Nanoscale

PAPER

Cite this: Nanoscale, 2018, 10, 11064

Received 5th March 2018,Accepted 11th May 2018

DOI: 10.1039/c8nr01855c

rsc.li/nanoscale

Cu atomic chains supported on β-borophenesheets for effective CO2 electroreduction†

Haoming Shen, a Yawei Li b and Qiang Sun*a,c

The good performance of Cu displayed in CO2 conversion promotes the study on how to disperse Cu

into 2D materials for better catalysis. Inspired by the recent studies on new 2D porous B sheets [Angew.

Chem., Int. Ed., 2017, 56, 10093; Adv. Mater., 2018, 30, 1704025; Phys. Rev. Lett., 2017, 118, 096401], here

for the first time we have explored the catalytic properties of Cu atomic chains on β-borophene sheets,

and have found that the Cu–B sheet can break the scaling relationship through providing secondary

adsorption sites, thus leading to small overpotentials in the preferable reaction pathway CO2 → COOH* →

CO* → CHO* → CH2O* → CH3O* → CH3OH. The Cu atomic chains also lower the energy barrier by

forming assistant adsorptions of H*. Electronic structure analyses further show that the Cu atomic chain

structure stabilizes the CHO* bonding through an enhanced σ bonding–π back-bonding mode. Our

study not only sheds light on the design of new catalysts for effective CO2 conversion but also expands

the applications of B sheets.

1. Introduction

Due to the global crisis of energy and the environment, numer-ous efforts have been made to find efficient and economicmethods for converting CO2 to other useful chemicals. Amongthem, electrocatalytic CO2 reduction shows some advantagesin high product selectivity and mild operation conditions overother methods.1–3 Currently, Cu is identified as one of the bestcatalysts in electrocatalysis. However, even with the ability ofdirectly producing hydrocarbons and oxygenates at a highcurrent efficiency, the catalytic performance of a copper elec-trode is still impeded by the high overpotential (∼1 V)4–6

because of the mechanism involved. The carboxyl mechanismwith COOH* as the key intermediate is most commonlyaccepted, and especially, stronger COOH and CHO adsorptionsas well as a weaker CO adsorption are needed for better cata-lytic performance. However, adsorption energies of the inter-mediates with the same binding atom attached to the metalsurfaces exhibit linear scaling relationships.1,7–11 Therefore, itis difficult to stabilize COOH and CHO adsorption without sta-bilizing CO adsorption too much on the same adsorption site.One possible solution is to introduce a secondary adsorption

site.1 Li et al.12 reported that when a Cu dimer is introduced tographene with adjacent single vacancies (Cu2@2SV), the finalproduct of electroreduction is CO and the catalytic perform-ance is much better than the single-doping ones. Similarimprovement has also been found in the phthalocyanine sheetincorporated with metal dimers in our previous work.13

Boron has a small covalent radius and flexible bond con-figurations from the common two-center two-electron bondsto multi-center two-electron bonds; the resulting structurescan display rich physical properties from semiconducting tosuperconducting,14–16 or from metallic to Dirac semi-metal-lic.17,18 However, as compared to other 2D materials like gra-phene, MXenes and metal dichalcogenides, much less efforthas been made on B sheets for CO2 conversion. Here, weexplore Cu supported on two-dimensional porous borophenesthat show a rich variety of bonding configurations and uniqueelectronic behaviors.19–21 Mannix et al.22 first reported the syn-thesis of borophenes (δ6 sheet) in 2015 and later onβ-borophene (β12 sheet) was also synthesized by Feng et al.23 in2016. Despite the fact that bulk boron is a semiconductor, two-dimensional borophene materials turn out to be metallic.24

