Copper-based materials have been found that efficiently convert carbon monoxide and water to ethanol using electricity. The discovery is a major advance towards storing renewable energy in the form of a liquid fuel. See Letter p.504
A remarkable improvement in a catalyst for the electrochemical production of carbon-based fuels from carbon monoxide and water is reported by Li et al.1 on page 504 of this issue. Although electrodes made from copper have been known for many years to catalyse the transformation of carbon dioxide to chemicals and fuels2, through intermediates such as CO, the authors have demonstrated that the method used to prepare copper electrodes has a substantial effect on the activity and energy efficiency of these catalysts. This result may lead to improvements in how renewable energy sources are exploited.
Renewable sources such as sunlight and wind provide an opportunity to power society without the negative consequences that accompany the use of fossil fuels, but their intermittent availability is a major limitation. Efficient means of storing energy from these sources must therefore be found in order to facilitate their widespread use3. An attractive approach is the use of electricity to drive the production of fuels from water and CO2 (refs 3,4,5,6; Fig. 1). Unlike conventional, centralized fuel production, electrochemical systems can operate at mild pressures and temperatures in small-scale reactors — making them ideal for producing fuels at sites of renewable energy sources, which are inherently dispersed. However, the development of efficient catalysts will be essential for the intended chemical transformations.
The electrochemical conversion of CO2 into fuels is a multi-step process that has many potential products and intermediates. The choice of catalyst has a profound effect on the selectivity and energy efficiency of this process. The first product is often CO: this gas has a low energy density, so further transformation is required to make an effective fuel. Although copper catalysts are known to work for the initial reduction of CO2 (refs 2,7), Li and colleagues focused on the subsequent transformation of CO. They report that, when copper is oxidized and then reduced back to copper in a controlled way, the resulting catalytic metal surface not only produces ethanol more selectively than previous catalysts, but is also more energy efficient. This discovery is a great step towards the cost-effective production of renewable liquid fuels with high energy densities.
One of the long-standing challenges in heterogeneous catalysis, in which reactions typically occur at the surface of a material, is to understand the causes of changes in catalytic performance. The surface of a material usually bears a variety of structural features, which are potential catalytic sites for the key reactions. Identifying which of these is responsible for catalysis requires the characterization of materials by numerous techniques, followed by attempts to correlate changes in surface features with changes in catalytic performance.
Li et al. analysed the copper catalysts produced by their oxidation–reduction process, and found that the size of the particles formed does not seem to explain their catalytic activity — similarly sized copper particles prepared using a different method did not have similar catalytic activity. The researchers propose that the different activities of the catalysts produced using the two methods derive from grain boundaries, the junctions between crystals within the particles. Although particles prepared using different methods may be similar in size, their grain boundaries vary substantially. These multidimensional 'defects' may be the key to the significant enhancements seen in selectivity and energy efficiency.
The insight gained from Li and co-workers' results lays the groundwork for further advances. As with all electrocatalysts, the energy efficiency of ethanol formation in the authors' system could be improved by decreasing the electric potential required for reaction and by increasing the product selectivity. But a more remarkable advance may come from studying the carbon–carbon bond-forming process that connects CO molecules to form two-carbon molecules (such as ethanol). The selective formation of carbon–carbon bonds is a key step in a variety of catalytic transformations, not only for industrial applications, but also in biological systems. Enzymes contain precisely positioned chemical groups that control bond-forming and bond-breaking reactions8, and the variations in activity and product selectivity observed by Li et al. could be due to the formation of similarly multifunctional catalyst structures. An understanding of this process could lead to the production of fuels that contain more carbon atoms (such as butanol), which have an even greater energy density than ethanol.
Liquid fuels produced from CO2 and its derivative CO face considerable challenges in the fuel market. One of the biggest problems is that fossil fuels have an inherent advantage, because energy was stored in them — by prehistoric photosynthesis — at no cost. By contrast, the energy used to produce fuels from renewable sources must be paid for. At present, this expense is substantial, but the cost relative to that of fossil fuels will probably decrease with time. Because of the challenges of predicting fossil-fuel prices, renewable-energy production costs, and incentives (such as carbon taxes) to use renewables in place of fossil fuels, the timetable for widespread adoption of renewable fuels is not clear. But the production of fuels from non-fossil sources will certainly require effective catalysts. Li and colleagues' work is an excellent step in that direction.
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Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects
Angewandte Chemie International Edition (2018)