Chemistry

Selling electrocatalytic CO 2 discount to formate through sulfur-boosting water activation on indium surfaces

CO2RR performances of sulfur-doped indium catalysts

Sulfur-doped indium (denoted as S−In) catalysts had been fabricated by electroreduction of sulfur-containing In2O3 precursors, which grew on carbon fibers by a solvothermal technique. The obtained catalysts with sulfur contents of zero, 2.5, four.9, 9.four, and 14 mol%, which had been decided by Auger electron spectroscopy (AES) (Supplementary Fig. 1), had been denoted as S0−In, S1−In, S2−In, S3−In, and S4−In, respectively. The electrocatalytic examine confirmed that our In2O3-derived metallic catalyst on carbon fibers exhibited increased exercise for CO2RR to formate than the industrial In foil at a possible of −zero.98 V versus reversible hydrogen electrode (RHE) (Fig. 1a). The formation price of formate elevated considerably with a rise in sulfur content material as much as four.9 mol% (S2−In), whereas the formation charges of H2 and CO solely modified barely on the similar time. The FE of formate additionally elevated with sulfur content material. An extra improve in sulfur content material to >four.9 mol% somewhat decreased the formation price of formate. Thus, one of the best efficiency was achieved over the S2−In catalyst. The formation price and FE of formate over the S2−In catalyst reached 1002 μmol h−1 cm−2 and 93% at −zero.98 V versus RHE, respectively, which had been about 17 and 1.6 instances these over In foil.

Fig. 1Fig. 1

CO2RR performances of sulfur-doped indium catalysts. a Formation charges of H2, CO and HCOO− and FE of formate for In foil and S−In catalysts at −zero.98 V (versus RHE) for 1 h. b Present density for S2−In catalyst over 1 h of response at every given potential (versus RHE). c ECSA-corrected present density and FE of formate for In foil and S2−In catalyst at every given potential for 1 h. d Plot of FE of formate versus present density for S2−In catalyst and a few typical catalysts reported thus far (see Supplementary Desk 1 for particulars). Response circumstances: CO2-saturated zero.5 M KHCO3 resolution in H-type electrochemical cell with platinum plate because the counter electrode and saturated calomel electrode (SCE) because the reference electrode. The experiments in every case had been carried out a minimum of for thrice. The error bar represents the relative deviation

We carried out 13CO2 labeling experiments for the S2−In catalyst. The merchandise obtained at a possible of −zero.98 V versus RHE had been analyzed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. A 1H NMR doublet was noticed at eight.5 ppm, which was attributable to the proton coupled to 13C in H13COO− (Supplementary Fig. 2a). A sign at 168.5 ppm was noticed within the 13C NMR spectrum, which might be ascribed to H13COO− (Supplementary Fig. 2b)11. These observations affirm that formate is fashioned from CO2 discount.

We carried out additional research for essentially the most environment friendly S2−In catalyst at completely different cathodic potentials. The CO2RR began to happen at a possible of −zero.33 V versus RHE (overpotenial, zero.14 V) with FE of formate of three% (Supplementary Fig. 3a), akin to that over oxidized Co catalyst11. Eighty % FE of formate was achieved at −zero.63 V versus RHE (overpotential, zero.44 V), higher than these over a lot of the non-noble catalysts beneath such a decrease overpotential (Supplementary Desk 1). The change within the utilized potential from −zero.33 to −1.23 V versus RHE resulted in a variation in present density in a broad vary from zero.15 to 100 mA cm−2, and the present density stored secure in the course of the electrocatalysis at every given potential (Fig. 1b). The present density ascribed to CO2RR, which was calculated by contemplating the FE of CO2RR, elevated considerably from zero.03 to 86 mA cm−2 by altering potential from −zero.33 to −1.23 V versus RHE after which grew to become nearly saturated (Supplementary Fig. four). It’s noteworthy that the present density of CO2RR of 86 mA cm−2 approaches the utmost worth (90 mA cm−2) evaluated by assuming the mass-transport limitation beneath our response circumstances (Supplementary Strategies). The efficiency of the S2−In catalyst was additional in contrast with that of In foil, a reference catalyst, at completely different potentials. The S2−In catalyst exhibited considerably increased present density, FE and formation price of formate than In foil at every potential (Supplementary Fig. 3a–c). For a greater comparability, now we have normalized the formation price of formate primarily based on the electrochemical floor space (ECSA) (Supplementary Desk 2), which was decided by double-layer capacitance (Cdl) technique (Supplementary Fig. 5). The S2−In catalyst exhibited increased normalized formation price of formate than In foil (Fig. 1c). The prevalence of the S2−In catalyst for the formation of formate grew to become much less vital at potentials extra damaging than −1.03 V versus RHE in all probability due to the mass-transport limitation at a excessive present density.

