Kjartansdóttir, Cecilía Kristín

Andelen af vedvarende energi på verdensplan forventes at stige med 38% i perioden mellem 2011 og 2035. Derfor vil andelen af energi fra fluktuerende energikilder, såsom vind, sol og vandkraft eskalere, og der vil blive behov for nye alternativer til energi konvertering, energilagring og load management. Produktion af hydrogen via elektrolyse fra overskydende elproduktion fra vedvarende energi giver mulighed for øget produktionskapacitet og load management uden drivhusgasser.Hydrogen er en værdifuld energibærer, der på mange måder er i stand til at bidrage til forskellige former for energikilder såsom; produktion af el via brændselsceller, brændstof til forbrændingsmotorer eller gasturbiner, eller som råstof til fremstilling af syntetiske brændstoffer via Sabatier eller Fischer–Tropsch processen. I nogle situationer kan det accepteres blot at injiceres hydrogen i den eksisterende naturgas infrastruktur. Alkalisk elektrolyse (AWE) er den nuværende standard (state of the art) for store industrielle vand elektrolyse -systemer. Et af de vigtigste kriterier for industriel AWE er effektive og holdbare elektroder. Formålet med nærværende Ph.d.-projekt var således at udvikle elektrode materialer til brintproduktion med henblik på at forbedre effektiviteten og holdbarheden, samt mindske omkostningerne, der er forbundet med industriel AWE. I forlængelse heraf blev der udviklet nye test setups for evaluering af effektivitet og holdbarhedstests. I forbindelse med nærværende ph.d.-projekt blev nye brint elektroder med stor elektrokatalytisk aktiv udviklet. Elektroderne blev fremstillet ved (PVD) deponering af 20 μm aluminium på et nikkel substrat efterfulgt af termokemisk diffusion og selektiv fjernelse af aluminium via ætsning. Det nye elektrodemateriale var særdeles porøst på såvel mikro som nanoskala, og opnåede en aktiv overflade op til 2300 gange større end en poleret nikkel overflade.De elektrokatalytiske overflader udviste unik adhæsion til substratet, som er afgørende for en efterfølgende industriel anvendelse. High Resolution Scanning Electron Microscope (HRSEM) billeder bekræftede en robust skelet-struktur med porer ned til nogle få nanometer. Halv-celle potentiodynamisk recordede polarisationskurver ved 25°C og 200 mA/cm2 viste at elektroderne havde 385 mV lavere brintoverspændings potentiale og 50 mV lavere ilt overspændings potentiale sammenlignet med poleret nikkel. Elektrodernes stabilitets test blev udført i en industriel bipolar, ikke zero-gap AWE stak hvor elektroderne blev anvendt både som anode og katode. Stakken var forsynet med 30 vægt % KOH elektrolyt og havde en maksimal drifts temperatur på 80°C ved et gas tryk på 22 bar. Varigheden af testen var omkring 2 år, hvor stakken var i drift over 9000 timer. Sammenligning af driftsdata registreret fra første måneds drift til data registreret efter 9000 timer viser ingen signifikant deaktivering af elektrodernes katalytiske aktivitet. Stakkens effektivitet blev målt til at være 81% (HHV) efter perioden test på 200 mA/cm2 ved 80°C Det bemærkes, at elektrolyse test stakken var i udviklingsfasen, og nogen effektivitets tab var at forvente i stakken grundet fejl-strømme.Elektrodetest målinger blev ligeledes udført i en ikke zero-gap, enkelt celle elektrolyse setup med 50 vægtprocent % KOH elektrolyt, ved 120°C og 200 mA/cm2 i 1600 timer. De udviklede elektroder blev anvendt både som katode og anode. Cellens effektivitet blev målt til at være 88% (HHV) i løbet af de første to timers drift. Efter omkring 100 timer var effektiviteten faldet til 84% og effektiviteten var efterfølgende konstant i den resterende del af testperioden. Årsagen til reduktionen i effektivitet tillægges nedbrydning af elektroderne, forårsaget af gaserosion eller korrosion, sammen med en mulig dannelse af nikkel hydrider, hvad angår katoden. Materialet nedbrydningen var mere omfattende ved anoden i end ved katoden. Single celle målinger indikerede ingen deaktivering af elektroder under pauser - uden elektrolyse.Mikrostruktur undersøgelser af PVD Al-Ni diffusion ved 610°C indikerer at diffusion mekanismen er domineret af en korn grænse diffusion af nikkel-rige faser i den PVD deponerede aluminium fase. Den første intermetalliske fase der dannes er fastslået til at være AlNi3. Denne optræder i de søjleformede korngrænser af aluminium strukturen efter kun nogle minutters varmebehandling. Det er særdeles overraskende og uventet at konstatere at nikkel og nikkelholdige faser er de mest mobile faser under diffusionsglødningen af den PVD deponerede aluminium på et nikkel basismateriale. Iagttagelserne er i kontrast til hvad der fremgår af den videnskabelige litteratur.Udover at der er AlNi3i korngrænserne af aluminium strukturen, observeres der Al3Ni og Al3Ni2 i diffusionslaget efter op til 30 minutters varmebehandling. Efter 2 timers varmebehandling kan dannelsen af en meget porøs γ - Al2O3 fase ved den øverste overflade iagttages. Kun Al3Ni2 og tynde lag af AlNi og AlNi3 observeres i diffusionszone, mendens AlNi3 fase ikke