Electrodeposition of Pt Electrocatalysts for MEAs

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum 

For large cathodic current densities smaller than the limiting current density, the deposition overpotential increases with increasing current density and vice versa, according to the Bulter-Volmer equation  

The catalytic activity of carbon supported electrodes was further improved by replacing the conventional carbon supports like Vulcan XC-72R with carbon nanotubes (CNTs) and carbon nanofibers (CNFs). Carbon nanostructures including CNT and CNF have extremely higher electronic conductivity and specific surface area in comparison with the carbon black.4 The traditional Vulcan XC-72R exhibits an electronic conductivity of 4.0 S cm-1, while the electronic conductivity of CNT and CNF are in the range of 103-104 S cm-1. The high electronic conductivity of CNT and CNF ensures a reliable conductive path connecting Pt particles to supporting electrodes and increases the Pt utilization 27 

Electrodeposition of Pt on multiwall carbon nanotubes (MWNT) has been reported by Wang et al.27 In their process, a layer of Co was first electrodeposited on a carbon paper. The purpose of Co film was to catalyze the growth of the subsequent MWNTs layer. A chemical vapor deposition (CVD) technique was used to deposit MWNTs on the Co film. Platinum was then electrodeposited on the MWNT layer. The electrolyte used contained H2PtCl6 and H2SO4. The electrodeposition of Pt was performed under potentiostatic condition using 0 V vs. saturate calomel electrode (SCE). The particle size achieved with this technique was approximately 25 nm that is too large in comparison with other chemical and electrochemical methods. 

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