Recovery of Polymer Electrolyte Fuel Cell exposed to sulfur dioxide

Biraj Kumar Kakatia,b, Anusree Unnikrishnanc, Natarajan Rajalakshmic, RI Jafric, KS Dhathathreyanc, Anthony RJ Kucernaka†

aDepartment of Chemistry, Imperial College London, London SW7 2AZ, UKcCentre for Fuel Cell Technology, ARCI, Taramani, Chennai 600 113, India

Abstract

Sulfur dioxide (SO2) is a common atmospheric contaminant which has a deleterious effect on fuel cells. The performance of a Polymer Electrolyte Fuel Cell (PEFC) utilising a Pt on nitrogen doped graphene support as the cathode catalyst was studied in the presence of air contaminated with known levels of SO2. The nitrogen doped graphene supported platinum was synthesized by a hydrothermal method. At levels of 25ppm SO2 in air there was within 15 minutes a 28 % reduction in the PEFC performance at 0.5 V. The performance degradation was more severe at higher SO2 concentrations. At 100 ppm SO2 in air the performance degraded by 91% at the same potential. The power loss of the fuel cell could not be recovered by externally polarising the PEFC at 1.6 V. Even after continuous potential cycling of the cell for 9 h only 80% of the initial performance could be recovered. However, a 15 minute treatment with 0.4% O3 in air showed almost a 100% performance recovery of the 100ppm SO2 contaminated fuel cell. The enhanced recovery of the fuel cell is related both to the chemical reaction of O3 with the adsorbed sulphur contaminant, and an increase of cathode potential during the electrochemical treatment.

Keywords: Catalyst, Contamination, Fuel Cell, Graphene, Ozone, Recovery

1. Introduction

The Polymer Electrolyte Fuel Cell (PEFC) is one of the most promising alternative power sources for automotive and portable applications. However, the high capital cost and durability are limiting the widespread commercialisation of PEFCs. To date, considerable efforts have been made to study the performance degradation of fuel cell due to the presence of contaminants in fuel and oxidant. PEFCs are particularly susceptible to the performance degradation by airborne contaminant as the system is open to the ambient atmosphere. St-Pierre (2009) proposed a list of 97 airborne contaminants which was later extended to 260 during research activities supported by US, Department of Energy [1]. Sulphur dioxide (SO2) is one of the major contaminants present in the air and is detrimental for the fuel cell performance even at the sub-ppm level [2-5].

The effects of low concentration SO2 on the durability of PEFC have been studied by various researchers [6-8]. Contamination studies with SO2 concentrations of about 500ppm have been reported using platinum based catalysts in OCP and at various load conditions. The effect of concentration, relative humidity, and temperature has also been reported [9, 10].The degradation effects of SO2 contamination and the mechanism of its poisoning and recovery has been widely discussed on platinum based catalysts both by electrochemical and single cell studies.b Current address: Department of Energy, Tezpur University, Tezpur 784 028, India

†† Corresponding author: Anthony@imperial.ac.uk; Ph: +44 30 75945831, Fax: +44 20 75945804

Currently, commendable efforts have been made to develop materials that can overcome the slow and sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode [11-21]. Among them, nitrogen doped graphene has attracted attention both as a catalyst support material to enhance ORR kinetics on platinum and as a catalyst in its own right [22-32]. Nitrogen doped graphene facilitates the favourable four electron mechanism for ORR reaction instead of the two electron mechanism via the undesired hydrogen peroxide intermediate formation. To the best of our knowledge, all previous work on sulphur tolerance of platinum has only considered high surface area carbon supported catalysts (e.g. Vulcan XC-72R or Ketjen Black) and no work has been performed on the effect of other supports on the degradation of catalysts in the presence of sulphur containing species. Nor has such work been reported on the recovery of such contaminated electrodes by chemical treatment.

In the present work, we have studied the SO2 tolerance of nitrogen doped graphene supported Pt cathode catalyst in a PEFC. The SO2 contaminated PEFC was later recovered by successive external polarisation and by chemical treatment using ozone as reported elsewhere [33, 34].

2. Experimental

2.1 Catalyst Preparation:

2.1.1 Synthesis of NG

Modified Hummers’ method was used to synthesize graphite oxide (GO) from graphite powder, procured from Sigma Aldrich (<45μm particle size) [35]. The nitrogen doped graphene (NG) was synthesized by the hydrothermal method [36]. As an example, 200mg GO was dispersed and sonicated in water for 1h. The pH of the solution was later adjusted to 10 by adding 25% NH4OH, followed by 5ml hydrazine hydrate. The thoroughly mixed dispersion was later transferred to a Teflon lined SS autoclave and subjected to a temperature of 180 °C for 3h. The reduced graphene was then filtered and washed several times with DI water.

