Atomic layer deposition of Manganese oxide on Carbon substrate for electrocatalytic water splitting
Stability is one of the key issues that determine the applicability of catalysts in industrial application. In this thesis, a composite heterogeneous catalyst was synthesized and its stability was scrutinized in electrocatalytic water splitting conditions. The conventional preparation of drop coating the electrocatalyst on glassy carbon was interchanged with Atomic layer deposition (ALD) to incorporate covalent interaction of the electrocatalyst with the carbon substrate. Manganese oxide was chosen for the Atomic layer deposition process to constitute the composite electrocatalyst. Prior to ALD process, the carbon substrates were functionalized where the functional groups act as the anchoring group for the ALD precursor. The main framework of the thesis was perceived to unify the concepts of carbon functionalization and ALD to improve the stability of the whole electrocatalyst system. Initially, the functionalization of glassy carbon substrate was explored by means of atmospheric pressure plasma treatment. A plasma admixture of Oxygen with Helium in comparison pure Helium was found to be beneficial for producing oxygen functional group on the glassy carbon surface and studied in detail afterwards. From the results, it could be comprehended that plasma induced functionalization is modulated by an etching process which dominates with longer duration of plasma treatment. However, the plasma modified carbon appeared to be unstable and experience further changes under the electrocatalytic condition of oxygen evolution reaction (OER). Considering such outcome, the alteration of pristine glassy carbon under the electrocatalytic condition was investigated. The experimental work included the influence of different electrochemical parameters (pH and potential) on the structural and chemical properties of glassy carbon. The results indicated that acidic pH is favorable than alkaline pH for producing oxygen functional group on the glassy carbon surface. Moreover, the study revealed that high oxidation potential such as 1.8 VRHE and above is capable of affecting the structure of glassy carbon in acidic medium. For analyzing the atomic layer growth of Manganese oxide, Silicon substrate was considered first. Although the initial film thickness analysis demonstrated an ideal self-saturated ALD growth of Manganese oxide on Silicon substrate, but later the results from the dissolution method revealed the co-existence of physisorbed Mn (less than 10%) with the ALD deposited MnOx. In case of carbon substrate, the ALD deposition of MnOx displayed no effective chemical interaction with functional group of carbon substrate. This was supported by multiple analyses such as TG-MS, TEM and ICP-OES. Under these circumstances, an additional step of post deposition annealing was implemented to re-establish the covalent link between the ALD deposited MnOx catalyst and carbon support. Acknowledging different functionalization and post deposition annealing techniques, multiple synthesis strategies were examined for electrochemical characterization. Among the considered strategies, it was found that the composite electrocatalyst synthesized by the synergistic method (A pre-functionalized carbon is further modified with a post deposition annealing after ALD deposition) performed better than the others. But in the stability measurement, the electrochemical performance gets degraded regardless of the synthesis routes of the electrocatalysts. Overall, the electrochemical results demonstrated that MnOx suffers from inevitable detachment from the carbon electrode surface during the electrochemical process. This was further confirmed by post analysis of the electrocatalysts. In conclusion, it can be stated that the change of synthesis strategy merely slows down the detachment process but is not capable of complete prevention. The lack of immobilization of MnOx on the carbon surface is mainly attributed to such deterioration. The complied research work of this thesis substantiates that a composite electrocatalyst consisting of MnOx and carbon is considerably distant from performing as a sustainable electrocatalyst for oxygen evolution reaction. The inability of the electrocatalyst however stems from the both carbon and ALD deposited MnOx as both of them are unstable in the harsh electrochemical condition of OER.
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