Electrochemical Reduction of Oxygen on Hydrophobic

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Electrochemical Reduction of Oxygen on Hydrophobic Ultramicroporous polyHIPEs Carbon Mykola Seredych, Andrzej Szczurek, Vanessa Fierro, Alain Celzard, and Teresa J. Bandosz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01497 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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Electrochemical Reduction of Oxygen on Hydrophobic Ultramicroporous polyHIPEs Carbon Mykola Seredych,1 Andrzej Szczurek,2 Vanessa Fierro,2 Alain Celzard2 and Teresa J. Bandosz1* 1

Department of Chemistry, The City College of New York, 160 Convent Ave, New York, 10031,

USA 2

Institut Jean Lamour, UMR Université de Lorraine – CNRS n°7198. ENSTIB, 27 rue Philippe

Séguin, BP 1041, 88051 Epinal cedex 9, France

ABSTRACT: A new kind of polyHIPEs (polymerized High Internal Phase Emulsions) based carbon derived from co-reacted furfuryl alcohol and tannin was tested as an ORR catalyst. To understand the reduction process, their surface was extensively characterized from the point of view of texture and chemistry. The prepared materials show subtle differences in the chemistry but marked in the porosity. The best performing sample had a very high volume of ultramicropores and the highest degree of defects on the surface. The oxygen was present on the surface mainly in epoxy and ether configurations. Those oxygen groups located in large pores promoted transfer of O2 dissolved in water/electrolyte to small pores of hydrophobic surface. There, a strong adsorption of oxygen was energetically favorable. This led to weakening of O-O

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bonds, subsequent dissociation of oxygen, and its reduction/protonation. The presented polyHIPEs carbons show high electrochemical stability and better tolerance to methanol than Pt/C. On them high kinetic current density was measured.

KEYWORDS: O2 reduction, carbon, electrocatalysis, porosity, surface chemistry, specific interactions

1. INTRODUCTION Until now Platinum/carbon (Pt/C) based cathodes are the best catalysts for an oxygen reduction reaction (ORR). This process is of significant importance for fuel cells, metal-air batteries and, in general, for the development of new sources of energy.1,2 Owing to the high cost of Pt catalysts, their low tolerance to fuel/methanol crossover and poor operation durability the extensive research efforts continue on the non-noble metal or non-metal electrocatalysts for the reduction of oxygen.3-16 The latter group of materials includes heteroatom-doped nanoporous carbons.13-15,17-25 Besides them, also other rather nonporous carbonaceous materials including graphene oxide or graphene have been investigated as efficient electrocatalysts for ORR.3,4,69,11,16

Nitrogen,5,6,8,10,13,14,23 sulfur,7,9,11,15,24,25 phosphorus,18,21,22 boron,21,26 alone or in co-

doping9,15,17 have been identified as important modifiers of carbons. They provide catalytic active sites for oxygen electrosorption and reduction.3,9,11,17,26 From all heteroatoms doped to carbon and tested for ORR, nitrogen brings the most promising results in terms of the highest number of electron transfer (n) and a high density of a kinetic current (Jk).13,27 The values of n close to 4e− and the kinetic current reaching 4 mA/cm2 (n = 3.96e− and Jk = 4.02 mA/cm2 at -0.50 V vs. SCE (0.51 V vs. RHE)) have been reported on this