It’s fairly distinctive that the excessive FE of formate (>85%) might be maintained in a wide variety of present density (25–100 mA cm−2) over the S2−In catalyst (Supplementary Figs. 3a and 1d). For comparability, the behaviors of some typical catalysts, which have been reported as high CO2RR-to-formate catalysts, are additionally displayed in Fig. 1d. The FE of formate drops considerably to <60% at a current density of >60 mA cm−2 over all of the electrocatalysts reported thus far even in ionic liquid or natural electrolyte (Supplementary Desk 1). Moreover, the S2−In catalyst confirmed wonderful stability in 10 h operation (Supplementary Fig. 6). All these details display that the current S2−In catalyst, which reveals excessive selectivity at excessive present density and thus excessive response price, may be very promising for CO2RR to formate.

Characterizations of sulfur-doped indium catalysts

The X-ray diffraction (XRD) patterns confirmed that In2O3 was the one crystalline section in precursors and In2O3 was lowered to metallic In after electroreduction (Supplementary Fig. 7). Solely diffraction peaks ascribed to metallic In with tetragonal section might be noticed for the S−In catalysts with completely different sulfur contents. The scanning electron microscopy (SEM) measurements confirmed that In particles had been uniformly distributed on carbon fibers in every pattern (Fig. 2a and Supplementary Fig. 8a–e). The common diameters of In particles had been evaluated to be related (110–131 nm) within the S−In samples with completely different sulfur contents (Supplementary Fig. 9a–e). After electrocatalytic response, the imply dimension of In particles within the S2−In catalyst maintained nearly unchanged (Supplementary Figs. 8f and 9f). The high-resolution transmission electron microscopy (HRTEM) measurements for the S−In samples displayed lattice fringes with an interplanar spacing of zero.272 nm (Fig. 2a and Supplementary Fig. 10), which might be ascribed to the In (101) side. The catalyst loading on carbon fibers was zero.5 ± zero.1 mg cm−2 for every catalyst. These recommend that there are not any vital variations in non-chemical parameters for the S−In collection of catalysts, equivalent to catalyst loading, dimension or dispersion of In particles and catalyst porosity. Thus, these parameters don’t account for the improved present density and the formation price of formate after the modification of In catalysts by sulfur.

Fig. 2Fig. 2

Characterizations of morphologies and chemical states for S−In catalysts. a SEM picture and HRTEM picture (insert) of S2−In catalyst. b STEM picture of S2−In catalyst and the corresponding EDS elemental mapping. c In Okay-edge XANES spectra for S2−In catalyst earlier than and after response. d In situ In Okay-edge XANES spectra for S2−In catalyst at −zero.98 V versus RHE. e and f In 3d and S 2p XPS spectra of S−In catalysts

