New hybrid chitosan derivative-carbon black was prepared and used as support for Pt nanoparticles. These catalysts improved the activity toward oxygen reduction reaction (ORR), compared with those of commercial Pt/C catalyst. The biopolymer chitosan provided an efficient and sustainable surface nitrogen source associated with the superior performance of the catalysts.Peer reviewe

Aghabarari, B.

García Fierro, José Luis

Ghiaci, M.

Martínez Huerta, M.ª Victoria

Peña Jiménez, Miguel Antonio

Aghabarari, Behzad

Martínez Huerta, M. Victoria

Digital.CSIC

Cite this: RSC Advances, 2013, 3, 5378Received 5th October 2012,Accepted 21st February 2013Hybrid chitosan derivative–carbon support for oxygenreduction reactions3DOI: 10.1039/c3ra22416cwww.rsc.org/advancesB. Aghabarari,ab M. V. Martı´nez-Huerta,*a M. Ghiaci,c J. L. G. Fierroa and M. A. Pen˜aaNew hybrid chitosan derivative–carbon black was prepared andused as a support for Pt nanoparticles. These catalysts showedimproved activity toward the oxygen reduction reaction (ORR),compared with a commercial Pt/C catalyst. The biopolymerchitosan provided an efficient and sustainable surface nitrogensource associated with the superior performance of the catalysts.The oxygen reduction catalysts used in current proton exchangemembrane fuel cell (PEMFC) prototypes are carbon supportedplatinum nanoparticles, but cost and supply constraints for large-scale applications require a factor of .4 increase in catalyticactivity per unit mass of precious metal.1,2 To overcome thisbarrier, much effort has been made in developing an effective Pt-based alloy with higher precious metal utilization,3 and evenalternative non-precious metal catalysts.4 Unfortunately, untiltoday, although substantial progress has been achieved, theperformance of the best non-precious metal catalysts (generallycarbon supported Fe– and/or Co–N catalysts) is still inferior whencompared to Pt-based catalysts in terms of both activity andstability.5During the last few years, nitrogen-doped carbon seems to bethemost suitable ORR active material to support Pt particles in thecathode catalyst.6,7 There has been increasing evidence showingthat nitrogen dopants have a profound impact on the surfacestates, electron transfer rates, and adsorption for electrocatalysisprocesses. Futhermore, it has been shown that the density ofstates at the Fermi Level and the work function will increaselinearly with increasing nitrogen content, creating a metal-likeconductivity.8,9In an effort to incorporate N species on the carbon supportsurface, carbonmaterials coated with N-containing polymers, suchas polypyrrole and polyaniline, and programmable peptides havebeen investigated.4,10,11 The polymer, as a nitrogen precursor,promises a more uniform distribution of nitrogen sites on thesurface and an increase in the active-site density. Additionally, thepresence of a polymer film has been shown to protect otherwiseunstable catalysts from degradation in acid media, which isabsolutely critical for the low pH environment of PEMFCs.12 Thesehybrid materials can possess more suitable properties for their useas catalyst supports than their individual components, which hasbeen ascribed to the higher available surface area and electronicconductivity of the support and easier charge-transfer at thepolymer–electrolyte interface allowing a high utilization ofdeposited metal nanoparticles. This is particularly important foroptimizing the synergistic nanoparticle–support interactions andfor maximizing the mass activity of expensive Pt catalysts.13Herein, we describe a significant improvement for the ORR byanchoring platinum nanoparticles (PtNPs) onto a new hybridchitosan derivative–carbon black support. Chitosan is a veryabundant, nontoxic and low cost biopolymer, constituted ofglucosamine and acetylglucosamine monomers, which has goodcomplexing ability due to the presence of –OH and –NH2 groupson the chain.14,15 The presence of the amine groups explains itsunique properties among biopolymers: (a) its cationic behaviourin acidic solutions, and (b) its affinity for metal ions. However, dueto the high viscosity and the limited capacity for incorporation of ahigh amount of metal, chitosan can easily be modified bychemical or physical