Carbon dioxide electroreduction (CO2RR) powered by renewable-electricity to chemicals and fuels is a promising route to store the intermittent renewable energy. Renewable methane (CH4) produced from CO2RR, a carbon-neutral alternative to fossil fuels, attracts interest in view of the well-established infrastructure for natural gas storage, distribution, and utilization.
Prior progress in methane selectivity in CO2RR has mainly been made below current density 50 mA cm-2.[3-7] Technoeconomic analyses suggest that practical CO2RR systems require current density above 100 mA cm-2 to make systems profitable. This motivated us to seek to increase, simultaneously, the current density and selectivity of methane production from CO2RR.
In CO2RR, after the generation of *CO intermediate, *CO protonation to *CHO is the potential-determining step for methane formation, competing with carbon-carbon (C-C) coupling for C2 products as well as the hydrogen evolution reaction (HER).[9-11] Thus, the key to improve methane selectivity is to suppress C-C coupling and HER simultaneously.
In a prior CO2RR study, we found that, on Cu surface, decreasing *CO coverage improved methane selectivity, but with prominent HER. To circumvent the favorable HER and increase the selectivity to methane, we developed a suite of Au-Cu bimetallic catalysts and presented a strategy wherein we controlled *CO availability on Au-Cu catalysts, enabling selectivity to methane at high production rates in CO2RR.
We first fabricated Au-Cu catalysts, based on Cu catalysts supported on polytetrafluoroethylene (PTFE) nanofibers, using a galvanic replacement enabled by the differing reduction potentials of Au and Cu. Using this approach, we obtained a series of Au-Cu catalysts with different atomic percentages of Au in Cu.
In CO2RR, we regulated *CO availability on Au-Cu catalysts through controlling the CO2 concentration and reaction rate. By supplying gas streams consisting of different volume ratios of CO2 to N2, we evaluated CO2RR performance on Au-Cu catalysts under different current densities. Compared to pure CO2, the methane selectivity is promoted on Au-Cu catalysts in CO2–N2 mixed streams at high current densities while the selectivity to ethylene – the main C2 product – decreases dramatically. Relative to Cu catalysts, HER on Au-Cu catalysts is suppressed with increased methane selectivity under low *CO coverage; this leads to a 1.6× improvement in the methane:H2 selectivity ratio compared with prior reports[12-16] having a total current density above 100 mA cm-2. We as a result achieve a methane Faradaic efficiency (FE) of (56 ± 2) % at a partial current density of (112 ± 4) mA cm-2. With the aid of density functional theory calculations and operando X-ray absorption spectroscopy, we found that, under low *CO coverage, the introduction of Au in Cu favors *CO protonation vs. C−C coupling while suppresses HER.
These findings in this work suggest a promising strategy to directly convert dilute CO2 stream to carbon-neutral methane with a combination of high selectivity and high reaction rate.
If you are interested in our work, you may find the full paper here: https://www.nature.com/articles/s41467-021-23699-4
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