(544hj) Enhanced CO2 Electroreduction to CH4 and C2H4 Via Selective Proton Transfer | AIChE

(544hj) Enhanced CO2 Electroreduction to CH4 and C2H4 Via Selective Proton Transfer


Schreier, M. - Presenter, Massachusetts Institute of Technology
Surendranath, Y., Massachusetts Institute of Technology

CO2-derived fuels, synthesized using renewable energy,
present an attractive route towards fuel formation from sustainable energy. The
role of proton transfer in dictating the efficiency and selectivity of the
two-electron, two-proton reduction of CO2 to CO has been well-studied.
Indeed, impressive progress has been achieved in the production of CO from CO2.1–3 However, the role of proton
transfer to surface-bound CO intermediates in governing the selective
electrosynthesis of higher order products beyond CO, such as methane or
ethylene, remains poorly understood.

Here, we investigate the electrochemical reduction of CO at Cu
electrodes in nonaqueous electrolytes at low temperatures. The choice of the
electrolyte and reaction conditions allows fine-tuned control of the proton
donor and the CO binding strength, enabling for the first time activation-controlled
kinetic studies over an extended parameter range. Our studies lead to the
surprising finding that increasing the concentration of CO serves to suppress
methane production (Figure 1a), in addition to hydrogen (Figure 1b). In
contrast, ethylene production remains unaffected by CO concentration and
depends predominantly on the electrochemical potential (Figure 1c).

These observations provide unprecedented insight into the mechanism
of electrochemical CO and CO2 reduction (Figure 2). Among others, we
are able to show that the rate of methane and hydrogen formation is governed by
the competition of CO and H for surface sites and that ethylene formation is
taking place from sites which are saturated with CO, indicating that simply
increasing the supply of CO may not be a viable strategy for improving CO and
CO2 reduction systems. Based on our results, we subsequently
demonstrate how knowledge of the reaction mechanism can be exploited to selectively
suppress hydrogen evolution relative to CO reduction by rational tuning of the
proton donor.

1: Tafel plots for methane (a), hydrogen (b) and ethylene formation (c) as a
function of the partial pressure of CO.

2: Proposed mechanistic model for methane, ethylene and hydrogen production
during the electrochemical reduction of CO.

(1)      Schreier, M., Grätzel, M. et al. Nature
2017, 2 (7), 17087.

(2)      Schreier, M.; Grätzel, M. et al. Nature
2015, 6, 7326.

(3)      Schreier, M.; Grätzel, M. et al. J. Am. Chem. Soc. 2016, 138 (6),