Carl M. Stoots

Syngas Production via High-Temperature Coelectrolysis of Steam and Carbon Dioxide

This paper presents results of recent experiments on simultaneous high-temperature electrolysis (coelectrolysis) of steam and carbon dioxide using solid-oxide electrolysis cells. Coelectrolysis is complicated by the fact that the reverse shift reaction occurs concurrently with the electrolytic reduction reactions. All reactions must be properly accounted for when evaluating results. Electrochemical performance of the button cells and stacks were evaluated over a range of temperatures, compositions, and flow rates. The apparatus used for these tests is heavily instrumented, with precision mass-flow controllers, on-line dewpoint and CO2 sensors, and numerous pressure and temperature measurement stations. It also includes a gas chromatograph for analyzing outlet gas compositions. Comparisons of measured compositions to predictions obtained from a chemical equilibrium coelectrolysis model are presented, along with corresponding polarization curves. Results indicate excellent agreement between predicted and measured outlet compositions. Cell area-specific resistance values were found to be similar for steam electrolysis and coelectrolysis. Coelectrolysis significantly increases the yield of syngas over the reverse water gas shift reaction equilibrium composition. The process appears to be a promising technique for large-scale syngas production.

Hydrogen Production Performance of a 10-Cell Planar Solid-Oxide Electrolysis Stack

An experimental study is under way to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production over a temperature range of 800 to 900oC. Results presented in this paper were obtained from a ten-cell planar electrolysis stack, with an active area of 64 cm2 per cell. The electrolysis cells are electrolytesupported, with scandia-stabilized zirconia electrolytes (~140 µm thick), nickel-cermet steam/hydrogen electrodes, and manganite air-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed over a range of steam inlet mole fractions (0.1 - 0.6), gas flow rates (1000 - 4000 sccm), and current densities (0 to 0.38 A/cm2). Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Hydrogen production rates up to 100 Normal liters per hour were demonstrated. Values of area-specific resistance and stack internal temperatures are presented as a function of current density. Stack performance is shown to be dependent on inlet steam flow rate.

Performance of planar high-temperature electrolysis stacks for hydrogen production from nuclear energy

An experimental program is under way to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production in a temperature range from 800 to 900°C. This temperature range is consistent with the planned coolant outlet temperature range of advanced nuclear reactors. Results were obtained from two multiple-cell planar electrolysis stacks with an active area of 64 cm2 per cell. The electrolysis cells are electrolyte-supported, with scandia-stabilized zirconia electrolytes (~140 μm thick), nickel-cermet steam/hydrogen electrodes, and manganite oxygen-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed in a range of steam inlet mole fractions (0.1 to 0.6), gas flow rates (1000 to 4000 standard cubic centimeters per minute), and current densities (0 to 0.38 A/cm2). Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Values of area-specific resistance and stack internal temperatures are presented as a function of current density. Initial stack-average area-specific resistance values <1.5 Ω·cm2 were observed. Hydrogen production rates in excess of 200 normal liters per hour (NL/h) were demonstrated. Internal stack temperature measurements revealed a net cooling effect for operating voltages between the open-cell potential and the thermal neutral voltage. These temperature measurements agreed very favorably with computational fluid dynamics predictions. A continuous long-duration test was run for 1000 h with a mean hydrogen production rate of 177 NL/h. Some performance degradation was noted during the long test. Stack performance is shown to be dependent on inlet steam flow rate.

Degradation Issues in Solid Oxide Cells During High Temperature Electrolysis

Idaho National Laboratory (INL) is performing high-temperature electrolysis (HTE) research to generate hydrogen using solid oxide electrolysis cells (SOECs). The project goals are to address the technical and degradation issues associated with the SOECs. This paper provides a summary of ongoing INL and INL-sponsored activities aimed at addressing SOEC degradation. These activities include stack testing, post-test examination, degradation modeling, and issues that need to be addressed in the future. Major degradation issues relating to solid oxide fuel cells (SOFC) are relatively better understood than those for SOECs. Some of the degradation mechanisms in SOFCs include contact problems between adjacent cell components, microstructural deterioration (coarsening) of the porous electrodes, and blocking of the reaction sites within the electrodes. Contact problems include delamination of an electrode from the electrolyte, growth of a poorly (electronically) conducting oxide layer between the metallic interconnect plates and the electrodes, and lack of contact between the interconnect and the electrode. INL’s test results on HTE using solid oxide cells do not provide clear evidence as to whether different events lead to similar or drastically different electrochemical degradation mechanisms. Post-test examination of the SOECs showed that the hydrogen electrode and interconnect get partially oxidized and become nonconductive. This is most likely caused by the hydrogen stream composition and flow rate during cooldown. The oxygen electrode side of the stacks seemed to be responsible for the observed degradation because of large areas of electrode delamination. Based on the oxygen electrode appearance, the degradation of these stacks was largely controlled by the oxygen electrode delamination rate. Virkar et al. [19–22] have developed a SOEC model based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic nonequilibrium. This model is under continued development. It shows that electronic conduction through the electrolyte, however small, must be taken into account for determining local oxygen chemical potential within the electrolyte. The chemical potential within the electrolyte may lie out of bounds in relation to values at the electrodes in the electrolyzer mode. Under certain conditions, high pressures can develop in the electrolyte just under the oxygen electrode (anode)/electrolyte interface, leading to electrode delamination. This theory is being further refined and tested by introducing some electronic conduction in the electrolyte.Copyright

