Sulfur-doped carbon nanotubes as catalysts for the oxygen reduction reaction in alkaline medium

Emerging Nitrogen and Sulfur Co-doped Carbon Materials for Electrochemical Energy Storage and Conversion

The growing global energy demands, coupled with the imperative for sustainable environmental challenges, have sparked significant interest in electrochemical energy storage and conversion (EESC) technologies. Metal-free heteroatom-doped carbon materials, especially those codoped with nitrogen (N) and sulfur (S), have gained prominence due to their exceptional conductivity, large specific surface area, remarkable chemical stability, and enhanced electrochemical performance. The strategic incorporation of N and S atoms into the carbon framework plays a pivotal role in modulating electron distribution and creating catalytically active sites, thereby significantly enhancing the EESC performance. This review examines the key synthetic strategies for fabricating N, S codoped carbon materials (NSDCMs) and provides a comprehensive overview of recent advancements in NSDCMs for EESC applications. These encompass various electrochemical energy storage systems such as supercapacitors, alkali-ion batteries, and lithium-sulfur batteries. Energy conversion processes, including hydrogen evolution, oxygen reduction/evolution, and carbon dioxide reduction are also covered. Finally, future research directions for NSDCMs are discussed in the EESC field, aiming to highlight their promising potential and multifunctional capabilities in driving further advancements in electrochemical energy systems.

A Simple Method for Preparation of Highly Conductive Nitrogen/Phosphorus-Doped Carbon Nanofiber Films

Heteroatom-doped conductive carbon nanomaterials are promising for energy and catalysis applications, but there are few reports on increasing their heteroatom doping content and conductivity simultaneously. In this manuscript, we use 2-(4-aminophenyl)-5-aminobenzimidazole as the diamine monomer to prepare polyamic acid with asymmetric structural units doped with phosphoric acid (PA) and polyacrylonitrile (PAN) as innovative composite precursors, which are then electrospun into nanofiber films. After stabilization and carbonization, the electrospun fibers are converted into N/P co-doped electrospun carbon nanofiber films (ECNFs) with high heteroatom content, including 4.33% N and 0.98% P. The morphology, structure, and conductivity of ECNFs were systematically characterized. The ECNFs doped with 15 wt.% PA exhibited conductivity that was 47.3% higher than that of the ECNFs undoped with PA, but the BET surface area decreased by 23%. The doped PA in the precursor nanofibers participated in the cyclization of PAN during thermal stabilization, as indicated by infrared spectroscopy and thermogravimetric analysis results. X-ray diffraction and Raman results indicate that a moderate amount of PA doping facilitated the formation of ordered graphitic crystallite structures during carbonization and improved the conductivity of ECNFs.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Sulfur-Doped Graphdiyne as a High-Capacity Anode Material for Lithium-Ion Batteries

Heteroatom doping is regarded as a promising approach to enhance the electrochemical performance of carbon materials, while the poor controllability of heteroatoms remains the main challenge. In this context, sulfur-doped graphdiyne (S-GDY) was successfully synthesized on the surface of copper foil using a sulfur-containing multi-acetylene monomer to form a uniform film. The S-GDY film possesses a porous structure and abundant sulfur atoms decorated homogeneously in the carbon skeleton, which facilitate the fast diffusion and storage of lithium ions. The lithium-ion batteries (LIBs) fabricated with S-GDY as anode exhibit excellent performance, including the high specific capacity of 920 mA h g⁻¹ and superior rate performances. The LIBs also show long-term cycling stability under the high current density. This result could potentially provide a modular design principle for the construction of high-performance anode materials for lithium-ion batteries.

Transition-Metal- and Nitrogen-Doped Carbide-Derived Carbon/Carbon Nanotube Composites as Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Transition-metal- and nitrogen-codoped carbide-derived carbon/carbon nanotube composites (M-N-CDC/CNT) have been prepared, characterized, and used as cathode catalysts in anion-exchange membrane fuel cells (AEMFCs). As transition metals, cobalt, iron, and a combination of both have been investigated. Metal and nitrogen are doped through a simple high-temperature pyrolysis technique with 1,10-phenanthroline as the N precursor. The physicochemical characterization shows the success of metal and nitrogen doping as well as very similar morphologies and textural properties of all three composite materials. The initial assessment of the oxygen reduction reaction (ORR) activity, employing the rotating ring-disk electrode method, indicates that the M-N-CDC/CNT catalysts exhibit a very good electrocatalytic performance in alkaline media. We find that the formation of HO2 - species in the ORR catalysts depends on the specific metal composition (Co, Fe, or CoFe). All three materials show excellent stability with a negligible decline in their performance after 10000 consecutive potential cycles. The very good performance of the M-N-CDC/CNT catalyst materials is attributed to the presence of M-N x and pyridinic-N moieties as well as both micro- and mesoporous structures. Finally, the catalysts exhibit excellent performance in in situ tests in H2/O2 AEMFCs, with the CoFe-N-CDC/CNT reaching a current density close to 500 mA cm-2 at 0.75 V and a peak power density (P max) exceeding 1 W cm-2. Additional tests show that P max reaches 0.8 W cm-2 in an H2/CO2-free air system and that the CoFe-N-CDC/CNT material exhibits good stability under both AEMFC operating conditions.

Heteroatom-doped carbon electrocatalysts prepared from marine biomass cellulose nanocrystals and bio-inspired polydopamine for the oxygen reduction reaction

Heteroatom-doped carbon electrocatalysts were prepared from marine biomass cellulose nanocrystals as potential electrocatalysts for an efficient oxygen reduction reaction.

