Nano-electrocatalyst materials for low temperature fuel cells: A review

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Catalysts for Oxygen Reduction Reaction in the Polymer Electrolyte Membrane Fuel Cells: A Brief Review

This mini-review presents a short account of materials with exceptional activity towards oxygen reduction reaction. Two main classes of catalytic materials are described, namely platinum group metal (PGM) catalyst and Non-precious metal catalyst. The classes are discussed in terms of possible application in low-temperature hydrogen fuel cells with proton exchange membrane and further commercialization of these devices. A short description of perspective approaches is provided and challenging issues associated with developed catalytic materials are discussed.

Highly efficient ethylene glycol electrocatalytic oxidation based on bimetallic PtNi on 2D molybdenum disulfide/reduced graphene oxide nanosheets

In this paper, a two-dimensional (2D) hybrid material of molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) is facilely synthesized and used as an ideal support for the deposition of Pt nanoparticles. The as-prepared Pt/MoS₂/RGO composites are further worked as electrocatalysts towards ethylene glycol oxidation reaction (EGOR). In addition, when alloying with Ni, the composite shows obvious enhancement in electrocatalytic performance for EGOR. Specifically, the optimized molar ratio of Pt to Ni is 3:1, namely Pt₃Ni/MoS₂/RGO performs the strongest current density of 2062 mA mg^-1_Pt, which is 11.1, 5.80 and 2.40 times higher than those of Pt, Pt₃Ni and Pt/MoS₂/RGO electrodes, respectively. The systematically electrochemical measurements indicate that the largely promoted electrocatalytic performances of Pt₃Ni/MoS₂/RGO are mainly attributed to the synergistic effect of Ni and Pt, and 2D sheets of MoS₂/RGO. This excellent performance indicates that the reported electrocatalytic material could be an efficient catalyst for the application in direct ethylene glycol fuel cell and beyond.

CoPtx/γ-Al2O3 bimetallic nanoalloys as promising catalysts for hydrazine electrooxidation

Stable bimetallic catalysts composed of CoPtx/γ-Al2O3 (x = Pt/Co molar ratio) were synthesized by wet impregnation method followed by calcination and the H2 reduction. The powders were characterized using XRD, AAS, BET, SEM, EDX, TPR, and TPO techniques. The prepared catalysts were drop casted on the glassy carbon electrode (GCE) and catalytic performance was examined for hydrazine electrooxidation in alkaline medium via cyclic voltammetry (CV). All the compositions in CoPtx/γ-Al2O3 series showed high responses towards hydrazine electrooxidation, however; high activity of CoPt0.034/γ-Al2O3 catalyst inferred it as a best material with an anodic peak current (iP) response of 200 μA at 0.86 V. The prominent electrochemical (EC) responses for this composition are attributed to better accessible surface area resulting in a fast electron transfer. The CoPtx/γ-Al2O3 catalysts are reported as the robust and superior prospective materials for extensive electroanalytical and catalytic studies.

Transition Metal–Nitrogen–Carbon (M–N–C) Catalysts for Oxygen Reduction Reaction. Insights on Synthesis and Performance in Polymer Electrolyte Fuel Cells

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) have attracted increasing interest as potential candidates to replace Pt, in the view of a future widespread commercialization of polymer electrolyte fuel cell (PEFC) devices, especially for automotive applications. Among different types of PGM-free catalysts, M–N–C materials appear to be the most promising ones in terms of activity. These catalysts can be produced using a wide variety of precursors containing C, N, and one (or more) active transition metal (mostly Fe or Co). The catalysts synthesis methods can be very different, even though they usually involve at least one pyrolysis step. In this review, five different synthesis methods are proposed, and described in detail. Several catalysts, produced approximately in the last decade, were analyzed in terms of performance in rotating disc electrode (RDE), and in H2/O2 or H2/air PEFC. The catalysts are subdivided in five different categories corresponding to the five synthesis methods described, and the RDE and PEFC performance is put in relation with the synthesis method.

The effect of varying solvents for MoS2 treatment on its catalytic efficiencies for HER and ORR

MoS₂ has been investigated intensively in the field of catalysis for the hydrogen evolution reaction (HER) in particular. Much effort has been made by various research teams worldwide to look into the specific catalyst design such as nano-structuring, defect engineering or hybrid structures. But what evades us is the fundamental preparation method for the dispersion of powdered MoS₂. Individual research teams with their best practices might be subjective and not validated by extensive experimental results. In this report, we find that the overpotential for the catalysis of HER varies from 0.57 to 0.72 V (freshly prepared) when different dispersion media are used, such as acetonitrile, N,N-dimethylformamide, ethanol, methanol and water. In terms of oxygen reduction reaction (ORR) catalysis, less significant differences were found. With both HER and ORR pertinent to the fuel cell industry, this report would serve as an insight to readers when comparing the results of MoS₂ catalysis across the literature from different research groups when different solvents were used as the dispersion medium.

Nitrogen/sulfur dual-doped reduced graphene oxide supported CuFeS2 as an efficient electrocatalyst for the oxygen reduction reaction

At present, low-cost and efficient electrocatalysts for accelerating the oxygen reduction reaction in fuel cells are highly desired.

Preparation of bacterial cellulose based nitrogen-doped carbon nanofibers and their applications in the oxygen reduction reaction and sodium–ion battery

3D N-doped carbon nanofibers were fabricated from low-cost biomass bacterial cellulose and used as electrodes for both ORR and SIBs.

Pt/Co Alloy Nanoparticles Prepared by Nanocapsule Method Exhibit a High Oxygen Reduction Reaction Activity in the Alkaline Media

Oxygen reduction reaction (ORR) catalysts are one of the main topics for fuel cells and metal/air batteries. We found that the platinum-cobalt alloy nanoparticles prepared by our original nanocapsule method exhibited a high ORR catalytic activity in alkaline solution, compared with that of the existing alloy nanoparticles prepared by different methods. The effect of alloy composition on the ORR activity was investigated to find the optimum composition (approximately 40 atom %). We also found that the enhancement of the catalytic activity in alkaline solution appeared in a very narrow range of Co content compared with that in acidic solution.

Germanium and phosphorus co-doped carbon nanotubes with high electrocatalytic activity for oxygen reduction reaction

Germanium and phosphorus co-doped carbon nanotubes (Ge–P-CNTs) were prepared by a simple and scalable approach.

Nitrogen(N)-doped activated carbon materials with a narrow pore size distribution derived from coal liquefaction residues as low-cost and high-activity oxygen reduction catalysts in alkaline solution

Coal liquefaction residues with a high content of nitrogen were used to prepare N-doped activated carbon as low-cost and high-activity oxygen reduction reaction catalyst.