A nitrogen and fluorine enriched Fe/Fe3C@C oxygen reduction reaction electrocatalyst for anion/proton exchange membrane fuel cells

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.

d-Orbital Electron Delocalization Realized by Axial Fe4 C Atomic Clusters Delivers High-Performance Fe-N-C Catalysts for Oxygen Reduction Reaction

Fe-N-C catalyst for oxygen reduction reaction (ORR) has been considered as the most promising nonprecious metal catalyst due to its comparable catalytic performance to Pt in proton exchange membrane fuel cells (PEMFCs). The active centers of Fe-pyrrolic N4 have been proven to be extremely active for ORR. However, forming a stable Fe-pyrrolic N4 structure is a huge challenge. Here, a Cyan-Fe-N-C catalyst with Fe-pyrrolic N4 as the intrinsic active center is constructed with the help of axial Fe4 C atomic clusters, which shows a half-wave potential of up to 0.836 V (vs. RHE) in the acid environment. More remarkably, it delivers a high power density of 870 and 478 mW cm-2 at 1.0 bar in H2 -O2 and H2 -Air fuel cells, respectively. According to theoretical calculation and in situ spectroscopy, the axial Fe4 C can provide strong electronic perturbation to Fe-N4 active centers, leading to the d-orbital electron delocalization of Fe and forming the Fe-pyrrolic N4 bond with high charge distribution, which stabilizes the Fe-pyrrolic N4 structure and optimizes the OH* adsorption during the catalytic process. This work proposes a new strategy to adjust the electronic structure of single-atom catalysts based on the strong interaction between single atoms and atomic clusters.

Development of Hydrogen-Oxygen Fuel Cells Based on Anion-Exchange Electrolytes and Catalysts with Reduced Platinum Content

Studies have been carried out to optimize the composition, formation technique and test conditions of membrane electrode assemblies (MEA) of hydrogen-oxygen anion-exchange membranes fuel cells (AEMFC), based on Fumatech anion-exchange membranes. A non-platinum catalytic system based on nitrogen-doped CNT (CNT_N) was used in the cathode. PtMo/CNT_N catalysts with a reduced content of platinum (10-12 wt.% Pt) were compared with 10 and 60 wt.% Pt/CNT_N at the anode. According to the results of studies under model conditions, it was found that the PtMo/CNT_N catalyst is significantly superior to the 10 and 60 wt.% Pt/CNT_N catalyst in terms of activity in the hydrogen oxidation reaction based on the mass of platinum. The addition of the Fumion ionomer results in minor changes in the electrochemically active surface area and activity in the hydrogen oxidation reaction for each of the catalysts. In this case, the introduction of ionomer-Fumion leads to a partial blocking of the outer surface and the micropore surface, which is most pronounced in the case of the 60Pt/CNT_N catalyst. This effect can cause a decrease in the characteristics of MEA AEMFC upon passing from 10PtMo/CNT_N to 60Pt/CNT_N in the anode active layer. The maximum power density of the optimized MEA based on 10PtMo/CNT_N was 62 mW cm^-2, which exceeds the literature data obtained under similar test conditions for MEA based on platinum cathode and anode catalysts and Fumatech membranes (41 mW cm^-2). A new result of this work is the study of the effect of the ionomer (Fumion) on the characteristics of catalysts. It is shown that the synthesized 10PtMo/CNT_N catalyst retains high activity in the presence of an ionomer under model conditions and in the MEA based on it.

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 MN_x, 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.

Nickel-induced charge redistribution in Ni-Fe/Fe3C@nitrogen-doped carbon nanocage as a robust Mott-Schottky bi-functional oxygen catalyst for rechargeable Zn-air battery

Designing earth-abundant and advanced bi-functional oxygen electrodes for efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are extremely urgent but still ambiguous. Thus, metal-semiconductor nanohybrids were developed with functionally integrating ORR-active Ni species, OER-active Fe/Fe₃C components, and multifunctional N-doped carbon (NDC) support. Expectantly, the resulted NDC nanocage embedded with Ni-Fe alloy and Fe₃C particles, as assembled Mott-Schottky-typed catalyst, delivered a promoted half-wave potential of 0.904 V for ORR and a low overpotential of 315 mV at 10 mA/cm² for OER both in alkaline media, outperforming those of commercial Pt/C and RuO₂ counterparts. Most importantly, the optimized Ni-Fe/Fe₃C@NDC sample also afforded a peak power density of 267.5 mW/cm² with a specific capacity of 773.8 mAh/g_Zn and excellent durability over 80 h when used as the air electrode in rechargeable Zn-air batteries, superior to the state-of-the-art bi-functional catalysts. Ultraviolet photoelectron spectroscopy revealed that the introduction of Ni into the Fe/Fe₃C@NDC component could well manipulate the electronic structure of the designed electrocatalyst, leading to an effective built-in electric field established by the Mott-Schottky heterojunction to expedite the continuous interfacial charge-transfer and thus significantly promote the utilization of electrocatalytic active sites. Therefore, this work provides an avenue for the designing and developing robust and durable Mott-Schottky-typed bi-functional catalysts for promising energy conversion.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Synthesis of unused-wood-derived C-Fe-N catalysts for oxygen reduction reaction by heteroatom doping during hydrothermal carbonization and subsequent carbonization in nitrogen atmosphere

