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Direct ethanol fuel cells with superior ethanol-tolerant nonprecious metal cathode catalysts for oxygen reduction reaction Kyeng-Bae Ma, Da-Hee Kwak, Sang-Beom Han, Hyun-Suk Park, Do-Hyoung Kim, JiEun Won, Suk-Hui Kwon, Min-Cheol Kim, Sang-Hyun Moon, and Kyung-Won Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00405 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Direct ethanol fuel cells with superior ethanoltolerant non-precious metal cathode catalysts for oxygen reduction reaction Kyeng-Bae Ma1, Da-Hee Kwak1, Sang-Beom Han1, Hyun-Suk Park1, Do-Hyoung Kim1, Ji-Eun Won1, Suk-Hui Kwon1, Min-Cheol Kim1, Sang-Hyun Moon1, and Kyung-Won Park1,*

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Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea

*Corresponding Author: Prof. Dr. Kyung-Won Park Tel: +82-2-820-0613; Fax: 82-2-812-5378 E-mail: [email protected]

KEYWORDS DEFC; Ethanol tolerance; Non-precious metal catalyst; Oxygen reduction reaction; Doped carbon

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ABSTRACT: Direct ethanol fuel cells (DEFCs), which use ethanol as a fuel, have attracted considerable attention due to their relatively high energy density, the non-toxicity of ethanol, and the abundance of ethanol sources. However, since the cross-over of ethanol as a fuel in DEFCs can deteriorate the cell performance due to the oxidation of ethanol (EtOH) at the cathode during oxygen reduction reaction (ORR), non-precious metal (NPM) cathode catalysts for ORR have been studied using carbon-based nanostructures as promising alternatives to Pt-based catalysts. Herein, the doped carbon nanostructures (C/Fe-TMPP and C/Fe-Pc) as cathode catalysts were synthesized using a template method with iron(III) 5, 10, 15, 20-(tetra-4-methoxyphenyl) porphyrin chloride (Fe-TMPP) and iron(II) phthalocyanine (Fe-Pc). In the half-cell test, C/FeTMPP exhibited an enhanced ORR activity in 0.5 M H2SO4 (i.e. high half-wave potential and specific current density) and maintained ORR performance in the presence of crossover ethanol, compared to a commercial Pt/C. Moreover, C/Fe-TMPP exhibited high performance in the DEFC supplied with high-concentrated EtOH as a fuel at the cathode. The excellent ORR activity of C/Fe-TMPP for the DEFC can be attributed to ethanol tolerance in the ORR and low ethanol adsorption energy of the active sites for the ORR of C/Fe-TMPP.

Introduction Proton exchange membrane fuel cells (PEMFCs) are electrochemical power sources that can be operated using electrochemical reactions such as the oxidation reaction of fuels at an anode and the oxygen reduction reaction (ORR) at a cathode.1-4 In particular, among the PEMFCs, direct ethanol fuel cells (DEFCs), which use ethanol as a fuel, have attracted considerable attention due to their relatively high energy density (8.3 kWh kg-1), the non-toxicity of ethanol, and the abundance of ethanol sources.5,6 In particular, the utilization of Pt-based catalysts for the ethanol oxidation reaction and ORR at the anode and cathode, respectively, is required in the


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DEFCs.7-10 In general, since the cross-over of ethanol as a fuel in DEFCs can deteriorate the cell performance due to the oxidation of ethanol at the cathode, on which oxygen reduction reaction (ORR) occurs, ethanol tolerant ORR catalysts for high-performance DEFCs have been extensively studied.11-15 Moreover, since the deteriorated stability and limited reserves of Pt have hindered the commercialization of PEMFCs including DEFCs, an innovative approach that can significantly reduce the loading amount of Pt catalysts needs to be proposed.16-20 Non-precious metal (NPM) cathode catalysts for ORR have been intensively studied using carbon-based nanostructures as promising alternatives to Pt-based metal catalysts.21-24 In general, the doped carbon-based cathode catalysts are stable in acid media, electrically conductive, and inexpensive.25-27 In particular, among the doped carbon-based materials, porphyrin-like carbon nanostructures with transition metal-bonded nitrogen (M-N4) are well-known as the best NPM catalysts for ORR comparable to a commercial Pt catalyst.28-31 In previous studies, porphyrinlike carbon nanostructures with high specific surface areas have been prepared using a template method with various doping and carbon sources.32-37 In this current study, high-performance DEFCs were fabricated using doped carbon-based nanostructure cathode catalysts with low ethanol adsorption energy and enhanced tolerance to cross-over ethanol, compared to typical Ptbased catalysts. The doped carbon nanostructures were synthesized using a SBA-15 template method with iron(III) 5, 10, 15, 20-(tetra-4-methoxyphenyl) porphyrin chloride (Fe-TMPP) and iron(II) phthalocyanine (Fe-Pc) as both doping and carbon sources. In addition, PtRu black as an anode catalyst for ethanol oxidation reaction (EOR) was prepared using a borohydride reduction method. The membrane-electrode-assembly (MEA) was fabricated using the doped carbon nanostructures as cathode catalysts, the PtRu black as an anode catalyst, and proton exchange membrane. The MEA was then evaluated using a computer-controlled unit cell station. Figure 1


