On The Scarcity or Not of Carbon-Based Overall Water Splitting Photocatalysts

Since the discovery in 2010 that carbon nitride combined with suitable co-catalysts could in the presence of appropriate electron donors and acceptors drive both the reduction of protons to molecular hydrogen and the oxidation of water to molecular oxygen, carbon-based materials have received an enormous amount of interest as potential light absorbers for photocatalytic water splitting. Now probably more than 200 different carbon-based materials, mostly conjugated polymers, are known to be active for hydrogen evolution in the presence of a sacrificial donor such as triethanolamine or ascorbic acid, and platinum or palladium nanoparticles that act as co-catalyst and site of the hydrogen evolution.¹ In contrast, a much smaller number of carbon-based materials have been reported to be active for the oxygen evolution half-reaction. The number of carbon-based materials experimentally active for overall water splitting can be counted on the fingers of one or two hands.<br/><br/>Based on the scarcity of materials that are active for overall water splitting one could naively assume that such activity is a rare property of carbon-based materials. That carbon-based materials generally just do not have the ionisation potential / valence band maximum position to drive water oxidation. In my contribution I’ll discuss the results of computational high-throughput screening of thousands of conjugated polymers which shows that this is not necessarily the case, or at least not for polymers.² Approximately, a similar fraction of the library of 3000+ conjugated polymers is predicted to be thermodynamically able to drive overall water splitting as is predicted to be active for sacrificial hydrogen evolution. Although the ability to drive overall water splitting combined with a small enough optical gap that a large part of the solar spectrum is absorbed is predicted to be relatively rare for this material class. Finally, I’ll discuss why we think the real reason behind the scarcity of carbon-based overall water splitting photocatalysts is not thermodynamics but the need for well-optimised co-catalysts and why thus future work on co-catalysts and/or combining polymers into heterojunctions, like recently explored experimentally by Kosco and co-workers,³ can rescue large numbers of carbon-based phoocatalysts from the proverbial scrap heap of history.<br/><br/>¹ Y. Wang et al. Nat. Energy 2020, 5, 633.<br/>² B. Saunders et al. Sustainable Energy Fuels, 2022, 6, 2233.<br/>³ J. Kosco et ak. Nat. Mater. 2020, 19, 559.