Titanium‐Containing Raney Nickel Catalyst for Hydrogen Electrodes in Alkaline Fuel Cell Systems

Abstract

In alkaline hydrogen-oxygen fuel cells Raney nickel is employed as catalyst for hydrogen electrodes. The rate of anodic hydrogen conversion has been increased significantly by using a titanium-containing Raney nickel. The properties of the catalyst powder, the influence of particle diameter, and the behavior of electrodes under load are described. Impedance measurements have been used to characterize the electrodes. In fuel cell systems the supported electrodes are normally operated at current densities up to 0.4 A'cm-2; the overload current density of 1 A.cm -~ can be maintained for several hours.The economic factors of the H2-O2 fuel cell are determined essentially by the costs of the catalysts employed in the electrodes. Hence it is rational for commercial applications to avoid the expensive and rare materials such as noble metals. In alkaline electrolyte, the metals nickel and silver are suitable as catalysts for the anode and the cathode, respectively (1, 2). Catalyst powder with large interior surface is used to have a large reactive surface area available. Raney nickel has thus proved useful for the H2 electrode (2) and it is considered in detail. The active material can be depyrophorized by a slow surface oxidation and the catalytic activity could be increased by annealing at 300~ in hydrogen atmosphere (3). The attainable current density of the electrodes depends strongly on the pretreatment of Raney nickel (2, 4).A further increment in catalytic activity could be achieved by changing from a binary Raney alloy to a ternary system in which an additional metal such as iron, molybdenum, or titanium has been alloyed to nickel and aluminum (5-7). The objective of the alloy additives is to produce ternary phases in which Ni, A1, and the additive metal are distributed homogeneously. Only such phases yield homogeneous alloy catalysts with highest activity. The additive metals lead to a lower filling of d bands and to lattice defects.The properties of titanium-containing Raney nickel catalyst are reported here. ExperimentalPreparation of the cataIyst.~The Raney alloy has a mass fraction of 50% A1; the rest consists of Ni and Ti in which the titanium content can be varied from 0 to 5%. When the liquid alloy is quenched in a water bath titanium remains dissolved in the two compounds NiA13 and Ni2Ala. A granulate is formed which can be powdered easily in a mill. Powder with a particle diameter of less than 50 ~m is employed.Aluminum is extracted to an extent of 95% in 6M potassium hydroxide at 80~ titanium remains completely in the catalyst. After washing, the powder is dried in vacuum and subjected to surface oxidation by

Structure and phase-composition of Ti-doped gas atomized Raney-type Ni catalyst precursor alloys

“…The findings presented here are contrary to those of both Mund et al [17] and Gros et al [20] NiAl is a B2 cubic phase with a wide compositional stability field extending from 42 -69 at.% Ni. On the Al-rich side, the deviation from stoichiometry is accommodated by the inclusion of vacancies on the Ni lattice sites [20], with the number of unoccupied Ni-sites approaching one third as the Ni 2 Al 3 stoichiometry is approached.…”

“…Mund et al [17] found that Ti can substitute for Ni in both the Ni 2 Al 3 and NiAl 3 phases, although the formation of the additional phase TiAl 3 has also been observed [19]. Based on a study of melt spun ribbons of Ti-doped Raney Ni, Gros et al [20] reported the formation of dendritic morphologies in the as-solidified material, these structures being similar to those observed in undoped Raney-type Ni.…”

“…The polarisation resistance of the electrodes is at least halved by the addition of small amounts of Ti, with the decrease in polarisation resistance corresponding to an increase in catalytic activity. However, above 2 wt.% Ti the catalytic activity tends to decrease again due to the greater extent of the oxide layer formed on the catalyst [17].…”

“…The use of titanium as a dopant has not been the subject of much research for hydrogenation reactions, although it is known that the addition of Ti to Raney-type nickel increases catalytic activity for fuel cell [17] and hydrogen production [18] applications. This is because the presence of Ti significantly increases the rate of anodic hydrogen conversion [17].…”

“…This is because the presence of Ti significantly increases the rate of anodic hydrogen conversion [17]. The polarisation resistance of the electrodes is at least halved by the addition of small amounts of Ti, with the decrease in polarisation resistance corresponding to an increase in catalytic activity.…”