There is an urgent need to develop renewable sources of energy and use existing resources in an efficient manner. In this study, in order to improve the utilization of unused biomass and develop green processes and sustainable technologies for energy production and storage, unused Douglas fir sawdust (SD) was transformed into catalysts for the oxygen reduction reaction. Fe and N were doped into SD during hydrothermal carbonization, and the N- and Fe-doped wood-derived carbon (Fe/N/SD) was carbonized in a nitrogen atmosphere. After the catalyst had been calcined at 800°C, its showed the highest current density (-5.86 mAcm^-2 at 0.5 V versus reversible hydrogen electrode or RHE) and E_onset value (0.913 V versus RHE). Furthermore, its current density was higher than that of Pt/C (20 wt% Pt) (-5.66 mA cm^-2 @0.5 V versus RHE). Finally, after 50 000 s, the current density of sample Fe/N/SD (2 : 10 : 10) remained at 79.3% of the initial value. Thus, the synthesized catalysts, which can be produced readily at a low cost, are suitable for use in various types of energy generation and storage devices, such as fuel cells and air batteries. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal (M@C) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of M@C type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.

Co/MoC Nanoparticles Embedded in Carbon Nanoboxes as Robust Trifunctional Electrocatalysts for a Zn-Air Battery and Water Electrocatalysis

To meet the application needs of rechargeable Zn-air battery and electrocatalytic overall water splitting (EOWS), developing high-efficiency, cost-effective, and durable trifunctional catalysts for the hydrogen evolution reaction (HER), oxygen evolution, and reduction reaction (OER and ORR) is extremely paramount yet challenging. Herein, the interface engineering concept and nanoscale hollowing design were proposed to fabricate N-doping carbon nanoboxes confined with Co/MoC nanoparticles. Uniform zeolitic imidazolate framework nanocube was employed as the starting material to construct the trifunctional electrocatalyst through the conformal polydopamine-Mo layer coating and the subsequent pyrolysis treatment. The Co@IC/MoC@PC catalyst displayed superior electrochemical ORR performances with a positive half-wave potential of 0.875 V and a high limiting current density of 5.89 mA/cm². When practically employed as an electrocatalyst in regenerative Zn-air battery, a high specific capacity of 728 mAh/g, a large peak power density of 221 mW/cm², a high open-circuit voltage of 1.482 V, and a low charge/discharge voltage gap of 0.41 V were obtained. Moreover, its practicability was further exploited by overall water splitting, affording low overpotentials of 277 and 68 mV at 10 mA/cm² for the OER and HER in 1 M KOH solution, respectively, and a decent operating potential of 1.57 V for EOWS. Ultraviolet photoelectron spectroscopy and density functional theory calculation revealed that the Co/MoC interface synergistically facilitated the charge-transfer, thereby contributing to the enhancements of electrocatalytic ORR/OER/HER processes. More importantly, this catalyst design concept can offer some interesting prospects for the construction of outstanding trifunctional catalysts toward various energy conversion and storage devices.

Enhanced electrocatalytic oxygen reduction reaction for Fe-N4-C by the incorporation of Co nanoparticles

Oxygen reduction reaction (ORR) catalytic activity can be improved by means of enhancing the synergy between transition metals. In this work, a novel porous Fe-N₄-C nanostructure containing uniformly dispersed Co nanoparticles (CoNPs) is prepared by an assisted thermal loading method. The as-prepared Co@Fe-N-C catalyst shows enhanced ORR activity with a half-wave potential (E_1/2) of 0.92 V vs. RHE, which is much higher than those of the direct pyrolysis CoNP-free sample Fe-N-C (E_1/2 = 0.85 V) and Pt/C (E_1/2 = 0.90 V) in alkaline media. It exhibits remarkable stability with only a 10 mV decrease in E_1/2 after 10 000 cycles and an outstanding long-term durability with 85% current remaining after 60 000 s. In acidic media, this catalyst exhibits catalytic activity with an E_1/2 of 0.79 V, comparable to Pt/C (E_1/2 = 0.82 V). X-ray absorption fine spectroscopy analysis revealed the presence of active centres of Fe-N₄. Density functional theory calculations confirmed the strong synergy between CoNPs and Fe-N₄ sites, providing a lower overpotential and beneficial electronic structure and a local coordination environment for the ORR. The incorporation of CoNPs on the surface of Fe-N₄-C nanomaterials plays a key role in enhancing the ORR catalytic activity and stability, providing a new route to prepare efficient Pt-free ORR catalysts.

