German Edition: DOI: 10.1002/ange.201809255Electrocatalysis Very Important PaperInternational Edition: DOI: 10.1002/anie.201809255

Composition Tailoring via N and S Co-doping and Structure Tuning byConstructing Hierarchical Pores: Metal-Free Catalysts for High-Performance Electrochemical Reduction of CO2

Hengpan Yang, Yu Wu, Qing Lin, Liangdong Fan, Xiaoyan Chai, Qianling Zhang,Jianhong Liu, Chuanxin He,* and Zhiqun Lin*

Abstract: A facile route to scalable production of N and S co-doped, hierarchically porous carbon nanofiber (NSHCF)membranes (ca. 400 cm2 membrane in a single process) isreported. As-synthesized NSHCF membranes are flexible andfree-standing, allowing their direct use as cathodes for efficientelectrochemical CO2 reduction reaction (CO2RR). Notably,CO with 94% Faradaic efficiency and @103 mAcm@2 currentdensity are readily achieved with only about 1.2 mg catalystloading, which are among the best results ever obtained bymetal-free CO2RR catalysts. On the basis of control experi-ments and DFT calculations, such outstanding CO Faradaicefficiency can be attributed to the co-doped pyridinic N andcarbon-bonded S atoms, which effectively decrease the Gibbsfree energy of key *COOH intermediate. Furthermore, hier-archically porous structures of NSHCF membranes imparta much higher density of accessible active sites for CO2RR,leading to the ultra-high current density.

Electrochemical reduction of CO2 in aqueous solution hasbeen widely recognized as a promising strategy for alleviatingthe concerns on the increasing level of atmospheric CO2

concentration. This is because CO2 reduction can be readilyconducted in aqueous solution, electrically driven fromrenewable energy sources, and produce value-added fuels orchemicals.[1–3] Electrochemical catalytic CO2RR, especiallyfor potential industrial-scale applications, depends heavily onstable, recyclable, and sustainable catalysts, which can retaintheir good performance at large current density(> 100 mAcm@2).[4, 5]

The past decades have witnessed metals (such as Ag,[6]

Pd,[7] Au,[8] Co[9]) and their derivatives (that is, metal oxidesand metal complexes) as the most commonly used electro-catalysts for efficient CO2RR.[6–15] However, these traditional

metal-based catalysts often suffer from some inevitableproblems, such as relatively high cost, poor stability inaqueous solution, and potential environmental toxicity.[16,17]

Recently, nanostructured carbon materials have emerged asalternative electrocatalysts to metal- and metal oxide-basedcounterparts for CO2RR. These metal-free carbon electro-catalysts are cost-effective and possess good conductivity andallow for the doping with heteroatoms.[18–26] However, it isnotable that typical heteroatom-doped carbon catalysts areoften powder-like and difficult to recycle, and have lowcatalyst productivity.[21] Although good product selectivity orhigh Faradaic efficiencies were occasionally reported, nano-structured carbon catalysts that maintain good efficiency athigh current density are still comparatively few and limited inscope.[18, 19]

Herein, we report a robust and scalable strategy forsynthesis of nitrogen and sulfur co-doped, hierarchicallyporous carbon nanofiber (NSHCF) membranes via electro-spinning for high-efficiency, metal-free CO2RR. Intriguingly,these NSHCF membranes carry good productivity and can beeasily tailored to specific shapes and then directly used ascathode for CO2RR. The nitrogen defects, particularlypyridinic N, doped into the pristine carbon structure arefound to significantly reduce the free energy barrier towardsthe binding of *COOH intermediate, which is the rate-determining step during the course of CO2RR.[27] Moreover,the introduction of S defects feature higher spin density andcharge delocalization within carbon structure, thereby leadingto the further decrease of free energy barrier.[19] It is worthnoting that control experiments and DFT calculations sub-stantiate that co-doping of N and S atoms significantly reducethe Gibbs free energy of rate-determining step for the CO2-to-CO conversion. More importantly, well-developed hier-archically porous structures of NSHCF provide sufficientchannels, which in turn result in a higher density of accessibleactive sites.[28, 29] The synergy of composition engineering (thatis, N and S co-doping) and structure tuning (that is,hierarchical pores) as noted above afford remarkable controland flexibility over the crafting of nanostructured carbonmaterials for high-performance CO2RR, manifesting a 94%CO Faradaic efficiency and @103 mAcm@2 current density atvery low catalyst loading (ca. 1.2 mg).

