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A Multisensor Device for Highly Efficient Diffusivity Measurements and Overall‐Concentration‐Polarization Evaluation in Fuel Cells

As the global population continues to expand, the fossil-fuelbased economy has become unsustainable due to increasing energy consumption. A hydrogen-based economy has been proposed to meet the increasing need for sustainable energy.[1–3] A promising direction towards the proposed economy is to enhance the performance of hydrogen-based fuel cells, including proton-exchange-membrane fuel cells (PEMFCs), solid-oxide fuel cells (SOFCs) and molten-carbonate fuel cells (MCFCs).[4–9] One of the major ongoing efforts is to measure and reduce the energy losses at the operating conditions of fuel cells, such as Ohmic, activation and concentration-polarization (CP) losses.[1,9,10] In electrode-supported fuel cells, gas transport is largely impeded by thick electrodes, and a slow gas transport results in a large CP. Gas transport in electrodes is mainly accomplished by diffusion, the coefficient of which, the diffusivity, correlates with the properties of the electrodes, such as the tortuosity and the porosity. With a given temperature, current density (i) and fixed gas partial pressures, the CP varies with the electrode’s limiting current density (LCD), which is proportional to the gas diffusivity and inversely proportional to the electrode thickness (l).[9,11] To analyze and improve the efficiency of gas transport in fuel cells, the diffusivities of the reagent gases transporting through the porous electrodes should be measured at the operating conditions. Voltage–current measurements on fuel cells have previously been conducted to fit the gas diffusivities indirectly, and CPs were calculated using the fitted diffusivities.[12,13] To measure the effective binary diffusivities in the electrodes of SOFCs directly, an electrochemical cell with an oxygen sensor and an oxygen pump was developed.[9,11] Most recently, a bisensor electrochemical cell was proposed to double the efficiency of diffusivity measurements in the anode and cathode of SOFCs.[14] The direct measurement of hydrogen and oxygen gases using an electrochemical cell allows the LCDs and CPs of SOFCs to be evaluated more reliably before the electrodes are assembled into intact fuel cells. However, only one sample can