Both theory and experiment have found that borophenes withhexagon holes are stable, and the borophene sheets areinert to oxidation and weakly interact with thesubstrates.16,19–21,23–26 In β-borophene sheets, the hexagonholes form a linear pattern with a tunable length. Thesehexagon holes provide the natural adsorption sites for copperatoms as the electron transferred from metal atoms to B net-works can effectively stabilize the system and form a uniquehexacoordinate Cu atom for adsorption.27 The one-dimen-

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr01855c

aDepartment of Materials Science and Engineering, Peking University, Beijing

100871, China. E-mail: sunqiang@pku.edu.cnbDepartment of Chemical Engineering, The Pennsylvania State University, University

Park 16801, USAcCenter for Applied Physics and Technology, Peking University, Beijing 100871, China

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sional holes in β-borophene sheets are therefore favorable toform Cu atomic chains for introducing a secondary adsorptionsite. In our previous work, we have proved that incorporatingCu dimers into graphene can improve the catalytic perform-ance of CO production,12 and Cu edge-decorated graphenenanoribbons are excellent catalysts for CO2 reduction as well.28

The β-borophene structures have natural adsorption sites withmetallicity, so there is no need to introduce vacancies or edgesfor supporting Cu atoms as explored in previous studies,12,28

and the intrinsic metallicity is favorable for enhancing the per-formance of electrocatalysis which is different from the semi-conducting features in defective graphene and graphene nano-ribbons. Therefore, we can expect good catalytic performancein this system that has been confirmed by our extensive theore-tical simulations as discussed below.

2. Models and methods

Experimentally, borophenes have been synthesized on metalsubstrates such as Cu(111),26 Ag(111)22,23 and Au(111)20 due tothe stabilization of sp2 hybridization by metal passivation.Various borophene structures have been found in experimentsor predicted by theory. In order to obtain the Cu atomic chainsthat we need, we choose the structures with hexagon holes dis-tributed in certain patterns. Cu atoms are placed in the centerof h-holes slightly above the borophene plane. The optimizedstructures are discussed in detail in the results.

Structure relaxations and single-point energy calculationsare performed using density functional theory as implementedin the Vienna ab initio Simulation Package (VASP);29 the projec-tor-augmented-wave (PAW)30 pseudopotential is utilized totreat the core electrons, while the Perdew–Burke–Ernzerhof(PBE)31 exchange–correlation functional of the generalized gra-dient approximation (GGA) is used for describing the electroninteractions. We applied the vdW(optB88) functional whichprovides a reasonable description of van der Waals forceswhen calculating the interaction of the solvent model and tran-sition states. A plane-wave cutoff energy of 400 eV is adoptedfor all the calculations. The vacuum space in the z-direction isset as 20 Å to minimize the interaction between image layers.The criteria for convergence in energy and force are 1 × 10−6

eV and 5 × 10−5 eV Å−1, respectively, for Cu-borophene struc-ture optimization and the residual force is switched to 0.02eV Å−1 for adsorbate optimization. The reciprocal space wassampled using different Monkhorst–Pack meshes32 due to thesize and shape differences between different borophene unitcells. To investigate the reaction pathway, all possible struc-tures are simulated. In our work, the computational hydrogenelectrode model (CHE)33 is applied in calculating the free ener-gies of the proton–electron transfer steps. The CHE modeldeals with reaction conditions at pH = 0 in the aqueous elec-trolyte and 1 bar of H2 in the gas phase at 298.15 K. Furtherdetails are listed in the ESI.†

3. Results and discussion3.1. Catalyst structures

Borophene structures which are used for Cu-doping and theside views of the supported Cu-borophenes are shown inFig. 1.24,25 The Cu atoms lie in the center of the hexagon holesof the borophenes. The optimized structural parameters of allthe catalyst candidates are shown in Table 1. For α-borophenebased structures (Fig. 1a), the Cu atoms are stabilized in iso-lated h-holes slightly above the borophene plane, forming asingle-metal-doped structure. Catalysts based on β-borophenes(Fig. 1b–f ) with lined-up h-holes have Cu atomic chains withdifferent lengths. Chains with a limited length have curvedstructures due to the strong interaction between Cu atomswhile the curvature decreases with an increase in the Cu chainlength. The Cu chains on β-borophene (Fig. 1g) with an infi-nite length remain straight with a longer distance between Cu

