Excellent Sulfur Dioxide Electrooxidation Performance and GoodStability on a Fe-N-Doped Carbon-Cladding Catalyst in H2SO4
Qing Zhao,a,b,z Ming Hou,a,z Shangfeng Jiang,a,b Jun Ai,a Limin Zheng,a and Zhigang Shaoa
aFuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalian 116023, People’s Republic of ChinabUniversity of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
Manuscript submitted March 9, 2017; revised manuscript received April 19, 2017. Published May 10, 2017.
Sulfur dioxide electrooxidation is an important reaction in thefields of flue gas purification, hydrogen production, energy genera-tion, sodium sulfate electrolysis and corrosion.1–5 The reaction alwaystakes place on anodic electrodes with direct oxidation of SO2 undercertain applied potential.6–8 SO2 anodic oxidation mostly perform onprecious metal catalysts such as the gold, platinum, palladium, irid-ium and their alloys,9–14 whose catalysis behaviors are extensivelyinvestigated and reported.15–18 Platinum is the most commonly usedcatalyst for its excellent SO2 catalytic activity and good stability inacid.5,6,9,19,20 Umran Tezcan Un et al. studied the SO2 electrooxida-tion desulfurization with a platinum expanded mesh anode in H2SO4
and achieved a high SO2 removal efficiency.19 Zhai et al. proposed agaseous SO2 electrooxidation process with Pt/C catalyst, showing theprospect in disposing the low concentration SO2.14 Lu and Ammonconducted the kinetic and electrochemical analysis of precious metalsin the electrooxidation of SO2. They believed that palladium and pal-ladium oxide are the better catalysts for SO2 electrooxidation than thegenerally used platinum, as the reaction initiates more easily at loweranodic potentials with easier oxygen-containing species derived onpalladium catalysts.21 Pillay et al. found that Pt3Co also displayed abetter catalytic performance for SO2, because the active sites on thesurface of Pt3Co (111)-Ll2 were more favorable for the adsorption ofOH and S, owing to their closer d-band center to Fermi energy.22
However, the Pt, Ru and Ir et al. precious catalysts generate pas-sivated layers under high oxidation potentials, covering the electrodesurface and inhibiting the electrooxidation of SO2. This is because thepartially adsorbed oxidates on the catalyst transform into metal ox-ides under high oxidation potentials.3 What’s more, the high price andlimited resource also restrict the application of these precious metalcatalysts in SO2 electrooxidation. Therefore, explorations of the cost-effective SO2 electrooxidation catalysts are meaningful for practicalapplication. As reported by Py. et al., SO2 can be oxidized with car-bon catalysts.23 The nitrogen doped graphite is also believed to bebeneficial for the electrooxidation of SO2.24 Currently, references ofnon-precious SO2 electrooxidation catalysts are very limited. Thus,more and more investigations of nonprecious SO2 electrooxidationcatalysts initiate to be concerned recently, and the imidazole modifiedFe-N-C catalyst is one of the minority recent publications.25
The metal-nitrogen-carbon catalysts are demonstrated highly ac-tive in many electrochemistry processes,26–28 while they are seldomdiscussed or reported in the process of SO2 electrooxidation. Withexcellent catalytic performance toward 2e− or 4e− reaction as well asthe cost-effective characteristics, the metal-nitrogen-carbon catalystis a promising candidate for the 2e− SO2 electrooxidation process.Herein, we prepared a Fe-N-doped carbon-cladding catalyst, which
was synthesized through pyrolyzing the BP2000 supported precur-sors under certain annealing temperatures. During the preparation,the Fe3+ ions were well-dispersed in the surfactant micelles, hav-ing better contact with the nitrogen source and carbon support. Theiron was successfully developed into the tiny metal particles in thecladding carbon lattices during carbonization. The well combinationof N and C elements as well as the sufficient contact between the metaland carbon lattices make it a good non-precious SO2 electrooxidationcatalyst. This carbon-cladding catalyst not only exhibits comparableSO2 electrooxidation performance to the state-of-the-art Pt/C, but alsodemonstrates a better stability in H2SO4.
Experimental
Materials.—The melamine was provided by the Tianjin FuchenChemical Reagent Factory, and the FeCl3 solution was prepared withthe FeCl3 · 6H2O (Tianjin Bodi Chemical Co., Ltd, China) reagent.Pluronic F127 was purchased from sigma-aldrich, Co., USA. Carbonmaterials BP2000, served as the initial support and basis before thepreparation of Fe-N-doped carbon-cladding catalyst, was got fromthe Cabot. And the compared state-of-the-art catalyst was the com-mercially available Pt/C (20%, JM), which was commonly referred inelectrochemical tests.
