Electrochemical Reduction of Carbon Dioxide on Graphene-Based Catalysts

Review Electrochemical Reduction of Carbon Dioxide on Graphene-Based Catalysts

Stefan Delgado , María del Carmen Arévalo, Elena Pastor * and Gonzalo García *

Instituto de Materiales y Nanotecnología, Departamento de Química, Universidad de La Laguna, P.O. Box 456, 38200 La Laguna, Santa Cruz de Tenerife, Spain; [email protected] (S.D.); [email protected] (M.d.C.A.) * Correspondence: [email protected] (E.P.); [email protected] (G.G.)

Abstract: The current environmental situation requires taking actions regarding processes for energy production, thus promoting renewable energies, which must be complemented with the development

of routes to reduce pollution, such as the capture and storage of CO2. Graphene materials have been chosen for their unique properties to be used either as electrocatalyst or as catalyst support (mainly for non-noble metals) that develop adequate efﬁciencies for this reaction. This review focuses on comparing experimental and theoretical results of the electrochemical reduction reaction of carbon

dioxide (ECO2RR) described in the scientiﬁc literature to establish a correlation between them. This work aims to establish the state of the art on the electrochemical reduction of carbon dioxide on graphene-based catalysts.

Keywords: CO2; graphene; HER; electrocatalysts; catalysis; Cu; nanoparticles

1. Introduction Citation: Delgado, S.; Arévalo, Over the last few decades, the drastic increase of carbon dioxide into the atmosphere M.d.C.; Pastor, E.; García, G. has caused global warming and the acidiﬁcation of the oceans, among other environmental Electrochemical Reduction of Carbon problems, thus leading to climate change. The accelerated consumption of fossil fuels Dioxide on Graphene-Based has been the priority factor in these problems, but it has not only caused a crisis in the Catalysts. Molecules 2021, 26, 572. environmental ﬁeld but also brought an energy crisis. This is a fact that also affects health https://doi.org/10.3390/ because air pollution is the sixth-leading cause of death, causing over two million prema- molecules26030572 ture deaths worldwide in addition to increasing asthma, respiratory diseases, diseases cardiovascular disease, cancer, etc. [1]. To put an end to this environmental situation, the Academic Editor: Byoung-Suhk Kim Paris Agreement, which aims to keep the global temperature rise below 2 ◦C by 2100 Received: 1 December 2020 and limit the temperature increase to 1.5 ◦C above pre-industrial levels, was drawn up in Accepted: 18 January 2021 Published: 22 January 2021 2015 [2]. For this, complementary strategies are needed to the transition towards renewable

Publisher’s Note: MDPI stays neutral energies, including the capture and storage of CO2, which help, together with natural with regard to jurisdictional claims in processes (photosynthesis and underground mineralization), to balance the presence of CO2 published maps and institutional affil- in the atmosphere [3]. Carbon capture and use (CCU) has arisen to reduce carbon emissions iations. by using CO2 as a raw material. Some examples include the production of methanol, dimethyl carbonate, and sodium carbonate; however, not all of these are sustainable. In this regard, most of these processes require significant energy inputs that carry carbon footprints themselves. Due to these problems related to economic viability, it is difficult to analyze the sustainability of CCU technology [4]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Therefore, it is necessary to develop innovative strategies that are capable of con- This article is an open access article verting carbon dioxide into products of energy value (e.g., hydrocarbons and alcohol), distributed under the terms and thus increasing its applications while reducing emissions. Among the possible conversion conditions of the Creative Commons products, formic acid has been proposed as a suitable material for the combination of Attribution (CC BY) license (https:// hydrogen storage and CO2 fixation. This compound is easy to store and transport, can be creativecommons.org/licenses/by/ used directly in direct formic acid fuel cells (DFAFCs), and releases H2 at room temperature 4.0/). using suitable metal catalysts [5–7]. Other oxygenated products, such as methanol and

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ethanol, are promising due to their high energy density, safety, and easy storage; additionally, like formic acid, they present the possibility of working directly in fuel cells. On the other hand, ethylene has enormous added value [8,9]. Of the different ways of converting CO2, chemical, photochemical, and electrochemical reductions stand out. Speciﬁcally, the electrochemical reduction process has been extensively investigated in recent years [10]. CO2 is a very stable molecule under environmental conditions, and its electrochemical reduction reaction (ECO2RR) is an endothermic process that requires the participation of multiple electrons and protons. Furthermore, the reaction intermediates of the different stages form various products in turn, depending on the material used as cathode and the applied potential (reduction potential values for Equations (1)–(10) are given relative to a reversible hydrogen electrode (RHE) at pH 7, 1.0 atm, and 25 ◦C in aqueous solution) [11,12]. Due to this, the selectivity for the desired products is not usually very high, and it is also conditioned by the hydrogen evolution reaction (HER; E0 = 0.000 V), which competes with the reaction under study. This currently raises the following challenges for the ECO2RR: the need for an increased reaction efﬁciency, the need for a decreased overpotential to overcome energy barriers and suppress the HER, and the need to obtain moderately high current densities for commercial applications [13,14].

+ − 0 CO2(g) + 2H + 2e → eHCOOH (l) E = −0.02 V (1)

+ − 0 CO2(g) + 2H + 2e → eeq (g) + H2O (l) E = −0.10 V (2) + − 0 CO2(g) + 8H + 8e → eCH4(g) + 2H2O (l) E = 0.17 V (3) + − 0 2CO2 + 12H + 12e → 2C2H4(g) + 4H2O (l) E = 0.08 V (4) + − 0 CO2(g) + 6H + 6e → eCH3OH(l) + H2O (l) E = 0.02 V (5) + − 0 2CO2(g) + 12H + 12e → 2C2H5OH(l) + 3H2O (l) E = 0.09 V (6) + − 0 CO2(g) + 4H + 4e → eeq Eq(l) + H2O (l) E = −0.07 V (7) + − 0 2CO2(g) + 2H + 2e → eH2C2O4(l) E = −0.50 V (8) + − 0 2CO2 + 14H + 14e → 4C2H6(g) + 4H2O (l) E = 0.14 V (9) − •− 0 CO2(g) + e → CO2 E = −1.900 V (10) For these reasons, current research is focused on the development of electrocatalysts that are chemically stable and inexpensive, have an appropriate shelf life, have an adequate product selectivity to promote their formation, and (ultimately) have the ability to operate at industry-acceptable current densities [13,15]. Various catalysts have been developed to improve the energy efficiency of the electrochemical reduction of CO2 with metals from the d-block (Cu, Co Pt, Fe, Au, Pd, Ag, Zn, Ni, etc.) and from the p-block (In, Sn, Pb, Bi, etc.) [10,16–20]. Specifically, Cu is a unique metallic catalyst for this reaction because it can efficiently reduce CO2 to hydrocarbons and oxygenates such as alcohols (either methanol or ethanol), carbon monoxide, and formic acid/format [8,9,21]. However, in certain cases, excessive overpotentials are required and current densities are not very high. Additionally, for other reasons such as toxicity and the cost of metals, it is intended to develop catalysts without noble metals that can generate the products with acceptable efficiency and conditions [7,22–24]. To achieve these catalysts, carbon- based materials are being investigated with special interest due to their wide range of possible nanostructures and because they may adsorb hydrogen and suppress the HER at low overpotentials [25]. Among the wide variety of these materials, graphene (G) and graphene materials (GMs) have become particularly important. In terms of electrochemical activity, pristine graphene is not active for the CO2 electroreduction. However, the electrocatalytic activity is enhanced after doping with oxygenated groups and other heteroatoms (Figure1 )[7,26]. Nitrogen was used with the first metal-free G-based catalyst for Molecules 2021, 26, x FOR PEER REVIEW 3 of 13 Molecules 2021, 26, 572 3 of 12

