Experimental Investigation of Proton Exchange Membrane Fuel Cell With Platinum and Nafion Along the In-Plane Direction

For the catalyst layer based on platinum, platinum particles must be sufficiently dispersed and reasonably distributed inside the catalyst layer to ensure its high electrochemical active surface area and catalytic activity. When the fuel cell is operated at various loads, the current density inside the catalyst layer will be uneven, which can accelerate the degradation of the electrode. Besides, the cost of platinum catalyst is also an important problem for the further development and commercialization of proton exchange membrane (PEM) fuel cells. So far, most studies focus on the design of through-plane direction of Pt and Nafion.

In this study, the effect of in- plane gradient distribution of Pt and Nafion on PEM fuel cell is investigated experimentally. One type of membrane electrode assembly (MEA) has a gradient distribution, in which the loud of Pt catalyst and Nafion of inlet region is more than that of outlet region. In the other type of MEA, Pt and Nafion are uniformly distributed throughout the catalyst layer. These two types of MEAs are compared under different operating conditions including flow rate, backpressure and relative humidity. In order to ensure the comparability, MEA with gradient and uniform distribution along the in-plane direction in the cathode are prepared with the same total loads of platinum and Nafion. The catalyst coated membrane (CCM) method is used in this study. The catalyst ink is sprayed on both sides of the Nafion membrane through a pneumatic automatic spraying device manufactured by ourselves. The CCM and GDL are prepared as MEA by hot pressing. During fuel cell operation, the cathode and anode are given air and hydrogen respectively. The fuel cell testing system is utilized for setting and logging the fuel cell operation parameters to get the polarization curves. Electrochemical impedance spectra (EIS) is tested simultaneously using Zahner electrochemical workstation.

The results show that the performance of gradient distribution is better than uniform distribution at high current density. As the air flow rate decreases, it is found that the activation loss and concentration loss of gradient distribution are lower than those of uniform distribution by EIS. This indicates that more reactants participate in the reaction of outlet region and generate more current. But when the flow rate is low enough, these losses of gradient distribution are higher, which suggests that catalyst efficiency decreases because of water flooding. Increasing the back pressure can improve the performance of the fuel cell and the above phenomenon becomes more obvious. Exchanging the inlet and outlet positions of gradient distribution, the cell performance of gradient distribution is higher at low relative humidity. EIS shows its lower ohmic loss, which indicates uniform distribution of water at low relative humidity. The gradient design of Pt and Nafion along the in-plane direction is a promising strategy to improve fuel cell performance. Reasonably controlling the gradient distribution of Pt in the plane direction can reduce the amount of catalysts and improve efficiency.