Studies On High Performance Pt-based Catalysts For PEMFC

Proton Exchange Membrane Fuel Cell(PEMFC) is one of the promising energy conversion equipments for portable electronic devices because of low environmental impact, high energy density, and simple structure. At present, Pt-based nanocatalysts are still the universal choice of electrocatalysts for PEMFC. Nevertheless, the limited resource and high cost of Pt pushes up the price of PEMFC, which cannot be accepted by the consumers. Moreover, the slow kinetics of Pt for oxygen reduction reaction and the poor performance of Pt for anti-poisoning also hinder the large-scale practical application and commercialization of PEMFC. Furthermore, under the corrosive operating conditions, the durability of Pt-based nanocatalysts cannot meet the requirement for the long-term operation of PEMFC. More importantly, the conventional catalyst layer of PEMFC is usually prepared by randomly incorporating a certain amount of non-conducting Nafion into the electrode. Howerver, Nafion for protons transport tends to wrap or isolate Pt NPs in the electrode, leading to the interruption of proton and electron channels, the increase of gas transport resistance and finally to the low Pt utilization. To resolve the above-mentioned issues, new attempts have been achieved in the design of catalyst structure and the modification of catalyst support to improving the catalytic performance and the Pt utilization of Pt-based catalyst.Fistly, a simple and efficient strategy to porous Pt Cu nanodendrites(Pt Cu-ND) with high electrocatalytic performance for PEMFC has been developed. The Pt Cu-ND are investigated by transmission electron microscopy(TEM), high-angle annular dark-field scanning TEM(HAADF-STEM), X-ray diffraction(XRD) and inductively couple plasma atomic emission spectrophotometry(ICP-AES). The formation mechanism of the Pt Cu-ND has been explored through by comparing the results obtained by different conditions. Duo to the unique structure with high surface area and rich step/edge atoms, the Pt Cu-ND catalyst shows impressive oxygen reduction reaction(ORR) activity and methanol oxidation activity(MOR). The ORR test manifests that the half-wave potentials of Pt Cu-ND(0.891V) is 70 m V higher than that of JM Pt/C. The MOR test demonstrates that the Pt Cu-ND catalyst has the high MOR current(1.001 m A·g-1Pt)and the low onset potential of MOR(0.579V) comparing with JM Pt/C.Secondly, a novel thiolated CNTs(SH-CNTs) has been developed through by the covalent bond link, which is used as support to prepare Pt Ru/SH-CNTs catalyst with a simple wetting chemical strategy. The morphology and structure of Pt Ru/SH-CNT are characterized by TEM and XRD. The TEM and XRD results demonstrate that Pt Ru nanoparticles are highly dispersed on SH-CNTs and have a high alloying extent. The MOR activity and anti-poisoning performance of Pt Ru/SH-CNT catalyst are evaluated by the electrochemical techniques and XPS. The MOR tests manifest that the methanol oxidation onset potential of Pt Ru/SH-CNT catalyst(~0.37V) is lower than that of Pt Ru/COOH-CNT catalyst(~0.41V), and the methanol oxidation peak current of Pt Ru/SH-CNT catalyst is approximately 1.7 times higher than Pt Ru/COOH-CNT catalyst. The CO-stripping experiment results show that the CO oxidation onset potential of Pt Ru/SH-CNT catalyst(0.387V) is also lower than that of Pt Ru/COOH-CNT catalyst(0.512V), confirming that Pt Ru/SH-CNT have more powerful anti-poisoning performance compared to Pt Ru/COOH-CNTs catalyst. The XPS results show the enhanced electrocatalytic activity of Pt Ru/SH-CNT catalyst should be attributed to the strong Pt-SH electronic interaction, which weakens the σ-π bond between CO and Pt.Thirdly, we present a facile strategy to efficiently embed Pt nanocrystals into N-doped porous carbon/carbon nanotubes(Pt@CNx/CNT). The N-doped porous carbon shells not only effectively prevented Pt nanocrystals from detachment, dissolution, migration, and aggregation during accelerated durability tests(ADT), but also allowed the access of electrolyte to the Pt surface and preserved the good electron transfer of CNT by avoiding the structural damage of carbon nanotubes(CNTs). Compared to JM Pt/C, the specific and mass activity of Pt@CNx/CNT are ～1.6 and ～4 times larger after ADT, respectively. The interaction between the embedded Pt nanocrystals and the encapsulating CNx layer was found in Pt@CNx/CNT, which markedly affected the electronic structure of Pt nanocrystals and contributed to the improvement on the catalytic activity and stability of Pt@CNx/CNT. As a result, the Pt@CNx/CNT catalyst exhibited an excellent thermal stability, durability, and su?cient catalytic activity for ORR.Finally, we have synthesized a hybrid catalyst with the high Pt utilization and durability, in which the Pt nanoparticles are deposited on the sulfonated carbon nanotubes(SO3H-Ar-CNTs) with conducting proton and electron. Electrochemical performance of the catalyst(Pt/SO3H-Ar-CNTs) is evaluated by the cyclic voltammograms, linear sweep voltammetry and fuel cell tests, respectively. The experimental results demonstrate that Pt/SO3H-Ar-CNTs catalyst exhibits good oxygen reduction reaction activity, enhanced durability and excellent Pt utilization compared with Pt/COOH-CNTs catalyst. The single cell test show that the maximum power output of Pt/SO3H-Ar-CNTs with 15% Nafion is 860 m W·cm-2, which is higher than that of Pt/COOH-CNT with 35% Nafion(766m W·cm-2), confirming that the Pt/SO3H-Ar-CNTs catalyst in the single cell have less dependence on the Nafion relative to the Pt/COOH-CNTs catalyst. The ADT tests show that ECSA of Pt/SO3H-Ar-CNTs catalyst only decreased approximately 26% after ADT, whereas Pt/COOH-CNT lost about 91% of their initial ECSA, revealing that the Pt/SO3H-CNTs catalyst was more electrochemically durable than the commercial Pt/C catalyst. The improved performance of Pt/SO3H-Ar-CNTs catalyst is ascribed to the introduction of the sulfonic acid group onto the CNTs surface by the covalent modification method, which ensures Pt NPs locate in triple-phase zones to increase Pt utilization and reduces the corrosion of the SO3H-Ar-CNTs.