2.1.2 Synthesis of Pt/NG

The polyol reduction method was used for synthesizing NG supported Pt. catalyst [37]. Around 60 mg reduced NG was dispersed and ultrasonicated in a mixture of ethylene glycol and DI water. The mixture was subjected to sonication for 0.5 h followed by vigorous stirring for another hour. A solution of 2% H 2 PtC l6 ∙ x H 2 Oin ethylene glycol was added slowly under continuous stirring till 30% Pt loading was achieved. The pH of the solution adjusted to 12 and heat treated at 130°C for 4 h to achieve complete reduction. The reduced catalyst was then filtered from the solution and washed with sufficient amount of water to neutralise its pH. The reduced catalyst was then dried in an oven at 80°C under an inert environment. The catalyst obtained at the end of this process is termed as nitrogen-doped graphene supported platinum (Pt/NG) catalyst.

2.2 Characterization of Catalyst

The morphology of the Pt/NG catalyst was studied with the help of Field Emission Scanning Electron Microscope (make: Hitachi, Japan; model: S-4800), Transmission Electron Microscope (make: Tecnai, USA; model: G2 20 STWIN). The crystallographic analysis of the catalyst was carried out using XRD (make: Phillips, UK; model: X'pert Pro) and X-Ray

Photoelectron Spectroscopy (make: Omicron Nanotechnology, Germany; model: SPECS-Phoibos100).

2.3 MEA fabrication

Fuel cell anodes with a nominal platinum loading of 0.25 mg cm−2 were procured from Arora-Matthey, India. The catalyst ink was prepared by mixing 3.2mg Pt/NG and 1.6mg Multi-walled carbon nanotube (MWNT) in 200 ml isopropyl alcohol. The mixture was ultrasonicated for around 20min and aproximately 10ml of 5% nafion was added to it. The mixture was again ultrasonicated for another 20min before going for spray-coating it over a carbon cloth (AvCarb® 1071HCB). The nominal platinum loading on the cathodes was maintained at 0.50 mg cm−2. MWNT was used with the catalyst to avoid restacking of the Pt decorated NG sheets and to maintain sufficient porosity for fuel cell application. 50 µm thick chemically stabilised nafion membranes (212CS) were used to make the MEA. The anode and the cathode gas diffusion electrodes were placed on either side of the membrane and hot pressed at 130°C for 90 s, under 100 kg cm−2 pressure.

2.3.4 Fuel cell operation cum contamination studies

All the experiments were performed in a single cell with an active area of 5 cm 2. High purity hydrogen (99.9999% Air Products, BIPA) and compressed air were used as anode and cathode gases, respectively. The fuel cell was operated at 70°C under 100% RH. The performance of the fuel cell and the polarisation studies were carried out using a fuel cell load/impedance analyser (make: Kikusui, Japan; model: KFM2030). For contamination studies, the compressed air supplied to the cathode side of the fuel cell was premixed with 25, 50, 75, and 100 ppm SO2, produced by diluting a stream of 1000ppm SO2 in Ar (BOC). The SO2 contaminated fuel cell was purged with nitrogen before recovering it either by external polarisation or by chemical cleaning. The chemical cleaning was carried out using O3

generated by an O3 generator (make: BMT Messtechnik GmbH, Germany; model: BMT801). The concentration of O3 in the cleaning gas was controlled with by controlling the applied voltage, current, and percentage of O2 in the inlet gas. The O3 concentration produced by the generator was calibrated with the help of a UV/VIS dual beam spectrometer (make: ATI-Unicam; model: UV4-200), at the known O3 UV adsorption peak (λmax = 253.7; ε = 3000 ± 30 dm3 mol−1 cm−1) [38]. The fuel cell was later purged with nitrogen before performing any polarisation and performance studies. The recovery of the fuel cell was also carried out by external polarisation under the same operating conditions. The external polarisation was carried out at 1000 sccm air flow rate.

Figure 1: SEM images of (a) graphene sheets and (b) multi-walled nanotubes (MWNT) used in this study. (c) TEM and (d) HRTEM images of the Pt/NG.

