Investigating sputtered thin film Pt-containing electrocatalysts for SO2(aq) electro-oxidation

Abstract

Environmental concerns, social, economic and political pressure, and new technologies are the main factors driving change in energy systems around the globe. Human life and development are inevitably dependent on energy and the world is currently significantly reliant on mainly fossil fuels to meet its energy requirements. Fossil fuels together with nuclear energy both have negative implications for the health of humans and the quality of the environment. A worthy competitor that can serve as a solution to the depleting and destructive nature of fossil fuels and nuclear energy is renewable energy. Hydrogen, not only a noncarbon- containing energy carrier, but the most abundant atom on earth is considered the ultimate clean energy carrier to be generated from renewable resources. As hydrogen is only found as part of compounds on earth, it is fairly energy intensive to separate hydrogen into its molecular form, which goes hand in hand with huge amounts of environmental pollutants being emitted to the atmosphere. The hybrid sulphur (HyS) cycle, a thermo-electrochemical water splitting process, through the electrochemical oxidation of SO2, serves as a means of producing hydrogen in a usable form without emitting any harmful pollutants. Although there are various ways to produce hydrogen, interest in the non-carbon-based HyS cycle as a potential large scale hydrogen production process, results from the fact that, whereas the anodic reaction for regular water electrolysis, as another means of producing hydrogen, occurs at a standard potential of 1.23 V (SHE), the anodic reaction in the SO2 depolarised electrolyser (SDE1) occurs at a standard potential of 0.17 V (SHE), which translates into an energy gain of more than one volt that makes the HyS cycle more favourable. Insufficient electrocatalyst activity, stability and economic viability are among the most challenging issues related to technologies for electrochemical energy conversion. An aspect that can improve the SDE performance and economic viability is improving the anodic reaction of electrochemically oxidising aqueous SO2. This can be achieved by improving the electrocatalyst for the anodic reaction. In an effort to address these barriers, combinatorial sputtering, high-throughput screening, and traditional methods were employed to investigate various thin film electrocatalyst combinations containing alternately varying content of platinum (Pt), palladium (Pd) and aluminium (Al), towards the electro-oxidation of aqueous SO2. Throughout the investigation the thin film electrocatalysts were exposed to different physical and electrochemical treatments and characterisation techniques, resulting in new insights gained. Included in the list of techniques and methods are combinatorial sputtering, photolithography, high-throughput screening, cyclic voltammetry, linear polarisation, rapid thermal annealing treatment (RTA), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The combinatorial sputtering approach, based on magnetron enhanced plasma sputtering and photolithography, was developed and employed in the syntheses of the thin film electrocatalysts. A multichannel potentiostat, connected to a custom manufactured multi-working electrode electrochemical cell, allowed for high-throughput parallel screening of the deposited electrocatalysts towards the electrooxidation of aqueous SO2. Employing onset potential and current output as the screening criteria together with stability tests and the results obtained from physical characterisation (by employment of the above mentioned techniques), thin films exhibiting satisfactory performance were identified. A Pt3Pd2 thin film, annealed at 800 °C, and a ternary combination of Pt40Pd57Al3, annealed at 900 °C, were identified as potential contenders to compete with pure Pt that is currently being employed as the anode material for electrochemically catalysing the electro-oxidation of SO2. Both Pt3Pd2 and Pt40Pd57Al3 thin films contain less Pt than a pure Pt thin film, while exhibiting increased electrocatalytic activity, and can serve as a basis for future studies