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.

Pyrolysis-Free Covalent Organic Polymers Directly for Oxygen Electrocatalysis.

ConspectusOxygen electrode catalysis is crucial for the efficient operation of clean energy devices, such as proton exchange membrane fuel cells (PEMFCs) and Zn-air batteries (ZABs). However, sluggish oxygen electrocatalysis kinetics in these infrastructures put forward impending requirements toward seeking efficient oxygen-electrode catalytic materials with a clear active-site configuration and geometrical morphology to study in depth the structure-property relationship of materials. Although transition-metal-nitrogen-carbon (M-N-C) electrocatalysts have shown great prospects currently and potential in oxygen electrocatalysis as promising platinum group metal-free catalysts, the universal pyrolysis operation in the preparation process often inevitably brings about randomness and diversity of active sites, for which it is difficult to determine the structure-activity relationship, understand the catalytic mechanism, and further improve facilities performance.Covalent organic polymers (COPs) are a class of molecular geometric constructs linked by irreversible kinetic covalent bonds through reticular chemistry. Unique structural tailorability, diverse design principles, and inherent well-defined construction in pristine COPs naturally provide a great platform to study the structure-property relationship of active sites and exhibit unique features for application. In this Account, we afford an overview of our recent attempts toward the utilization of COP materials as free-pyrolysis oxygen electrode catalysts, enabling accurate construction of oxygen electrodes with clear active site and geometrical morphology characteristics in PEMFC and ZAB devices yet without enduring any high-temperature pyrolysis treatments. Starting from the needs of modern electrocatalysis, we discussed the unique properties for the design and development of pyrolysis-free pristine COPs as high-performance oxygen electrode catalytic materials in terms of intrinsic electronic structure properties and membrane-electrode-assembly (MEA) application distinguished from pyrolysis M-N-C catalysts. First, the pyrolysis-free COP catalysts provide a viable molecular model catalyst platform, which is conducive to mechanism comprehension for the relationship between catalyst activity and structure. Second, the simple and low-energy consumption synthesis process for pyrolysis-free catalysts lays the foundation for the large-scale production of catalysts, oxygen electrodes, and even the entire stack assembly without considering numerous complicated factors as traditional pyrolytic catalysts. Besides, most traditional COPs are difficult to dissolve and solution process due to their cross-linked skeleton. Our newly developed COP materials with solution processability bring about new opportunities to the process and assemble oxygen electrodes into device. These properties are unparalleled and have not been systematically reviewed and analyzed by any research reports so far. Here, we have clarified the specific advantage and potential of pyrolysis-free COP materials as oxygen electrodes applied in PEMFC and ZAB devices in response to the latest progress and requirements of current electrocatalytic research.

Atomically Dispersed Isolated Fe-Ce Dual-Metal-Site Catalysts for Proton-Exchange Membrane Fuel Cells.

Atomically dispersed single-metal-site catalysts are hailed as the most promising category for the oxygen reduction reaction (ORR) with full metal utilization and complete exploitation of intrinsic activity. However, due to the inherent electronic structure of single-metal atoms in MNx, it is difficult to break the linear relationship between catalytic activity and adsorption energy of reaction intermediates, and the performance of such catalysts still falls short of expectations. Herein, we change the adsorption structure by constructing Fe-Ce atomic pairs to modulate the iron d-orbital electron configuration, breaking the linear relationship based on single-metal sites. The 4f cruise electrons of cerium element reduce the d-orbital center of iron in the synthesized FeCe-single atom dispersed hierarchical porous nitrogen-doped carbon (FeCe-SAD/HPNC) catalyst, and more orbital-occupied states appear near the fermi level, which weakens the adsorption strength in the active center and oxygen species, so that the rate-determining step was shifted from *OH desorption to *O > *OH, rendering the excellent ORR performances of the FeCe-SAD/HPNC catalyst. The synthesized FeCe-SAD/HPNC catalyst shows excellent activity, with a half-wave potential as high as 0.81 V for ORR in 0.1 M HClO4 solution. Additionally, by constructing a three-phase reaction interface with a hierarchical porous structure, the H2-O2 proton-exchange membrane fuel cell (PEMFC) assembled with FeCe-SAD/HPNC as cathode catalyst achieves a maximum power density of 0.771 W cm-2 and good stability.

Fundamental Understanding of Electronic Structure in FeN4 Site on Electrocatalytic Activity via dz2 -Orbital-Driven Charge Tuning for Acidic Oxygen Reduction.

The structural diversity of active sites resulting from traditional pyrolysis hinder our understanding of the local coordination environment (LCE) around the active site, and its effects on performance in the oxygen reduction reaction (ORR). We created a series of FeN4 active-site configurations via a pyrolysis-free approach where LCEs are defined by covalent organic polymers (COPs). Our results suggest a more positive charge on iron atoms in the vicinity of an electron-withdrawing side-chain; that is, a high-valence configuration (FeH+ N4 ) that is achieved with a COPBTC @Cl-CNTs catalyst subject to d z 2 ${{d}_{{z}^{2}}}$ -orbital tuning. A new descriptor ξ, defined as the band center of iron atoms projected on the 3 d z 2 ${{3d}_{{z}^{2}}}$ -orbital, was introduced to quantitively explain a volcano-like regulation mechanism. When ξ is distributed between -1.887 and -1.862 eV, the catalytic performance of the COPBTC @Cl-CNTs electrocatalyst is optimized.

Porous Covalent Organic Polymer Coordinated Single Co Site Nanofibers for Efficient Oxygen-Reduction Cathodes in Polymer Electrolyte Fuel Cells.