Fig. 1 Geometric structures of unsupported borophenes24,25 (top view)and Cun@B (side view). (a) α’ (B8) sheet and Cu1@B; (b) β5 (B26) sheet andCu2@B; (c) β31 (B36) sheet and Cu3@B; (d) β32 (B46) sheet and Cu4@B; (e)β33 (B56) sheet and Cu5@B; (f ) B66 sheet and Cu6@B; (g) β12 (B5) sheetand Cu∞@B.

Table 1 Structure properties of unadsorbed catalysts (c = 20 Å)

Atomic Chain Length

1 2 3 4 5 6 ∞

Symmetry P6MM CMM2 CMM2 CMM2 CMM2 CMM2 PMM2aa/Å 5.141 5.139 5.146 5.149 5.155 5.157 5.174ba/Å 8.905 14.616 20.278 25.952 31.648 37.445 2.958dCu–Cu/Å — 2.485 2.583 2.624, 2.717 2.671, 2.754 2.685, 2.736, 2.838 2.958

a The structure properties of the rectangular unit cells.

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atoms. These catalysts are named after the length of theatomic chains as Cun@B (n = 1, 2, 3, 4, 5, 6, ∞) forconvenience.

The cell parameters of the rectangular unit cells are listedin Table 1 and the distances between the parallel chains andCu atoms in Cu1@B are given. The distances between the par-allel chains increase from 5.139 Å to 5.174 Å with the lengthincreasing except for Cu1@B with a distance of 5.141 Å. TheCu–Cu distances within chains also increase together with thechain length. It is interesting to note that the Cu–Cu distancesin curved structures are close to the Cu–Cu distance on theCu(111) surface (2.539 Å).

To better understand the catalytic mechanism involved inthe studied systems, we first study the electronic structures ofthe systems. The electronic structures of Cu1@B, Cu4@B andCu∞@B are shown in Fig. 2, from which one can see that theyare metallic. Similar features are also found for Cu2–6@B. Onecan expect that the metallic systems would enhance the cata-lytic performance in CO2 electrocatalysis, which has been con-firmed by detailed calculations as discussed below.

Due to the difference in electron affinity between Cu and B,electron transfers from metal atoms to B networks wouldoccur as calculated by the Bader charge analysis.34 On Cu1@B,the charge transfer is 0.231 with Cu atoms being the positivecenter. On Cu∞@B, the charge transfer is 0.263. On otherCun@B structures, the charge distributions on Cu atoms areuneven, for example, on the Cu3 chain, the middle Cu atomcarries a charge of +0.255e while the charges on the other twoCu atoms at the ends are +0.229e. According to our calculation,the Cu atoms in the middle carry more positive charge. Theseregularly distributed Cu chains with charges supported on Bsheets would provide desirable platforms for CO2 adsorptionand conversion.

3.2. Free energies and reaction pathway analyses

The adsorption models on Cu atomic chains with limitedlengths are easy to build. Each chain provides an active site forone adsorbate. The distances between these adsorbates areabove 5 Å according to the structure information from Table 1.Different sites in these chains have different adsorption abil-

ities and we find that the adsorbates prefer to associate withthe center of the chains. The adsorption site preference iscoincident with the charge analyses. However, the model foradsorption on Cu∞@B is difficult to determine. Adsorptionswith high coverage show a high energy barrier for the COOH*adsorption while interlaced adsorptions between chains canlower the energy barrier (see Table S2 and Fig. S2†). Therefore,we use a low-coverage adsorption model to avoid the inter-action between adsorbates. The distances between adsorbatesare also above 5 Å. The adsorption model can be realized witha low concentration of reactants in the experiment.