processes to prepare chitosan derivatives.16,17To this end, we are conceiving a novel strategy to gain highernitrogen content and higher affinity for Pt ions on the chitosanbiopolymer by incorporation of a new compound with a highatomic N/C ratio. In the approach used, the chitosan derivativewas prepared by a Michael reaction with methyl acrylate followedby an amidation reaction with diethylentriamine to obtainchitosan-(N-(2-(2-aminoethylamino)ethyl) propanamide) (CSD)(Fig. 1). FTIR and NMR studies confirm the formation of thenew chitosan derivative (CSD) (see ESI3 for details).The chitosan derivative obtained was coated on the carbonblack (Vulcan XC 72R) with different mass ratios of CSD/carbon(CSD.C) by two different methods to obtain 5 wt% and 10 wt%CSD.C (method 1), and 13 wt% CSD.C (method 2). Method 1aInstitute of Catalysts and Petroleochemistry, CSIC, Marie Curie 2, 28049, Madrid,Spain. E-mail: mmartinez@icp.csic.esbDepartment of Chemistry, Faculty of Science, Islamic Azad University, ShahrekordBranch, P.O.Box: 166, Shahrekord, IrancDepartment of Chemistry, Isfahan University of Technology, Isfahan, 8415683111,Iran3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra22416cRSC AdvancesCOMMUNICATION5378 | RSC Adv., 2013, 3, 5378–5381 This journal is  The Royal Society of Chemistry 2013consisted of mixing the carbon black dispersed in isopropanolwith the appropriate amount of chitosan derivative in 2 wt% aceticacid aqueous solution over 2 h, after which the solvent wasremoved in a rotary evaporator. Method 2 was used to maximizedthe amount of biopolymer incorporated by mixing the chitosanderivative in 2 wt% acetic acid aqueous solution with carbon black(mass ratio CSD/carbon = 5/1), dispersing with ultrasonictreatment for 30 min, stirring the mixture at room temperaturefor 24 h and then separating the obtained CSD.C by centrifuga-tion. Finally, Pt nanoparticles (30 wt%) were anchored onto thehybrid CSD.C by impregnation of an appropriate aqueous solutioncontaining chloroplatinic acid, followed by addition of a NaBH4solution to reduce the metal at room temperature. The electro-catalysts derived from 5 wt% CSD.C, 10 wt% CSD.C and 13 wt%CSD.C are denoted as Pt.5.CSD, Pt.10.CSD and Pt.CSD.M2. Acommercial Johnson Matthey Pt/C catalyst with a nominal Ptloading of 40 wt% was employed for a comparison.An increase of the chitosan loading decreases the BET surfacearea of the carbon substrate. At the same time, elemental analysisof Pt catalysts showed an increase in the nitrogen content with thecontent of biopolymer (Fig. 2). This implies that chitosanderivative remains on the carbon surface after incorporation ofPt metal.High-resolution transmission electron microscopy (HRTEM)images are shown in Fig. 3. Method 1 (Fig. 3A and 3B) producesflower-like Pt nanoparticles with sizes ranging from 10 to 30 nm.However, Pt nanoparticles of the Pt.CSD.M2 catalyst (Fig. 3C and3D) show some areas with a homogeneous distribution centred at2 nm and other distribution areas with nanoflowers. The high-magnification image (Fig. 3D) displays well oriented 3D flower-likenanoparticles consisting of a core with many small and irregularplatinum nanocrystals.The electrocatalytic activity of the Pt nanoparticles supportedon the new hybrid chitosan derivative–carbon black for the ORRwas evaluated with a rotating ring disk electrode (RRDE) system in0.5 M H2SO4 at 2 mV s21. The stability of the hybrid was alsochecked with cyclic voltammetry, which showed the peak currentsremain constant after 100 cycles (Fig. S43). On the basis of CO-stripping the specific electrochemical active surface areas (ECSAs)of the Pt.5.CSD, Pt.10.CSD, Pt.CSD.M2 catalysts with Pt/C (JM) as areference, were determined to be 49, 29, 21 and 84 m2 g21Pt,Fig. 2 Nitrogen content and BET surface area of the chitosan-based catalysts.Fig. 3 HRTEM images of Pt.5.CSD (A), Pt.10.CSD (B) and Pt.CSD.M2 (C, D).Fig. 4 Polarization curves on a glassy carbon rotating ring-disk electrode (A) andH2O2 yield (B) of Pt/C (JM) (1), Pt.5.CSD (2), Pt.10.CSD (3) and Pt.CSD.M2 (4).Fig. 1 Chitosan (N-(2-(2-aminoethylamino)ethyl) propanamide) (CSD).This journal is  The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 5378–5381 | 5379RSC Advances Communicationrespectively. The lower ECSA of the CSD-based catalysts comparedwith commercial catalyst was mostly due to the singularnanocrystallites obtained.The ORR activity and four-electron selectivity (H2O2 yield) datafor Pt.5.CSD, Pt.10.CSD, Pt.CSD.M2 and Pt/C (JM) are shown inFig. 