Studies Of Ni-Based Anodic Catalysts For Alkaline Polymer Electrolyte Fuel Cells

In order for low temperature fuel cells to become widely applicable, it is necessary to get rid of the dependence on noble metals. Alkaline polymer electrolyte fuel cell (APEFC) is a promising approach to achieve the goal. The APEFC is expected to combine the merits of alkaline fuel cell (AFC) and proton exchange membrane fuel cell (PEMFC):for electrolyte, a solid electrolyte replaces the liquid solution and for catalysts, transition metals or their oxides may replace noble metals. However, the development of APEFC is not as smooth as expected. In addition to the difficulty encountered in synthesizing the high-performance alkaline polymer electrolyte, the stable and highly efficient non-noble metal catalysts present a very tough challenge.This thesis focuses on the Ni-based catalysts for the anodic hydrogen oxidation reaction (HOR) in APEFC, including some basic aspects of the electrochemistry of HOR, the preparation, characterization and activity evaluation of a variety of Ni-based catalysts. The main results are as follows:1. Derivation of a new equation for calculating i0 for the hydrogen reactions at Pt The exchange current density i0 is an important parameter describing the activity of an electrode to a reaction of interest. For reactions with fast kinetics like the hydrogen electrode reactions at Pt, it is necessary to correct for the influence of concentration change in the vicinity of the electrode surface for calculating i0 from polarization measurements. In order to describe the polarization behavior of the hydrogen electrode reactions and calculate i0 to a better approximation, we derived a new equation:η=(iRT/2F){1/[i0(1-i/ia,d)0.5]+1/ia,d} where ia,d is the anodic limiting current density controlled by the transport of dissolved hydrogen. This new approximation takes into account the influence of the concentration change in the vicinity of the electrode surface on both the equilibrium potential and exchange current density. The validity of this equation was checked by the experimental data of hydrogen electrode reactions at Pt rotating disk electrode in alkaline solutions. The results showed that the equation is able to describe the line shape of polarization curve in the low polarization region to a rather high precision. The exchange current density at Pt in 0.1 mol/L KOH and 25℃was found to be 0.10 mA/cm2 with an activation energy 33.5 kJ/mol.2. Preparation and characterization of a series of Ni-based catalystsNi-based catalysts with Ti, V, Cr, Mn, Cu, Ag or Au added were prepared with chemical reduction method and characterized with x-ray diffraction (XRD), high resolution electron transmission microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), oxygen and hydrogen temperature-programmed desorption (O-and H-TPD). Results indicated that the added Ti, V, Cr and Mn existed as oxides on the surfaces of Ni nanoparticles while Cu, Ag and Au formed alloys with Ni to certain extents. All these modifications did not much alter the interaction of Ni surfaces to hydrogen, but appreciably weakened the interaction of Ni surfaces to oxygen, with the modification by Cr oxides being most effective.3. Activity evaluation of the Ni-based catalystsAfter experimental comparison of different approaches, a procedure was established and adopted to estimate the surface area of the Ni-based catalysts. The sample was first reduced at-0.5 V (vs. RHE) for 20 min and then the integrated anodic charge between 0.10-0.88V on anodic linear potential scan was used to calculate the surface area, assuming 0.52 mC/cm2. With thus obtained surface area and the above mentioned new equation of i0, the exchange current density for the hydrogen electrode at Ni/C in 0.1 mol/L KOH at 30℃was found to be 0.87μA/cm2 with an activation energy 41 kJ/mol. The modification by adding Cu and Ti led to an increasing in i0 by a factor of 2.5 and an order of magnitude, respectively.4. Tailoring surface electronic structures for selective resistance to oxidation of Ni surfaces Based on the density functional theory (DFT) calculation and experimental results, it was proposed that the Ni surface might be rendered selective resistance toward oxidation by tailoring the surface electronic structures. DFT calculations showed distinct differences between 0 and H adsorption on Ni. While Hads was found to interact with Ni mainly through sp-band, Oads mainly through d-band. Calculation also revealed that surface decoration with transition metal oxides, such as CrOx, could selectively depress the d-band reactivity of Ni surface while keep sp-band reactivity essentially unchanged. The idea of tailoring electronic structures for selective resistance to oxidation was supported by the O-TPD and hydrogen activation experiments of Ni-based catalysts. For example, Cr oxide modification increased significantly the resistance to air oxidation but retained the activity to hydrogen. After oxidation treatment, the Cr oxide modified Ni catalyst could be reductively activated by hydrogen normally at room temperature.