Construction of Heteroatom-Doped Porous Carbon Architectures for Energy and Sensing Applications

In this chapter, we have concentrated on the main electrocatalytic oxygen processes, oxygen reduction reaction (ORR) and water splitting oxygen evolution reaction (OER), and biosensors based on porous carbon architectures, which are more important areas of research because of the rise in demand for energy management, supply, and disease diagnosis. Heteroatom-doped carbon hollow spheres are very useful because they have a large surface area, mesoporosity, spherical wall thicknesses, edge plane defect sites, catalytic active sites, and fast heterogeneous electron-transfer rates. These properties are very important for making commercial devices. This chapter provides an overview of hollow carbon nanospheres that are doped with single and double heteroatoms, as well as cobalt oxide. These carbon compounds function as dual catalysts for OER and ORR, as well as an effective electrocatalyst for the oxygen reduction process in both acidic and alkaline media. Electrocatalytically, heteroatom-doped carbon sphere-modified electrodes can simultaneously and specifically identify and determine the analytes, while also validating the target species in real samples. N-doped hollow carbon spheres coated-Co3O4 functioned as an efficient dual-function oxygen electrocatalyst for oxygen evolution and oxygen reduction processes and also as a biosensor for highly effective electrochemical sensing of acetaminophen. A symmetric supercapacitor using dual heteroatom-doped and SBA-15 templated porous carbon was also discussed

A Perspective on the Recent Amelioration of Co₃O₄ and MnO₂ Bifunctional Catalysts for Oxygen Electrode Reactions

Metal-air batteries with the aid of high theoretical energy density and affability are trusted as propitious energy storage systems in today’s energy research. However, enforcement of the technology is still hindered by the sluggish kinetics of their electrode reactions, that is, oxygen evolution and oxygen reduction reaction (OER/ORR). Developing a catalyst with inherently greater bifunctional activity and durability is the finest solution to confront the aforementioned challenges. Transition metal oxides (TMOs) are the most appropriate choice of materials for that purpose since they are highly active, inexpensive, abundant and non-hazardous. Among the various transition metal oxides, MnO2 and Co3O4 are gaining much attention due to their superior bifunctional performance and alkaline stability owing to their structural features and physicochemical properties. With the inspiration from promoted catalytic activity of MnO2 and Co3O4, this chapter is fully devoted to these two catalysts. The activity structural relationship, recent developments and future directions of these materials for bifunctional catalysis have been discussed in more detail. Besides, the significant parameters judging the bifunctional activity, that is, phase, crystal facets, morphology, defects, strains and mixed metals oxide formations, have been illustrated with suitable evidence. In addition, the fundamentals of water oxidation and reduction reactions are explained with the mechanisms. Moreover, the physiochemical properties of MnO2 and Co3O4 materials and their influence on the catalytic activity are related for a better understanding of bifunctional catalysis. This collective perception will be highly useful for the comprehension and designing of advanced metal oxide catalysts to further improve bifunctional catalysis

The effects of morphology, microstructure and mixed-valent states of MnO2 on the oxygen evolution reaction activity in alkaline anion exchange membrane water electrolysis

In this work, we focused on the evaluation of oxygen evolution reaction (OER) activity of three different shapes of α-MnO2 nanowires (NWs), nanorods (NRs) and nanotubes (NTs) in alkaline anion exchange water electrolyser. We have attempted to separate the effect of shape, surface area, Mn3+ content and crystal facets on OER activity and stability. X-ray Photoelectron Spectroscopy (XPS) measurements showed that NTs had the highest surface concentration of Mn3+ on the as prepared samples with average Mn oxidation state of 3.33. However, after activation an increase in the average oxidation state of all three shapes to 3.9 was confirmed by XPS. X-Ray Diffraction (XRD) showed surface restructuring after testing. MnO2 NWs showed the highest OER mass activity of 60.6 A g−1 (10 mA cm−2 at 1.67 V (RHE)) due to the higher surface area of 72.2 m2 g−1. While NTs showed the highest specific activity due to highest content of 211 facet, high Mn3+ surface concentration/surface defects. Similar trend was observed in electrolyser testing with 2 mg cm−2 loading. Poor electronic conductivity of MnO2 resulted in decrease in performance with increased loading to 4 mg cm−2. All the studied shapes showed good stability over 36 h of electrolyser testing

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Acetaminophen on N, P‑Doped Hollow Mesoporous Carbon Nanospheres

