Through this project, we discovered a remarkably convenient and inexpensive means by which copper electrodes are modified to promote the production of ethylene. Our approach uses specific organic additives that alter the chemical properties of the copper electrode by creating an organic film on it. The resulting organic/copper hybrid electrode generates ethylene with remarkably high selectivity and activities approaching the minimum threshold established for a successful industrial application.
In our first published work we report a novel and convenient method for nanostructuring copper electrodes using N,N′‐ethylene‐phenanthrolinium dibromide as molecular additive. Selectivities up to 70 % for C-C coupled products (ethylene, ethanol and propanol) are observed for more than 40 h without significant change in the surface morphology. Mechanistic studies reveal several roles for the organic additive, including: the formation of cube‐like nanostructures by corrosion of the copper surface, the stabilization of these nanostructures during electrocatalysis by formation of a protective organic layer, and the promotion of C-C products.
We have then reported a molecular tuning strategy that stabilizes intermediates for more selective CO2RR to ethylene. We found that the adhered molecules improve the stabilization of certain population of carbon monoxide intermediate thereby favouring further reduction to ethylene. As a result of this strategy, we reported the highest CO2RR to ethylene rate ever reported with selectivity of 72% at a partial current density of 230 mA/cm2. We also reported the most stable ethylene electrocatalyst with constant ethylene production for 190 hours.
In another study, we have reported an adlayer functionalization catalyst design: a catalyst/tetrahydro-phenanthrolinium/ionomer (CTPI) interface in which the catalytically active copper is functionalized using a similar phenanthrolinium-derived film and a perfluorocarbon-based polymeric ionomer. We found that this hierarchical adlayer augments both the local CO2 availability and the adsorption of the key reaction intermediate carbon monoxide on the catalyst surface. Using this CTPI catalyst, we achieved an ethylene selectivity of 66% at a partial current density of 208 mA/cm2 in a membrane-electrode assembly set-up.
To get closer to a potential industrial application of our device, we have recently reported a tandem electrolyzer which drastically reduces the energy input of the CO2RR. We developed an integrated electrochemical system that comprises a solid-oxide CO2-to-CO electrochemical cell (SOEC) and a CO-to-C2H4 MEA electrolyser. The MEA is equipped with a hierarchical catalyst composed of Cu, N-tolyl-tetrahydro-bipyridine, and a short-side-chain (SSC) ionomer. The MEA, as a result, delivered a C2H4 selectivity of 65% and an energy efficiency (EE) of 28% at a current density of 166 mA cm-2. In the integrated system, we achieved a peak C2H4 production rate of ~1.3 mmol per hour and a total energy requirement of ~120 GJ per tonne of ethylene, which represents a ~45% reduction in energy intensity compared to literature benchmarks.
We have also performed extended mechanistic studies of our approach to have a better understanding of the role of the organic film in altering the selectivity profile of the CO2RR. We have demonstrated how in situ coating of a silver electrode via electrodeposition of an organic film selectively inhibits proton versus CO2 mass transport, thereby affording extremely selective (>99%) electrochemical CO2-to-CO conversion.