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shows a schematic illustration of the direct ethanol fuel cell using the NPM cathode catalysts with the tolerance to ethanol. The ethanol in the DEFC can cross over a proton exchange solidstate membrane to the cathode due to the concentration gradient and electro-osmotic drag.

Experimental Section Synthesis and characterization of doped carbon nanostructures. To synthesize the SBA15 template, P123 (8 g, BASF) was dissolved in 60 mL de-ionized (DI) water (18.2 MΩ⋅cm) and 240 mL 2 M HCl at 35 oC with continuous stirring for 3 h. Tetraethyl orthosilicate (17 g, TEOS, SIGMA ALDRICH) was added in the solution, mixed with continuous stirring for 20 h, and then aged at 80 oC for 24 h without stirring. The precipitate was washed with DI water and ethanol and then dried in a 50 oC oven overnight. The dried white powder was heated under an air atmosphere at 600 oC for 4 h. The as-prepared silicate template (0.5 g, SBA-15) was mixed with FeTMPP (0.5 g, 98%, PorphyChem) or FePc (0.5 g, 90%, SIGMA ALDRICH) and then heated under an N2 atmosphere at 900 oC for 3 h. The heated samples were washed with continuous stirring in 10 vol% dilute hydrofluoric acid (HF) for 10 h in order to remove SBA-15 and impurities, rinsed with DI water and ethanol, and dried in a 50 oC oven overnight. Synthesis and characterization of PtRu black. To synthesize PtRu alloy anode catalyst for EOR, 100 mg H2PtCl6 (99.995%, SIGMA ALDRICH) and 50.7 mg RuCl3 (40~49%, SIGMA ALDRICH) were dissolved in 300 mL DI water with continuous stirring. Following the complete dissolution, the metal salts were reduced using a borohydride reduction method. 750 mg NaBH4 (99%, Aldrich) dissolved in 20 mL DI water was added into the salt solution with vigorous stirring for 12 h. The precipitate was washed with DI water and ethanol several times. The final product was obtained by drying in a 50 oC oven overnight.


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Structural analysis. The crystal structure and alloy formation of the catalyst were characterized using X-ray diffraction (XRD, Bruker, D2Phase System). In the XRD analysis, the radition source was Cu Kα (ߣ = 0.15406 nm) and the applied voltage and current of the tube were 30 kV and 10 mA, respectively. The particle size and structure for the catalyst were observed using a transmission electron microscope (TEM, JEM-ARM 200F) operating at 200 kV. The specimen for TEM analysis was prepared by dropping the catalyst solution dispersed in ethanol on a carbon-free Cu grid. To investigate the specific surface area and pore structure of the samples, nitrogen adsorption/desorption isotherms were obtained using a Micromeritics ASAP 2020 adsorption analyzer. The crystal structure was analysed using Raman spectroscopy (Horriba Jovin Yvon, LabRam Aramis) with Yag laser (λ=532 nm). The surface chemical state and composition were observed using X-ray photoelectron spectroscopy (XPS, Thermo U.K., Kalpha) with a beam source of Al Kα (1486.8 eV) and power of 200 W under a chamber pressure of 7.8×10-9 Torr. Electrochemical analysis. The electrochemical properties of the catalyst were characterized using a potentiostat (Eco Chemie, AUTOLAB). A graphite rod and Ag/AgCl (in 3 M KCl) were used as counter and reference electrodes, respectively. The catalyst inks for electrochemical analysis were prepared by mixing and sonicating the powder in DI water. 2 µL ink was dropped on a glassy carbon electrode and dried in a 50 oC oven for 10 min. The amount of all catalysts for EOR deposited on the electrode was ~0.56 mg cm-2. The ethanol oxidation of the catalysts was evaluated in Ar-saturated 2 M C2H5OH (EtOH) + 0.5 M H2SO4. In addition, the amount of catalyst for ORR deposited on the electrode was ~0.6 mg cm-2. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) of the catalysts for ORR were obtained in O2-saturated 0.5 M H2SO4. The EtOH tolerance test during ORR was conducted by linearly sweeping between