The energy-dispersive X-ray spectroscopy (EDS) evaluation for the S2−In catalyst indicated that In, S and O components existed within the catalyst, and these components had been distributed uniformly over the catalyst particle (Fig. 2b and Supplementary Fig. 11). The sputtering of the S2−In pattern with Ar ions resulted in a big lower within the sign of S however the sign of In somewhat elevated barely within the AES spectra (Supplementary Fig. 12). This remark means that sulfur species are primarily situated on the floor of In particles. The X-ray absorption near-edge construction spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) had been used to research the chemical states of indium and sulfur. The XANES measurements for the S2−In catalyst earlier than and after electrocatalytic response displayed the identical sample in In Okay-edge (Fig. 2c), indicating that the chemical state of indium didn’t change earlier than and after response. The comparability of the sample with these for reference samples, i.e., In foil and In2O3, means that the oxidation state of indium within the S2−In catalyst earlier than and after response lies between zero and +three. We additional carried out in situ XANES measurements for the S2−In catalyst beneath electrocatalytic CO2RR at −zero.98 V (versus RHE). The consequence signifies that indium can also be within the oxidation state between In0 and In3+ beneath response circumstances (Fig. 2nd). The In 3d5/2 and In 3d3/2 spectra obtained from XPS measurements might be deconvoluted into In0 and In3+ parts for the S−In samples with completely different sulfur contents (Fig. 2e)23. This additional means that In0 and In3+ species co-exist on the surfaces of S−In catalysts. The S 2p spectra for the S−In catalysts displayed a peak at 161.eight eV, which might be assigned to S2− in sulfides (Fig. 2f)26. The O 1 s spectra displayed a peak at 530.6 eV, which might be assigned to O2− in In2O3 (Supplementary Fig. 13)23,27. Thus, In2O3 species co-exist with metallic In on the catalyst floor along with sulfide species. The XPS outcomes for the S2−In catalyst after electrocatalytic CO2RR response revealed that the floor states of indium, sulfur and oxygen didn’t endure vital modifications in the course of the electrocatalysis (Supplementary Fig. 14).

Our electrochemical characterizations clarified that the ECSA for the S−In collection of catalysts didn’t change considerably with sulfur content material (Supplementary Desk 2). Thus, as talked about earlier than, the enhancing impact of sulfur is just not active-surface-area associated. The linear sweep voltammetry and electrochemical impedance spectra measurements in CO2-saturated zero.5 M KHCO3 aqueous resolution confirmed that the presence of sulfur on indium elevated the cathodic present density and accelerated the charge-transfer kinetics within the electrocatalysis (Supplementary Fig. 15).

Functioning mechanism of sulfur and results of indium state

Our current work has demonstrated that the sulfur-modified In catalyst may be very promising for electrocatalytic discount of CO2 to formate. To know the position of sulfur extra deeply, it’s essential to disentangle various factors which will contribute to CO2RR within the current system. Our outcomes present that the S0−In catalyst with out sulfur fabricated by electroreduction of In2O3 precursor rising on carbon fibers reveals increased FE of formate than In foil (Fig. 1a and Supplementary Fig. 16a). The exercise of the S0−In catalyst, expressed by the ECSA-corrected formation price of all merchandise (together with HCOO−, CO, and H2), is nearly the identical with that of In foil (Supplementary Fig. 16b). The S0−In catalyst reveals nanoparticulate morphology with a median diameter of 128 nm, whereas In foil has easy surfaces (Supplementary Fig. 8a and 8g). Furthermore, our XPS measurements reveal that a small fraction of In3+ (i.e., In2O3) species co-exists with metallic In on the S0−In floor. The nanostructured morphology and the presence of oxidized species on metallic catalysts had been reported to be helpful to CO2RR23,28,29,30. Particularly, In(OH)three was proposed to play an important position within the formation of formate or CO23,30. Our XPS outcomes indicated the co-existence of In2O3 however not In(OH)three with In0 in our case. To know the position of floor oxidized species, we additional pretreated In foil in air at 250 °C for three h to generate a protection of In2O3 on In surfaces. The electrocatalytic CO2RR consequence confirmed that the FE of formate elevated on the surface-oxidized In foil, though the formation price of all merchandise primarily based on ECSA didn’t change considerably (Supplementary Fig. 16). We carried out CO2 adsorption beneath gas-phase circumstances to match the CO2 adsorption capability amongst completely different catalysts. Our measurements revealed that the ECSA-corrected CO2 adsorption quantity elevated within the sequence of In foil < surface-oxidized In foil < S0−In (Supplementary Fig. 17), and this agrees with the sequence of FE of formate. Subsequently, we suggest that the co-existence of oxide species, in addition to the nanostructure morphology might account for the excessive FE of formate in the course of the CO2RR over the S0−In catalyst in all probability by enhancing the adsorption of CO2 onto catalyst surfaces.

