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Molten carbonate fuel cells for electrolysis
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
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 [en]
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: urn:nbn:se:kth:diva-185433ISBN: 978-91-7595-928-3OAI: oai:DiVA.org:kth-185433DiVA: diva2:920605
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
List of papers
1. Electrochemical performance of reversible molten carbonate fuel cells
Open this publication in new window or tab >>Electrochemical performance of reversible molten carbonate fuel cells
2014 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 39, no 23, 12323-12329 p.Article in journal (Refereed) Published
Abstract [en]

The electrochemical performance of a state-of-the-art molten carbonate cell was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes by using polarization curves and electrochemical impedance spectroscopy (EIS). The results show that it is feasible to run a reversible molten carbonate fuel cell and that the cell actually exhibits lower polarization in the MCEC mode, at least for the short-term tests undertaken in this study. The Ni hydrogen electrode and the NiO oxygen electrode were also studied in fuel cell and electrolysis cell modes under different operating conditions, including temperatures and gas compositions. The polarization of the Ni hydrogen electrode turned out to be slightly higher in the electrolysis cell mode than in the fuel cell mode at all operating temperatures and water contents. This was probably due to the slightly larger mass-transfer polarization rather than to charge-transfer polarization according to the impedance results. The CO2 content has an important effect on the Ni electrode in electrolysis cell mode. Increasing the CO2 content the Ni electrode exhibits slightly lower polarization in the electrolysis cell mode. The NiO oxygen electrode shows lower polarization loss in the electrolysis cell mode than in the fuel cell mode in the temperature range of 600-675 degrees C. The impedance showed that both charge-transfer and mass-transfer polarization of the NiO electrode are lower in the electrolysis cell than in the fuel cell mode.

Keyword
Molten carbonate electrolysis cell, Ni electrode, NiO electrode, Reversible molten carbonate fuel cell
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-150929 (URN)10.1016/j.ijhydene.2014.02.144 (DOI)000340328800055 ()2-s2.0-84904768004 (ScopusID)
Note

QC 20150623

Available from: 2014-09-12 Created: 2014-09-11 Last updated: 2016-04-20Bibliographically approved
2. Operating the nickel electrode with hydrogen-lean gases in the molten carbonate electrolysis cell (MCEC)
Open this publication in new window or tab >>Operating the nickel electrode with hydrogen-lean gases in the molten carbonate electrolysis cell (MCEC)
(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
hydrogen-lean gases, molten carbonate electrolysis cell, Ni electrode, polarization
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-185428 (URN)
Note

QC 20160419

Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-04-20Bibliographically approved
3. Performance and durability of the molten carbonate electrolysis cell (MCEC) and the reversible molten carbonate fuel cell (RMCFC)
Open this publication in new window or tab >>Performance and durability of the molten carbonate electrolysis cell (MCEC) and the reversible molten carbonate fuel cell (RMCFC)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The molten carbonate electrolysis cell (MCEC) provides the opportunity for producing fuel gases, e.g. hydrogen or syngas, in an environmentally friendly way, especially when in combination with renewable electricity resources such as solar, wind and/or hydropower. The evaluation of the performance and durability of the molten carbonate cell is a key for developing the electrolysis technology. In this study, we report that the electrochemical performance of the cell and electrodes somewhat decreases during the long-term test of the MCEC. The degradation is not permanent, though, and the cell performance could be partially recovered. Since conventional fuel cell materials consisting of Ni-based porous catalysts and carbonate electrolyte are used in the MCEC durability test, it is also shown that the cell can alternatingly operate as an electrolysis cell for fuel gas production and as a fuel cell for electricity generation, i.e. as a so-called reversible molten carbonate fuel cell (RMCFC). This study reveals that the cell performance improves after a long period of RMCFC operation. The stability and durability of the cell in long-term tests evidence the feasibility of the electrolysis and reversible operations in carbonate melts using a conventional fuel cell set-up, at least in lab-scale.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-185431 (URN)
Note