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-0.2 and 1.0 V in O2-saturated 0.5 M H2SO4 in the presence of different concentrations of EtOH. To evaluate the effect of EtOH during ORR, the LSVs of the cathode catalysts were measured at 0.6 V by instantaneously inserting 2 M EtOH (100 µL) in O2-saturated 0.5 M H2SO4. Fabrication of membrane-electrode-assembly and unit cell measurement. The membrane-electrode-assembly (MEA) was fabricated using a conventional decal method. The ink for the anode was prepared by mixing and sonicating the catalyst (0.3 g) in DI water (600 µL), isopropanol alcohol (30 mL), and Nafion solution (650 µL). The ink for the cathode was prepared by mixing and sonicating the catalyst (0.3 g) in DI water (600 µL), isopropanol alcohol (30 mL), and Nafion solution (3250 µL). The catalyst ink was sprayed on Teflon film using an ultrasonicator and the Nafion solution (100 µL) was then coated on the catalyst layer. The loading amounts of anode and cathode were ~2 and ~3 mgcatalyst cm-2, respectively. The MEAs were fabricated by hot-pressing the anode and cathode-coated Teflon films inserted by a pretreated Nafion membrane (117, DuPont) at 120 oC and 40 atm for 2 min. The MEAs with an active area of 5 cm2 were inserted into graphite plates with serpentine flow channels and were assembled with gold-coated copper plates as current collectors. The unit cell measurements using the MEAs under an ambient pressure were carried out using a unit cell station (CNL Energy Co., CNLPEM005-01). In the unit cell measurement, the flow rate of the 2 M ethanol solution at the anode was 2 mL min-1. The flow rates of O2 gas (humidified at 65 oC) were 300 mL min-1. Density functional theory (DFT) calculation. Density functional theory (DFT) calculations were carried out with Quantum-ESPRESSO code.38 The wavefunctions and charge density were expanded within plane wave basis sets with cutoff kinetic energies of 60 and 240 Rydberg, respectively. The interactions between electron and nuclei were expressed with norm-conserving


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pseudopotentials and the exchange-correlaction of electron was approximated by Perdew-BurkeErnzerhof functional.39,40 The FeN4-embedded graphene structure was modeled with 5×5 expanded hexagonal graphene sypercell, has sufficient lateral dimensions (12.31Å) to avoid the interaction with periodic cells. To model graphene quantum dot systems, we used a supercell constituted of 30 carbon atoms passivated by hydrogen atoms as the starting model with 12Åthick vacuum layers placed in all directions. Only the gamma point was taken into account in all of the DFT calculations for this study. The adsorption free energy (∆Gad) was calculated using: (1) where respectively, and

and

are the total energies of ethanol-adsorbed and clean substrates, is the total energy of an ethanol molecule.

Results and Discussion The XRD patterns of the as-prepared Pt and PtRu for EOR are shown in Figure S1(a), exhibiting the XRD peaks corresponding to a face-centered-cubic (fcc) crystal structure. As calculated by the Debye-Scherrer equation with the (220) planes, the average particle sizes of Pt and PtRu were determined as 4.6 and 3.0 nm (Figure S1(b)), respectively, demonstrating the formation of the nanoparticles. In particular, the more positive shift of the PtRu peaks implies the alloy formation between Pt and Ru. In addition, from the position of the shift peak, the smaller lattice parameter of the PtRu nanoparticle (NP) was calculated as 0.387 nm, compared to that of Pt (0.3907 nm). The variation of the crystal structure in the catalyst can lead to the modification of the surface structure for EOR.41-44 Based on Vegard’s law, the calculated elemental composition of Pt and Ru was 46 at% and 54 at%, respectively. As shown in Figures S2(a)-S2(c),