Nitrogen-coordinated single-cobalt-atom electrocatalysts, particularly ones derived from high-temperature pyrolysis of cobalt-based zeolitic imidazolate frameworks (ZIFs), have emerged as a new frontier in the design of oxygen reduction cathodes in polymer electrolyte fuel cells (PEFCs) due to their enhanced durability and smaller Fenton effects related to the degradation of membranes and ionomers compared with emphasized iron-based electrocatalysts. However, pyrolysis techniques lead to obscure active-site configurations, undesirably defined porosity and morphology, and fewer exposed active sites. Herein, a highly stable cross-linked nanofiber electrode is directly prepared by electrospinning using a liquid processability cobalt-based covalent organic polymer (Co-COP) obtained via pyrolysis-free strategy. The resultant fibers can be facilely organized into a free-standing large-area film with a uniform hierarchical porous texture and a full dispersion of atomic Co active sites on the catalyst surface. Focused ion beam-field emission scanning electron microscopy and computational fluid dynamics experiments confirm that the relative diffusion coefficient is enhanced by 3.5 times, which can provide an efficient route both for reactants to enter the active sites, and drain away the produced water efficiently. Resultingly, the peak power density of the integrated Co-COP nanofiber electrode is remarkably enhanced by 1.72 times along with significantly higher durability compared with conventional spraying methods. Notably, this nanofabrication technique also maintains excellent scalability and uniformity.

An Initial Covalent Organic Polymer with Closed-F Edges Directly for Proton-Exchange-Membrane Fuel Cells.

Covalent organic polymers (COPs) are a class of rising electrocatalysts for the oxygen reduction reaction (ORR) due to the atomically metrical control of the organic molecular components along with highly architectural robustness and thermodynamic stability even in acid or alkaline media. However, the direct application of pristine COPs as acidic ORR electrocatalysts, especially in device manner, e.g., in proton-exchange-membrane fuel cells (PEMFCs), remains a big challenge. Currently, the decoration toward electronic structures of active sites is considered a vital pathway to enhancing the acidic ORR activity of carbon-based electrocatalysts. Here, an initial F-decorated fully closed π-conjugated quasi-phthalocyanine COP (denoted as COPBTC -F) is reported. The introduction of the closed-F edges stepwise drags more electrons from FeN4 sites in COPBTC -F into the catalyst margin, which weakens the occupied numbers of bonding orbitals between COPBTC -F and OH* intermediates at the rate-determining step, exhibiting over five times intrinsic performance beyond the counterpart without F functionalities (termed as COPBTC ). Significantly, the maximum power density utilizing COPBTC -F as a cathode catalyst in PEMFCs is remarkably increased by an order of magnitude compared with COPBTC , which is a stride forward among catalysts based on a pyrolysis-free conjugated-polymer network in device manner to date.

Temperature-Dependent Morphologies of Precursors: Metal-Organic Framework-Derived Porous Carbon for High-Performance Electrochemical Double-Layer Capacitors.

In this work, three Cu metal-organic framework samples with tunable rhombic, squama, and trucated bipyramid morphologies have been synthesized at 0, 25, and 60 °C, respectively, and further employed as precursors to initially prepare Cu@C composites by the calcination-thermolysis procedure. Then Cu@C composites have been etched with HCl and subsequently activated with KOH to obtain activated porous carbon (APC-0, -25, and -60). Interestingly, APC-25 presents a loose multilevel morphology of cabbage and possesses the largest specific surface area (1880.4 m2 g-1) and pore volume (0.81 cm3 g-1) among these APC materials. Consequently, APC-25 also exhibits the highest specific capacitance of 196 F g-1 at 0.5 A g-1, and the corresponding symmetric supercapacitor cell (SSC) achieves a remarkable energy density of 11.8 Wh kg-1 at a power density of 350 W kg-1. Furthermore, APC-25 shows excellent cycling stability, and the loss of capacitance is only 7.7% even after 10000 cycles at 1 A g-1. Significantly, five light-emitting diodes can be lit by six SSCs, which proves that APC-25 can be used in energy storage devices.

Six Isomorphous Window-Beam MOFs: Explore the Effects of Metal Ions on MOF-Derived Carbon for Supercapacitors.

Six isomorphous metal-organic frameworks (MOFs) with a 3D window-beam architecture have been synthesized from solvothermal reactions, and are named Zn, Cd, Ni, Co, Mn and Cu-MOF, respectively. The series of MOFs was utilized as precursors to synthesize MOF-derived carbon with different morphologies. Zn and Cd-MOFs lead to the derivation of porous carbons (PCs), which exhibit remarkable BET specific surface areas. For derivates of Ni, Co and Mn-MOFs, graphitized carbons (GCs) show some carbon graphitization, but their BET specific surface areas are relatively small. C-Cu has the smallest BET specific surface area, and there is no carbon graphitization. Therefore, the metal ion of the parent MOF exerts a crucial effect on the preparation of MOF-derived carbon, such as the pore-forming effect of Zn and Cd species, and catalytic graphitization of Ni, Co, and Mn species. The capacitances of MOF-derived carbon follow the sequence of PCs>GCs>C-Cu, which reveals that the specific surface area plays a dominant role in the capacitive performance of electrical double layer capacitors (EDLCs), and that the graphitization could improve the capacitance. Significantly, PC-Zn exhibits the best specific capacitance (138 F g-1 at 0.5 Ag-1 ), and excellent life cycle, which can be applied as an electrode material in supercapacitors.