CeO₂稳定 PEMFC 阴极催化剂制备与性能研究

Proton exchange membrane fuel cell（PEMFC）is widely concerned by many researchers all over the world because of its high efficiency，no pollution to the environment and low working temperature. It is considered to be an important application as power source for both stationary and mobile applications to solve the world energy crisis. Platinum（Pt）has long been the efficient catalyst in the use of PEMFCs for both hydrogen oxidation reaction（HOR）at the anode and oxygen reduction reaction（ORR）at the cathode. Owing to the slower four-electron transfer process，ORR is five-orders-of magnitude slower than that of HOR，more Pt catalysts are needed in the cathode due to the poor kinetics of ORR and transport limitations of protons and reactants in cathode catalyst layers. While platinum is scarce and expensive，which hinders the commercial application of PEMFC. Conventional PEMFC using Pt/C cathode catalysts shows high oxygen reduction reaction activity，but its performance decreases very quick in long-term stability test. Therefore，incorporating catalyst supports such as cerium oxide（CeO₂）into Pt based PEMFC cathodes is developed to solve the problem. In this paper，the oxygen reduction activity and stability of Pt/C cathode catalysts were improved by using oxygen defects rich CeO₂ nanoparticles as carrier. The coprecipitation-acid treatment process of synthesis CeO₂ and infiltration preparation of CeO₂-based Pt/C catalyst were as follows：Firstly，Ag@CeO₂ core-shell spheres were obtained by filtration，washing and drying aqueous solution containing cerium nitrate，silver nitrate and aqueous ammonia. Secondly，the core-shell powders were calcined and excess concentrated HNO₃ treated，followed by repeated washing and infiltration to obtain the nanosized defective CeO₂. Thirdly，CeO₂ was mixed with commercial carbon black XC-72R recorded as CeO₂/C carrier，and Pt-CeO₂/C and Pt alloy-CeO₂/C catalysts were fabricated using wet reduction Pt or Pt alloy into C/CeO₂ carrier. The properties of CeO₂ and Pt based-CeO₂/C catalysts were characterized and analyzed using transmission electron microscope（TEM），X-ray diffraction（XRD），X-ray photoelectron spectroscopy（XPS）and Brunauer-Emmett-Teller （BET）. All the electrochemical tests were performed by a three-electrode cell using the device CHI 760E. A glassy carbon electrode（5 mm diameter，0.196 cm²）was used as the working electrode. The reversible hydrogen electrode（RHE）and Pt sheet（1 cm² area）were used as reference and counter electrode. Linear sweep voltammetry（LSV）was conducted in O₂-saturated 0.1 mol·L^-1 HClO₄ solution in the potential range of 0~1.2 V at a scan rate of 10 mV·s^-1 and cyclic voltammetry（CV）characterization of the samples was recorded between the voltage range of 0.081~1.281 V at a scan rate of 20 mV·s^-1 in N₂-saturated 0.1 mol·L^-1 HClO₄ solution. Accelerated durability testing（ADT）was performed by potential cycles between 0.6 and 1.0 V at a scan rate of 100 mV·s^-1 in O₂-saturated 0.1 mol·L^-1 HClO₄ solution. All potential values were corrected with ohmic loss. H₂-air PEMFC single cell tests were conducted in a standard fuel cell test system with spraying PtCo-CeO₂/C（2∶1），Pt₂Co-CeO₂/C（2∶1）and Pt/C（for comparison）into the cathode of membrane electrode assembly（MEA）with Pt loading of 0.2 mg·cm^-2，where the anode Pt loading was 0.1 mg·cm^-2. The results showed that the coprecipitation-acid treatment method synthesized CeO₂ was cubic fluorite structure，and its particle size was between 10~30 nm. The specific surface area of CeO₂ prepared by co-precipitation-acid treatment method was 44.08 m²·g^-1，and there was a large amount of Ce³⁺ in CeO₂ nanoparticles，indicating that CeO₂ prepared here was rich in oxygen defects. Energy dispersive spectrometer （EDS）results also showed that Pt，CeO₂ and C distributed evenly with a Pt size of 5 nm in the catalysts. As for the electrochemical results of Pt-CeO₂/C and Pt alloy -CeO₂/C samples，the ratio of CeO₂ to C was varied，and the electrochemical properties and stability were tested in a three-electrode system to investigate the optimal CeO₂ to C ratio. The results showed that the higher amount of CeO₂ in the sample，the poorer conductivity of the catalysts due to the low amount of good conductor C，resulting in a lower ORR activity. The paper listed the corresponding rotating disk electrode（RDE）test results for 20%Pt or Pt alloy loading on CeO₂/C at different C∶CeO₂ ratio. The catalysts using Pt or PtCo alloy nanoparticles on CeO₂/C showed good performance，for example，Pt-CeO₂/C catalyst showed the highest activity with the half-wave potential of 0.868 V at the mass ratio of C∶CeO₂=2∶1. The stability of Pt-CeO₂/C catalyst was greatly improved compared with 20%Pt/C as well，because the mass activity of 20%Pt/C decreases by 82.2%，while the mass activity attenuation of Pt-CeO₂/C catalyst was only 13.8% after 10000 consecutive cycles. Single cell with PtCo-CeO₂/C as cathode showed a better current density of 1.578 A·cm^-2 at 0.6 V and a maximum power density of 1.02 W·cm^-2 than that of commercial Pt/C（1.17 A·cm^-2 and 0.94 W·cm^-2 at the same voltage）. Taking into account the similar Pt loadings，this electrochemical performance tested in this paper was better than commercial 20%Pt/C，indicating that the doping of CeO₂ could improve the electrochemical activity and stability of the carbon-based carrier.