Physical characterization.—Transmission electron microscopy(TEM) images were recorded on a JEOL JEM-2000EX microscope.Samples were prepared by dropping a diluted isopropyl alcohol cat-alyst suspension onto a Cu grid, followed by drying in air. High-resolution TEM (HRTEM) images were examined by the Tecnai G2F20-TWIN under 200 kV. X-ray diffraction (XRD) patterns were ac-quired on a D/max-2500 PC diffractometer with Cu Kα (λ = 1.54 nm)radiation. Raman spectra were collected on a Raman spectrometer ofJobin Yvon LabRAM HR800 with a 532 nm laser. The laser powerwas 2 mW. Nitrogen adsorption/desorption method was performed toexamine the surface area and pore properties of the catalyst with theMicromeritics ASAP 2010 instrument at 77 K. X-ray photoelectronspectroscopy (XPS) measurements were carried out on the imagingphotoelectron spectrometer of Thermo Scientific ESCALab 250Xiinstrument with a monochromatic Al Kα X-ray source (1486.6 eV).
Electrochemical characterization.—The electrochemical cataly-sis and SO2 electrooxidation performances were carried out and com-pared on the rotating ring disk electrode in a typical three-electrode cellsystem on CHI 730D electrochemical workstation (CHI Instruments,Chenhua Co., China). A platinum foil was used as the counter elec-trode, and a saturated calomel electrode was served as the referenceelectrode (SCE). The background electrolyte in test was 0.5 M H2SO4.
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Journal of The Electrochemical Society, 164 (7) H456-H462 (2017) H457
Figure 1. Synthesis of the Fe-N-doped carbon-cladding catalyst.
SO2 was generated by dissolving appropriate amount of solid Na2SO3
in the N2 bubbling deoxygenate 0.5 M H2SO4, and the concentra-tion of Na2SO3 was 1 g L−1. The working electrode was prepared byevenly coating a quantitative well-dispersed catalyst suspension onglass carbon electrode (GCE, d = 4 mm), forming a uniform catalystfilm. The loaded carbon-cladding catalyst on the electrode was 0.379mg cm−2, and the referenced Pt/C electrode was 0.1 mg cm−2 (equal to20 μgPt cm−2). The electrochemical performances of SO2 oxidationwere analyzed with the linear sweep voltammograms (LSV), Tafeltests, cyclic voltammograms (CV) and LSV-rotating tests. LSV wasused to characterize the SO2 electrooxidation performances in 1 gL−1 Na2SO3, which was made of 0.5M H2SO4 (as the backgroundelectrolyte).
Preparation of carbon-cladding catalyst.—The Fe-N-dopedcarbon-cladding catalyst was synthesized through pyrolyzing a com-posite precursor of melamine and the micellization activated FeCl3
with Pluronic F127, supported on the high surface area carbon BP2000under certain annealing temperatures. The processes were illustratedin Fig. 1. First, 0.5 g surfactant Pluronic F127 was dissolved in 20 mL0.1 M FeCl3 aqueous solution, so as to have the Fe3+ better dispersedin the surfactant micelles. Second, 1 g melamine was dissolved in100 mL 100◦C deionized water sufficiently. Then, the dispersed Fe3+
solution was added to the dissolved melamine solution drop-wisely,incorporating with each other under stirring. After magnetic stirring
for 20 min, 160 mg BP2000 was added into the mixture, offering highadsorption surface area for reactants and providing excellent supportfor the newly formed carbon structures and composites. Meanwhile,the oil bath temperature was adjusted to 80◦C and kept until evaporatedto dryness. Subsequently, the mixture of precursor was obtained andcollected. Finally, subject the mixture to pyrolysis and carbonizationunder Ar atmosphere with the procedure of 2◦C min−1 to 240◦C, kept2 h and then followed by increasing to 700◦C with a rising rate of 1◦Cmin−1, pyrolyzing and holding under 700◦C ultimately. Foremost,the kept pyrolysis and carbonization time under 240◦C and 700◦Care all 2 h respectively to have the materials better transformed. Theas-prepared carbon-cladding catalyst was obtained after grinding.
