Researchers in Canada have demonstrated a new photochemical diode artificial photosynthesis system that can enable efficient, unassisted overall pure water splitting without using any sacrificial reagent.

As reported in an open-access paper in Nature Communications, the wafer-level photochemical diode arrays exhibited solar-to-hydrogen efficiency of ~3.3% in neutral (pH ~ 7.0) overall water splitting reaction. In part of the visible spectrum (400–485 nm), the energy conversion efficiency and apparent quantum yield reaches ~8.75% and ~20%, respectively—the highest values ever reported for one-step visible-light driven photocatalytic overall pure water splitting. The device could also be reconfigured to turn carbon dioxide back into fuel.

The wafer level photochemical diodes consist of vertically aligned InGaN nanosheets, with well-defined anode and cathode surfaces for water oxidation and proton reduction, respectively.

Schematic illustration of wafer-level unassisted photocatalytic overall water splitting on double-band nanowire arrays, which are vertically aligned on a planar substrate and decorated with co-catalysts for hydrogen evolution reaction (HER). Unlike tandem PEC cells or photovoltaic (PV) devices, this approach does not require any carrier recombination/transfer or current matching between the layers along vertical direction. Both water oxidation and proton reduction reaction occur on the radial non-polar surfaces of each layer. Chowdhury et al. Click to enlarge.

Previous direct solar water splitters have achieved a little more than 1 percent stable solar-to-hydrogen efficiency in fresh or saltwater. Other approaches suffer from the use of costly, inefficient or unstable materials, such as titanium dioxide, that also might involve adding highly acidic solutions to reach higher efficiencies.

The new device is made from the same widely used materials as solar cells and other electronics, including silicon and gallium nitride (often found in LEDs). With an industry-ready design that operates with just sunlight and seawater, the device could pave the way for large-scale production of clean hydrogen fuel.

Zetian Mi, a professor of electrical and computer engineering at the University of Michigan who led the research while at McGill University in Montreal, and his team built a nano-sized cityscape of gallium nitride towers that generated an electric field. The gallium nitride turns light, or photons, into mobile electrons and positively charged vacancies called holes. These free charges split water molecules into hydrogen and oxygen.

… we propose and demonstrate multi-band InGaN nanosheet photochemical diode (PCD) structures, which can spontaneously induce charge carrier separation and steer charge carriers toward the distinct redox sites for water oxidation and proton reduction. During the synthesis of InGaN photochemical diode nanosheet structure, p-type dopant (Mg) concentrations are rationally tailored, which induces a large built-in electric field between the two parallel surfaces. Consequently, the two surfaces are enriched with photo-generated holes and electrons to perform water oxidation and proton reduction reactions, respectively. In addition to the efficient charge carrier separation and extraction, the spatial separation of catalytic sites in such a nanoscale photochemical diode effectively reduces carrier recombination and back reaction.

—Chowdhury et al.

By precisely controlling charge carrier flow at the nanoscale, the wafer-level photochemical diode arrays delivered the observed solar-to-hydrogen efficiency.

The effective manipulation and control of charge carrier flow in nanostructured photocatalysts provides critical insight in achieving high efficiency artificial photosynthesis, including the efficient and selective reduction of CO₂ to hydrocarbon fuels.

—Chowdhury et al.

At present, the silicon backing of the chip does not contribute to its function, but it could be doing more. The next step may be to use the silicon to help capture light and funnel charge carriers to the gallium nitride towers.

Although the 3 percent efficiency might seem low, when put in the context of the 40 years of research on this process, it’s actually a big breakthrough. Natural photosynthesis, depending how you calculate it, has an efficiency of about 0.6 percent.

—Zetian Mi

Mi said that 5 percent efficiency is the threshold for commercialization, but that his team is aiming for 20 or 30 percent efficiency.

Mi conducts similar research to strip carbon dioxide of its oxygen to turn the resulting carbon into hydrocarbons, such as methanol and syngas. This research path could potentially remove carbon dioxide from the atmosphere, as plants do.

The work was supported by the Fuel Cell Technologies Office of the US Department of Energy and Emissions Reduction Alberta.

Resources

• Faqrul A. Chowdhury, Michel L. Trudeau, Hong Guo & Zetian Mi (2018) “A photochemical diode artificial photosynthesis system for unassisted high efficiency overall pure water splitting” Nature Communications volume 9, Article number: 1707 doi: 10.1038/s41467-018-04067-1