Redox Stable Cathodes for CO2-Steam Co-Electrolysis Process in Solid Oxide Electrolyzers for Syn-Gas Generation
Authors: Kulkarni Aniruddha Pramod , Kaur Gurpreet , Fini Daniel , Giddey Sarbjit , Hos Tomy , Herskowitz Moti 
Published: February 2020

Penetration of renewable energy (RE) in the consumable energy mix is expected to grow substantially in near future from its current share of 6.6 % of the total energy consumed globally.[i] In parallel, an allied energy conversion and storage technologies are also being developed and deployed globally. The batteries can provide short to medium term energy storage solution, however long term energy storage as well as transport of RE from the regions with high RE generation capacity to the regions with low RE capacity need innovative solutions. The conversion of RE into storable and transportable fuels is one of such approaches. The technologies such as electrolytic hydrogen production have already entered commercial space, however large scale storage and transport of hydrogen is energy intensive with about $15/kg cost just for the storage of compressed H2 at 700 bar.[ii] Further, the infrastructure for hydrogen transport is almost non-existent. Hence, the alternative electrochemical technologies which can produce transportable liquid fuels like blended hydrocarbons and methanol are being explored. These fuels are ready to use in the existing technologies like IC engines and diesel generators. As CO2 from industrial sources is used in the synthesis of these fuels, there is no net addition of CO2 in the cycle. It is worth noting that even with increased penetration of RE, there would be numerous industrial processes which generate vast amount of CO2 amounting to about 21 % of total CO2 emissions.[iii]
The solid oxide electrolyser (SOE) is an electrochemical membrane reactor based upon oxygen ion conducting ceramic electrolyte which can efficiently produce syn-gas (mixture of H2 and CO) as a feedstock for synthesis of liquid fuels. SOEs can be thermally integrated to liquid fuel synthesis reactors which boosts the overall efficiency of the system with heat input from an exothermic fuel synthesis reaction used in the SOE. The total synthesis and transport efficiencies of such integrated system can be above 75 % with cost in the range of US$ 0.8 to 0.9 per litre of the fuel using well established existing fuel transportation infrastructure.[iv]
Traditionally, yttria stabilised zirconia (YSZ) electrolytes are used in SOE with Ni-YSZ cermet electrodes for splitting of CO2 and H2O on cathode side (negative electrode) and a perovskite La0.80Sr0.20MnO3 (LSM) is used to facilitate oxygen evolution reaction on the anode side (positive electrode). While Ni-YSZ electrodes have demonstrated excellent compatibility with YSZ electrolytes and sufficient catalytic activity as a fuel cell electrode, its application in solid oxide electrolyser typically requires additional external supply of hydrogen as a reducing gas along with steam and CO2 mixtures. Further, redox and thermal stability of Ni-YSZ cathodes could be of a concern considering intermittent nature of renewable energy sources. Thus, alternative electrodes such as ceramic perovskites and ceramic-Ag composites are under development as an alternative to Ni-YSZ cathode.
In this presentation, an overview of the development work at CSIRO (Australia) and Ben Gurion University at Negev (Israel) would be presented along with the experimental data on the electrochemical performance of various ceramic and composite cathodes. Some of these newly developed cathodes have demonstrated excellent electrochemical performance with an operating voltage close to 1.25 V providing current densities up to 350 mA/cm2 in robust and scalable electrolyte supported cells, with 80% CO2-H2O conversion to syngas in a single pass at 800° C for over 500 h of operation. The presentation will also compare the energy efficiencies of the cells with different cathodes based upon the product energy content. The advantages/disadvantages of using alternative electrodes would be also elaborated based upon initial assessment.

References:

[i] REN21 Renewables Global Status Report, 2017.
[ii] Final Report: Hydrogen Storage System Cost Analysis, Strategic Analysis Inc for US DOE, September 2016.
[iii] Global Greenhouse Gas Emissions Data, US EPA, January 2010.
[iv] Syngas Production By Coelectrolysis of CO2/H2O: The Basis for a Renewable Energy Cycle, Energy Fuels, 23 (6), pp 3089–3096, DOI: 10.1021/ef900111f, 2010.