Results and Discussion
Physical characterization of carbon-cladding catalyst.—Mor-phology features and specific structural properties of the preparedcarbon-cladding catalyst were characterized with the transmissionelectron microscopy (TEM), high-resolution TEM (HRTEM) andhigh-angle annular dark-field scanning transmission electron mi-croscopy (HAADF-STEM) (Fig. 2 and Fig. S1). Comprehensively,this catalyst is made up of carbon-cladding structures with numerouswell-dispersed tiny Fe-N-doped nanoparticles distributed and inlayedin the graphitic carbon lattice. It is interesting to address that, basedon the specific STEM/EDS scan and elemental mappings (Fig. 2a),
Figure 2. HR-TEM images of the carbon-cladding catalyst. (a) HAADF-STEM elemental mapping of C, O, N, Fe; (b) specific area image of SAED pattern; (c)TEM image; (d-g) HRTEM; Scale bar: 5 nm.
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the surficial distribution of N is highly in accordance with that ofthe C element, demonstrating the successfully doped N element andwell interactions between C and N in the catalyst. HRTEM imagesshowed the fine structures of the carbon-cladding catalyst (Fig. 2d-2g).Graphitic-like layers with the interlayer spacing of 0.348∼0.411 nmall correspond to the (002) plane of graphitic carbon. The enlargedinterlayer spaces than the normal value of 0.34 nm come from thehetero-doped iron/ nitrogen elements to the carbon lattices and thewell-dispersed embedded nanoparticles in carbon walls. The uniformdistribution of nanoparticles in carbon could attribute to the confine-ment of graphitic layers (Fig. 2f), suppressing the agglomeration ofnanoparticles.29 Meanwhile, the confinement enhances the contact ofparticles and graphitic layers, improving the activity and stability ofthe catalyst. HRTEM images revealed some dense region of the em-bedded particles in the graphitic carbon-cladding catalyst (Fig. S1 (f)).The crystalline lattice spacing of 0.437 and 0.477 nm are correspond-ing to the (110) and (001) planes of Fe3C respectively. The relevantSAED pattern further indicates the (211) plane of Fe3C crystallinestructure (Fig. 2b). The lattices of 0.21 nm is consistent with the (211)plane of Fe3C. The encapsulated Fe3C nanoparticles are believed toactivate the graphitic cladding layers, making positive contributionsto the catalysis.30
As we know, catalytic performances are governed by the intrinsicactive sites, determined by the composition and interaction of chemi-cal components, as well as the accessibility of active sites and trans-port properties, influenced by the obtained surface area and porousstructures.30 So the pore properties, crystal structures, carbonizationcharacteristics, as well as the doped elements of this carbon-claddingcatalyst were further characterized with following measurements.
To further identify the structural and constituent properties ofcarbon-cladding catalyst, X-ray diffraction (XRD) measurement wasapplied to examine the crystalline phases (Fig. 3a). The results confirmthat nanoparticles in catalysts are coexistent of the metallic Fe (JCPDS
No. 87-0721), Fe3C (JCPDS No. 89-2867) and a small amount Fe3Nand Fe3O4 (JCPDS No. 87-0246). The small wide dispersed carbonpeak at around 25.6◦, accompanying with the strong metallic Fe andFe3C signals, suggests the embedded and encapsulated structures un-der carbon shells. The developed Fe3C, Fe3N and Fe3O4 substancescan activate the cladding carbon, enhancing the catalysis activity.Graphitic degree of the carbon-cladding catalyst was evaluated withRaman (Fig. 3b). The spectrum demonstrates a relatively high graphi-tization with an ID/IG value of 1.13. The intensive D band at around1332.1 cm−1 implies the disordering defects in catalysts and the shiftedG band at around 1597.4 cm−1 suggests the high graphitic propertyof carbon structures, which may be influenced by the doped sub-stances. Moreover, the noteworthy broad 2D band (typical 2650 cm−1)from 2275 cm−1 to 3386 cm−1, demonstrated the numerous devel-oped graphitic structures in catalyst. Fringes and defects of graphiticstructures will activate the catalysis processes on carbon.27,28,31,32 Theporosity was further assessed by N2 adsorption/ desorption isotherm(Fig. 3c), and the pore properties were analyzed with DFT and HKmethods (Fig. 3d). The prominent type-IV isotherm hysteresis loopindicates the abundant mesopore structure of the catalysts. The surfacearea is 706.3 m2 g−1, and total pore volume is 0.945 cm3 g−1. Poresare widely distributed in the range of 0.8–1.5 nm (based on the HKmethod), and 2.3–25.4 nm (based on the DFT method) respectively.The average pore diameter is 5.35 nm. Pores in this carbon-claddingcatalyst are beneficial for SO2 electrooxidation, as known that themesopores are good for reactants transportation and the micropores ataround 0.7–0.9 nm are active for SO2 catalysis based on the analysisfrom Maria Lezanska et al.33 In a word, this catalyst exhibits high sur-face area, desired mesoporous structure and large pore volume, whichare favorable for the active site exposure and reactants transport. Sothis obtained carbon-cladding catalyst can provide better transportproperty and desired active sites for SO2 electrooxidation catalysis.