3. Results and discussions

3.1 Characterization of catalyst

The FESEM images of the graphene and the MWNT used in this study are shown in fig. 1(a). The crumpled graphene sheets are visible in the micrographs. The average diameter of the MWNTs was recorded as 30 nm. MWNTs were used to avoid restacking of the graphene sheets under pressure inside the fuel cell. The micrograph of the MWNTs used is shown in fig 1(b). The TEM micrographs of the Pt/NG are shown in fig 1(c) and 1(d). It was observed that the Pt particles were uniformly decorated over the NG sheet.

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(c) (d)

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20 40 60 80

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C(002) Pt(311)Pt(220)Pt(200)Pt(111)

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Fig.2 (a) XRD plot of nitrogen doped graphene and Pt/NG respectively and (b) XPS spectrum of nitrogen doped graphene

The powder XRD analyses of NG and Pt/NG were carried out and the diffractograms are shown in fig. 2(a). The signature of graphite 002 is seen in the diffractogram of NG. The shifting of the 002 peak towards lower value and its broadening indicate that the van de Waals bonding between the graphene layers has been broken and graphene sheets are separated. The crystalline platinum signature is observed in the diffractogram of Pt/NG catalyst. The shifted graphite 002 peak is also visible in the diffractogram of Pt/NG. X-ray photoelectron spectroscopy (XPS) analysis of the NG was carried out to verify and quantify the doping/functionalization of the graphene sheets with nitrogen. It was found that the reduced graphene sheets were functionalised with around 3.8% nitrogen. The XPS spectrum of the NG is shown in fig. 2(b).

3.2 Effect of SO2 on the fuel cell polarisation curve

Figure 3 shows the effect of increasing concentration of SO2 on the polarisation characteristics of the fuel cell. The cathode gas was contaminated with 25, 50, 75 and 100 ppm SO2 respectively. A significant drop in the performance was observed due to the presence of SO2 in the cathode air. From the polarisation studies, it is seen that the SO2 is contaminating the active sites of the Pt-catalyst and the effect is cumulative. The power of the fresh cell at 0.5 V was recorded as around 1.52 W. It was recorded that the presence of 25 ppm SO2 in the air (for 15 minutes) degrades the power of the fuel cell to 1.10 W. A further 25ppm increase in the concentration of SO2 (for 15 minutes) degrades the power of the cell to 0.38 W. For 75 and 100 ppm SO2, the power of the fuel cell at 0.5V was decreased to 0.23 and 0.15 W, respectively after an exposure for 15 minutes at each concentration.

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Figure 3: Effect of increasing SO2 contamination on the performance of the developed PEFC. The cathode gas was contaminated with 25, 50, 75 and 100 ppm SO2 (for 15 minutes for each contamination) during this study. The flow field area of the cell was 5 cm2 and it was operated at 70°C and 100% RH.

3.2 Recovery of SO2 contaminated fuel cell

The recovery of fuel cell contaminated with 25, 50, 75 and 100 ppm SO2 were carried out by external polarisation as well as O3 cleaning. Figure 4 shows the representative polarisation characteristics of the fresh, 100ppm SO2 contaminated, and recovered fuel cell. The performance of the fresh fuel cell was analysed by galvanostatically polarising the cell between 0 and 1.0 A cm−2 current densities. The performance recorded in this polarisation studies was taken as a baseline for the rest of the experiments. It was observed that contaminating the cathode air with 100ppm SO2 degrades the performance of the fuel cell significantly. As reported earlier at 0.5V about 90% of the performance was lost. The cathode gas was replaced with fresh air after contaminating with 100 ppm SO2 for approximately 900 s. It was observed that the performance of the contaminated fuel cell partially recovered when the cathode was polarised in clean air.

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after O3 cleaning for 10 min after O3 cleaning for 15 min

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after O3 cleaning for 15 min

Figure 4: (a) Effect of 100ppm SO2 on performance of PEFC, with Pt/NG as cathode catalyst, and sequential recovery of the cell performance with O3. The total time for SO2 poisoning was 900 s. (b) Normalised power of PEFC after SO2 contamination, and after successive O3

cleaning. The SO2 contamination was performed during three galvanostatic polarization studies. The flow rate of O3/Air, during recovery, was 100 sccm. 70°C and 100% RH.

