Christina M. Johnston

High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt

Fuel cell catalysts synthesized from abundant metals approach the performance and durability of platinum at lower cost. The prohibitive cost of platinum for catalyzing the cathodic oxygen reduction reaction (ORR) has hampered the widespread use of polymer electrolyte fuel cells. We describe a family of non–precious metal catalysts that approach the performance of platinum-based systems at a cost sustainable for high-power fuel cell applications, possibly including automotive power. The approach uses polyaniline as a precursor to a carbon-nitrogen template for high-temperature synthesis of catalysts incorporating iron and cobalt. The most active materials in the group catalyze the ORR at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supported platinum, combining their high activity with remarkable performance stability for non–precious metal catalysts (700 hours at a fuel cell voltage of 0.4 volts) as well as excellent four-electron selectivity (hydrogen peroxide yield <1.0%).

Synthesis-structure-performance correlation for polyaniline-Me-C non-precious metal cathode catalysts for oxygen reduction in fuel cells

In this report, we present the systematic preparation of active and durable non-precious metal catalysts (NPMCs) for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temperature, metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphology. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst’s degree of graphitization through heat treatment versus the excessive loss of surface area that occurs at higher temperatures is a key to preparing an active catalyst. XPS and XAFS spectra are consistent with the presence of Me–Nx structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The average speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.

A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability

A non-precious metal catalyst for oxygen reduction in acid media, enriched in graphene sheets/bubbles during a high-temperature synthesis step, has been developed from an Fe precursor and in situ polymerized polyaniline, supported on multi-walled carbon nanotubes. The catalyst showed no performance loss for 500 hours in a hydrogen/air fuel cell. The improved durability is correlated with the graphene formation, apparently enhanced in the presence of carbon nanotubes.

Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells

Hydrogen produced from water and renewable energy could fuel a large fleet of proton-exchange-fuel-cell vehicles in the future. However, the dependence on expensive Pt-based electrocatalysts in such fuel cells remains a major obstacle for a widespread deployment of this technology. One solution to overcome this predicament is to reduce the Pt content by a factor of ten by replacing the Pt-based catalysts with non-precious metal catalysts at the oxygen-reducing cathode. Fe- and Co-based electrocatalysts for this reaction have been studied for over 50 years, but they were insufficiently active for the high efficiency and power density needed for transportation fuel cells. Recently, several breakthroughs occurred that have increased the activity and durability of non-precious metal catalysts (NPMCs), which can now be regarded as potential competitors to Pt-based catalysts. This review focuses on the new synthesis methods that have led to these breakthroughs. A modeling analysis is also conducted to analyze the improvements required from NPMC-based cathodes to match the performance of Pt-based cathodes, even at high current density. While no further breakthrough in volume-specific activity of NPMCs is required, incremental improvements of the volume-specific activity and effective protonic conductivity within the fuel-cell cathode are necessary. Regarding durability, NPMCs with the best combination of durability and activity result in ca. 3 times lower fuel cell performance than the most active NPMCs at 0.80 V. Thus, major tasks will be to combine durability with higher activity, and also improve durability at cell voltages greater than 0.60 V.

Aqueous-based synthesis of ruthenium–selenium catalyst for oxygen reduction reaction

Carbon-supported Se/Ru(Se) catalysts of a broad range of composition were synthesized via a reduction procedure in which a mixture of RuCl3, SeO2 and Black Pearl carbon was treated with NaBH4 in basic media at room temperature. Physical characterization of the catalyst was performed by X-ray diffraction, energy dispersive X-ray spectroscopy and by high resolution transmission electron microscopy. The effect of NaOH addition during the reduction by NaBH4 and the impact of a post-reduction thermal treatment at 500 degrees C were interrogated. The activity of the catalyst towards the oxygen reduction reaction was studied by the use of a rotating disk electrode. It was found that the half-wave potential for the oxygen reduction reaction was about 0.78 V vs. RHE. The Se-to-Ru ratio and metal loading on carbon were optimized for the oxygen reduction reaction and the optimized catalyst was tested at the cathode of a polymer electrolyte fuel cell. The stability of the Se/Ru(Se) catalyst was evaluated by electrochemical cycling and by leaching the catalyst in 0.5 M H2SO4 at 80 degrees C.

