Co-sputtered PtxPdyAlz thin film electrocatalysts for the production of hydrogen via SO2(aq) electro-oxidation

Abstract

A co-sputtered Pt x Pd y Al z ternary system was investigated for potential use as anode catalyst for the electrooxidation of aqueous sulphur dioxide (SO 2 ), a key reaction in the hybrid sulphur (HyS) process for splitting water into hydrogen and oxygen. Combining the noble metals Pt and Pd with Al resulted in no significant improvement in onset potential; however, current output was improved for the majority of the electrocatalysts evaluated. Of these electrocatalysts, only a single ternary composition exhibited improved stability when compared to pure Pt. It was found that a combination of Pt 40 Pd 57 Al 3 (annealed at 900°C) exhibited superior performance when compared to pure Pt and the previously determined best binary electrocatalyst, i.e. Pt 3 Pd 2 . Current density (mA.mg Pt −1 ) increased from 108.11 to 181.21 and finally to 396.73 for Pt, Pt 3 Pd 2 and Pt 40 Pd 57 Al 3 , respectively, indicating an increase in activity that correlates with a decrease in Pt content. Atomic force microscopy (AFM) revealed an increase in surface roughness for Pt, Pt 3 Pd 2 and Pt 40 Pd 57 Al 3 , while the occurrence of metal interaction and certain degrees of Al migration (a result of annealing) was confirmed for Pt 40 Pd 57 Al 3 by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD).

Review of Anodic Catalysts for SO2 Depolarized Electrolysis for “Green Hydrogen” Production

“…The next step in their research was to add a non-noble metal, aiming better catalyst activity and a lower price. Falch et al [41] developed a film composed of platinum, palladium and aluminium, which are co-sputtered on a support. They made improvements on the onset potential and in current density, obtaining 396.73 mA/mgPt with the ternary combination Pt 40 Pd 57 Al 3 .…”

The Effect of Sputtered Pt40Pd57Al3 Thin Film Thickness on SO2(aq) Electro-Oxidation

“…Pt (1768°C) and Pd (1555°C) are expected to be the least influenced by the annealing temperature of 900°C, which is well below the melting point of these two metals. However, as part of a previous investigation into this metal combination, an XPS analysis revealed that a degree of alloy formation occurs between Pt and Al and that contraction of the thin film also occurs [11]. This occurrence of contraction (exposure of the GC) is supported by the different degrees of agglomeration of the thin films as observed by the SEM analysis (Fig.…”

“…Trend (b) is an indication that, in terms of current output, the annealing treatment has no significant effect on increasing the current output of the respective films, which may suggest that no migration of a specific metal to the surface of the thin film occurred [11], i.e. the exposed AN surfaces available for SO 2 electro-oxidation have the same surface composition as the AD films.…”

“…5 (inset), it is clear that there is no significant change in electrocatalyst composition for all thicknesses. Although the EDX analysis is a bulk analysis technique and can therefore not provide any information on the surface composition of the samples, an earlier investigation [11] highlighted some surface enrichment of Al subsequent to RTA (also under an Ar atmosphere) that was ascribed to lattice relaxation and associated atom size differences. This surface enrichment was substantiated by X-ray photoelectron spectroscopy (XPS) depth profiling.…”

“…Hence, various attempts to reduce or eliminate the Pt content, whilst not compromising performance, have been made. A recent study has shown that a sputtered Pt 40 Pd 57 Al 3 thin film (60 nm) electrocatalyst annealed at 900°C proved to be competitive in comparison with its pure Pt competitor [11]. However, the effect that rapid thermal annealing (RTA) has on varying film thickness (a structure effect), and the subsequent effect on the electro-oxidation of SO 2 , was not investigated.…”