Previous studies have reported that the main products ofCO2 electroreduction are CH4 and C2H4 at copper-based elec-trodes.5,6,35 However, ethylene formation usually occurs inaqueous alkaline solution.36–38 In our work, we focus only onthe mono-carbon products to simplify the kinetic analyses.Consequently, here, we first investigate the first step of CO2

adsorption. By considering different bonding configurations,we find that the OCHO* adsorbates have the lowest energieson these catalysts. However, the stable OCHO* adsorbates aredifficult to dissociate which makes the formate mechanismunfavorable. The overpotential of formate production will bediscussed later. On the other hand, a carbon-end bonding asshown in Fig. 3a can also be found. Further analyses indicatethat the carboxyl mechanism is favored in the electroreductionwhich leads to hydrocarbon production.39 For the carboxylmechanism, the hydrogenation of CO2 or CO* as well as thedesorption of CO normally have high energy barriers.1,5,6

Therefore, the COOH*, CO* and CHO* adsorbates are the keyintermediates for identifying the limiting potential and therate-determining step. On the other hand, adsorbates like

Fig. 3 Geometric structure of adsorbates on Cu∞@B. (a) COOH*; (b)CO*; (c) CHO*; (d) CH2O*; (e) OCH3*; (f ) H*.

Fig. 2 Band structures of (a) Cu1@B, (b) Cu4@B and (c) Cu∞@B, wherethe Fermi level is shown in the dashed line.

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OCH2* and CH2OH* may also cause the variation of the finalproducts.6 Thus, we also take these adsorbates into consider-ation. The highest energy barrier in the pathway to eachproduct is referred to as the limiting potential.

By analyzing all possible adsorbates on these catalysts, wefind that the hydrogenation of the CO* adsorbate (Fig. 3b) isthe rate-limiting step with the highest overpotential on all Cu-borophenes. The free energy diagram of CO2 electroreductionis shown in Fig. 4. Despite the huge difference between thefree energy diagram on Cu1@B and on other catalysts, the rela-tive stabilities of the adsorbates remain the same. The mostfavorable reaction pathway is CO2 → COOH* → CO* → CHO*→ CH2O* → CH3O* → CH3OH. The free energies of the keyintermediates as well as the overpotentials are listed inTable 2. Further hydrogenation of CHO* has lower overpoten-tials than the hydrogenation of CO* which will not limit thereaction rate at the indicated potentials.

We find that COOH* and CHO* adsorption energies onCu1@B are much higher than those on the others while theadsorption energy reaches a minimum on Cu4@B with littledifference between each other. With no nearby Cu atoms, thesingle-atom active site on Cu1@B adsorbs the intermediateand a C–Cu bond forms. On the other hand, COOH* andCHO* adsorbates on Cu chains have extra O–Cu bonds whichhelp in lowering the free energies. The structure propertiesalso show good agreement with the adsorbate stabilities. TheC–Cu bond of CHO* on Cu1@B is 1.944 Å, while on Cu2@Band Cu4@B, the bond lengths are 1.924 Å and 1.917 Å, respect-ively. During the adsorption, the formation of π backbondinghelps to activate the adsorbates. The metal d-band electronsfill the π* antibonding molecular orbital. In the case of CO*and CHO*, a stronger adsorption comes with stronger σbonding and π backbonding. The increased filling of the anti-bonding π* state of CO corresponds to an increase in the C–Obond length and the adsorbate activation. That being said, the

longer C–O bond in CHO* (1.266 Å on Cu4@B, 1.214 Å onCu1@B) also indicates a stronger adsorption on Cu4@B.

In contrast, the free energies of CO* remain in a rangebetween −0.106 eV and −0.231 eV despite the drastic changeof COOH* and CHO* energies. The rather slight change can beattributed to the end adsorption of the CO* adsorbate. TheC–Cu bond on Cu4@B is 1.811 Å, only 0.01 Å shorter than thaton Cu1@B. On the other hand, the C–O bonds are around1.15–1.16 Å, which are longer than that in carbon monoxide.The structure properties suggest that the CO* bonding showslittle difference on the Cu-borophene catalysts.

