Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

In situ immobilization of Fe/Fe3C/Fe2O3 hollow hetero-nanoparticles onto nitrogen-doped carbon nanotubes towards high-efficiency electrocatalytic oxygen reduction

The rational design of affordable, efficient and robust electrocatalysts towards the oxygen reduction reaction (ORR) is of vital importance for the future advancement of various renewable-energy technologies. Herein, we develop a feasible and delicate synthesis of Fe/Fe3C/Fe2O3 hollow heterostructured nanoparticles in situ immobilized on highly graphitic nitrogen-doped carbon nanotubes (referred to as Fe/Fe3C/Fe2O3@N-CNTs hereafter) via a simple hydrogel-bridged pyrolysis strategy. The simultaneous consideration of interfacial manipulation and nanocarbon hybridization endows the formed Fe/Fe3C/Fe2O3@N-CNTs with sufficiently well-dispersed and firmly immobilized active components, regulated electronic configuration, enhanced electrical conductivity, multidimensional mass transport channels, and remarkable structural stability. Consequently, benefiting from the compositional synergy and architectural superiority, the as-obtained Fe/Fe3C/Fe2O3@N-CNTs exhibit excellent ORR catalytic activity, impressive durability and superior selectivity in an alkaline electrolyte, outperforming the commercial Pt/C catalyst and a majority of the previously reported Fe-based catalysts. Furthermore, the rechargeable Zn-air battery using Fe/Fe3C/Fe2O3@N-CNTs + RuO2 as an air-cathode exhibits a higher power density, larger specific capacity and better cycling stability as compared with the state-of-the-art Pt/C + RuO2 counterpart. The explored hydrogel-bridged pyrolysis strategy enabling the concurrent heterointerface construction, nanostructure engineering and nanocarbon hybridization may inspire the future design of high-efficiency electrocatalysts for diverse renewable energy applications.

A review of the α-Fe2O3 (hematite) nanotube structure: recent advances in synthesis, characterization, and applications

α-Fe2O3 nanotubes are exceptional one-dimensional transition metal oxide materials with low density, large surface area, promising electrochemical and photoelectrochemical properties, which are widely investigated in lithium-ion batteries, photoelectrochemical devices, gas sensors, and catalysis. They have drawn significant attention to the fields of energy storage and conversion, and environmental sensing and remediation due to the increase in the global energy crisis and environmental pollution. Many efforts have been made toward controlling the morphology or impurity doping to improve the intrinsic properties of α-Fe2O3 nanotubes. In this review, we introduce the synthesis methods and physicochemical properties of α-Fe2O3 nanotubes. The fabrication conditions, which are important for the physicochemical properties of materials, are also listed to describe the synthesis processes. Furthermore, the development and breakthrough of various applications in batteries, supercapacitors, photoelectrochemical devices, environmental remediation, and sensors are systematically reviewed. Finally, some of the current challenges and future perspectives for α-Fe2O3 nanotubes are discussed. We believe that this timely and critical mini-review will stimulate extensive studies and attract more attention, further improving the development of the α-Fe2O3 (hematite) nanotube structure.

Electrochemical Ammonia Generation Directly from Nitrogen and Air Using an Iron-Oxide/Titania-Based Catalyst at Ambient Conditions

Ammonia was produced electrochemically from nitrogen/air in aqueous alkaline electrolytes by using a Fe₂O₃/TiO₂ composite catalyst under room temperature and atmospheric pressure. At an applied potential of 0.023 V versus reversible hydrogen electrode, the rate of ammonia formation was 1.25 × 10^-8 mmol mg^-1 s^-1 at an overpotential of just 34 mV. This rate increased to 2.7 × 10^-7 mmol mg^-1 s^-1 at -0.577 V. The chronoamperometric experiments on Fe₂O₃/TiO₂/C clearly confirmed that Fe₂O₃ along with TiO₂ shows superior nitrogen reduction reaction activity compared to Fe₂O₃ alone. Experimental parameters such as temperature and applied potential have a significant influence on the rate of ammonia formation. The activation energy of nitrogen reduction on the employed catalyst was found to be 25.8 kJ mol^-1. Real-time direct electrochemical mass spectrometry analysis was used to monitor the composition of the evolved gases at different electrode potentials.