At potentials higher than 0.9 V, the sulfur adsorbed on Pt is easily electro-oxidized to sulfate (SO4

2−) [40, 41] by the following reaction:

2 Pt−S+4 H2O → SO42−¿+8 H +¿+6 e−¿+Pt ¿¿ ¿ (1)

After contamination with sulphur, the PEFC was recovered chemically by treating the cathode side of the fuel cell with 0.4% O3 in air under unpolarized conditions. The contaminated PEFC was initially O3 treated for 600 s and purged with nitrogen to remove any residual O3 inside the cell. The I-V performance of the O3 treated fuel cell was again monitored for the same current densities. It is observed that around 95% of the initial performance of the fuel cell could be recovered by treating the SO2 contaminated cell with 0.4% O3 for 600 s at room temperture. By treating the SO2 contaminated fuel cell for another 300 s we could recover ~100% of the initial performance. This can be observed in the normalised power densities plot of the fuel cell under different conditions shown in fig. 5. The maximum OCP of the PEFC during O3 treatment was 1.44 V. This high open circuit potential may have hastened the recovery process by oxidising the adsorbed species to

bisulfate as reported by Baturina et.al from XANES studies at high voltage of 0.9V [41], however there may also be a direct chemical attack of ozone or one of its decomposition intermediates (e.g. OH) on the poison [34]. Recovery of the contaminated fuel cell was also possible using higher concentrations of ozone (2 vol%) for the same treatment time. It appears that 0.4% ozone is close to the lower limit of suitable concentration when the cell is treated at room temperature, as the results become somewhat variable for even lower concentrations of ozone. It may be possible to use lower concentrations of ozone if the cell temperature is increased or the treatment time is extended, although this might contribute to increased carbon corrosion.

The O3 recovery mechanism can be attributed to both a direct and indirect and chemical processes. In the direct chemical process, O3, reacts with the adsorbed sulphur leading to formation of SO3. In the presence of H2O this product undergoes rapid hydrolysis to form H2SO4. In the indirect process, hydrolysis of O3 leads to the production of OH˙ radicals which react with the adsorbed S leading to many intermediates with varying S and O content. The mechanism of chemical reaction is well explained by both in-situ and ex-situ studies where although the contaminant studied was H2S at the anode, the adsorbed sulphur species on platinum and recovery mechanism by ozone remains the same [34]. Recovery of the PEFC contaminated with 100ppm SO2 (15 minutes) was also attempted by externally stepping the cell potential from 0V to 1.6 V for 900 s. It was also observed that even cyclic polarisation of the fuel cell, in I- V mode, for 9 hours only recovered 80% of the initial performance. This suggests that the O3 cleaning of the fuel cell is much more efficient than external polarisation.

4. Conclusion

The Pt/NG catalysts were developed and used as a cathode catalyst to study the effects of SO2

on the performance of PEFC. The Pt/NG showed good performance as a cathode catalyst in PEFC. The presence of SO2 in the air stream has a cumulative detrimental effect on the performance of the PEFC. The power of the fuel cell at 0.5 V cell potential decreased from the initial value of 1.52 W by 28% on exposure to 25ppm SO2 in the air for a period of 15 minutes. Similarly, the power loss for exposure to 50, 75, and 100 ppm SO2 concentrations for a period of 15minutes were 75, 84 and 91%, respectively. Switching the cathode gas from contaminated to fresh air recovers the performance of the PEFC partially. This suggests the removal of the subsurface sulphur containing compounds and the re-oxidation of adsorbed S Oand/orS O is difficult under normal fuel cell conditions. The external polarisation of the SO2 contaminated PEFC at 1.6 V, even for a long period, can only recover the performance of the cell to 80% of its initial performance could be recovered by cyclic polarisation of the cell between 0 and 1.6 V. Even cyclic polarisation for 9 h could not recover the initial performance of the PEFC. This may be attributed due to the presence of strongly adsorbed elemental S on the active sites of Pt. On the other hand, treating the PEFC contaminated with 100 ppm SO2 with 0.4% O3 can recover around 95% of the initial performance within 600 s. Further treatment of the fuel cell for another 300 s can completely recover the performance at 0.5V. The efficient and enhanced recovery by O3 is attributed due to the chemical and electrochemical cleaning of the catalyst. However it is necessary to optimise the concentration of O3, treatment time, operating temperature and humidity to achieve an effective and fast recovery. Excessive concentration of O3 and prolonged treatment can change the hydrophobicity of the electrodes leading to reduction of performance at high current densities[33].

5. Acknowledgements

The authors would like to acknowledge Dr. G. Sundararajan, Director, ARCI for his constant support and encouragement. Financial assistance from DST-RCUK and the Engineering and Physical Sciences Research Council under project EP/I037024/1 is acknowledged.

References

[1] St-Pierre J, Zhai YF, Angelo M. Quantitative ranking criteria for PEMFC contaminants. International Journal of Hydrogen Energy. 2012;37:6784-9.