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the as-prepared PtRu NP exhibited a high crystalline structure with an average size of ~3.0 nm. From the energy dispersive X-ray spectroscoy (EDX) data, the elemental composition of Pt and Ru was 45 at% and 55 at%, respectively (Figure S2(d)), which was in agreement with the data analyzed from the XRD pattern. Figure 2(a) shows the XRD patterns of the doped carbon nanostructures as NPM cathode catalysts synthesized using a pyrolysis process with Fe-TMPP and Fe-Pc under an N2 atmosphere at 900 oC (denoted as C/Fe-TMPP and C/Fe-Pc, respectively). The as-prepared samples exhibited peaks at 26.3o and 43o corresponding to the (002) and (101) planes of graphitic structure, respectively, demonstrating the formation of a graphitic carbon structure. The Raman spectra of the as-prepared samples consisted of G- and D-bands at 1584 and 1350 cm-1 corresponding to the graphitic sp2 carbon structure and disorder due to heteroatomic doping, respectively (Figure 2(b)). The intensity ratios of the D- to G-band for C/Fe-TMPP and C/Fe-Pc were 1.05 and 1.06, respectively, demonstrating the doping of Fe and N in the carbon structure.22,45,46 To characterize the specific surface area and pore size distribution, the nitrogen adsorption/desorption isotherms of the samples were obtained (Figures 2(c) and 2(d)). The C/FeTMPP and C/Fe-Pc exhibited the specific areas of 946 and 856 m2 g-1, respectively, and average pore sizes of 2.85 and 2.77 nm, respectively. The mesoporous structure with a high surface area can result in enhanced ORR activity due to the facilitating mass transport and increased active sites.47 Figure 3 shows the SEM and TEM images of C/Fe-TMPP and C/Fe-Pc as NPM cathode catalysts prepared using a pyrolysis process with Fe-TMPP and Fe-Pc under an N2 atmosphere at 900 oC. The as-prepared samples exhibited a mesoporous nanostructure transferred by a silicate template with a mesoporous structure. The uniform elemental distribution of Fe and N as dopants in the porous carbon structure was confirmed from EDX mapping images (Figures 3(e) and 3(f)).


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To confirm the chemical composition and state of the NPM cathode catalysts, XPS analysis of C/Fe-TMPP and C/Fe-Pc was performed. From the XPS survey spectra, the existence of N, Fe, and O as well as C could obviously be observed (Figure S3(a)). In particular, C/Fe-TMPP and C/Fe-Pc contained the chemical species of carbon such as C (284.6 eV), C-N (285.6 eV), C-O (286.2 eV), and O=C-O (288.5 eV) (Figures S3(b) and S3(c), respectively). The amounts of nitrogen in C/Fe-TMPP and C/Fe-Pc were 3.77 at% and 4.47 at%, respectively. Moreover, the chemical states of nitrogen in C/Fe-TMPP and C/Fe-Pc consisted of pyridinic N (398.3−399.5 eV), pyrrolic N (400.1−400.9 eV), graphitic N (401.2−402.0 eV), and oxidized nitrogen N (403.4−410.0 eV) (Figures 4(a) and 4(b), respectively). However, depending on the precursor for nitrogen, the relative ratio of nitrogen species varied (Figure 4(c)). The ratios of pyridinic N and pyrrolic N as essential electrocatalytic active sites for ORR in C/Fe-TMPP were 33.0% and 45.1%, respectively, whereas the ratios of pyridinic N and pyrrolic N in C/Fe-Pc were 31.3% and 41.6%, respectively. However, the ORR activity of C/Fe-TMPP with relatively high portions of pyridinic N and pyrrolic N can be superior to that of C/Fe-Pc. The amounts of iron species in C/Fe-TMPP and C/Fe-Pc were 0.17 at% and 0.14 at%, respectively, forming the Fe-pyridinic N structure for Fe-N4 and Fe-N3 to act as ORR active sites (Figures 4(d) and 4(e)). CVs for Pt and PtRu NPs as anode catalysts were measured in Ar-saturated 0.5 M H2SO4 and 2 M EtOH + 0.5 M H2SO4 in order to characterize the electrochemical properties and EOR in an acid medium (Figures S4). The current densities of Pt and PtRu NPs at 0.45 V, i.e. a voltage corresponding to an activation polarization region, were 61.1 and 75.3 mA cm-2, respectively, exhibiting a faster EOR of PtRu than that of Pt. In addition, the ratios of forward peak current density (If) to backward peak current density (Ib) for Pt and PtRu NPs were 0.754 and 0.997, respectively. Compared to the Pt NPs, the higher If/Ib of the PtRu NPs implies the enhanced


