As CO₂ levels in the atmosphere increase and it becomes clear that this has substantial consequences for our climate, developing alternatives to fossil fuels is more urgent than ever. Fuels are unrivaled for energy storage and we have an enormous infrastructure built to handle fuels for transportation and heating. In addition, the chemical industry and all the products and materials that we rely on in modern society are completely reliant on fossil feedstocks. This motivates the development of sustainable processes to generate fuels and chemical feedstocks from water and CO₂ using solar energy; such a process is analogous to photosynthesis in nature and is sometimes referred to as artificial photosynthesis.

There are large scientific and technical challenges involved in making even the simplest fuel, H₂, and even more so for carbon-based fuels by means of CO₂ reduction. Here we describe some of the recent progress and the obstacles that need to be surmounted to find economically viable solutions. In accordance with the theme of materials for sustainable energy, we focus on solid state devices for the production of solar fuels. There have also been important developments in molecular catalysis for fuels and we refer the interested reader to a recent review.

—Montoya et al.

Various device configuration powered by a renewable energy source perform electrochemical water splitting or CO₂ reduction. One such connects a photovoltaic (PV) to a separate electrolyzer with catalysts that drive the necessary conversion reactions (PV/electrolysis). Another possibility is to combine these two pieces into a fully integrated system in which the catalyst is deposited directly on top of the solar absorbers to create a photoelectrochemical (PEC) device. A range of intermediate configurations also exist that allow partial decoupling of the device components and set various bene fi and constraints on the device operation.

PV/electrolysis and PEC devices require photon absorbers that can harvest solar energy efficiently as well as catalysts for water-splitting and fuel production. In addition, PEC devices require effective integration of the photon absorber and the catalyst. Resulting from these requirements, the challenges for solar fuels and chemical production fall into three areas: photon capture, electrocatalysis and integration.

Among the team’s findings and suggestions:

• Materials discovery remains the biggest challenge for photoabsorber materials. While experimental and computational screening of new materials has begun, there are still many to be simulated and tested. Some insights into structure–property relationships have been derived from the large quantity of available band structure data, but there are likely to be a number of trends that might yet be discovered and leveraged into more effective searches. Stability, particularly in the aqueous environment at the electrode interface, will also be a key concern.

• While there has recently been “remarkable progress” in developing new, non-precious metal catalysts for hydrogen evolution, there is still significant room for improvement. A larger challenge is to improve the performance of catalysts for the oxygen evolution reaction (OER). Even the best OER catalysts have overpotentials of 0.3 V or higher, suggesting that OER catalysis faces a fundamental obstacle to reducing overpotential losses that is very difficult to break through theory has shown that this overpotential barrier may be explained by scaling relationships between the stability of important intermediates for large classes of catalysts.

  New catalyst design strategies are needed such that the stability of different intermediates and transition states can be varied independently. This likely will require precise engineering of individual active sites and thus new approaches to designing and synthesizing nanostructured materials.

• Creating solar fuels using CO₂ reduction is even more demanding, the authors noted. While reasonably efficient catalysts exist to produce CO from CO₂, there are no known catalysts producing hydrocarbons or alcohols with high rates and low overpotentials.

  The current state of the art for hydrocarbon synthesis is still Cu, which requires nearly 1 V of overpotential to produce 10 mA cm⁻² of products further reduced than CO. The understanding of CO₂ reduction electrocatalysis is still in its infancy. In general, an improved molecular-level understanding of chemical processes at the charged solid–liquid interface, and advanced descriptors (experimental and theoretical) of catalytic activity and selectivity will be necessary to screen for new catalysts.

• Integrated systems require engineered interfaces between photoabsorber materials and catalysts resulting in the lowest losses.

We are far from having solutions to the production of solar fuels and chemicals, and we cannot rely solely on trial-and-error strategies to solve the challenges ahead. However, by improving fundamental understanding of materials properties relevant to each aspect of the photoelectrocatalytic device that connects the incident photon to the desorbing fuel molecule, we may accelerate the process of making artificial photosynthesis a reality.

—Montoya et al.

Resources

• Joseph H. Montoya, Linsey C. Seitz, Pongkarn Chakthranont, Aleksandra Vojvodic, Thomas F. Jaramillo & Jens K. Nørskov (2016) “Materials for solar fuels and chemicals” Nature Materials doi: 10.1038/nmat4778