To display the intrinsic position of sulfur, now we have modified the S0−In catalyst with sulfur by a easy impregnation technique. The obtained S-impregnated S0−In catalysts with sulfur contents starting from zero to 7.1 mol% have been used for CO2RR. The formation price of formate elevated with a rise in sulfur content material as much as 2.6 mol% after which decreased (Supplementary Fig. 18a). We carried out related research utilizing In foil to additional exclude the influences of nanostructures and floor oxide species. The electrocatalytic CO2RR utilizing S-impregnated In foil catalysts with sulfur contents of zero–7.zero mol% confirmed related dependences of catalytic behaviors on sulfur content material (Supplementary Fig. 18b). The presence of sulfur on In foil with a correct content material (≤2.2 mol%) vital enhanced the formation price of formate, though the worth of formation price was a lot decrease as in contrast with that on the S-impregnated S0−In collection of catalysts. The change in FE of formate with sulfur content material was much less vital for each collection of catalysts (Supplementary Fig. 18). These outcomes are according to these noticed for the S−In collection of catalysts (Fig. 1a) and make sure that the sulfur species on In surfaces contributes to selling the exercise of CO2RR to formate.

Furthermore, after we added a small quantity of Zn2+ to dam the floor S2− websites by way of the robust interplay between Zn2+ and S2− websites31, the formation price of formate over the S2−In catalyst decreased from 1002 to 687 μmol h−1 cm−2 (Supplementary Desk three). This remark supplies additional proof that it’s the S2− species however not different components that performs a key position in accelerating the CO2RR to formate over the S−In catalysts.

As talked about above, the enhancement within the adsorption and activation of CO2 is significant for acquiring excessive CO2RR efficiency. Nonetheless, our outcomes reveal that the presence of sulfur doesn’t considerably promote CO2 adsorption (Supplementary Fig. 17). We suggest that the sulfur species might improve the CO2RR to formate by accelerating the activation of water. As proven in Eq. 1, the discount of CO2 to formate additionally consumes H2O, however up to now the activation of H2O has been neglected within the CO2RR. Particularly, the activation of H2O in alkaline media is a sluggish step, which even determines the kinetics of H2 evolution response (HER)32,33. It’s reported that the H2 formation exercise is one order of magnitude decrease beneath alkaline circumstances (pH = 13) than that in an acid electrolyte (pH = 1) in the course of the HER over Au(111) surfaces33, due to the problem within the discount of water in alkaline electrolyte as in comparison with the discharging of hydronium in acid electrolyte. The alkaline electrolyte is extensively employed in literature for CO2RR and likewise in our work. We think about that the activation of H2O would even be a sluggish step for CO2RR in alkaline medium.

To achieve additional insights into the position of the activation of H2O in CO2RR, now we have carried out research on the kinetic isotope impact (KIE) of H/D over the S2−In catalyst. When D2O was used to switch H2O in zero.5 M KHCO3 electrolyte, the formate fashioned was nearly within the type of DCOO– (538 μmol h−1 cm−2) as an alternative of HCOO– (10 μmol h−1 cm−2) (Supplementary Fig. 19). This means that the hydrogen in formate primarily originates from water somewhat than HCO3–. The KIE of H/D in CO2RR to formate was calculated to be 1.9. This KIE worth is attribute of main kinetic isotopic impact34. We’ve additionally measured KIE of H/D in zero.5 M K2SO4 electrolyte and obtained the identical consequence (Supplementary Fig. 19). This consequence supplies proof that the dissociation of water is concerned within the price figuring out step for CO2RR to formate over our S−In catalysts.