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tolerance to CO-related species formed during the EOR due to the bifunctional mechanism of the PtRu alloy structure.48,49 To characterize the electrochemical properties in an acid medium, CVs for a commercial Pt(20 wt%)/C (Premetek Co. Fuel cell grade, Comm.Pt/C), C/Fe-TMPP, and C/Fe-Pc as cathode catalysts were measured in Ar- and O2-saturated 0.5 M H2SO4 (Figures S5(a), S5(c), and S5(e), respectively). The Comm.Pt/C, C/Fe-TMPP, and C/Fe-Pc exhibited the ORR peak potentials at 0.845, 0.793, and 0.703 V, respectively, in O2-saturated 0.5 M H2SO4. LSVs for Comm.Pt/C, C/Fe-TMPP, and C/Fe-Pc were measured with different rotating speeds in O2-saturated 0.5 M H2SO4 (Figures S5(b), S5(d), and S5(f), respectively). In particular, the half-wave potentials (E1/2) for Comm.Pt/C, C/Fe-TMPP, and C/Fe-Pc were 0.86, 0.833, and 0.797 V, respectively, and the reduction current densities for Comm.Pt/C, C/Fe-TMPP, and C/Fe-Pc at 0.9 V were 0.669, 0.915, and 0.781 mA cm-2, respectively (Figures S6(a) and S6(b)). Furthermore, LSVs for Comm.Pt/C, C/Fe-TMPP, and C/Fe-Pc were measured using a rotating ring disc electrode (RRDE) in order to characterize the electron transfer number (n) and generation rate of hydrogen peroxide (%H2O2) during the ORR process in an acid medium (Figure S6(c)). The values of n and %H2O2 for the samples were determined from the following equations (Figure S6(d)): (2)

(3)

where ID, IR, and N are the disc current, Pt ring-disk current, and collecting coefficient (-0.4245), respectively. The values of the electron transfer number for Comm.Pt/C, C/Fe-TMPP, and C/FePc at 0.9 V were 3.84, 3.90, and 3.86, respectively, and %H2O2 for Comm.Pt/C, C/Fe-TMPP,


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and C/Fe-Pc at 0.9 V were 8.2%, 4.2%, and 6.4%, respectively (Figure S6(e)). Consequently, the NPM catalysts showed an ORR activity comparable to Pt, i.e high half-wave potential, high ORR current, and electron transfer number approaching 4. In particular, compared to C/Fe-Pc, the improved ORR performance of C/Fe-TMPP can be attributed to a large specific surface area with relatively high portions of pyridinic and pyrrolic N states, increasing the number of electrocatalytic active sites for ORR. In direct ethanol fuel cells (DEFCs), the crossover of ethanol as a fuel can deteriorate the performance of Pt-based catalysts at the cathode due to the poisoning effect by ethanol.50 However, it has been reported that the NPM cathode catalysts exhibit a tolerance of ethanol without mixed potentials by EOR. To investigate the ethanol tolerance of the cathode catalysts, LSVs for the catalysts were measured in O2-saturated H2SO4 containing different concentrations of ethanol at a rotating speed of 1600 rpm. As shown in Figure 5(a), in the case of Comm.Pt/C, the oxidation peaks evidently appeared despite the reduction potential region for ORR. The oxidation current densities increased with increasing concentration of EtOH, demonstrating the increased EOR on Comm.Pt/C. Thus, as shown in Figure 5(b), Comm.Pt/C exhibited the difference of half-wave potentials (∆E1/2) in the absence and presence of EtOH, i.e. ∆E1/2 ~ 0.356 V in the absence of EtOH and 0.1 M EtOH. This implies a significantly deteriorated ORR activity due to the mixed potential resulting from the oxidation of EtOH on Comm.Pt/C as a cathode catalyst. On the other hand, C/Fe-TMPP and C/Fe-Pc as NPM catalysts exhibited no serious variation in the LSVs with increasing EtOH concentration in O2-saturated H2SO4 (Figures 5(c) and 5(d), respectively), demonstrating the maintained ORR activity even in the presence of EtOH due to the tolerance to EtOH by the NPM catalyst. The variation of E1/2 of the cathode catalysts was expressed as a function of EtOH concentration in O2-saturated H2SO4


