Titichai Navessin

On the Micro-, Meso-, and Macroporous Structures of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

In this work, N(2) adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer-Emmett-Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles (<2 nm), mesopores ascribed to the void space inside agglomerates (2-20 nm), and meso- to macroporous space inside aggregates of agglomerates (>50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume, in contrast to Vulcan XC-72, for which the corresponding fraction of micropores was 15% of the total pore volume. The microstructure of the carbon support was found to be a significant factor in the formation of the microstructure in the three-component CLs, serving as a rigid porous framework for distribution of platinum and the ionomer. It was found that platinum particle deposition on Ketjen Black occurs in, or at the mouth of, the supports micropores, thus affecting its effective microporosity, whereas platinum deposition on Vulcan XC-72 did not significantly affect the supports microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm, which are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells I. Theoretical Modeling

The effect of Nafion loading on the electrode polarization characteristics of a proton exchange membrane fuel cell is studied with a macrohomogeneous model. The composition dependence of performance is rationalized by first relating mass fractions of the different components to their volume fractions and thereafter involving concepts of percolation theory to parameterize effective properties of the cathode catalyst layers. In particular, we explore systematically the effect of Nafion content on the performance. For a uniform layer, the best performance is obtained with a Nafion content of about 35 wt %, representing an optimum balance of proton transport, oxygen diffusion, and electrochemically active surface area. With the help of this modeling tool, we propose a nonuniform Nation catalyst layer and the modeling indicates that such a layer improves performance. Our preliminary experiments (to appear in Part II) confirm this claim. The two cases of nonuniform Nation distribution across the entire thickness include: a three-sublayer structure with equally thick layers, simulating a constant gradient, and a two-sublayer structure with variable thickness of the sublayers. Compared with the optimum Nafion content (35 wt %) in uniform distribution, the three-sublayer structure with higher Nation content on the membrane side exhibits significantly enhanced performance.

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells II. Experimental Study of the Effect of Nafion Distribution

Gas diffusion electrodes (GDEs) containing a graded distribution of Nafion were prepared and characterized, and their performance as fuel cell cathodes compared to GDEs possessing a uniform distribution of Nafion. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and porosimetry are used to characterize the variations in electrochemical properties, ionic conductivity, and microstructures. The cathodic performance was improved over uniform electrodes at intermediate and high levels of polarization when the Nafion content in the GDE was higher toward the catalyst layer/membrane interface and lower toward the catalyst layer/carbon paper interface since this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Fuel cell performance is much poorer when the gradient of Nafion content is reversed, i.e., highest at the catalyst layer/carbon paper interface since this distribution disfavors proton and gas transport in the regions where they need to be maximized.

Highly corrosion resistant platinum–niobium oxide–carbon nanotube electrodes for the oxygen reduction in PEM fuel cells

Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometre interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles from 0.5 V to 1.4 V, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3%, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.

PEMFC Catalyst Layers: The Role of Micropores and Mesopores on Water Sorption and Fuel Cell Activity

The effects of carbon microstructure and ionomer loading on water vapor sorption and retention in catalyst layers (CLs) of PEM fuel cells are investigated using dynamic vapor sorption. Catalyst layers based on Ketjen Black and Vulcan XC-72 carbon blacks, which possess distinctly different surface areas, pore volumes, and microporosities, are studied. It is found that pores <20 nm diameter facilitate water uptake by capillary condensation in the intermediate range of relative humidities. A broad pore size distribution (PSD) is found to enhance water retention in Ketjen Black-based CLs whereas the narrower mesoporous PSD of Vulcan CLs is shown to have an enhanced water repelling action. Water vapor sorption and retention properties of CLs are correlated to electrochemical properties and fuel cell performance. Water sorption enhances electrochemical properties such as the electrochemically active surface area (ESA), double layer capacitance and proton conductivity, particularly when the ionomer content is very low. The hydrophilic properties of a CL on the anode and the cathode are adjusted by choosing the PSD of carbon and the ionomer content. It is shown that a reduction of ionomer content on either cathode or anode of an MEA does not necessarily have a significant detrimental effect on the MEA performance compared to the standard 30 wt % ionomer MEA. Under operation in air and high relative humidity, a cathode with a narrow pore size distribution and low ionomer content is shown to be beneficial due to its low water retention properties. In dry operating conditions, adequate ionomer content on the cathode is crucial, whereas it can be reduced on the anode without a significant impact on fuel cell performance.

Influence of Membrane Ion Exchange Capacity on the Catalyst Layer Performance in an Operating PEM Fuel Cell

The effect of ion exchange capacity (IEC) of polymer electrolyte membranes (PEMs) on cathode catalyst layer operation is investigated using a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC) and a series of tetrafluoroethylene-g-polystyrene sulfonic acid (ETFE-g-PSSA) membranes. The electrochemically active surface area (ESA) of the catalyst layer reveals a slight dependence on IEC. The steady-state beginning-of-life polarization curves show an increase in fuel cell performance with increased IEC. The membranes IEC and molecular structure controls the water content within, and regulates the water balance in the complete MEA. Comparing half-fuel-cell and fuel cell systems reveals that the ESA in the latter is lower as a result of reduced wetting of the catalyst layer but this is offset by an order of magnitude improvement of the effective O 2 diffusion. Consequently oxygen reduction reaction (ORR) performance is higher in the fuel cell system. The balance between electro-osmotic flux and hydraulic counterflux in the membrane is employed to explain the distinct effects of IEC in the half fuel cell and fuel cell systems. The two types of measurements thus provide a convenient tool for studying the interplay of different mechanisms of water flux in the membrane.

Factors Influencing Electrochemical Properties and Performance of Hydrocarbon-Based Electrolyte PEMFC Catalyst Layers

Cathode catalyst layers (CLs) for proton exchange membrane fuel cells (PEMFCs) incorporating sulfonated poly(ether ether ketone) (SPEEK) solid polymer electrolyte were prepared and studied in a H 2 /O 2 fuel cell operated at 50°C and 100% relative humidity. SPEEK-based CLs were found to exhibit higher protonic resistance, lower effective usage of Pt, and lower fuel cell performance compared to Nafion-based cathodes. A method of fabrication was developed to achieve a homogeneous distribution of SPEEK and polytetrafluoroethylene (PTFE) throughout the catalyst layer. Homogeneously prepared SPEEK-based CLs exhibited a higher electrochemically active surface area and lower protonic resistance but lower porosity and inferior water management compared to those prepared using a traditional two-step fabrication method, wherein SPEEK ionomer was impregnated into a preformed catalyst layer incorporating sintered PTFE and Pt/C. The choice of SPEEK electrolyte over Nafion was shown to adversely affect the bulk proton conductivity of the electrolyte inside the catalyst layer. This can be offset by increasing the SPEEK content in the catalyst layer but with the penalty of increased flooding and a larger resistance to gas transport.