other heteroatoms (Figure 1) [7,26]. Nitrogen was used with the first metal-free G-based catalystthe for ECOthe ECO2RR,2RR, and and it is it the is mostthe most widely widely used used heteroatom heteroatom [22, 24[22,24,25],,25], although although there have there havebeen been studies studies with with others others such such as B as [23 B], [23], P [15 P], [15], S [7 ],S F,[7], Cl, F, Br,Cl, and Br, and I [11 I]. [11].

Figure 1. Different possibilities of heteroatom-doped graphene materials. Reproduced with permission [11]. Figure 1. Different possibilities of heteroatom-doped graphene materials. Reproduced with permission Reaction[11]. Mechanism: Key Intermediates, Rate-Determining Steps and Products The single electron reduction of CO to a CO •− radical occurs at E0 = −1.90 V vs. Reaction Mechanism: Key Intermediates, Rate-Determining2 Steps2 and Products RHE (Equation (10)) with a high energy contribution so that it accepts an electron, which Thecauses single the electron molecule reduction to lose itsof CO initial2 tolinear a CO2 structure●− radical andoccurs generate at E0 = a − bent1.90 radicalV vs. RHE anion. The (Equationlifetime (10)) ofwith this a radical high energy is very short,contribution thus making so that it it difficult accepts to an clearly electron, know which its structure. causes theDespite molecule this, itto has lose been its establishedinitial linear that, structure for Cu—which and generate is the mosta bent studied radical metal anion. (although The lifetimeit can of be this generalized radical is for very Ag andshort, Au, thus given making their similarityit difficult in to terms clearly of reactionknow its kinetics structure.and Despite electronic this, properties)—the it has been established bond angle that, andfor theCu—which distance is change, the most giving studied rise to the 2 •− metal (althoughcarboxylate it can intermediate be generalizedη (C,O)-CO for Ag2 andadsorbed Au, given on their the metal similarity surface in [terms11,27– 29of ]. reaction kineticsProton-coupled and electronic multi-electron properties)—the steps forbond the angle ECO2 RRandare the generally distance morechange, favorable giving risethan to single the carboxylate electron reductions intermediate due toη2 the(C,O)-CO more thermodynamically2•− adsorbed on the stablemetal moleculessurface that [11,27–29].are produced (Equations (1)–(9)). Furthermore, these reactions are influenced by the pH of Proton-coupledthe electrolyte, multi-electron and diverse species steps for may the therefore ECO2RR be are promoted generally by amore specific favorable concentration than singleof protons electron in reductions the solution due [30 ].to Thethe speciesmore thermodynamically produced depend onstable the catalystmolecules surface that nature, are producede.g., if (Equations the CO2 adsorption (1)–(9)). Furthermore, occurs trough these carbon, reactions OCHO* are is influenced obtained, by but the if itpH is trough of the electrolyte,oxygen, COOH* and diverse is formed species (* represents may therefore an adsorbed be promoted species). by Hydrogen a specific adsorptionconcen- and tration ofthe protons HER may in the also solution occur at [30]. the sameThe species potential produced range than depend the ECO on the2RR, catalyst and a competitionsur- face nature,between e.g., bothif the reactionsCO2 adsorption may consequently occurs trough happen. carbon, Then, OCHO* it is is possible obtained, to but reach if it a whole is troughseries oxygen, of C1 COOH*compounds is formed such as (* HCOOH, represents HCHO, an adsorbed CH3OH, species). and CH Hydrogen4 through bothad- routes. sorptionTo and produce the HER C2+ may(hydrocarbons also occur at and the oxygenate same potential species, range e.g., than C2H4 the,C2 ECOH6, and2RR, C and2H5 OH),a the competitioncoupling between of C1 bothintermediates reactions ismay required consequently so that a C–Chappen. bond Then, is formed it is [possible27,28,31 ].to reach a wholeAnother series important of C1 compounds adsorbed such intermediate as HCOOH, is *CO, HCHO, which isCH the3OH, common and CH species4 for throughmost both of routes. the products To produce that are C subsequently2+ (hydrocarbons generated and oxygenate because of species, its stability. e.g., Its C protonation2H4, C2H6, andcan C occur2H5OH), trough the coupling carbon in of order C1 intermediates to generate CHO*is required or trough so that oxygen a C–C to bond obtain is COH*, formed but[27,28,31]. the first option is more common. This protonation is one of the most important rate- Anotherdetermining important steps adsorbed [28]. It should intermediate be noted is that*CO, a strongwhich is CO* the bonding common poisons species the for catalyst, most ofand the products the HER that is preferred. are subsequently On the othergenerated hand, because weak CO*of its adsorptionstability. Its can protona- result in CO tion candesorption, occur trough and carbon the reduction in order will to therefore generate not CHO* continue or trough towards oxygen other Cto1 productsobtain [13]. COH*, but theFigure first2 depictsoption twois more pathways common. to generate This protonation *CHO. One ofis themone isof throughthe most the im- following portant species:rate-determining COOH*, CO*, steps and [28]. CHO*. It should The be other noted one that is through a strong the CO* following bonding species: poisons OCHO*, the catalyst,HCOOH* and the and HER CHO*. is preferred. In this regard, On thermodynamicsthe other hand, weak dictate CO that* adsorption OCHO* is morecan re- favorable than COOH*, although the latter has been more reported in the literature [27]. sult in CO desorption, and the reduction will therefore not continue towards other C1 products [13]. Figure 2 depicts two pathways to generate *CHO. One of them is through the following species: COOH*, CO*, and CHO*. The other one is through the following species: OCHO*, HCOOH* and CHO*. In this regard, thermodynamics dictate that OCHO* is

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Molecules 2021, 26, 572 4 of 12 more favorable than COOH*, although the latter has been more reported in the literature [27].

