Ethanol electro-oxidation at M/Pd/GC, Pd/PANI/GC and Pd/PANI fibrous electrodes in alkaline medium for direct ethanol fuel cells
thesisposted on 2021-07-29, 07:43 authored by Alexandros Symillidis
Global warming has been one of the major environmental problems of our planet since mid-20th century and it is attributed to the excessive emissions of greenhouse gases, such as CO2, by human activity. This, coupled with the finite nature of fossil carbonaceous fuels, has generated the need for alternative fuels, such as renewable hydrogen, bioalcohols etc., as well as, more efficient energy conversion devices, such as fuel cells. In this project several M/Pd and Pd/polyaniline (PANI)-based catalysts were investigated for the ethanol electro-oxidation reaction (EOR) in alkaline medium for the development of anode electrodes for alkaline direct ethanol fuel cells (ADEFCs). Specifically, PANI was tested both as a promoter for Pd and as a support material, while several transition metals were also investigated as additional promoters. Under this framework, a series of novel polyacrylonitrile (PAN) and ionic liquid (IL) core / Pd and PANI shell fibrous mat electrodes were developed and investigated in several variations and modifications, aiding to the development of polymeric electrodes for ADEFCs, that could exploit the advantages of a polymer in terms of mechanical properties, cost etc., as well as its promoting effects. All experimental samples of this project were tested mainly via cyclic voltammetry (CV) over a temperature range 25 – 60 oC. The main performance indices used was the forward peak current density at 25 oC, jP, and activations energy, Eα, as calculated by the Arrhenius plots. Also, several samples were characterised via SEM, EDX and XPS. Initially, several added metals were investigated under the same experimental conditions comparatively for simple Pd / glassy carbon (GC) electrocatalyst for the EOR in alkaline medium. Ag primarily and Bi secondarily performed the best and were distinguished as candidate promoters for polyaniline-based Pd electrocatalysts developed, tested and discussed in the following parts. Subsequently, the promoting effects of polyaniline were investigated via electrodeposited Pd/PANI/GC catalysts, that showed better catalytic performance compared to electrodeposited Pd/GC catalyst. Specifically, some of those catalysts were prepared in two steps (electropolymerisation / electrodeposition of PANI followed by electrodeposition of Pd – “2-Step”), some in one step (co-electrodeposition of PANI and Pd – “1-Step”) and some in one co-electrodeposition step followed by a second Pd electrodeposition step (“2-1-Step”). Most catalysts prepared with the 2-Step method demonstrated better performance than simple Pd/GC in terms of Eα. Catalysts prepared with the 1-Step method demonstrated better electrocatalytic activity in average in terms of both performance indices than the 2-Step catalysts. Some Pd/PANI catalysts prepared via the 2-1-Step method showed the highest values of peak current density. However, in terms of activation energy, there was almost no improvement compared to the control. In terms of stability though, a better catalytic stability was achieved for a co-electrodeposited Pd/PANI catalyst via this method. Also, a novel type of PAN/IL core / PANI shell fibrous mats was developed and tested as support electrode for Pd-electrodeposited catalyst for EOR in alkaline medium. Those electrodes demonstrated the promoting effects of PANI in terms Eα, but did not achieve significant values of jP compared to Pd/GC. However, just the fact that those mats were the first independent fibrous polymer electrodes-support for Pd that were active for EOR in alkaline medium, combined with their excellent robustness and morphology, set a mark for further development upon this path. Consequently, a series of modified mats were prepared in order to investigate ways of performance improvement. Three distinct modification paths were followed. The first involved the added promoting metals distinguished in the first part (Ag and Bi), which showed encouraging results in terms of activation energy and current density up to half of that of plain Pd/GC. The second-modification-path samples consisted of phytic acid (PA)-doped PANI shell instead of HCl-doped PANI, deposited with Pd. The aim of that was to shield PANI from deprotonation in alkaline environment and enhance the catalytic performance in the long run, which was achieved in both terms of Eα and jP. Finally, the third modification path involved co-polymerisation of Pd and ANI for fibre shell preparation in one step, in a strategy similar to that followed for “1-Step” coelectrodeposided samples. Those samples were characterised by fine catalyst dispersion and showed significant promoting enhancement in terms of Eα.
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