Gabriel A. Goenaga

Cobalt imidazolate framework as precursor for oxygen reduction reaction electrocatalysts.

We demonstrate a new approach of preparing a non-platinum group metal (PGM) electrocatalyst for oxygen reduction reaction through rational design by using cobalt imidazolate framework—a subclass of metal-organic framework (MOF) material—as the precursor with potential to produce uniformly distributed catalytic center and high active-site density. MOFs represent a new type of materials, and have recently been under broad exploration of various important applications due to their amenability to rational design for different functionalities at molecular level. In particular, their high surface areas, well-defined porous structures, and building block variety not only distinguish them from the conventional materials in gas adsorption and separation, but also offer new promises in catalysis application. However, the application of porous MOFs for electrocatalysis in fuel cell has yet to be exploited. The oxygen reduction reaction (ORR) at the cathode of a proton exchange membrane fuel cell (PEMFC) represents a very important electrocatalytic reaction. At present, the catalyst materials of choice are platinum group metals (PGMs). The high costs and limited reserves of PGMs, however, created a major barrier for large-scale commercialization of PEMFCs. Intensive efforts have been dedicated to the search of low-cost alternatives. The discovery of ORR activity on cobalt phthalocyanine stimulated extensive investigations of using Co–N4 or Fe–N4 macromolecules as precursors for preparation of transition metal (TM) based, non-PGM catalysts. The ORR activity over a cobalt–polypyrrole composite was observed, of which a Co ligated by pyrrolic nitrogens was proposed as the catalytic site. Activation in an inert atmosphere of the similar TM– polymer composite through pyrolysis further improved the catalytic activity. More recently, significant enhancement in ORR activity was demonstrated in a carbon-supported iron-based catalysts, and it was suggested that micropores (width <20 ) have critical influence on the formation of the active site with an ionic Fe coordinated by four pyridinic nitrogens after high-temperature treatment. The onset potential for an Fe-based catalyst is found to be 0.1 V higher than that of a Co-based system although the latter is more stable under PEMFC operating condition. These previous studies proposed the nitrogen-ligated TM entities either as the precursors or the active centers for the catalytic ORR process. Another challenge for non-PGM ORR catalysts is their relatively low turn-over-frequency in comparison with Pt. To compensate low activity without using excessive amount of catalyst, thus causing thick electrode layer and poor mass transport, it is desirable to produce the highest possible catalytic-site density, that are evenly distributed and accessible to gas diffusion through a porous framework. Herein we report the first experimental demonstration of porous MOF as a new class of precursor for preparing ORR catalysts. Different from previous approaches, MOFs have the following advantages when used to prepare non-PGM electrocatalysts: MOFs have clearly-defined three-dimensional structures. The initial entities such as TM–N4 can be grafted into MOFs with the highest possible volumetric density through regularly arranged cell structure. The MOF surface area and pore size are tunable by the length of the linker. The organic linkers would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need of a second carbon support or pore forming agent. Furthermore, the TM–ligand composition can be rationally designed with wide selection of metal–linker combinations for systematical investigation on the relationship between precursor structure and catalyst activity. Our studies demonstrate the initial step to achieve such advantages. [a] Dr. S. Ma, Dr. G. A. Goenaga, Dr. D.-J. Liu Chemical Sciences & Engineering Division Argonne National Laboratory, Argonne, IL 60439 (USA) Fax: (+1) 630-252-4176 E-mail : djliu@anl.gov [b] A. V. Call Department of Materials Science and Engineering Northwestern University, Evanston, IL 60208 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201003080.

New Approaches to Non-PGM Electrocatalysts Using Porous Framework Materials

The catalytic oxygen reduction reaction (ORR) at the cathode is a critical process to proton exchange membrane fuel cell (PEMFC) operation. The current catalyst materials of choice are platinum group metals (PGMs) with high costs and limited reserves. Reported herein are our recent efforts in developing non-PGM electrocatalyst materials using rational design and synthesis. A variety of porous organic materials were developed as the catalyst precursors for preparing of ORR catalysts with high surface area and active site density free of carbon support. Electrocatalytic activities and physical properties of the new catalysts were investigated by various techniques in the process of understanding of the active site formation.

Polymer Electrolyte Fuel Cell with Vertically Aligned Carbon Nanotubes as the Electrocatalyst Support

We successfully prepared membrane electrode assemblies (MEAs) for polymer electrolyte fuel cells using aligned carbon nanotubes (ACNTs) as the cathode catalyst support and evaluated their performances in single-cell tests. The ACNT-based MEAs demonstrated improved performance over a commercial carbon-black-based reference MEA in voltage-current polarization studies, particularly in the high current, mass-transport-limited region. The ACNT-MEAs also showed a significantly higher stability in conductivity and corrosion resistance than that of the reference during an accelerated aging test through potential cycling, which can be attributed to the vertical alignment and high graphiticity of the carbon nanotubes.

