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Bottling renewables

Production of fuels from water and CO2 using renewable energy could provide energy storage solutions and reduce our dependence on fossil fuels. This month, we showcase recent efforts to advance understanding in this area.

Advances in technology and the decreasing cost of renewable power present exciting opportunities to decarbonize our energy systems. However, due to their intermittency, the increased penetration of renewables requires us to reconsider how energy systems function and to seek new ways to store energy. While electrochemical storage technology has advanced enormously, significant progress is still necessary to provide energy storage at grid scale and over different timescales. Moreover, chemical fuels are still the dominant energy carrier in use today for mobile applications where energy density is a key consideration.

In this context, the production of chemical fuels from abundant molecules such as CO2 and water using renewable energy sources has become an area of intense study. Splitting water into hydrogen and oxygen in an electrolyser using wind or solar generated electricity enables storage of renewable energy in chemical form. Alternatively, solar energy can be harnessed — in much the same way as in a solar cell — but rather than collecting the current, the energy can be directly channelled to sites on a catalyst to drive the chemical reactions involved in splitting water into its constituents. The hydrogen can later be burned to produce heat or supplied to a fuel cell to generate electricity directly. Similar, albeit more complex, processes are also involved in the production of fuels from CO2.

This month we present a Collection of Reviews and opinion, alongside some of our favourite Articles, published in Nature Energy over the past year, on the production of fuels from water and CO2. The pieces span an array of different electrochemical and photochemical approaches and stages of technological readiness, from mechanistic understanding to device-level considerations.

In the near term, the easiest way to produce fuel electrochemically or photochemically at scale is to use an electrolyser to split water. In this arrangement, catalysts are required to increase the rate of the hydrogen evolution reaction (HER), which occurs at the cathode, and the oxygen evolution reaction (OER), which occurs at the anode. Recently, many researchers have targeted replacement of the expensive noble-metal catalysts that are the archetypal HER and OER catalysts. In their Comment, Kibsgaard and Chorkendorff discuss where attention should be focussed when considering scale-up of such systems. In particular, they feel that OER catalysts that are stable in acid and have improved performance per active site are a high priority. They also lay out recommendations for measuring and reporting catalytic performance.

Researchers are also considering how to augment traditional electrolysis technology to circumvent issues that arise when using intermittent power sources to drive electrolysers, including the mixing of hydrogen and oxygen, and poor efficiency. Addressing these challenges in their Article, Rothschild, Grader and colleagues report a two-stage electrolysis system, where hydrogen and oxygen are generated in separate steps with high efficiency.

Advances at the device level are also necessary in photoelectrochemical water splitting. Under this approach, the solar harvesting step is integrated more closely with the catalytic part of the process, forming a stand-alone device. A strength here is the mitigation of electrical and thermal losses that can occur in more separated systems. In their Article, Haussener and colleagues explore how active thermal management can be used in devices powered by concentrated solar light to cool the photoabsorber and provide heat to the electrochemical part of the device to boost overall performance.

Of course, nature already provides a blueprint for converting solar energy into chemicals via photosynthesis. For instance, in their Article, Reisner and team couple natural machineries with synthetic features to generate hydrogen photoelectrochemically. They integrate a synthetic dye-sensitized photoanode with photosystem II (a protein complex in green plants that oxidizes water during photosynthesis), allowing greater solar light absorption than the natural system alone would allow. The composite photoanode is wired to a natural hydrogenase enzyme that produces hydrogen.

Besides wired approaches where hydrogen and oxygen are generated at separate electrodes — whether electrochemically or photochemically — the production of hydrogen and oxygen in the same vicinity over particulate photocatalysts has been researched for many years. A challenge here is to design a material or composite that can harvest light as well as catalyse both HER and OER. Many works focus on only catalysing one of the two half reactions, making use of sacrificial agents to scavenge electrons or holes. However, in their Article, Würthner, Stolarczyk and colleagues demonstrate the power of precise catalyst design to achieve simultaneous oxygen and hydrogen production, using CdS nanorods as a light harvester along with nanoparticulate and molecular co-catalysts anchored to the surface to facilitate HER and OER, respectively. In their Review, Zwijnenburg, Durrant, Cooper, Tang and team explore how the properties of polymeric photocatalysts can be tuned through careful design and synthesis.

Compared to water electrolysis, electrochemical reduction of CO2 is relatively immature and faces challenges in terms of reaction selectivity, overall conversion rates and efficiency. However, its products are more similar to traditional fuels, facilitating integration into existing fuel infrastructure. In a Review, Calle-Vallejo, Koper and colleagues evaluate mechanistic elements of CO2 reduction and look at how catalyst design, as well as operational conditions, effect CO2 reduction performance, suggesting that a more integrated approach to research would be prudent in the future.

Improvements at the device level continue apace, moving technologies towards application. For example, in their Article, Wang and team incorporate solid-state electrolytes into CO2 electrochemical reduction cells, producing highly concentrated solutions of CO2-derived fuels, free from additional solutes found in typical set-ups. The cost of converting CO2 to fuels is also critical; Kenis and colleagues show in their Article that the electricity consumption requirements of CO2 electrolysers can be significantly reduced by changing the oxidation reaction from OER to oxidation of glycerol (a product of biodiesel synthesis).

While this Collection focusses on low-temperature conversion of water and CO2, higher temperature technologies, as well as nitrogen reduction, are also gaining momentum as routes to convert renewable energy to chemical energy. These approaches offer great promise, despite the hurdles still to be overcome, and there is much exciting work still to be done to realize this. We look forward to bringing you more of it in the near future.

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Bottling renewables. Nat Energy 4, 721 (2019).

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