Uso de la técnica de interrupción de corriente para la caracterización de una celda de combustible no humidificada con membrana de intercambio protónico

Abstract

Due to the energetic crisis, the US government has promoted the development of alternatives resources for the generation of electricity. Fuel Cells have emerged as one of the most promising technologies for the power source of the future. A fuel cell is an electrochemical reactor where the energy of a chemical reaction (hydrogen and oxygen) is converted directly into electricity using a proton exchange membrane. The Proton exchange membrane (PEM) has some inefficiencies that can be measured using a current interruption technique with hydrogen and oxygen as raw materials. The current interruption method instantaneously stops the current through the membrane. The response of the membrane during this change is used to characterize it. A Tektronix TDS 3032 oscilloscope was used to measure the voltage drop of the system. Three different fluxes of hydrogen and oxygen at stoichiometric rates were studied in this work (8.5, 5.3 and 2.7ml/min for hydrogen flow). The results are presented using two graphical approaches: one for open and another for the closed circuit. The graphs were classified according to regions. Region 1 is attributed to kinetics effects. Region 2 was attributed to kinetics and mass transfer effects. Region 3 was attributed to equilibrium phase. The information obtained with current interruption technique was used to find an equation that describe the open circuit and close circuit behavior. An exponential equation describe the closed circuit and a sixth order polinomial equation describe the open circuit graphs. Also, was find an equivalent circuit of the cathode region using Micro Sim® simulation program. Micro Sim® permits to find an equivalent circuit that simulates the current signal and open circuit voltage response. The equivalent circuit describes the region 1 and 3 behavior for the closed circuit using one resistance of 0.8Ω to simulate region 1 and other resistance of 0.8Ω and 250mF capacitor to simulate region 3. Region 2 was not possible to simulate because the system exhibited two contributions (charge and mass transfer effects) that convert this region in a non-linear second order behavior. This non-linear behavior could lead to postulate a reaction mechanism with the transfer of protons through the membrane as the rate-limiting step. In conclusion, the current interruption technique allowed for the action of offering a graphical represenation of fuel cell membrane equipment. For future works is recommended to repeat the same series of experiments for anode that will permit to find an equivalent representative circuit of the total PEM fuel cell.