Nikiforov, Aleksey

Vandelektrolyse baseret på polymer elektrolyt membraner (PEM) er en lovende teknologi, som tillader at produktion af brint. Da brint muligvis kan blive en fremtidig energibærer i en "Brint cyklus". Elektrolyse af vanddamp ved forhøjede temperaturer har flere fordele i forhold til en lavtemperatur elektrolyse proces. Dog betyder den forøgede temperatur samtidig et øget krav til konstruktionsmaterialerne i cellen, f.eks. kommer der et øget krav til dimensionsstabiliteten og den kemiske stabilitet under de stærkt korrosive forhold som ændes i systemet. Materialer der benyttes i lavtemperatur elektrolyse kan således ikke nødvendigvis anvendes i systemer, som virker ved temperaturer over 100 °C og nye materialer skal derfor undersøges. I afhandlingens kapitel 1 gives en generel introduktion til emnet, medens Kapitel 2 gennemgår teoretiske aspekter vedrørende elektrokatalyse. Desuden beskrives de teknikker, der er blevet benyttet til karakterisering af katalysatorerne og kontruktionsmaterialerne. I kapitel 3 gennemgås opbygningen af PEM elektrolysatorer, samt litteraturen vedrørende de forskellige materialer der indgår, dvs. membraner, katalysatorer og konstruktionsmaterialer. I kapitel 4 fremlæggers resultaterne af korrosionstests af forskellige typer kommercielle rustfrie stålmaterialer, nikkel-baserede legeringer, samt titanium og tantal for anvendelse som mulige bipolare plader og konstruktionsmateriale for højtemperatur PEM elektrolyseceller (HTPEMEC). Korrosionsresistensen er blevet målt under betingelser som simulerer en højtemperatur PEM dampelektrolysecelle. Steady state voltammetri er blevet anvendt i kombination med skanning elektron mikroskopi (SEM), og energi dispersiv røntgen spektroskopi (EDX) er benyttet for at evaluere stabiliteten af ovenfor nævnte materialer. Det blev fundet at blandt legeringerne var de rustfri ståltyper mindst modstandsdygtige overfor korrosion under stærk anodisk polarisering. Af nikkel-legeringerne udviste Inconel®625 størst potentiale som et muligt materiale i højtemperatur elektrolyse celler, da den var den mest korrosionsbestandig blandt de testede stålmaterialer og nikkellegeringer. Tantal-belagt rustfrit stål viste fremragende korrosionsbestandighed ved de undersøgte betingelser, derimod var der ringe passivering af titanium, som da også havde den højeste korrosionshastighed blandt de undersøgte materialer ved 120 °C. I dag er der stor interesse for nye katalysatormaterialer, som kan gøre det mulig at undgå ædelmetaller, men der mangler stadig et afgørende gennembrud. I dette arbejde er der benyttet en anden strategi, nemlig formindskelse af den anvendte katalysatormængde på anodesiden, ved brug af et bæremateriale. Som mulig anodekatalysatorbærer er her undersøgt et SiC-Si-materiale. Udførelsen og præsentationen af dette ændes i kapitel 5. Det katalytisk aktive IrO2 er blevet påført på SiC-Si ved "Adams fusion synthesis", og karakteriseret med forskellige teknikker. XRD og BET målinger viste ind ydelse af bærematerialet på egenskaberne af de dannede IrO2 partikler. De fremstillede katalysatorer blev karakteriserede gennem cyklisk voltammetri og andre elektrokemiske metoder ved 25, 80, 120 og 150 °C. I overensstemmelse med den observerede variation i partikelstrrelsen, vil en 80 % loading af IrO2 på bærematerialet give en maximal forbedring af aktivitet af katalysatoren. Målinger af elektrisk ledningsevne af pulverene viste en indflydelse af bærematerialet på pakningen af IrO2 partiklerne. I dette tilfælde kunne det konkluderes, at et materiale med dårlig elektrisk ledningsevne godt kunne bidrage til en forøgelse af udnyttelsen af en elektrokatalysator. Resultaterne peger således på fremtidige perspektiver ved brug af lavt ledende keramiker som bærematerialer, og for videre forskning inden for dette emne. En væsentlig del af arbejdet har været helliget udvikling af en metode til elektrokemisk karakterisering af katalysatorer ved forhøjede temperaturer. Kapitel 6 består af afsluttende bemærkninger og forslag til fremtidigt arbejde. Kapitel 7 udgøres af to artikler der er publicerede i løbet af projekt perioden.Proton exchange membrane (PEM) water electrolysis presents an attractive technology allowing to produce hydrogen for further use as a renewable energy source in