Research from US’ Oak Ridge uses carbon and copper nano-spikes to turn carbon dioxide and water directly into ethanol. The results could be of great significance for energy storage.
The energy industry has long looked to bio-ethanol as a replacement for conventional fuels, and as a method of lowering CO2 emissions from transport. Other ethyl chains like methanol are often valuable by-products, or products in their own right. But were ethanol to be made efficiently and economically enough, its potential extends far beyond fuel and chemical production. A team at the US Department of Energy’s (DoE) Oak Ridge National Laboratory (ORNL) in the US may have stumbled on just such an opportunity. In researching the first step of a proposed reaction to convert CO2 and water to methanol, the team discovered that the catalyst used was causing the entire reaction. The study’s lead author, ORNL’s Adam Rondinone, added: “We discovered somewhat by accident that this material worked.” The reaction requires a nanostructured catalyst made of carbon, copper and nitrogen. When a voltage was applied, the catalyst reacted with the CO2 and water solution to produce ethanol – a method which the team says “essentially reverses the combustion process.” Moreover, the reaction is achieved at room temperature – the only inputs required, apart from the catalyst, are CO2 and water.
Lightning rods This kind of electrochemical reaction is not new. Previous studies have produced smaller amounts of several different products – including carbon monoxide (CO), formic acid (HCOOH), methane (CH4) and ethylene (C2H4) or ethane (C2H6). However, as the researchers note in their paper, the efficiency of these methods is not great, and the chains produced are limited – most reactions will only produce “single-digit percentages” of liquid hydrocarbons, and are unlikely to produce any molecule heavier than methane in sufficient quantities to be practical. Yet the ORNL reaction appears to be reasonably efficient, at least in terms of the end product. The process converts water and CO2 into ethanol with a yield of 63% at 1.2 V, with other marginal amounts of pure hydrogen (H2), CO and CH4. Depending on the sample size, the yield in some tests was as high as 70%. Despite some of the media bluster around the process, it is not the answer to the world’s CO2 emission problems. CO2 is stored temporarily in the form of ethanol, and the process is a net consumer of energy. Nevertheless, if its relative simplicity can be paired with economic and technical efficiency, the possibilities are intriguing. “We’re pushing [the] combustion reaction backwards with very high selectivity to a useful fuel,” Rondinone said. “Ethanol was a surprise – it’s extremely difficult to go straight from carbon dioxide to ethanol with a single catalyst.” The design and structure of the catalyst is the secret to the team’s success. Copper nanoparticles are embedded in “randomly oriented nanospikes” of carbon around 50-80 nanometres long, and which end in a curled tip around 2 nanometres wide. According to Rondinone, these spikes “are like 50-nanometer lightning rods that concentrate electrochemical reactivity at the tip of the spike.” According to the team’s paper on the subject, these reactive sites “work in tandem to control the electrochemical reduction of carbon monoxide dimer to alcohol,” ensuring that the conversion happens efficiently and uniformly, and produces the specific alcohol as desired. The paper adds: “The Cu/CNS catalyst is unusual because it primarily produces ethanol rather than methane or ethylene. Ethanol, as a C2 product, requires carbon-carbon coupling between surface-adsorbed intermediates at some point during the reduction reaction. Recent calculations on C−C coupling on Cu(211) surfaces suggest the kinetic barriers for the coupling are strongly influenced by the degree of the adsorbed CO hydrogenation.”
Storage hunters By using commonly available elements and better structuring, the hope is that such catalysts could also cheaper and easier to produce than current iterations, which typically rely on expensive and rare metals. That opens the door to new catalytic conversions which may have previously been considered uneconomic. Because the reaction can be performed at room temperature, there is hope that the process could be scaled up into industrial applications – this is where energy storage comes in. The researchers suggest that excess electricity from variable power sources – renewables such as wind and solar especially – could be used to drive large-scale plants using the catalyst, enabling them to store energy in the form of ethanol. “A process like this would allow you to consume extra electricity when it’s available to make and store as ethanol,” Rondinone said. “This could help to balance a grid supplied by intermittent renewable sources.” Re-deploying that energy again is another issue. It is unlikely to be used in utility-scale power generation, although it could be effective as a means of powering mini-grids or small generators. It could also be blended into gasoline or used directly as a transport fuel, or it could simply be sold as a useful product much like the methanol produced by gas-to-liquids (GTL) systems. Most importantly, however, that fuel would be effectively carbon-neutral (provided it was generated by renewable sources). That provides a valuable incentive for refiners and hydrocarbons producers to get on board, if the process could be made economic. For now, however, the process is still in the experimental stage. The researchers now plan to refine their approach to improve the overall production rate and further study the catalyst’s properties and behaviour. One key limitation is outlined in the paper’s conclusion. High over-potential in the reaction means that less energy is recovered than is used for the transformation, the majority of which is lost through heat. The team notes that although this could be lowered “with the proper electrolyte, and by separating the hydrogen production to another catalyst,” this loss “probably precludes economic viability for this catalyst.” Yet other designed catalysts may not. Increased “selectivity” in future nanostructure designs could enable catalysts capable of manufacturing other hydrocarbon chains like methane, ethylene and ethane. In a curious twist, the future of efficient energy storage could yet lie in hydrocarbons.