Synthesis and Evaluation of Heat-treated, Cyanamide-derived Non-precious Catalysts for Oxygen Reduction

Although polymer electrolyte fuel cells (PEFCs) have long been recognized as a potential power source for zero-emission vehicles, they remain economically uncompetitive with internal combustion engines. The necessary cost reductions can only be achieved by greatly lowering the content or eliminating the platinum catalyst required for the oxygen reduction reaction (ORR) at the cathode, the electrode process that demands substantially higher Pt loadings than the much faster hydrogen oxidation reaction (HOR) at the anode.

A Non-Precious Electrocatalyst for Oxygen Reduction Based on Simple Heat-Treated Precursors

Some active non-precious catalyst was made by simple heat treated novel precursors. The power density obtained in the H{sub 2}/O{sub 2} fed PEM fuel cells is as high as 0.22 W/cm{sup 2}{sub MEA} at 0.65 V. The active site seems to have little relevance with the nitrogen. We are now working on to figure out the active site(s) in this catalyst.

Graphene-Riched Co9S8-N-C Non-Precious Metal Catalyst for Oxygen Reduction in Alkaline Media

In this work, a non-precious metal catalyst consisting of Co9S8 nanoparticles surrounded with nitrogen-doped graphene-like carbon (Co9S8-N-C) was developed for oxygen reduction in alkaline media. Improved activity has been measured with the Co9S8-N-C catalyst relative to Pt/C and a non-precious metal catalyst based on Fe instead of Co (Fe-N-C). An extensive physical characterization, including XRD, SEM, TEM, and XPS and electrochemical kinetic analysis was conducted to provide insight into the catalyst morphology and structure.

Anode Catalysts for the Direct Dimethyl Ether Fuel Cell

For the last decade, dimethyl ether (DME) has been considered a promising alternative fuel for direct fuel cells because of several advantages over methanol. In this work, PtRu catalysts with a wide range of Pt-to-Ru ratios were screened for the direct DME fuel cell (DDMEFC) performance for the first time. Overall, Pt50Ru50 performs best in the high and middle voltage ranges, whereas Pt80Ru20 performs best at low voltages. A maximum power density of 0.10 W/cm 2 is achieved at a current density of 0.37 A/cm 2 . In spite of using gaseous DME feed, the measured performance exceeds the best previously published results. The anode activity and fuel crossover of a DDMEFC were investigated in this work and compared to those of a DMFC. The adsorption and electrooxidation of DME on Pt black catalyst were also studied in halfcell electrochemical measurements.

Spontaneous Deposition of Noble Metal Films onto Hexaboride Surfaces

The use of the spontaneous deposition of metals onto surfaces is widespread in industry and research. Here, we report that metal hexaborides (e.g., LaB 6 , YB 6 , EuB 6 ) spontaneously deposit noble metals from solution to form robust layers. We observed spontaneous deposition of noble metals from inorganic salts or acids onto a variety of hexaboride compounds. No pretreatment of the hexaboride is necessary, and noble metal depositions ranging from complete and highly uniform nanometer-scale layers to relatively heavy mirrorlike coatings are easily obtained. We find that the hexaborides both facilitate the formation of and sustain the electrocatalytic activity of nanolayer platinum depositions. To provide such activity and dispersion, the hexaboride supports appear to have unusual promotion and adhesion qualities, possibly due to their low work functions, which are 2-3 eV lower than the noble metals. This variety of unique material and process properties may have benefits in electrocatalysis, heterogeneous catalysis, electronic devices, and coatings.