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A Rotating Disk Electrode Study Gustav K.H Investigation of the oxygen reduction activity on Silver – a rotating disk electrode study Gustav K.H. Wiberg, Karl J.J. Mayrhofer, Matthias Arenz To cite this version: Gustav K.H. Wiberg, Karl J.J. Mayrhofer, Matthias Arenz. Investigation of the oxygen reduction activity on Silver – a rotating disk electrode study. Fuel Cells, Wiley-VCH Verlag, 2010, 10 (4), pp.575. 10.1002/fuce.200900136. hal-00552362 HAL Id: hal-00552362 https://hal.archives-ouvertes.fr/hal-00552362 Submitted on 6 Jan 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Fuel Cells Investigation of the oxygen reduction activity on Silver – a rotating disk electrode study For Peer Review Journal: Fuel Cells Manuscript ID: fuce.200900136.R3 Wiley - Manuscript type: Original Research Paper Date Submitted by the 12-Mar-2010 Author: Complete List of Authors: Wiberg, Gustav; Technische Universität München, Lehrstuhl Physikalische Chemie Mayrhofer, Karl; Technische Universität München, Lehrstuhl Physikalische Chemie Arenz, Matthias; Technische Universität München, Lehrstuhl Physikalische Chemie Ag Catalyst, Alkaline Media, Fuel Cell Electrode, PEM Fuel Cell, Keywords: Oxygen Reduction Reaction Wiley-VCH Page 1 of 15 Fuel Cells 1 2 3 4 Investigation of the oxygen reduction activity on Silver – a 5 6 rotating disk electrode study 7 8 9 10 11 12 Gustav K.H. Wiberg a, Karl J.J. Mayrhofer a, b and Matthias Arenz a, c * 13 14 15 a Technische Universität München, Garching, D-85748, Germany 16 17 Lehrstuhl für Physikalische Chemie 18 b present address: MPI für Eisenforschung 19 20 Abt. Grenzflächenchemie undFor Oberflächentechnik Peer Review 21 c new affiliation: Department of Chemistry, CS06, University of Copenhagen, Universitetsparken 5, DK-2100 22 23 Copenhagen Ø Denmark 24 25 26 * to whom correspondence should be addressed: [email protected] 27 28 29 Keywords: oxygen reduction reaction (ORR), Silver (Ag), alkaline electrolyte, non-platinum metal catalyst, 30 electrochemically accessible surface area (ECA) 31 32 33 Abstract 34 35 In this study the oxygen reduction reaction (ORR) is investigated on a nanoparticulate Silver electrocatalyst in 36 alkaline solution. The catalytic activity of the catalyst is determined both in terms of mass activity as well as 37 38 specific activity and turn over frequency, respectively. It is demonstrated that the established mass activities are 39 independent of the applied catalyst loading, an essential requirement for a reasonable analysis. The 40 41 determination of the electrochemically active surface area (ECA) or the number of electrochemically accessible 42 sites (N ECAS ), respectively, is performed by the underpotential deposition of lead. The obtained value of the 43 44 activity is compared to activities of polycrystalline Silver and Platinum measured in the same electrolyte, as well 45 as to literature data. 46 47 48 Introduction 49 50 Even though fuel cells (FC) were invented a long time ago and foreseen to be “tomorrows” energy converter of 51 52 choice since the last decades, they have not yet arrived on a broad front. In the case of mobile applications, with 53 size scaling from handheld devices up to automobiles, the cost of energy conversion of a state of the art proton 54 55 exchange membrane (PEM) FC so far exceeds that of other competitive mobile technologies. One of the main 56 reasons for this is the high price of the noble metal catalyst of FC’s. In an attempt to reduce costs, non-platinum 57 catalysts might be a possible alternative [1, 2]. Therefore also alkaline based FC have recently re-gained 58 59 attention due to the fact that a variety of cheaper non-platinum metals, such as silver, nickel and cobalt, or even 60 metal-oxides are reported to be active for catalyzing the essential oxygen reduction reaction [3-5]. In order to compare different electrocatalysts, a common methodology for their analysis is required. As pointed out in the review on high surface area catalysts by Gasteiger et al. [6], even for the case of Pt based 1 Wiley-VCH Fuel Cells Page 2 of 15 1 2 3 electrocatalysts the reported activities deviate from each other by more than one order of magnitude; regardless 4 5 of originating from rotating disk electrode (RDE), flow cell or membrane electrode assembly (MEA) 6 measurements. Bearing in mind that these discrepancies impede an efficient comparison, Mayrhofer et al. [7] 7 8 recently proposed a set of guidelines for the characterization of Pt based catalysts by the thin-film RDE 9 methodology. These guidelines include, that the electrochemically active surface area (ECA) has to be evaluated 10 11 by