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Journal of The Electrochemical Society, 164 (7) H456-H462 (2017) H459
Figure 4. XPS analyses. Deconvolution of (a) C1s, (b) O1s, (c) N1s, and (d) Fe 2p spectra.
X-ray photoelectron spectroscopy (XPS) was conducted to eluci-date compositions and bonding configurations of different elementsin the carbon-cladding catalyst (Fig. 4). Generally, this catalyst iscomposed of C (92.2 at.%), O (4.96 at.%), N (2.3 at.%) and Fe (0.54at.%), demonstrating the successfully doped N and Fe elements incatalyst. For C 1s spectra, the intensive sp2 C peak, centering at 284.3eV (Fig. 4a), implies the high graphitization of this catalyst, which isin accordance to Raman analysis. Graphitization property enhancesthe catalytic activity by increasing the electrical conductivity.34 ForO 1s spectra (Fig. 4b), the carbonate peak at around 530.2 eV (0.91at.%) suggests the appearance of some iron oxides, agreeing with theelemental mapping and XRD analysis. Moreover, a certain amountof C-O and O-C=O groups developed in the catalyst enhances thetransport ability of electrolytes, benefiting from the interaction of cat-alysts and electrolytes. The complex N 1s spectra is deconvolutedinto five peaks, assigning to pyridinic N, Fe-N, pyrrolic N, graphiticN and oxidized N respectively (Fig. 4c). Nevertheless, there is onlya small amount less effective oxidized N formed, suggesting the ef-fectively doped N element.30,31,34 The peak binding at around 398.7eV is ascribed to the formation of N-Fe structure, demonstrating theformation of Fe-Nx active sites and verifying the XRD results. Thedeconvoluted Fe 2p spectra reveals a pair of doublets of the Fe 2p3/2
and Fe 2p1/2 peaks at the 711.0 eV and 723.9 eV and at the 712.5eV and 726.3 eV with a satellite peak at 719.0 eV(Fig. 4d), sug-gesting the present of iron carbide and oxidized iron species. Thepeak at 711.0 eV in Fe 2p3/2 indicates the coordination of Fe ionsand N, confirming the existence of Fe-N bonding and Fe-Nx ac-tive sites. Thus, this carbon-cladding catalyst is a well-establishedFe-N doped catalyst, on which the effectively doped N, proper com-bined Fe-Nx, and the dispersive embedded carbon-cladding struc-tures display synergetic effects for catalysis.30,31,34 Moreover, the highsurface area, typical mesoporous characteristics and large pore vol-umes are also beneficial for SO2 electrooxidation, due to their great
contributions to the formation of active sites and fast transport ofreactants.