FigureFigure 2.2. SchemeScheme ofof thethe reactionreaction pathwayspathways andand keykey intermediatesintermediates for for different different CO CO22electroreduction electroreduction products.products. ReproducedReproduced withwith permission permission [ 32[32].]. HER:HER: hydrogenhydrogen evolution evolution reaction. reaction.

− − AnotherAnother crucialcrucial anionanion thatthat is is formed formed as as an an intermediate intermediate is is the the dimer dimer (CO) (CO)2 2,, which which hashas aa relevantrelevant rolerole in in thethe mechanismsmechanisms for for the the formation formation of of C C22species. species. ProtonProton transfertransferonly only occursoccurs onceonce the the dimer dimer has has formed; formed; consequently, consequently, dimer dimer formation formation is the is stepthe step that that limits limits the ratethe rate of the of reactionthe reaction for the for formation the formation of these of these species species with with two Ctwo atoms C atoms [30]. [30].

2.2. Graphene-BasedGraphene-Based CatalystsCatalysts 2.1. Metal-Free Graphene-Based Electrocatalysts 2.1. Metal-Free Graphene-Based Electrocatalysts Most of the works reported in the literature have studied the ECO2RR with graphene Most of the works reported in the literature have studied the ECO2RR with graphene materials before preparing the composite with the metal, thus serving as a reference. In materials before preparing the composite with the metal, thus serving as a reference. In general, the ECO2RR results of metal-free graphene materials are bad, and the efficiencies general, the ECO2RR results of metal-free graphene materials are bad, and the efficiencies are low. Thus, in the work of Huang et al., partially-oxidized, 5-nm cobalt nanoparticles are low. Thus, in the work of Huang et al., partially-oxidized, 5-nm cobalt nanoparticles (PO-5 nm Co) dispersed on a single-layer nitrogen-doped graphene (SL-NG) (denoted as (PO-5 nm Co) dispersed on a single-layer nitrogen-doped graphene (SL-NG) (denoted as PO-5 nm Co/SL-NG) were synthesized and employed for the ECO2RR [19]. SL-NG activity PO-5 nm Co/SL-NG) were synthesized and employed for the ECO2RR [19]. SL-NG activ- is even worse than for PO-Co, and only PO-Co/SL-NG is interesting due to the synergistic ity is even worse than for PO-Co, and only PO-Co/SL-NG is interesting due to the syn- effect that occurs after its combination. Something similar was obtained in the work of ergistic effect that occurs after its combination. Something similar was obtained in the Ning et al. with cuprous oxide supported on reduced graphene oxide (Cu2O/rGO) and work of Ning et al. with cuprous oxide supported on reduced graphene oxide cuprous oxide supported on nitrogen-doped reduced graphene oxide (Cu2O/N-rGO) [9]. The(Cu authors2O/rGO) added and cuprous additional oxide information supported with on data nitrogen-doped for rGO and N-rGO.reduced The graphene results wereoxide (Cu2O/N-rGO) [9]. The authors added additional information with data for rGO and not good either because for both GMs, the main product was H2. Because of these data, it N-rGO. The results were not good either because for both GMs, the main product was H2. can be concluded that these GMs are not active materials for the ECO2RR, even after being dopedBecause with of these N [9]. data, it can be concluded that these GMs are not active materials for the ECOOther2RR, even studies after have being confirmed doped with this N fact [9]. [ 33–35]. In the work of Xiong et al. [34], Cu- Sn catalystsOther studies were supported have confirmed on N-doped this fact graphene [33–35]. In (NG). the work For this of Xiong material, et al. the [34], majority Cu-Sn catalysts were supported on N-doped graphene (NG). For this material, the majority product in the ECO2RR is H2, and the faradaic efficiency (FE) for this gas reaches 79% at −product1.0 V vs. in RHE, the ECO while2RR it is decreases H2, and tothe 68% faradaic and 42%, efficiency respectively, (FE) for for this Cu/NG gas reaches and Sn/NG 79% at following−1.0 V vs. theRHE, previous while it trend decreases that proves to 68% the and low 42%, activity respectively, of GMs for theCu/NG reaction and underSn/NG studyfollowing [34]. the Additionally, previous trend in the that work proves of Hossain the low et activity al. [33], of in GMs which for they the reaction deposited under Cu nanoparticlesstudy [34]. Additionally, (NPs) on rGO, in the the work researchers of Hossain showed et al. that [33], the in catalytic which they activity deposited of rGO Cu is verynanoparticles similar to (NPs) that of on unsupported rGO, the researchers Cu NPs, both showed being that very the low catalytic (Figure activity3). of rGO is very similar to that of unsupported Cu NPs, both being very low (Figure 3).

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Figure 3. Linear sweep voltammogram (LSV) for glassy carbon (GC), rGO, Cu nanoparticles (NPs), Figure 3. Linear sweep voltammogram (LSV) for glassy carbon (GC), rGO, Cu nanoparticles (NPs), Figure 3. Linear sweep voltammogram (LSV) for glassy carbon (GC), rGO, Cu nanoparticles−1 (NPs), and Cu/rGO in a 0.1 M NaHCO3 solution at a scan rate of 20 mVs . Reproduced−1 with permission and Cu/rGO in a 0.1 M NaHCO solution at a−1 scan rate of 20 mVs . Reproduced with permis- and Cu/rGO in [33].a 0.1 RHE:M NaHCO reversible3 solution hydrogen at a scan electrode. rate3 of 20 mVs . Reproduced with permission [33]. RHE: reversiblesion hydrogen [33]. RHE: electrode. reversible hydrogen electrode. Chen et al. developed a 2D/0D composite catalyst formed from bismuth oxide Chen et al. developedChen et a al. 2D/0D developed composit a 2D/0De catalyst composite formed catalystfrom bismuth formed oxide from bismuth oxide nanosheets and nitrogen-doped graphene quantum dots (Bi2O3-NGQDs) [35]. The elec- nanosheets and nanosheetsnitrogen-doped and nitrogen-doped graphene quantum graphene dots quantum(Bi2O3-NGQDs) dots (Bi [35].2O3-NGQDs) The elec- [35]. The electro- trochemical response of Bi2O3 and Bi2O3-NGQDs is shown in Figure 4 and is compared chemical response of Bi2O3 and Bi2O3-NGQDs is shown in Figure4 and is compared with trochemical responsewith that of of Bi the2O3 grapheneand Bi2O 3material,-NGQDs NGQDs.is shown By in comparingFigure 4 and the is current compared densities and the that of the graphene material, NGQDs. By comparing the current densities and the FEs for with that of theFEs graphene for formate material, production, NGQDs. it was By foundcomparing that thethe activity current ofdensities the NGQDs and theis extremely low, FEs for formate production,formate production, it was found it was that found the thatactivity the of activity the NGQDs of the NGQDsis extremely is extremely low, low, with an withFE ofan aroundFE of around 15%, while 15%, while the Bi theO Biand2O3 Bi andO Bi-NGQD2O3-NGQD present present FEs closeFEs close to 85% to 85% and 100%,and with an FE of around 15%, while the Bi2O3 and 2Bi23O3-NGQD2 3present FEs close to 85% and 100%, respectively, at a potential− of −0.9 V (vs. RHE) [35]. 100%, respectively,respectively, at a potential at a potentialof −0.9 V of(vs. RHE)0.9 V (vs.[35]. RHE) [35].

