Coking resistant Pd-Au composite membranes and Cu-based electrodes by electroless plating for H₂ separation and purification

Abstract

In this work, electroless plating (ELP) was used to fabricate thin metal films for hydrogen separation and purification applications. The first section focused on Pd-Au alloy composite membranes because pure Pd was susceptible to sulfur and carbon poisoning which adversely affected the throughput (H2 flux) and selectivity (H2 purity). Dense Pd-Au bilayer films fabricated by sequential ELP of Pd and Au were annealed at different temperatures and pressures of H2 gas to assist in the formation of a homogenous alloy. The highest temperature (550°C) and pressure (3.0MPa H2) reduced the annealing time from five days to nine hours because of a lower activation energy for inter-metallic diffusion. Additionally, there was a need to identify an in situ parameter to confirm annealing as ex situ analysis techniques were too time intensive. This was achieved by calculating the apparent activation energy for H2 permeation (Eact) which had a characteristic value depending on the Au composition. Alloys in the 0–41wt% Au range were annealed and their calculated Eact values were found to agree well with literature data for cold-rolled foils. Eact initially decreased from 12.4 to 7.5kJmol-1 (0 to 21wt% Au) before increasing to 9.0kJmol-1 (41wt% Au). For the second section, it was desired to fabricate cheap and coking resistant anodes on protonic ceramic membranes for hydrogen separation from hydrocarbon-rich feeds. Cu is a good candidate because of its stability at high temperatures in reducing and hydrocarbon-containing gas environments. But Cu electrodes fabricated using commercial pastes are thick (≥10μm) and tend to delaminate from the ceramic. Cu ELP was shown to be a better alternative as thin (~1μm) and well-adhered electrodes were fabricated with Pd, Ru, and Cu as activation catalysts. Amongst them Pd/Cu and Ru/Cu had the lowest area specific resistances (two orders of magnitude lower than the pastes). Furthermore, during galvanic hydrogen pumping, the membrane-electrode assemblies demonstrated high current efficiencies (>80%). Pd/Cu and Ru/Cu also showed low power requirements: ≤34mW/(NmL H2 min-1) in 10%H2 in Ar. However, after testing Pd/Cu in the presence of CH4, the power requirement in 10%H2 in Ar tripled and that change was irreversible.