Nanostructure Engineering of Cathode Layers in Proton Exchange Membrane Fuel Cells: From Catalysts to Membrane Electrode Assembly

The membrane electrode assembly (MEA) is the core component of proton exchange membrane fuel cells (PEMFCs), which is the place where the reaction occurrence, the multiphase material transfer and the energy conversion, and the development of MEA with high activity and long stability are crucial for the practical application of PEMFCs. Currently, efforts are devoted to developing the regulation of MEA nanostructure engineering, which is believed to have advantages in improving catalyst utilization, maximizing three-phase boundaries, enhancing mass transport, and improving operational stability. This work reviews recent research progress on platinum group metal (PGM) and PGM-free catalysts with multidimensional nanostructures, catalyst layers (CLs), and nano-MEAs for PEMFCs, emphasizing the importance of structure-function relationships, aiming to guide the further development of the performance for PEMFCs. Then the design strategy of the MEA interface is summarized systematically. In addition, the application of in situ and operational characterization techniques to adequately identify current density distributions, hot spots, and water management visualization of MEAs is also discussed. Finally, the limitations of nanostructured MEA research are discussed and future promising research directions are proposed. This paper aims to provide valuable insights into the fundamental science and technical engineering of efficient MEA interfaces for PEMFCs.

Surface Hydrophobicity Engineering of Pt-Based Noble Metal Aerogels by Ionic Liquids toward Enhanced Electrocatalytic Oxygen Reduction

Modulating the surface properties of electrocatalysts with ligands could effectively regulate their catalytic properties, while limited in-depth understanding of the surface ligands restricted their rational combination. Herein, ionic liquids (ILs) with different lengths of hydrophobic side chains were employed to regulate the surface hydrophobicity of noble metal aerogels, for comprehending the relationship between surface hydrophobicity and oxygen reduction reaction (ORR) activity and enhancing electrocatalytic ORR. The volcano-like trends between the hydrophobicity and the ORR activity for various Pt-based aerogels indicated that a suitable hydrophobic surface constructed by ILs was most favorable for contacting with oxygen molecules and the desorption of oxygen intermediates. Typically, the PtPd aerogel modified by ILs (PtPd aer-[MTBD][PFSI]) exhibited an inspiring ORR activity, with a 70 mV increase in half-wave potential and a 7.1-fold mass activity compared to the commercial Pt/C. Therefore, the regularity between the surface hydrophobicity and ORR activity of noble metal aerogels was uncovered and will facilitate the modulation of electrocatalysts for practical applications.

Recent progress on the synthesis of metal alloy nanowires as electrocatalysts

Benefiting from both one-dimensional (1D) morphology and alloy composition, metal alloy nanowires have been exploited as advanced electrocatalysts in various electrochemical processes. In this review, the synthesis approaches for metal alloy nanowires are classified into two categories: direct syntheses and syntheses based on preformed 1D nanostructures. Ligand systems that are of critical importance to the formation of alloy nanowires are summarized and reviewed, together with the strategies imposed to achieve the co-reduction of different metals. Meanwhile, different scenarios that form alloy nanowires from pre-synthesized 1D nanostructures are compared and contrasted. In addition, the characterization and electrocatalytic applications of metal alloy nanowires are briefly discussed.

Advances in Low Pt Loading Membrane Electrode Assembly for Proton Exchange Membrane Fuel Cells

Hydrogen has the potential to be one of the solutions that can address environmental pollution and greenhouse emissions from traditional fossil fuels. However, high costs hinder its large-scale commercialization, particularly for enabling devices such as proton exchange membrane fuel cells (PEMFCs). The precious metal Pt is indispensable in boosting the oxygen reduction reaction (ORR) in cathode electrocatalysts from the most crucial component, i.e., the membrane electrode assembly (MEA). MEAs account for a considerable amount of the entire cost of PEMFCs. To address these bottlenecks, researchers either increase Pt utilization efficiency or produce MEAs with enhanced performance but less Pt. Only a few reviews that explain the approaches are available. This review summarizes advances in designing nanocatalysts and optimizing the catalyst layer structure to achieve low-Pt loading MEAs. Different strategies and their corresponding effectiveness, e.g., performance in half-cells or MEA, are summarized and compared. Finally, future directions are discussed and proposed, aiming at affordable, highly active, and durable PEMFCs.

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.

Deep oxidization of glucose driven by 4-acetamido-TEMPO for a glucose fuel cell at room temperature

Exploiting suitable oxidation catalysts is of great importance in the development of sugar-based fuel cells (SFCs). Herein, a novel room-temperature glucose/O2 fuel cell (GFC), which employs 4-acetamido-2,2,6,6-tetramethylpiperidin-1-oxyl (ACT) as an anodic electrocatalyst and air-breathing Pt-C as a cathode, is demonstrated. Under room temperature operation, the as-assembled GFCs are capable of delivering a maximum power density of 100 μW cm-2 in the presence of 50 mM glucose. Bulk electrolysis products of glucose identified by mass spectrum and Fourier transform infrared spectroscopy include gluconic acid and glucaric acid, suggesting that the aldehyde and primary hydroxy groups of glucose can be deeply oxidized into carboxyl groups through a 6e- pathway. The deep glucose oxidation capability makes ACT a promising anodic electrocatalyst for SFCs.

N-Rich hetero-porous defective carbon induced by trace B-doping enables efficient oxygen reduction

A facile boron doping strategy to improve the nitrogen retention in carbon-based catalysts is developed toward efficient oxygen reduction reaction via offsetting the electron migration. By considering the hetero-porous structure at the same time, this B, N co-doped catalyst outperforms commercial Pt/C in terms of both activity and stability.