[2] Nagahara Y, Sugawara S, Shinohara K. The impact of air contaminants on PEMFC performance and durability. Journal of Power Sources. 2008;182:422-8.

[3] Tsushima S, Kaneko K, Hirai S. Two-stage Degradation of PEMFC Performance Due to Sulfur Dioxide Contamination. Polymer Electrolyte Fuel Cells 10, Pts 1 and 2. 2010;33:1645-52.

[4] Gould BD, Baturina OA, Swider-Lyons KE. Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and COS. Journal of Power Sources. 2009;188:89-95.

[5] Zhai JX, Hou M, Zhang HB, Zhou ZM, Fu J, Shao ZG, et al. Study of sulfur dioxide crossover in proton exchange membrane fuel cells. Journal of Power Sources. 2011;196:3172-7.

[6] Mohtadi R, Lee WK, Van Zee JW. Assessing durability of cathodes exposed to common air impurities. Journal of Power Sources. 2004;138:216-25.

[7] Jing FN, Hou M, Shi WY, Fu J, Yu HM, Ming PW, et al. The effect of ambient contamination on PEMFC performance. Journal of Power Sources. 2007;166:172-6.

[8] Zhai J, Hou M, Liang D, Shao Z, Yi B. Investigation on the electrochemical removal of SO2 in ambient air for proton exchange membrane fuel cells. Electrochemistry Communications. 2012;18:131-4.

[9] Tsushima S, Kaneko K, Morioka H, Hirai S. Influence of SO2 Concentration and Relative Humidity on Electrode Poisoning in Polymer Electrolyte Membrane Fuel Cells. Journal of Thermal Science and Technology. 2012;7:619-32.

[10] Zhai Y, Bender G, Bethune K, Rocheleau R. Influence of cell temperature on sulfur dioxide contamination in proton exchange membrane fuel cells. Journal of Power Sources. 2014;247:40-8.

[11] Zhang J, Sasaki K, Sutter E, Adzic RR. Stabilization of platinum oxygen-reduction electrocatalysts using gold clusters. Science. 2007;315:220-2.

[12] Yang GX, Chen Y, Zhou YM, Tang YW, Lu TH. Preparation of carbon supported Pd-P catalyst with high content of element phosphorus and its electrocatalytic performance for formic acid oxidation. Electrochemistry Communications. 2010;12:492-5.

[13] Wu G, More KL, Johnston CM, Zelenay P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science. 2011;332:443-7.

[14] Wang D, Xin HL, Hovden R, Wang H, Yu Y, Muller DA, et al. Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat Mater. 2013;12:81-7.

[15] Wakisaka M, Mitsui S, Hirose Y, Kawashima K, Uchida H, Watanabe M. Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrolyte fuel cells studied by EC-XPS. J Phys Chem B. 2006;110:23489-96.

[16] McGuire R, Dogutan DK, Teets TS, Suntivich J, Shao-Horn Y, Nocera DG. Oxygen reduction reactivity of cobalt(II) hangman porphyrins. Chemical Science. 2010;1:411-4.

[17] Ma JA, Tang YW, Yang GX, Chen Y, Zhou Q, Lu TH, et al. Preparation of carbon supported Pt-P catalysts and its electrocatalytic performance for oxygen reduction. Applied Surface Science. 2011;257:6494-7.

[18] Liu X, Fu G, Chen Y, Tang Y, She P, Lu T. Pt-Pd-Co trimetallic alloy network nanostructures with superior electrocatalytic activity towards the oxygen reduction reaction. Chemistry. 2014;20:585-90.

[19] Liang HW, Cao X, Zhou F, Cui CH, Zhang WJ, Yu SH. A free-standing Pt-nanowire membrane as a highly stable electrocatalyst for the oxygen reduction reaction. Adv Mater. 2011;23:1467-71.

[20] Li Y, Zhou W, Wang H, Xie L, Liang Y, Wei F, et al. An oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes. Nat Nanotechnol. 2012;7:394-400.

[21] Guo S, Zhang S, Sun S. Tuning nanoparticle catalysis for the oxygen reduction reaction. Angew Chem Int Ed Engl. 2013;52:8526-44.

[22] Zhang L, Xia Z. Theoretical study of nitrogen boron co-doped graphene as efficient oxygen reduction reaction catalysts for fuel cell. Abstracts of Papers of the American Chemical Society. 2013;245.

