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Title: Electrochemical treatment of stainless steel for application in polymer electrolyte fuel cell bipolar plates
Author: Gabreab, E. M.
ISNI:       0000 0004 5359 5036
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2015
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Polymer electrolyte fuel cell (PEFC) performance is typically considered to have reached the targets required for most applications; however, maintaining this performance throughout the operational lifetime of the fuel cell, or restricting its degradation to an acceptable level, is still a challenge. One of the most important yet understudied areas identified as a source of fuel cell performance degradation is the bipolar plate (BPP). The BPP typically accounts for more than 80% of the weight of a stack and almost all of the volume. Thus BPP weight and size reduction, while maintaining the desired properties is crucial in increasing the performance, making fuel cells more commercially viable. Stainless steels BPPs are a popular choice, with the advantage of low cost, high strength and good corrosion resistance as well as ease of incorporating a flow field via stamping or embossing. The main criterion for material suitability with regards to stainless steel in particular, is large voltage drops at the interface; a consequence of corrosion protective insulating surface oxides. Surface modification/treatment techniques of inexpensive and widely available metals are capable of improving the performance of metals in fuel cell environments and are an alternative to the use of costly precious metal coatings. Electrochemical surface treatments involve modification of the oxide layer by enriching its chromium content, leading to higher corrosion resistance and lower interfacial contact resistance (ICR). This project explores the use of electrochemical surface treatment to improve the corrosion and contact resistance of 316 stainless steel (316 SS). The process is an anodic treatment, whereby the material is polarised positive of the transpassive region, under process conditions of H2SO4-to-glycerol ratio at varying treatment times. The surface treatment demonstrates an increase in the corrosion resistance, enhanced durability of the passive film and significantly reduces ICR (interfacial contact resistance), to values well below the DoE 2017 targets at a compression of 200 psi (138 N cm-2). While ex-situ near-surface characterisation suggests the improved properties of the electrochemically treated 316 SS are primarily as a consequence of Cr enrichment at the near-surface of the material. AFM analysis of surface roughness and morphology demonstrated increased Ra values of treated samples; which although has been previously associated with improved ICR, was not significant enough in this study to affect the ICR. The addition of glycerol as a viscosity-enhancing agent did not demonstrate a significant improvement of the process, while shorter treatment times demonstrated greater process current efficiencies, which consequently impact the economic costs. The process demonstrates that the use of anodized treated 316 SS to improve low ICR and satisfactory corrosion resistance of low cost metallic bipolar plate PEFCs is feasible.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available