Artificial photosynthesis – from sunlight to fuels and valuable products for a sustainable future

Photosynthesis harvests the most abundant energy source on planet Earth, splitting the most abundant chemical on the Earth’s surface and utilizing the electrons extracted from water splitting for the synthesis of complex organic products. From nearly every aspect, there is plenty to appreciate about this ingenious process, such as the ability to power thermodynamically uphill reactions at room temperature, the reaction specificity of CO₂ reduction and the near unity quantum efficiency in using harvested photons for O₂ evolution. The most significant of all, however, is the sheer scale of this reaction. It not only provides the energy and food source for nearly all life on Earth, but also supports the enormous energy needs of human activities through stored photosynthetic products from millions of years ago in the form of fossil fuels. The rapid consumption of the latter has been the main issue that has inspired debate and motivated tremendous research efforts lately. Such consumption liberates CO₂ at a rate that has never been seen before, leading to uncertainties in climate change that may threaten the very existence of humanity. On a more immediate time scale, the combustion of fossil fuels has been recognized as a direct cause of a large number of environmental issues. The impact on both the long-term, global scale of climate change and short-term, local scale of environmental issues is so daunting that geologists are arguing that we have officially entered the Anthropocene.

A sustainable future requires society to break the dependence on fossil fuels and shift our energy needs toward renewable energy sources, of which solar energy represents the ultimate renewable resource. An important technological challenge for most renewable energy supplies that needs immediate solutions is storage. Consider solar energy as an example. In addition to the diurnal nature, we face seasonal variations in most places on a scale of 50% or more, not including variations due to weather conditions. Of the storage technologies that have been assessed, chemicals (e.g. H₂ or hydrocarbons) feature much higher energy densities and hold promise for terawatt-scale implementations. When the energy source for the synthesis of these chemical fuels is from the Sun, we may regard the fuels as solar fuels and, hence, the term artificial photosynthesis. On the one hand, it is hoped that artificial photosynthesis will be able to harvest solar energy and store it in chemicals at much higher efficiencies than natural photosynthesis. More importantly, artificial photosynthesis is pursued for the synthesis of fuels that are only able to form through long geological processes. On the other hand, artificial photosynthesis may be designed such that it does not compete with natural photosynthesis, which will be dedicated to meeting our basic needs such as food sources. While these goals may appear aligned with the dream of large scale solar energy storage for a sustainable future, artificial photosynthesis produces more than just energy storage media. When coupled with highly selective catalysis, valuable products may be readily produced through artificial photosynthesis as well.

For all the promises it holds, artificial photosynthesis represents an exciting area with great opportunities. It draws inspiration from natural photosynthesis but strives to carry out the reactions at higher efficiencies. It may be used to refer to processes where sunlight is the direct energy source; it is also suitable to borrow the term for discussions on processes where sunlight is the indirect energy source, such as electrochemical reactions, so long as the electricity comes from solar energy in one way or another. As manifested by the collection of articles in this special issue, the topic brings together researchers from a diverse background.

The first broad research direction that has been energized by artificial photosynthesis is materials science. For the combined goals of high efficiency, low cost and long durability, researchers have been in search of inorganic materials that can be readily utilized. Within this context, Kudo et al. (10.1039/C8SE00079D) report a study of complex sulfides (CGIZS) in the form of powders that enable a solar-to-hydrogen (STH) efficiency of 1.1% (in combination with BiVO₄ for oxidation). Similarly, Domen et al. (10.1039/C8SE00101D) demonstrate that complex selenides also enable an STH efficiency of 1.1%. In a slightly different implementation configuration, Maeda et al. (10.1039/C8SE00191J) employ [Co(bpy)₃]^2+/3+ as a redox shuttle for the demonstration of Z-scheme solar water splitting using TiO₂ co-doped with N and F. These studies display a common feature where solar light is directly used, and the materials examined have the potential to be readily used at a low cost.

The second area that has attracted great attention for the purpose of artificial photosynthesis is catalysis. After all, artificial photosynthesis first and foremost presents challenges involving chemical synthesis. In these efforts, the energy source may directly come from sunlight by, for example, the utilization of photo-sensitizers (see Hung et al. (10.1039/C8SE00253C)) or a light harvesting unit in the form of photoelectrodes (see Berardi, Gimbert-Surinach and Llobet et al. (10.1039/C8SE00146D)). It could also come from sunlight indirectly by, for example, solar electricity. Indeed, electrocatalysis is arguably more impactful for the purpose of utilizing renewable energy sources for the synthesis of fuels or valuable products because the energy can be from other renewable sources such as wind. Within this context, homogeneous catalysts are reported by Hung et al. (Co²⁺–porphyrin) for hydrogen evolution, where sacrificial electron donors (ascorbic acid) are involved. Inoue et al. (10.1039/C8SE00102B) study Si–porphyrin-based catalysts that are specific for the formation of H₂O₂. For large scale industrial implementations, heterogeneous catalysts are generally preferred over homogeneous ones for benefits such as durability, ease of separation and recycling, among others. Streb and Kurz et al. (10.1039/C8SE00155C) learn from molecular catalysts and study MoS_x-based catalysts for hydrogen evolution reactions. A key advantage offered by molecular catalysts is the reaction specificity. Within this context, Amao (10.1039/C8SE00209F) reviews how biocatalysts can be used for CO₂ utilization. Berardi, Gimbert-Surinach and Llobet et al. present a strategy to integrate photoelectrodes with molecular catalysts.

A great challenge that has limited the development of heterogeneous catalysts for artificial photosynthesis is the lack of understanding of the working mechanisms, especially at the molecular level. A few articles included in this issue address this problem. For instance, Dau et al. (10.1039/C8SE00114F) employed time-resolved X-ray absorption spectroscopy to advance the understanding of NiFeOOH catalysts for O₂ evolution. Boettcher et al. (10.1039/C8SE00187A) further advance the application of the dual-working electrode approach for the study of the interface between Ni(Fe)OOH and a photoelectrode. For a similar system, albeit a molecular catalyst, Streb and Rau et al. (10.1039/C8SE00328A) discuss an operando tool for the study of O₂ evolution.

A third grand challenge the community has to address concerning artificial photosynthesis is engineering on a system level. Due to the difficulties in scaling up, electrochemical cells are traditionally expensive. The implementation of various artificial photosynthesis advances developed in the lab using electrochemical approaches will need to address this issue. New engineering strategies are likely needed to specifically take advantage of the uniqueness of artificial photosynthesis. To this end, Takanabe et al. (10.1039/C8SE00272J) report a detailed study on mass transport at the nanoscale, with the implementation of powder-based artificial photosynthesis in pure water in mind. Vermaas and Smith et al. (10.1039/C8SE00118A) study how bipolar membranes may be used to enable redox reactions in incompatible solutions. Berlinguette et al. (10.1039/C8SE00175H) review new developments in using organic media instead of water as electron donors for easy H₂ generation. A new direction in this area is to engineer the oxidation of organic compounds for the production of value-added chemicals.

Research has continued to reveal new details about natural photosynthesis, which helps us appreciate the process at new levels every day. How to successfully mimic it at higher efficiencies and lower cost will not be an easy task. However, we have few other options available to develop a sustainable future. As such, the community is motivated to solve the various challenges and enable artificial photosynthesis as a practical route toward renewable energy storage and valuable product preparation. The collections presented here represent a humble snapshot of these efforts. They will prove important pieces of the great puzzle that we hope to be solved soon.

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