Plants have long cracked the knack of storing energy from the Sun, and it is this energy that ends up as fossil fuels. Solar water splitting can be described as “artificial photosynthesis” and as such is “a promising approach to storing solar energy in the form of hydrogen on a global scale,” according to the roadmap authors Sheng Chu, Wei Li, Yanfa Yan, Thomas Hamann, Ishiang Shih, Dunwei Wang and Zetian Mi. They set out the three main strategies: photochemical, photovoltaic electrolysis, and the most promising approach so far, photoelectrochemical cells, which they suggest provides the greatest potential for an agreeable compromise between cost and efficiency.

Photoelectrochemical cells feature semiconductor electrodes – the “artificial leaf” – with semiconductor/liquid junctions. Under sunlight, a semiconductor with the right bandgap can facilitate the electrolysis of water, typically in two stages – water oxidation followed by proton reduction. To best accommodate the two stages, researchers have developed tandem solar water splitters that cover the energies of the two reactions.

Researching in tandem

The benefits of tandem cell structures for improving energy generation efficiency has been widely recognized in solar cell research and development. In his review of the "Prospects for photovoltaic efficiency enhancement using low-dimensional structures" in 2000, Martin Green at the University of New South Wales in Australia stated that while the performance of traditional bulk solar cells was limited to 33%, approaches such as those based on "stacked" or tandem cells, could double efficiency limits to 68%. Green explains that in tandem solar cells this increase in solar to electrical energy conversion efficiency is due to the reduction of losses caused by thermal relaxation of carriers. "By sending light of a narrow bandwidth range to different cells of an appropriately matched bandgap…this loss can be reduced substantially."

Combining materials with different bandgaps opens up a broader range of the Sun’s electromagnetic radiation, prompting researchers to investigate innovative materials and structures with optimized optoelectronic properties. Silicon, perovskite and III-V semiconductors all feature in Mi and co-authors’ appraisal of electrode materials in tandem structures for solar water splitting, and the same materials remain prominent in tandem solar cell research. Several groups have attempted to combine the qualities of perovskites with crystalline silicon in tandem solar cells. While crystalline silicon solar cells continue to lead in terms of efficiencies, perovskites have exploded on the photovoltaics scene recently with their low processing costs and soaring efficiencies, as well as broadband absorption across the visible range. Pairing the two materials into a monolithic tandem cell is far from trivial, generally requiring high temperatures that are incompatible with optimized silicon technology. However, researchers in Germany and Switzerland have recently demonstrated a way around these processing temperature requirements. Steve Albrecht, Michael Saliba, Juan Pablo Correa Baena and colleagues at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH in Germany, and École Polytechnique Fédérale de Lausanne and the Laboratory for Photomolecular Science, Station 6, in Lausanne, Switzerland, reported their results in the journal Energy and Environmental Science in 2016.

As well as innovations with materials, researchers are exploring how to bring new material designs to optimized structures such as doped InP nanowires. These nanostructures shorten the diffusion distance, which is an asset for both solar cells and water splitting. Again, fabrication presents a challenge, but as Magnus Borgström and colleagues at Lund University, with Lars Samuelson, recently reported in Nanotechnology, careful control of the precursor ratios for the nanowire synthesis can enable optimized carrier gradients without introducing stacking faults from different crystalline phases.

Catalysing progress

As well as cross fertilization with solar cell research, Mi and co-authors emphasize in their roadmap the overlap between solar water splitting and photocatalytic research. Efficient water splitting must optimize light absorption, and charge separation, while also driving the relevant reactions for hydrogen and oxygen evolution. The same is true for other processes, such as organic dye degradation. Reporting on the ultra-high rate of photodegradation of organic dyes under visible light illumination on Ag2O-nanoparticle-decorated porous pure B-phase TiO2 nanorods, Kamal Kumar Paul, Ramesh Ghosh and P K Giri at the Indian Institute of Technology Guwahati conclude, "The major improvement in photocatalytic efficiency has been explained on the basis of enhanced visible light absorption and band-bending-induced efficient charge separation in the heterostructure."

Yafei Zhang, Zhi Yang and colleagues at Shanghai Jiao Tong University in China studied the photocatalytic properties of cubic cuprous oxide-reduced graphene oxide nanocomposites for breaking down the common dye pollutant from textiles, foodstuffs, paper and leather industries – methyl orange. Following investigations of the photocatalytic mechanisms at play, they attribute the enhanced photocatalytic performance to increased charge transportation to drive the reaction, and effective separation of photoelectrons from vacancies, as well as the improved contact area. The common attributes identified exemplify that research into PEC cells can offer useful insights for other photocatalytic systems. As Mi and colleagues point out in the roadmap, "Careful studies of the SCLJ [semiconductor liquid junction] can help us understand which parts of the system are responsible if a system fails to deliver the expected performance."

The roadmap concludes with the promising prospect of integrating III-V materials with established manufacturing processes – in particular, III-nitride nanostructures – with silicon, already successful as a low-cost large-area solar-cell platform. However, Mi and co-authors emphasize the potential of transition metal dichalcogenides and organic materials, also highlighting how many materials are yet to be investigated. "For example, 8000 and 700,000 compound materials are available for ternary and quaternary metal oxides, respectively, with most of them yet to be investigated for PEC water splitting. The large number of untested elemental combinations gives hope that ideal materials are still ahead of us." This should offer hope not only for solar water splitting but also solar cells, photocatalysis and any other fields based on similar processes. Despite the evident utility of solar water splitting research for energy storage, the question "what will the results of this research ultimately lead to?" remains as true as ever.

The Roadmap on solar water splitting: current status and future prospects is available in Nano Futures.