In our system, in N2-saturated 1.zero M KOH resolution with out CO2, the formation price of H2 was additionally discovered to extend with a rise in sulfur content material within the collection of S−In catalysts or within the S-impregnated S0−In and In foil catalysts (Supplementary Fig. 20). A number of latest research have additionally reported the position of adsorbed anions together with Sδ− species on metallic surfaces in accelerating the activation of H2O in alkaline media31,32,33. It’s proposed that Sδ−−hydrated cation (Okay+(H2O)n) networks might be fashioned within the double layer by way of non-covalent Coulomb interactions between the floor anionic sulfur species and the hydrated cation. This will promote the dissociation of H2O to type adsorbed hydrogen intermediate (H*), i.e., the Volmer step (2H2O + M + 2e− → 2M−H* + 2OH−), which is believed to be a sluggish step in HER31,32,33. Nonetheless, to one of the best of our data, there is no such thing as a report back to correlate the CO2RR exercise with the enhancement within the activation of H2O, as a result of the present consensus is the enhancement in HER would lower the CO2RR selectivity.

To acquire additional proof for the position of H2O activation in CO2RR, now we have investigated the impact of pH of electrolyte on electrocatalytic CO2RR over S0−In and S2−In catalysts. Three completely different electrolytes, i.e., K2HPO4, KHCO3, and K2SO4, had been employed to control the pH worth, as a result of it’s identified that the native pH on the cathode/electrolyte interface will increase within the following sequence: K2HPO4 < KHCO3 < K2SO435,36. Our electrocatalytic outcomes present that the formation price and FE of formate improve within the sequence of K2HPO4 < KHCO3 < K2SO4 over each catalysts (Supplementary Fig. 21a and 21b), indicating that a increased native pH surroundings favors the formation of formate. As in comparison with the S0−In, the S2−In catalyst exhibited increased formation price and FE of formate utilizing all of the three electrolytes. Moreover, the ratio of formation charges of formate for the S2−In and S0−In catalysts, i.e., RateS2−In/RateS0−In, elevated from 1.four to 1.9 and additional to 2.1 upon altering the electrolyte from K2HPO4 to KHCO3 and additional to K2SO4 (Supplementary Fig. 21a). This means that the position of sulfur in enhancing the formation of formate is extra vital at the next pH worth. This helps our hypothesis that the sulfur modification enhances formate formation by accelerating the activation of H2O, which turns into harder at the next pH32,33.

To acquire additional info on the marketing impact of sulfur on indium surfaces, now we have carried out density useful idea (DFT) calculations for CO2RR to HCOOH and CO on indium sole and sulfur-doped indium surfaces, and the outcomes are summarized in Supplementary Desk four. The optimized adsorption configurations of reactants, intermediates and merchandise on indium and sulfur-doped indium surfaces are displayed in Fig. 3a and Supplementary Fig. 22. The activation of CO2 happens on indium websites and the switch of a proton/electron pair or adsorbed H intermediate to CO2 results in the formation of certain formate intermediate (HCOO*) on two indium websites through two oxygen atoms (Fig. 3a) or certain carboxylate intermediate (*COOH) on single indium website through carbon atom (Supplementary Fig. 22). HCOO* and *COOH are believed to be intermediates for the formations of HCOOH and CO, respectively37,38,39. For the HCOOH pathway, the Gibbs free energies (ΔG) for the formations of HCOO* and HCOOH* are zero.29 and zero.67 eV, respectively on indium solely surfaces (Fig. 3b). The presence of sulfur on indium considerably decreases the corresponding Gibbs free energies for HCOO* and HCOOH* to −zero.16 and zero.10 eV, respectively (Fig. 3b). These outcomes recommend that the doping of sulfur on indium surfaces makes the HCOOH pathway considerably energy-favorable. For the CO pathway, the Gibbs free energies for the formation of *COOH are 1.49 and zero.82 eV on pure and sulfur-doped indium surfaces, respectively (Fig. 3c). Thus, the HCOOH pathway is extra energy-favorable than the CO pathway, and thus can interpret why each pure and sulfur-doped indium surfaces possess increased selectivity of HCOOH than that of CO.