Electrochemical characterization of carbon-cladding catalyst.—To evaluate the SO2 electrooxidation behaviors, linear sweep voltam-mograms (LSV) and cyclic voltammetries (CV) were performed onthe carbon-cladding catalyst, and compared with the commercial Pt/C(20%, JM). As shown in Fig. 5a, carbon-cladding catalyst displaysan excellent SO2 electrooxidation activity and its observed current-voltage relationships are very close to Pt/C below 0.7 V. Onset oxida-tion potential of the carbon-cladding catalyst is 0.516 V, about 32 mVhigher than that of Pt/C. The half-wave oxidation potential is 0.629 Vvery close to the 0.627 V of Pt/C. LSV plot of this catalyst shows aplateau in the medium potential region between 0.7 V and 1.05 V, andthen goes through a continuous increase beyond 1.05 V. While Pt/Cdisplays a relatively higher current plateau between 0.7 V and 1.05 V,then displays a sharp performance drop with the increase of oxidationpotential, ascribing to the restraint effects of derived Pt-O species.The different limiting currents in medium oxidation potential regionsattribute to the influence of intrinsic active sites and transport propertyof the catalysts. The oxidation behaviors of carbon-cladding catalystsurpass that of Pt/C when the potential is above 1.194 V (B, Fig. 5a).The strong interaction of Pt-O on Pt /C catalyst partially covers the cat-alytic surface and restrains the electrooxidation of SO2. Whereas, thecarbon-cladding catalyst is mainly made up of carbon, on which theoxygen species are easily formed and these species can make positivecontributions to SO2 electrooxidation. Thus, it’s reasonable to believethat this carbon-cladding catalyst is an excellent candidate for SO2
electrooxidation especially above 1.194 V. CV measurements werefurther applied to examine the SO2 electrooxidation activity on thecarbon-cladding catalyst (Fig. 5b). CV curve of the carbon-claddingcatalyst in 0.5 M H2SO4 shows a pseudo rectangle shape with wideelectric double layer characteristics, indicating the high surface area of
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H460 Journal of The Electrochemical Society, 164 (7) H456-H462 (2017)
Figure 5. Electrochemical tests. (a) LSV of Pt/C and carbon-cladding catalyst; in 1 g L−1 Na2SO3; 1600 r min−1, 5 mV s−1. (b) CV of carbon-cladding catalyst.
this catalyst. CV in Na2SO3 presents a sharp oxidation peak at 0.644 V,suggesting the fast oxidation process on catalysts under the relativelylow oxidation potentials. The dropped oxidation performances afterthe current peak indicate the hysteretic process of reactants transport.It is further proved by the highly enhanced oxidation currents in thewhole oxidation potential range under the continuous rotating process(1600 r min−1). This carbon-cladding catalyst exhibits the highestSO2 electrooxidation activity as compared with other SO2 oxidationcatalysts in the Table S1.
To get further insight into the kinetic activities of SO2 electroox-idation, catalytic behaviors of this carbon-cladding catalyst and Pt/Cwere further investigated by the LSVs–rotating and Tafel polariza-tion tests (Fig. 6) and analyzed on the basis of the Koutecky-Levich(K-L, i−1 = ik
−1+id−1 = ik
−1+� · ω−1/2) equations. Typically, SO2
oxidation is a rigorously irreversible reaction, controlled by both theelectrochemical and mass transfer processes. The oxidation currentsof carbon-cladding catalyst increase with the elevation of rotatingspeeds in the medium and high potential regions due to the shortened
Figure 6. Kinetic analyses and comparisons. (a) LSVs of 20% Pt/C in 1 g L−1 Na2SO3 with different rotation rates, 20 mV s−1; (b) LSVs of the carbon-claddingcatalyst in 1 g L−1 Na2SO3 with different rotation rates, 20 mV s−1; (c) K-L comparison of 20% Pt/C and carbon-cladding.
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Journal of The Electrochemical Society, 164 (7) H456-H462 (2017) H461
Figure 7. Stability comparison of Pt/C and the carbon-cladding catalyst. (a) CV tests on 20% Pt/C. (b) CV tests on carbon-cladding catalyst. CV: H2SO4,N2-saturated, 50 mV s−1. (c) LSVs before and after accelerated durability test; Na2SO3, 5 mV s−1, 1600 r min−1. Accelerated durability.
diffusion distance, but the oxidation currents are not interfered by therotation rates in the low potential parts, implying the electrochemicalcontrolling process in low potential region (Fig. 6b). However, therotation rates seemed to have a more serious effect on Pt/C in the lowpotential parts for SO2 electrooxidation, which may be ascribed tothe stronger adsorption and interaction of Pt and SO2 (Fig. 6a). Thecorresponding K-L plots of carbon-cladding catalyst show a fairlygood linearity and near coincidence when the rotation rate is equalor larger than 600 r min−1, indicating the first-order reaction kinet-ics toward SO2. Moreover, the K-L plot slopes are resemble at 0.8V and 1.2 V on the carbon-cladding catalyst, indicating the similarelectron transfer processes of SO2 electrooxidation under these con-ditions. The decreased slope under 1.4 V suggests the slightly lowerelectron transfer performance on this catalyst on the basis of the K-Lequation (Fig. 6c). This may be influenced by the competition reac-tions of water activation and the SO2 electrooxidation. In contrast,Pt/C displays the similar linear K-L plots at 1.2 V and 1.4 V, and thekinetic currents are close under these oxidation potentials based onthe qualitative analyses by extrapolating the K-L plots. However, thecarbon-cladding catalyst has the higher kinetic currents than Pt/C un-der all the selected potentials (0.8 V, 1.2 V, and 1.4 V), suggesting theexcellent SO2 oxidation activity on this catalyst. The Tafel polariza-tion characteristics of carbon-cladding catalyst and Pt/C are comparedin Fig. 6d. The initiate oxidation potentials are very close on the twocatalysts. In the relatively low polarization potential region (below 0.9V), the carbon-cladding catalyst exhibits the relatively lower oxida-
tion potentials under the same reaction rates (A, Fig. 6d) and higheroxidation current under the same oxidation potential (B, Fig. 6d).Above 0.9 V, the polarization enters into the limiting region, showingsimilar polarization features on catalysts. In the high potential region,the carbon-cladding catalyst displays the better SO2 electrooxidationperformance. So the kinetic performance of SO2 oxidation on thecarbon-cladding catalyst is relatively better in the test.
