Relative humidity and platinum content are crucial factors that affect proton and oxygen transport behavior in the heterogeneous architecture of the fuel cell cathode. There is a need to understand the concurrent effects of these factors on fuel cell performance, considering recent commercialization efforts aimed at reducing the costs of fuel cell systems. This body of work presents a systematic account of the effect of cathode platinum loading and operating relative humidity on a matrix of structure, property, and performance parameters. Results demonstrate that RH-sensitivity is governed by the parameters of the polarization domain in which the system is operated, and in turn, by the inherent material properties that drive these parameters. Results further suggest that MEAs with lower Pt loadings are less responsive to fluctuation in humidification, compared to high platinum-loaded counterparts, and in turn could offer a broader operating window or support the usage of less complex auxiliary humidification systems.

Porosity is one of the critical parameters governing mass transport of reagents and products in the heterogenous architecture of a fuel cell electrode. Techniques for the measurement thereof are required that are rapid, cost-effective, and simple, and yet capable of the highest levels of accuracy, precision, and stability. This body of work presents a comprehensive account of the measurement system analysis (MSA) of the densometer technique for the ex situ determination of total porosity and mean thickness of thin film porous materials by way of hydrostatic principles. The MSA involved a four phased approach which systematically tested several process assumptions before performing gage precision and accuracy studies and, finally, benchmarking of the system against several conventional industry techniques. Results confirmed statistically that the densometry technique, in conjunction with a standardized measurement procedure, can be used for the precise and accurate measurement of porosity as well as thickness across a representative range for porous materials deployed in fuel cells and similar technologies.

We report the synthesis of core-shell Ni-Pt nanoparticles (NPs) with varying degrees of crystallographic facets and surface layers rich in Pt via a seed-mediated thermolytic approach. Mixtures of different surfactants used during synthesis resulted in preferential surface passivation, which in turn dictated the size, chemical composition, and geometric evolution of these PtNi NPs. Electrochemical investigations of these pristine core-shell Ni-Pt structures in the oxygen reduction reaction (ORR) show that their catalytic functionalities outperform the commercial Pt/C reference catalyst. The enhanced electrocatalytic ORR performances of these Pt-based PtNi NPs are correlated with the weakened oxygen binding strength or surface-adsorbed hydroxyl (OH) species on active Pt surface sites induced by the downshift of the d-band center as a result of compressive strain effects. Our studies offer a robust synthetic approach for the development of core-shell nanostructures for enhanced ORR catalysis.

This work follows our recent study of titanium fibre felts applied as a cathode gas diffusion layer (GDL) in a proton exchange membrane fuel cell. Here, an in-house hydrophobic treatment of PTFE (0−20 wt%) is applied to these felts and the effects on cell performance, gas diffusion, and water transport are studied with comparison to conventional commercial carbon GDLs. The titanium fibre felt with a low PTFE loading of 5 wt% consistently outperformed the untreated substrate and all carbon GDLs tested across all six sets of operating conditions. This loading is shown to improve flooding resistance due to the increased hydrophobicity while not majorly reducing porosity, gas permeability, or ohmic resistance. The low loading also maintains a fraction of hydrophilic pathways within the substrate which has been shown to contribute to improved liquid water transport under wetter operating conditions. An increase in PTFE loading to 10 wt% and higher results in reduced porosity and permeability, and saturation of the internal structure of the substrate with PTFE resulting in poorer performance. Finally, the full range of treated and untreated felts are shown to exhibit a good balance between water retention, water rejection, and hydrophobicity based on oxygen transport resistance.

