Analysis on Membrane Electrode Assembly for Intermediate Temperature Proton Exchange Membrane Fuel Cell and Alkaline Anion Exchange Membrane Water Electrolysis

Abstract

With the increase in interest of hydrogen energy as an alternative energy source, the research on electrochemical devices to generate electricity using hydrogen as fuel and to produce hydrogen using water as reactant are actively pursued. Proton exchange membrane fuel cells are electrochemical devices that convert hydrogen and oxygen to electricity and water. The higher thermodynamic efficiency (> 40%) than conventional internal combustion engines (20 – 30%) and no greenhouse gas emission during operation are the great advantages of the proton exchange membrane fuel cells. The hydrogen production is dominantly performed through steam reforming from natural gas or methane, which process creates carbon dioxide or hydrocarbons as byproducts. Water electrolysis is a technique that decomposes water into oxygen and hydrogen gas only through electrochemical reaction. This technique primarily using renewable power sources and make it possible to yield high purity hydrogen without greenhouse gas emission. To establish green technology system from hydrogen production to converting hydrogen into electricity, the intensive research on developing fuel cell and water electrolysis technology are conducted. The intermediate-temperature proton exchange membrane fuel cells are operating at higher temperature (100–120 °C) than low-temperature proton exchange membrane fuel cells (~ 80 oC). The higher operating temperature of the system improves the reaction kinetics, CO tolerance and heat/water management. Therefore, many advantages are expected with proton exchange membrane fuel cells operating at intermediate temperature compared to that at low temperature. Alkaline anion exchange membrane water electrolysis has advantages over the conventional alkaline water electrolysis that uses alkaline solution electrolyte with porous diaphragm separators. The polymer electrolyte membrane based systems offer advantages with regard to safety, efficiency, and separation of product gases. Moreover, the alkaline operating condition makes it possible to use inexpensive non-noble metal catalysts towards oxygen evolution and hydrogen evolution reaction unlike in proton exchange membrane water electrolysis. Even though with these great advantages, intermediate temperature proton exchange membrane fuel cells or alkaline anion exchange membrane water electrolysis show relatively low performances than low temperature proton exchange membrane fuel cells or proton exchange membrane water electrolysis, respectively due to their operational characteristics. To overcome this issue and obtain high performance and durability, the research on developing polymer electrolyte and catalysts are actively conducted. Most of the researches are oriented to the material development. Even though the developed polymer electrolyte or catalysts showed great material property, their performances are not reflected in a cell performance. A single cell is containing a membrane electrode assembly which is consisted with a polymer electrolyte membrane and catalyst layer on both side of the membrane. The catalyst layer, which consists of the metal catalysts and binders, is an important component of the membrane electrode assemblies since the electrochemical reactions are occurred in there involving mass transport of reactants and products. Therefore, the catalyst layer structure and properties should be optimized regarding on each operating condition of the electrochemical devices to obtain high performance and stability. For intermediate temperature proton exchange membrane fuel cell operation, membrane electrode assembly drying is a dominant factor influencing the cell performance since the ionic conductivity and mass transport of the catalyst layer are strongly dependent on the ionomer films layered on the catalyst particles. In the research of intermediate temperature proton exchange membrane fuel cell, the drying of electrode membrane electrode assembly at 120 oC and ≤ 35% RH operating condition is experimentally proven and the contents of ionomeric binder, which has water retention ability, is controlled to 20 – 40 wt.% to construct effective catalyst layers for high performance. The optimum ionomer content increases with decreasing current density where the drying of membrane electrode assembly is dominant over flooding. However, at high current density region where the water production is high and sufficient hydration is provided, the maximum performance is obtained with 30 wt% ionomer content due to flooding. For alkaline anion exchange membrane water electrolysis operation, the effect of pressing of membrane electrode assembly and feed supplying methods are investigated. Through introducing pressing procedure, higher water splitting current density is obtained due to the improvement is mass transport. Additionally, the characteristics of double-side feed (supplying reactant solutions to both anode and cathode) and single-side feed operation (supplying a reactant solution to only anode) are studied with various anode binder content. In double-side feed operating condition, the cell performance is dominantly affected by electrochemical activation site of the catalyst and porosity of the catalyst layers. The optimal content is found to be 9 wt.% between 5 – 20 wt.%. In single-side operating condition, the water splitting current density has been greatly increased by improvement in mass transport and catalyst dissolution during the cell operation due to the high water splitting current resulted in massive oxygen production dominantly affects the long-term performance stability. Therefore, the highest binder content in this experiment, 20 wt.%, exhibits the best durability and cell performance. In this study, the electrochemical factors affecting cell performances are analyzed and research directions of membrane electrode assembly for high performance are proposed regarding on the operating conditions of electrochemical devices.