QC 20160419

Available from: 2016-04-18 Created: 2016-04-18 Last updated: 2016-04-20Bibliographically approved
4. Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in a Molten Carbonate Electrolysis Cell (MCEC)
Open this publication in new window or tab >>Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in a Molten Carbonate Electrolysis Cell (MCEC)
2015 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 162, no 9, F1020-F1028 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study was to elucidate the kinetics of a porous nickel electrode for hydrogen production in a molten carbonate electrolysis cell. Stationary polarization data for the Ni electrode were recorded under varying gas compositions and temperatures. The slopes of these iR-corrected polarization curves were analyzed at low overpotential, under the assumption that the porous electrode was under kinetic control with mass-transfer limitations thus neglected. The exchange current densities were calculated numerically by using a simplified porous electrode model. Within the temperature range of 600-650 degrees C, the reaction order of hydrogen is not constant; the value was found to be 0.49-0.44 at lower H-2 concentration, while increasing to 0.79-0.94 when containing 25-50% H-2. The dependence on CO2 partial pressure increased from 0.62 to 0.86 with temperature. The reaction order of water showed two cases as did hydrogen. For lower H2O content (10-30%), the value was in the range of 0.47-0.67 at 600-650 degrees C, while increasing to 0.83-1.07 with 30-50% H2O. The experimentally obtained partial pressure dependencies were high, and therefore not in agreement with any of the mechanisms suggested for hydrogen production in molten carbonate salts in this study.

Place, publisher, year, edition, pages
Electrochemical Society, 2015
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-173295 (URN)10.1149/2.0491509jes (DOI)000359177100077 ()2-s2.0-84937054026 (ScopusID)
Note

QC 20150909

Available from: 2015-09-09 Created: 2015-09-09 Last updated: 2016-04-20Bibliographically approved
5. Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC)
Open this publication in new window or tab >>Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC)
2015 (English)In: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 182, 493-509 p.Article in journal (Refereed) Published
Abstract [en]

The performance of a molten carbonate electrolysis cell (MCEC) is to a great extent determined by the anode, i.e. the oxygen production reaction at the porous NiO electrode. In this study, stationary polarization curves for the NiO electrode were measured under varying gas compositions and temperatures. The exchange current densities were calculated numerically from the slopes at low overpotential. Positive dependency on the exchange current density was found for the partial pressure of oxygen. When the temperature was increased in the range 600-650 degrees C, the reaction order of oxygen decreased from 0.97 to 0.80. However, there are two different cases for the partial pressure dependency of carbon dioxide within this temperature range: positive values, 0.09-0.30, for the reaction order at lower CO2 concentration, and negative values, -0.26-0.01, with increasing CO2 content. A comparison of theoretically obtained data indicates that the oxygen-producing reaction in MCEC could be reasonably satisfied by the reverse of oxygen reduction by the oxygen mechanism I, an n = 4 electron reaction, assuming a low coverage of oxide ions at high CO2 content and an intermediate coverage for a low CO2 concentration.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-177977 (URN)10.1039/c5fd00011d (DOI)000364153200027 ()26211875 (PubMedID)2-s2.0-84946561934 (ScopusID)
Note

QC 20151202

Available from: 2015-12-02 Created: 2015-11-30 Last updated: 2016-04-20Bibliographically approved
6. A model for gas phase mass transport on the porous nickel electrode in the molten carbonate electrolysis cell
Open this publication in new window or tab >>A model for gas phase mass transport on the porous nickel electrode in the molten carbonate electrolysis cell
(English)Manuscript (preprint) (Other academic)
Abstract [en]

A one-dimensional model based on the Maxwell-Stefan diffusion equations was applied to evaluate the effect of the reverse water-gas shift reaction and the influence of the gas phase mass transport on the performance of the porous nickel electrode in the molten carbonate electrolysis cell. The concentration gradients in the current collector are larger than in the electrode for the inlet gases not in equilibrium, due to the shift reaction taking place in the electrode. When the humidified 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 in the electrolysis cell when the inlet gases have low contents of hydrogen. The mass-transfer limitations at low contents of water and carbon dioxide are captured in the model, but the effect on the electrode polarization, especially of carbon dioxide, is overestimated. Despite an overestimation in the calculations, the experimental data and the modeling results are still consistent in that carbon dioxide has a stronger effect on the gas phase mass transport than other components, i.e. water and hydrogen.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-185432 (URN)
Note

QC 20160419

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

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