Devices

Overview | Semiconductors| Catalysts | Protection | Structures | Devices | Systems 

Goal

Scanning electron micrographs of (top) an array of a microwire-array device based on silicon microwires coated with tungsten oxide, and (bottom) a cross-sectional view of a single microwire in the array, showing the silicon core coated by transparent conductive oxide (ITO) and tungsten oxide shells.¹

Build an integrated device that makes fuels, chemicals, or electricity from sunlight.

Strategy

We focus on building integrated devices based on architectures that are both inherently safe and compatible with inexpensive and flexible materials.  We incorporate materials developed in the course of our work on semiconductors, catalysts, protective coatings, three-dimensional structures, and surface modification into complete devices, and measure the performance and stability of these devices under operating conditions.

By constructing full devices, we can observe interactions between components, liquid phases, separators, and supporting materials.  Likewise we can observe the effects of product-gas bubbles which form on the electrodes, collect products, and measure the stability of the full device.

Highlights

Incorporation of a Bipolar Membrane into an Efficient Sunlight-Driven Water-Splitting Device

A water-splitting cell contains a liquid electrolyte and is divided into two compartments by a separator that: 1) ensures the safety of the device by preventing mixing of hydrogen and oxygen gases; and, 2) prevents efficiency losses due to recombination or back-reactions of products; while, 3) allowing an ionic current to flow between the anode and cathode.  Traditionally, the same electrolyte is used on both sides of the membrane.  Strongly alkaline or strongly acidic electrolytes are required for efficient water splitting, but they challenge the stability of other materials in the system, particularly semiconductors and catalysts.

(top) Cross-sectional scanning electron micrograph of an InGaP/GaAs tandem light absorber coated by a protective layer of titanium dioxide bearing a nickel water-oxidation catalyst.³ (bottom) Relationship between the current density at the bipolar membrane (J_membrane)and at the photoelectrode (J_electrode) as a function of the total membrane voltage (V_membrane), and the voltage loss (V_{membrane,loss}) across the bipolar membrane.²

Bipolar membrane separators allow an acidic electrolyte to be used on one side of the membrane and an alkaline electrolyte to be used on the other side.  Thus bipolar membranes relax the materials-stability constraints for water-splitting by removing the requirement that the materials at the cathode be stable in the same electrolyte as the materials at the anode.

We built an efficient device that splits water by coupling a tandem photoanode, indium gallium phosphide/gallium arsenide (InGaP/GaAs) protected by a titanium dioxide coating bearing a nickel water-oxidation catalyst, to a titanium cathode coated with a cobalt phosphide water-reduction catalyst.  The photoanode was placed in contact with a buffered potassium borate solution (pH = 9.3) and the cathode was placed in contact with a sulfuric acid solution (pH = 0).  The two solutions were separated using a bipolar membrane.²

The cell spontaneously split water at a solar-to-hydrogen efficiency of 10% for over 100 hours.  The use of the buffered solution increased the stability of the photoanode significantly.  The increased stability was reflected by the size of the photoanode (1 cm²) that could be used in this cell versus in cells using a strongly alkaline (pH = 14) electrolyte (0.031 cm²), because the stability of the photoanode depends on the absence of defects in the titanium dioxide protective coating.  However, to operate at 10% efficiency, the area of the bipolar membrane needed to be 4.5 times that of the photoelectrode due to voltage losses across the membrane.  Additional efficiency losses were associated with the increased overpotential required to oxidize water at pH = 9.3 relative to strongly alkaline conditions.  Thus, bipolar membranes, particularly when used with a near-neutral electrolyte, present a trade-off between relaxed materials constraints and loss of efficiency.

References

1. Shaner, M. R.; Fountaine, K. T.; Ardo, S.; Coridan, R. H.; Atwater, H. A.; Lewis, N. S., Photoelectrochemistry of core-shell tandem junction np⁺-Si/n-WO₃ microwire array photoelectrodes. Energy Environ. Sci. 2014, 7 (2), 779-790.
2. Sun, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C., A stabilized, intrinsically safe, 10% efficient, solar-driven water-splitting cell incorporating Earth-abundant electrocatalysts with steady-state pH gradients and product separation enabled by a bipolar membrane. Adv. Energy Mater. 2016, in press.
3. Verlage, E.; Hu, S.; Liu, R.; Jones, R. J. R.; Sun, K.; Xiang, C.; Lewis, N.; Atwater, H. A., A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III-V light absorbers protected by amorphous TiO₂ films. Energy Environ. Sci. 2015, 8, 3166-3172.

_{by Kimberly Papadantonakis, June 2016.}