Context

Emission of greenhouse gases, particularly CO₂, is widely considered to be a major cause of global warming and related detrimental changes in the environment.  This has led to growing interest in carbon capture and utilization (CCU), in which CO₂ is recycled into carbon-neutral liquid fuels. CCU processes are carbon-neutral so long as the energy required to drive them comes from renewable or nuclear sources.One possible route to CO₂ recovery is co-electrolysis of CO₂ and H₂O to make a mixture of CO and H₂, known as synthesis gas or syngas for short. Syngas can be transformed into a wide range of liquid fuels and polymers by Fischer-Tropsch synthesis.

Many technological hurdles remain before co-electrolysis can be commercially feasible.  These include insufficient current, current efficiency, and durability. These issues depend strongly on the nature of surface reactions at the electrode, yet little is known about their mechanisms, kinetics, and sensitivity to different electrodes. The use of high temperature increases reaction kinetics, though further improvements must be made.

Technical Aspects

The diagram below shows the processes in a solid oxide electrolysis cell (SOEC, left) and chemical reactions (right). In CO₂/H₂O co-electrolysis, CO₂ is converted to CO, and H₂O to H₂, at an electrocatalytic cathode such as gadolinium-doped ceria (GDC). These are charge transfer reactions whereby electrons are consumed and doubly-charged oxygen ions (O^2–) produced. The O^2– ions migrate through the solid oxygen electrolyte — a typical one being yttrium-stabilized zirconia (YSZ) — and recombine to form O₂ at a Pt anode for discharge to the atmosphere.

CO2 electrolysis	CO2 + 2 e–	—>	CO + O2–
H2O electrolysis	H2O + 2 e–	—>	H2 + O2–
RWGS	CO2 + H2	—>	CO + H2O
Anode	2 O2–	—>	O2 + 4 e–
Overall	CO2 + H2O	—>	CO + H2 + 1/2 O2

 

 

At the high temperatures of the SOEC, the reverse water gas shift reaction (RWGS) also occurs. The RWGS consumes H₂ by reaction with CO₂, to form CO and H₂O. The term “shift” refers to the conversion of H₂ to CO and can be used to advantage to adjust the H₂/CO ratio to optimize the Fischer-Tropsch reaction that occurs downstream of the SOEC. A key point in our research is to determine the relative extents of the co-electrolysis and RWGS reactions as a function of cathode material, temperature, pressure, and gas phase composition.

Approach

We study co-electrolysis reactions in a SOEC that operates at high temperature (700-900 °C). The diagram below illustrates the cell configuration used in our research. Reactions occur inside a closed-ended YSZ tube. A GDC cathode is painted onto the inside of the tube and a Pt anode on the outside. The YSZ tube is the electrolyte: doubly-charged oxygen anions (O^2–) migrate from the cathode to the anode. The CO₂/H₂O reactant gas enters the cell through the inner annulus and products exit through the outer annulus. The center tube provides wires for temperature measurement by a thermocouple (TC) and the electrical connection to the cathode. The outlet gas is measured by frequency response mass spectrometry (FRMS) to determine the amounts of CO₂, H₂O, CO, and H₂ and to establish the relative rates of co-electrolysis and the RWGS.