Reversible solid oxide cell (rSOC) and solid oxide electrolysis (SOE) are gaining growing interest because they can convert both electricity into a chemical fuel and fuel back to electricity with high efficiency. This work summarizes for the first time the electrochemical performance characterization of Elcogen E350 stack both in fuel cell and electrolysis modes. Elcogen E350 stack is composed of 15 unit layers all having an 121 cm2 active electrode area. The electrochemical characterization is conducted in temperature region 650 – 700 °C and with varying current densities, multiple steam-to-hydrogen ratios, reactant utilizations and air flow rates. Results indicate that the stack can be used not only as a fuel cell but also in electrolysis mode and it is capable to be operated up to 1 A/cm2current density already at 650 °C and can be operated also with high reactant utilization even in electrolysis mode. Figure 1. (left) Elcogen E350 polarization at conditions: T[furnace] = 650°C, T[air,inlet] = T[fuel,inlet] = 640 °C, V[steam] = 21.1 Nl/min, V[H2] = 2.24 Nl/min, V[air] = 67 Nl/min. (right) Elcogen E350 polarization at conditions: T[furnace] = 650°C, T[air,inlet] = T[fuel,inlet] = 640 °C, V[steam] = 7.76 Nl/min, V[H2] = 7.76 Nl/min, V[air] = 33 Nl/min. Figure 1

A 500W SOFC stack provided by Topsoe Fuel Cells A/S was run with reformed diesel fuel for 1,200h. Diesel fuel used was Swedish Environmental Class 1 containing <10ppm of sulfur. The fuel was reformed with an autothermal reformer (ATR). The reformer consisted of a mixing chamber and a reactor chamber containing a noble metal catalyst provided by SudChemie. The reformer used the diesel fuel without any pre-treatment such as sulfur adsorbent. The reformer unit was able to achieve high conversion of reactants with only ppm levels of higher hydrocarbons at the reformer outlet. Particulate emissions of the reformer were measured to be 30gm3, a very low value even below daily exposure limits in ambient air set by the European Comission directives. The stack was first run with 3% humidified H2+N2 fuel to obtain baseline performance and then switched to reformate. Diesel reformate was found not to induce significantly more long-term degradation on the stack compared to hydrogen case. However, an initial degradation of about 20mVcell1 was observed during the first 150h on diesel fuel reformate. This could be an indication of a poisoning mechanism taking place, but needs further research to be verified.

A technical description and experimental analysis of a SOFC demonstration unit is presented. The unit contains most of the primary BoP-components of a complete SOFC system, except of air and fuel recirculation equipment or fuel system compressor. Natural gas is used as the fuel and electricity is supplied to the electric grid. A 5 kW power class planar SOFC stack from Research Centre Julich is assembled to the demo unit and a long-term experiment is conducted to assess the characteristic performance and durability of different components of the unit (e.g. the SOFC stack, the fuel pre-reformer and air heat exchangers). The evolution of absolute voltage drop of the stack over time is found to be of the same magnitude when compared to short stack experiments. Thus, other system components are not observed to cause an increase in the characteristic voltage drop of the stack. Two BoP-components, the afterburner and the power conversion unit failed to operate as designed. The performance of other BoP-components i.e. fuel pre-reformer and heat exchangers were satisfactory during the test run, and no significant performance loss could be measured.

A non-isothermal, two-phase model for a polymer electrolyte fuel cell (PEFC) is presented, analyzed, and solved numerically under three different thermal, and two hydrodynamic, modeling assumptions; the consequences of these are then discussed in terms of thermal and water management and cell performance. The study is motivated by recent experimental results that suggest the presence of previously unreported, and thus unmodeled, thermal contact resistances between the components of PEFCs and the discrepancy in the value for the capillary pressure that is used by different authors when modeling the two-phase flow in PEFCs. For the three different thermal assumptions (assuming effective heat conductivities, isothermal flow, and interfacial and bulk conductivites), liquid saturations of around 10% are obtained at the cathode active layer for 1000 mA cm(-2) and a cell voltage of 0.6 V. When lowering the capillary pressure (hydrodynamic assumption), liquid saturations of almost 30% and locally up to 100% are observed at the active layer of the cathode. At this current density and voltage, temperature differences across the cell of around 9 degrees C are predicted. In addition, the effect of varying clamping pressure within the framework of the model is touched upon. The benefits of the scaling analysis conducted here, to predict correctly, prior to numerical computations, important characteristic cell performance quantities such as current density and temperature drop are also highlighted. (c) 2005 The Electrochemical Society.

