Proton Transfer Promoted Electrochemical CO₂ Reduction With Aminocarboxylic Acids As Proton Relays

Rapid and convenient lifestyle improves the quality of life and makes people more and more enjoy it. Thus people pay much more attention to the environment they live in. One problem that we face now is the rising CO2 concentration in the air caused by the fossil fuel-dependent manner. It is not only lead to greenhouse effect which bringing us the gloable warming but also lead us to a series of negative environmental hazards. Therefore, the number of people who focus on how to transfer and use CO2 in an environmental friendly and energetic sustainable way is on the increase.It has a great potential and prospect in electrochemical reduction of CO2. However, on the other hand, there are also some challenges for electroreduction of CO2, such as energy-consumption and inefficiency. So developing efficient way of the electrochemical reduction of CO2 is imminent.Proton-coupled electron transfers (PCET) are ubiquitous in natural and synthetic processes. The association of proton transfer and electron transfer is a common and valid way to avoid the charge accumulation and energy waste that could occur upon successive reactions. CO2-, e- and H+ are all very important for the electrochemical reduction of CO2. Many researchers concerned more about the transformation from CO2 to CO2-which may neglect the equal importance of the following CO2- transforming to the product involved with e" and H+. Among the existing reports on PCET, we can see that PCET processes are often applied in oxygen reduction, while it seldom mentioned in electrochemical reduction of CO2. Thus, the proton transfer relay promotion strategy may have the potential to achieve efficient conversion of CO2 to the product. we may not only reduce the occurrence of hydrogen evolution but also increase the Faradaic efficiency. It has important academic significance and potential application value to carry out this work.In this paper, we mainly discussed about proton transfer promoted electrochemical reduction of CO2 with aminocarboxylic acids as proton relay. We picked EDTA as representative. Using bulk indium as the working electrode, we found the reduction efficiency was 2.8 times higher with the addition of EDTA. By the test of elecroactivity, the examination of rate-determined step and detection of the radicals by electron paramagnetic resonance (EPR) technique, we proposed EDTA as a proton relay to transfer the protons to CO2 radicals, thus promote CO2 reduction. In addition, we use several other aminocarboxylic acids to conduct the experiment for mechanism study. And it’s seemed the protonated amine played the key role as proton relay. What’s more, except bulk indium, several other bulk metals can also be improved the ability of reducting CO2 with no changing of the selectivity of products.For the deep understanding of mechanism the aminocarboxylic acids as proton relay enhancing the reduction of CO2, we use nano-Sn as the working electrode to reduct the CO2 within or with no EDTA. We found 2.3 times improvement of reduction of CO2 with EDTA. Using the technique including the test of elecroactivity and the examination of rate-determined step and joining with related aminocarboxylic acids, we thought EDTA could be as proton relay to enhance CO2 transforming to the product. After that, we compared two systems. It seemed the bulk indium had better result in reducting CO2 with the help of EDTA. We considered that because of weaker adsorption of EDTA on bulk indium which would not affect the adsorption of CO2.