Layered bismuth oxide/bismuth sulfide supported on carrageenanderived carbon for efficient carbon dioxide electroreduction to formate

Xiangzhao Hu,Junjie Sun,Wanzhen Zheng,Sixing Zheng,Yu Xie,Xiang Gao,Bin Yang,Zhongjian Li,Lecheng Lei,Yang Hou,2,4,*

1 Key Laboratory of Biomass Chemical Engineering of Ministry of Education,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

2 Institute of Zhejiang University– Quzhou,Quzhou 324000,China

3 Department of Material Chemistry,Nanchang Hangkong University,Nanchang 330063,China

4 School of Biological and Chemical Engineering,Ningbo Technology University,Ningbo 315100,China

Keywords:Nanomaterials Catalyst Selectivity Bi2S3/Bi2O3 CO2 electroreduction Formate

ABSTRACT Electrochemical reduction of carbon dioxide (CO2ER) into formate plays a crucial role in CO2 conversion and utilization.However,it still faces the problems of high overpotential and poor catalytic stability.Herein,we report a hybrid CO2ER electrocatalyst composed of layered bismuth sulfide (Bi2S3) and bismuth oxide(Bi2O3)supported on carrageenan derived carbon(Bi-CDC)prepared by a combined pyrolysis with hydrothermal treatment.In such 3D hybrid,layered Bi2O3 and Bi2S3 are uniformly grown on nanocarbon supports.Benefiting from strong synergistic effect between Bi2O3/Bi2S3 and nanocarbon,Bi-CDC-1:2 displays a high Faradic efficiency (FE) of >80% for formate production in the range of-0.9 V to -1.1 V with the maximum formate FE of 85.6% and current density of 14.1 mA.cm-2 at-1.0 V.Further,a positive onset potential of -0.5 V,a low Tafel slope of 112.38 mV.dec-1,and a slight performance loss during long-term CO2ER tests are observed on Bi-CDC-1:2.Experimental results shows that the better CO2ER performance of Bi-CDC-1:2 than that of Bi2O3 can be attributed to the strong interfacial interactions between nanocarbons and Bi2O3/Bi2S3.In situ ATR-FTIR measurements reveal that the rate-determining step in the CO2ER is the formation of HCOO* intermediated.Compared with carbon support,Bi-CDC-1:2 can promote the production of HCOO* intermediate and thus promoting CO2ER kinetic.

1.Introduction

The concentration of carbon dioxide in the atmosphere has continued to rise due to the massive combustion of fossil fuels,causing a series of environmental problems such as rising sea levels and global warming [1–4].Electrochemical CO2reduction can use green energies such as wind,solar and tidal energies to convert CO2into high value-added chemicals [5],such as CO [6,7],CH4[8],HCOOH[9]and CH3CH2OH[10],etc.,which can not only reduce the greenhouse effect,but also alleviate the energy crisis.However,considering that the conversion of CO2requires high activation energy,and a variety of reduction products can be obtained through different reaction paths,it is necessary to explore suitable catalysts to promote the activity and selectivity of CO2electroreduction [11].

Among various products of CO2electrochemical reduction,formate is particularly desirable due to its ease of transportation and storage,high volumetric hydrogen density and wide versatility in multiples practical applications [12].To date,several metal catalysts (such as Sn [9],In [13] and Pb [14]) and some of their oxides(such as,SnO2[15,16] and In2O3[17]) have been investigated for converting CO2into formate via CO2ER.Among them,bismuth(Bi)-based materials are considered as promising electrocatalysts toward CO2ER owing to its low toxicity and low cost (compared to In and Pb) [18].In addition,it is reported that the Bi materials can efficiently inhibit the hydrogen evolution reaction because it has a positive hydrogen adsorption free energy[19].Certain efforts have been made in developing Bi-based materials.For instance,Zhang et al.prepared ultrathin Bi nanosheets via a liquid-phase exfoliation of bulk Bi in an isopropanol solution of NaOH.Compared with bulk Bi,exfoliated Bi nanosheets presented an enhanced formate FE and increased current density [20].Also,nanostructured Bi2O3catalysts with different morphologies have been reported for the electroreduction of CO2to formate in aqueous solutions [21].