Hierarchical Heterostructures of Ultrasmall Fe2O3-Encapsulated MoS2/N-Graphene as an Effective Catalyst for Oxygen Reduction Reaction

In this study, a facile approach has been successfully applied to synthesize a hierarchical three-dimensional architecture of ultrasmall hematite nanoparticles homogeneously encapsulated in MoS₂/nitrogen-doped graphene nanosheets, as a novel non-Pt cathodic catalyst for oxygen reduction reaction in fuel cell applications. The intrinsic topological characteristics along with unique physicochemical properties allowed this catalyst to facilitate oxygen adsorption and sped up the reduction kinetics through fast heterogeneous decomposition of oxygen to final products. As a result, the catalyst exhibited outstanding catalytic performance with a high electron-transfer number of 3.91-3.96, which was comparable to that of the Pt/C product. Furthermore, its working stability with a retention of 96.1% after 30 000 s and excellent alcohol tolerance were found to be significantly better than those for the Pt/C product. This hybrid can be considered as a highly potential non-Pt catalyst for practical oxygen reduction reaction application in requirement of low cost, facile production, high catalytic behavior, and excellent stability.

Well-defined Fe, Fe3C, and Fe2O3 heterostructures on carbon black: a synergistic catalyst for oxygen reduction reaction

Constructing Fe-based composites with a smart heterostructure, by using metallic Fe, Fe₃C, Fe₂O₃ as building blocks, may open a door to rationally designing high-active and low-cost transition metal-based catalysts for the oxygen reduction reaction.

Mesoporous anodic α-Fe2O3 interferometer for organic vapor sensing application

Mesoporous α-Fe₂O₃ interferometers with well-resolved optical fringes can display high sensitivity to organic vapors.

Alkaline electrochemical advanced oxidation process for chromium oxidation at graphitized multi-walled carbon nanotubes

Alkaline electrochemical advanced oxidation processes for chromium oxidation and Cr-contaminated waste disposal were reported in this study. The highly graphitized multi-walled carbon nanotubes g-MWCNTs modified electrode was prepared for the in-situ electrochemical generation of HO₂^-. RRDE test results illustrated that g-MWCNTs exhibited much higher two-electron oxygen reduction activity than other nanocarbon materials with peak current density of 1.24 mA cm^-2, %HO₂^- of 77.0% and onset potential of -0.15 V (vs. Hg/HgO). It was originated from the highly graphitized structure and good electrical conductivity as illustrated from the Raman, XRD and EIS characterizations, respectively. Large amount of reactive oxygen species (HO₂^- and ·OH) were in-situ electro-generated from the two-electron oxygen reduction and chromium-induced alkaline electro-Fenton-like reaction. The oxidation of Cr(III) was efficiently achieved within 90 min and the conversion ratio maintained more than 95% of the original value after stability test, offering an efficient and green approach for the utilization of Cr-containing wastes.

Hierarchical oxygen-implanted MoS2 nanoparticle decorated graphene for the non-enzymatic electrochemical sensing of hydrogen peroxide in alkaline media

Owing to the extensive applications of hydrogen peroxide (H₂O₂) in biological, environmental and chemical engineering, it is of great importance to investigate sensitive and selective sensing platform towards the detection of H₂O₂. Herein, oxygen-implanted MoS₂ nanoparticles decorated graphene nanocomposite is synthesized via a facile one-pot solvothermal method for the sensitive detection of H₂O₂ in alkaline media. The structure and morphology of the MoS₂/graphene nanocomposites were systematically characterized, showing that Mo-O bonds are formed and oxygen is implanted into the crystal structure in the nanocomposite. As a result, the MoS₂/graphene composite exhibited enhanced electron transfer kinetics and excellent electro-reduction performance towards H₂O₂ in alkaline media. Under optimum conditions, the fabricated sensor demonstrated a wide linear response towards H₂O₂ in the range of 0.25-16mM with a low detection limit of 0.12μM and high sensitivity of 269.7μAmM^-1cm^-2. Besides, the constructed sensor presented a good selectivity to H₂O₂ with the presence of other interfering species. Therefore, the proposed sensor was successfully applied for the detection and determination of H₂O₂ in real sample, indicating great potential for the practical applications.

W-doped MoS2 nanosheets as a highly-efficient catalyst for hydrogen peroxide electroreduction in alkaline media

Novel W-doped MoS₂ electrocatalysts have been successfully fabricated through a facile one-pot solvothermal method and employed for the hydrogen peroxide reduction reaction (HPRR) in emerging alkaline H₂O₂-based fuel cells.

Thermal evolution of MnxOy nanofibres as catalysts for the oxygen reduction reaction

The present study shows how, starting from green and low-cost precursors, nanostructured manganese oxides with good catalytic efficiencies for the oxygen reduction reaction can be fabricated through the electrospinning technique. The role of the crystalline phase and morphological features, on the electro-catalytic behaviour, is discussed.