Dual heteroatom-doped carbon hollow spheres have attracted attention for their intriguing properties, including high surface areas, mesoporosity, sphere wall thicknesses, edge plane defect sites, catalytic active sites, and fast heterogeneous electron-transfer rates. Understandably, the material finds widespread attention in the field of electrochemical sensors. In this work, we have successfully synthesized nitrogen (N) and phosphorus (P) dual doped hollow mesoporous carbon spheres (NPHMCS) by a simple self-polymerization process. The effect of loading a P precursor, namely, phytic acid (PA), on the electrochemical sensing of bioanalytes is investigated in detail. The investigation revealed that 0.6 g loading of PA (NPHMCS-0.6) resulted in an enhanced surface area of 940 m2 g–1, a higher pore volume of 0.40 cm3 g–1, enriched defect sites of 0.99, pyridinic sites of 24.90%, and moderate P–C + P–N sites of 74.65%. The synergistic effect of nitrogen and phosphorous doping along with the abovementioned properties is taken advantage in the fabrication of electrochemical sensors for the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA), and acetaminophen (AC). The fabricated sensors displayed a wide linear range of sensing over a concentration from 5 to 6000 μM for AA, 0.5 to 2000 μM for DA, 0.5 to 5000 μM for UA, and 5 to 1200 μM for AC. Based on the calibration plot, the limits of detection (LOD) were calculated to be 0.032, 0.002, 0.005, and 0.020 μM for AA, DA, UA, and AC, respectively. The electroanalytical performance of the fabricated sensor was successfully validated for the analysis of target species in real samples. The developed methodology offers prospective advantages in clinical diagnostics for the simultaneous analysis of small biomolecules and drugs

Simple Immersion in Polar Solvents Induces Targeted 1T Phase Conversion of MoS₂ for HER: A Greener Approach

Molybdenum disulfide (MoS2) with metallic 1T phase is perceived as the possible alternative to benchmark Pt in the hydrogen evolution reaction (HER). The dominance prevailing in the density of active sites, electrical conductivity, and hydrophilicity of 1T phase improves HER kinetics. In the construction of MoS2, 2H is the stable phase due to its thermodynamic stability over the 1T phase. On other hand, the Li+ ion intercalation/exfoliation technique converts 2H to a metastable 1T phase. Pyrophoric Li+ ion, an organic solvent, suffers from sluggish intercalation/exfoliation kinetics and is still a setback in attaining conversion. Based on these investigations, herein, for the very first time, such a phase conversion was reached by soaking the 2H-MoS2 in only polar solvents. Among the various polar solvents, extended phase conversion (70.1%) was attained in N,N-dimethylformamide (DMF) when 2H-MoS2 was immersed for 90 days, i.e., DMF-90D catalyst. Upon the prolonged immersion, the electrostatic forces between solvent and 2H-MoS2 occur, and the van der Waals forces between the layers of MoS2 are diminished. Under such conditions, solvent molecules intercalate into MoS2 and induce phase conversion from the 2H to 1T phase. The density functional theory (DFT) study further verified that only the electrostatic interactions are present between the solvent and MoS2 and that no charge transfer emerged. In acidic HER, the DMF-90D catalyst to secure 10 mA cm–2 required 263 mV with a lower Tafel slope value (59 mV/dec). This facile and greener approach can be further extended to other impressive 2D materials in the future

Tailoring of 1T Phase-Domain MoS₂ Active Sites with Bridging S₂^2–/Apical S^2– Phase-Selective Precursor Modulation for Enriched HER Kinetics

Molybdenum disulfide (MoS2) is a promising alternative electrocatalyst for hydrogen evolution reaction (HER) due to its relatively near zero hydrogen adsorption free energy (ΔGH = 0.08) and availability as a metallic (1T) phase. The superior catalytic activity of the 1T phase over 2H is owing to the availability of dense active sites, 107 fold higher conductivity, and greater hydrophilicity. However, in the synthesis of 1T-MoS2, a highly controlled proficient method is indispensable due to its metastable nature. Besides, phase enrichment is greatly sensitive to experimental parameters such as precursor, temperature, reaction time, and solvent. In the context of precursors, to date, no single precursor has been recognized as a selective precursor for the synthesis of 1T-MoS2. In this work, MoS2 with high content of 1T phase (79.4%) and excessive bridging S22–/apical S2– sites has been formulated from a single precursor, that is, ammonium tetrathiomolybdate ((NH4)2MoS4), ATTM). In HER, it displayed an inspired activity, that is, achieving 10 mA cm–2 current density, it requires just 248 mV overpotential with a minimal Tafel slope value (56 mV/dec). The maximum enrichment of the 1T phase, abundant accumulation of catalytically active bridging S22–/apical S2– sites, and the complete reduction of Mo+6 to Mo+4 (absence of Mo+6) are root causes for the outstanding activity of the synthesized 1T phase-domain MoS2. To the best of our knowledge for the very first time, here, we declare that the single source, that is, ATTM is an exclusive precursor for the selective synthesis of 1T-MoS2 with advantageous structural features. Moreover, this expedient precursor could be more pertinent for the industrial-scale preparation of 1T phase-domain MoS2 in near future