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kind of materials.27 On the polyaniline-derived N- and O-doped mesoporous carbons the onset potential was shifted to more positive value compare to Pt and the kinetic current density reached ~29 mA/cm2 and stable n value of 2.7e−.13 The ORR activity was linked to the synergistic effect of nitrogen and oxygen groups such as pyridone. Co-doping (N, S) of pre-synthesized graphitic microporous carbon nanospheres resulted in a high ORR activity (n = 3.8e− and Jk = 27.0 mA/cm2 at -0.70 V vs. Ag/AgCl (0.28 V vs. RHE).15 Mesoporous co-doped graphene (S and N) was also investigated and its performance (n = 3.3e− and Jk = 24.5 mA/cm2 at -0.50 V vs. Ag/AgCl (0.48 V vs. RHE) was linked to the synergistic effects of charge and spin densities.9 Even though the performance of those materials is still considered as worse than that of Pt/C and in the majority of studies on ORR an incomplete 4e− electron pathway is followed,28 heteroatomdoped carbons show a high tolerance to methanol crossover.9,11,27 Recently, we have indicated that important assets of sulfur-doped nanoporous carbons are their conductivity, hydrophobicity, and porosity.24,25,29,30 The particular role of the latter is in the dispersion/distribution of heteroatoms on the surface. We suggested that reduced sulfur groups (in bisulfide and thiophenic configurations), when located in very small pores provide the hydrophobicity and this, combined with the small size of pores results in an efficient withdrawal of oxygen from water/electrolyte and its strong adsorption on a surface.24,25,29,30 This process might be an important step for ORR. On the other hand, more bulky oxidized sulfur species such as sulfoxide and sulfones can exist in larger pores providing hydrophilicity and thus promoting a transport of water (with dissolved oxygen) to the small pores where oxygen reduction is enhanced. Following this line of research the objective of this paper is to investigate the performance of new polyHIPEs carbons as ORR catalysts. PolyHIPEs carbons are specific carbonaceous materials having micro/mesoporous texture and a high volume of macropores.31 An advance


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compared to state-of-the-art in the field is in addressing new carbons of unique hierarchical structure containing mainly oxygen in epoxy/ether groups as a heteroatoms as ORR catalysts. Since these carbons are mainly hydrophobic with very limited content of oxygen containing acidic groups and they have a marked volume of small ultramicropores where oxygen adsorption can be enhanced, they were chosen as suitable candidates to further support our hypothesis on the importance of these features, not necessary linked to specific groups on the carbon surface as reported previously, for ORR.7,9,13,24,25 So far, mainly hydrophilic carbonaceous surfaces of low porosity have been considered as beneficial for ORR. The behavior of the catalysts is linked to their surface features and specific porosity governing ORR is discussed. 2. EXPERIMENTAL SECTION 2.1. Materials. The preparation of the organic, emulsion–templated, precursors was detailed elsewhere.32 Briefly, furfuryl alcohol (FA) and tannin (T) (base of the resin) were mixed and coreacted in the presence of antifoam and various amounts of Tween 80 (surfactant). After thorough stirring, cyclohexane (internal phase, so as to obtain a final volume fraction of 70%) was incorporated with 4-hydroxybenzenesulfonic acid (catalyst). The resultant emulsions were cured and, when the polymerization was completed, the internal phase was evaporated. The as-obtained, highly porous, cellular monoliths were finally pyrolysed in a tubular furnace equipped with a quartz tube flushed at 100 mL/min with high-purity nitrogen and heated at 1 °C/min up to 900 °C. After the final temperature was held for 2 h, the materials were slowly cooled down to room temperature in the flow of inert gas. Such pyrolysis process resulted in black carbon monoliths, called CFAT-S, where S is the initial wt. % of surfactant in the formulation. Here 1.4, 3.9 and 6.5 wt. % are represented by A, B and C, respectively. As


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reported previously, the cell size distribution was shifter to lower values at rather constant overall porosity when the amount of surfactant was increased.32 The variation in the surfactant amount was related to the requirement of getting a stable emulsion before the latter turns hard and can be dried for recovering the porosity derived from the evaporation of cyclohexane droplets. The choice of surfactant amount was based on this limit: one close to the lower limit -1.4%, one close to the upper limit - 6.5%, and one in the middle3.9%). For reference Pt/Vulcan catalytic carbon with 20 wt. % Platinum (Sigma Aldrich) was used. 2.2. Methods. 2.2.1. Electrochemical Measurements. The performance of our materials for electrochemical ORR was investigated in 0.10 M KOH using a three-electrode cell with Ag/AgCl/NaCl (3 M) as a reference electrode. The ORR stability tests of the catalysts were run in potentiostaticpotentiodynamic mode. After stabilization of the catalysts by cycling in the potential range of 1.17 V to 0.18 V vs. RHE for both polyHIPEs carbons and Pt/C, the chronoamperometry tests were run at the constant potential (0.71 V vs. RHE for polyHIPEs carbons and 0.85 V vs. RHE for Pt/C) under O2 until ORR current reach the stability. The potentials were chosen as those at which the peak maximum from the static cyclic voltammetry for ORR was reached. After this, the cyclic voltammetry (CV) was run again with a scanning rate of 5 mV/s and then chronoamperometry. This sequence was repeated until 1200 cycles were reached. The chronoamperometry and long-term stability tests were carried out on VersaSTAT MC (AMETEK, Princeton Applied Research). To study tolerance to methanol 0.2 mL were added into the electrochemical cell. The final concentration of methanol (0.2 mL, three times) added to the electrochemical cell (40 mL, 0.10 M KOH) was 0.36 M. The working electrode was prepared