A measurement system for current distribution mapping for a PEFC has been developed. The segmented anode is constructed so as to have high thermal conductivity in order to prevent the formation of large temperature gradients between the electrodes. The construction is therefore feasible for use at high current densities. Both segmented and unsegmented gas diffusion layers are used. The effect of inlet humidification and gas composition at the cathode side is studied. In addition, two different flow geometries are studied. The results show that the measurement system is able to distinguish between current distribution originating from differences in proton conductivity, species concentration and gas diffusion layer properties.

A two-dimensional, non-isothermal, two-phase model et a polymer electrolyte fuel cell (PEFC) is presented. The model is developed tor conditions where variations in the stream-wise direction are negligible. In addition, experiments were conducted with a segmented cell comprised of net flow fields. The, experimentally obtained, current distributions were used to validate the PEFC model developed. The PEFC model includes species transport and the phase change of water, coupled with conservation of momentum and mass, in the porous backing of the cathode, and conservation of charge and heat throughout the fuel cell. The current density in the active layer at the cathode is modelled with an agglomerate model, and the contact resistance for heat transfer over the material boundaries is Liken into account. Good agreement was Obtained between the modelled and experimental polarization curves. A temperature difference of 6 degrees C between the bipolar plate and active layer on the cathode, ad a liquid saturation of 6% at the active layer in the cathode were observed at 1 A cm(-2).

In small fuel cell applications, it is desirable to take care of the management of reactants, water and heat by passive means in order to minimize parasitic losses. A polymer electrolyte membrane fuel cell, in which air flow on the cathode was driven by free convection, was studied by experimental and modelling methods. The cathode side of the cell had straight vertical channels with their ends open to the ambient air. A two-dimensional, isothermal and steady state model was developed for the cathode side to identify the limiting processes of mass transport. The modelled domain consists of the cathode gas channel and the gas diffusion layer. Experimental data from current distribution measurements were used to provide boundary conditions for oxygen consumption and water production. The model results indicate that at the cell temperature of 40 °C the performance of the cell was limited by water removal. At the cell temperature of 60 °C, the current distribution was determined by the partial pressure of oxygen.

The performance and current distribution of a free-breathing polymer electrolyte membrane fuel cell (PEMFC) was studied experimentally in a climate chamber, in which temperature and relative humidity were controlled. The performance was studied by simulating ambient conditions in the temperature range 10 to 40 °C. The current distribution was measured with a segmented current collector. The results indicated that the operating conditions have a significant effect on the performance of the fuel cell. It was observed that a temperature gradient between the fuel cell and air is needed to achieve efficient oxygen transport to the electrode. Furthermore, varying the air humidity resulted in major changes in the mass diffusion overpotential at higher temperatures.

The ohmic voltage loss in a fuel cell can be determined with the current interruption method. The method was utilized to measure the ohmic voltage loss in an individual cell of a fuel cell stack. This was achieved by producing voltage transients and monitoring them with a digital oscilloscope connected in parallel with the individual cell. In this study, the method was applied to a small polymer electrolyte membrane fuel cell (PEMFC) stack in which different air supply levels were employed on the cathode side. In the case of higher air-feed rate, the results revealed an increase of ohmic losses in the middle of the stack by up to 21% at 400mAcm−2, compared to the unit cell with the lowest ohmic loss. This probably resulted from the decrease of membrane conductivity because of drying. Comparison to individual cell voltages showed that the decrease of conductivity would not be observed if only the individual cell voltages alone were measured. The total ohmic loss in the stack was measured using the same method to verify the reliability of the measurement system. The results indicate a good agreement between the total ohmic loss and the combined ohmic losses in the individual cells.

A measurement system for the mapping of current distribution in a free-breathing polymer electrolyte membrane fuel cell (PEMFC) is introduced. In the measurement system, the ridges of the flow-field are made of gold-plated stainless steel and the rest of the measurement plate is made of a non-conducting material. The gas diffusion layer is not segmented and the error resulting from this is analyzed computationally. The effect of the cell temperature on the current distribution is studied with the measurement system. It appears that the measurement system is useful for PEMFC characterization and even large spatial variations of current density can be measured with it. According to the results, the optimum operating temperature for the studied cell is around 60°C without external humidification. In addition, it is concluded that the molecular diffusion is dominating mass transport mechanism at low temperatures but the current density profile is more homogeneous at elevated temperatures.

The mass diffusion overpotential distribution in a free-breathing proton exchange membrane fuel cell (PEMFC) was determined from current distribution measurements using a flow pulse approach. The current distribution measurements were conducted with a segmented flow-field plate. Flow pulses were fed to the cathode channels to form a uniform oxygen concentration distribution along the channels. Simultaneously, the cell resistance was monitored using the current interruption method. From the experimental data, the mass diffusion overpotential distribution was calculated using the Tafel equation. The results show that the mass diffusion overpotential in different parts of the cell may vary considerably, for example, at 180 mA cm−2 the mass diffusion overpotential difference between the bottom and top part of the cell was 0.1 V.