Fuel Cell Testing

Fuel cell testing was carried out on both microtubular cells and simple button cells. The test rig was custom built using insulating fire blocks (Thermal Ceramics) and ceramic rods. Nichrome ribbon (Advent Ni80/Cr20 1.5 mm x 0.13 mm) was used for heating elements, wound around the ceramic rods. The blocks were hollowed to allow a heating chamber to be formed. A Eurotherm 2404 controller was used to control the formed furnace. Unit Instruments 7300 mass flow controllers (MFCs) regulated gas flow to the system, via Swagelock piping. The tubes undergoing testing were inserted onto an inlet pipe with an external diameter of approx 1.5 mm which allowed fuel to be supplied to the anode layer on the inside wall of the mSOFC. Gases were supplied by BOC, with 99.95% pure H2 and 50:50 He/H2 being used. A schematic of the equipment used can be seen in Figure 10.

Figure 10. Schematic diagram of the fuel cell test equipment used in this thesis

The button cells required additional equipment in order to make them gas tight. Cells prepared were coated with porous silver ink paste (SPI) over the surface of both electrolyte layers, and used to attach a short length of silver wire (Silver Wire Co, 99.98%) to either side. The button cell was then cemented onto a ceramic firing tube with an outer diameter matching the size of cell, using ceramabond (Aremco) and left to set for 6 hours. Once set, the same test procedure for the m-SOFCs was followed; the firing tube with the button cell was placed over a length of Swagelock^® tubing within the furnace, allowing hydrogen to be supplied to the anode side sealed within the firing tube and allowing air to reach the cathode side still exposed. A schematic showing the test setup in more detail can be seen in Figure 11.

Figure 11. Schematic diagram of the pellet cell test setup used in this thesis

A Solartron 1400 was used to record all electrical measurements and to control electrical demands on the cell. The Solartron CellTest suite of software was used to program a test profile and to analyse results obtained.

3.9.3.1 Fuel Cell Thermodynamics

As a fuel cell is an energy conversion device, the efficiency of conversion is of paramount importance. This can be calculated theoretically and also verified experimentally. The energy available for conversion to electrical work is given by the Gibbs free energy change for the overall cell reaction, which is the difference in the energy of formation of the products and reactants and is the maximum amount of energy

that can be produced from the reaction under the conditions specified. The Gibbs free energy change at STP for the reaction between hydrogen and oxygen to give liquid water can be seen in Equation 20.

ΔG = G^Φf , products - G^Φf , reactants

kJmol^-1

(20)

Equation 20. Gibbs free energy change of the reaction of hydrogen and oxygen to form water

The Gibbs free energy change is itself a function of temperature and reactant concentration (equated to partial pressure for gaseous reactants) and can be seen in Equation 21, where is the standard Gibbs free energy change, R and T are the universal gas constant (8.134 J K^-1 mol^-1) and absolute temperature, and / / are the partial pressures of H₂O, H₂ and O₂ respectively.

(21)

Equation 21. Gibbs free energy change for the reaction of hydrogen and oxygen to form water, with respect to partial pressure of gases

If the system is assumed to be operated reversibly, with all Gibbs free energy being converted to electrical energy, then the electromotive force between the anode and

cathode takes the value of E in Equation 22, with n being the number of electrons involved in the reaction and F being Faraday's constant (96485 C mol^-1).

(22)

Equation 22. Calculation of electromotive force between anode and cathode in a fully reversible system

Combining Equation 21 and Equation 22 leads to Equation 23, the Nernst equation, in which E^Φ represents the reversible potential under standard conditions (1.23 V for a hydrogen/oxygen system).

(23)

Equation 23. Nernst equation for the reaction of hydrogen and oxygen to form water

The Nernst equation relates the open circuit voltage (OCV, represented by E) of the fuel cell to the temperatures an (in this case) partial pressures of reactants and products.

Typically for an SOFC running on hydrogen at 800 °C the open circuit potential is between 0.9 - 1 V.

3.9.3.2 Fuel Cell Polarisation Curve

A fuel cell undergoing evaluation is usually subject to testing to determine the current-voltage response of the cell. By recording the response of the cell to difference loads

placed upon it, it is possible to form a fuel cell polarisation curve, an example of which can be seen in Figure 12.

Figure 12. Example fuel cell polarisation curve showing the three main sources of voltage losses

The voltage losses within a fuel cell polarisation curve can be split into 3 main categories: activation polarisation losses, ohmic losses and mass transport losses.

 Activation polarisation losses are primarily caused by the kinetics of the reactions taking place at the electrodes; for example, the oxidation of hydrogen to protons.

 Ohmic losses are primarily caused by resistances in the materials used within the cell; for example, the electronic resistance of the electrode materials.

 Mass transport losses are primarily caused by the inability for enough reactants to be supplied to the active layers of the electrodes to supply demand.

Developments in SOFC anode materials may assist in reducing losses caused in any of these three sections. For example, improvements in electronic or oxide ion conductivity would potentially reduce ohmic losses, while improvements in catalytic activity may bring about a lowering of activation polarisation losses.