A team of Brown University researchers has fine-tuned a copper catalyst to produce complex hydrocarbons—C2+ products—from CO2 with high efficiency. An open-access paper on the work is published in Nature Communications.
The electrochemical CO2 reduction reaction (CO2RR), driven by renewable energy, is a promising strategy to reduce CO2 accumulation. By converting CO2 into products of higher value, a closed-loop carbon economy begins to emerge. To make CO2RR economically viable, efficient electrocatalysts with high selectivity for targeted products at scale are needed. Among the metals studied, copper is the only metal known for its intrinsic ability to convert CO2 into hydrocarbons and alcohols via electrochemical CO2RR.
… Advances made in the previous studies inspired us to study each parameter influencing catalyst performance separately (i.e., chemical species present and their reactivity/solubility, applied potential, pH, and roughness) in order to develop a novel electrochemical method that utilizes these parameters to produce a Cu electrocatalyst selective for C2+ products. In this study, electrocatalysts with a balance of high density of defect sites (i.e., under-coordinated Cu) and low roughness are shown to efficiently convert CO2 to C2 and C3 products (FEC2+ of 72%) via electrochemical CO2RR. The electrochemical method used to produce these electrocatalysts consists of three steps: (i) anodic halogenation of Cu foils, (ii) subsequent oxide formation in a KHCO3 electrolyte, and (iii) electroreduction.—Kim and Palmore (2020)
The researchers say the preparation process can be scaled up to an industrial level fairly easily, which gives the new catalyst potential for use in large-scale CO2 recycling efforts.
Two-dimensional visualization of under-coordinated Cu atoms and surface roughness with the corresponding outcomes when used for the electrochemical CO2RR. This schematic is simplified but face-centered cubic Cu has a coordination number of 12. Kim and Palmore (2020)
There had been reports in the literature of all kinds of different treatments for copper that could produce these C2+ with a range of different efficiencies. What Taehee did was a set of experiments to unravel what each of these treatment steps was actually doing to the catalyst in terms of reactivity, which pointed the way to optimizing a catalyst for these multi-carbon compounds.—Professor Tayhas Palmore, who co-authored the paper with Ph.D. student Taehee Kim
There have been great strides in recent years in developing copper catalysts that could make single-carbon molecules, Palmore says. For example, Palmore and her team at Brown recently developed a copper foam catalyst that can produce formic acid efficiently, an important single-carbon commodity chemical. But interest is increasing in reactions that can produce C2+ products.
There had been evidence from prior research that halogenation of copper—a reaction that coats a copper surface with atoms of chlorine, bromine or iodine in the presence of an electrical potential—could increase a catalyst’s selectivity of C2+ products. Kim experimented with a variety of different halogenation methods, zeroing in on which halogen elements and which electrical potentials yielded catalysts with the best performance in CO2-to-C2+ reactions. He found that the optimal preparations could yield faradaic efficiencies of between 70.7% and 72.6%, far higher than any other copper catalyst.
The research helps to reveal the attributes that make a copper catalyst good for C2+ products. The preparations with the highest efficiencies had a large number of surface defects—tiny cracks and crevices in the halogenated surface—that are critical for carbon-carbon coupling reactions. These defect sites appear to be key to the catalysts’ high selectivity toward ethylene, a C2+ product that can be polymerized and used to make plastics.
This work shows that efficient conversion of CO2 to C2+ products requires a Cu catalyst with a high density of defect sites that promote adsorption of carbon intermediates and C–C coupling reactions while minimizing roughness.—Kim and Palmore (2020)
The research was funded by the National Science Foundation (CHE-1240020).
Kim, T., Palmore, G.T.R. (2020) “A scalable method for preparing Cu electrocatalysts that convert CO2 into C2+ products. Nat Commun 11, 3622 doi: 10.1038/s41467-020-16998-9