[23] Wen Q, Wang S, Yan J, Cong L, Chen Y, Xi H. Porous nitrogen-doped carbon nanosheet on graphene as metal-free catalyst for oxygen reduction reaction in air-cathode microbial fuel cells. Bioelectrochemistry. 2014;95:23-8.

[24] Sun Q, Kim S. Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells. Electrochimica Acta. 2015;153:566-73.

[25] Lu Z-J, Bao S-J, Gou Y-T, Cai C-J, Ji C-C, Xu M-W, et al. Nitrogen-doped reduced-graphene oxide as an efficient metal-free electrocatalyst for oxygen reduction in fuel cells. Rsc Advances. 2013;3:3990-5.

[26] He Q, Li Q, Khene S, Ren X, Lopez-Suarez FE, Lozano-Castello D, et al. High-Loading Cobalt Oxide Coupled with Nitrogen-Doped Graphene for Oxygen Reduction in Anion-Exchange-Membrane Alkaline Fuel Cells. Journal of Physical Chemistry C. 2013;117:8697-707.

[27] Fu R, Yang L, Feng L, Guo W. One-pot Low-temperature Synthesis of Nitrogen-doped Graphene and It Application as Cathode Catalyst in Microbial Fuel Cells for Electricity Generation. Chemical Journal of Chinese Universities-Chinese. 2014;35:825-30.

[28] Bordjiba T, Aguibi H, Mahmoudi O, Guetteche Y, Ieee. Substitutional doping of bore, aluminum, silicon, phosphor and nitrogen in graphene for fuel cell Density Functional Theory Study. 2013 International Conference on Renewable Energy Research and Applications (Icrera). 2013:923-6.

[29] Ahmed MS, You J-M, Han HS, Jeong D-C, Jeon S. A Green Preparation of Nitrogen Doped Graphene Using Urine for Oxygen Reduction in Alkaline Fuel Cells. Journal of Nanoscience and Nanotechnology. 2014;14:5722-9.

[30] Jafri RI, Rajalakshmi N, Dhathathreyan KS, Ramaprabhu S. Nitrogen doped graphene prepared by hydrothermal and thermal solid state methods as catalyst supports for fuel cell. International Journal of Hydrogen Energy. 2015;40:4337-48.

[31] del Cueto M, Ocon P, Poyato JML. Comparative Study of Oxygen Reduction Reaction Mechanism on Nitrogen-, Phosphorus-, and Boron-Doped Graphene Surfaces for Fuel Cell Applications. Journal of Physical Chemistry C. 2015;119:2004-9.

[32] Zhang LP, Xia ZH. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. Journal of Physical Chemistry C. 2011;115:11170-6.

[33] Kakati BK, Kucernak ARJ. Sulphur dioxide contamination and Ozone recovery of Polymer Electrolyte Fuel Cell: catalyst, single cell and stack studies. Energy and Environmental Science. 2016;Submitted manuscript.

[34] Kakati BK, Kucernak ARJ. Gas phase recovery of hydrogen sulfide contaminated polymer electrolyte membrane fuel cells. Journal of Power Sources. 2014;252:317-26.

[35] Hummers WS, Offeman RE. Preparation of Graphitic Oxide. Journal of the American Chemical Society. 1958;80:1339-.

[36] Long D, Li W, Ling L, Miyawaki J, Mochida I, Yoon SH. Preparation of nitrogen-doped graphene sheets by a combined chemical and hydrothermal reduction of graphene oxide. Langmuir. 2010;26:16096-102.

[37] Li WZ, Liang CH, Zhou WJ, Qiu JS, Zhou ZH, Sun GQ, et al. Preparation and characterization of multiwalled carbon nanotube-supported platinum for cathode catalysts of direct methanol fuel cells. Journal of Physical Chemistry B. 2003;107:6292-9.

[38] Rakness K, Gordon G, Langlais B, Masschelein W, Matsumoto N, Richard Y, et al. Guideline for measurement of ozone concentration in the process gas from an ozone generator. Ozone-Science & Engineering. 1996;18:209-29.

[39] Contractor AQ, Lal H. The nature of species adsorbed on platinum from SO2 solutions. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1978;93:99-107.

[40] Loučka T. Adsorption and oxidation of sulphur and of sulphur dioxide at the platinum electrode. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1971;31:319-32.

[41] Baturina OA, Gould BD, Korovina A, Garsany Y, Stroman R, Northrup PA. Products of SO2 adsorption on fuel cell electrocatalysts by combination of sulfur K-edge XANES and electrochemistry. Langmuir. 2011;27:14930-9.