Figure 1a depicts the synthetic strategy for NSHCF (fordetails see the Supporting Information). First, polymer nano-fiber was electrospun from a solution containing ZIF-8nanoparticles (Supporting Information, Figure S1), trithio-cyanuric acid (TA), and polyacrylonitrile (PAN). After

[*] Dr. H. P. Yang, Y. Wu, Q. Lin, Dr. L. D. Fan, Dr. X. Y. Chai,Prof. Q. L. Zhang, Prof. J. H. Liu, Prof. C. X. HeCollege of Chemistry and Environmental EngineeringShenzhen UniversityShenzhen, Guangdong, 518060 (China)E-mail: hecx@szu.edu.cn

Prof. Z. Q. LinSchool of Materials Science and EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332 (USA)E-mail: zhiqun.lin@mse.gatech.edu

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201809255.

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carbonization of ZIF-8, TA, and PAN at 900 88C in an argonatmosphere, the nitrogen and sulfur co-doped NSHCF(denoted NSHCF900) was yielded. NSHCF900 containshierarchically porous structures, which is due to the collapseof ZIF-8 and the evaporation of Zn species. NSHCFs werealso obtained after carbonization under 800 88C and 1000 88C,denoted NSHCF800 and NSHCF1000, respectively. Apartfrom ZIF-8 nanoparticles, ZnCl2 powders were also used aspore-forming agent for electrospinning, and the correspond-ing membrane was referred to as NSCF900.

It is notable that the initial polymer nanofiber membranecan be produced over an area of 24 X 18 cm2 in a singleelectrospinning process, which was then divided into 12 piecesfor the subsequent carbonization treatment. Figure 1 b pres-ents a piece of as-synthesized, 12 X 3 cm2 NSHCF900, which isflexible and self-supporting without the need of additionalcarriers or binders, resembling a carbon cloth (SupportingInformation, Figure S2). As revealed by field-emission scan-ning electron microscopy (FESEM, Figure 1c,d) and high-resolution transmission electron microscopy (HRTEM, Fig-ure 1e,f), NSHCF900 nanofibers are connected with eachother, forming a network-like structure with the diameter ofnanofiber ranging from 500 to 1000 nm. Compared with thesmooth and solid NSCF900 nanofibers (Supporting Informa-tion, Figures S3, S4), NSHCF900 have an obviously roughsurface and well-distributed hollow nanocages within thecarbonaceous nanofibers (Figure 1d,e). These nanocageshave an average size of 100 nm (Figure 1 f), similar to thatof ZIF-8 nanoparticles. NSHCF800 and NSHCF1000 havesimilar hollow structures as NSHCF900 (Supporting Infor-mation, Figures S3, S4). N2 sorption isotherms (SupportingInformation, Figures S5, S6) further confirms thatNSHCF900, NSHCF800, and NSHCF1000 have a mesoporousstructure, while NSCF900 displays a microporous structure.

The lattice distance marked in the HRTEM image (inset ofFigure 1 f) corresponds to the (002) plane of graphiticcarbon.[21] Moreover, the EDX mappings (Figure 1g) of anindividual NSHCF900 nanofiber indicate that C, N, and Selements are uniformly distributed.