Other intermediates are also taken into consideration.COH* adsorption with hydroxyl has been considered as one ofthe hydrogenation products of CO* but the high free energymakes it unlikely to get generated. CH2O* shows advantages instabilities against CHOH* (0.69 eV lower on Cu∞@B) in CHO*hydrogenation, but both are commonly seen in the carboxylmechanism.6 The CH2O* free energies also follow a similartendency with CHO*. Formaldehyde formation is also likely tooccur on Cu1@B as the desorption is spontaneous but the de-sorption on other catalysts is unfavorable against furtherhydrogenation. OCH3* is found to be more stable with a twiston the bonding atom. The CH2OH* free energy is much higherthan that of OCH3*. Interestingly, the OCH3* intermediatesprefer to adsorb on the bridge site between Cu atoms.Therefore, the free energy on the single atom catalyst Cu1@Bis almost 0.7 eV higher than the others. In the reaction processfrom *CO to *OCH3, the O atom associates with the active sitenext to the original site and eventually forms a Cu–O–Cubridge adsorption. The mechanism is similar to CO2 electrore-duction on Cu(211). Considering all the possible desorptionproducts like HCOOH, CH2O, CH3OH and CH4, we findCH3OH to be the most favorable product throughout the reac-tion pathway. Although the formate adsorption is stable, theoverpotential needed for formic acid production is 1.27 eV onCu∞@B while methanol production needs only 0.434 eV.

In this case, Cu atomic chains supported on borophenestructures are able to stabilize COOH* and CHO* without low-ering the adsorption energy of CO*. The atomic chain struc-tures provide a nearby adsorption site for the O atom bonding.The bridge adsorption changes the bonding atom of COOH*and CHO* structures. Due to the change of the bondingatoms, the scaling relationship no longer exists on these Cuchain catalysts.1,7–10 Compared to the catalytic performance on

Fig. 4 Energy diagrams of intermediates on Cun@B. The dashed line isthe most favorable pathway.

Table 2 Free energies of bare catalysts, COOH*, CO* and CHO* inter-mediates as well as the overpotential on each catalyst

Free energy/eV Cat. COOH* CO* CHO* Overpotential/eV

Cu1@B 0.000 0.660 −0.111 0.896 1.007Cu2@B 0.000 0.105 −0.149 0.417 0.566Cu3@B 0.000 0.029 −0.195 0.317 0.512Cu4@B 0.000 −0.031 −0.202 0.268 0.470Cu5@B 0.000 −0.013 −0.210 0.276 0.486Cu6@B 0.000 0.027 −0.231 0.308 0.539Cu∞@B 0.000 0.035 −0.106 0.328 0.434

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Cu1@B, the catalytic abilities of Cu atomic chains are superior.The overpotential is lowered by more than 0.5 eV which willgreatly improve the energy efficiency. The stabilized CHO*intermediates bring the overpotential down and the overpoten-tials of CO2 electroreduction on Cu4@B and Cu∞@B are 0.470and 0.434 eV, respectively, which are much lower than the over-potential on Cu surfaces (0.74 eV on Cu(211)6). In our previouswork, we found that the Cu dimer doped into graphene(Cu2@2SV) and Cu edge-decorated graphene nanoribbons(n-AGNR) are excellent catalysts for CO2 electroreduction; theoverpotential needed for methanol production is 0.82 eV onCu2@2SV12 and 0.44 eV–0.58 eV on n-AGNR.28 The calculatedcatalytic performance of Cu4@B and Cu∞@B has proved thatthese copper–borophene catalysts might have huge potentialsin applications.

