Abstract
This thesis presents the development and application of electrochemical half-cell setups to
study the catalytic reactions taking place in High Temperature Polymer Electrolyte Fuel Cells
(HTPEM-FCs): (i) a pressurized electrochemical cell with integrated magnetically coupled
rotating disk electrode (RDE) and (ii) a gas diffusion electrode (GDE) setup designed for
experiments in conc. H3PO4. The pressurized cell is demonstrated by tests on polycrystalline
platinum electrodes up to 150 ºC. Functionality of the RDE system is proved studying the
oxygen reduction reaction (ORR) at temperatures up to 140 ºC and oxygen pressures up to
~100 bar at room temperature. The GDE cell is successfully tested at 130 ºC by means of
direct oxidation of methanol and ethanol, respectively. In the second part of the thesis, the
emphasis is put on the ORR in H3PO4 with particular focus on the mass transport of
dissolved oxygen. A potential step method (hydrodynamic chronocoulometry) is evaluated
for simultaneous measurement of diffusivity and solubility of oxygen by means of RDE.
Finally, the ORR tests are extended to conc. H3PO4 at more relevant working temperatures
and under increased oxygen pressure.
Direct oxidation of ethanol is in principle a promising concept to supply HTPEM-FCs with a
sustainable and on large scale available fuel (ethanol from biomass). However, the
intermediate temperature tests in the GDE setup show that even on Pt-based catalysts the
reaction rates become first significant at potentials, which approach the usual cathode
potentials of HTPEM-FCs. Therefore, it seems that H3PO4-based fuel cells are not much
suited to efficiently convert ethanol in accordance with findings in earlier research papers.
Given that HTPEM-FCs can tolerate CO containing reformate gas, focusing research
activities on catalysts for reformate oxidation appears more rational.
Improvements of the ORR activity at the cathode can have large influence on the
performance of HTPEM-FCs. The measurements of oxygen diffusivity and solubility
contribute to the understanding of oxygen mass transport at the interface of platinumphosphoric
acid. At room temperature, a relative slow ORR hindering process is active,
which requires using a fast method (cyclic voltammetry with high scan rate / hydrodynamic
chronocoulometry) to accurately measure the diffusion limited currents, and thus, oxygen
diffusivity and solubility. In conc. H3PO4 at 100 ºC and under increased oxygen pressure, the
issue is apparently much less pronounced. Further examination is underway. Finally, a
fluorinated electrolyte additive, C6F13SO3K, is examined by RDE. Preliminary results at 100
ºC indicate that small amounts of the additive moderately increase the diffusion limited
currents
study the catalytic reactions taking place in High Temperature Polymer Electrolyte Fuel Cells
(HTPEM-FCs): (i) a pressurized electrochemical cell with integrated magnetically coupled
rotating disk electrode (RDE) and (ii) a gas diffusion electrode (GDE) setup designed for
experiments in conc. H3PO4. The pressurized cell is demonstrated by tests on polycrystalline
platinum electrodes up to 150 ºC. Functionality of the RDE system is proved studying the
oxygen reduction reaction (ORR) at temperatures up to 140 ºC and oxygen pressures up to
~100 bar at room temperature. The GDE cell is successfully tested at 130 ºC by means of
direct oxidation of methanol and ethanol, respectively. In the second part of the thesis, the
emphasis is put on the ORR in H3PO4 with particular focus on the mass transport of
dissolved oxygen. A potential step method (hydrodynamic chronocoulometry) is evaluated
for simultaneous measurement of diffusivity and solubility of oxygen by means of RDE.
Finally, the ORR tests are extended to conc. H3PO4 at more relevant working temperatures
and under increased oxygen pressure.
Direct oxidation of ethanol is in principle a promising concept to supply HTPEM-FCs with a
sustainable and on large scale available fuel (ethanol from biomass). However, the
intermediate temperature tests in the GDE setup show that even on Pt-based catalysts the
reaction rates become first significant at potentials, which approach the usual cathode
potentials of HTPEM-FCs. Therefore, it seems that H3PO4-based fuel cells are not much
suited to efficiently convert ethanol in accordance with findings in earlier research papers.
Given that HTPEM-FCs can tolerate CO containing reformate gas, focusing research
activities on catalysts for reformate oxidation appears more rational.
Improvements of the ORR activity at the cathode can have large influence on the
performance of HTPEM-FCs. The measurements of oxygen diffusivity and solubility
contribute to the understanding of oxygen mass transport at the interface of platinumphosphoric
acid. At room temperature, a relative slow ORR hindering process is active,
which requires using a fast method (cyclic voltammetry with high scan rate / hydrodynamic
chronocoulometry) to accurately measure the diffusion limited currents, and thus, oxygen
diffusivity and solubility. In conc. H3PO4 at 100 ºC and under increased oxygen pressure, the
issue is apparently much less pronounced. Further examination is underway. Finally, a
fluorinated electrolyte additive, C6F13SO3K, is examined by RDE. Preliminary results at 100
ºC indicate that small amounts of the additive moderately increase the diffusion limited
currents
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2016 |