This work compares titanium fibre felts and conventional carbon gas diffusion layers (GDLs), varying in thickness, as cathode GDLs for polymer electrolyte membrane fuel cells (PEMFCs) in terms of i) overall performance, ii) mass transport properties, and iii) water management. The study presents a comprehensive approach to understanding GDL properties which affect performance. Titanium fibre felts exhibit improved performance across thicknesses and operating conditions, and both GDL types exhibit optimised performance at a thickness of 400 mm. Greater porosity and pore size of titanium fibre felts contribute to greater air permeability than carbon GDLs. Ohmic resistance for titanium fibre felts remains low across thicknesses, indicating good electrical conductivity of the titanium substrate and good interfacial contact with the catalyst layer. In contrast, ohmic resistance for carbon GDLs is higher and sensitive to thickness. Finally, flooding resistance is optimised at 400 mm for both GDL types, consistent with the observed performance. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Exploring platinum (Pt)-solute nanoparticles (NPs) with multifaceted geometries, multifunctionality, and extended stability is key for several electrocatalytic applications. Here, we demonstrate that Pt-based alloy NPs modified with molybdenum (Mo) exhibit superior electrocatalytic properties compared with pure Pt NPs. Electrocatalytic testing of these Mo-modified Pt nanoalloys shows excellent catalytic performance in the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), and the methanol oxidation reaction (MOR). We show that the catalytic functionalities in the ORR and the MOR achieved by the Mo-modified Pt nanoalloys are 15-25 times greater than the standard commercial Pt/C nanocatalyst. These nanoalloys also show enhanced resistance to poisoning by carbon monoxide (CO) adlayers and stability after accelerated durability tests (ADTs). We associate the origin of these exceptional electrocatalytic performances of the Pt-solute-Mo nanoalloys with their high degree of concavity, variations in vertex chemistry, and Pt enrichment along the corners and edges. Our findings offer a facile synthetic approach for the synthesis of Pt-solute nanoalloys as high-performance multifunctional electrocatalysts.

The durability and long-term applicability of catalysts are critical parameters for the commercialization and adoption of fuel cells. Even though a few studies have been conducted on hollow carbon spheres (HCSs) as supports for Pt in oxygen reduction reactions (ORR) catalysis, in-depth durability studies have not been conducted thus far. In this study, Pt/HCSs and Pt/nitrogen-doped HCSs (Pt/NHCSs) were prepared using a reflux deposition technique. Small Pt particles were formed with deposition on the outside of the shell and inside the pores of the shell. The new catalysts demonstrated high activity (>380 μA cm−2 and 240 mA g−1) surpassing the commercial Pt/C by more than 10%. The catalysts demonstrated excellent durability compared to a commercial Pt/C in load cycling, experiencing less than 50% changes in the mass-specific activity (MA) and surface area-specific activity (SA). In stop-start durability cycling, the new materials demonstrated high stability with more than 50% retention of electrochemical active surface areas (ECSAs). The results can be rationalised by the high BET surface areas coupled with an array of meso and micropores that led to Pt confinement. Further, pair distribution function (PDF) analysis of the catalysts confirmed that the nitrogen and oxygen functional groups, as well as the shell curvature/roughness provided defects and nucleation sites for the deposition of the small Pt nanoparticles. The balance between graphitic and diamond-like carbon was critical for the electronic conductivity and to provide strong Pt-support anchoring.

There is currently a disconnect between the high electrocatalyst oxygen reduction reaction (ORR) performance measured ex situ, using the rotating disc electrode (RDE), and the in situ membrane electrode assembly (MEA) performance. The disconnect in the electrocatalyst performance raises questions both about the pitfalls of the RDE technique at extrapolating the performance to higher overpotentials and how to improve the in situ catalyst layer performance to meet ambitious fuel cell targets. This work aims to bridge the gap by measuring the ORR ex situ performance under high mass transport conditions, at high overpotentials, using the floating electrode (FE) technique. Here, we determine the performance of three Pt/C electrocatalysts using the FE in 1 M HClO4 and 1 M H2SO4 to show that the MEA activities measured at 80 degrees C, 150 kPa(g) were substantially lower than the room temperature and pressure performance of the same catalyst in 1 M HClO4 using the RDE and FE methods and also lower than the FE in 1 M H2SO4, implying MEA limitations are not purely due to sulfonate adsorption from the Nafion. Finally, FE and MEA data was modeled which obtained j(o) values on the FE (oxide free conditions) which were 4-6x larger, at 11-26 mu A cm(-2), than those measured on the MEA. The difference is interpreted as due to better water removal in the FE system. This work shows that MEA catalyst layers are vastly underutilized, due to poor water transport, and current densities equivalent to 10-16 A cm(-2) at 0.65 V for 400 mu g(Pt) cm(-2) (25-40 A mg(-1)) are achievable, whereas the current mass activity of MEAs is <40% of this value at 25 and 80 degrees C, 150 kPa(g).

