Electrocatalytic properties of platinum nanocenter

Authors: Aneta KolaryZurowska Artur Zurowski Sonia Dsoke Beata Dembinska Sylwia Zoladek Malgorzata Kiliszek Roberto Marassi Pawel J Kulesza

Publish Date: 2014/06/19

Volume: 18, Issue: 11, Pages: 2993-3001

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Abstract

A unique preparation method of obtaining stable composite film with ultralow platinum content highly active towards oxygen reduction and hydrogen oxidation is presented here The matrix for platinum centers consists of highsurfacearea zeolitetype acidic salt of cesium phosphododecatungstate Cs25H05PW12O40 admixed with carbon Vulcan XC72 carriers Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings on the level of 2–5 μg cm−2 demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation particularly when compared to the performance of the Cs25H05PW12O40free system ie containing only Vulcan support prepared and examined under analogous conditions The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles 1–2 nm diameters inside the pores as well as to high surface acidity of the polyoxometalatebased salt providing protonrich environment at the electrocatalytic interfacePlatinum is so far the most effective electrocatalytic system with respect to its high activity towards reduction of oxygen 1 2 3 4 5 and oxidation of hydrogen 6 7 8 9 10 Nevertheless due to high cost of the noble metal there has been growing interest in minimizing platinum content and development of materials containing highly dispersed platinum nanoparticles that can provide large surface areas necessary for efficient heterogeneous electrocatalysis 11 12 13 14 One of possible approaches to achieve this goal is to utilize robust conducting electronically and ionically matrices eg certain metal oxides which are capable of immobilizing and separating physically catalytic metal centers thus reducing their rate of agglomeration and degradation Obviously an ideal matrix for heterogeneous electrocatalysis would interact specifically with dispersed noble metal centers to affect their structural electronic chemisorptive and interfacial properties 15 16 17 18With respect to the reduction of oxygen a good matrix support should also be reactive towards reduction of hydrogen peroxide ie the undesirable oxygen reduction intermediate For instance tungsten oxide was demonstrated to activate traces of dispersed platinum and promote oxygen reduction through the existence of metal–support interactions 17 18 leading to the hydrogen spillover and formation of highly conductive hydrogen tungsten oxide bronzes 12 13 14 15 16 19 reactive towards reduction of hydrogen peroxide 12 13 The enhancement phenomenon in the oxygen reduction was also observed when Ptbased electrocatalysts were modified with ultrathin films of heteropolytungstates 20 21 22 which can be considered as electrochemical analogs of tungsten oxides which are known for fast electron and proton transfer capabilities 20 23 Adsorption of H3PW12O40 on Pt resulted in the positive shift of the surface voltammetric peaks related to the formation of Ptoxo PtOH or PtO species 22 thus enlarging the potential window where bare metallic Pt sites eg for adsorption and activation of oxygen molecules existed Further interactions between heteropolyanions and Pt surface atoms were demonstrated to involve mostly corner oxygen atoms from heteropolyanions and only a few percent of interfacial reactive platinum atoms were believed to be blocked with respect to the access of oxygen molecules 22Tungsten heteropolyacids were considered for fuel cell research as both catalysts 24 25 26 and membrane compounds 27 28 29 30 but most of them were readily soluble in acid media and tended to be desorbed from Pt surface during the longterm operation of the systems Therefore there is a need to increase polyoxometalates overall stability and interfacial rigidityIt has been recently established that salts of heteropolyacids may be produced by partial exchange of protons in parent heteropolyacid with a small Li+ Na+ or large Cs+ Rb+ K+ NH4 + cation 31 The resulting systems are characterized by different solubility and morphology in comparison to the parent heteropolyacid structures 32 33 34 35 For example the partial exchange of protons in H3PW12O40 with Cs+ cations transforms the watersoluble acid into the waterinsoluble acid salt Cs25H05PW12O40 Further the surface area of the latter system 100 m2 g−1 is much higher than that of H3PW12O40 5 m2 g−1 The increase of x ie the Cs content in Cs x H3 − x PW12O40 from 0 to 2 results in lowering the population of surface protons but their availability significantly enlarges when x is changed from 2 to 3 the system shows the highest surface acidity at x = 25 Thus acidic strength of Cs25H05PW12O40 is similar to that of H3PW12O40 32 35 36 Further membranes composed of Cs25H05PW12O40 and Nafion relative to plane Nafion exhibit superior stability against oxidative agents and better performance in PEMFCs 37 38 39 It has also been suggested that Cs25H05PW12O40 present in the membrane acts as active catalyst towards H2O2 decomposition 38 39 Coprecipitation or simple admixing of Cs25H05PW12O40 with PtCo alloys results in the improved electrocatalytic performance toward oxygen reduction 40 Moreover similar enhancement effect was observed after impregnation of such matrix with chloroplatinic acid followed by its chemical or electrochemical reduction 41In the present work we describe a unique preparation method and discuss electrocatalytic properties towards oxygen reduction and hydrogen oxidation of composite utilizing the Cs25H05PW12O40 salt functioning as stable and active matrix for highly dispersed platinum nanocenters at ultratrace level obtained via anodic dissolution corrosion of the platinum counter electrode 13 42 43 44 Since heteropolytungstates are better catalysts for reduction of oxygen than their molybdenum counterparts 20 21 22 we have focused here exclusively on the tungstencontaining compound The addition of carbon nanoparticles Vulcan XC72 has ensured good electronic conductivity of the catalytic layer The main advantages of the utilized salt are as follows high surface acidity and presence of mobile protons in the vicinity of catalytic centers mesoporous morphology and ability to entrap very small catalytic nanoparticles ca 1–2 nm diameters The reactivity of Cs25H05PW12O40 towards reduction decomposition of hydrogen peroxide may also result in dimishing of the formation of undesirable H2O2 intermediateThe electrochemical measurements were carried out in a threeelectrode cell utilizing a platinum flag as the counterelectrode and a saturated calomel electrode SCE as the reference electrode placed in a separate compartment and connected to the main cell by a Luggin capillary A rotating glassy carbon disk electrode of 01256 cm2 surface area was used as the working electrode Before deposition of catalytic layers the glassy carbon disk electrode substrate was polished on a cloth wetted with aqueous suspension of Al2O3 particle size 03–005 μm Electrochemical measurements were done with CH Instrument Model 660 B workstation Rotating disk electrode voltammetric measurements were executed using a variable speed rotator from Pine Instruments All measurements were carried out at room temperature 22 ± 2 °C As a rule all potentials were expressed vs RHEHeteropolytungstic acid H3PW12O40 was obtained from Fluka Salt of cesium nitrate V was from Aldrich Vulcan XC72 carbon nanoparticles were purchased from ETEK Nafion® 5  alcoholic solution was from Ion Power and ethanol 999  was from Baker Electrolytes were prepared using H2SO4 of 99999  purity Aldrich All solutions were prepared using ultrapure water Millipore MilliQ

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