FigureFigure 4. 4.FaradaicFaradaic efficiency efﬁciency (FE) (FE) and and formate partialpartial currentcurrent densitydensity for for Bi Bi2O2O3-NGQD3-NGQD (nitrogen- (nitro- 2 3 Figure 4. Faradaicgen-dopeddoped efficiency graphene graphene (FE) quantumand quantum formate dot), dot), partial Bi2 BiO32O, and3current, and NGQD NGQD density at differentat fordifferent Bi potentialsO -NGQDpotentials (upper(nitro- (upper panels) panels) and and long- 2 3 gen-doped graphenelong-termterm quantum electrolysis electrolysis dot), experiments BiexperimentsO , and ofNGQD Biof2 BiO23 Oat-NGQD3 -NGQDdifferent recorded recorded potentials at at− −0.7(upper0.7 V V and and panels)− −1.0 Vand (bottom panel) in a long-term electrolysis experiments of Bi2O3-NGQD recorded at −0.7 V and −1.0 V (bottom panel) in a 0.50.5 M M KHCO KHCO33 electrolyte.electrolyte. Reproduced Reproduced with with permission permission [35]. [35]. a 0.5 M KHCO3 electrolyte. Reproduced with permission [35].

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Molecules 2021, 26, x FOR PEER REVIEW 6 of 13 Despite the low activities shown in the works mentioned above, there are several Molecules 2021, 26, x FOR PEER REVIEW 6 of 13 strategies to design efficient metal-free electrocatalysts for the ECO2RR. For instance, Wu et al.Despite prepared the low NGQDs activities thatshown exhibit in the aworks high mentioned total carbon above, dioxide there are reduction several efficiency of strategiesup to 90%, to design with efficient selectivity metal-free for conversions electrocatalysts to for ethylene the ECO and2RR. ethanolFor instance, reaching Wu 45% in 1 M Despite the low activities shown in the works mentioned above, there are several etKOH al. prepared [36]. Furthermore,NGQDs that exhibit the amountsa high total of carbon C2 and dioxide C3 species reduction produced efficiency areof up comparable to strategies to design efficient metal-free electrocatalysts for the ECO2RR. For instance, Wu tothose 90%, with of electrocatalysts selectivity for conversions based on Cuto ethylene NPs. The and great ethanol difference reaching in45% the in results 1 M employing et al. prepared NGQDs that exhibit a high total carbon dioxide reduction efficiency of up KOH [36]. Furthermore, the amounts of C2 and C3 species produced are comparable to tosimilar 90%, with materials selectivity may for be conversions related to to the ethylene synthetic and method,ethanol reaching the pH, 45% and in the1 M counterion, thoseamong of electrocatalysts other factors. based on Cu NPs. The great difference in the results employing similarKOH [36].materials Furthermore, may be relatedthe amounts to the ofsy ntheticC2 and method,C3 species the produced pH, and arethe comparablecounterion, to Other significant work is reported by Han and col. that prepared graphene (G), amongthose ofother electrocatalysts factors. based on Cu NPs. The great difference in the results employing similarnitrogen-dopedOther materials significant may graphene work be relatedis reported (NG), to the edge-richby syHannthetic and graphene col.method, that preparedthe (EG), pH, and andgraphene defectivethe counterion, (G), ni- graphene (DG). trogen-dopedamongDG was other synthesized graphenefactors. (NG), through edge-rich the elimination graphene (EG), of theand nitrogen defectiveatoms graphene from (DG). an NG structure, DGwhich wasOther synthesized caused significant many through work topological theis reported elimination defects by ofHan the thatand nitrogen col. offered that atoms prepared abundant from an graphene NG catalytically structure, (G), ni- active sites, trogen-doped graphene (NG), edge-rich graphene (EG), and defective graphene (DG). whichhigh caused electronic many conductivity, topological defects and strongthat offered CO2 adsorptionabundant catalytically [7]. This active work sites, revealed the great DG was synthesized through the elimination of the nitrogen atoms from an NG structure, highpotential electronic of conductivity, DG as a metal-free and strong GMCO2 adsorption for the ECO [7]. ThisRR. work DG revealed exhibited the angreat excellent FE of which caused many topological defects that offered abundant2 catalytically active sites, potential84% for of CODG as production a metal-free at GM− 0.6for the V (vs.ECO RHE),2RR. DG with exhibited a smaller an excellent overpotential FE of 84% and a higher forhigh CO electronic production conductivity, at −0.6 V (vs. and RHE), strong with CO 2a adsorption smaller overpotential [7]. This work and revealed a higher the elec- great electrocatalytic activity toward the ECO22RR in comparison with the rest of materials trocatalyticpotential of activity DG as towarda metal-free the ECO GM2 RRfor inthe comparison ECO RR. DG with exhibited the rest anof materialsexcellent FE(Figure of 84% 5).for(Figure CO production5). at −0.6 V (vs. RHE), with a smaller overpotential and a higher electrocatalytic activity toward the ECO2RR in comparison with the rest of materials (Figure 5).

