Hydrophilic-hydrophobic dual catalyst layers for proton exchange membrane fuel cells under low humidity

Air oxidation of carbon supports boosts the low-humidity fuel cell performance

We introduce a straightforward, yet effective strategy to combat the performance decline of proton-exchange membrane fuel cells in low-humidity environments. Our method centers on air-oxidizing carbon supports, significantly improving proton and oxygen transport within the cathode catalyst layer.

Perovskite-Carbon Joint Substrate for Practical Application in Proton Exchange Membrane Fuel Cells under Low-Humidity/High-Temperature Conditions

Highly active catalysts with promising water retention are favorable for proton exchange membrane fuel cells (PEMFCs) operating under low-humidity/high-temperature conditions. When PEMFCs operate under low-humidity/high-temperature conditions, performance attenuation rapidly occurs owing to reduced proton conductivity of dehydrated membrane electrode assemblies. Herein, we load platinum onto a perovskite-carbon joint substrate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-σ-XC-72R) to construct a highly active and durable catalyst with good water retention capacity. We propose that the Pt/(BZCYYb-C) catalyst layer at cathode can promote the water back diffusion of produced water from the cathode to the membrane, thus preventing the decay of fuel-cell performance under low-humidity/high-temperature conditions. The catalyst exhibited outstanding mass activity of 0.542 A mg_pt^-1 at 0.9 V vs RHE. PEMFCs with such a catalyst delivered very high peak power densities (1.70/1.14 W cm^-2 under H₂-O₂/air conditions at 70 °C) and kept 85.3%/92.1% of initial performance values under low-humidity/high-temperature conditions (relative humidity 60%@70 °C/100 °C).

Electrochemical removal of tetrabromobisphenol A by fluorine-doped titanium suboxide electrochemically reactive membrane

This study reports fluorine-doped titanium suboxide anode for electrochemical mineralization of hydrophobic micro-contaminant, tetrabromobisphenol A. Fluorinated TiSO anode promoted electro-generated hydroxyl radicals (•OH) with higher selectivity and activity, due to increased O₂ evolution potential and more loosely interaction with hydrophobic electrode surface. For electro-oxidation process, fluorine doping had an insignificant impact on outer-sphere reaction and exerted inhibition on inner-sphere reaction, as indicated by cyclic voltammogram performed on Ru(NH₃)₆^3+/2+, Fe(CN)₆^3-/4- and Fe^3+/2+ redox couple. This facilitated electrochemical conversion of TBBPA and intermediates via more efficient outer-sphere reaction and hydroxylation route. Additionally, generated O₂ micro-bubbles could be stabilized on hydrophobic F-doped TiSO anode, which extended the three-phase boundary available for interfacial enrichment of TBBPA and subsequent mineralization. Under action of these comprehensive factors, 0.5% F-doped TiSO electrochemically reactive membrane could achieve 99.7% mineralization of TBBPA upon energy consumption of 0.52 kWh m^-3 at current density of 7.8 ± 0.24 mA cm^-2 (3.75 V vs SHE) and flow rate of 1628 LHM based on flow-through electrolysis. The modified anode exhibited superior performances compared with un-modified one with more efficient TBBPA removal, less toxic intermediate accumulation and lower energy consumption. The results may have important implications for electrochemical removal and detoxification of hydrophobic micro-pollutants.

Electrocatalyst Performance at the Gas/Electrolyte Interface under High-Mass-Transport Conditions: Optimization of the "Floating Electrode" Method

The thin-film rotating disk electrode (TF-RDE) is a well-developed, conventional ex situ electrochemical method that is limited by poor mass transport in the dissolved phase and hence can only measure the kinetic response for Pt-based catalysts in a narrow overpotential range. Thus, the applicability of TF-RDE results in assessing how catalysts perform in fuel cells has been questioned. To address this problem, we use the floating electrode (FE) technique, which can facilitate high-mass transport to a catalyst layer composed of an ultralow loading of catalyst (1-15 μg_Pt cm_geo^-2) at the gas/electrolyte interface. In this paper, the aspects that have critical effects on the performance of the FE system are measured and parametrized. We find that, in order to obtain reproducible results with high performance, the following factors need to be taken into account: system cleanliness, break-in procedure, hydrophobic agent, ionomer type, and the measurements of catalyst surface area and loading. For some of these parameters, we examined a range of different approaches/materials and determined the optimum configuration. We find that the gas permeability of the hydrophobic agent is an important factor for improving the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) performance. We provide evidence that the suppression of the HOR and ORR introduced by the Nafion ionomers is more than a local mass transport barrier but that a mechanism involving the adsorption of the sulfonate on Pt also plays a significant role. The work provides intriguing insights into how to manufacture and optimize electrocatalyst systems that must function at the gas/electrolyte interface.

Modifying the Catalyst Layer Using Polyvinyl Alcohol for the Performance Improvement of Proton Exchange Membrane Fuel Cells under Low Humidity Operations

A proton exchange membrane fuel cell (PEMFC) system for the application of unmanned aerial vehicles is equipped without humidifiers and the cathode channels of the stack are open to the environment due to limited weight available for power sources. As a result, the PEMFC is operated under low humidity conditions, causing membrane dehydration, low performance, and degradation. To keep the generated water within the fuel cell to humidify the membrane, in this study, polyvinyl alcohol (PVA) is employed in the fabrication of membrane electrode assemblies (MEAs). The effect of PVA content, either sprayed on the gas diffusion layer (GDL) or mixed in the catalyst layer (CL), on the MEA performance is compared under various humidity conditions. The results show that MEA performance is increased with the addition of PVA either on the GDL or in the CL, especially for non-humidified anode conditions. The result suggested that 0.03% PVA in the anode CL and 0.1% PVA on the GDL can improve the MEA performance by approximately 30%, under conditions of a non-humidified anode and a room-temperature-humidified cathode. However, MEAs with PVA in the anode CL show better durability than those with PVA on the GDL according to measurement with electrochemical impedance spectroscopy.

Optimizing the structural design of a nanocomposite catalyst layer for PEM fuel cells for improving mass-specific power density

For the purpose of redesigning a PEM fuel cell with ultralow Pt loading, this review comprehensively summarizes and comments on recent important findings on ultrathin catalyst layer structures. We introduce recent advances in electrocatalyst research and development (R&D), highlighting the urgency of ultralow Pt loading in the total design of PEM fuel cells. Following that, the reason for a thinner and more ordered electrode structure is presented for the next generation of PEM fuel cells. We then review recent progress in methods for preparing Pt nanoparticles on high-aspect-ratio supports, extended surface area of nanowires (confined agglomerates and nanowires) and ordered arrays. Regarding the ordered arrays, we expatiate on proton conductor arrays and electron conductor arrays, including carbon nanotube-assisted arrays, TiO₂ nanotube-assisted arrays, Co-OH-CO₃ nanowire-assisted arrays, and pigment red 149-assisted arrays. Challenges related to proton transport and transfer, electron conduction and mass transport are then discussed to supply further research direction.