Shanna Knights

High oxygen-reduction activity and durability of nitrogen-doped graphene

Nitrogen-doped graphene as a metal-free catalyst for oxygen reduction was synthesized by heat-treatment of graphene using ammonia. It was found that the optimum temperature was 900 °C. The resulting catalyst had a very high oxygen reduction reaction (ORR) activity through a four-electron transfer process in oxygen-saturated 0.1 M KOH. Most importantly, the electrocatalytic activity and durability of this material are comparable or better than the commercial Pt/C (loading: 4.85 µgPt cm−2). XPS characterization of these catalysts was tested to identify the active N species for ORR.

Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.

Extremely Stable Platinum Nanoparticles Encapsulated in a Zirconia Nanocage by Area‐Selective Atomic Layer Deposition for the Oxygen Reduction Reaction

Encapsulation of Pt nanoparticles (NPs) in a zirconia nanocage by area-selective atomic layer deposition (ALD) can significantly enhance both the Pt stability and activity. Such encapsulated Pt NPs show 10 times more stability than commercial Pt/C catalysts and an oxygen reduction reaction (ORR) activity 6.4 times greater than that of Pt/C.

Multigrain Platinum Nanowires Consisting of Oriented Nanoparticles Anchored on Sulfur‐Doped Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst

Direct growth of multigrain platinum nanowires on sulfur-doped graphene (PtNW/SG) is reported. The growth mechanism, including Pt nanoparticle nucleation on SG, followed by nanoparticle attachment with orientation along the <111> direction is highlighted. PtNW/SG demonstrates improved Pt mass and specific activity compared with commercial catalysts toward oxygen reduction, in addition to dramatically improved stability through accelerated durability testing.

Ionomer Degradation in Polymer Electrolyte Membrane Fuel Cells

The structural degradation of the polymer electrolyte in both the bulk membrane and the cathode catalyst layer (CL) was investigated. An accelerated stress test (AST) was developed to degrade the ionomer in two membrane electrode assembly (MEA) designs, with cathode catalyst structures comprised of 23 and 33 wt % Nafion, respectively. During the AST the air cathode potential was held at 1.0 V RHE (where RHE is reference hydrogen electrode) at 90°C and 100% relative humidity for up to 440 h. The MEA with 33 wt % Nafion had a greater platinum content in the membrane and a higher fluoride washout rate, suggesting the higher ionomer content in the cathode CL facilitated the mass transfer of contaminants (such as dissolved platinum) into the membrane. It is proposed that H 2 O 2 was produced at the anode, diffused into the membrane, and decomposed at the platinum and iron sites bound in the membrane structure. The decomposition products attacked the ionomer both in the bulk phase and CL, causing (i) membrane thinning, which exacerbated the H 2 crossover, (ii) lower membrane conductivity, and (iii) CL structure degradation manifested by enhanced reaction penetration depth into the CL and decreased effective oxygen diffusivity due to the changes in CL water content. These effects acting in synergy had profound negative repercussions on the fuel cell polarization for the MEA with 33 wt % Nafion in the cathode CL.

Transient Analysis of Hydrogen Sulfide Contamination on the Performance of a PEM Fuel Cell

A transient kinetic model for the anode catalyst layer of a proton exchange membrane (PEM) fuel cell is proposed to describe the performance loss introduced by the hydrogen sulfide contaminant. Reaction rates are considered as functions of cell current density and contamination level and are estimated based on the available experimental data. It is found that at a constant cell current density the surface coverage of Pt-S increases faster with time and the anode overpotential rises sharply when increasing the contamination level from 1 to 10 ppm, leading to a faster and more severe cell performance degradation. At a constant contamination level, the surface coverage of Pt-S also increases faster with time when increasing the cell current density from 0.1 to 1.0 A cm -2 , resulting in faster and more severe cell performance degradation. Simulation shows that the contaminant surface coverage at steady state is governed by current density. With the same contaminant concentration, to maintain a higher current density output, a significant decrease of steady-state contaminant coverage is required, while at a lower current density, steady-state contaminants coverage increases significantly.

