Basic Research Needs for Solar Energy Utilization - Office of ...

SOLAR-POWERED CATALYSTS FOR ENERGY-RICH FUELS
FORMATION
All methods of producing solar fuels must involve coupling photo-driven single electron steps
with fuel-forming, multi-electron transfer processes. No inexpensive, man-made systems come
close to the performance of naturally found enzymes, which perform such processes with high
turnover and minimal energy loss. Practical solar fuel formation requires construction of
currently unknown catalyst systems to form hydrogen and oxygen from water and to efficiently
reduce carbon dioxide from the air.
EXECUTIVE SUMMARY
Significant scientific challenges confront the design and synthesis of efficient, high-turnover,
solar-powered catalysts for the conversion of solar energy into energy-rich fuels. Important
reactions include the splitting of water into oxygen and hydrogen and the reduction of carbon
dioxide to methane. Guideposts for the development of new systems will come in part from the
understanding acquired from bioenergetic proteins involved in fuel-producing reactions,
especially the water-splitting reaction of Photosystem II and hydrogen-producing reaction of
hydrogenase. The performance of the current generation of catalysts is far from that required for
a solar fuels production system with even modest efficiency, so that the development of a new
generation of fuel-forming catalysts is necessary for integration into both higher-order artificial
photosynthetic assemblies and photoelectrochemical devices. To achieve this objective, several
important goals must be attained: (1) identify new methods for unraveling the mechanisms of
complex, coupled reactions for the solar production of fuels; (2) develop a fundamental
understanding of excited-state bond making and breaking processes yielding oxygen and
hydrogen; (3) understand the rates and mechanisms of multielectron/atom transfer reactions
using new theoretical and experimental approaches; (4) understand how proton-coupled electron
transfer reactions including H atom and hydride transfers reduce the energy requirements for
catalytic processes; (5) understand at a molecular level how catalytic reactions occur at interfaces
and surfaces; and (6) develop molecular design and synthesis strategies to produce robust
functional catalytic systems that mimic biological processes.
SUMMARY OF RESEARCH DIRECTION
Any practical technology for the decomposition of water into hydrogen and oxygen needs to
circumvent the need for sacrificial reagents (i.e., those that are consumed and are not part of a
catalytic cycle). Fabrication of all of the components for large-scale solar energy utilization must
be inexpensive, a requirement arising from the large surface areas needed for future solar fuels
plants. Most of the catalysts that have been explored are based on noble metals that may be too
expensive for practical deployment. It is therefore important to use catalysts that are based on the
first-row transition metals. Biological catalytic systems demonstrate that this is an achievable
goal. The catalyst must be robust, having a high turnover coefficient, rapid cycling, and chemical
stability under the harsh conditions of prolonged irradiation. A practical catalyst should consist
of synthetically accessible components with favorable physical characteristics, such as solubility,
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