Solar energy is currently dominated by photovoltaic devices, which have ridden massive economies of scale to price dominance. But these devices are not necessarily the best choice in all circumstances. Unless battery technology improves, it’s quite expensive to add significant storage to solar production. And there are types of transportation—long-distance rail, air—where batteries aren’t a great solution. These limitations have made researchers maintain interest in alternate ways of using solar energy.

One alternative option is to use the energy to produce a portable fuel, like a hydrocarbon or hydrogen itself. This is possible to do with the electrons produced by photovoltaic systems, but the added steps can reduce efficiency. However, systems that convert sunlight more directly to fuel have suffered from even worse efficiencies.

But a Japanese group has decided to tackle this efficiency problem. The team started with a material that’s not great—it only absorbs in the UV—but is well understood. And the researchers figured out how to optimize it so that its efficiency at splitting water to release hydrogen runs right up against the theoretical maximum. While it’s not going to be useful on its own, it may point the way toward how to develop better materials.