3D boron doped carbon nanorods/carbon-microfiber hybrid composites: synthesis and applications in a highly stable proton exchange membrane fuel cell

Boron-doped carbon nanorods (BCNRs) were directly grown on carbon-microfiber by the spray pyrolysis chemical vapour deposition method. The stability of the deposited Pt nanoparticles was found to be increased by more than three times with substitutional boron dopants in 3D carbon nanomaterials. Our work will be of great technological significance for developing highly stable electrode materials in fuel cells.

3D Porous Fe/N/C Spherical Nanostructures As High-Performance Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media

Exploring inexpensive and high-performance nonprecious metal catalysts (NPMCs) to replace the rare and expensive Pt-based catalyst for the oxygen reduction reaction (ORR) is crucial for future low-temperature fuel cell devices. Herein, we developed a new type of highly efficient 3D porous Fe/N/C electrocatalyst through a simple pyrolysis approach. Our systematic study revealed that the pyrolysis temperature, the surface area, and the Fe content in the catalysts largely affect the ORR performance of the Fe/N/C catalysts, and the optimized parameters have been identified. The optimized Fe/N/C catalyst, with an interconnected hollow and open structure, exhibits one of the highest ORR activity, stability and selectivity in both alkaline and acidic conditions. In 0.1 M KOH, compared to the commercial Pt/C catalyst, the 3D porous Fe/N/C catalyst exhibits ∼6 times better activity (e.g., 1.91 mA cm-2 for Fe/N/C vs 0.32 mA cm-2 for Pt/C, at 0.9 V) and excellent stability (e.g., no any decay for Fe/N/C vs 35 mV negative half-wave potential shift for Pt/C, after 10000 cycles test). In 0.5 M H2SO4, this catalyst also exhibits comparable activity and better stability comparing to Pt/C catalyst. More importantly, in both alkaline and acidic media (RRDE environment), the as-synthesized Fe/N/C catalyst shows much better stability and methanol tolerance than those of the state-of-the-art commercial Pt/C catalyst. All these make the 3D porous Fe/N/C nanostructure an excellent candidate for non-precious-metal ORR catalyst in metal-air batteries and fuel cells.

Relative Humidity Effect on Anode Durability in PEMFC Startup/Shutdown Processes

Ru dissolution and crossover to the cathode from PtRu anode catalysts, which are commonly used in polymer electrolyte membrane fuel cells (PEMFCs) to provide CO tolerance for reformate-based fuel cell applications, have been previously identified as critical durability issues. In this investigation, the effect of relative humidity (RH) on Ru dissolution and crossover was studied using an anode accelerated stress test (AST) to mimic anode potential variation that occur during fuel cell start-ups and shut-downs. Stress testing at lower RH resulted in less Ru degradation, which was indicated by changes in cyclic voltammetry (CV), namely CO stripping peak shifts. The relative degree of Ru crossover was also reflected by the decreases in cell performance and CO tolerance. The results highlight the critical role of water in the Ru degradation mechanism and indicate that controlling the RH could be an effective strategy to mitigate Ru crossover.

PEMFC MEA and System Design Considerations

Proton exchange membrane fuel cells (PEMFCs) are being developed and sold commercially for multiple near term markets. Ballard Power Systems is focused on the near term markets of backup power, distributed generation, materials handling, and buses. Significant advances have been made in cost and durability of fuel cell products. Improved tolerance to a wide range of system operation and environmental noises will enable increased viability across a broad range of applications. In order to apply the most effective membrane electrode assembly (MEA) design for each market, the system requirements and associated MEA failures must be well understood. The failure modes associated with the electrodes and membrane degradation are discussed with respect to associated system operation and mitigating approaches. A few key system considerations that influence MEA design include expected fuel quality, balance-of-plant materials, time under idle or open circuit operation, and start-up and shut-down conditions.