Recently,combining Bi-based nanostructures with conductive carbonaceous supports represents an effective strategy to further promote the electrochemical performance.The carbon supports not only provide a stable base and improve the stability of catalyst,but also enable the effective electron transfer from carbon support to reactive sites,which strengthen the adsorption of key reaction intermediate and thus promote CO2ER [22].For instance,BiOxnanoparticles supported on Vulcan XC-72R carbon black was synthesized by a solvothermal method [23].Similarly,Liu et al.reported Bi2O3nanosheets grown on carbon nanofiber and used it as the cathode catalyst for CO2ER [24].The synergistic effect between Bi2O3and carbon supports was believed to enhance the binding of * OCHO intermediate on catalyst surface.Unfortunately,currently reported Bi-based CO2ER materials are all with high overpotentials and poor catalytic stability,which limit their industrial application[25].Great efforts have been made to improve the catalytic performances of Bi-based electrocatalysts for CO2ER,especially for the strategy by doping foreign element in Bi-based electrocatalysts.Previous reports revealed that S doped bismuth oxide could obviously promote the formate production via CO2ER,attributed to the enhanced adsorption capacity of CO2and decreased energy barrier for the formation of * HCOO intermediate[26].However,the overall conductivity of S doped bismuth oxide is unsatisfactory.To overcome this problem,the carbon materials with high conductivity can be used as the supports to enhance the conductivity of Bi-based CO2ER electrocatalysts.Among various carbon-based supports,Carrageenan is the sulfated polygalactan,which can be directly used as precursor to synthesize the S doped porous carbon [27].However,so far,there have been no reports on using the carrageenan derived carbon as support to enhance the conductivity of electrocatalyst,and as precursor to prepare the S element modified CO2ER electrocatalyst,simultaneously.

Herein,we reported a hybrid CO2ER catalyst with layered Bi2O3/Bi2S3supported on the surface of carrageenan derived carbon (Bi-CDC) by pyrolyzing carrageenan to obtain carbon supports,followed by hydrothermal treatment with Bi(NO3)3.5H2O to form Bi2O3/Bi2S3.Layered Bi2O3/Bi2S3with thickness of～10 nm was supported on porous nanocarbons with average diameter of～3 μm,which was denoted as Bi-CDC.Owing to the strong interfacial interactions between Bi2O3/Bi2S3and porous nanocarbons,asprepared Bi-CDC-1:2 catalyst exhibited an outstanding CO2ER performance for producing formate,featured by a small onset potential of -0.5 V,and high FE value of 85.6% at -1.0 V.Further in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR)measurements revealed that the formation of HCOO*intermediate was the rate-determining step toward formate generation.Systematic characterization and comprehensive electrochemical results demonstrated that the strong synergistic effect between Bi2O3/Bi2S3and carbon supports could promote the production of HCOO*intermediate,thus enhancing its CO2ER performance.

2.Materials and Methods

2.1.Materials

Carrageenan was obtained from Shanghai yuanye Bio-Technology Co.,Ltd;bismuth nitrate pentahydrate(Bi(NO3)3.5H2O,≥99%),and sulfuric acid were bought from Sinopharm Chemical Reagent Co.,Ltd.All the chemicals were used as obtained without further purification.

2.2.Synthesis of carrageenan derived carbon (CDC)

0.4 g of carrageenan was added into porcelain boat and heated at 600 °C for 2 h at a heating rate of 5 °C.min-1under N2atmosphere.Then,the calcined sample was washed with 2.0 mol.L-1H2SO4for 12 h to completely remove metal impurities.After filtration and drying,the target product of CDC was obtained.

2.3.Synthesis of Bi-CDC-(1:4/1:2/4:1)

40 mg of CDC and 20 mg of Bi(NO3)3.5H2O were added into 30 ml of deionized water.The mixture solution was stirred for 1 h.Then,the resulting mixture was transferred into a Teflonlined autoclave with 50 ml capacity and then heated to 180 °C for 12 h.The resultant precipitate was then filtered and washed three times with deionized water.The target product of Bi-CDC-1:2 was obtained after drying in vacuum at 60°C for 10 h.The procedures for synthesis of Bi-CDC-1:4 and Bi-CDC-4:1 were similar to that of Bi-CDC-1:2,expect for changing the amount of Bi(NO3)3-.5H2O to 10 mg and 160 mg,respectively.The control catalyst of Bi-CDC-1:2 without CDC adding was prepared by using the same synthesis method as that of Bi-CDC-1:2,except without added CDC [28].