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(Figure 5(e)). Furthermore, chronoamperometric responses to ethanol introduced to an O2saturated electrolyte were measured at 0.6 V, as shown in Figure 5(f). C/Fe-TMPP and C/Fe-Pc showed relatively constant reduction current densities despite the introduction of 2 M EtOH (100 µL) during the ORR process, implying the maintained ORR activity in the presence of EtOH. On the other hand, Comm.Pt/C exhibited the sudden current drop in the direction of the oxidation of EtOH with the insertion of EtOH. Therefore, the tolerance and inactivity of cross-over ethanol for the NPM cathode catalysts could be suitable for their practical application to DEFCs. To characterize the MEAs prepared using C/Fe-TMPP and Comm.Pt/C as cathode catalysts and PtRu as an anode catalyst for DEFCs, polarization curves were obtained at 80 oC by supplying 2 M EtOH at the anode and humidified O2 at the cathode (Figure 6(a)). The open circuit voltages (OCVs) and maximal power densities of the DEFCs with C/Fe-TMPP were 0.70 V and 8 mW cm-2, respectively, and those of Comm.Pt/C were 0.58 V and 10 mW cm-2, respectively (Figure 6(b)), representing the DEFC performance similar to that using a typical PtRu anode catalyst for alcohol oxidation.18 Compared to Comm.Pt/C, the higher OCV of C/FeTMPP can be attributed to the tolerance of C/Fe-TMPP to the crossover EtOH in the DEFC. However, the lower power density of the DEFC with C/Fe-TMPP might result from the increased ohmic loss due to the significantly thicker catalyst layer with the higher loading amount, compared to the DEFC with Comm.Pt/C. The relatively higher loading amount for C/Fe-TMPP as an NPM catalyst might be attributed to a low volumetric mass density of C/FeTMPP.51 Remarkably, at a relatively high potential range between OCV and 0.35 V (i.e. the region in which an activation loss can be predominant), as shown in Figure 6(c), the DEFC with C/Fe-TMPP exhibited superior cell performance, representing higher power densities due to the excellent EtOH tolerance and low voltage drop by C/Fe-TMPP.52


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To investigate the effect of the concentration of EtOH as a fuel on the DEFC performance, polarization curves of the DEFCs with C/Fe-TMPP and Comm.Pt/C as cathode catalysts were obtained at different EtOH concentrations (Figures 7(a)-7(d)). The OCVs of the DEFC with C/Fe-TMPP supplied with 2, 5, and 10 M EtOH at the anode were 0.711, 0.699, and 0.689 V, respectively, whereas the OCVs of the DEFC with Comm.Pt/C supplied with 2, 5, and 10 M EtOH at the anode were 0.59, 0.55, and 0.52 V, respectively (Figure 8(a)). In general, with increasing concentration of EtOH as a fuel at the anode, the crossover of EtOH to the cathode supplied at the anode can increase, assuming that the pressure at both the anode and cathode are identical to the atmosphere, which might be mainly attributed to the concentration gradient of EtOH and electro-osmotic effect.53-54 Thus, the ethanol crossover (the ethanol flux) through PEM caused by the diffusion and electro-osmotic drag can be expressed as follows55: (4) where JEtOH, Jdiff, and Jelectro are the total flux, and the fluxes resulting from the diffusion and the electro-osmosis, respectively. The Jdiff and Jelectro can be dependent on the concentration gradient and the cell current density, respectively. However, the effect of ethanol concentration and current density on the ethanol crossover rate might be complicated and competitive. With increasing current density, since more ethanol molecules could be electrochemically oxidized at the anode, the concentration gradient of EtOH could be decreased, reducing the ethanol crossover rate. On the other hand, at higher current densities, additional protons could be transported from the anode to the cathode, resulting in an increased ethanol crossover rate due to the electro-osmotic drag. Moreover, for relatively low EtOH concentrations, the EtOH crossover might be dominantly controlled by the concentration gradient, whereas in the case of higher EtOH concentrations, the EtOH crossover might be dominantly controlled by the electro-osmotic


