(584g) Hydrogen and Carbon Monoxide Production By Co-Electrolysis of H2O and CO2 Using Solid Oxide Cells

Hydrogen is currently considered as one of the most interesting energy carrier because of limited reserves of fossil fuels, the increasing price of oil and environmental regulations concerning the emission of green-house gases.However, conventional hydrogen production routes such as steam reforming, partial oxidation of heavy hydrocarbons and coal gasification requires the use of fossil fuels.Alternative processes for hydrogen production include reforming and pyrolysis using biomass, direct methanol reforming, fermentation of biomass and biological production. On the other hand, production of hydrogen and synthetic hydrocarbons from renewable energy sources is a very attractive solution to reduce both oil consumption and carbon dioxide emissions without the need for modifications of existing infrastructures.Renewable energy resources such as solar energy, wind power, hydropower or geothermal power have gained considerable attention in recent years due to their large availability and the possibility to use the excess energy for an efficient production of fuels by high temperature steam electrolysis (HTSE). The hydrogen, produced by HTSE, can be stored and directly used as fuel for combined heat and power generation (CHP) by power systems based on solid oxide fuel cells (SOFC). Reversible systems based on solid oxide cells can be integrated with renewable sources to produce gaseous fuel by using solid oxide electrolysis (SOE) that can be reused to convert the produced fuel into electricity when the energy source is not available. It is worth noticing that, among all methods to produce hydrogen, water electrolysis combined with renewable sources can be considered the cleanest route to produce electricity since no hydrocarbons are consumed and no CO₂ is released in the whole process. However, while hydrogen production by electrolysis has been under development in the last twenty years, only few studies on production of synthesis gas (H₂+CO) by simultaneous electrolysis of steam and carbon dioxide (coelectrolysis) has been reported in the last few years. Synthesis gas is the raw gas mixture for production of liquid hydrocarbon fuels via the Fischer Tropsch synthesis (FTS) and it is conventionally produced by coal gasification or steam reforming of natural gas. Nevertheless, these conventional methods for syngas production also release significant amount of CO₂ needs to be separated before feeding the mixture to the FTS reactor. Coelectrolysis is a very attractive technology for the production of syngas from renewable energy sources without consuming fossil fuels or releasing green-house gases. Conversion of CO₂ into precursors for FTS can be considered as a clean process suitable to sustain a closed carbon cycle. In addition, after removing H₂O and CO₂, the H₂/CO mixture obtained from coelectrolysis is free from gas impurities such as tar, aromatics and organic impurities which are, on the other hand, typical of syngas derived from biomasses.Coelectrolysis in SOC has only been demonstrated in few studies and there is not a general agreement whether CO is exclusively produced via the reverse water gas shift reaction and that no electrolysis of CO₂ occurs or if CO is produced both via the reverse water gas shift reaction and via electrolysis of CO₂.The total amount of Hydrogen downstream the cell depends on the steam conversion (which is proportional to the current applied to the system) but also on the H₂ percentage which is normally included in the feed gas in order to maintain  reducing conditions. The composition of the syngas which is produced by coelectrolysis and therefore the ratio between H₂ and CO downstream the system should be tailored depending on the structure of the liquid fuel to be produced by FTs. A H₂ to CO molar ratio between 2 and 3 is normally required to produce light hydrocarbons.The present work is focused on production of H₂ and H₂/CO-rich mixtures by electrolysis and coelectrolysis in SOC-based stacks. The stack consists of an assembly of 6 Ni-YSZ supported cells interfaced with proprietary SOFCONNEX™ gas diffusion layers and Crofer 22 APU metallic interconnectors. The latter are coated on the air side to reduce Cr evaporation.The aim of the study is to investigate the suitability of this stack to operate in electrolysis and coelectrolysis mode and also to run under reversible SOE/SOFC operation.Measurements were performed on a proprietary test bench designed to test stacks in the power range of 20-200W.The performances of the stack were initially investigated in SOFC-mode, performing current-potential polarization curves in the range 650°C - 750°C. After that, stack operation was reversed in SOE-mode and the stack was tested under 90%H₂O/10%H₂ and 50%H₂O/10%H₂ cathode atmosphere. In order to assess the suitability of the stack to be integrated with renewable energy sources, fast on/off load cycles as well as SOFC/SOE cycles were performed and a very good performance stability was observed.Co-electrolysis measurements were performed at 750°C and using gas mixtures containing different CO₂/(H₂+H₂O) ratios. The composition of the gases, downstream the stack and after H₂O condensation, was analysed by gas chromatography (GC) in order to investigate the influence of the operating parameters and fuel composition on the relative amount of H₂, CO and CO₂ produced in the stack. As expected, the amount of H₂ in the mixture linearly increase with the increase of current density. A generally similar trend was also observed for CO percentage, while CO₂ percentage rapidly decreases with the increase of the electrolysis current. The rate of CO production was observed to depend on the relative CO₂ amount in the gas mixture entering in the stack. Thermochemical calculations were also performed in order to correlate the results of the GC analysis to the composition of the cathode gas atmosphere.