−1 −1 FigureFigure 5. LSV 5. LSVcurves curves in a 0.1 inM KHCO a 0.1 M3 electrolyte KHCO3 (theelectrolyte scan rate was (the 10 scan mVs rate) and was faradaic 10 mVs effi- ) and faradaic

cienciesefﬁciencies to CO at to different CO at applied different potentials applied on potentials pristine graphene, on pristine nitrogen-doped graphene, graphene nitrogen-doped (NG), graphene edge-richFigure 5. graphene LSV curves (EG), in anda 0.1 de Mfective KHCO graphene3 electrolyte (DG). (the Reproduced scan rate was with 10 permission mVs−1) and [7]. faradaic effi- (NG), edge-rich graphene (EG), and defective graphene (DG). Reproduced with permission [7]. ciencies to CO at different applied potentials on pristine graphene, nitrogen-doped graphene (NG), edge-richFurthermore, graphene the (EG), DG and electrocatalyst defective graphene was relati (DG).vely Reproduced stable after with 10 permission h of the continuous [7]. ECO2RRFurthermore, at −0.6 V (vs. theRHE) DG (Figure electrocatalyst 6a). In addition, was relatively Figure 6b stablereveals after the operating 10 h of the continuous mechanismECOFurthermore,2RR and at − the0.6 thebest V DG (vs.performance electrocatalyst RHE) (Figure of the was DG6 relatia). catalyst. Invely addition, stable Indeed, after Figurea Tafel10 h of 6slopeb the reveals continuousvalue of the operating 139ECOmechanism mV·dec2RR at−1 −was0.6 and Vobtained the(vs. bestRHE) for performance DG,(Figure which 6a). indicates In of addition, the the DG initial Figure catalyst. electronic 6b reveals Indeed, transfer the a operating Tafel to the slope value of − •− adsorbedmechanism139 mV CO·dec 2and molecule1 thewas best obtained to performance form the for intermediate DG, of the which DG catalyst.CO indicates2 as Indeed, the the rate-determining initiala Tafel electronicslope value step transfer of to the (RDS).139 mV·dec The rest−1 was of theobtained catalysts for DG,showed which higher indicates Tafel theslope initial •−values electronic that indicated transfer tothe the adsorbed CO2 molecule to form the intermediate CO2 as the rate-determining step (RDS). •− sloweradsorbedThe restkinetics ofCO the 2for molecule catalyststhe RDS to[7]. showed form the higher intermediate Tafel slope CO2 values as the that rate-determining indicated the step slower kinetics (RDS). The rest of the catalysts showed higher Tafel slope values that indicated the for the RDS [7]. slower kinetics for the RDS [7].

Figure 6. (a) Time-dependent total current density curve (left y-axis) and faradic efﬁciency for CO

(right y-axis) at DG at −0.6 V (vs. RHE) in 0.1 M KHCO3.(b) Tafel plots for all catalysts. Reproduced with permission [7]. Molecules 2021, 26, x FOR PEER REVIEW 7 of 13

Figure 6. (a) Time-dependent total current density curve (left y-axis) and faradic efficiency for CO Molecules 2021, 26, 572 (right y-axis) at DG at −0.6 V (vs. RHE) in 0.1 M KHCO3. (b) Tafel plots for all catalysts. Reproduced7 of12 with permission [7].

2.2. Graphene-Supported Metal Nanoparticles 2.2. Graphene-SupportedThe results described Metal in Nanoparticlesthe bibliography show that each metal catalyst exhibits differencesThe results in terms described of efficiency in the and bibliography selectivity showto reduce that eachCO2 to metal different catalyst products. exhibits However,differences when in terms graphene of efficiency materials andare used selectivity as metal to support, reduce COthere2 to is a different reduction products. in the freeHowever, energy when of the graphene reaction in materials comparison are used with asunsupported metal support, metal there catalysts is a reduction [16,18,27,37]. in the Byfree applying energy of DFT the reaction(density in functional comparison theory) with unsupportedcalculations, Lin metal et catalystsal. showed [16 ,that18,27 the,37 ].in- By teractionapplying DFTwith (densitygraphene functional produces theory) a strong calculations, influence Linon etthe al. formation showed that of theCOOH* interaction and withCHO* graphene species, as produces well as athe strong suppression influence of onHCOOH the formation (Figure of7) COOH*[18]. With and a higher CHO* Fermi species, levelas well than as CO the2 molecules, suppression the of upper HCOOH graphene (Figure layer7)[ injects18]. With electrons a higher into the Fermi nonbonding level than orbitalsCO2 molecules, of COOH* the and upper CHO*, graphene resulting layer in injects charge electrons redistribution into the and nonbonding electrostatic orbitals inter- of actionCOOH* between and CHO*, the molecules resulting inand charge graphene. redistribution The last andreduces electrostatic the free energies interaction of betweenthe reactionthe molecules and the onset and graphene. potentials Theof crucial last reduces reaction the steps free (Figure energies 7b). of Figure the reaction 7a shows and the the decreaseonset potentials in the free of crucialenergy reactiondue to the steps incorporation (Figure7b). of Figuregraphene;7a shows the reaction the decrease is conse- in quentlythe free more energy thermodynamically due to the incorporation favorable of for graphene; both pathways the reaction (COOH* is consequently and CHO*), with more thethermodynamically effect on the production favorable of forhydrocarbons both pathways being (COOH* more significant and CHO*), in comparison with the effect with on HCOOHthe production [18]. Interestingly, of hydrocarbons these being theoretical more significant results have in comparisonbeen confirmed with with HCOOH experi- [18]. mentalInterestingly, studies these [9,38]. theoretical results have been confirmed with experimental studies [9,38].

Figure 7. (a) A schematic freefree energyenergy diagram.diagram. ((bb)) OnsetOnset potentialspotentials for for the the electrochemical electrochemical reduction reduc- tionreaction reaction of carbon of carbon dioxide dioxide (ECO (ECO2RR)2RR) on on metal metal surfaces surfaces with with and and withoutwithout aa graphene overlay- overlayer. er.Reproduced Reproduced with with permission permission [18 [18].].