applying a proper method that also considers the capacity of the support, and that the ECA has to be linear 12 dependent on the loading [7]. The comparison of electrocatalysts of different classes, i.e. non-platinum and 13 platinum based electrocatalysts, and the judgment of their prospect is even more difficult. In this respect the 14 15 necessity of a common methodology in catalyst characterization as well as the importance of benchmark 16 activities cannot be enough emphasized. One fundamental problem in the comparison of different catalysts is the 17 18 definition and determination of the ECA used to evaluate the specific activity; as the meaning of real surface 19 area depends on the method of measurement, on the theory of this method, and on the conditions of application 20 For Peer Review 21 of the method [8]. For Pt based catalysts, the ECA is determined by the stripping charge of underpotential- 22 deposited hydrogen (H ) or oxidation of an adsorbed CO monolayer assuming a one to one ratio of the 23 upd 24 adsorbates and Pt atoms [9]. However, for non-platinum metal catalysts these rather straightforward methods are 25 in general not feasible. A range of alternatives has been reviewed by Trasatti and Petrii [8], suggesting for 26 27 example under potential deposition (upd) of foreign metals for metal nanoparticulate systems. The methodology 28 of metal upd for the ECA determination has already been successfully applied by Green and Kucernak [10] in 29 30 the case Cu upd on PtRu alloy catalyst. For Ag based catalysts, Pbupd [11] as well as Tlupd [12] are feasible options, 31 both yielding corresponding results. 32 33 34 The scope of this article is to present a thorough characterization of Silver based electrocatalysts for the ORR in 35 36 alkaline electrolyte. Extending the guidelines for the activity determination by the thin-film RDE technique of Pt 37 based catalysts and applying an active compensation of the solution resistance, we characterize a silver 38 39 nanoparticle catalyst and polycrystalline (PC) Ag in 0.1M KOH solution. An underpotentially deposited lead 40 monolayer is stripped of the catalyst in a linear potential sweep for the determination of the number of 41 42 electrochemical accessible surface sites. The obtained results of the ORR activity are compared to those of PC Pt 43 measured in the same electrolyte as well as literature values. 44 45 46 47 Experimental 48 49 All electrochemical measurements were conducted in two separate, 3-compartment Teflon cells, previously 50 51 described in refs. [9, 13, 14], using a rotating disk electrode setup with rotation control (Radiometer Analytical, 52 France) and a Potentiostat (Princeton Applied Research, USA). The electrolyte solutions: 0.1 M KOH and 0.1 M 53 -1 54 KOH + 125 µM Pb(NO 3)2 were prepared using Millipore water (Resistivity = 18.2 MΩcm , TOC < 5 ppb), 55 KOH pellets (Merck, Suprapure) and Lead(II)nitrate salt (Merck, analytical grade). During measurements, the 56 57 electrolyte was purged with either Argon or Oxygen (Airgas, purity grade 5.0). A Standard Calomel Electrode 58 (SCE) was employed as a reference electrode. However, the reversible potential of hydrogen oxidation/reduction 59 60 was determined for each measurement, and all presented data are given with respect to the Reversible Hydrogen Electrode (RHE). The working electrodes used are Ag, Pt and Glassy Carbon (GC) disks (each Ø=5 mm, A=0.196 cm²), the sides coated with epoxy-resin (to minimise parasitic currents) and mounted into Teflon RDE 2 Wiley-VCH Page 3 of 15 Fuel Cells 1 2 3 tips. The glassy carbon disk was cleaned by polishing, placing into a sonic cleaner, and finally placing it for 5 4 5 min into concentrated perchloric acid. 6 The Silver catalyst was provided in powder form by Umicore AG & Co. KG having an average particle size of 7 8 about 100 nm. It was suspended in Millipore water, dispersed by a 2 seconds-insertion into an ultra sonic cleaner 9 and thereafter kept on rotation by a magnetic stirrer. An aluminium foil covered the glassware in order to protect 10 11 the silver catalyst from light. By these measures, agglomeration of the particles was reduced and the upper limit 12 of the catalyst solution lifetime was one week; however, by keeping the solution longer in the sonic cleaner or 13 unprotected from light, a visible degradation appeared already after a few hours. The catalyst layers were 14 15 prepared by pipetting 20-40 µl catalyst solution onto an electrochemically oxidised GC disk (100 mA for 5 min 16 in 2 electrode setup). Thereafter the electrode tip was put in a light sheltered dryer under N -atmosphere.
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