The reaction mechanism of quinuclidine mediated C-H oxidation of unactivated C-H bonds has been elucidated. In-situ cathodically generated H 2 O 2 was shown to diffuse to the anode where it is oxidized and participates in the ketonization reaction. Further oxidation of H 2 O 2 to H 2 O, O 2 and H + leads to glassy carbon surface degradation. Oxidation of quinuclidine was shown be kinetic-diffusion control limited and shown to be irreversible at 0 and 1 M 1,1,1,3,3,3-hexafluoroisopropan-2-ol solutions, and quasi-reversible at 0.1 M. Competing side reactions of quinuclidine with hexafluropropanolate and hydrogen peroxide were identified that lead to decreased reaction efficiency, which explains why quinuclidine needs to be used stoichiometrically.

One drawback of alkaline water electrolysis (AWE) is the sluggish oxygen evolution reaction (OER) kinetics at the anode of the electrolyser and improvement of the anode material is still a necessity. Various Ir1−yNiyOxelectrocatalyst combinations were prepared by reactive sputtering and subjected to high-throughput electrochemical screening using a Au/SiO2wafer. The best performing electrocatalysts were identified and subjected to single electrode electrochemical and physical characterisation using glassy carbon disk electrodes as the substrate. Rotating disk electrode (RDE) analysis included linear sweep voltammetry (LSV) before and after short term chronopotentiometry (CP). Energy dispersive X-ray spectroscopy and X-ray photo-electron spectroscopy (XPS) confirmed the controlled synthesis of thin film electrocatalysts by utilising reactive sputtering, and confirmed Ir dissolution which occurred during CP measurements. Overall, the Ir1−yNiyOxelectrocatalyst combinations containing higher amounts of Ir (Ir92Ni8Ox, Ir68Ni32Oxand Ir62Ni38Ox) performed the best of the evaluated mixed metal oxide electrocatalysts. However, evident from this study was that excellent mass-specific activity (current per mass of noble metal) was not exhibited by the mixed metal oxides when compared to Ni.

A recent development in photocatalytic reactor systems is the introduction of catalyst coated optical fibre for improved illumination efficiency. A high catalyst loading per unit reactor volume would require a large amount of coated fibre. Hence, an automated optical fibre catalyst coating process (AOFCCP) was developed. Here, we report on the effect of process variables in the deposition of TiO2 (P25 and anatase) on the coating thickness and surface morphology. The thickness of the photocatalytic layer increases with an increase in the withdrawal speed, which is attributed to a reduced flow back of the slurry prior to the drying process. This is further substantiated by the observed increase the catalyst layer thickness upon increasing the drying/calcination temperature (200 - 500 degrees C) upon coating P25 on to the optical fibre. The surface of the layer created by dip-coating through a P25-slurry was often corrugated, whereas the anatase surface obtained by gel-coating was smooth.

The development of active and durable proton exchange membrane fuel cell catalysts with high loading (ca. 40%) is critical for the commercialization of hydrogen fuel cells. Herein we report on the synthesis of a novel Pt/C catalyst using a novel bowl-like broken hollow carbon sphere (and N-doped sphere) support (carbon shell thickness ~ 4.6 nm). Highly dispersed Pt nanoparticles (dPt ~ 4 nm) were deposited on both supports and within the carbon shell. The Pt particles in the pores were exposed on both sides of the shell, while the shell porosity ensured pore confinement of the Pt. Both catalysts exhibited high electrochemical surface areas (60–65 m2 g−1) and cycling durability (6000 cycles) that was superior to a commercial benchmark Pt/C catalyst. These studies indicate that high loadings of confined small Pt particles on both sides of thin interconnected carbons can lead to high oxygen reduction reaction activities and durability.