Hossain et et al. al. designed designed an an easy easy procedure procedure to to synthesize synthesize a ananostructured nanostructured thin thin film ﬁlm comprising NPsNPs ofof Cu Cu on on rGO rGO obtained obtained by theby directthe direct electrochemical electrochemical reduction reduction of a mixture of a mixtureof copper of and copper GO precursorsand GO precursors [33]. The Cu[33]. (NP)/rGO The Cu (NP)/rGO ﬁlm exhibited film excellent exhibited stability excellent and stabilitycatalytic and activity catalytic for the activity electrochemical for the electrochemical reduction of CO reduction2 in an aqueous of CO2 solution,in an aqueous almost solution,tripling the almost current tripling densities the current in comparison densities with in comparison unsupported with Cu NPsunsupported (Figure3 ).Cu Carbon NPs (Figuremonoxide 3). Carbon and formate monoxide were and found formate as the were main found products as the by main chromatography, products by chroma- and the tography,achieved FEand was the close achieved to 69% FE at was−0.6 close V (vs. to 69% RHE, at reference−0.6 V (vs. hydrogen RHE, reference electrode) hydrogen [33]. On electrode)the other hand, [33]. On nanocubes the other of hand, cuprous nanocubes oxide (Cu of2 O)cuprous incorporated oxide (Cu into2O) rGO incorporated and Cu2O (rGO)into rGOrevealed and promisingCu2O (rGO) results revealed [9]. Figurepromising8a,b compareresults [9]. the Figure FEs achieved 8a,b compare at glassy the carbon- FEs achievedsupported at Cuglassy2O and carbon-supported Cu2O/rGO, respectively. Cu2O and Cu The2O/rGO, main respectively. outcomes indicated The main that out- the comesincorporation indicated of that the graphenethe incorporation material of raised the graphene the production material of rais COed and the H 2productionand increased of COthe stabilityand H2 and of Cuincreased2O nanocubes. the stability of Cu2O nanocubes. The incorporation of a second metal has been considered in theoretical calculations [31,39]. In order to modulate the interaction between the bimetallic catalyst and the graphene-based support (and therefore modify the catalytic activity), Cu, Ni, Pd, Pt, Ag, and Au were combined and compared with monometallic dimers by calculation [31]. It was found that Pt2, AgNi, Pd2, and AgPt supported on defective graphene revealed the lowest overpotential values for the ECO2RR [31]. On the other hand, transition metal dimers (Cu2, CuMn, and CuNi) introduced into single graphene vacancies (called @ 2SV) were studied for the ECO2RR by DFT [39]. All samples revealed a low overpotential, a high FE, and selectivity for species production during the ECO2RR. In this regard, Cu2 @ 2SV,MnCu @ 2SV,and NiCu @ 2SV promoted the formation Molecules 2021, 26, 572 8 of 12

Molecules 2021, 26, x FOR PEER REVIEW 8 of 13 of CO, CH4, and CH3OH, respectively. Results were explained in terms of the oxophilicity of Ni and Mn [39].

FigureFigure 8. 8. FaradaicFaradaic efficiency efﬁciency of of the the gas gas products products for for ( (aa)) Cu Cu22O,O, (b (b) )Cu Cu2O/rGO,2O/rGO, and and (c) ( cCu) Cu2O/N-rGO2O/N-rGO inin a a 0.1 0.1 M M KHCO KHCO3 3solution.solution. Reproduced Reproduced with with permission permission [9]. [9 ].

TheBimetallic incorporation catalysts of havea second also metal been employedhas been considered in experimental in theoretical studies. calculations For instance, [31,39].Pd-Cu/graphene In order to catalysts modulate with the diverseinteraction Pd-Cu between wt% ratiosthe bimetallic were synthesized catalyst and from the graphite graphene-basedoxide suspensions support and (and metal theref precursorore modify salts the by applyingcatalytic activity), the sodium Cu, borohydride Ni, Pd, Pt, Ag, reduction and Aumethod were [combined38]. Particle and sizes compared ranging with from monome 8 to 10tallic nm were dimers acquired, by calculation and the [31]. best It catalytic was foundperformance that Pt2, towardAgNi, Pd the2, and ECO AgPt2RR supported was achieved on defective with 1 graphene wt% Pd–2 revealed wt% Cu/graphene the lowest overpotentialmaterial [38]. values for the ECO2RR [31]. On the other hand, transition metal dimers (Cu2, InCuMn, addition and toCuNi) copper-based introduced catalysts, into single WC graphene [13], Bi vacancies [17], In2O (called3 [37], @ and 2SV) Pd-In were [40 ] studiedmaterials for were the ECO supported2RR by onDFT GMs, [39]. andAll samples a good catalytic revealed performance a low overpotential, toward a the high ECO FE,2RR andhas beenselectivity accomplished. for species Forproduction instance, during three-dimensional the ECO2RR. In Pd/graphene this regard, (Pd/3D-rGO),Cu2 @ 2SV, MnCuIn/3D-rGO, @ 2SV, and and Pd-In/3D-rGO NiCu @ 2SV promoted catalysts were the formation prepared byof aCO, mild CH method4, and CH that3OH, combined respectively.chemical and Results hydrothermal were explained steps in [40 terms]. 3D-rGO of the oxophilicity materials are of 3DNi and structures Mn [39]. with a high densityBimetallic of interconnected catalysts have pores also and been metal employed NPs anchored in experimental on the folds. studies. Interestingly, For instance, Pd 0.5- Pd-Cu/grapheneIn0.5/3D-rGO showed catalysts the with smallest diverse particle Pd-C size,u wt% and ratios its dispersion were synthesized on rGO sheets from was graph- found iteto beoxide more suspensions homogeneous and thanmetal monometallic precursor sa catalystslts by applying of Pd and the In.sodium These borohydride characteristics reductionenhance the method performance [38]. Particle for the sizes ECO ranging2RR, decreasing from 8 to 10 the nm overpotential were acquired, and and increasing the best the catalyticFE to 85.3% performance at −1.6 V (vs.toward Ag/AgCl) the ECO in 0.52RR M KHCOwas achieved3 for formate with production 1wt% Pd–2 [40 ].wt% Cu/graphene material [38]. 2.3. MetalIn addition Nanoparticles to copper-based Supported ca ontalysts, Heteroatom-Doped WC [13], Bi Graphene [17], In2O Materials3 [37], and Pd-In [40] materialsThe introductionwere supported of heteroatoms on GMs, and into a good the graphenecatalytic performance structure may toward improve the the ECO catalytic2RR has been accomplished. For instance, three-dimensional Pd/graphene (Pd/3D-rGO), activity towards the ECO2RR. For example, Figure8c shows that Cu 2O supported on N- In/3D-rGO, and Pd-In/3D-rGO catalysts were prepared by a mild method that combined doped rGO (Cu2O/N-rGO) enhances the FE up to 70%, raises the production of ethylene chemicalat −1.4 Vand (vs. hydrothermal RHE), and increasessteps [40]. the3D-rGO catalytic materials stability. are 3D Based structures on these with results, a high the density of interconnected pores and metal NPs anchored on the folds. Interestingly, authors concluded that pyridine nitrogen improves the catalytic stability of Cu2O due to N Pd(pyridine)–Cu0.5-In0.5/3D-rGO interactions showed the [9 ].smallest particle size, and its dispersion on rGO sheets was foundCuSn to be alloymore nanoparticleshomogeneous supportedthan monometa on anllic NG catalysts material of Pd stand and out In. dueThese to charac- their low teristics enhance the performance for the ECO2RR, decreasing the overpotential and in- cost and unique catalytic activities towards the ECO2RR. Nitrogen doping into graphene creasing the FE to 85.3% at −1.6 V (vs. Ag/AgCl) in 0.5 M KHCO3 for formate production [40].

