Development of Measurement Method and Simulation of Water Content Distribution of CCM of Fuel Cell
Authors: Nagumo Kenji , Chiba Hiroto 
Published: February 2020

The durability of fuel cells is closely related to water content of the catalyst coated membrane (CCM). When the water content is high, the electrode catalysts become swollen due to dissolution, agglomeration, and redeposition, and the decline in the catalyst surface area accelerates. When the water content is low, impurities and hydrogen peroxide accumulate in the PEM, promoting chemical degradation and reducing the thickness of the PEM.

To help ensure durability of automotive fuel cells, it is therefore necessary to identify the in-plane distribution of the water content of CCM in the cells and to control it within the proper range. The in-plane distribution of water content is also affected by the operating condition parameters of the cells. In addition to the fact that a large number of combinations of operating condition parameters are possible, measurements also take time, and it has been challenging to determine parameters with an adequate level of accuracy.

 The development of the fuel cell stack for the 2016 model FCV sought to realize increased fuel cell stack durability by clarifying the distribution of the water content of the CCM in the cells through the development of a multi-segment impedance sensor system and the prediction of the optimum water content distribution using simulations.

 There are various measurement methods of water content inside fuel cell. For example nuclear magnetic resonance (NMR), neutron radiography, X-ray and so on. In order to clarify CCM water content in an actual vehicle, we need a method that would make it possible to obtain measurements without influencing the in-plane humidity distribution and other such factors during electric generation.

 Based on the results of examination, the impedance method was selected to measure in-plane distribution of water content　during actual vehicle operation and development carried out.  The sensor is intended for incorporation in a fuel cell stack and was therefore designed with the same shape as the MEA. As a result, it becomes possible to obtain CCM water content measurements without any need to prepare special separators or seals. The　surface on either side of the sensor has 75 square sensor pads which used to make impedance measurements of the CCM arranged. With measurement using the sensor, the distribution of CCM water content could be measured.

 In order to develop a method of simulation of the in-plane water content of CCM in cells, original Honda functional modifications were made to commercially available polymer electrolyte fuel cell simulation software. Simulation results correspond well with results of measurements of multi-segment impedance sensor system and the average predictive accuracy for the 75 segments was within 5.0%. Following the optimization of the operating condition parameters with the simulation, variations in the distribution of water content in the CCM were reduced by about 30%. In addition, the use of the developed simulation made it possible to determine operating conditions in several days that would previously have necessitated several weeks using tests.

The in-plane water content distribution of the CCM varies due to instantaneous changes in operating conditions, considerably during running mode, increasing during acceleration, and decreasing during deceleration. Because of this, it was necessary to verify whether the in-plane water content distribution of the CCM, which was optimized by simulation above, was maintained within the scope of the upper and lower limit values during transient current generation in an actual on-board generation system. During acceleration, In-plane water content of the CCM reaches its maximum value in the center of the cell in the direction of gas flow but remains below the upper limit value. In the transition from acceleration to deceleration, the in-plane water content of the CCM at the cathode inlet declines to close to the lower limit value. It was verified, however, that the in-plane water content of the CCM during deceleration remained within the upper and lower limit values.