Caltech team proposes taxonomy for solar fuels generators; different approaches to converting sunlight to chemical fuels

22 December 2014

Researchers at the California Institute of Technology are proposing a nomenclature and taxonomy for solar fuels generators—devices that harness energy from sunlight to drive the synthesis of chemical fuels. A number of different approaches to this technology are being pursued, many of which can be differentiated by the physical principles on which they are based, according to the Caltech team, led by Dr. Nathan Lewis.

In an open-access paper published in the RSC journal Energy Environmental Science, Dr. Lewis and colleagues outlined their method of using the source of the asymmetry that separates photogenerated electronics and holes as the basis for their taxonomy. They identify three basic device types: photovoltaic cells, photoelectrochemical cells, and photoelectrosynthetic particulate/molecular photocatalysts.

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An understanding of the inherent operating principles and the advantages and challenges associated with each of these device types will facilitate clear comparisons between devices as well as help guide research efforts toward improving these devices and achieving the ultimate goal of sustainable fuel production.

Illustrations of Nielander et al.’s different categories of solar fuels generators.

(a) Semiconductor/electrolyte junction in the dark and prior to equilibration in which the photovoltage and photocurrent are determined in whole or in part by the difference between Fermi level of the semiconductor (EF) and the electrochemical potential of the electrolyte solution (Eredox), denoted as DE.

(b) Semiconductor buried junction in the dark and prior to equilibration in which the photovoltage and photocurrent are determined by the difference between the Fermi levels (EF) of the two solid-state contacting phases (DE), shown here as two semiconductors. The DE is independent of any difference between the Fermi level of the solid contacting the electrolyte and the electrochemical potential of the electrolyte. The highly doped phase (in red) allows for ohmic contact between it and the contacting electrolyte phase.

(c) Particulate/molecular photocatalysts suspended or dissolved in solution. Each unit individually absorbs light, generates excited carriers and effects the desired chemical reactions at the particulate/molecular electrolyte interface.

Credit: RSC, Nielander et al. Click to enlarge.

All solar fuels generators require an electrical asymmetry to separate and transport photogenerated charge carriers vectorially, the authors explain. Without this vectorial separation and transport, the charge carriers—and the chemical products—would have no net directionality and thus would undergo no net separation. This would result in recombination of charge carriers and/or a loss of chemical potential in the resulting fuel/oxidant mixture.

The required vectorial separation can be effected by chemical and/or electrical potential gradients as well as by kinetic asymmetries at the interface between two unlike materials. We refer to this interface as a “junction”. We note that our usage of the term “junction” differentiates such an interface from an interface between two unlike materials that does not result in an asymmetry which produces a vectorial charge separation.

We propose that the various solar-fuels generators can be differentiated at a fundamental level based on the underlying principles used to accomplish vectorial charge separation and by the method in which the separated charge is used to effect the synthesis of chemical fuels.

Photovoltaic cells (PV). PV cells produce electricity from sunlight are widely available, and are referred to as solar electric cells. PV cells that produce fuels are referred to as “PV-biased electrosynthetic cells” and can consist of any number of buried junctions arranged electrically in series with electrocatalysts submerged in an electrolyte.

In all such systems, the Caltech team explained, the photovoltage generated by the structure is independent of the nature of the electrocatalyst/electrolyte interface.

• Advantages are the high reported solar-to-fuels efficiencies and the independence of the power-producing junction with respect to the formal potential for the reactions of interest.

• Issues include achieving a cost advantage for a system with the functioning photovoltaic cell immersed in the electrolyte, relative to a system that utilizes a discrete photo- voltaic cell in dry conditions wired to a discrete fuel-forming device, as well as finding catalyst/electrolyte interfaces that are transparent, conductive, and stable under operational, fuel- forming conditions.

• Key research needs involve the development of cost-competitive photovoltaic cells, the integration of components, discovery of materials, development of low-cost fabrication methods, and the stabilization of electrodes through the use of materials that act as transparent and conductive protecting layers.

Photoelectrochemical cells. (PEC) Devices utilizing solid/ionic-conductor junctions, also referred to as solid/electrolyte junctions, are called photoelectrochemical (PEC) cells. PEC cells that only produce electricity are referred to as regenerative photoelectrochemical cells.

PEC cells that produce fuels at the semi-conductor/electrolyte junction are referred to as photoelectrosynthetic cells.

The performance of photoelectrodes consisting of semiconductor/electrolyte junctions is determined by the energetics and kinetics of the semiconductor/electrolyte interface. Commonly, an electrocatalyst is incorporated at the semiconductor/electrolyte interface to improve the interfacial charge-transfer kinetics; however, for the device to remain categorized as a PEC cell, the nature of the electrolyte must affect the performance of the cell.

• Advantages of PEC cells are their simplicity of fabrication and the finding that inexpensive polycrystalline semiconductor/electrolyte junctions can often perform nearly as well as their single crystalline counterparts.

• Issues associated with PEC cells include obtaining a combination of materials that are operationally stable and also possess appropriate interfacial energetics and band gaps, as well as the development and integration of electrocatalysts into the semiconductor/electrolyte junction.

• Key research needs for solar fuels generators based on PEC cells involve the discovery and development of semiconducting materials that possess both the proper band gaps for effective sunlight absorption and well-positioned band energetics, and the development of methods for incorporating efficient electrocatalysts into semiconductor/electrolyte interfaces that are stable under operational, fuel-forming conditions.

Photovoltaic-biased photoelectrochemical cells. Coupling a PV cell with a PEC cell results in a cell that contains both a buried junction and a semiconductor/electrolyte junction—a PV-biased PEC cell. Like their parents, PV-biased PEC cells can produce electricity or fuel. PV-biased PEC cells that produce fuels and that include at least one buried junction may fall into a number of categories, which are systematically named based on whether fuel formation occurs at a solid/electrolyte junction in the device and the presence or absence of additional two-terminal regenerative PEC cells.

• PV-biased PEC cells in which fuel formation occurs at the solid/electrolyte junction are “PV-biased photoelectrosynthetic cells”.

• PV-biased PEC cells that produce fuels that are formed away from a solid/electrolyte junction, but include at least one isolated regenerative PEC cell, are referred to as “Regenerative PEC- and PV-biased electrosynthetic cells”.

Photoelectrosynthetic particulate/ molecular photocatalysts. The semiconducting material can be in a dispersed particulate form as opposed to a solid electrode. This approach support both the buried junction and the semiconductor/electrolyte junction motifs.

The particulate versions of PV and PEC cells, as well as the related photo-driven molecular photocatalysts wherein inorganic molecular compounds are dispersed in solution, share many of the same research challenges as their parent categories, with the added challenge of developing methods to physically separate the products of the fuel-forming reactions. The term cell does not apply to particulate schemes that employ neither addressable electrodes nor a built-in means to enforce the separation of products. For these reasons, we consider all three of these strategies to comprise members of the general category of photoelectrosynthetic particulate/molecular photocatalysts.

Resources

• Adam C. Nielander, Matthew R. Shaner, Kimberly M. Papadantonakis, Sonja A. Francis and Nathan S. Lewis (2015) “A taxonomy for solar fuels generators,” Energy Environ. Sci., 8, 16 doi: 10.1039/C4EE02251C