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A A R O N M . A P P E L

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remarkable improvement in a catalystfor the electrochemical production

of carbon-based fuels from carbonmonoxide and water is reported by Li et  al .1 ina paper published on Nature’s website today.Although electrodes made from copper havebeen known for many years to catalyse thetransformation of carbon dioxide to chemi-cals and fuels2, through intermediates such asCO, the authors have demonstrated that themethod used to prepare copper electrodeshas a substantial effect on the activity andenergy efficiency of these catalysts. This resultmay lead to improvements in how renewableenergy sources are exploited.

Renewable sources such as sunlight andwind provide an opportunity to power societywithout the negative consequences thataccompany the use of fossil fuels, but theirintermittent availability is a major limitation.Efficient means of storing energy from thesesources must therefore be found in order tofacilitate their widespread use3. An attrac-tive approach is the use of electricity to drivethe production of fuels from water and CO2 (refs 3–6; Fig. 1). Unlike conventional, central-ized fuel production, electrochemical systemscan operate at mild pressures and temperaturesin small-scale reactors — making them idealfor producing fuels at sites of renewable energysources, which are inherently dispersed. How-

ever, the development of efficient catalystswill be essential for the intended chemicaltransformations.

The electrochemical conversion of CO2 into fuels is a multi-step process that hasmany potential products and intermediates.The choice of catalyst has a profound effecton the selectivity and energy efficiency of thisprocess. The first product is often CO: this gashas a low energy density, so further transfor-mation is required to make an effective fuel.Although copper catalysts are known to workfor the initial reduction of CO2 (refs 2, 7), Liand colleagues focused on the subsequent

transformation of CO. They report that,

when copper is oxidized and then reduced backto copper in a controlled way, the resultingcatalytic metal surface not only producesethanol more selectively than previous cata-

lysts, but is also more energy efficient. This dis-covery is a great step towards the cost-effectiveproduction of renewable liquid fuels with highenergy densities.

One of the long-standing challenges inheterogeneous catalysis, in which reactionstypically occur at the surface of a material, isto understand the causes of changes in cata-lytic performance. The surface of a materialusually bears a variety of structural features,which are potential catalytic sites for the keyreactions. Identifying which of these is respon-sible for catalysis requires the characterizationof materials by numerous techniques, followedby attempts to correlate changes in surface fea-tures with changes in catalytic performance.

Li et al . analysed the copper catalysts pro-duced by their oxidation–reduction process,and found that the size of the particles formeddoes not seem to explain their catalytic activ-ity — similarly sized copper particles preparedusing a different method did not have similar

catalytic activity. The researchers propose thatthe different activities of the catalysts producedusing the two methods derive from grainboundaries, the junctions between crystals

within the particles. Although particles pre-pared using different methods may be similarin size, their grain boundaries vary substan-tially. These multidimensional ‘defects’ may bethe key to the significant enhancements seenin selectivity and energy efficiency.

The insight gained from Li and co-work-ers’ results lays the groundwork for furtheradvances. As with all electrocatalysts, theenergy efficiency of ethanol formation in theauthors’ system could be improved by decreas-ing the electric potential required for reactionand by increasing the product selectivity. Buta more remarkable advance may come from

studying the carbon–carbon bond-formingprocess that connects CO molecules to formtwo-carbon molecules (such as ethanol). Theselective formation of carbon–carbon bondsis a key step in a variety of catalytic transfor-mations, not only for industrial applications,but also in biological systems. Enzymes con-tain precisely positioned chemical groups thatcontrol bond-forming and bond-breakingreactions8, and the variations in activity andproduct selectivity observed by Li et  al . couldbe due to the formation of similarly multi-

functional catalyst structures. An understand-ing of this process could lead to the productionof fuels that contain more carbon atoms (suchas butanol), which have an even greater energydensity than ethanol.

Liquid fuels produced from CO2 and itsderivative CO face considerable challenges inthe fuel market. One of the biggest problemsis that fossil fuels have an inherent advan-tage, because energy was stored in them — by

E L E C T R O C H E M I S T R Y  

Catalysis at theboundariesCopper-based materials have been found that efficiently convert carbonmonoxide and water to ethanol using electricity. The discovery is a majoradvance towards storing renewable energy in the form of a liquid fuel.

Figure 1 | Storing energy from renewable sources. Renewable energy sources, such as solar energy,are only intermittently available, so a means of storing the energy is needed. If the energy is convertedinto electricity, this could be used to drive electrochemical reactions that convert carbon dioxide intoliquid fuel, often with carbon monoxide (not shown) as an intermediate; when the fuel is later used todo work, CO2 is released, and can be recycled into fuel. Li et al.1 report improved copper catalysts for the

electrochemical conversion of CO to ethanol, a potential liquid fuel.

Renewableenergy

Electricity

Fuel

CO2

Work

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prehistoric photosynthesis — at no cost. Bycontrast, the energy used to produce fuels fromrenewable sources must be paid for. At present,this expense is substantial, but the cost rela-tive to that of fossil fuels will probably decreasewith time. Because of the challenges of pre-dicting fossil-fuel prices, renewable-energyproduction costs, and incentives (such as car-

bon taxes) to use renewables in place of fossilfuels, the timetable for widespread adoption ofrenewable fuels is not clear. But the production

of fuels from non-fossil sources will certainlyrequire effective catalysts. Li and colleagues’work is an excellent step in that direction. ■

Aaron M. Appel is in the Catalysis ScienceGroup, Pacific Northwest National Laboratory,Richland, Washington 99352, USA.e-mail: aaron.appel@pnnl.gov 

1. Li, C. W., Ciston, J. & Kanan, M. W.Nature http://dx.doi.org/10.1038/nature13249 (2014).

2. Hori, Y. in Modern Aspects of Electrochemistry Vol. 42

(eds Vayenas, C. G. et al.) 89–189 (Springer, 2008).3. Cook, T. R.et al. Chem. Rev. 110, 6474–6502

(2010).4. Olah, G. A., Prakash, G. K. S. & Goeppert, A. J. Am.

Chem. Soc. 133, 12881–12898 (2011).5. Kondratenko, E. V., Mul, G., Baltrusaitis, J.,

Larrazábal, G. O. & Pérez-Ramírez, J. Energy Environ.Sci. 6, 3112–3135 (2013).

6. Thoi, V. S., Sun, Y., Long, J. R. & Chang, C. J. Chem.Soc. Rev. 42, 2388–2400 (2013).

7. Li, C. W. & Kanan, M. W. J. Am. Chem. Soc. 134, 

7231–7234 (2012).8. Appel, A. M. et al. Chem. Rev. 113, 6621–6658

(2013).

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