Fig. threeFig. 3

DFT calculation outcomes and response scheme. a Optimized configurations of I CO2, II HCOO*, III HCOOH*, IV HCOOH on (101) side of pure indium (In) and V CO2, VI HCOO*, VII HCOOH*, VIII HCOOH on (101) side sulfur-doped indium (S–In). b Gibbs free power diagrams for CO2RR to HCOOH on In (101) and S−In (101) surfaces. c Gibbs free power diagrams for CO2RR to CO on In (101) and S−In (101) surfaces. d Gibbs free energies for the formation of H* on pure In (101), In and S websites of S−In (101) surfaces. e Schematic illustration for the position of S2− in selling water dissociation and H* formation for the discount of CO2 to formate. Free energies of b, c and d are proven relative to fuel CO2 and H2. The inexperienced, yellow, grey, crimson, and blue balls signify In, S, O, C, and H

We’ve additional calculated the Gibbs free energies for the HER within the absence of CO2 on pure In and sulfur-doped In surfaces. The formation power of H* species is zero.21 eV on sulfur websites of sulfur-doped In, considerably decrease than that on In websites of sulfur-doped In (zero.69 eV) and pure In (zero.82 eV) (Fig. 3d). The decrease formation power of H* species means the next exercise of H2O dissociation on the electrocatalyst floor40,41,42,43. Subsequently, our calculation outcomes point out that the sulfur modification can improve the HER and the sulfur website on In surfaces is chargeable for the dissociation of H2O to type the adsorbed H* intermediate. Alternatively, within the presence of CO2, our DFT calculation reveals that the doping with sulfur has turned the formation of HCOO*, the precursor of formate, on the S−In floor to be exergonic (Fig. 3b), whereas the formation of H* from H2O alone, nonetheless stay endergonic. We consider that that is the key purpose for why the formation of formate however not the formation of H2 is preferentially enhanced within the case of CO2RR after the modification of In by sulfur (Fig. 1a), though sulfur can enhance the activation of H2O.

On the idea of the outcomes and dialogue described above, we suggest that the floor S2− species serves as an anchor to maintain the Okay+(H2O)n cation near indium surfaces within the double layer through Coulomb interactions (Fig. 3e). The near-surface H2O molecules might be activated facilely, forming adsorbed H* intermediate and releasing an OH− anion. The H* intermediate can subsequently react with the adsorbed CO2 to type a certain HCOO* intermediate. After accepting an electron, HCOO* is remodeled to formate and desorbs from indium surfaces. These proposed elementary steps are displayed in Supplementary Word 1.

It’s noteworthy that Pérez-Ramírez and associates lately reported a promotion impact of sulfur modification on the discount of CO2 to formate over Cu catalyst21,44. The doping of sulfur primarily modified the product selectivity and the FE of formate elevated from 26% to 78% after sulfur modification over the Cu catalyst. The sulfur adatom on Cu surfaces is proposed to take part actively in CO2RR as a nucleophile both by transferring a hydride or by tethering CO2, thus suppressing the formation of CO.44 The completely different behaviors of sulfur doping on In and Cu catalysts reveal diversified functioning mechanisms of sulfur for CO2RR.

Generality of chalcogenide-modified metal-catalyzed CO2RR

Platinum is a robust HER catalyst with robust skill for the formation of adsorbed atomic hydrogen species32,41, and thus Pt may additionally work as a promoter for CO2RR in response to our speculation. We discovered that the doping of small quantity of Pt onto indium may promote the formation of formate to some extent (Supplementary Fig. 23a), however the FE of formate decreased as a result of the formations of H2 and CO had been accelerated extra considerably (Supplementary Fig. 23b). Pt not solely enhances the formation of adsorbed H* species but in addition accelerates the recombination of H* to H2, and thus is just not an excellent promoter for CO2RR to formate. Alternatively, sulfur accelerates the activation of H2O with out considerably enhancing the recombination of H* intermediates to H2.