Molecules 2021, 26, x FOR PEER REVIEW 9 of 13

2.3. Metal Nanoparticles Supported on Heteroatom-Doped Graphene Materials The introduction of heteroatoms into the graphene structure may improve the catalytic activity towards the ECO2RR. For example, Figure 8c shows that Cu2O supported on N-doped rGO (Cu2O/N-rGO) enhances the FE up to 70%, raises the production of ethylene at −1.4 V (vs. RHE), and increases the catalytic stability. Based on these results, the authors concluded that pyridine nitrogen improves the catalytic stability of Cu2O due Molecules 2021, 26, 572 to N (pyridine)–Cu interactions [9]. 9 of 12 CuSn alloy nanoparticles supported on an NG material stand out due to their low cost and unique catalytic activities towards the ECO2RR. Nitrogen doping into graphene sheets results in the strong binding of CuSn NPs to the catalyst support. The strong sheetsbinding results prevents in thealloy strong NP bindingdetachment of CuSnand NPsagglomeration, to the catalyst thereby support. improving The strongthe bindinglong-term prevents stability. alloy Furthermore, NP detachment electron and transfer agglomeration, from NG to thereby CuSn NPs improving appears the to long-be termbeneficial stability. for the Furthermore, adsorption electron of CO2 transferand/or fromhydrogen, NG to thus CuSn enhancing NPs appears the toCO be2RR beneﬁcial per- forformance the adsorption [34]. of CO2 and/or hydrogen, thus enhancing the CO2RR performance [34]. Figure 99 depictsdepicts thethe distributiondistribution of produc productsts generated generated during during this this reaction reaction as as a a function of of the the applied applied potential potential for for differen differentt copper–tin copper–tin ratios ratios on on NG, NG, and and an an optimal optimal composition was was obtained obtained for for Cu-Sn Cu-Sn0.1750.175. This. This alloy alloy generated generated C1 C products1 products with with an anFE FE close to 93% at −−1.01.0 V V (vs. (vs. RHE), RHE), which which was was the the highest highest of ofall all prepared prepared materials. materials. This This led led to considerable improvements improvements over over Cu Cu and and Sn Sn separately, separately, which which were were found found to todevelop develop FEs of 32%32% andand 58%,58%, respectivelyrespectively [34[34].]. ToTo clarify clarify the the experimental experimental results, results, DFT DFT simulations simula- oftions the of free the energy free energy for hydrogen for hydrogen adsorption adsorption and CO 2andreduction CO2 reduction were carried were out carried (Figure out 10 ). The(Figure main 10). results The main indicated results that indicated hydrogen that canhydrogen be easily can adsorbed be easily onadsorbed Sn surfaces, on Sn but sur- the freefaces, energies but the becomefree energies higher become after Cu higher introduction, after Cu andintroduction, the HER isand accordingly the HER is suppressed. accord- Afteringly suppressed. that, more H After atoms that, can more participate H atoms in can the participate ECO2RR processes in the ECO [342RR]. processes [34].

Molecules 2021, 26, x FOR PEER REVIEWFigure 9. FaradaicFaradaic efficiency efﬁciency of of Cu-Sn Cu-Sn NPs NPs on on NG NG with with various various ratios ratios of of Cu Cu to to Sn Sn at at −−1.01.0 V V (vs.10 (vs. of RHE)13

forRHE) the for ECO the2 RRECO (0.52RR M (0.5 KHCO M KHCO3). Reproduced3). Reproduced with with permission permission [34]. [34].

Figure 10. Left panel: Schematic Gibbs free energyenergy proﬁleprofile forfor thetheH H adsorption adsorption on on different different surfaces. surfaces.Right panel:Right panel: schematic schematic Gibbs freeGibbs energy free proﬁleenergy forprofile the detailed for the COdetailed2RR pathway CO2RR pathway on the surface on the (101) surfaceof three (101) models. of three Reproduced models. Re withproduced permission with [permission34]. [34].

OnOn the the other other hand, hand, Figure Figure 1 111aa illustrates illustrates the the synergy synergy between between the the catalyst catalyst and and the the catalyst support support as as an an important important increment increment of of the the faradaic faradaic current current discerned discerned at atPO-5 PO-5 nm nm Co/SL-NG catalyst. catalyst. Additionally, Additionally, the the catalyst catalyst revealed revealed remarkable remarkable stability, stability, even evenafter 10 after h10 of h ofCO CO2 electrolysis,2 electrolysis, both both in current in current and and in inmorphology, morphology, particle particle size, size, structure, structure, and and metal content, with an FE above 70% at −0.90 V (vs. SCE (saturated calomel electrode)) for methanol production (Figure 11b) [19].

Figure 11. (a) Linear sweep voltammograms of single-layer nitrogen-doped graphene (SL-NG), 5-nm cobalt nanoparticles (PO-5 nm Co) and PO-5 nm Co/SL-NG in 0.1 M NaHCO3 at 20 mV s−1. (b) FE of methanol and current density for PO-5 nm Co/SL-NG under various electrolysis potentials for 10 h. Reproduced with permission [19].

As stated above, Chen et al. developed a 2D/0D composite catalyst formed by Bi2O3-NGQDs [34]. This material exhibits an FE of almost 100% for formate production with a moderate potential of −1.0 V (vs. RHE) and good stability (Figure 4). Results were attributed through DFT calculations to an increment of the adsorption energy of CO2 and OCHO* after the combination of Bi2O3 with NGQDs (Figure 12b). Figure 12a illustrates the free energy diagram for the HCOOH formation process on Bi2O3 and Bi2O3-NGQDs materials, which reveals the highest energy barrier for the OCHO* formation that intensely falls for Bi2O3-NGQDs. Furthermore, the energy barrier for the HER increases for MG, thus explaining the preference of Bi2O3-NGQDs material to generate formate over H2 [35].

Molecules 2021, 26, x FOR PEER REVIEW 10 of 13

Figure 10. Left panel: Schematic Gibbs free energy profile for the H adsorption on different surfaces. Right panel: schematic Gibbs free energy profile for the detailed CO2RR pathway on the surface (101) of three models. Reproduced with permission [34].

Molecules 2021, 26, 572 On the other hand, Figure 11a illustrates the synergy between the catalyst and the10 of 12 catalyst support as an important increment of the faradaic current discerned at PO-5 nm Co/SL-NG catalyst. Additionally, the catalyst revealed remarkable stability, even after 10 h of CO2 electrolysis, both in current and in morphology, particle size, structure, and metal content, with an FE above 70% at −0.90 V (vs. SCE (saturated calomel electrode)) for metal content, with an FE above 70% at −0.90 V (vs. SCE (saturated calomel electrode)) for methanol production (Figure 11b) [19]. methanol production (Figure 11b) [19].

