Electrochemically Synthesized Cu/Pt Core/Shell Catalysts For PEMFC

Polymer electrolyte membrane fuel cells (PEMFC) offer low weight and high power density and are being considered for automotive and stationary power applications. However, the intolerable cost and poor catalysis of the catalyst Pt block PEMFC from commercialization.Electrodeposition of metal catalysts on a porous carbon electrode (PCE) in an aqueous solution opens a promising way in increasing utilization of metal catalysts practically. Electrodeposition in an aqueous bath can selectively deposit platinum catalysts only onto the electrode regions, where ions and electrons are accessible because only in those regions there are electron transport channels and metal ion transport channels. However, during the Pt electrodeposition, Pt prefers to deposit on the surface of Pt particles that are already existed, which leads to a larger size of deposited Pt particles, In addition, the current efficiency for Pt electrodeposition is extremely low in a simple acidic aqueous solution, in which the hydrogen evolution is unavoidable.The direct deposition of Pt on the PCE is not feasible for the preparation of catalytic electrode. In order to achieve high catalytic performance, Cu/Pt Core/Shell like structured catalysts was electrochemically synthesized by two steps. Firstly, highly dispersed Cu nanoparticles were electrochemically deposited on a substrate of an uncatalyzed porous carbon electrode (PCE) using a modulated pulse current electrodeposition method. Compared to 13% current efficiency for direct Pt deposition on PCE, the current efficiency for Cu deposition is as high as 95%. Cu nanoparticles with an average diameter of about 80-90 nm have been successfully deposited on the PCE and were uniformly distributed throughout the whole PCE including the inner sites of the PCE. Secondly, the shell of Pt was configured outside Cu nanoparticles through replacement. In this step, to obtain Cu/Pt Core/Shell like structured catalyst, adding thiourea (TU) to H2PtCl6 solution and increasing its PH is very essential to prevent Cu core from self-dissolving. The atomic ratios of Cu to Pt in Cu/Pt Core/Shell like structure catalyst increase with TU introduction and pH value adjustment. As a matter of fact, the replaced Pt catalysts would be close to a solid Pt spHere without TU addition and pH value adjustment. The new approach reported in this thesis inherits the advantages of electrodeposition, which site-selectively deposit metal catalysts on the interface of carbon/electrolyte of PCE, but avoids the problems caused by direct Pt deposition on PCE, such as, larger Pt particles, uneven Pt dispersion serious hydrogen evolution and so on.The results show that Cu/Pt coreshell like structured catalysts exhibit better catalytic activity than the widely recognized commercial catalysts even at lower Pt loadings. The catalytic activities of the electrodes with Cu/Pt Core/Shell structured catalysts are much better than that of the electrode with commercial Pt/C catalyst even though the Pt loadings in Cu/Pt Core/Shell structured catalysts is not as high as that in commercial Pt/C catalyst (0.10mg·cm-2 in Cu/Pt/PCE against 0.27mg·cm-2 in Pt/C). It demonstrated that the single cell with Cu/Pt Core/Shell structured catalysts gave better performance for ORR. The peak power density is up to 0.9W·cm-2 at 0.24 mg·cm-2 Pt for each cathode and anode. The results about Cu/Pt core-shell-like structured catalysts are encouraging. It implys that there were possibility to fabricate an electrode with high performance for the PEMFCs using the method invented in this thesis.