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by mixing the active material with polyvinylidene fluoride (PVDF) and commercial carbon black (carbon black, acetylene, 50 % compressed, Alfa Aesar) (8:1:1) in N-methyl-2-pyrrolidone (NMP) until a homogeneous slurry. The slurry was coated on a Ti foil (current collector) with the total surface area of 1 cm2 of an active material. Linear sweep voltammograms were obtained in 0.1 KOH using 757 VA Computrace (Metrohm) at various rotation rates (from 0 to 2000 rpm) with Ag/AgCl (3 M KCl) and Pt wire as a reference and a counter electrode, respectively. The measurements of cyclic voltammetry were carried out under pure O2 or N2 saturation in the electrolyte in the potential range of 0.19 to -0.8 V vs. Ag/AgCl (1.17 V to 0.18 V vs. RHE) for both polyHIPEs carbons and Pt/C. A scan rate of 5 mV/s was chosen to compare the result with those addressed in the literature on carbonaceous catalysts. The working electrode was prepared by dispersing 5 mg of the catalyst in 1 ml of deionized water and 0.5 ml of 1 wt. % Nafion aqueous solution. About 5 µL of the prepared slurry was dropped (three times) on a polished glassy carbon electrode (Metrohm, Switzerland, diameter 2 mm) and dried at 50 ºC in air. The potential was swept from 0.19 to -0.8 V vs. Ag/AgCl (1.17 V to 0.18 V vs. RHE) at a scan rate of 5 mV/s. After each scan, the electrolyte was saturated with pure O2 for 20 minutes. All the experiments were carried out at room temperature. The current densities in the manuscript, unless otherwise specified, refer to the projected geometric surface area of 1 cm2. RHE conversion The measured potentials versus the Ag/AgCl (3M KCl) reference electrode were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation: ‫݃ܣܧ = ܧܪܴܧ‬/‫ ݈ܥ݃ܣ‬+ 0.059‫ ܪ݌‬+ ‫݃ܣ݋ܧ‬/‫݈ܥ݃ܣ‬


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where ERHE is the converted potential versus RHE, EAg/AgCl is the experimental potential measured against the Ag/AgCl reference electrode, and EoAg/AgCl is the standard potential of Ag/AgCl (3 M KCl) at 25 °C (0.210 V). The electrochemical measurements were carried out in 0.10 M KOH (pH = 13) at room temperature; therefore, ERHE = EAg/AgCl + 0.977 V. 2.2.2. Scanning Electron Microscopy (SEM). SEM images were obtained using a FEI Quanta 600 FEG system. The samples were coated by an ultra-thin layer of carbon by vacuum sputtering for ensuring a good electrical contact with the sample holder. For all samples, the secondary electrons detector was used for observing the topological contrast and measuring the pore sizes in the best conditions. 2.2.3. Evaluation of Porosity. Sorption of nitrogen at -196 oC was carried out using an ASAP 2020 (Micromeritics, Surface Area and Porosity Analyzer). Before the experiments, samples were out-gassed at 250 oC to constant vacuum (2 × 10-6 Torr) for more than 48 h. This temperature and long degassing time were chosen based on the results of thermal analysis indicating that no surface groups decompose at lower temperatures from these carbons.33 The BET surface area, total pore volumes, Vt, (from the last point of isotherm at relative pressure equal to 0.99), micropore volume, volume of pores less than 0.7 nm and 1 nm, V = ~3e−). Thus small hydrophobic pores might be those responsible for 4e− reduction and their involvement might be potential dependent. The ratio of the volume of the pores smaller than 0.7 nm to the total surface area indicates that their contribution is higher for CFAT-C than CFAT–B (3.5 x 10-4 cm3/m2 and 3.0 x 10-4 cm3/m2). To account for the capacitance effect we also calculated the ratios of the cathodic current densities at 0.71 V vs. RHE to the capacitance in the cathodic range. They are 14, 33 and 35 mA/F for CFAT-A, CFAT-B and CFAT-C, respectively, still indicating the slight superiority of the CFAT-C surface even at more positive potential. The results suggest that oxygen reduction process on our catalyst might be not only by surface area or volume of pores but also by the pore size distribution. The sloping of RDE plateaus observed in Figure 3 might be due to the diffusion of oxygen/reactant transport to the very small pores and the involvement of the latter in the reduction process. A similar trend was observed by Mullen and coworkers for the highly porous nitrogen-doped carbon materials in 0.1 M KOH51 and Li and coworkers.27 Taking this into consideration, we analyzed the trend between the kinetic current density at the different potentials and the volume of pores smaller than 0.7