We report a rapid solution-phase strategy to synthesize alloyed PtNi nanoparticles which demonstrate outstanding functionality for the oxygen reduction reaction (ORR). This one-pot coreduction colloidal synthesis results in a monodisperse population of single-crystal nanoparticles of rhombic dodecahedral morphology with Pt-enriched edges and compositions close to Pt1Ni2. We use nanoscale 3D compositional analysis to reveal for the first time that oleylamine (OAm)-aging of the rhombic dodecahedral Pt1Ni2 particles results in Ni leaching from surface facets, producing aged particles with concave faceting, an exceptionally high surface area, and a composition of Pt2Ni1. We show that the modified atomic nanostructures catalytically outperform the original PtNi rhombic dodecahedral particles by more than two-fold and also yield improved cycling durability. Their functionality for the ORR far exceeds commercially available Pt/C nanoparticle electrocatalysts, both in terms of mass-specific activities (up to a 25-fold increase) and intrinsic area-specific activities (up to a 27-fold increase).

We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.

In moving towards a greener global energy supply, hydrogen fuel cells are expected to play an increasingly significant role. New catalyst support materials are being sought with increased durability. MAX phases show promise as support materials due to their unique properties. The layered structure gives rise to various potential (001) surfaces. DFT is used to determine the most stable (001) surface terminations of Ti2AlC, Ti3AlC2 and Ti3SiC2. The electrical resistivities calculated using BoltzTraP2 show good agreement with the experimental values, with resistivities of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2 and 0.268 µΩ m for Ti3SiC2. Surfaces with Al or Si at the surface and the corresponding Ti surface show the lowest cleavage energy of the different (001) surfaces. MAX phases could therefore be used as electrocatalyst support materials, with Ti3SiC2 showing the greatest potential.

Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance. We report on the preparation of binary PtNi NPs via a co-thermolytic approach in which we optimize the synthesis variables, which results in significantly improved catalytic performance. We used scanning transmission electron microscopy to characterise the range of morphologies produced, which included spherical and concave cuboidal core-shell structures. Electrocatalytic activity was evaluated using a rotating disc electrode (1600 rpm) in 0.1 M HClO4; the electrocatalytic performance of these Ni@Pt NPs showed significant (∼11-fold) improvement compared to a commercial Pt/C catalyst. Extended cycling revealed that electrochemical surface area was retained by cuboidal PtNi NPs post 5000 electrochemical cycles (0.05-1.00 V, vs. SHE). This is attributed to the enclosure of Ni atoms by a thick Pt shell, thus limiting Ni dissolution from the alloy structures. The novel synthetic strategy presented here results in a high yield of Ni@Pt NPs which show excellent electro-catalytic activity and useful durability.

A simple, modified Metal-Organic Chemical Deposition (MOCD) method for Pt, PtRu and PtCo nanoparticle deposition onto a variety of support materials, including C, SiC, B4C, LaB6, TiB2, TiN and a ceramic/carbon nanofiber, is described. Pt deposition using Pt(acac)2 as a precursor is shown to occur via a mixed solid/liquid/vapour precursor phase which results in a high Pt yield of 90-92% on the support material. Pt and Pt alloy nanoparticles range 1.5-6.2 nm, and are well dispersed on all support materials, in a one-step method, with a total catalyst preparation time of ∼10 hours (2.4-4× quicker than conventional methods). The MOCD preparation method includes moderate temperatures of 350 °C in a tubular furnace with an inert gas supply at 2 bar, a high pressure (2-4 bar) compared to typical MOCVD methods (∼0.02-10 mbar). Pt/C catalysts with Pt loadings of 20, 40 and 60 wt% were synthesised, physically characterised, electrochemically characterised and compared to commercial Pt/C catalysts. TEM, XRD and ex situ EXAFS show similar Pt particle sizes and Pt particle shape identifiers, namely the ratio of the third to first Pt coordination numbers modelled from ex situ EXAFS, between the MOCD prepared catalysts and commercial catalysts. Moreover, electrochemical characterisation of the Pt/C MOCD catalysts obtained ORR mass activities with a maximum of 428 A gPt -1 at 0.9 V, which has similar mass activities to the commercial catalysts (80-160% compared to the commercial Pt/C catalysts).

