New Insight into the Electrochemical CO2 Reduction Reaction: Radical Reactions Govern the Whole Process

In present paper, we wish to suggest a new reaction mechanism for the electrochemical reduction of carbon dioxide (CO2). This mechanism can provide a reasonable interpretation to the formation of widely range of products on copper (Cu) catalyst.

(O=C • -O-Cu) is the first step for the electrochemical CO 2 reduction, we can obtain an insightful explanation for the formation of a wide range of products.

Future Needs and Prospects
The electrochemical reduction of CO 2 yields not only carboncontaining products but also hydrogen (H 2 ), as Kuhl et al. observed.We also assume that another important radical, atomic hydrogen radical (H • ), is formed in the process of CO 2 reduction via an electron-transfer to the proton.It is assumed that the H 2 product is formed through the dimerization of two H • radicals.
In general, the H • radicals act as potent reducing agents, 11 which can react with other radicals or spin-paired intermediates as well.Taking into account the production of all products-CO, C 1 , C 2 , C 3 and H 2 , it is our assumption that each step in the product formation is a radical reaction in electrochemical CO 2 reduction, with the exception of the formation of the H • radicals, which is an electrochemical reaction step.
In this paper we will focus just on the formation of CO, C 1 , and C 2 products, which Kuhl et al. experimentally observed. 2The possible pathways for product formation are illustrated in the figure for relevant radical reactions shown.The reaction process of C 3 products or other C 1 and C 2 products reported by other researchers will be discussed in future publications.
In view of the formation of CO, we suggest it is formed by a radical elimination reaction.The radical (O=C  We assume that the formation of C 2 products initiates with radical dimerization-neighboring radicals (O=C In our scheme, two radicals (O=C • -O-Cu) must be adjacent to dimerize for C 2 product formation.However, the majority of radicals (O=C • -O-Cu) are isolated in actual reaction process because the electrochemical CO 2 reduction requires a media to dissolve gaseous CO 2 and those media could reduce the statistical probability of two O=C • -O-Cu radical forming on neighboring surface Cu atoms.This probably is the main reason that C 2 products usually have lower yields than CO and C 1 products.
Thus far, the formation pathways of CO, C 1 , and C 2 products from the radical (O=C • -O-Cu) have been displayed, clearly and simply.It seems to be quite clear and reasonable that radical reactions occurring on the radical (O=C • -O-Cu) govern the formation of various products: radical elimination forms CO, coupling with H • radical forms C 1 products, and dimerization of neighboring radicals (O=C • -O-Cu) forms C 2 products.The formation of C 1 products requires H • radicals at this determinative step, indicating that a high proton concentration in electrolyte may be of benefit for C 1 product formation due to the higher production of H • radicals.Conversely, electrolytes with low proton concentration might favor the formation of CO and C 2 products as the concentration of H • radicals would be lower in such conditions and can effectively avoid the O=C • -O-Cu reaction with H • radicals.
Our analyses, based on the proposal assumptions, seem to have an excellent agreement with experimental observations previously reported by other researchers.For example, Dinh et al. reported the high selectivity formation of CO and C 2 H 4 using the concentrated potassium hydroxide (KOH) electrolyte; 12 we believe this result occurs due to a scarcity of H • radicals, as low proton concentration in electrolyte hinders the formation of O=CH-O-Cu in determinative step, causing the radical (O=C • -O-Cu) predominantly undergo radical elimination and dimerization reactions.When using common aqueous electrolytes, such as KHCO 3 solution, the proton concentration in the vicinity of Cu surface is reduced under constant current conditions due to proton consumption.The reaction pathway thus gradually shifts from coupling with H • radicals to radical elimination or radical dimerization.This is the possible reason that a wide range of CO 2 reduction products is observed when using aqueous electrolyte.

Conclusions
Understanding the mechanisms responsible for product formation can provide critical insights into the design of effective reaction systems.In this paper, we have proposed the formation of the radical (O=C • -O-Cu) as initial step in CO 2 reduction process and discussed various product formation pathways.We believe all products (except hydrogen gas) observed from Cu catalyst should be derived from a singular intermediate and the proposed radical (O=C • -O-Cu) formed in initial step can provide clear and straightforward pathways for various product formation.
Moreover, electrochemical CO conversion could also occur over Cu metal catalyst. 13It is possible to extend our assumption to the reduction of CO into other chemicals and fuels.The initial conversion step may be the formation of radical ( • C=O-Cu), and subsequent radical reactions would lead to the formation of various products.We will discuss a fully detailed reaction process in future work.
The assumption of the radical (O=C • -O-Cu) formed in initial step and radical reactions involved in the subsequent steps as discussed above may be considered as reasonable interpretation in the description of the pathways of various product formation in electrochemical CO 2 reduction reaction.A correlation between them needs to be confirmed by further investigation.We believe that breakthroughs in catalytic mechanisms will significantly contribute to our ability to electrochemical recycle CO 2 in further centuries.

ORCID
Youyi Sun https://orcid.org/0000-0002-8960-0533 4 ).It is our opinion that the formation of these products starts with a radical coupling reaction in which the radical (O=C • -O-Cu) couples with a H • radical to create a spin-paired intermediate (O=CH-O-Cu).The formic acid may then be formed by abstraction of the radical (O=CH-O • ) from this intermediate by a H • radical.In addition, the O=CH-O-Cu intermediate also may undergo a radical addition reaction where the H • radical adds to the O=C group resulting in a new, carbon-centred radical (HO-C • H-O-Cu).This radical might then lead to the formation of the other two C 1 products (CH 3 OH and CH 4 ) via various stepwise reactions with the H • radicals as shown in Fig. 1 (Pathways for C 1 products).
Radical reaction pathways for the formation of CO, C 1 , and C 2 products in electrochemical CO 2 reduction reaction.The radical (H • ) is formed via electrochemical step.The solid ball represents the copper (Cu) atom.radical dimerize, a new spin-paired intermediate [Cu-OC(OH)=C (OH)O-Cu] would be generated.This intermediate may lead to the formation of ethylene (CH 2 CH 2 ) or vinyl alcohol (CH 2 CHOH) via further reactions with the H • radicals.Kuhl et al. did not observed vinyl alcohol in their study, but acetaldehyde (CH 3 CHO) instead, we assume that it is possibly due to the tautomerization of vinyl alcohol into acetaldehyde.
• -O-Cu) could z E-mail: yysunzjut@outlook.comECSAdvances, 2023 2 040503 spontaneously dimerize to form a spin-paired intermediate (Cu-OOC-COO-Cu) as showed in figure (Pathways for C 2 products).This intermediate contains two O=C groups, when one of which is reduced by the H • radical (via addition of H • radical to O=C), a radical [Cu-OC • (OH)-COO-Cu] would be generated.Acetic acid (CH 3 COOH) and glycolaldehyde (HOCH 2 CHO) may then be formed via stepwise reactions with the H • radicals.If both O=C groups are entirely reduced, a new radical [Cu-OC • (OH)-C • (OH)O-Cu] possessing two radical-centers would form.Then, following reactions with the H • radicals, partial C 2 products-ethylene glycol (HOCH 2 CH 2 OH) and ethanol (CH 3 CH 2 OH) may be produced via additional stepwise radical reactions.If, however, the formation of the Cu-OC • (OH)-C • (OH)O-Cu radical is followed by a radical dimerization-where the two radical centers of Cu-OC • (OH)-C • (OH)O-CuFigure 1.