The elementary composition of NSHCF900 was furtherinvestigated by X-ray diffraction (XRD), Raman spectra, andX-ray photoelectron spectroscopy (XPS) measurements. Asshown in the Supporting Information, Figure S7, two broaddiffraction peaks centered at 24.188 and 44.188 were seen inNSHCF800, NSHCF900, NSHCF1000, and NSCF900,indexed to the (002) and (100) planes of carbon, confirmingthe crystalline nature of all of the samples.[17] It is notable thatno extra diffraction peaks, such as Zn or ZnO, were observedin the XRD patterns, indicating the complete etching of Znspecies. As shown in Figure S8, the intensive peaks at 1340and 1575 cm@1 associated with the D and G band wereobserved in Raman spectra, respectively.[20] It is important tonote that NSHCF900 showed the highest intensity ratio of theD to G bands (ID/IG ; 1.14), compared to that of NSHCF800(1.05), NSHCF1000 (0.96), and NSCF900 (0.98), which maybe attributed to the doping of sulfur and nitrogen defects.Furthermore, N 1s XPS spectra (Supporting Information,Figure S9) visualized the presence of pyridinic N (398.8 eV)and graphitic N (401.2 eV).[21,22] The S 2p spectra (SupportingInformation, Figure S9) can be decomposed into severaldifferent peaks, representing carbon-bonded sulfur (C@S@C;163.5 and 164.7 eV) and oxidized sulfur species (C@SOx@C;167.4 and 168.6 eV).[30] The atomic structures of N and Sspecies are illustrated in Figure S10. The XPS spectra ofNSCF900, NSHCF800, and NSHCF1000 are also presented inFigure S11. NSCF900 was found to have similar XPS spectrato NSHCF900. Slightly different from NSHCF900, no obviouspeaks and only carbon-bonded sulfur were observed in theN 1s and S 2p spectra, respectively, of NSHCF1000, while theN 1s spectra of NSHCF800 exhibited pyridinic N, graphitic N,and pyrrolic N (399.8 eV), as well as carbon-bonded sulfur.

On the basis of a suite of characterizations describedabove, as-prepared NSHCF composites possess hierarchicallyporous structures and abundant heteroatom doping, whichmay facilitate enhanced activity for CO2RR. In this context,the electrochemical CO2RR by NSHCF composites was firstevaluated by linear sweep voltammetry (LSV). Compared toN2-saturated 0.1m KHCO3 solution, NSHCF900 displayeda markedly higher current density in CO2-saturated solution(Figure 2a), signifying that electrochemical CO2RR increasedthe cathodic current density.[6, 11] NSHCF900 also showeda significantly higher current density than those of othercathodes (Figure 2b; Supporting Information, Figure S12),demonstrating the enhanced catalytic activity in CO2RR. Thecatalytic activities for CO2RR was further investigated by thecontrolled potential electrolysis method. It is noteworthy thatas NSHCF composites are flexible and self-supporting, theycan be readily cut into specific size and shape (ca. 1.2 mg percm2) and directly used as cathode for electrolysis. First, theeffect of electrolysis potential on Faradaic efficiency forCO2RR was performed at the cathode potential from @0.3 to@1.1 VRHE. For NSHCF900 (Figure 2c), the Faradaic effi-ciency for CO increased from @0.3 to @0.7 VRHE and reached

Figure 1. a) Synthesis of NSHCF. 1) electrospinning of polymer nano-fibers, 2) being carbonized at 90088C. b) Digital images of a flexibleNSHCF900 membrane. c) Low- and d) high-resolution (where through-holes are marked with red arrows) FESEM images of NSHCF900.e),f) HR-TEM images of NSHCF900. Inset in (f) shows the latticefringe. g) EDX mapping of a single NSHCF900 nanofiber.

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the maximum value (94%) at @0.7 VRHE, along with thesignificant dropping of H2 Faradaic efficiency from 67% at@0.3 VRHE to 6% at @0.7 VRHE. Furthermore, the CO and H2

Faradaic efficiencies of other catalysts were also evaluatedand presented in Figure 2d and the Supporting Information,Figure S13. Clearly, in comparison to those of NSHCF800(76 %), NSHCF1000 (22 %), and NSCF900 (89 %),NSHCF900 achieved the highest Faradaic efficiencies forCO with a maximum value of 94 %.

It is widely recognized that the stability of an electro-catalyst is key for practical applications.[16] In our experi-ments, all four catalysts were evaluated for 36 h durabilitytests at a constant @0.7 VRHE cathode potential. The outletgases were analyzed every 3 h by GC and the correspondingFaradaic efficiencies of CO were also calculated. The currentdensity of NSHCF900 maintained a steady value at approx-imately @100 mAcm@2 with no significant decay (Figure 2e),and the corresponding Faradaic efficiency of CO onlydecreased slightly to 93% (Figure 2 f) throughout the stabilitytest. NSHCF800 and NSHCF1000 also showed good long-time durability. As for NSCF900, approximately 50% loss ofcurrent density and 20% loss of CO Faradaic efficiency wereobserved during long-term tests. After 36 h of electrolysis,SEM, TEM, and EDX analysis of the NSHCF900 samplewere conducted. Figure S14 shows that NSHCF900 retainedwell-distributed hollow nanocages within the carbonaceousnanofibers. The EDX mappings of an individual NSHCF900nanofiber demonstrated that C, N, and S elements were stilluniformly distributed, indicating the robust electrochemical