2.4.Characterizations

The morphologies of as-prepared catalysts were characterized by field-emission scanning electron microscopy (FESEM,SU-8010) and high-resolution TEM (HRTEM,200 kV,JEM-2100).The crystal structures of as-prepared catalysts were determined by Xray powder diffractometer (XRD,ZETIUM DY 2186,4 kW).Raman spectra of as-prepared catalysts were determined by a Raman spectrometer (HR Evolution,532 nm).The electronic states in the as-prepared catalysts were determined by X-ray photoelectron spectroscopy (XPS).

2.5.Electrochemical measurements

To prepare the working electrode,10 mg of as-prepared catalysts was dispersed into mixture solution containing 100 μl 0.5%(-mass) Nafion solution and 900 μl ethanol.After sonication for 2 h and stirred for 12 h,the obtained ink (100 μl) was dropped onto the carbon paper and used as working electrode (WE,active area was 1 cm×1 cm).Electrochemical measurements were performed on a three-electrode system and used an electrochemical workstation (CHI 760E).Counter electrode (CE) was a Pt wire,and reference electrode (RE) was an Ag/AgCl (saturated KCl) electrode.The 0.5 mol.L-1KHCO3solutions pre-saturated with CO2were used as electrolytes.During the CO2ER measurements,CO2gas was transported at a constant rate of 20 ml.min-1.The linear sweep voltammogram (LSV) curves were measured with a scan rate of 5 mV.s-1.The electrochemically active surface area (ECSA) were tested between -0.8 V and -0.9 V (vs.Ag/AgCl).Electrochemical impedance spectroscopy (EIS) was tested in a frequency range between 0.01 Hz and 105Hz.Liquid and gas products were analyzed by1H nuclear magnetic resonance (BRUKER AVIII500M)and gas chromatograph(FuLi 9790II).All potentials involved in this work were converted to reversible hydrogen electrode (RHE)according to the following formula:

where the experimental temperature is 25 °C;pH of 0.5 mol.L-1KHCO3solution is 7.2;is 0.199 V at 25 °C.

2.6.In-situ ATR-FTIR experiments

In-situ ATR-FTIR tests used a BRUKER INVENIO R spectrometer and a liquid nitrogen-cooled MCT detector.The reflective element was a silicon facet crystal.The ultra-thin gold foil was deposited on the silicon surface to enhance the signal.The electrocatalyst was dropped on the Au membrane with a loading amount of 0.1 mg.cm-2and used as the working electrode.Counter electrode was a Pt wire,and reference electrode was an Ag/AgCl (saturated KCl)electrode.The 0.5 mol.L-1KHCO3solutions pre-saturated with CO2were used as electrolytes.The potential was set from 0 V to-1.2 V.

3.Results and Discussion

3.1.Structural characterization of Bi-CDC-1:2

Fig.1(a) illustrated the fabrication process of Bi-CDC-1:2 catalysts.Typically,carrageenan derived carbon was prepared through a pyrolysis of carrageenan at 600°C for 2 h,followed by acid treatment,which resulted in the formation of porous nanocarbons(CDC,Fig.S1,Supplementary Material) [29].Then,the CDC and Bi(NO3)3.5H2O were mixed in water and hydrothermally treated at 180 °C for 12 h.As a result,the Bi2O3and Bi2S3hybrids supported on CDC (Bi-CDC-1:2) were obtained.The morphologies of asprepared Bi-CDC-1:2 were characterized by field emission scanning electron microscopy (FESEM).As displayed in Fig.1(b) and(c),the surface of carbon supports was well covered by layered Bi2O3and Bi2S3.The diameter of nanocarbons was～3 μm and the thickness of layered Bi2O3/Bi2S3was～10 nm.Further,highresolution transmission electron microscopy (HRTEM) clearly displayed that Bi2O3and Bi2S3were confined into the carbon support(Fig.1(d)).The lattice fringes were about 0.319 nm and 0.374 nm,in consistent well with the Bi2O3(201)and Bi2S3(101)planes[30],respectively.