The electrochemical stability of carbon-cladding catalyst was stud-ied with the accelerated durability test through CV circulations. Aftercirculating 2383 cycles between 0.1 and 1.2 V in 0.5 M H2SO4, therewas no obvious H-adsorption/ desorption peaks left for the Pt/C, anda pair of symmetry peaks appeared because of the damage of carbon(Fig. 7a). Similarly, CV curve of the carbon-cladding catalyst alsopresented a similar symmetrical peak because of the transformationof carbon after the durability test (Fig. 7b). Influence of the accelerateddurability tests for the electrooxidation of SO2 was shown in Fig. 7c.Both the onset and half-wave oxidation potentials of the two catalystsare obviously changed after aging 2383 cycles, indicating the loss ofactive sites and damage of carbon structures. However, transforma-tion on the Pt/C was more notably (Fig. 8). Concretely, the limitingoxidation current of Pt/C reduced 39.56 % after aging 2383 cycles,while the limiting current of carbon-cladding catalyst decreases 12.41% in the meantime. Therefore, the prepared carbon-cladding catalystexhibits a better stability than that of Pt/C, which may be attributed tothe well protection of the high graphitic carbon-cladding structures tothe Fe-N constituents and inlaid particles.
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H462 Journal of The Electrochemical Society, 164 (7) H456-H462 (2017)
Figure 8. TEM images of 20% Pt/C and the carbon-cladding catalyst before and after the accelerated durability test. (a-b)TEM images and particle size distributionof Pt/C before the accelerated durability test, (c-d) TEM images and particle size distribution of Pt/C after the accelerated durability test ; (e-f) TEM images ofcarbon-cladding catalyst after the accelerated durability test.
During the accelerated durability test, the well dispersed Pt onporous carbon was largely degraded after circulating 2383 cycles be-tween 0.1 and 1.2 V, exhibiting remarkable dissolution and migrationcharacteristics. As shown in Figs. 8c–8d, the Pt nanoparticles are seri-ously loss and agglomerated after the accelerated durability test, andthe left Pt particles are in the larger size. What’s more, the left Pt/C cat-alyst displays the typical characteristics of high graphite carbon (Fig.7a). In contrast, the damage on carbon-cladding catalyst are not thatmuch severely (Figs. 8e–8f). The inlaid and encapsulated substancesin the carbon-cladding structures are well kept after the acceleratedtest. These carbon-cladding structures are supposed to enhance thestability of this catalyst.
Conclusions
In summary, based on the physical and electrochemical character-izations, the effectively doped Fe-N and well embedded Fe3C sub-stances in the catalyst activate the graphitic carbon-cladding struc-tures, which are believed to be synergetic responsible for the excel-lent performance of SO2 electrooxidation and better stability of theas-synthesized carbon-cladding catalyst in the durability test. More-over, the high surface area, mesoporous structures and large porevolumes also contribute greatly to the well formation of active sitesand fast transport of reactants, which are beneficial for SO2 electroox-idation. Overall, this nonprecious carbon-cladding catalyst exhibitscomparable SO2 oxidation performance and good electrochemicalstability to that of Pt/C in H2SO4, indicating a potential application ofnonprecious catalyst in SO2 electrooxidation with excellent catalysisperformance.
Acknowledgments
This work was financially supported by the National Key Technol-ogy Support Program (No. 2015BAG06B00) and the Major Programof the National Natural Science Foundation of China (No. 61433013).
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