (i.e., catalytic) production of H₂ via nature's biocatalysts, hydrogenase enzymes.³ Indeed, despite certain challenges, nature provides an inspirational model and materials for constructing such systems (see Figure 1). Hydrogenases may be merged with materials to develop catalytic biohybrids that could lead to cost-effective, renewable alternatives to the expensive noble-metal catalysts (e.g., platinum) that are currently used in artificial devices.

Figure 1. Components for coupling solar-driven, photosynthetic water oxidation to hydrogen (H₂) production in photobiological systems are shown on the left. Solar-driven water splitting by the photosynthetic apparatus generates charge that is transferred to a mobile charge carrier, ferredoxin, and ultimately to hydrogenase for catalytic H₂ production. On the right, components of an artificial, solar biohybrid H₂ production device. If used as a cathode in a solar capture device (black arrows), charge generation and transfer from the solar device to the cathode drives catalytic H₂ production. If the biohybrid is composed of semiconducting materials of appropriate energetics, the material itself generates the charge for catalytic H₂ production (red arrows). e–: Photogenerated charge. D, D⁺: Reduced, oxidized state, respectively, of a sacrificial dono molecule.

Hydrogenase biosynthesis using recombinant technology^4,5 can provide the suitable raw material needed to develop and investigate model biohybrids and artificial H₂-manufacturing devices.⁶ The technology will also facilitate investigation of hydrogenases with a view to eventually engineering structures adapted to a biohybrid context.⁵ Though hydrogenases are easily immobilized onto electrodes as films, the process results in a so-called Poisson distribution of the enzymes in random orientations. Attaining high-coverage hydrogenase-material interfaces that are both spatially and electronically controlled will minimize, for example, charge-scavenging side reactions and lead to higher catalytic rates of H₂ generation.

Methods of synthesis and engineering techniques can help in governing the hydrogenase-materials interface. For example, the architecture and energetics of carbon nanomaterials can be synthetically tuned⁷ for separation into nearly homogeneous samples.⁸ Carbon-based materials are also biocompatible and electrically conductive. We have recently shown⁹ that, when combined in solution, single-walled carbon nanotubes (SWNTs) and hydrogenases spontaneously form biohybrid, catalytic complexes. These remain stable for at least several days, competent for repeated exchange of charge between hydrogenases and SWNTs. Potentially, they could function as either standalone H₂ factories or be active in immobilizing hydrogenases on an electrode in a solar capture device (see Figure 2).

Figure 2. Computational model illustrating SWNT hydrogenase biohybrid.⁹ The hydrogenase and the SWNT are shown to scale.

In sum, hydrogenase-material biohybrids hold out the promise of inexpensive yet powerful catalysts for use in fabricating the next generation of solar-powered H₂ production devices. Critical to the hydrogenase-material interface are catalytic performance and stability. Our ongoing research efforts are aimed at understanding how to engineer both of these characteristics in a manner that supports the generation of biohybrid architectures suitable for large-scale use in artificial solar devices.

The author would like to acknowledge the following collaborators: Maria L. Ghirardi, Drazenka Svedruzic-Chang, Jeffrey L. Blackburn, Timothy J. McDonald, ShengBai Zhang, and Yong-Hyun Kim (National Renewable Energy Laboratory); Michael Hambourger, Miguel Gervaldo, Thomas A. Moore, Ana L. Moore, and Devens Gust (Arizona State University); and Matthew C. Posewitz (Colorado School of Mines). This work was supported by the US Department of Energy under contract DEAC36-99GO10337 with the National Renewable Energy Laboratory, and the US Department of Energy Office of Science, Basic Energy Sciences program.