4. The kinetic current curves normalized by the ECSA of allcatalysts exhibit two distinguishable potential regions, with well-defined diffusion limiting currents (0.2–0.5 V) followed by a mixedkinetic-diffusion control region in the potential window of 0.6–0.9V with an onset potential of around 1 V. The half-wave potentialsof the Pt.5.CSD, Pt.10.CSD, Pt.CSD.M2 and Pt/C (JM) are 0.86, 0.85,0.84 and 0.85 V, respectively. The specific activity was derived inthe mixed-control domain at +0.85 V to be 0.36 mA cm22 forPt.5.CSD, 0.47 mA cm22 for Pt.10.CSD, 0.42 mA cm22 forPt.CSD.M2 and 0.22 mA cm22 for the commercial catalyst. Theonset potential for the ORR in the chitosan-based catalysts isapproximately 20 mV lower than commercial catalyst and does notshift suggesting that the nature of the active site has not changed.The results exhibit that the chitosan-based catalysts posses a betterperformance than the commercial catalyst.Based on the ring current generated from the oxidation ofH2O2, we can see that both the Pt.CSD.M2 and commercialcatalyst produced a lower level of H2O2 than the Pt.5.CSD andPt.10.CSD catalysts. Nevertheless, all of them show a H2O2 yieldbelow 1.2% over the potential range from 0.2 to 0.8 V versus RHE.For instance, at +0.4 V, the yield of H2O2 was only 0.52%, 0.59%,0.93% and 1.05% on the Pt.CSD.M2, Pt/C (JM), Pt.5.CSD andPt.10.CSD, respectively, corresponding to the numbers of electronstransferred being 3.989, 3.988, 3.981 and 3.979, respectively,signalling virtually complete reduction of O2 and H2O in a four-electron process.18Since the nitrogen incorporated in the carbon is considered tobe part of the ORR active sites either with or without a boundmetal center,19 the effect of platinummetal on nitrogen speciationin CSD.M2 and Pt.CSD.M2 was studied using XPS as shown inFig. 5. The component at values close to 400 eV can be assigned toamine groups and to –NH–CLO bonds, while the component at401.9 eV could correspond to some protonated amines. The peakat low binding energy at 398.7 eV in Pt.CSD.M2 can be assigned toa nitride, therefore indicating the presence of a metal–nitrogenbond.20The contribution of the chitosan derivative to the electro-catalytic activity is likely to be caused by different cooperativefactors: (i) the existence of a good interfacial contact betweenchitosan molecules, carbon and Pt nanoparticles, which promotethe unique nanocrystalite shape; (ii) the ability of the chitosanderivative to produce protonated amine groups, which enableproton transfer on the catalyst surface by performing as anionomer;21 (iii) the presence of nitrogen can have an affect on theelectron transfer rates and adsorption processes of the ORR.In summary, a new chitosan derivative was successfullymodified and coated on the surface of carbon black, and thehybrids were employed as a catalyst support in this paper. TEMand electrochemical results demonstrate that the hybrid chitosanderivative–carbon is suitable as a support for Pt electrocatalysts.The enhanced catalytic activity toward the ORR may be correlatedwith the amount of chitosan derivative on the surface. Moreover,given the prevalence of chitosan in many other applications, thesehybrid chitosan derivative–carbon supports also have the potentialto be generally useful for improving properties and performancein areas beyond catalysts,22 and in bioelectrochemical systems,such as cathodic catalysts in microbial fuel cells.23AcknowledgementsThis work was supported partially by the Ministry of Scienceand Innovation, Spain (Project ENE2010-15381) and theComunidad de Madrid (Project P2009/ENE-1743)Notes and references1 H. A. Gasteiger and N. M. Markoviç, Science, 2009, 324, 48.2 J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson,H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff andJ. K. Norskov, Nat. 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Hybrid chitosan derivative-carbon suport for oxygen reduction reactions

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