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Solid Polymer Electrolyte Fuel Cells

Fuel cells convert the available energy of a fuel and oxidant into electrical power. Improvements in fuel cell technology combined with environmental concerns about conventional power generation have sparked widespread interest in Proton Exchange Membrane (PEM) fuel cells for portable, transportation and stationary applications. A schematic description of their operation is shown below for the H2/O2 system. The advantages of PEM fuel cells are high power density, the absence of corrosive liquid electrolytes, the relative simplicity of stack designs (no moving parts), the sturdiness of the system, and the relatively low operating temperatures and operating pressure. In PEM fuel cells, the central component is a polymeric membrane that provides an ionic path between the anode and the cathode of the galvanic cell and serves to separate the two reactant gases. The properties of the PEM are paramount to the successful operation and commercialization of fuel cells. The challenge is to find a low cost membrane with high ionic conductivity; zero electronic conductivity; low gas permeability; dimensional stability; mechanical strength; resistance to dehydration; and chemical stability towards oxidation, reduction and hydrolysis.

Membrane Design and Synthesis: Our goal is to understand structure-property relationships of proton exchange membranes and ultimately design superior, enabling materials. This is achieved through control of the membrane’s chemical microstructure, in conjunction with evaluation of properties such as proton conductivity, water retention and transport, and mechanical stability, environmental stability and fuel cell electrochemistry. The figure below shows an electron micrograph of a PEM prepared in our laboratory and illustrates how the microstructure can be controlled so that high proton conductivities can be obtained. The proton conducting regions have dimensions ~ 10 nm. Membranes exhibiting promising properties are being studied as an integral component of fuel cell systems. The information obtained from these studies further advances the scientific basis for designing fuel cell membranes and fuel cell systems.

Figure 1. (left) Schematic diagram of a PEM Fuel Cell. (Right) A transmission electron micrograph of a Proton Exchange Membrane showing how the acidic regions (dark) can be spatially-connected to achieve highly conductive materials. The acidic clusters have dimensions of ~ 10-100 nm.

Several projects are under way involving the synthesis of novel membranes comprising block copolymers of fluorpolymers and sulfonated polyarylene sulfones, sulfonated poly ether ether ketones, and polyimides. Examples of polymer membranes studied in our laboratory are listed below.

Examples of polymer structures synthesized in our laboratory are include

These studies are leading to materials with interesting bicontinous morphologies in which a percolation network of acid sites are encapsulated by non-ionic sites to yield mechanically robust materials with high proton conductivity.

Figure 2. Morphologies (TEMs) of novel block copolymers PEMs prepared at SFU.

Proton Conductivity: Studies of the conductivity of ionomer membranes is essential because of its importance to fuel cell operation but accurate measurement of specific conductivity poses a significant challenge in terms of experimental methodology. A.C. impedance spectroscopy is being used in our laboratory as a means for measuring conductivity values. A dedicated electrochemical cell has been developed which enables conductivity to be measured up to 100°C and relative humidity up to 100% RH. Trends of conductivity, and conductivity anisotropy, are correlated with polymer structure and membrane morphology in order that a rationale for membrane design can be achieved. The example below shows the precipitous reduction of proton conductivity in PEMs when the water inside is frozen, and the role of membrane morphology on the anisotropy of conductivity (see J. Amer. Chem Soc., 129, 15106-15107).

Figure 3. Conductivity of PEMS as a function of temperature.

The relationship between proton conductivity and morphology is actively examined as polymer structure has been shown to dramatically affect conductivity as illustrated below.

Figure 4. Conductivity of PEMS as a function of polymer structure. (See J. Mater. Chem., 2007, 17, 3255–3268.)

Solid State Electrochemistry at Microelectrodes: Within the cathode catalyst later of a proton exchange membrane fuel cell, the oxygen reduction reaction proceeds orders of magnitude slower than its anode counterpart. Due to these limiting kinetics, investigations into the influence of local environmental conditions are important for a deeper understanding at the interface of the catalyst and ionomer.

Figure 5. Catalyst layer structure emphasizing the role of ionomer in mass transport.