Four different diffraction rings and scattered dots shown in selected area electron diffraction (SAED) pattern of Bi-CDC-1:2 could be attributed to Bi2O3and Bi2S3(Fig.1(e)) [31,32].The elemental composition of Bi-CDC-1:2 was characterized by energydispersive X-ray spectroscopy (EDX) mapping images (Fig.1(f)),and the results displayed the homogeneous distribution of S,O and Bi species on the carbon matrix in the Bi-CDC-1:2.The crystal structures of Bi-CDC-1:2 were determined by X-ray diffraction(XRD) and Raman spectroscopy [33].As shown in Fig.1(g),no diffraction peaks belong to carbon was observed for Bi-CDC-1:2,possibly due to the formed amorphous structure of carbon supports.The diffraction peaks located at 27.9°,32.7°,41.3°,46.2°,55.5°,and 77.9° were associated to the (201),(220),(212),(222),(421),and (620) planes of Bi2O3(JCPDS No.27-0050) [31],and the peaks located at 11.1°,23.7°,31.8°,35.6°,48.2°,and 59.1°were associated to the(110),(101),(221),(240),(060),and(242)planes of Bi2S3(JCPDS No.17-0320) [32],respectively.

The Raman spectrum of Bi-CDC-1:2 was shown in Fig.2(a).The D and G bands were found at 1342 and 1573 cm-1,respectively.The ID/IGvalue of Bi-CDC-1:2 was calculated to be 0.91.Two Raman bands centered at 234 and 256 cm-1were associated with Bi2S3[32],while the peak located at 313 cm-1was associated with Bi2O3[34].X-ray photoelectron spectroscopy (XPS) of Bi-CDC-1:2 revealed the co-existence of Bi,S,C,N and O elements (Fig.2(b)).In high resolution Bi 4f XPS spectrum (Fig.2(c)),two clear peaks at binding energies of 159.4 and 164.7 eV could be attributed to Bi3+4f7/2and Bi3+4f5/2,respectively,agree well with the XRD results.The high resolution O 1s XPS spectrum displayed the presence of Bi-O and Bi-OH bonds at 531.8 and 533.3 eV,respectively(Fig.2(d)) [31].

Fig.1.(a) Schematic fabrication process of Bi-CDC.(b–c) FESEM images of Bi-CDC-1:2.(d–e) HRTEM image and SAED pattern of Bi-CDC-1:2.(f) EDX elemental mapping images of S,O and Bi in Bi-CDC-1:2.(g) XRD pattern of Bi-CDC-1:2.

Fig.2.(a) Raman spectrum,(b) XPS full spectrum,(c) high resolution Bi 4f XPS spectrum,and (d) high resolution O 1s XPS spectrum of Bi-CDC-1:2.

