Dustin Banham

Effect of carbon support nanostructure on the oxygen reduction activity of Pt/C catalysts

This work is focussed on establishing the effect of the nanostructure of a series of ordered mesoporous carbon (OMC) support materials, after Pt loading, on the oxygen reduction reaction (ORR) performance, for application in proton exchange membrane fuel cells (PEMFCs). Hexagonal mesoporous silica (HMS) templates were prepared using alkylamine surfactants with varying carbon chain lengths, producing wormhole pore diameters of 1.5–3.1 nm and silica wall thicknesses of ca. 2.3 nm. The HMS pores were then filled either with sucrose or an aromatic carbon precursor (anthracene or naphthalene), followed by carbonization and removal of the HMS in NaOH, leaving behind an interconnected carbon structure (“nano-strings”), 1.5–3.1 nm in diameter. These OMCs all have a similar, bimodal, pore size distribution, with the smaller pores (1.8 nm) attributed to removal of the HMS walls and the larger pores (∼3.5 nm) arising from incomplete filling of the HMS pores with carbon precursor. The OMCs were loaded with 20 wt% Pt, resulting in very similar Pt particle sizes (ca. 5 nm), as confirmed by XRD, TEM, and electrochemical surface area measurements. The ORR activity was found to decrease as the carbon nano-string diameter decreased, proposed to be due to a higher electronic resistance, while the degree of OMC graphitization, determined by XRD analysis, had only a minor impact on the ORR activity.

First time investigation of Pt nanocatalysts deposited inside carbon mesopores of controlled length and diameter

This work represents the first-reported synthesis and evaluation of Pt-loaded colloid-imprinted carbon (CIC) supports consisting of a porous shell surrounding a solid core, for use as catalysts for cathodes in proton exchange membrane fuel cells (PEMFCs). Increasing the temperature of imprinting during the synthesis of CIC supports, from 250 to 400 °C, gave a five times increase in porous shell thickness (average pore depth), as confirmed by gas sorption and transmission electron microscopy studies. The CIC supports, all with a 26 nm pore diameter, were loaded with 20 wt% Pt and characterized with 3D transmission electron microscopy and electron tomography, showing that the Pt nanoparticles are uniformly deposited throughout the CIC pores. Using 3-electrode electrochemistry in 0.5 M H2SO4, it was found that the rate (per active Pt surface area) of the oxygen reduction reaction is independent of the pore length, with no transport limitations encountered. This demonstrates that full utilization of both the Pt and the CIC internal surface area was achieved under these experimental conditions, which promises benefits in terms of enhanced Pt utilization, thus lowering the cost and improving the durability of PEMFCs. Furthermore, this work has opened up an entirely new direction for fuel cell catalyst layer design by allowing the controlled modification of both carbon support pore diameter and pore length, also of relevance for battery and capacitor applications.

Is the rapid initial performance loss of Fe/N/C non precious metal catalysts due to micropore flooding?

The activity of non-precious metal catalysts (NPMCs) has now reached a stage at which they can be considered as possible alternatives to Pt for some proton exchange membrane fuel cell (PEMFC) applications. However, challenges still remain in achieving acceptable stability (performance during potentiostatic or galvanostatic experiments). The most widely reported hypotheses for the instability of NPMCs include de-metalation, protonation/anion binding, and generation of H2O2. Recently, it has been proposed that the largest contribution to the instability of NPMCs is from flooding of micropores within the catalyst particles leading to significant mass transport limitations. While indirect evidence has been obtained that appears to support this hypothesis, no study has yet been performed to directly target micropore flooding. In this work, a systematic study is performed to investigate micropore flooding in situ before and after stability testing. The results do not support micropore flooding as being a large contributor to instability, at least for the family of NPMCs evaluated in this work. The protocol outlined here can be used by other researchers in the NPMC community to diagnose micropore flooding in their own respective catalysts.

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.

Atomic layer deposited tantalum oxide to anchor Pt/C for a highly stable catalyst in PEMFCs

Tantalum oxide (TaOx) nanoparticles (NPs) are deposited on a commercial Pt/C catalyst by an area-selective atomic layer deposition (ALD) approach to enhance the stability of the catalyst in proton exchange membrane fuel cells (PEMFCs). Due to the application of a blocking agent for protecting the Pt surface, TaOx particles are selectively nucleated and grown around Pt NPs. The TaOx loading on the Pt/C surface could be controlled precisely by varying the number of ALD cycles. When deposited on the Pt/C surface with 35 ALD cycles, the TaOx-anchored Pt NPs formed an excellent triple-junction structure of TaOx–Pt–carbon. The electrochemical durability tests indicated that the TaOx-anchored Pt/C catalyst showed comparable catalytic activity and superior long-term stability to Pt/C. Moreover, the long-term stability test in membrane electrode assembly (MEA) indicated a very low power density loss (12%) after a 120 h accelerated durability test. The significantly enhanced catalyst stability during PEMFCs operation is due to the anchoring effect of TaOxvia strong metal oxide–support interactions. This strategy shows great potential for developing highly stable catalysts for PEMFCs.

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.

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

The first commercially viable hydrogen/air performance for a nonprecious metal catalyst–based PEMFC is demonstrated. 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.