A mass-producible integrative structure Pt alloy oxygen reduction catalyst synthesized with atomically dispersive metal-organic framework precursors

Oxygen reduction reaction (ORR) catalyst is one of the most significant influential factors for the application of proton exchange membrane fuel cells. This work introduces a mass-producible high-performance PtZn alloy integrative structure ORR catalyst, synthesized with the atomically dispersive metal-organic framework precursors. This PtZn catalyst displays excellent catalytic activity with the onset reduction potential of 1.0 V_RHE@ 0.16 mA cm^-2 (Reversible hydrogen electrode; RHE) and the half-wave potential of 0.934 V_RHE for the ORR catalysis. The calculated specific activity and mass activity at 0.9 V are 9.44 A m^-2 and 544 A g_Pt^-1, respectively, which are 5.62 times and 5.77 times as high as the commercial Pt/C. The mass activity is remarkably higher than the target put forward by the Department of Energy (DOE; 440 A g_Pt^-1). Furthermore, this PtZn catalyst also exhibits outstanding stability after the 10,000 potential cyclic degeneration test. The ORR current is much higher than Pt/C in the whole potential range not only before but also after the 10,000 potential cycles with identical Pt loading. This catalyst has a multifarious active-site catalytic structure with PtZn alloyed particles and atomically dispersive metal-N active sites on the N-doped graphited carbon matrix, exhibiting appealing ORR catalytic activity and sound stability for the application and scalable production of fuel cell catalysts.

Enhanced oxygen reduction activity of bimetallic Pd–Ag alloy-supported on mesoporous cerium oxide electrocatalysts in alkaline media

Currently, the rational design and fabrication of Pt-free electrocatalysts towards the oxygen reduction reaction for extensive applications in fuel cells is a challenging task.

Micelles of Mesoporous Silica with Inserted Iron Complexes as a Platform for Constructing Efficient Electrocatalysts for Oxygen Reduction

Iron, N-codoped carbon materials (Fe-N-C) are promising electrocatalysts toward oxygen reduction reactions due to their high atom utilization efficiency and intrinsic activity. Nanostructuring of the Fe-N-C materials, such as introducing porosity into the carbon structure, would be conducive to further increasing the exposure of active sites as well as improving the mass transfer. Herein, we explore the potential of iron complex-functionalized micelles of mesoporous SiO₂ as a platform for constructing porous Fe-N-C materials. The classical three-dimensional MCM-48 was selected as a proof-of-concept example, which was utilized as the hard template, and cetyltrimethylammonium bromide micelles inside it played the role of the main carbon source. Fe-N_x sites were derived from Fe-1,10-phenanthroline complexes in the micelles introduced by in situ incorporation of 1,10-phenanthroline and post Fe²⁺ insertion in an aqueous solution. After thermal annealing in a nitrogen atmosphere and subsequent removal of the MCM-48 framework, a carbon material that possesses porous structural features with uniformly dispersed Fe-N_x sites (MPC@PhFe) was obtained, which shows superior ORR activity in a 0.1 M KOH solution and great potential for Zn-air battery applications as well. This work demonstrates the feasibility as well as the effectiveness of turning micelles of mesoporous SiO₂ into porous carbon structures and might offer a universal strategy for manufacturing carbon materials for future application in energy storage and conversion.

Encapsulation of Fe nanoparticles into an N-doped carbon nanotube/nanosheet integrated hierarchical architecture as an efficient and ultrastable electrocatalyst for the oxygen reduction reaction

The exploration of cost-effective, highly efficient and robust electrocatalysts toward the oxygen reduction reaction (ORR) is of paramount significance for the advancement of future renewable energy conversion devices, and yet still remains a great challenge. Herein, we demonstrate a straightforward one-step pyrolysis strategy for the scalable synthesis of an iron-nitrogen-carbon hierarchically nanostructured catalyst, in which Fe-based nanoparticles are encapsulated in bamboo-like N-doped carbon nanotubes in situ rooted from porous N-doped carbon nanosheets (Fe@N-C NT/NSs). The delicate fabrication of such an 0D/1D/2D integrated hierarchical architecture with encased Fe species and open configuration renders the formed Fe@N-C NT/NSs with sufficient confined active sites, reduced charge transfer resistance, improved diffusion kinetics and outstanding mechanical strength. As such, compared with commercial Pt/C, the optimized Fe@N-C NT/NSs catalyst exhibits efficient ORR activity, superior durability and strong tolerance to methanol in KOH medium. More impressively, when assembled as a cathode catalyst in a microbial fuel cell, the Fe@N-C NT/NSs electrode displays significantly enhanced power density and output voltage in comparison with commercial Pt/C, holding great promise in practical energy conversion devices. What's more, the simple yet reliable synthesis strategy developed here may shed light on the future design of advanced high-efficiency hierarchical architectures for diverse electrochemical applications and beyond.