3.2.Electrochemical performance measurements

The CO2ER catalytic activities of as-prepared samples were carried out in CO2-saturared 0.5 mol.L-1KHCO3solution using a three-electrode system.All potentials were converted versus reversible hydrogen electrode (RHE) in this work.The gaseous products for CO2ER were tested by on-line gas chromatograph(GC),the liquid products were collected and measured by1H nuclear magnetic resonance(1H NMR)(Fig.S2).The generated formate was the main CO2ER product,together with minor amount of H2and CO gases.As shown in Fig.3(a),linear sweep voltammetry(LSV)curve in CO2-saturated 0.5 mol.L-1KHCO3solution displayed that the Bi-CDC-1:2 featured a current density of 14.1 mA.cm-2at-1.0 V,much higher than that in Ar-saturated KHCO3electrolyte at-1.0 V.Further,the onset potential was estimated to be-0.5 V for Bi-CDC-1:2,smaller than that in Ar-saturated electrolyte,which indicated a high CO2ER performance of Bi-CDC-1:2.The corresponding formate FE values of Bi-CDC samples were given in Fig.3(b).Specifically,the Bi-CDC-1:2 exhibited the maximum formate FE of 85.6% at -1.0 V,which was about 1.2 and 1.1 times higher than that of Bi-CDC-1:4 (71.3%) and Bi-CDC-4:1 (77.9%),respectively.Notably,the Bi-CDC-1:2 possessed a wide potential window of formate FE >80% from -0.9 V to -1.1 V as compared to other control samples,further confirming the superior CO2ER activity of Bi-CDC-1:2 catalysts.Additionally,partial current densities) of formate production for Bi-CDC samples were displayed in Fig.3(c).Theof three samples exhibited a gradually increasing trend with the negative change of applied potentials.The Bi-CDC-1:2 presented the highestof 10.1 mA.cm-2at -1.0 V,which was 1.77 and 1.39 times higher than those of Bi-CDC-1:4 (5.7 mA.cm-2) and Bi-CDC-4:1(7.7 mA.cm-2)at the same potential,respectively.For comparison,the control catalyst of Bi-CDC-1:2 without carrageenan derived carbon (CDC) added was synthesized (Fig.S3).As shown in Fig.S4 and Fig.3(d),the Bi-CDC-1:2 without CDC adding possessed a small ECSA value of 0.39 mF.cm-2and a poor catalytic performance for CO2ER (formate FE <70%).Clearly,the Bi-CDC-1:2 possessed a much higher ECSA value of 11.97 mF.cm-2(Fig.S4) and larger formate FE (maximum formate FE=85.6%) than that of Bi-CDC-1:2 without CDC adding.Thus,the layer structure of Bi-CDC-1:2 played an important effect on increasing the ECSA of electrocatalysts,thus exposing more active sites for boosting the whole CO2ER activities [35].Further,the formate FE of CDC was also investigated (Fig.3(d)).It was noteworthy that the Bi-CDC-1:2 always gave the higher formate FE than of bare CDC at different potentials were lower than 30%.The above results concluded that the strong synergistic effect between Bi2O3/Bi2S3and CDC played a positive effect on enhancing the CO2ER performance [36].

Fig.3.(a)LSV curves of Bi-CDC-1:2 in Ar-saturated or CO2-saturated 0.5 mol.L-1 KHCO3 solutions,(b–c)FE values and partial current densities for foramte over Bi-CDC-1:4,Bi-CDC-1:2,and Bi-CDC-4:1 in CO2-saturated 0.5 mol.L-1 KHCO3 solutions,(d) FE values for foramte over Bi-CDC-1:2,CDC,and Bi-CDC-1:2 without CDC adding.

Further electrochemical measurements were performed to evaluate the CO2ER performance of as-prepared Bi-CDC samples.As shown in Fig.4(a),the measured Tafel slop for Bi-CDC-1:2 was 112.38 mV.dec-1,lower than that of Bi-CDC-1:4(125.01 mV.dec-1) and Bi-CDC-4:1 (128.36 mV.dec-1),indicating the fastest reaction kinetics of Bi-CDC-1:2 for CO2ER to produce formate[37].In addition,the ECSA values of Bi-CDC samples were also estimated through the double-layer capacitance (Cdl) value(Fig.S6)[38].The Bi-CDC-1:2 owned the highest Cdlwith a value of 11.97 mF.cm-2,which was much higher than that of Bi-CDC-1:4(4.94 mF.cm-2)and Bi-CDC-4:1(4.34 mF.cm-2)(Fig.4(b)),respectively,demonstrating the greatly promoted effective active surface of Bi-CDC-1:2 for CO2ER,through binding a suitable amount of the carbon supports.Moreover,EIS measurements of Bi-CDC samples were conducted at a potential of -0.6 V.In Fig.4(c),the Bi-CDC-1:2 showed the smaller electron transfer resistance compared to other control samples,indicating excellent conductivity and charge transfer rate.Finally,a long term CO2ER stability of Bi-CDC-1:2 catalyst was carried out at -1.0 V associated with corresponding FE tested at each one hour (Fig.4(d)),and the results showed that negligible changes in the formate FE and total current density could be observed over a 10 h test period,supporting the superior CO2ER stability of Bi-CDC-1:2.Additionally,the morphology of Bi-CDC-1:2 after 10-hour CO2ER stability test was characterized by FESEM imaging.As shown in Fig.S7,the Bi-CDC-1:2 still kept the well layered structure after 10-hour CO2ER test,demonstrating a favorable structural stability of Bi-CDC-1:2.