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nm. As seen in the Figure 11b R2 increases with the shift of the potential to more negative. The same scenario was observed for carbon nanotubes by Joo and coworkers.52

4. CONCLUSIONS The results presented in this paper show an important role of small pores with hydrophobic surface for oxygen reduction reactions on carbon catalysts in alkaline environment. The new polyHIPEs carbons tested here show a hierarchical pore structure and relatively low oxygen content mainly in epoxy and ether groups. This results in favorable surface features with oxygen groups being populated enough on the surface in large pores to promote the transport of electrolyte with dissolved oxygen to small pores and from there its release from water to hydrophobic pores of high adsorption energy. It is proposed that there, owing to strong physical adsorption forces and charge transfer from a carbon surface to adsorbed oxygen O-O bond weakens and dissociation of oxygen takes place. This is accompanied by an electron transfer to oxygen from the electrode and the protonation process. The size of pores, their volume and chemistry are crucial factors for the ORR process on these catalysts. ASSOCIATED CONTENT Supporting Information. Cyclic voltammograms on modified glassy carbon RDE in 0.10 M KOH at scan rate of 5 mV/s for the Pt/C; LSVs on modified glassy carbon RDE in O2-saturated electrolyte for ORR before and after correction for the capacitive current. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Author Contributions 1

M.S. and T.J.B. contributed equally to this work. A.S. prepared the materials under supervision

of A.C., A.C. did Raman spectroscopy studies and V.F. carried out adsorption experiments and related data analysis. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors appreciate experimental help of Mr. Jimmy Encalada. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The French group gratefully acknowledges the financial support of the CPER 2007-2013 “Structuring the Competitiveness Fibre Cluster”, through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds. REFERENCES (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345-352. (2) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652−657. (3) Liu, M.; Zhang, R.; Chen, W. Chem. Rev. 2014, 114, 5117-5160. (4) Li, Y.; Zhou, W.; Wang, H.; Xie, L.; Liang, Y.; Wei, F.; Idrobo, J.-C.; Pennycook, S. J.; Dai, H. Nat. Nanotechnol. 2012, 7, 394-400. (5) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Science 2009, 323, 760-764. (6) Yang, S.; Zhi, L.; Tang, K.; Feng, X.; Maier, J.; Mullen, K. Adv. Funct. Mater. 2012, 22, 3634-3640.


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Caption to the Tables Table 1. pKa values of the groups detected on the surface using potentiometric titration. The numbers of groups in [mmol/g] are parentheses. Table 2. Atomic concentration of elements on the surface determined by XPS for the materials studied. Table 3. The results of deconvolution of C 1s, O 1s, and N 1s core energy levels. Table 4. The parameters of porous structure calculated from nitrogen adsorption measurements using 2D-NLDFT model (SBET – BET surface area; Vt – total pore volumes; Vmeso - volume of mesopores; V

Electrochemical Reduction of Oxygen on Hydrophobic

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