Donald W. Kirk

Nickel Cathode Passivation in Alkaline Water Electrolysis

During the electrolysis of water using alkaline electrolytes, nickel cathodes are observed to passivate over time. The cause of the passivation remains controversial, though the formation of a nickel hydride layer is a strong contender. The addition of certain metal ions such as iron or vanadium to the alkaline electrolytes have been observed to mitigate the passivation effect. These metallic ions can act at the electrode-electrolyte interface through oxidation reduction couples but are also observed to deposit on the electrode. In this work, we focused on determining whether the deposit had a significant role in passivation prevention. We used a thin sputtered coating of metallic iron on nickel electrodes to observe the passivation effect.

Characterization of Ag-Mn-base Nanostructured Catalysts

Literature has reported that Ag and various forms of Mn have a high activity towards the oxygen reduction reaction (ORR). [1-4] The ORR is complex involving 4 electrons per oxygen molecule. Depending on the electrocatalyst used, this reaction may produce hydrogen peroxide as an intermediate which then must be decomposed to produce hydroxide ions and oxygen. A bifunctional Ag/Mn-base catalyst system would promote direct ORR and the decomposition of hydrogen peroxide. In this work, nanowires were selected as the high surface area catalyst structure. Nanowire catalysts have been studied due to the ease of fabrication, controllable size, and unique morphologies. Ag nanowires from 10 to 100 nm in diameter were successfully synthesized using a modified template-base deposition technique.[5] The metal deposition took place in two steps. First, a nanoporous polycarbonate membrane was sensitized in a Sn-base solution. Next, the membrane was immersed into a two-part bath containing an ammonium hydroxide complexing agent and a rochelle salt reducing agent. The deposition of elemental Ag resulted from a redox reaction between the Ag and Sn ions. Volmer Weber growth was identified as the mechanism for Ag deposition. Figure 1 shows a TEM image of these structures. [4] The following studies have been conducted on the synthesis of bimetallic Ag/Mn structures and their effect on electrochemical performance.[1-3] Research showed that bulk Ag/Mn catalysts had a higher activity towards the ORR compared to a pure bulk Ag electrode.[1] A Mn concentration between 1 and 10wt% produced a significant increase in electrocatalytic activity.[1] It was proposed that Mn altered the Ag d orbital electronic structure, increasing the bond strength between Ag and adsorbed O2 and the kinetics of O2 splitting.[1] In other work, carbon nanotubes (CNTs) were deposited with Ag and MnO2 to observe the bifunctional effect of both metals. [2-3] A higher electrocatalytic activity for the ORR was achieved. These studies concluded that improvements in electrochemical performance have been observed when elemental Mn and various Mn-oxides were deposited with Ag. For this work, the synergistic effect of Ag/Mn nanowire catalysts are compared to the performance of a pure Ag system. The approach taken was to adapt the original electroless deposition bath for Ag in order to codeposit Mn. The main challenge was to develop a stable bath chemistry for both metals. Ag deposition is ideal at a pH of 10 to 11. According to the Pourbaix diagram for Mn, Mn ions are stable in acid solutions. Complexing Mn ions is thus required to avoid precipitation in the basic silver deposition solution. The membrane surface was observed under an SEM to analyze the change in the growth mechanism of Ag with the addition of Mn in the bath. A pure Ag deposition can be seen in Figure 2. The addition of Mn to the bath altered the growth mechanisms and structure of a pure Ag deposit as seen in Figures 3 to 4. It was proposed that the Mn was depositing on the surface and causing Ag to alter its growth mechanisms on the polymer membrane. The membranes were then dissolved and the nanowires were characterized using scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy to determine the chemical and structural form of Mn present in the nanostructures.