3.3. HER reaction analyses

The hydrogen evolution reaction (HER) is one of the mostcommon side reactions in the electroreduction of CO2. Thereduction takes place in aqueous solution and the proton isprovided by the solution. The existence of the HER cannot beignored. Therefore, we investigated the H* adsorption. For acatalyst with multiple active sties, there are a few differentadsorption states of the hydrogen atom. On graphene sup-ported metal dimers,12 DFT calculation results suggest thathydrogen atoms prefer to adsorb on the bridge sites of thetransition metal dimers. On Cun@B catalysts, the hydrogenatoms also prefer to adsorb on the bridge sites except onCu1@B (Fig. 3f). The hydrogen evolution reaction involvesproton adsorption and proton coupling to adsorbed H*.Overpotentials of the hydrogen evolution reaction on Cu1@B,Cu4@B and Cu∞@B are calculated to be 0.110 eV, 0.386 eVand 0.154 eV, respectively. Hydrogenations on the borophenesare also considered and the energy barrier is 0.727 eV onCu4@B, which makes it unlikely to occur.

The energy barriers of the HER are lower than those of CO2

reduction but the gap is particularly very small on Cu4@B.Compared to the energy barriers of hydrogen evolution on atypical metal electrode (0.05–0.15 meV),40 the energy barriersof the HER are rather high. The calculated overpotential of theHER on Cu(211) is 0.03 eV, which is a lot smaller than that ofCO2 reduction (0.74 eV).6 But the Cu surfaces are still con-sidered to be suitable catalysts for CO2 electroreduction.

On Cu4@B, the energy barrier for the first step is −0.031 eVwhile the energy barrier is 0.386 eV for the HER. Therefore,when increasing the applied potential, the surface of Cu4@Bwill be covered with COOH*. Due to the negative free energychange in COOH* → CO*, the surface will be quickly coveredwith CO*. When the applied potential reaches 0.386 eV, theactive sites are blocked by CO*, so HER side reaction will notbe able to proceed. Furthermore, experimental adjustmentslike tuning the partial pressure of the CO2 gases and the temp-erature can also influence the reaction rates and reduce theHER side reaction. With a low overpotential of CO2 reductionand relatively high HER overpotential, the Cu4@B catalyst

might show superior performance in the experiment withgood energy efficiency and high selectivity.

3.4. CO adsorbate hydrogenation analyses

The free energy diagram shows that the elementary reactionstep involving *CO hydrogenation to *CHO has the highestfree energy barrier. Therefore, we focus on the transition statesof H–CO* and perform a detailed calculation about the hydro-genation. In the free energy calculation, the CHE method isused to obtain the free energies of the proton–electron transfersteps. However, the CHE method cannot simulate the tran-sition states. Moreover, in actual circumstances, the protonscome in the form of hydrated protons and the solvent also par-ticipates in the proton transfer and the transition state for-mation. Thus, in transition state calculation, a more practicalmethod is needed to simulate the effect of hydrated protonsand the solvent effect. The solvent model with several watermolecules above the adsorption surface is introduced to simu-late the proton transfer. Transition states were simulated usingthe climbing-image nudged elastic band (CINEB) method.

Interestingly, we find that the Cu atomic chains are alsocapable of assisting the hydrogenation of CO*. On Cun@B (n >3), the Cu atom beside the CO* adsorption site provides anassistant adsorption as the proton moves towards the catalystsurface. With the charge transfer, a H* + CO* intermediateforms. This intermediate is able to lower the energy barrierand provides a perfect angle for proton attachment. Duringthe hydrogenation, the proton approaches the CO* along withthe Cu atomic chains and the C–H bond forms. The C–O bondgenerally leans toward the Cu atom on the other side as shownin the background of Fig. 5. With the help of the assistantadsorption, the proton is able to get close to the CO* adsorbateand the energy barrier is lowered. The calculated transitionstate energies are listed in Table 3 and the energy diagrams areshown in Fig. 5.