For these experiments, various chemical compositions of ionomer are tested under different temperature and/or relative humidity in a specially designed electrochemical cell. Using potential step coulometry, where the potential is rapidly shifted to a region where ORR occurs, the overall permeability of reactant gas (oxygen) can be determined. Slow sweep potentiometry can be used to determine exchange current densities, which can be used to find activation energies for specific environmental conditions.

In this project the kinetics and mass transport of oxygen reduction in polymer electrolytes are investigated because oxygen reduction is a rate limiting process in fuel cells. These properties are determined by ultra-microelectrode techniques. Microelectrodes are particularly useful because mass transport and Ohmic contributions are minimized. We use a custom-built electrochemical cell designed for rapid measurements, and to have control over temperature, humidity, the nature of the polyelectrolyte and electrocatalyst (see J. Electroanal. Chem. 568C (2004) 247).

Figure 6. Solid State electrochemical cell for microelectrode electrochemistry. Lower Figure: Oxygen solubility and diffusion coefficients extracted from chronoamperometric data.

Electrochemistry at gas diffusion electrodes, and fuel cell analysis: At the heart of a proton exchange membrane fuel cell (PEMFC) is a membrane electrode assembly (MEA), comprising of a proton conducting membrane sandwiched between two gas diffusion electrodes. The porous gas diffusion electrode structure consists of Pt electrocatalyst dispersed on high surface area carbon black, held together with a binding agents such as polytetrafluoroethyene (PTFE) and Nafion (see below). It has been long established that optimal electrochemical kinetics is achieved when a three-phase interface exits between the membrane, the electrode and the reactant gas. Only catalyst located in this region is electrochemically active and not all catalyst present is utilized. MEA fabrication facilities include a suite of automated systems for catalyst layer deposition such as screen printing, spray coating, decal transfer for catalyst coated gas diffusion layers or catalyst coated membranes.

Several research projects aimed at understanding and advancing fuel cell technology are under way at NRC. One such project investigates the interplay between the PEM, active catalyst area, and catalyst utilization at membrane/GDE interfaces. This requires in depth knowledge of the microstructure and nanostructure of the various components, which are examined using a variety of techniques – including SEM and TEM as illustrated below.

Figure 7. Solid State electrochemical cell for microelectrode electrochemistry. Lower Figure: Oxygen solubility and diffusion coefficients extracted from chronoamperometric data.

Membrane–electrode assemblies are evaluated using extensive electrochemical analysis and fuel cell polarization techniques in conjunction with electrochemical impedance spectroscopy. In conjuction with the fuel cell modeling team. It has been possible to simulate fuel cell performance from externally measured materials properties as shown below. Good simulation is observed in the kinetic and Ohmic regions. Current research is directed towards evaluating water transport in MEAs in order to simulate the whole operating region.

Figure 8. Polarization Curves simulated from materials properties for different gas diffusion electrodes (solid and dashed lines), and experimental polarization curves (points).

Detailed knowledge of the physico-chemical attributes of fuel cell materials and their relationship to fuel cell electrochemical performance has led to substantial improvements in current and power densities as evidenced below for MEAs fabricated at NRC-IFCI.

Figure 9. Improvements of MEA fuel cell Polarization Curves. The cell is operated at 70°C.

Alkaline Anion Exchange Membranes for Fuel Cells: While there has been a large amount of work on proton exchange membrane (PEM) fuel cells due to their ability to generate clean energy, they require the use of platinum to function. On the other hand, alkaline anion exchange membrane (AAEM) fuel cells, which do not require platinum theoretically, have recently gained a large amount of attention due to the potentially lower costs of construction. However, AAEMs generally suffer from degradation at high pH and elevated temperatures, thus hindering fuel cell studies. Our goal is the synthesis and characterization of stable AAEMs, which ideally are stable at high pH and high temperatures (60-80°C) and are electrically insulating, gas impermeable, highly anion conducting, and possess strong mechanical properties. Some examples that we have developed are shown in Figure 1 below (for “mes-PDMBI-OH-” see O. D. Thomas, K. J. W. Y. Soo, T. J. Peckham, M. P. Kulkarni, S. Holdcroft, J. Am. Chem. Soc. 2012, 134, 10753–10756; for “HMT-PDMBI-OH-” see A. G. Wright, S. Holdcroft, ACS Macro Lett. 2014, 444–447).

Figure 10. Chemical structures of stable AAEMs from our group.