(625u) Issues of Planar Solid Oxide Fuel Cell Stack

Issues of Planar Solid Oxide Fuel Cell Stack

Tapobrata Dey^a,b, Debanand Singdeo^a,Manaswita Bose^a, Prakash C Ghosh^a

^aDepartment ofEnergy Science and Engineering, Indian Institute of Technology Bombay, Mumbai-400076, India

Present Address

^bDepartment of Mechanical Engineering, D. Y. Patil College of Engineering, Akurdi,Pune-411044, India

Abstract:

Solid oxide fuel cell (SOFC) is one of the possible solutions to the challenge posed by the increasing energy requirement, as the core competency of SOFC is based on efficiency and clean energy conversion. In order to achieve wide spread acceptance, the technology has to perform reliably over a long period of time. Long term performance is governed by materials of construction and design of the cell as well as the environment under which it is operated. SOFC performances are affected by different types of losses such as activation, ohmic and concentration. Activation losses depend on electrochemical properties of the materials present at the electrode electrolyte interface, ohmic losses depend on bulk resistance of the electrolyte as well as contact resistance which is further related to the compressive pressure applied on the active area. Mass transfer or concentration loss depends on fuel and oxidant flow distribution on the active area. Among these losses, ohmic and concentration play a significant role when a laboratory scale fuel cell is further scaled up. This is accomplished by studying the polarisation behaviour of both coupon cell and large area cell. The performance obtained is compared to understand the way in which the ohmic and mass transfer losses affect a large area cell.  

The objective of the present work is to challenges faced by SOFC technology towards the commercialisation lies in the issues related to scale up. The anode and cathode contact with the interconnect in case of the scaled up SOFCs, are found to be about 2.5% and 6.7% respectively as opposed to 50%, in the coupon cell under 0.2 kN load. A maximum contact area about 17% has been achieved on the application of 0.7 kN load on a single cell assembly. The poor contact at the interface can be attributed to the uneven surface morphology of the cell and interconnect. Improvements in contact area at the interfaces are necessary in order to operate the scaled up SOFC at higher current densities. Finally, suggestions are made to sustain the performance during the scale up.

The nanomechanical properties are estimated to optimize the contact pressure inside the SOFC stack. The macro- and nano-mechanical properties play a vital role in SOFC to ensure the mechanical stability of the cell. This study investigates both the macro- and nano-mechanical properties of all the three component layers (anode, cathode and electrolyte) of a planar SOFC. The flexural fracture strength experiments in three point bending mode are performed to study the macro-mechanical failure behavior of the single cell. Further, the nanoindentation technique is utilized in both pre- and post-reduced conditions to evaluate the nanomechanical properties e.g. nanohardness, Young’s modulus, mean contact pressure, relative stiffness and relative spring back. The characteristic values of the various nanomechanical properties are analyzed using Weibull distribution for anode, electrolyte and cathode layers of the SOFC.

Ohmic resistance at the interface of the electrode and interconnect in fuel cells influence the overall performance of a cell. In the present work, contact resistances between interconnect and electrodes in planar type SOFC under various compression loads and at different temperatures are measured in laboratory scale experiments. The roughness of the electrode and interconnect surfaces is characterized and a mathematical model to determine the contact resistance at the interfaces with known morphology, is proposed. The experimental results are found to be in good agreement with the values obtained from the model.

The concentration losses depend on the distribution of fuel and oxidant over the cell active area. The flow distribution in any fuel cell is ensured by designing appropriate flow-configuration. The present work aims to investigate the flow uniformity and mass transport losses in SOFC and the role of flow geometry on improving performance of the unit. The preliminary design consists of an outlet manifold, channel feeding the active area region and an inlet manifold. The simulation of the entire model on the computational platform is proved to be counter-productive. So, the geometry is subdivided into three parts (inlet manifold, straight channel and outlet manifold) and the flow field is sequentially simulated. Pressure drop in the channel is determined assuming the flow to be fully developed and laminar in the channel, which is valid owing to the large length to hydraulic diameter (l/D >> 1) ratio for each channel. Different types of manifold modifications are used in order to achieve better flow uniformity.

Performance of the scaled up SOFC is drastically affected by ohmic and concentration losses. The ohmic losses increase with reduction in both macro and microscopic contact area whereas, the concentration losses depend upon the distribution of fuel and oxidant over the cell active area. The present work aims to investigate the influence of the interfacial resistance (ohmic) both in terms of the external compression load, i.e., the interaction at the microscopic level and macroscopic contact area. The effect of the uniformity of the flow distribution on the performance of the scaled up SOFC is also studied. To that end, the base configuration is modified at different stages and the performance of the cell is noted.  The electrochemical performance is observed to enhance by 44% with application of the optimal external contact pressure estimated based on the materials properties, which is further improved by 108% with an increase in the apparent contact area. Another 15% enhancement in performance is observed with better uniformity in the flow distribution. Overall 167% enhancement in the performance is achieved with the modified design of the cell.

Further, a CFD based simulation is carried out to optimize the planar SOFC design and fuel and oxidant flow configuration for reducing the concentration losses. Various factors of SOFC like, the flow field design and kinetics of chemical and electrochemical reactions are investigated by CFD model. The simulated result obtained using CFD model is validated with the experimental results. The model is valuable for optimizing the SOFC flow field design.