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Operating the nickel electrode with hydrogen-lean gases in the molten carbonate electrolysis cell (MCEC)
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-9203-9313
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0002-2268-5042
(English)Manuscript (preprint) (Other academic)
Abstract [en]

If a molten carbonate electrolysis cell (MCEC) is applied for fuel gas production it is important to know the polarization of the nickel electrode when operated at low concentration of hydrogen. Thus, the electrochemical performance of the Ni electrode was investigated under hydrogen-lean gases containing 1/24.5/24.5/50%, 1/49.5/24.5/25%, 1/24.5/49.5/25% and 1/49.5/49.5/0% H2/CO2/H2O/N2 in the temperature range of 600–650 °C and was then compared to the reference case with 25/25/25/25% H2/CO2/H2O/N2. The electrochemical measurements included polarization curve coupled with current interrupt, and electrochemical impedance spectroscopy. Polarization resistances of the Ni electrode obtained by the two different techniques agreed well. For the inlet gases containing low amounts of hydrogen the Ni electrode exhibited higher polarization losses than when using the reference case in the electrolysis cell. The electrochemical impedance measurements showed that both charge-transfer and mass-transfer polarizations were higher for hydrogen-lean gases at all measured temperatures. Except under the condition with 1/49.5/49.5% H2/CO2/H2O at 650 °C, the Ni electrode exhibited lower mass-transfer polarization when compared to the reference case. Furthermore, the mass-transfer polarization was strongly dependent on temperature under H2-lean gases, differing from the reference case when the temperature has almost no effect on mass-transfer polarization. The activation energy for hydrogen production was calculated to be in the range of 69–138 kJ·mol-1 under all measured gases, indicating that the Ni electrode is under kinetic and/or mixed control in the MCEC.

Keyword [en]
hydrogen-lean gases, molten carbonate electrolysis cell, Ni electrode, polarization
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-185428OAI: oai:DiVA.org:kth-185428DiVA: diva2:920565
Note

QC 20160419

Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-04-20Bibliographically approved
In thesis
1. Molten carbonate fuel cells for electrolysis
Open this publication in new window or tab >>Molten carbonate fuel cells for electrolysis
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The molten carbonate fuel cell has evolved to current megawatt-scale commercial power plants. When using the fuel cell for electrolysis, it provides a promising option for producing fuel gases such as hydrogen and syngas. The cell can thereby operate reversibly as a dual energy converter for electricity generation and fuel gas production. The so-called reversible molten carbonate fuel cell will probably increase the usefulness of the system and improve the economic benefits.

This work has investigated the performance and durability of the cell in electrolysis and reversible operations. A lower polarization loss is found for the electrolysis cell than for the fuel cell, mainly due to the NiO electrode performing better in the MCEC. The stability of the cell in long-term tests evidences the feasibility of the MCEC and the RMCFC using a conventional fuel cell set-up, at least in lab-scale.

This study elucidates the electrode kinetics of hydrogen production and oxygen production. The experimentally obtained partial pressure dependencies for hydrogen production are high, and they do not reasonably satisfy the reverse pathways of the hydrogen oxidation mechanisms. The reverse process of an oxygen reduction mechanism in fuel cell operation is found to suitably describe oxygen production in the MCEC.

To evaluate the effect of the reverse water-gas shift reaction and the influence of the gas phase mass transport on the porous Ni electrode in the electrolysis cell, a mathematical model is applied in this study. When the humidified inlet gas compositions enter the current collector the decrease of the shift reaction rate increases the electrode performance. The model well describes the polarization behavior of the Ni electrode when the inlet gases have low contents of reactants. The experimental data and modeling results are consistent in that carbon dioxide has a stronger effect on the gas phase mass transport than other components, i.e. water and hydrogen.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 57 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:18
Keyword
Durability, electrode kinetics, gas phase mass transport, molten carbonate electrolysis cell, molten carbonate fuel cell, performance, reversible.
National Category
Chemical Sciences
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-185433 (URN)978-91-7595-928-3 (ISBN)
Public defence
2016-05-20, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160419

Available from: 2016-04-20 Created: 2016-04-18 Last updated: 2016-04-20Bibliographically approved

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