Micromodification of the Catalyst Layer by CO to Increase Pt Utilization for Proton-Exchange Membrane Fuel Cells.

Improving the utilization of platinum in proton-exchange membrane (PEM) fuel cells is critical to reducing their cost. In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed. However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level. The failure of these direct translations from RDE to MEA has been well documented with several key reasons having been previously identified. One of them is the resistance caused by complex mass transport pathways in the MEA. Herein, we improve the proton and oxygen transportations in the MEA by building a thin and uniform distribution of ionomer on the catalyst surface. As a result, a PEM fuel cell design is capable of showing a current density improvement of 38% at the same voltage (0.6 V) under the H2/air operation.

Native Ligand Carbonization Renders Common Platinum Nanoparticles Highly Durable for Electrocatalytic Oxygen Reduction: Annealing Temperature Matters.

Current protocols for synthesizing monodisperse platinum (Pt) nanoparticles typically involve the use of hydrocarbon molecules as surface-capping ligands. Using Pt nanoparticles as catalysts for the oxygen reduction reaction (ORR), however, these ligands must be removed to expose surface sites. Here, highly durable ORR catalysts are realized without ligand removal; instead, the native ligands are converted into ultrathin, conformal graphitic shells by simple thermal annealing. Strikingly, the annealing temperature is a critical factor dictating the ORR performance of Pt catalysts. Pt nanoparticles treated at 500 °C show a very poor ORR activity, whereas those annealed at 700 °C become highly active along with exceptional stability. In-depth characterization reveals that thermal treatment from 500 to 700 °C gradually opens up the porosity in carbon shells through graphitization. Importantly, such graphitic-shell-coated Pt catalysts exhibit a superior ORR stability, largely retaining the activity after 20 000 cycles in a membrane electrode assembly. Moreover, this ligand carbonization strategy can be extended to modify commercial Pt/C catalysts with substantially enhanced stability. This work demonstrates the feasibility of boosting the ORR performance of common Pt nanoparticles by harnessing the native surface ligands, offering a robust approach of designing highly durable catalysts for proton-exchange-membrane fuel cells.

Integrating PGM-Free Catalysts into Catalyst Layers and Proton Exchange Membrane Fuel Cell Devices.

While proton exchange membrane fuel cells (PEMFCs) continue to expand into commercial markets, there is still pressure to decrease cost. One of the largest opportunities to reducing cost is to reduce the amount of platinum-group metal (PGM) catalysts used in the electrodes (particularly the cathode). Over the past decade, exciting advances in the Fe/N/C family of PGM-free oxygen reduction reaction (ORR) catalysts has provided great optimism that not only can PGMs at the cathode be reduced but possibly be completely eliminated. In fact, in September 2017, Ballard Power Systems announced the commercialization of the world's first PEMFC product to utilize a PGM-free catalyst at the cathode (FCgen-micro (non-precious-metal catalyst, NPMC)). However, for these catalysts to be used in more demanding applications, an improved understanding and new design approaches for PGM-free catalyst layers will be required. Herein, some of the latest research on both modeling and experimental studies in the field of PGM-free catalyst layer research are discussed. In addition, a short discussion on Ballard's new NPMC is provided.

Critical advancements in achieving high power and stable nonprecious metal catalyst-based MEAs for real-world proton exchange membrane fuel cell applications.

Despite great progress in the development of nonprecious metal catalysts (NPMCs) over the past several decades, the performance and stability of these promising catalysts have not yet achieved commercial readiness for proton exchange membrane fuel cells (PEMFCs). Through rational design of the cathode catalyst layer (CCL), we demonstrate the highest reported performance for an NPMC-based membrane electrode assembly (MEA), achieving a peak power of 570 mW/cm2 under air. This record performance is achieved using a precommercial catalyst for which nearly all pores are <3 nm in diameter, challenging previous beliefs regarding the need for larger catalyst pores to achieve high current densities. This advance is achieved at industrially relevant scales (50 cm2 MEA) using a precommercial NPMC. In situ electrochemical analysis of the CCLs is also used to help gain insight into the degradation mechanism observed during galvanostatic testing. Overall, the performance of this NPMC-based MEA has achieved commercial readiness and will be introduced into an NPMC-based product for portable power applications.

Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups.

The in situ diazonium reduction reaction is a reliable and well-known approach for the surface modification of carbon materials for use in a range of applications, including in energy conversion, as chromatography supports, in sensors, etc. Here, this approach was used for the first time with mesoporous colloid-imprinted carbons (CICs), materials that contain ordered monodisperse pores (10-100 nm in diameter) and are inherently highly hydrophilic, using a common microporous carbon (Vulcan carbon (VC)), which is relatively more hydrophobic, for a comparison. The ultimate goal of this work was to modify the CIC wettability without altering its nanostructure and also to lower its susceptibility to oxidation, as required in fuel cell and battery electrodes, by the attachment of pentafluorophenyl (-PhF5) groups onto their surfaces. This was shown to be successful for the CIC, with the -PhF5 groups uniformly coating the inner pore walls at a surface coverage of ca. 90% and allowing full solution access to the mesopores, while the -PhF5 groups deposited only on the outer VC surface, likely blocking its micropores. Contact angle kinetics measurements showed enhanced hydrophobicity, as anticipated, for both the -PhF5 modified CIC and VC materials, even revealing superhydrophobicity at times for the CIC materials. In contrast, water vapor sorption and cyclic voltammetry suggested that the micropores remained hydrophilic, arising from the deposition of smaller N- and O-containing surface groups, caused by a side reaction during the in situ diazonium functionalization process.

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.