Development of a Durable Cathode Material for PEFC By Encapsulating of Pt Particles into Carbon Mesopores

Abstract

Electrocatalysts have an important role to determine the performance of PEFC, which is a promising energy device for our future sustainable society. For PEFC electrocatalysts, Pt particles are generally dispersed on carbon supports. Reduction of Pt active sites during the long-time use becomes a serious problem for practical use of PEFC. Pt particles have tendency to agglomerate, or dissolve and re-precipitate under the potential cycles, resulting in the growth of particles. We are focusing of suppressing such Pt particle growth and increasing the durability of electrocatalysts by encapsulating Pt into the nanochannnels of mesoposous carbon (MC). This idea comes from reducing Pt mobility on the curved surface comparing to the flat surface, which is also supported by theoretical study.1 Electrocatalysts were synthesized from platinum(II) acetylacetonate as a precursor by depositing ca. 30 wt% Pt on our original MC, which was made from the self-organization formaldehyde/resorcinol and Pluronic F127.2 The resulting electrocatalyst was named as Pt/MC and used as a cathode catalyst. For the anode, a standard Pt/Ketjen black catalysts (TEC10E50E, TKK Corp.) was used. Membrane electrode assemblies (MEAs) were prepared by spray-printing the dispersion containing a catalyst and Nafion ionomer on both sides of Nafion membrane. For the comparison, Pt was also deposited on commercial available Vulcan XC72 carbon black, named as Pt/V, and used as a cathode catalyst.Current-voltage (IV) characteristics of MEAs were measured in this study. Then, ohmic loss, concentration loss, and overvoltage were further analyzed separately after ohmic resistance was found by impedance measurements. In order to evaluate durability of MEAs, accelerating degradation tests were performed using potential cycles between 0.6 and 1.0 V vs RHE, where dissolution and re-precipitation of Pt particles occur.3 When the change in overvoltage was compared, the overvoltage was lower for Pt/MC-MEA than Pt/VC-MEA in all the cases after 5000, 20000, and 40000 cycles. Therefore, encapsulation of Pt nanoparticles into carbon nanochannels successfully increased durability. Change in size of Pt particles before and after 40000 potential cycles was evaluated by both surface and transmitted images at the same position. As seen Figure 1, large particles seen in STEM image are mostly identified as staying on the MC surface observed from SEM image Therefore, the growth of Pt particles was found suppressed within the nanochannels, which is most likely due to reduced Pt mobility inside the nanochannels.