A series of clever design tweaks have boosted the efficiency of solar-powered hydrogen production, using cheap and readily available materials.

After COVID, hailstorms and bushfire smoke disrupted plans to develop entirely new solar components, the team from ANU Research School of Physics and ANU College of Engineering, Computing and Cybernetics (CECC) had to rethink their plans.

The result? Existing components deployed in such an innovative and efficient way, that the team achieved a record 20.7 percent efficiency, reported in Advance Energy Materials.

“It’s a massive step towards a green hydrogen economy,” said lead author Dr Joshua Butson, who did the work during his PhD at the RSPhys Electronic Materials Engineering (EME) Department.

“Although we used some expensive components they were deployed sparingly and effectively.”

Hydrogen has the potential to fuel a green revolution by replacing fossil fuels in transport and industry, something the United States government has acknowledged by devoting $9.5B to grants for hydrogen development. 

It’s a much-needed step - currently less than one percent of the global hydrogen production is green - that is, produced with renewable energy. Hydrogen sourced from fossil fuel is significantly cheaper and much more prevalent.

This inspired the EME team to put their skills in solar cell design towards developing much more efficient hydrogen production, for which they received support from the Australian Renewable Energy Agency.

In previous work they had developed one part of the puzzle, cheap and efficient catalysts for the hydrogen cell electrodes. 

But efficient use of these electrodes was hampered by a mismatch between solar photovoltaic (PV) cells, which output up to about 1.0 volts (depending on the exact material) and hydrogen production by electrolytic water splitting, which needs 1.6 volts. 

One solution is stacked layers of PV cells, known as tandem cells, built from two or even three layers of different materials. The team hoped to design a tandem cell that could match the voltage needed for water splitting closely – most tandems generate too much voltage, resulting in excess power being thrown away, which reduces the conversion efficiency.

However during enforced thinking time – thanks to a string of extended lab closures in 2020 – Associate Professor Siva Karuturi began considering commercially available technology and realised that by multiplying the voltages they could get the match they wanted. 

To generate almost exactly the right voltage to drive four water-splitting cells in series they could use three tandem triple-junction solar cells in series, using a commercially available cell with layers of indium gallium phosphide, indium gallium arsenide and germanium.

These tandem PV cells can obtain solar energy generation efficiency of more than 30 percent; to deploy this voltage to split water and form hydrogen the team connected them to electrodes with their previously-developed catalysts. At the anode, a porous nickel-iron hydroxide alloy, whose high surface areas makes for good catalysis, mounted on an impermeable layer of nickel foil to protect the internal photovoltaic layers. 

At the cathode they used nickel-molybdenum alloy on nickel foam, and dunked the whole setup in sodium hydroxide solution as the electrolyte (and the water to be split).

The catalyst choices reduce costs significantly, as they are made from cheap and abundant materials that operate with alkaline electrolyte solution, in contrast with previous acidic solutions that corroded all but the most expensive metal catalysts, such as palladium or platinum.

The new system gave a more than 50 percent improvement over the previous efficiency, jumping from 13.6 percent solar to hydrogen efficiency to 20.7 percent.

This is just the start - modelling by CECC researcher Dr Astha Sharma showed the efficiency can be improved to as high as 28 percent. 

Further gains are possible by using lenses to concentrate the solar radiation, which could push the efficiency above 30 percent. Fresnel lenses are regularly used with PV cells to multiply the radiation intensity up to 500 times that of the sun, but these setups are dogged by overheating.

However the ANU device sidesteps this potential problem because the cells are fully immersed in electrolyte, and that doubles as a coolant; in fact the electrolyte performs better at elevated temperatures, so the concentrated sunlight could have benefits beyond just the greater number of photons.  

CECC researcher Dr Julie Tournet says the results are promising.

“It shows you don’t need to overcomplicate things. You can take an existing commercial cell and with simple engineering turn it into a very efficient device,” Dr Tournet said.

To find out if the device is efficient enough to be able to compete with fossil fuel alternatives, Dr Tournet analysed the hydrogen market, but says the answer is complex.

“It is technically feasible. The timing of it though, depends on how fast the whole industry will ramp up and how supportive policies will be worldwide. 

“At the moment there are a lot of incentives for the photovoltaic industry, but not for the hydrogen industry, which is still in its infancy,” she said.

“The analysis gets complex very quickly - financing and regulations such as insurance make a significant part of the capital costs. Several analyses in the literature use carbon credits in their calculation to bring the costs down but that is not a reality for most regions in the world at the moment. 

“Government and regulatory bodies’ support will be really crucial to allow this type of technology to reach market, beyond what technical innovations can bring,” Dr Tournet said.