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drag.55 With increasing EtOH feed concentration at the anode, the DEFC with Comm.Pt/C exhibited significantly decreased OCV due to the mixed potential of the Comm.Pt/C cathode catalyst with the poor tolerance to EtOH crossed over to the cathode. On the other hand, as the EtOH concentration was increased, the DEFC with C/Fe-TMPP exhibited significantly slight variation of OCV due to the superior EtOH tolerance of C/Fe-TMPP. Moreover, the maximal power densities of the DEFC with C/Fe-TMPP supplied with 2, 5, and 10 M EtOH at the anode were 8.6, 7.0, and 6.0 mW cm-2, respectively, whereas the maximal power densities of the DEFC with Comm.Pt/C supplied with 2, 5, and 10 M EtOH at the anode were 10.7, 6.0, and 4.0 mW cm-2, respectively (Figure 8(b)). Remarkably, the DEFC with C/Fe-TMPP supplied with high concentrated EtOH such as 5 and 10 M exhibited superior performance, i.e. higher maximal power densities due to the excellent electrocatalytic activity of C/Fe-TMPP for ORR in the presence of EtOH.16 Figures 8(c)-8(e) show plots of current density versus applied voltage of C/Fe-TMPP and Comm.Pt/C with 2, 5, and 10 M EtOH, respectively, at the anode. C/Fe-TMPP supplied with 2 M EtOH at the anode exhibited higher current densities up to 0.35 V due to the effect of high OCV and lower current density at 0.30 V, compared to Comm.Pt/C (Figure 8(c)). However, the DEFC with C/Fe-TMPP supplied with 5 and 10 M EtOH maintained significantly high current densities at all voltages, compared to Comm.Pt/C (Figures 8(d) and 8(e), respectively). In particular, the difference of current density between C/Fe-TMPP and Comm.Pt/C gradually increased with decreasing cell voltages, resulting from the superior EtOH tolerance of C/Fe-TMPP and the decreased ORR efficiency of Comm.Pt/C with the increased EtOH crossover. The significantly deteriorated power and current densities of the DEFC with Comm.Pt/C with increasing concentration of EtOH can be mainly attributed to the decreased active sites of Comm.Pt/C for ORR, due to the electrocatalytic nature of Pt for both EOR and


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ORR. On the other hand, the relatively slight reduction of the performance of the DEFC with C/Fe-TMPP with increasing EtOH concentration could result from the maintained active sites of C/Fe-TMPP for ORR because of the electrocatalytic properties of the NPM catalyst for the ORR, i.e. the superior EtOH tolerance of C/Fe-TMPP. The crossover of ethanol as a fuel in the DEFCs could significantly affect the performance of the cathode catalyst for ORR. In particular, the EOR as well as ORR can simultaneously occur on the electrocatalytic active sites in the Pt-based cathode catalysts. During the catalytic reaction, oxygen and ethanol molecules could be competitively adsorbed on the identical metallic active sites provided by the Pt-based catalysts for EOR and ORR, respectively. However, according to many studies, oxygen molecules rather than ethanol could be selectively adsorbed on the active sites of the NPM catalysts.18 Thus, the adsorption energies of O2 and EtOH in the gas phase on the representative active sites, such as Fe-N4, pyridinic N, and pyrrolic N of C/Fe-TMPP as a NPM catalyst were calculated using the density function theory (DFT) calculation (Figures 9(a)9(c)). According to the literature, the adsorption energies of O2 and ethanol on a typical pure Pt catalyst were -0.44~-0.81 and -0.23~-0.81 eV, respectively, demonstrating a competitive catalytic process between ORR and EOR due to the similar adsorption energy values of O2 and ethanol.18 This demonstrates that the ORR on a Pt catalyst can be affected by the ethanol oxidation, which can simultaneously occur in the presence of EtOH, resulting in the mixed potential between ORR and EOR, i.e. the deteriorated ORR performance. The adsorption energies of O2 on pyridinic N and Fe-N4, as previously reported, were -0.53 and -1.01 eV, respectively, whereas the adsorption energy of ethanol on Fe-N4 was calculated to be -0.18 eV with no adsorption of ethanol on pyridinic N and pyrrolic N, representing the relatively low adsorption energy of ethanol on the active sites of C/Fe-TMPP, i.e. the tolerance of ethanol as a


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fuel in DEFCs, compared to Pt catalyst.18,56 However, in the DFT calculation, the solvent and pH effects need to be considered to find out more actual adsorption energy values of O2 and ethanol on a catalyst.

