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 (FE_C2+ 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).

Resources

• 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