Performance improvement of fuel cells using platinum-functionalised aligned carbon nanotubes

The focus of this research was towards the improvement of the performance of proton exchange membrane fuel cells. The overarching goals were: (1) providing guidelines for design of new catalysts; (2) promoting nanocatalyst applications towards alternative energy applications; and (3) integrating advanced instrumentation into nanocharacterisation and fuel cell (FC) electrochemical behaviour. In tandem with these goals, the cathode catalysts were extensively refined to improve FC performance and minimise noble metal usage. In this study, the major accomplishment was producing aligned carbon nanotubes (ACNT), which were then modified by platinum (Pt) nanoparticles via a post-synthesis colloidal chemistry approach. The Pt-ACNTs demonstrated improved cathodic catalytic activity, as a result of incorporation of the nanotubes with the additional advantage of decreased Pt loading. It was also determined that surface mechanical properties, such as elastic modulus and hardness were increased. Collectively, these enhancements provided an improved FC performance.

Performance Improvement in PEMFC using Aligned Carbon Nanotubes as Electrode Catalyst Support

A novel membrane electrode assembly (MEA) using aligned carbon nanotubes (ACNT) as the electrocatalyst support was developed for proton exchange membrane fuel cell (PEMFC) application. A multiple-step process of preparing ACNT-PEMFC including ACNT layer growth and catalyzing, MEA fabrication, and single cell packaging is reported. Single cell polarization studies demonstrated improved fuel utilization and higher power density in comparison with the conventional, ink based MEA.

Investigation of a microporous iron(III) porphyrin framework derived cathode catalyst in PEM fuel cells

In Polymer Electrolyte Membrane Fuel Cells (PEMFCs) the thickness, structure and morphology of the electrode layer play an important role in the cell performance. This effect becomes particularly significant when the cathode catalyst is based on a non-precious metal due to the higher catalyst loadings required to compensate for the lower catalytic activity when compared to Pt based catalysts. In this study, an iron(III) porphyrin framework material was synthesized and pyrolyzed and its catalytic activity towards the oxygen reduction reaction (ORR) was evaluated using rotating disk electrode (RDE) experiments and single cell testing. Single cell performance was evaluated as a function of the electrode catalyst loading (electrode thickness) and oxygen partial pressure. As expected, the ORR kinetic overpotential was the major contributor to the overall voltage loss. However, the mass transport contribution to the voltage loss became more prominent with small increases in the cathode catalyst loading. The observed performance is discussed in the context of structure and morphology of the catalyst layer (CL), analyzed through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD).

Electrocatalytic Activity and Stability Enhancement through Preferential Deposition of Phosphide on Carbide

Phosphides and carbides are among the most promising families of materials based on earth‐abundant elements for renewable energy conversion and storage technologies such as electrochemical water splitting, batteries, and capacitors. Nickel phosphide and molybdenum carbide in particular have been extensively investigated for electrochemical water splitting. However, a composite of the two compounds has not been explored. Here, we demonstrate preferential deposition of nickel phosphide on molybdenum carbide in the presence of carbon by using a hydrothermal synthesis method. We employ the hydrogen evolution reaction in acid and base to analyze the catalytic activity of phosphide‐deposited carbide. The composite material also shows superior electrochemical stability in comparison to unsupported phosphide. We anticipate that the enhanced electrochemical activity and stability of carbide deposited with phosphide will stimulate investigations into the preparation of other carbide–phosphide composite materials.

Transport Properties of Perfluorosulfonate Membranes Ion Exchanged with Cations

In this work, the properties of univalent, that is, Li+, Na+, NH4+, and TEA+ form perfluorosulfonate (PFSA) membranes are studied and compared to the properties of H+ form materials. Properties of these polymer membranes including water uptake, density and conductivity, were investigated for membranes exposed to various water activity levels. The water uptake by the membranes decreased in the order H+ > Li+ > Na+ > NH4+ > TEA+, the same order as the hydration enthalpy (absolute values) of cations. Conductivity values did not strictly follow this order, indicating the importance of different factors besides the hydration level. The partial molar volume of water is derived from the density data as a function of water content for the various membrane forms. This provides further insight into the water, cation, and polymer interactions. Factors that contribute to the conductivity of these membranes include the size of cations, the electrostatic attraction between cations and sulfonate group, and the ion-dipole and hydrogen bonding interactions between cations and water. NH4+ transport is surprisingly high given the low water uptake in NH4+ form membranes. We attribute this to the ability of this ion to develop hydrogen bonded structures that helps to overcome electrostatic interactions with sulfonates. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was used to measure the diffusion coefficient of water in the membranes. FT-IR spectroscopy is employed to probe cation interactions with water and sulfonate sites in the polymer. Overall, the results reflect a competition between the strong electrostatic interaction between cation and sulfonate versus hydration and hydrogen bonding which vary with cation type.