Protection-driven selectivity swap from ethylene to acetate in high-rate CO2/CO electrolysis

As an rising carbon seize, utilization, and storage expertise, CO₂ electrolysis to beneficial chemical compounds and fuels with energy provide derived from renewable vitality is of nice significance¹. To push the CO₂ electrolysis course of in direction of sensible utility, reaching excessive selectivity of multicarbon (C₂₊) merchandise (e.g., ethylene, acetate, ethanol, and n-propanol) at industrial present densities is very fascinating². Aside from rational design of catalytically lively nanostructures, tuning the microenvironments close to catalyst surfaces has additionally been demonstrated to be efficient in facilitating C–C coupling and enhancing C₂₊ manufacturing. Catalyst microenvironments will be modified by way of tuning electrolyte compositions (e.g., cation, anion, and pH) in addition to introducing natural compounds and ionomer layers on catalyst surfaces^3–7. A key position of those tailor-made microenvironments is to range the native concentrations of CO₂, H₂O, OH⁻ and H⁺ in addition to the floor coverages of key intermediates equivalent to adsorbed CO (*CO). An alternate strategy to modify catalyst microenvironments is to make use of combined CO/CO₂ feeds which might be often derived from incomplete industrial combustion of fossil fuels. But, the microenvironment within the presence of combined CO/CO₂ feeds and its results on the formation of C₂₊ merchandise are poorly understood. Presently, most of mechanistic research deal with pure CO₂ and CO electrolysis, and just a few of them take account of CO-CO₂ co-electrolysis^8,9. For instance, a current research proposed a CO/CO₂ cross-coupling mechanism that successfully explains improved ethylene manufacturing in an H-cell with a impartial electrolyte⁹. A membrane electrode meeting (MEA) electrolyzer is taken into account as essentially the most promising electrochemical system for CO₂ electrolysis, nonetheless, up to now basic understandings on the response mechanism of CO/CO₂ co-electrolysis below alkaline MEA circumstances are nonetheless missing.

Within the current work, we investigated CO/CO₂ co-electrolysis over nanoporous CuO nanosheets in an alkaline MEA electrolyzer to fill the data hole of each electrocatalytic reactivity and mechanistic understanding. Within the pure CO₂ feed, CO₂ electrolysis delivered a peak ethylene Faradaic effectivity (FE) of 52.5% at an utilized present density of 0.6 A cm⁻². When CO/CO₂ feeds had been fed to the electrolyzer, the product selectivity shifted from ethylene to acetate. With rising CO stress within the feed, acetate changed ethylene as the key product and in the meantime the overall present density considerably elevated. In 0.6 MPa CO feed, the acetate FE achieved 48% with a complete present density of three A cm⁻². Below optimized circumstances, the FE and partial present density of C₂₊ merchandise reached 90.0% and three.1 A cm⁻², similar to a carbon selectivity of 100.0% and yield of 75.0%, outperforming thermocatalytic CO hydrogenation (Determine 1).

Determine 1. CO₂/CO electrolysis efficiency. a–d, FE and cell voltage as a operate of utilized present density over CuO nanosheet catalyst measured in 0.1 M KOH below pure CO₂ feed (a), CO/CO₂ (3:1) co-feed (b), pure CO feed (c) and 1 M KOH below 0.6 MPa pure CO feed (d). e, Comparability of CO discount by way of electrocatalysis and thermal catalysis. CO circulation charges of 30 ml min⁻¹ (crimson) and 60 ml min⁻¹ (orange).

To determine the selectivity swap from ethylene to acetate, we firstly characterised the CuO nanosheet catalyst in its as-prepared state, throughout and after CO₂/CO electrolysis. Whereas the CuO nanosheet catalyst suffered from vital reconstruction, the adjustments within the morphology and construction had been impartial on feed composition. The ¹³CO isotopic labelling experiment adopted by GC-MS product evaluation indicated that solely 2.4% of the CO₂ within the CO/CO₂ (3:1) co-feed went to merchandise and the CO/CO₂ cross-coupling mechanism couldn’t clarify the improved ethylene manufacturing below alkaline MEA circumstances. Reducing CO partial stress and native pH are two roles performed by CO₂ within the co-feeds, and their results had been decoupled by changing CO₂ with Ar and pressurizing CO. The upper native pH did play a task in acetate manufacturing, nonetheless, the selectivity swap from ethylene to acetate was primarily pushed by CO stress (*CO protection).

Operando Raman spectroscopy measurements had been carried out to look at floor adsorbate species throughout electrolysis. The product distribution measured within the operando cell virtually resembled that measured in our MEA electrolyzer. We discovered that with rising CO stress the atop-adsorbed *CO band shifted in direction of increased wavenumber and the world ratio of high-frequency-band (HFB)-*CO_atop and low-frequency-band (LFB)-*CO_atop elevated from 0.73 within the CO/CO₂ (1:1) co-feed to 2.74 in 0.4 MPa pure CO feed. Together with literature¹⁰, we postulate that CO tends to preferentially adsorb on terrace websites at low *CO protection and step websites are supplied with extra CO at excessive *CO protection. Density practical principle (DFT) calculations additional point out that the ethylene formation is favorable on Cu(100) side (terrace websites) whereas the acetate formation is considerably improved on Cu(511) side (step websites). The identification of response websites by operando Raman spectroscopy and the evaluation of response pathways after C−C coupling step by DFT calculations rationalize the *CO coverage-driven selectivity swap from ethylene to acetate in high-rate CO₂/CO electrolysis (Determine 2).

Determine 2. *CO-coverage-dependent response pathways. a, Operando *CO_atop Raman peaks of the CuO nanosheet catalyst measured in 0.1 M KOH at 0.05 A cm⁻². b, Peak space ratio of HFB-*CO_atop to LFB-*CO_atop. c,d, Activation free-energy barrier of ethylene in addition to acetate by way of H assault and −OH dissociation on Cu(100) (c) and Cu(511) (d).

To push CO₂/CO electrolysis in direction of sensible utility, the scale-up of the electrolysis is indispensable. We firstly scaled up the electrolysis course of by rising the geometric electrode space from 4 to 100 cm² after which assembled an electrolyzer stack with 4 100 cm² MEAs. For CO₂ electrolysis, a peak ethylene FE of 49.9% was achieved at a complete present of 40 A. For CO electrolysis, the C₂₊ FE was nonetheless increased than 65% at a complete present of 250 A, with the very best ethylene formation charge of 457.5 mL min^–1 at 150 A and acetate formation charge of two.97 g min^–1 at 250 A. The electrolysis performances are comparable within the 4 and 100 cm² electrolyzers in addition to 100 cm² stack, highlighting the wonderful effectiveness of our scale-up demonstration (Determine 3).

Determine 3. Scale-up demonstration. a,b, Schematic (a) and {photograph} (b) of the electrolyser stack with 4 100 cm² MEAs used on this work. c,d, FE and cell voltage as a operate of utilized present density over CuO nanosheet catalyst measured in 0.1 M KOH below pure CO₂ feed (c) and 1 M KOH below pure CO feed (d).

Our work highlights the promise of tuning catalyst microenvironments for the selective manufacturing of single C₂₊ merchandise equivalent to acetate and ethylene, and presents an efficient scale-up demonstration of high-rate CO₂/CO electrolysis.

If you need to know extra particulars, please check out our article revealed in Nature Nanotechnology: https://doi.org/10.1038/s41565-022-01286-y

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