Developing solid solution nanoparticles with complex faceted geometries and unusual composition sectoral zoning can enhance their catalytic performance. In a solution-phase synthesis of PtNi nanoparticles, we show that a mixture of surfactants results in surface functionalization, which in turn controls the morphological evolution of nanoparticles. The nanoparticles exhibited complex chemical growth zoning, rich in Pt geometric topologies, which varied as a function of surfactant mixture. Compositional mapping, using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy dispersive X-ray spectroscopy (EDS), highlights core-shell structures (similar to 1 nm Pt coating thickness) with edge-vertex Pt-enrichment and Ni-rich faces. These core-shell Ni-Pt nanoparticles demonstrated enhanced activities toward the oxygen reduction reaction (ORR) compared to commercial Pt/C, even after extended potential cycles (5000). Our synthetic approach, which utilizes the surfactants' array of distinct functional groups, offers new avenues toward the formation of concentric core-shell structures with multifaceted topologies. These materials show considerable promise as electrocatalysts.

The development of support structures for electrocatalysts has received a great deal of attention over the last decade, with carbon structures (i.e. nanostructures, Vulcan carbon (VC)) having been studied extensively. Carbon support structures increase the surface area, stability and activity of electrocatalysts in most cases, and can be used to overcome the delamination of thin films. In an attempt to (i) obtain surface structures and areas on SiO2 wafer pads, for combinatorial high-throughput sputtering and screening, that are comparable to glassy carbon (GC), (ii) eliminate delamination of the electrocatalyst and (iii) increase activity and stability, this study focused on VC:Nafion support preparation techniques. Four VC inks were prepared and used as carbon support on GC electrode inserts to analyse their effect on the activity of sputtered Ni thin films (40 nm) towards the oxygen evolution reaction (OER) in alkaline media. Linear sweep voltammetry (LSV) and chronopotentiometry (CP) were employed to compare the catalytic activity and stability of these sputtered Ni thin films on the various VC supports. Results suggest that similar activity compared with IrO2 and RuO2 could be achieved by sputtered Ni on VC:Nafion support, indicating improved Ni utilisation as well as improved short-term stability of the Ni thin films. These results validate the use of VC:Nafion support as substrate for sputtered electrocatalysts.

Nanoalloys with anisotropic morphologies of branched and porous internal structures show great promise in many applications as high performance materials. Reported synthetic approaches for branched alloy nanostructures are, however, limited by the synthesis using a seed-growth process. Here, we demonstrate a conveniently fast and one-pot solution-phase thermal reduction strategy yielding nanoalloys of Pt with various solute feed ratios, exhibiting hyperbranched morphologies and good dispersity. When Pt was alloyed with transition metals (Ni, Co, Fe), we observed well-defined dendritic nanostructures in PtNi, PtCo and Pt(NiCo), but not in PtFe, Pt(FeNi) or Pt(FeCo) due to the steric hindrance of the trivalent Fe(acac)3 precursor used during synthesis. In the case of Pt-based nanoalloys containing Ni and Co, the dendritic morphological evolution observed was insensitive to large variations in solute concentration. The functionality of these nanoalloys towards the oxygen reduction reaction (ORR); however, was observed to be dependent on the composition, increasing with increasing solute content. Pt3(NiCo)2 exhibited superior catalytic activity, affording about a five- and 10-fold enhancement in area-specific and mass-specific catalytic activities, respectively, compared to the standard Pt/C nanocatalyst. This solution-based synthetic route offers a new approach for constructing dendritic Pt-based nanostructures with excellent product yield, monodispersity and high crystallinity.