stability and reusability of NSHCF900. The catalyst loadingamount required for achieving the maximum Faradaicefficiency and the partial current density of products forNSHCF900 compared with previously reported catalysts inthe Supporting Information, Table S1. For the NSHCF900composite, the achievable maximum Faradaic efficiency(94 %) of CO is comparable to those of state-of-the-artelectrocatalysts, regardless of metal-based or metal-freecatalysts. Compared to those reported state-of-the-artmetal-free electrocatalysts,[18, 19] using relatively low catalystloading, NSHCF900 yielded one of the highest currentdensities (@103 mAcm@2), which is one of the best metal-free electrocatalysts for electrochemical reduction of CO2 toCO.

Kinetics for the outstanding performance of NSHCF900were first analyzed by the Tafel slopes of the logarithm of COpartial current density (log jco), as shown in Figure 3a. The

measured Tafel slope for NSHCF900 is 105 mVdec@1, which islower than that of NSHCF800 (159 mVdec@1), NSHCF1000(192 mVdec@1), and NSCF900 (132 mVdec@1). According toa previous study, the rate-determining step of CO2RR wasdemonstrated to be a single electron transfer to a CO2

molecule with a 118 mVdec@1 Tafel slope.[21] The Tafel slopeof 159, 192, and 132 mVdec@1 indicated the poor kinetics onNSHCF800, NSHCF1000, and NSCF900. For NSHCF900, theTafel slope (105 mVdec@1) is lower than 118 mVdec@1,suggesting accelerated electron transfer and enhanced cata-lytic activity in CO2RR.[20,31]

We hypothesize that the doped species within NSHCF isthe origin of the disparity in the catalytic activity (forexample, N-doped carbons have excellent CO2RR capabil-ities).[18–25] In our study, N-doped hierarchically porous carbonnanofiber (NHCF900; Supporting Information, Figure S15)only yields 63% Faradaic efficiency for CO in CO2RR, whichis significantly lower than that of NSHCF900, signifying theimportance of S species. The relationship between thenitrogen and sulfur contents in catalysts and the associated

Figure 2. a) LSV curves in CO2-saturated (cc) and N2-saturated(aa) 0.1m KHCO3 solution using NSHCF900 as cathode. b) LSVrecorded in CO2-saturated solution by utilizing various cathodes.c) Faradaic efficiencies of CO and H2 using NSHCF900 as cathode.d) Faradaic efficiencies of CO by capitalizing on various cathodes atpotentials ranging from @0.3 to @1.1 VRHE in CO2-saturated 0.1mKHCO3 solution. e) Long-time and f) reuse tests performed at@0.7 VRHE.

Figure 3. Comparisons of a) Tafel plots, b) free energy diagram ofCO2RR to CO on N-doped graphene and N, S co-doped graphene,c) double-layer capacitance, and d) CO2 adsorption amount of variouscatalysts.

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CO2RR activity is illustrated in the Supporting Information,Figure S16, which demonstrates that pyridinic N is thedominant active sites for CO2RR, and the presence ofcarbon-bonded S can greatly enhance the catalytic activity.DFT calculations based on the structure model in theSupporting Information, Figure S17 and Table S2 were con-ducted to further explore the possible role of nitrogen- andsulfur-doping played in CO2RR. It has been reported that theformation of adsorbed *COOH is the rate-determining stepfor the CO2-to-CO reduction, and the CO generation can beinhibited by high Gibbs free energy of *COOH.[17] Ascalculation results shown in Figure 3b, the lower Gibbs freeenergy barrier of 1.01 eV for *COOH occurs at pyridinic Nadjacent to carbon-bonded S atom, consistent with105 mVdec@1 Tafel slope and 94 % Faradaic efficiency ofCO at @0.7 VRHE using NSHCF900. In contrast, the higherGibbs free energy (1.34 eV) for pure pyridinic N atomsrequires extra energy to overcome the barrier of *COOHadsorption, corresponding to 118 mVdec@1 Tafel slope inprevious reports.[17, 21] This difference can be attributed to thehigher spin density and charge delocalization caused bygreater polarizability and larger atomic size of doped Satoms.[19]

