Assessment of trends in the electrochemical CO 2 reduction and H 2 evolution reactions on metal nanoparticles
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Research Letter
Assessment of trends in the electrochemical CO2 reduction and H2 evolution reactions on metal nanoparticles Dominic R. Alfonso and Douglas R. Kauffman, National Energy Technology Laboratory, United States Department of Energy, Pittsburgh, Pennsylvania 15236, USA Address all correspondence to Dominic R. Alfonso at [email protected] (Received 1 June 2017; accepted 1 August 2017)
Abstract We used density functional theory to investigate the electrochemical CO2 reduction and competing hydrogen evolution reaction on model Au, Ag, Cu, Ir, Ni, Pd, Pt, and Rh nanoparticles. On the coinage metal, the free energy of adsorbed COOH, CO, and H intermediates generally becomes more favorable with decreasing particle size. This pattern was also observed on all transition metals with the binding of the intermediates observed to be stronger on almost all of these metals. Comparative studies of the reaction profile reveal that H2 evolution is the first reaction to be energetically allowed at zero applied bias.
Introduction Rising atmospheric CO2 levels associated with fossil fuel use pose serious social and environmental concerns across the globe. One particularly appealing mitigation strategy involves recycling the CO2 into industrially relevant chemicals, such as CH4, C2H2, or syngas for Fischer–Tropch processes.[1,2] Electrochemical CO2 conversion is especially appealing because it can be done at ambient temperature and pressure using excess or “stranded” renewable energy.[2–5] Carried out on a large scale, the technology could potentially recycle sizeable percentage of CO2 from emissions produced from the industrial process. Large-scale deployment of electrochemical CO2 conversion systems will require developing improved electrocatalysts with lower energy requirements. CO2 can be theoretically converted to CO, alcohols, and hydrocarbons at very low applied potentials, but the experimental work has shown that realistic systems require large overpotentials to promote the CO2 reduction reaction (CO2RR).[6] Another related concern is that the large overpotentials allow competitive H2 evolution reaction (HER) from water splitting, thus reducing overall efficiency and product selectivity. Experiments have indicated that Au nanoparticle (NP) electrocatalysts demonstrate higher CO2RR activity than bulk, polycrystalline metals. Studies of 4–10 nm diameter monodispersed Au NPs for CO2 to CO conversion showed that the 8 nm yields the highest selectivity with ∼90% Faraday efficiency (FE) at −0.67 V versus RHE.[7] Density functional theory (DFT) calculations showed that steps on (211) facets were more active than closed packed surfaces. These undercoordinated surface sites significantly stabilized COOH,
which explains why nanostructured catalysts have lower overpotentials than planar, bulk surfaces. On an Au13 NP model, much stronger binding of the other relevant CO intermediate was predicted. This result suggests less favorable product liberation since NP active sites must stabilize COOH more than CO for the catalyst to show
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