the "Hydrogen cycle". Electrolysis of water steam at elevated temperatures has several advantages over the low temperature process. However, at the same time it involves increased demands to dimensional and chemical stability of components against corrosion environment. Therefore, materials utilized in low temperature PEM electrolyzers cannot be used in systems operating above 100 °C and new candidates should be tested. The materials in question are those for bipolar plates, gas diusion layers (GDLs), catalysts and catalyst supports. This work is focused on developing bipolar plate, GDL and catalyst support materials for the anode compartment of PEM electrolyzers, operating at elevated temperatures. The thesis starts with Chapter 1, which gives an introduction into the subject and Chapter 2 subsequently presents the theoretical background of the topic and describes techniques used to characterize catalysts and construction materials. Chapter 3 presents general principles and overview of materials used for PEM water electrolysis. Chapter 4 reports results of testing dierent types of commercially available stainless steels, Ni-based alloys as well as titanium and tantalum as possible metallic bipolar plates and construction materials for HTPEMEC. The corrosion resistance was measured under simulated conditions of high temperature PEM steam electrolyzer. Steady-state voltammetry was used in combination with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to evaluate the stability of the mentioned materials. It was found that stainless steels were the least resistant to corrosion under strong anodic polarization. On the contrary, Ni-based alloys showed higher corrosion resistance in the simulated PEM electrolyzer medium. In particular, Inconel®625 was the most promising among the tested corrosionresistant alloys for the anodic compartment of high-temperature steam electrolyzer. The tantalum coated stainless steel showed outstanding resistance to corrosion in selected media, while passivation of titanium was weak, and the highest rate of corrosion among all tested materials was observed for titanium at 120 °C. Today, there is a high interest in the eld towards investigation of new catalyst materials, which can make it possible to avoid noble metals. However, this work suggests a different approach of decreasing the loading of the active component at the oxygen electrode by using a catalyst support. In order to achieve that, investigation of a novel SiC-Si compound was performed and is presented in Chapter 5. The active iridium oxide was deposited on the SiC-Si in-situ by the Adams fusion synthesis and characterized by different techniques. XRD and nitrogen adsorption experiments showed an influence of the support on surface properties of the IrO2 particles, affecting IrO2 particle size. The prepared catalysts were electrochemically characterized by cyclic voltammetry experiments at 25, 80, 120 and 150 °C. In accordance with the observed variation in particle size, a support loading of up to 80% improved the activity of the catalyst. Powder conductivity measurements were also performed, which showed the in uence of the support on the packing of IrO2 particles. Investigation showed that even a support material with poor electrical conductivity contributes beneficially to the electrocatalyst active surface area, increasing its utilization. Results demonstrated potential perspectives of using low conductive ceramics as a catalyst support which means that further research in this eld is of high interest. An essential part of the study was devoted to the development of a method of elevated temperature catalyst electrochemical characterization, which was implemented for the evaluation of the performance of the synthesised catalysts. Chapter 6 consists of concluding remarks and proposals for the future research. Chapter 7 contains two articles, which were published during the project period

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New Construction and Catalyst Support Materials for Water Electrolysis at Elevated Temperatures

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New Construction and Catalyst Support Materials for Water Electrolysis at Elevated Temperatures

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