Electrochimica Acta 111 (2013) 152–159 Contents lists available at ScienceDirect Electrochimica Acta jou rn al hom ep age: www.elsevier.com/locate/elec tacta The impact of iridium on the stability of platinum on carbon thin-film model electrodes Maria Wesselmark a,2 , Björn Wickman b , Carina Lagergren a,∗,1 , Göran Lindbergh a,1 a Applied Electrochemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden b Department of Applied Physics, Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden a r t i c l e i n f o Article history: Received 7 March 2013 Received in revised form 11 July 2013 Accepted 13 July 2013 Available online xxx Keywords: Fuel cell PEMFC Oxygen reduction Iridium oxide Platinum a b s t r a c t Increasing the stability and lifetime of the electrodes is one of the most important factors in order to realise a large scale use of polymer electrolyte membrane fuel cells (PEMFC). By using well-defined thin-film model electrodes, the stability of Pt and Pt on Ir were examined as cathode catalysts in a single cell PEMFC setup. The electrodes were fabricated by evaporating thin layers of Pt and Pt on Ir onto the microporous layer of a gas diffusion layer. The amount of Pt deposited was equivalent to 3 nm (about 6.3 g cm -2 ) and the amount of Ir was varied between 1.5 nm and 20 nm (between 3.4 g cm -2 and 45.3 g cm -2 ). All samples with Ir showed an increased stability over samples with sole Pt during cyclic corrosion test between 0.6 V and 1.2 V vs. the reversible hydrogen electrode. For thin layers of Ir, the initial activity for the oxygen reduction reaction was equal to or superior to that of sole Pt but for thicker Ir films it was somewhat lower. Hydrogen underpotential deposition and CO stripping were used to estimate the electrochemical surface area during the experiments and physical characterisation using scanning electron microscopy and X-ray photoelectron spectroscopy were used to determine the structure of the samples. The results suggest that Ir can stabilise Pt in the cathode electrode. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Today, high amounts of Pt catalyst are needed on the cathode in the polymer electrolyte fuel cell due to the sluggish oxygen reduction reaction. In addition, the loss of electrochemically active surface area in the catalyst layer causes a considerable performance loss over time. The degradation is more severe on the cathode than on the anode, partly due to carbon corrosion, and it is therefore important to find a way to improve the activity and stability of the cathode catalyst layer [1]. By using alternative materials as support for Pt, an enhanced activity and stability may be obtained [2–8]. An increased ORR activity of a non-alloy, or catalyst on support, can be explained by a spill-over mechanism of reactants or intermediates between the two materials or by the adlineation mechanism, where new sites with enhanced activity is created in the interfacial area between the two materials [2,3,6]. The difference between Pt on a support and a Pt alloy are sometimes subtle and the effects of Pt on a ∗ Corresponding author. E-mail address: carinal@kth.se (C. Lagergren). 1 ISE member. 2 Present adress: Intertek Semko AB, Torhamnsgatan 43, Box 1103, SE-164 22 Kista, Sweden. support could be similar to the ones of an alloy. Recently there has been a strong development of Pt monolayer catalysts, with a very good Pt utilisation, improved activity and good stability [4]. The improved activity may be related to a change in the Pt–Pt dis- tance, changes in the electron structure or an increased repulsion of OH ad on Pt [4,7,8]. Metal oxides are generally rather stable in the fuel cell environment and have been investigated both as sup- port and combined with Pt in the catalyst layer in order to increase the stability of the electrode and the activity for ORR [9–18]. The advantage of using a metal oxide as catalyst support instead of car- bon is a higher oxidation resistance, but additional effects such as better adhesion of the Pt and higher ORR activity on the alternative support material could also be achieved. Ir (or Ir oxide) is a very good catalyst for the oxygen evolu- tion reaction (OER) in acidic solution [19] and is therefore often added to the Pt catalyst of the cathode in unitised regenerative fuel cells [20–30]. Higher Ir content improves the OER whereas too high amounts of Ir have a negative effect on the ORR [26,27]. Due to the high potentials obtained during electrolysis of water in the regenerative mode it is important with an electrode which is stable at those potentials. With Ti and Ir oxide as gas diffusion layer, an improved stability of the catalyst layer has been achieved [30]. Improved stability as well as higher activity for oxygen reduc- tion was predicted in DFT calculations by Balbuena et al. [31] of ternary alloys with Pt, Ir and Co or Cr. The improvements were 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.108