Systematic Study of Pt-Ru/C Catalysts Prepared by Chemical Deposition for Direct Methanol Fuel Cells

Electrocatalysis DOI 10.1007/s12678-017-0359-9 ORIGINAL RESEARCH Systematic Study of Pt-Ru/C Catalysts Prepared by Chemical Deposition for Direct Methanol Fuel Cells C. Jackson 1 & O. Conrad 1 & P. Levecque 1 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract In this research, the activity and stability for methanol electro-oxidation on Pt-Ru/C catalysts was increased by optimising the catalyst preparation method. The Pt-Ru/C catalysts were synthesised using Pt(acac)2 and Ru(acac)3 precursors for chemical deposition of the metals. Performance of the catalyst was examined by cyclic voltammetry and chronoamperometry in a methanolcontaining electrolyte. TEM, EDS, X-ray photoelectron spectroscopy and XRD were used to physically characterise the catalysts. The parameters investigated were precursor decomposition phase, synthesis temperature and Pt/Ru ratio. Precursor deposition from the liquid phase was more active for methanol electro-oxidation, predominantly due to particle size and degree of alloying achieved during this precursor decomposition phase. Synthesis temperature affected the particle size, active surface area, ruthenium oxidation state and degree of alloying which in turn affected catalyst stability and activity for methanol electro-oxidation. The Pt/Ru ratio greatly affects the performance of the catalyst. The catalyst with the highest activity for methanol electro-oxidation was the catalyst synthesised at 350 °C with a Pt/Ru ratio of 50:50. Keywords Direct methanol fuel cell . Platinum . Ruthenium . Electrocatalysis . Thermally induced chemical deposition * P. Levecque 1 HySA/Catalysis Centre of Competence, Centre for Catalysis Research, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa Introduction Methanol is considered to be the most promising alcohol for portable and microfuel cell applications since methanol is a liquid under atmospheric conditions, synthesised easily and inexpensively, with a specific energy density of 6 kWh kg−1 [1]. Therefore, the direct methanol fuel cell (DMFC) is a promising alternative to conventional batteries, as they offer longer run times and methanol can be easily replenished from the fuel storage. This would translate into a longer battery life and more power available on portable devices. In addition, the DMFC would have the advantage of instantaneous refuelling, unlike the rechargeable battery which requires hours to restore power. Despite the many advantages of DMFC’s over hydrogen polymer electrolyte fuel cells (PEFC’s), the drawbacks of DMFC’s are the high cost of materials used in fabrication, the crossover of methanol from the anode to the cathode, ruthenium dissolution and crossover from the anode to the cathode, low efficiency and low power density [2]. Due to the low activity of the catalyst at the anode, catalyst loading at the anode is approximately ten times that of the catalyst loading in the hydrogen PEMFC. The high catalyst loading increases mass transfer limitations which further decreases the efficiency at the anode [3]. Carbon-supported Pt-Ru catalysts are considered to currently be the best catalysts for the anode of the DMFC because of their tolerance of the carbon oxygenate intermediate of the methanol electro-oxidation reaction and activity towards the water splitting reaction [4]. These Pt-Ru/C catalysts are usually prepared by chemical reduction of H2PtCl6 and RuCl3 precursors with an atomic ratio of Pt0.5Ru0.5 [5]. However, it has been proposed that catalyst precursors containing chloride have lower activity and stability than non-chloride precursors since the chloride deactivates the active sites on the catalyst [6]. This Electrocatalysis optimum ratio of Pt/Ru, morphology, degree of alloying and particle size is highly contested since optimum conditions are easily influenced by slight variations in preparation methods [5]. The organo-metallic chemical vapour deposition (OMCVD) synthesis method has many advantages over wet synthesis. Namely, it is a ‘one-step’ process which is less time consuming since it allows lengthy stages, involved in the wet chemistry method, to be avoided [7]. In addition, the mixing of catalyst precursors in the OMCVD method occurs in the vapour phase. This allows for small particle production, excellent uniformity and an enhanced level of control over metal loading, since the decomposition occurs at the same time and in a more controlled manner [8]. The CVD process is a promising catalyst synthesis method because small particles are produced which show excellent electrochemical properties in PEFC’s [9]. The aim of this study was to investigate the characteristics and electrochemical performance for methanol electro-oxidation of PtRu/C catalysts prepared by OMCVD method and a new method which involves precursor decomposition before vapourisation. The effect of varied precursor decomposition phase, synthesis temperature and Pt/Ru ratio was investigated. Experimental Preparation of Catalysts Pt(acac)2 and Ru(acac)3 were used as precursors for Pt and Ru, respectively, supported on carbon black (Vulcan XC-72R). The precursors and carbon black were mixed well to produce 0.25 g of Pt-Ru/C catalysts with varying Pt/Ru ratios by thermally induced chemical deposition [10, 11]. The catalysts were prepared in a tubular furnace, under argon (2 bars) and vacuum (0.01 bar) atmospheres at 350 °C for 4 h. Catalysts were prepared with varying operating temperatures for 4 h under a 2-bar argon atmosphere. Catalysts prepared with different Pt/Ru ratios were prepared at 350 °C for 30 min. Preparation of the Working Electrode The catalyst ink was prepared in a glass vial by adding 5 mg of the catalyst to 5.5 mL of 18.2 mΩ cm deionised water (MilliQ), 1 mL of isopropanol (Kimix) and 50 μL of 5 wt.% Nafion solution. The mixture was sealed in the vial, the vial placed in a beaker of ice and sonicated for 30 min. A micropipette was used to place 10 μL of the catalyst ink onto the working electrode, which was a 5-mm diameter glassy carbon disc electrode, polished with 1 and 0.05 μm alumina paste. The electrode was left in air to dry. Electrochemical Experiments The electrochemical characterisation experiments were conducted in a three-electrode electrochemical cell. A glassy carbon electrode coated with catalyst ink was used as the working electrode; a Pt wire as a counter electrode and Hg/HgSO4 reference electrode were used for the electrochemical experiments. All potentials were corrected and reported using the standard hydrogen electrode (SHE). A 0.5-M H2SO4 (95– 98% H2SO4 Sigma-Aldrich Reagent Grade) electrolyte solution was used for cyclic voltammetry experiments and prepared using 18.2 mΩ cm deionised water and concentrated H2SO4. A 0.5-M H2SO4 and 1 M MeOH (99.9% SigmaAldrich CHROMASOLV) electrolyte solution was used for the methanol oxidation cyclic voltammetry, prepared using 18.2 mΩ cm deio (...truncated)