Fig. 5 Energy diagrams of the hydrogenation of CO* on Cun@B (n > 3).Energies of intermediates CO*, H* + CO* and CHO* and the energies ofCO–H* transition states are shown. The atom movement during theCO* hydrogenation on Cu4@B is presented in the background, watermolecules above the surface are not shown.

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The CINEB calculation shows the transition states of theC–H bond formation. The free energies of CO–H transitionstates are in a close range between 0.69 and 0.77 eV. Theenergy barrier on Cu∞@B is slightly higher than that on theothers, which is in agreement with the transition state scalingrelationships41 as the CO* free energy is higher on Cu∞@B.

3.5 Adsorption electronic structure analyses

The d band center theory is taken into consideration to explainthe trend of H* stabilities.42 The increase of the gap betweenthe Fermi level and the d band center corresponds to thedestabilization of the metal–adsorbate interaction. The calcu-lated d band centers of the catalysts as well as the free energiesof H*, CO* and CHO* are listed in Table 4. The trend of H*and the end-adsorbed CO* free energies match well with the dband centers. On the other hand, the free energies of CHO*show less relevance to the d band centers. The differences canbe concluded to be due to the change of adsorption modes asthe structure-related adsorptions of CHO* are less influencedby the d band center than the end-adsorbed CO* adsorptions.

The adsorbate–metal binding can be considered as twolocalized states according to a similar adsorption on metal sur-faces.43 The metal–ligand bonding theory of organometalliccatalysts indicates that the σ bonding and π backbonding arethe main interactions between the catalyst and the adsorbate.Therefore, the density of states (DOSs) of the Cu1@B, Cu4@Band Cu∞@B with CHO* and CO* adsorbate are calculated. Byanalyzing the density of states, the influence brought by intro-ducing Cu atomic chains can be explained.

Projected density of states of CHO* adsorbates are shown inFig. 6. The σ bonding state energy is slightly lower than that ofthe π backbonding state, while both bonding states are locatedaround E − Ef = −5.7 to −8.0 eV. Therefore, by analyzing the pro-

jected density of states (PDOS), we can see that the adsorbate-to-metal σ bonding and metal-to-adsorbate π backbonding are atE − Ef = −7.21/−5.76 eV and E − Ef = −7.10/−5.80 eV for CHO*adsorption on Cu4@B and Cu∞@B (Fig. 6c–f) and at E − Ef =−6.82/−5.80 eV on Cu1@B (Fig. 6a and b). The projected densityof states analyses suggest that the σ bonding is contributed toby the carbon pz orbital and the Cu dz2 orbital while the π back-bonding is contributed to by carbon px/py orbitals and Cu dxz/dyz orbitals. The interactions of O and Cu atoms are alsoincluded and are shown as the lines above in Fig. 6c–f. The rela-tively lower energy range on Cu4@B and Cu∞@B indicates thatthe interaction is stronger, resulting in a lower free energy.

On the other hand, the PDOSs of CO* adsorptions on Cu1@B,Cu4@B and Cu∞@B show little difference between each other(Fig. 7) with a σ bonding at E − Ef = −8.02 to −7.85 eV (Fig. 7a, c

Table 3 Calculated d band centers and free energies of H*, CO* andCHO* intermediates

Free energy/eV H+ + CO* H* + CO* CO–H* (TS) CHO*

Cu3@B 0.00 0.05 0.72 0.51Cu4@B 0.00 0.04 0.69 0.47Cu5@B 0.00 0.10 0.70 0.49Cu6@B 0.00 0.07 0.72 0.54Cu∞@B 0.00 0.01 0.77 0.43

Table 4 Calculated d band centers and free energies of H*, CO* andCHO* intermediates

Catalysts d-band center/eV

Free energy/eV

H* CO* CHO*

Cu1@B −6.29 −0.110 −0.111 0.896Cu2@B −5.26 −0.354 −0.149 0.417Cu3@B −4.67 −0.390 −0.195 0.317Cu4@B −4.86 −0.386 −0.202 0.268Cu5@B −5.02 −0.368 −0.210 0.276Cu6@B −5.26 −0.350 −0.231 0.308Cu∞@B −5.78 −0.154 −0.106 0.328