Further to heteroatom doping by nitrogen and sulfur,hierarchically porous structures of NSHCF900 may alsopartially account for the high-performance CO2RR. Ascontrol experiment, NSCF900 composite with typical micro-porous structure (Supporting Information, Figures S3–S5)was also employed in electrochemical CO2RR under the samereaction conditions as NSHCF composites. CO Faradaicefficiency (89 %) obtained using NSCF900 cathode (ca.1.7 mg per cm2) at @0.7 VRHE is comparable to NSHCF900(94 %), owing to the similar pyridinic N and carbon-bonded Scontent. However, NSCF900 cathode showed far less currentdensity than NSHCF900, although the catalyst loadingamount (1.7 mg) was larger than NSHCF900 (1.2 mg).Hence, these differences in catalytic activity may be attrib-uted to the hierarchically porous structures of NSHCF, whichprovide abundant channels between the active sites and theelectrolyte.[28, 29] To verify this hypothesis, we first estimatedthe electrochemical active surface areas (ECSA) of thesecomposites by double-layer capacitance (Cdl). According tothe Supporting Information, Figure S18 and Figure 3c, the Cdl

of NSCF900 (8.5 mF cm@2) is much lower than NSHCF900(38.8 mF cm@2), although the specific surface area (493 m2 g@1)is slightly larger than NSHCF900 (465 m2 g@1). Similarly,NSHCF800 and NSHCF1000 also exhibited higher Cdl thanNSCF900. Moreover, CO2 adsorption measurements of allcatalysts at 298 K from 0 to 1 atm were also studied (Fig-ure 3d). For NSHCF800, 900, and 1000, the CO2 adsorptioncapacity is 58.4, 76.2, and 42.7 cm3 g@1 at 1 atm, respectively,which is far larger than 5.1 cm3 g@1 for NSCF900. SinceNSHCF composites possess hierarchical porosities, the pres-ence of mesopore channels can enhance the gas transporta-tion for better accessibility to the micropores where CO2 ismainly adsorbed and reduced. Despite NSCF900 havingcomparable pyridinic N and carbon-bonded S contents toNSHCF900, the lack of mesopore channels resulted in a muchlower density of accessible active sites for CO2RR, which

ultimately led to the significantly lower current densityobserved for NSCF900.

In summary, we developed an effective strategy forscalable production of N and S co-doped, hierarchicallyporous carbon membranes. They are flexible and free-stand-ing, and they can thus be directly utilized as cathode forelectrochemical CO2RR. Remarkably, NSHCF900 manifestsa 94% CO Faradaic efficiency and @103 mAcm@2 currentdensity at the low catalyst loading. Notably, control experi-ments and DFT calculation substantiate that the co-doping ofN and S atoms significantly reduce the Gibbs free energy ofrate-determining step for the CO2-to-CO conversion. Fur-thermore, NSHCF membranes possess judiciously craftedhierarchical pores, rendering abundant accessible active sitesfor CO2RR. Consequently, the composition tailoring via Nand S co-doping in conjunction with structure tuning byconstructing hierarchical pores synergistically lead to theultra-high current density and Faradaic efficiency. Thisstrategy may be readily extended to produce a wide rangeof hetero-atoms-doped carbon membranes for high-perfor-mance electrolysis and energy storage devices.

Acknowledgements

The financial support of the National Natural ScienceFoundation (NNSF) of China (21574084 and 21571131), theNatural Science Foundation of Guangdong Province(2015A030313554 and 2017A040405066), and Shenzhen Gov-ernmentQs Plan of Science and Technology(JCYJ20160308104704791, JCYJ20170817095041212, andJCYJ20170818091657056) are gratefully acknowledged.

Conflict of interest

The authors declare no conflict of interest.

Keywords: CO2 reduction · electrocatalysis · hierarchical pores ·metal-free catalysts · ultrahigh current density

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Manuscript received: August 10, 2018Revised manuscript received: September 14, 2018Accepted manuscript online: October 3, 2018Version of record online: October 25, 2018

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