Synthesis and oxygen reduction activity of shape-controlled Pt(3)Ni nanopolyhedra

Platinum-based alloys have been extensively shown to be effective catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Most of these catalysts are nanoparticles without shape control. Recently, extended Pt(3)Ni(111) surfaces prepared in ultrahigh vacuum were demonstrated to possess enhanced ORR catalytic activity as compared to the state-of-the-art carbon supported Pt (Pt/C) nanoparticle catalysts. How and whether this promising surface can be transformed into practical nanoscale electrocatalysts used in PEMFCs remain a challenge. We report a new wet-chemical approach of preparing monodisperse Pt(3)Ni nanoctahedra and nanocubes terminated with {111} and {100} facets, respectively. We further show that the ORR activity on the Pt(3)Ni nanoctahedra is approximately 5-fold higher than that of nanocubes with a similar size. Comparison of ORR activity between carbon-supported Pt(3)Ni nanoctahedra and commercial Pt/C reveals that the Pt(3)Ni nanoctahedra are highly active electrocatalysts. This synthetic strategy may be extended to the preparation of other shape-controlled fuel cell electrocatalysts.

Impedance analysis for hydrogen adsorption pseudocapacitance and electrochemically active surface area of Pt electrode

Electrochemically active surface area (ECA) of a polycrystalline Pt electrode is measured from the pseudocapacitance (Cp) values that are associated with hydrogen underpotential deposition. The potential-dependent Cp values are extracted from the raw impedance data by removing the interferences coming from the double-layer charging and hydrogen evolution. Three different approaches have been made: (i) by using the proportionality between the capacitance and area of the capacitive peak on imaginary capacitance plots, (ii) by complex nonlinear least-squares (CNLS) fitting on both the imaginary and real part of complex capacitance with appropriate equivalent circuits, and (iii) by using the modified Kramers-Kronig (K-K) relation. The first approach is the simplest one for the Cp measurement but cannot be used in the hydrogen evolution region (<0.05 V vs RHE), whereas the measurement can be extended down to -0.01 V with the second method. The isotherm fitting on the Cp(E) profile shows that the saturation of adsorbed hydrogen is reached at -0.1 V vs RHE. Faster data acquisition is possible with the third approach since the data analysis can be made without the time-consuming low frequency data (<100 Hz). The roughness factor and ECA of the Pt electrode are calculated from the electric charge that is obtained by integrating the potential-dependent Cp values; the roughness factor (1.4-1.5) lies within the normal range for planar electrodes.

Effectiveness factor of Pt utilization in cathode catalyst layer of polymer electrolyte fuel cells

In this work, we analyze effectiveness factors of Pt utilization in perfluorosulfonate ionomer (PFSI) bonded thin film cathode catalyst layers of polymer electrolyte fuel cells. We define the effectiveness factor of Pt utilization as the apparent rate of current conversion exhibited by a specific catalyst layer design divided by the ideal rate obtained if all Pt atoms were used equally in electrochemical reactions at the specified electrode overpotential and externally provided reactant concentrations. This definition includes statistical factors at all relevant scales as well as non-uniformities of reaction rate distributions under operation. Our model is based on the random composite agglomerated morphology of the catalyst layer. It accounts for the interplay of transport phenomena and electrochemical kinetics. At the mesoscopic scale, limited effectiveness of Pt utilization in agglomerates is mainly an electrostatic effect. We determined spatial distributions of effectiveness factors of agglomerates in the through-plane direction, and thereafter calculated overall effectiveness factors of the cathode catalyst layer. Our results show that small agglomerate radius, low operating current density, high operating temperature, and high oxygen partial pressure result in high effectiveness factors of Pt utilization. Finally, we compared PFSI-bonded thin film cathode catalyst layers with ultrathin two-phase cathode catalyst layers in terms of effectiveness factors. Including the surface to volume atom ratio of Pt nanoparticles, the two different types of structures exhibit similar effectiveness factors of Pt utilization, which are found to be distinctly below 10%.Key words: polymer electrolyte fuel cells, fuel cell modeling, cathode catalyst layer, Pt utilization, effectiveness factor.