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Thomas MalloukProfessor in Energy Research; Professor of ChemistryDepartment of ChemistryUniversity of PennsylvaniaPhiladelphia, PA
Future solar energy conversion systems (both photovoltaic and fuel-producing solar cells) must be efficient, stable, and inexpensive in order to be competitive with fossil energy sources. In biological photosynthesis, the internal quantum efficiency for light-induced charge separation is near unity, and similarly high efficiencies can be achieved in photovoltaic cells based on dye sensitization of semiconductors. However it remains a challenge to adapt these kinds of molecular photosystems to the efficient production of energy-dense fuels.
By coupling the photosensitizers of dye cells to nanoparticulate or molecular oxygen evolution catalysts, we can now make solar cells that split water with visible light. Using related design ideas we have also made visible light-powered “Z-schemes” in which dye-sensitized hydrogen-evolving particles are coupled to oxygen evolving particles by redox mediators. The low solar conversion efficiency of these systems is a consequence of the kinetics of charge separation and recombination at the dye-sensitizer interface, which in turn are related to the management of protons generated in the water oxidation reaction.
This problem of proton management at the anode of water splitting solar cells led us to investigate system-level issues that arise with (photo)electrochemical fuel production near neutral pH. In buffer-based water splitting cells, losses from solution resistance and electrochemically generated pH gradients become substantial in cells that run continuously for periods of hours. This problem can however be addressed by using bipolar membrane-based cells in which the cathode and anode operate at low and high pH, respectively. Bipolar membranes enable efficient water splitting and CO2 electrolysis, and are also interesting for other membrane-based electrochemical energy conversion devices such as fuel cells and redox flow batteries. The catalytic reaction enabling these applications is water association/dissociation at the bipolar polymer interface, which is still not fully understood.