The High-Temperature Electrolysis Integrated Laboratory-Scale Experiment

Abstract The High-Temperature Electrolysis Integrated Laboratory-Scale experiment was designed at the Idaho National Laboratory (INL) and Ceramatec during 2006 and early 2007 and constructed in the spring and summer of 2007. A “half-module,” two stacks of 60 cells each, was tested at Ceramatec for 2040 h in June–September 2006 and a full module, four stacks of 60 cells each, was completed in March 2007. Initial shakedown testing of the INL Integrated Laboratory-Scale (ILS) experimental facility commenced on August 22, 2007. Heatup of the first ILS module started at 4:10 PM on September 24, 2007, and ran for 420 h. The test average H2 production rate was ~1.3 N.m3/h (Normal cubic meters per hour, where Normal conditions are 273 K and 1 atm) (0.116 kg H2/h), with a peak measured H2 production rate of over 2 N.m3/h (0.179 kg H2/h). Significant module performance degradation was observed over the first 250 h, after which no further degradation was noted for the remainder of the test. Once all test objectives had been successfully met, the test was terminated in a controlled fashion.

Computational Fluid Dynamics Model of a Planar Solid-Oxide Electrolysis Cell for Hydrogen Production from Nuclear Energy

A three-dimensional computational fluid dynamics (CFD) model has been created to model high-temperature steam electrolysis in a planar solid-oxide electrolysis cell (SOEC). The model represents a single cell as it would exist in an electrolysis stack. Details of the model geometry are specific to a stack tested at the Idaho National Laboratory (INL). Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density, and hydrogen production in a range of stack operating conditions. Mean model results are shown to compare favorably with experimental results obtained from an actual ten-cell stack tested at INL.

Thermal and Electrochemical Three Dimensional CFD Model of a Planar Solid Oxide Electrolysis Cell

A three-dimensional computational fluid dynamics (CFD) model has been created to model high-temperature steam electrolysis in a planar solid oxide electrolysis cell (SOEC). The model represents a single cell, as it would exist in an electrolysis stack. Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc. and tested at the Idaho National Laboratory. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Mean model results are shown to compare favorably with experimental results obtained from an actual ten-cell stack tested at INL.Copyright

Integrated Operation of the INL HYTEST System and High-Temperature Steam Electrolysis for Synthetic Natural Gas Production

Abstract The primary feedstock for synthetic fuel production is syngas, a mixture of carbon monoxide (CO) and hydrogen. Current hydrogen production technologies rely upon fossil fuels and produce significant quantities of greenhouse gases as a by-product. This is not a sustainable means of satisfying future hydrogen demands given the current projections for conventional world oil production and future targets for carbon emissions. For the past 6 yr, the Idaho National Laboratory (INL) has been investigating the use of high-temperature steam electrolysis (HTSE) to produce the hydrogen feedstock required for synthetic fuel production. HTSE water-splitting technology, combined with non-carbon-emitting energy sources, can provide a sustainable, environmentally friendly means of large-scale hydrogen production. Additionally, laboratory facilities are being developed at the INL for testing hybrid energy systems composed of several tightly coupled chemical processes (HYTEST program). The first such test involved the coupling of HTSE, a CO2 separation membrane, the reverse-shift reaction, and the methanation reaction to demonstrate synthetic natural gas production from a feedstock of water and either CO or a simulated flue gas containing CO2. This paper will introduce the initial HTSE and HYTEST testing facilities, overall coupling of the technologies, testing results, and future plans.

Performance Characterization of Solid-Oxide Electrolysis Cells for Hydrogen Production

An experimental study has been completed to assess the hydrogen-production performance of single solid-oxide electrolysis cells operating over a temperature range of 800 to 900°C. The experiments were performed over a range of steam inlet partial pressures (2.3 – 12.2 kPa), carrier gas flow rates (50–200 sccm), and current densities (−0.75 to 0.25 A/cm2 ) using single electrolyte-supported button cells of scandia-stabilized zirconia. Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Values of area-specific resistance and hydrogen production rate are presented as a function of current density. Cell performance is shown to be continuous from the fuel-cell mode to the electrolysis mode of operation. The effects of steam starvation and thermal cycling on cell performance parameters are discussed. Laboratory capabilities are currently being expanded to allow for testing and characterization of multiple-cell electrolysis stacks. Some fundamental differences between the fuel-cell and electrolysis modes of operation have been summarized.Copyright

The Concept and Experimental Investigation of CO2 and Steam Co-electrolysis for Resource Utilization in Space Exploration

CO 2 acquisition and utilization technologies will have a vital role in designing sustainable and affordable life support and in situ fuel production architectures for human and robotic exploration of the Moon and Mars. For long-term human exploration to be practical, reliable technologies have to be implemented to capture the metabolic CO 2 from the cabin air and chemically reduce it to recover oxygen. Technologies that enable the in situ capture and conversion of atmospheric CO 2 to fuel are essential for a viable human mission to Mars. This paper describes the concept and mathematical analysis of a closed-loop life support system based on combined electrolysis of CO 2 and steam (co-electrolysis). Products of the coelectrolysis process include oxygen and syngas (CO and H 2 ) that are suitable for life support and synthetic fuel production, respectively. The model was developed based on the performance of a co-electrolysis system developed at Idaho National Laboratory (INL). Individual and combined process models of the co-electrolysis and Sabatier, Bosch, Boudouard, and hydrogenation reactions are discussed and their performance analyses in terms of oxygen production and CO 2 utilization are presented.