kan identificeres. Varmebehandlinger længere end 2 timer resulterer i langsom opbygning af AlNi, AlNi3 og γ- Al2O3 fase samt kornvækst i Al2Ni3 fasen. Den noget særprægede diffusionsmekanisme kan vise sig at være årsagen til de gode egenskaber af den udviklede PVD Al/Ni elektrode. Elektroder fremstillet med korte diffusions tider, 10-30 minutter, er konstateret at være lettere at ætse selektivt i alkalisk miljø med et rest aluminium indhold på 4-5 vægt% i den udætsede skeletal struktur. For elektroder der varmebehandles i 24 timer, kan der observeres op til 15 vægt % rest aluminium i det resterende nikkel struktur. Selektiv udætsning af PVD Al-Ni struktur efter kort tids diffusion, resulterer i dannelse af revner vinkelret på nikkel substratet. Revnerne formindsker den mekaniske styrke af belægninger. Elektroder varmebehandlet i korte tid viser sig ikke at være stabile under OER. Selektiv ætsning af aluminium fra elektroder varmebehandlet i 24 timer viser resulterer derimod i tætte, krakfrie og mere mekanisk stabile strukturer. Elektrokemisk karakterisering af overfladearealets for de nye udviklede elektroder viser, at den elektrokatalytiske aktivitet stiger proportional til det porøse lags tykkere, op til 20 μm.Med henblik på at reducere produktionsomkostningerne i forbindelse med fremstilling af elektroder, blev fire mulige procesteknologier, foruden den allerede undersøgte afprøvet. Disse var: (1) varmaluminering af nikkel, efterfulgt af termo- kemisk diffusion, (2) direkte termo- kemisk diffusion af aluminium folie med nikkel plader, (3) elektrolytisk deponering af aluminium belægninger fra ioniske væsker på et nikkelsubstrat efterfulgt af termo- kemisk diffusion og slutteligt (4) PVD belægninger af aluminium på en elektrolytisk deponeret sulfamate nikkeloverflade efterfulgt af termo- kemisk diffusion. På grund af den høje affinitet af oxygen til aluminium og dermed også smeltet aluminiums høje affinitet til vand og den efterfølgende høje opløsning af hydrogen i smeltet aluminium kan fremstilling af AlNi legeringer for elektroder betragtes som en udfordring. Det blev hurtigt klart at termokemisk diffusion, henholdsvis mellem en aluminium folie og nikkel i en argon atmosfære og en PVD deponeret aluminium belægning på en elektropletteret sulfamate nikkel var blandt de mest lovende procesteknologier til fremstilling af skeletal nikkel.The share of renewable energy worldwide is expected to increase by 38% in the period between 2011 and 2035. Consequently, the share of energy from fluctuating energy sources, such as wind, sun and hydro, will escalate and new alternatives for energy conversion, energy storage and load management will be needed. Producing hydrogen via water electrolysis using surplus, low cost, power from renewables offers the possibility of increased production capacity and load management with no greenhouse emissions. Hydrogen is a valuable energy carrier, which is able to contribute to various forms of energy, such as, production of electricity via fuel cells, fuel for internal combustion engines or gas turbines, or as a raw material for the production of synthetic fuels via Sabatier or Fischer - Tropsch process. In some situations it may be suitable to simply inject hydrogen into the existing natural gas based infrastructure. Alkaline water electrolysis (AWE) is the current standard (stat of the art) for industrial large-scale water electrolysis systems. One of the main criteria for industrial AWE is efficient and durable electrodes. The aim of the present PhD study was to develop electrode materials for hydrogen production in order to improve the efficiency and durability, and decrease the costs associated with industrial AWE. The primary effort was reserved to the hydrogen electrodes. Additionally, a new test setup for efficiency and durability measurements was to be designed and constructed. During the present PhD study, new hydrogen electrodes with large electrocatalytic active surface area were developed. The electrodes were produced by physical vapour deposition (PVD) of about 20 μm of aluminium onto a nickel substrate followed by thermo-chemical diffusion and selective aluminium leaching. The obtained electrode surfaces were found to be highly porous; both at micro- and nano-scale, and surface roughness factors of up to 2300 times that of polished nickel were measured. The electrocatalytic surfaces were characterized to have unique adhesion to the substrate, which is a critical criterion for industrial applications. High Resolution Scanning Electron Microscope (HR-SEM) images reveal highly skeletal structure with pores down to a few nanometres. Half-cell potentiodynamic polarisation curves, recorded at 25°C, 200 mA/cm2, show the electrodes to have 385 mV lower hydrogen overpotential and 50 mV lower oxygen overpotential, when compared to polished nickel. Durability test was carried out in an industrial sized bipolar, non-zero gap AWE stack where the developed electrodes were applied both as anode and cathode. The stack was operated with 30 wt.