Low-cost solar-to-hydrogen cell achieves breakthrough 17.6% efficiency

Hydrogen's impressive energy density offers some compelling advantages that could see it make a huge difference in the electric aviation and eVTOL sectors, as well as in renewable energy, where it's a lightweight and transportable, if not particularly efficient, way to store clean energy that's not necessarily generated where or when you need it. It's also being pushed as a means of exporting green energy, and Japan and Korea in particular are investing heavily in the idea of a hydrogen energy economy powering everything from vehicles to homes and industry.

For this to come about in a globally positive way, it's imperative that clean, green hydrogen production becomes cheaper, because right now, the easiest and cheapest ways to get yourself a tank full of hydrogen are things like steam reforming, which produces up to 12 times as much carbon dioxide as it does hydrogen by weight.

Green, renewable production methods are thus hot topics for researchers and industry, and a new breakthrough from scientists at the Australian National University (ANU) could make a significant contribution.

It's a photoelectrochemical (PEC) solar-to-hydrogen (STH) cell – a cell that takes in solar energy and water, and directly outputs hydrogen instead of powering an external electrolysis system. In this case, it puts a cutting-edge perovskite photovoltaic cell in tandem with a photoelectrode, and it works better than any similar devices that have been built, using relatively inexpensive semiconductors.

“The voltage generated by a semiconductor material under sunlight is proportional to its bandgap," said project lead Dr. Siva Karuturi, PhD, lead researcher at ANU's College of Engineering and Computer Sciences. "Silicon (Si), the most popular PV material in the market now, can only produce a third of the voltage needed to split water directly. If we use a semiconductor with a bandgap twice that of Si, it can provide sufficient voltage, but there is a trade-off. The higher the bandgap, the lower the sunlight capturing ability of a semiconductor. To break this trade-off, we use two semiconductors with smaller bandgaps in tandem that not only capture the sunlight light efficiently, but together produce the necessary voltage to spontaneously generate hydrogen."

One key metric here is solar-to-hydrogen efficiency, and the ultimate target, as laid out by the US Department of Energy nearly a decade ago, is 25 percent, with a 2020 milestone of 20 percent. And while cells have been designed previously that hit 19 percent, these have used prohibitively expensive semiconductor materials. Nothing that could be called affordable has managed to break the 10 percent mark until this design, which lab simulations under accepted conditions have pegged at an impressive 17.6 percent efficiency using a silicon/titanium/platinum photoelectrode.

The team says its results suggest "immense opportunities" for further optimization. The design can be made more efficient by fine-tuning the individual component designs, and it can be made even cheaper by replacing the precious catalytic metals with more abundant materials.

The end game in this space is to get truly clean, renewable hydrogen production down to prices around US$2.00 per kilogram, where it can compete head to head with dirty hydrogen and indeed fossil fuels. "Significant cost benefits could be achieved through the use of the solar-to-hydrogen approach," says Dr. Karuturi, "as it avoids the need for added power and network infrastructure necessary when hydrogen is instead produced using an electrolyser. And by avoiding the need to convert solar power from DC to AC power and back again, in addition to avoiding power transmission losses, the direct conversion of solar energy into hydrogen can achieve a higher overall efficiency for the total process."

The paper is available in Advanced Energy Materials.

Source: Australia National University via RenewEconomy