S/N Co-Doped Hollow Carbon Particles for Oxygen Reduction Electrocatalysts Prepared by Spontaneous Polymerization at Oil-Water Interfaces

We herein report that sulfur and nitrogen co-doped hollow spherical carbon particles can be applied to oxygen reduction reaction (ORR) electrocatalysts prepared by calcination of polydopamine (PDA) hollow particles. The hollow structure of PDA was formed by auto-oxidative interfacial polymerization of dopamine at the oil and water interface of emulsion microdroplets. The PDA was used as the nitrogen source as well as a platform for sulfur-doping. The obtained sulfur and nitrogen co-doped hollow particles showed a higher catalytic activity than that of nonsulfur-doped particles and nonhollow particles. The high ORR activity of the calcined S-doped PDA hollow particles could be attributed to the combination of nitrogen and sulfur active sites and the large surface areas owing to a hollow spherical structure.

Intrinsic Effect of Carbon Supports on the Activity and Stability of Precious Metal Based Catalysts for Electrocatalytic Alcohol Oxidation in Fuel Cells: A Review

Electrocatalyst supports, in particular carbonaceous materials, play critical roles in the electrocatalytic activity and stability of precious metal group (PMG)-based catalysts such as Pt, Pd, and Au for the electrochemical alcohol oxidation reaction (AOR) of fuels such as methanol and ethanol in polymer electrolyte membrane fuel cells (PEMFCs). Carbonaceous supports such as high surface area carbon provide electronic contact throughout the catalyst layer, isolate PMG nanoparticles (NPs) to maintain high electrochemical surface area, and provide hydrophobic properties to avoid flooding of the catalyst layer by liquid water produced. Compared to high surface area carbon, PMG catalysts supported on 1D and 2D carbon materials such as graphene and carbon nanotubes show enhanced activity and durability due to the intrinsic effect of the underlying carbonaceous supports on the electronic states of PMG NPs. The modification of the electronic environment, in particular the d-band centers of PMG NPs, weakens the adsorption of AOR intermediates, facilitates breaking of the C-C bonds, and thus enhances the electrocatalytic activity of PMG catalysts. The doping of heteroatoms further facilitates the electrocatalytic activity for the AOR through the structural, bifunctional, and electronic effects, in addition to the enhanced dispersion of PMG NPs in the carbon support. The prospects for the development of effective PMG-based catalysts for high-performance alcohol-fuel-based PEMFCs is discussed.

Synergistic effect on BCN nanomaterials for the oxygen reduction reaction – a kinetic and mechanistic analysis to explore the active sites

The rGO doped with boron and nitrogen reduce the oxygen via the dissociative four-electron pathway whereas the two-electron oxygen reduction reaction is more predominant on the rGO doped with either of the two individual heteroatoms.

MnCo2O4 nanoparticles supported on nitrogen and sulfur co-doped mesoporous carbon spheres as efficient electrocatalysts for oxygen catalytic reactions

The development of efficient bifunctional electrocatalysts for the oxygen reduction and oxygen evolution reactions is essential to address the challenge of sluggish reaction kinetics.

Heteroatom-Doped Carbon Nanotube and Graphene-Based Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is a key step that determines the performance of a variety of energy storage and conversion devices, such as fuel cells and metal-air batteries. Heteroatom-doped carbon nanotubes (CNTs) and graphenes have attracted increasing interest and hold great promise as efficient ORR catalysts to replace noble-metal-based catalysts, owing to their unique structure characteristics, excellent physicochemical properties, low cost, and rich resources. In this review, recent progress on the design, fabrication, and performance of heteroatom-doped CNT- and graphene-based catalysts is summarized, aiming to provide insights into the working mechanism of these heteroatom-doped nanocarbons in ORR. The advantages, challenges that remain, and possible solutions of these nanocarbon-based electrocatalysts are discussed. Finally, future developing trends of the CNT- and graphene-based ORR catalysts are proposed.

Study of iron oxide nanoparticle phases in graphene aerogels for oxygen reduction reaction

Four iron oxide phases incorporated in a graphene support were examined; differences in their catalytic properties depended on their phases.

Towards high-efficiency nanoelectrocatalysts for oxygen reduction through engineering advanced carbon nanomaterials

We summarize and discuss recent developments of different-dimensional advanced carbon nanomaterial-based noble-metal-free high-efficiency oxygen reduction electrocatalysts, including heteroatom-doped, transition metal-based nanoparticle-based, and especially iron carbide (Fe₃C)-based carbon nanomaterial composites.

A sulfur doped carbon nanotube as a potential catalyst for the oxygen reduction reaction

The catalytic ability of SCNT in a four-electron oxygen reduction reaction (ORR) of fuel cells was studied by DFT calculations.

Tuning α-Fe2O3 nanotube arrays for the oxygen reduction reaction in alkaline media

α-Fe₂O₃ nanotube arrays were fabricated and employed as low cost non-noble electrocatalysts for the oxygen reduction reaction (ORR). As-prepared α-Fe₂O₃ nanotube arrays exhibit excellent ORR catalytic activity and durability in alkaline media.

In situ incorporation of a S, N doped carbon/sulfur composite for lithium sulfur batteries

A novel sulfur–nitrogen co-doped carbon material (SNC), which is obtained by taking polyaniline as the nitrogen-containing carbon precursor and then incorporating sulfur atomsin situas the matrix material for lithium sulfur batteries, is investigated.