% KOH electrolyte at a maximum temperature of 80°C and a pressure of 22 bar. The duration of the test was about 2 years where the stack was operated for approximately 9000 hours. Comparison of data captured from the first month of operation to data captured after the durability testing period indicates no significant deactivation/deterioration in performance of the electrodes during the whole operation period. The stack efficiency was measured to be 81% (HHV), after the test period, at 200 mA/cm2 and 80 °C. It is noted that the electrolysis test stack was in the development stage and some losses were expected in the stack itself due to stray currents. Durability measurements were also carried out in a non-zero gap, single cell electrolysis setup with 50 wt.% KOH electrolyte, at 120°C and 200 mA/cm2 for 1600 hours. The developed electrodes were applied both as cathode and anode. The cell efficiency was measured to be 88 % (HHV) during the first two hours of operation. After about 100 hours, the efficiency had decreased to 84% and was thereafter constant throughout the remaining test period. The reason for the efficiency drop is proposed to be degradation of the electrodes, caused by gas-erosion corrosion, together with formation of nickel hydrides in electrode metal lattice. The material degradation was considerably more severe at the anode compared to the cathode. The durability single-cell measurements indicate no deactivation of electrodes after shut-downs. Microstructure investigations on the PVD Al-Ni diffusion couples at 610°C indicate the diffusion mechanism to be dominated by grain boundary diffusion of nickel-rich phases into the aluminium-rich PVD structure. The first intermetallic phase formed is determined to be AlNi3. The phase is observed as small particles in the columnar grain boundaries of the aluminium structure, after only a few minutes of heat treatment. Due to the high mobility of aluminium at the annealing temperature, finding nickel and nickel-rich species to be the most mobile during the heat treatment is highly unexpected and is in contrast with what is stated in the current scientific literature. Together with the AlNi3 particles observed in the aluminium residue, only Al3Ni and Al3Ni2 are present in the diffusion layer for up to 30 minutes of heat treatment. Over 30 minutes of heat treatment results in depletion of the aluminium and formation of highly porous γ-Al2O3 phase at the top surface. After 2 hours, of heat treatment only Al3Ni2 and thin layers of AlNi and AlNi3 are observed in the diffusion zone, with no traces of the AlNi3 phase. Heat treatments performed for longer than 2 hour result in gradual thickening of the AlNi, AlNi3 and γ-Al2O3 diffusion layers and grain growth in the Al2Ni3 phase. The diffusion mechanism can be the key to good properties of the developed PVD Al/Ni electrodes. Electrodes produced with shorter time of diffusion, 10-30 minutes, are found to be more prone to alkaline aluminium leaching and only 4-5 wt.% of aluminium residue is found in the leached skeletal nickel structure. For the electrodes heat treated for 24 hours, up to 15 wt.% aluminium residue is observed in the skeletal nickel structure. However, leaching of the PVD Al-Ni structure, after short periods of diffusion, results in formation of cracks perpendicular to the Ni substrate. The cracks reduce/affect the mechanical strength of the treated coatings. Electrodes heat treated for short times are found not to be stable under OER. However, selective aluminium leaching of electrodes heat treated for 24 hours results in dense, crack free and more mechanically stable/stronger structure. Electrochemical characterisation on the effect of surface area of the developed electrodes, indicate that the electrocatalytic activity increases in proportion to the porous layer up to the whole 20 μm investigated. With the aim of reducing the production costs associated with the electrode manufacturing, four process techniques and combinations of these were screened. These were; (1) hot dip aluminising of nickel followed by thermo-chemical diffusion, (2) direct thermo-chemical diffusion of aluminium and nickel sheets, (3) aluminium ionic liquid electroplating on a nickel plate followed by thermo-chemical diffusion and (4) physical vapour deposition of aluminium onto electroplated sulfamate nickel substrate followed by thermo-chemical diffusion. Due to the high affinity of aluminium towards hydrogen and oxygen, producing oxide free AlNi alloy coatings in an inexpensive and simple manner was found to be challenging. Only the direct diffusion between aluminium and nickel sheets in argon atmosphere and the PVD Al onto electroplated sulfamate nickel were found to give promising coatings

Online Research Database In Technology

Development of Hydrogen Electrodes for Alkaline Water Electrolysis

http://orbit.dtu.dk/ws/files/100552811/2014_07_08_Thesis_combined.pdf

Development of Hydrogen Electrodes for Alkaline Water Electrolysis

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