Indium catalysts doped with different chalcogen species have additionally been investigated for the CO2RR. A collection of selenium-doped and tellurium-doped indium catalysts, denoted as Se−In and Te−In, had been fabricated by the same technique to that for the S−In catalysts. The electrocatalytic outcomes confirmed that the doping of a correct quantity of selenium or tellurium may additionally promote the formation of formate (Fig. 4a). The FE of formate additionally barely elevated by the modification of In with Se or Te (Supplementary Fig. 24a), suggesting that selenium or tellurium modifier performed related roles to sulfur. The formation price of formate decreased within the sequence of S−In > Se−In > Te−In (Fig. 4a). The lower within the electronegativity alongside the chalcogen group from sulfur to tellurium would lower the interplay between chalcogenide and the hydrated cation43, and thus would decrease the power to activate H2O to adsorbed H* species.

Fig. fourFig. 4

A number of promoted metal-catalyzed CO2RR methods. a Formation charges of formate over S−In, Se−In and Te−In catalysts at −zero.98 V (versus RHE) for 1 h. b Impact of alkali metallic cations (Na+, Okay+, and Cs+) in MHCO3 (for CO2RR) or MOH (for HER) electrolyte on CO2RR and HER performances for S0−In and S2−In catalysts at −zero.98 V (versus RHE) for 1 h. c Impact of alkali metallic cations (Na+, Okay+, and Cs+) in MHCO3 for CO2RR on common present density and FE of formate at −zero.98 V (versus RHE) for 1 h. d Formation charges of formate over S−Bi and S−Sn catalysts at −zero.98 V (versus RHE) for 1 h. The experiments in every case had been carried out a minimum of for thrice. The error bar represents the relative deviation

The Okay+ cation within the electrolyte might be changed by different alkali metallic cations, however the CO2RR efficiency was affected by the metallic cation employed. Upon altering the cation from Na+ to Okay+ and additional to Cs+, the formation price of formate elevated considerably from 789 to 1002 and additional to 1449 μmol h−1 cm−2 at a possible of −zero.98 V versus RHE over the S2−In catalyst (Fig. 4b). The present density elevated from 47 to 57 and additional to 84 mA cm−2 on the similar time, whereas the FE of formate stored at 91–93% (Fig. 4c). Alternatively, the change within the response price was very restricted over the S0−In catalyst with out sulfur and the present density was in 32–36 mA cm−2 by altering the metallic cation from Na+ to Cs+ (Fig. 4b, c). We speculate that the smaller ionic hydration quantity and radius of hydrated cation of Cs+ (H2O)n (n = 6 for Cs+ versus n = 7 for Okay+ and 13 for Na+)45,46 lead to stronger interactions with S2− on In surfaces and thus increased skill to activate H2O. These outcomes present additional proof for our speculation that sulfur on indium surfaces features for H2O activation through interplay with hydrated metallic cations within the double layer (Fig. 3e). Furthermore, the usage of Cs+ as an alternative of Okay+ can additional improve the CO2RR efficiency of the S2−In catalyst. A formation price of formate of 1449 μmol h−1 cm−2 with a formate FE of 93% might be achieved at −zero.98 V versus RHE, considerably higher than these reported thus far (Supplementary Desk 1).

Moreover indium, we discovered that the technique to reinforce the CO2RR to formate by doping sulfur might be prolonged to different metals equivalent to bismuth and tin. The doping of sulfur onto Bi and Sn surfaces with a correct quantity considerably promoted the formation price of formate (Fig. 4d). The formation charges of formate reached 767 and 640 μmol h−1 cm−2 over S1−Bi and S1−Sn catalysts at −zero.98 V versus RHE, which had been 1.four and 1.5 instances increased that of Bi and Sn catalysts with out sulfur, respectively. The FE of formate stored nearly unchanged or barely elevated on the similar time (Supplementary Fig. 24b). These outcomes affirm the generality of our technique to speed up the CO2RR to formate by enhancing H2O activation by way of modifying metallic surfaces with an applicable quantity of chalcogenide species.


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