Conclusions In summary, the doped carbon nanostructure cathode catalysts for high-performance DEFCs were fabricated and compared to a commercial Pt/C cathode catalyst. C/Fe-TMPP exhibited superior ORR activity in both the absence and presence of crossover ethanol, whereas the ORR properties of Comm.Pt/C were significantly affected by the absence and presence of crossover ethanol. Moreover, compared to the Comm.Pt/C catalyst, C/Fe-TMPP used as a NPM cathode catalyst exhibited the high performance in the DEFC supplied with high concentrated EtOH as a fuel at the cathode, i.e. the relatively high OCV and power/current densities. The excellent ORR activity of C/Fe-TMPP for the DEFC can be due to the ethanol tolerance in the ORR and a low adsorption energy of ethanol. Thus, C/Fe-TMPP as a NPM catalyst structure can be applied as an alcohol-tolerant cathode catalyst for high-performance direct alcohol fuel cells used as an alcohol supplied at the anode.


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FIGURES

Figure 1. Schematic illustration of direct ethanol fuel cell with doped carbon nanostructures as non-precious

metal

cathode

catalysts

for


oxygen

reduction

reaction.

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Figure 2. (a) XRD patterns and (b) Raman spectra of the as-prepared C/Fe-TMPP and C/Fe-Pc. Nitrogen adsorption-desorption characteristic curves of (c) C/Fe-TMPP and (d) C/Fe-Pc.


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Figure 3. SEM, TEM, and elemental mapping images of C/Fe-TMPP (a, c, and e) and C/Fe-Pc (b, d, and f).


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Figure 4. XPS spectra of N1s for (a) C/Fe-TMPP and (b) C/Fe-Pc. Comparison of the relative ratio of N species in C/Fe-TMPP and C/Fe-Pc. XPS spectra of Fe2p for (d) C/Fe-TMPP and (e) C/Fe-Pc.


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Figure 5. (a) LSVs of Pt/C for ORR measured in O2-saturated 0.5 M H2SO4 with different ethanol concentrations at 25 oC. (b) LSVs of Pt/C for ORR measured in O2-saturated 0.5 M H2SO4 and O2-saturated 0.5 M H2SO4 containing 0.1 M EtOH at 25 oC. LSVs of (c) C/Fe-TMPP and (d) C/Fe-Pc for ORR measured in O2-saturated 0.5 M H2SO4 with different ethanol concentrations at 25 oC. (e) Comparison of half-wave potentials of Pt/C, C/Fe-TMPP, and C/FePc measured in O2-saturated 0.5 M H2SO4 containing different concentrations of EtOH. (f) Chronoamperometric curves of Pt/C, C/Fe-TMPP, and C/Fe-Pc measured at 0.6 V for ORR in O2-saturated 0.5 M H2SO4. The 2 M EtOH (100 µL) solution was instantaneously injected during the ORR process.


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Figure 6. (a) Polarization curves of the DEFCs with C/Fe-TMPP and Pt/C as cathode catalysts and PtRu as an anode catalyst supplied with 2 M EtOH and humidified O2 gas at the anode and cathode, respectively, at 80 oC. (b) Comparison of OCVs and maximal power densities of the DEFCs with C/Fe-TMPP and Pt/C. (c) Comparison of power densities of the DEFCs with C/FeTMPP and Pt/C measured in the potential range of 0.70−0.35 V.


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Figure 7. Polarization curves of the DEFCs with Pt/C (a, c) and C/Fe-TMPP (b, d) as cathode catalysts and PtRu as an anode catalyst supplied with 2, 5, and 10 M EtOH and humidified O2 gas at the anode and cathode, respectively, at 80 oC.


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Figure 8. Comparison of (a) OCV and (b) maximal power density versus EtOH concentration measured in the DEFCs with C/Fe-TMPP and Pt/C as cathode catalysts. Plots of current density versus applied voltage measured in the DEFCs supplied with (c) 2, (d) 5, and (e) 10 M EtOH at the anode.


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Figure 9. The adsorption models of EtOH on the representative active sites, such as (a) Fe-N4, (b) pyridinic N, and (c) pyrrolic N of C/Fe-TMPP as a NPM catalyst.


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AUTHOR INFORMATION Supporting Information. Additional electrochemical data, TEM images and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A2B2016033) and the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT (NRF-2017M1A2A2086648).


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Table of content

C/Fe-TMPP as an NPM catalyst exhibits high performance in the DEFC supplied with highconcentrated ethanol as a fuel at the cathode.


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Direct Ethanol Fuel Cells with Superior Ethanol ... - ACS Publications

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