Figure 11. (a) Linear sweep voltammograms of single-layer nitrogen-doped graphene (SL-NG), 5-nm Figure 11. (a) Linear sweep voltammograms of single-layer nitrogen-doped graphene (SL-NG),−1 cobalt nanoparticles (PO-5 nm Co) and PO-5 nm Co/SL-NG in 0.1 M NaHCO3 at 20 mV s .(b) FE 5-nm cobalt nanoparticles (PO-5 nm Co) and PO-5 nm Co/SL-NG in 0.1 M NaHCO3 at 20 mV s−1. (b) of methanol and current density for PO-5 nm Co/SL-NG under various electrolysis potentials for FE of methanol and current density for PO-5 nm Co/SL-NG under various electrolysis potentials 10forh. 10 Reproduced h. Reproduced with with permission permission [19 [19].].

As statedstated above,above, ChenChen etet al. al. developed developed a 2D/0Da 2D/0D composite composite catalyst catalyst formed formed by by Bi 2O3- NGQDsBi2O3-NGQDs [34]. [34]. This This material material exhibits exhibits an FEan ofFE almostof almost 100% 100% for for formate formate production production with awith moderate a moderate potential potential of −of1.0 −1.0 V V (vs. (vs. RHE)RHE) and good good stability stability (Figure (Figure 4).4 ).Results Results were were attributed through through DFT DFT calculations calculations to to an an increment increment of ofthe the adsorption adsorption energy energy of CO of2 CO and2 and OCHO* after the the combination combination of of Bi Bi2O2O3 with3 with NGQDs NGQDs (Figure (Figure 12b). 12 b).Figure Figure 12a 12illustratesa illustrates thethe free energy diagram diagram for for the the HCOOH HCOOH formation formation process process on on Bi Bi2O23O and3 and Bi2O Bi32-NGQDsO3-NGQDs materials, whichwhich revealsreveals thethe highesthighest energyenergy barrier barrier for for the the OCHO* OCHO* formation formation that that intensely in- Molecules 2021, 26, x FOR PEER REVIEWfalls for Bi O -NGQDs.2 3 Furthermore, the energy barrier for the HER increases11 of for 13 MG, tensely falls2 for3 Bi O -NGQDs. Furthermore, the energy barrier for the HER increases for thusMG, explainingthus explaining the preference the preference of Bi 2ofO 3Bi-NGQDs2O3-NGQDs material material to generate to generate formate formate over over H2 [35]. H2 [35].

Figure 12. (a) Free-energy diagram and (b) adsorption energy of CO , OCHO*, and HCOOH (ads) Figure 12. (a) Free-energy diagram and (b) adsorption energy of CO2, OCHO*,2 and HCOOH (ads) forfor Bi Bi22OO3 3andand Bi Bi2O23O-NGQDs.3-NGQDs. Reproduced Reproduced with with permission permission [35]. [35].

3.3. Perspective Perspective In this review, the general concept of the CO reduction reaction was discussed, In this review, the general concept of the CO2 reduction2 reaction was discussed, with specialwith special attention attention paid paidto low to lowtemperature temperature electrolysis electrolysis with withgraphene-based graphene-based electrode electrode materials.materials. TheThe conversion conversion of of CO CO2 to2 to chemicals chemicals usable usable as fuels as fuels via viaroom room temperature temperature electrolysis electrolysis with graphene-based graphene-based catalysts catalysts is isa very a very attr attractiveactive technology technology for forenergy energy conversion conversion and and storage.storage. This This technology technology has has the the po potentialtential to to reduce reduce the the produced produced CO CO2 2throughthrough its its con- conver- versionsion into into valuable valuable energy energy carriers carriers (hydrocarbons, (hydrocarbons, alcohols, alcohols, or or CO-rich CO-rich feeds) feeds) thatthat willwill help helpto improve to improve environmental environmental contamination. contamination. WithWith the the aim aim to to solve solve the the principal principal catalytic catalytic problem problem at the at the cathodes cathodes of low of low temper- tempera- atureture electrolyzers,electrolyzers, thethe fundamentalfundamental study of carbon carbon dioxide, dioxide, as as well well as as hydrogen hydrogen evolu- evolution tionon graphene-based on graphene-based materials materials in ain wide a wide pH pH range, range, was was considered. considered. In general, the results discussed in this manuscript confirm that graphene materials are excellent supports, producing synergistic effects that cause improvements in the efficiency and selectivity of the catalysts used for the electrochemical reduction of CO2. However, in most cases, the activity of graphene not modified by metal species is very low. All the new advances in the fundamental understanding of reactions occurring on graphene-based catalysts presented here may help to improve the fabrication of novel electrodes in order to enhance the performance and decrease the cost of this technology.

Author Contributions: S.D., M.C.A., E.P., and G.G. contributed to the design and writing of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The Spanish Ministry of Economy and Competitiveness (MINECO) supported this work under project ENE2017-83976-C2-2-R (co-funded by FEDER). Financial support from E3TECH (CTQ2017-90659-REDT) and REPICOMES (ENE2017-90932-REDT) Excellence Networks is acknowledged. G.G. acknowledges the Viera y Clavijo program (ACIISI and ULL) for financial support. Acknowledgments: G.G. acknowledges NANOtec, INTech and Cabildo de Tenerife for laboratory facilities. Conflicts of Interest: The authors declare no conflicts of interest.

Molecules 2021, 26, 572 11 of 12

In general, the results discussed in this manuscript confirm that graphene materials are excellent supports, producing synergistic effects that cause improvements in the efficiency and selectivity of the catalysts used for the electrochemical reduction of CO2. However, in most cases, the activity of graphene not modified by metal species is very low. All the new advances in the fundamental understanding of reactions occurring on graphene-based catalysts presented here may help to improve the fabrication of novel electrodes in order to enhance the performance and decrease the cost of this technology.

Author Contributions: S.D., M.d.C.A., E.P., and G.G. contributed to the design and writing of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: The Spanish Ministry of Economy and Competitiveness (MINECO) supported this work under project ENE2017-83976-C2-2-R (co-funded by FEDER). Financial support from E3TECH (CTQ2017-90659-REDT) and REPICOMES (ENE2017-90932-REDT) Excellence Networks is acknowledged. G.G. acknowledges the Viera y Clavijo program (ACIISI and ULL) for financial support. Acknowledgments: G.G. acknowledges NANOtec, INTech and Cabildo de Tenerife for laboratory facilities. Conflicts of Interest: The authors declare no conflict of interest.

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