Bipolar plates are one of the mechanical components of these devices and responsible for over 80% of the weight in a stack. They also provide the cells forming it with the mechanical stability they require by physically separating them and acting as electrical connectors between them. Flow geometries also ensure correct distribution of reagents on the catalyst layer and help remove the water generated during the electrochemical reaction to the exterior. The heat generated in the cathodic catalyst layers by the reaction is exchanged with the exterior through these elements. Consequently, a good design of the channel geometry and appropriate materials are a must.
Study of new materials and durability analysis
Graphite is the material most often used in commercial PEM fuel cells. It is a very light material with limited thermal and electrical conductivity in the plane perpendicular to the reaction zones. That is why the use of metal plates can be quite advantageous. However, metals are heavier than graphite and can degrade in acidic atmospheres when moisture is present.
At LIFTEC we study the performance of surface coatings used in low-density metal plates as an alternative to graphite plates by assessing their durability in prolonged tests in actual operating conditions. High-resolution images or SEM-EDX are used to analyse the degradation of the plates and study the formation of pitting or fractures in the coating. This same technique is applied to the MEAs to detect the migration of metal ions from the plates to the diffusion or catalyst layer.
Damage observed in a nerve in the reaction zone of an aluminium bipolar plate covered with chemical nickel
SEM image of a fissure in the chromium nitride and zirconium two-layer coating deposited by PVD on aluminium plates
DX analysis of a MEA in which we can observe the migration of Al and Ni ions from the bipolar plate surface
Design of new flow geometries and optimisation of gas distribution systems in bipolar plates
The uniform flow distribution of reactant gases from the main collectors to each of the plates and their homogenous distribution over the entire area of the catalyst layers are essential for optimal fuel cell operation. That is why channel sizing protocols and flow geometry have become basic pillars of bipolar plate design.
The group has several two and three-dimensional codes to optimise the design of the flow geometries of bipolar plates and general collectors used to distribute reagent gases to each cell. These codes are based on numerically solving Navier-Stokes (NS) equations describing the movement of incomprehensible, Newtonian and laminar fluids, such as the gases in cathodes and anodes in PEM fuel cells.
Numerical simulation results of the flow distribution of reagent gases to each cell in a stack from general collectors
Over time, the level of complexity has increased and the description of other complex processes has been added, for example transport of gases, protons and ions through porous media (diffusion and catalyst layers and proton conducting membrane), formation, condensation and management of water in the fuel cell, heat transfer, etc. (see line 3).
Based on the outcomes of the numerical simulations, the group has patented two new flow geometries: “cascade” and “herringbone”.<0} Our experimental results have been very good so far, especially compared with those achieved using serpentine, parallel-serpentine or straight-parallel channel geometries.
|Bipolar plates with the two flow geometries designed and patented by the group: “cascade” (left) and “herringbone” (right)
Study of water generation and management inside the cells
Water is essential for the proton conduction of polymer membranes in low-temperature PEM cells. However, the water present inside the cell also affects gas transport and distribution through flow channels, the diffusion layer and the catalyst layer. Although a high water content favours proton conductivity in the membrane, in excess in liquid form it can obstruct pores or channels making reagent transfer difficult. In fact, the phenomenon known as pooling is usually more severe in the cathode than in the anode since the water generated there makes it difficult for oxygen (or air) to diffuse towards the electrodes.
LIFTEC focuses on experimental analysis of how water is generated and managed in different types of flow geometries in operating single cells using direct display with CCD cameras. This makes it possible to detect condensation or pooling zones that prevent the correct distribution of the gas in the diffusion layer and to improve channel designs or create strategies to extract water during operation.
Transparent bipolar plate used for displays with a CCD camera
Single cell used for the actual operation test and evaluation of the generation of water using direct display
Image taken with CCD camera of the cathode in a single cell during a test showing the condensation of water on the plate surface