3.3.Reaction mechanism analysis

To further explore reaction mechanism of CO2electroreduction to formate via CO2ER over Bi-CDC-1:2 catalyst,in situ ATR-FTIR tests were employed(Fig.5(a))[39].In Fig.5(b)and 5(c),a characteristic peak located at 1390 cm-1could be observed,which was assigned to the vibration of O-C-O bonds in the HCOO*,and the characteristic peak located at 1648 cm-1was assigned to adsorbed H2O [40].Meanwhile,the peak intensity of HCOO*intermediate centered at 1390 cm-1increased gradually with the applied potentials increased from-0.4 V to-1.2 V as shown in the enlarge spectra (Fig.5(d)).The calculated peak area of intermediate HCOO*located at 1390 cm-1showed a first-order dependence with the applied potentials (Fig.5(e)),indicating that the formation of HCOO*intermediate was the rate-determining step.Therefore,a possible pathway for the CO2ER on Bi-CDC-1:2 was proposed(Fig.5(f)):1) CO2was absorbed on the surface of Bi-CDC-1:2;2)the absorbed CO2received an electron and combined with a proton to form HCOO*intermediate;3)the intermediate HCOO*obtained an electron to produce absorbed HCOO-;4) the HCOO-desorbed from the Bi-CDC-1:2 surface [41].In addition,we also compared in situ ATR-FTIR results of Bi-CDC-1:2 with bare CDC (Fig.S8),the results showed that the peak intensity of intermediate HCOO*in the CDC sample was lower than that of Bi-CDC-1:2,which demonstrated that the synergistic effect between Bi2O3/Bi2S3and CDC promoted the production of HCOO*intermediate and thus promoting CO2ER reaction kinetic.

4.Conclusions

In summary,we developed a hybrid CO2ER electrocatalyst composed of layered Bi2O3/Bi2S3with a thickness of～10 nm supported on porous nanocarbons.The Bi-CDC-1:2 displayed an impressive CO2ER performance for formate production,featured by the highest formate FE of 85.6% at -1.0 V,a high current density of-14.2 mA.cm-2,a small onset potential of -0.5 V and a low Tafel slope of 112.38 mV.dec-1.The in situ ATR-FTIR analysis unveiled that the formation of HCOO*intermediate was the ratedetermining step.The excellent CO2ER performance of Bi-CDC-1:2 could be attributed to the synergistic effects between Bi2O3/Bi2S3and nanocarbons,which accelerated the formation of intermediate HCOO*during the CO2ER process.The layered Bi2O3/Bi2S3supported on nanocarbons hybrid designed in this work can pave a promising pathway for designing low cost and highly efficient Bi-based CO2ER electrocatalysts,which will provide innovative ways for a wider range of electrochemical applications,such as hydrogen evolution,oxygen reduction,and ammonia oxidation reactions.

Fig.4.(a–c) Tafel slops,calculated ECSA values,and Nyquist plots of Bi-CDC-1:4,Bi-CDC-1:2,and Bi-CDC-4:1,(d) 10 h CO2ER electrolysis of Bi-CDC-1:2 at -1.0 V.

Fig.5.(a)Schematic configuration of in situ ATR-FTIR test,(b–c)in situ ATR-FTIR spectra and corresponding contour image of Bi-CDC-1:2,(d)in situ ATR-FTIR spectra of Bi-CDC-1:2 at-0.4,-0.8,and-1.2 V,(e)peak area of HCOO* intermediate located at 1390 cm-1 over Bi-CDC-1:2 at different potentials,(f)a proposed reaction mechanism of Bi-CDC-1:2 for CO2ER.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21922811,21878270,22178308,and 21961160742),Jiangxi Province ‘‘double thousand plan”project(205201000020),the Zhejiang Provincial Natural Science Foundation of China(LR19B060002),the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (2019R01006),Zhejiang Key Laboratory of Marine Materials and Protective Technologies (2020K10),Key Laboratory of Marine Materials and Related Technologies,CAS,and the Startup Foundation for Hundred-Talent Program of Zhejiang University.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.02.009.