Fig. 6 Projected density of states of CHO* adsorbed on Cu1@B (a, b),Cu4@B (c, d) and Cu∞@B (e, f ). Lower energy peaks A, A’ and A’’ are thepz–dz2 σ bonding (a, c, and e) and the higher energy peaks B, B’ and B’’are the px/y–dxz/yz π backbonding (b, d, and f). The red lines are thedensity of Cu and the blue lines are C or O atoms. The interactions of Oand Cu atoms are also included and are shown as the lines above (c–f ).

Fig. 7 Projected density of states of CO* adsorbed on Cu1@B (a, b),Cu4@B (c, d) and Cu∞@B (e, f ). The red lines are the density of Cu andthe blue lines are C atoms.

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and e) and a metal-to-adsorbate π backbonding at E − Ef = −7.74to −7.60 eV (Fig. 7b, d and f). The similar electronic structuresare in good agreement with the close CO* free energy range.

On traditional metal surfaces, due to the scaling relation-ships, CHO* adsorbates are stabilized with CO* only and thusthe overpotential is difficult to reduce. However, on Cun@B(n > 1) structures, the chain structures provide not only adsorp-tion sites for C-end adsorbates, but extra bonds between Cuand O atoms as well. The extra bonds only exist in adsorbateslike COOH*, CHO* and a few other adsorbates with O and low-coordinate C atoms. The Cu–O bonds also help in enhancingthe metal-to-adsorbate π backbonding. The electron transfer-ring back to the adsorbate π* antibonding state can destabilizethe C–O bond and lower the energy barriers for further hydro-genation. On the other hand, the CO* adsorption moderemains unchanged. With the stabilized CHO* adsorption andunchanged CO* adsorption, we are able to achieve a loweroverpotential for CO* hydrogenation.

4. Conclusions

In summary, we have investigated the CO2 electroreduction per-formance of Cu supported borophene structures. The uniquechain structures of Cu atoms play a key role in lowering theoverpotential of CO2 electroreduction. The catalytic reactions onCun@B proceed with the reduction pathway of CO2 → COOH*→ CO* → CHO* → CH2O* → CH3O* → CH3OH. The hydrogen-ation of the CO* adsorbate is the rate-limiting step in this reac-tion. Cu4@B and Cu∞@B have overpotentials of 0.470 and0.434 eV, respectively, showing their great catalytic abilities forCO2 electroreduction. The chain structures cause a significantdecrease of free energies for the COOH* and CHO* adsorbateswithout lowering the CO* free energies. Detailed investigationalso reveals that Cu atomic chains are capable of lowering theenergy barrier by offering an assistant adsorption for CO*hydrogenation. The Cu-borophene catalysts show the followingadvantages: (1) β-borophene sheets have intrinsic holes withrich patterns, offering the feasibility and flexibility to introduceCu atoms and tune the catalytic properties; (2) the intrinsicmetallicity of β-borophene sheets enhances the performance ofCO2 electrocatalysis; (3) the light mass of β-borophene sheets isan additional merit as compared with other substrates; (4) dis-playing low overpotentials for CO2 reduction and high over-potentials for HER side reaction as compared with bulk Cu aswell as other Cu containing catalysts; (5) compared with othertypical single-atom catalysts, the tunable atomic chain struc-tures have the advantage of forming multiple adsorption modesto break the scaling relationship. We hope this study wouldmotivate more theoretical and experimental work on Cu–Bbased materials for more effective CO2 conversion.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is partially supported by grants from the NationalNatural Science Foundation of China (21573008 and 21773003),and from the Ministry of Science and Technology of China(2017YFA0204902). This work is also supported by the High-per-formance Computing Platform of Peking University.

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