Development of α‐MnO2 Nanowire with Ni and (Ni, Co) – Cation Doping as an Efficient Bifunctional Oxygen Evolution and Oxygen Reduction Reaction Catalysts

Manganese oxides (MnO 2 ) with nanowire morphology materials are a promising candidate for improving oxygen evolution and oxygen reduction reaction (OER/ORR) performance. In this work, we developed transition metal cation doping strategy into the α-MnO 2 tunnel structure to tune the Mn oxidation states and control the uniform nanowire morphology, crystalline structure in order to investigate the effect of doping over bifunctional activity. The single Ni 2+ cation doping in α-MnO 2 with various loading concentrations resulted in 8Ni-MnO 2 exhibiting remarkable OER and ORR activity owing to their excessive concentration of Mn 3+ and Mn 4+ octahedral sites respectively. Further, Co 2+ cation doping in 8Ni-MnO 2 leads to an enhanced synergistic effect that significantly improves the fraction of Mn 3+ quantity which is confirmed by average oxidation state. For, electrochemical OER performance of 8Co-8Ni-MnO 2 exhibits a potential of 1.77 V, Tafel slope value of 68 mV dec -1 and lower charge transfer resistance and it is active in ORR with more positive onset potential of 0.90 V, half-wave potential of 0.80 V, better current density (4.7 mA cm -2 ) and a four-electron pathway. Moreover, bifunctional activity (ΔE = E OER @10 mA cm -2 – ORR@E 1/2 ) of 8Co-8Ni-MnO 2 demonstrated 0.97 V, indicates an excellent activity in alkaline electrolyte solution

One-Pot Synthesis of Ni0.05Ce0.95O2−δ Catalysts with Nanocubes and Nanorods Morphology for CO2 Methanation Reaction and in Operando DRIFT Analysis of Intermediate Species

The valorization of CO2 via renewable energy sources allows one to obtain carbon-neutral fuels through its hydrogenation, like methane. In this study, Ni0.05Ce0.95O2−δ catalysts were prepared using a simple one-pot hydrothermal method yielding nanorod and nanocube particles to be used for the methanation reaction. Samples were characterized by XRD, BET, TEM, H2-TPR, and H2-TPD experiments. The catalytic activity tests revealed that the best performing catalyst was Ni0.05Ce0.95O2−δ, with nanorod morphology, which gave a CO2 conversion of 40% with a selectivity of CH4 as high as 93%, operating at 325 ◦C and a GHSV of 240,000 cm3 h−1 g−1. However, the lower activation energy was found for Ni0.05Ce0.95O2−δ catalysts with nanocube morphology. Furthermore, an in operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was performed flowing CO2:H2 or CO:H2 mixture, showing that the main reaction pathway, for the CO2 methanation, is the direct hydrogenation of formate intermediate

One-Pot Synthesis of Ni0.05Ce0.95O2−δ Catalysts with Nanocubes and Nanorods Morphology for CO2 Methanation Reaction and in Operando DRIFT Analysis of Intermediate Species

The valorization of CO2 via renewable energy sources allows one to obtain carbon-neutral fuels through its hydrogenation, like methane. In this study, Ni0.05Ce0.95O2−δ catalysts were prepared using a simple one-pot hydrothermal method yielding nanorod and nanocube particles to be used for the methanation reaction. Samples were characterized by XRD, BET, TEM, H2-TPR, and H2-TPD experiments. The catalytic activity tests revealed that the best performing catalyst was Ni0.05Ce0.95O2−δ, with nanorod morphology, which gave a CO2 conversion of 40% with a selectivity of CH4 as high as 93%, operating at 325 °C and a GHSV of 240,000 cm3 h−1 g−1. However, the lower activation energy was found for Ni0.05Ce0.95O2−δ catalysts with nanocube morphology. Furthermore, an in operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was performed flowing CO2:H2 or CO:H2 mixture, showing that the main reaction pathway, for the CO2 methanation, is the direct hydrogenation of formate intermediate

Dual Heteroatom-Doped Carbon Monoliths Derived from Catalyst-free Preparation of Porous Polyisocyanurate for Oxygen Reduction Reaction

Tris­(4-isocyanatophenyl)­methane (TIPM) and <i>N</i>,<i>N</i>′-dimethylformamide react at room temperature with no externally added catalyst to yield polyisocyanurate (PIR) gels. The obtained PIR gels were converted to N- and S-doped porous carbon monoliths by thermal treatment at 1000 °C with elemental sulfur under inert conditions. The PIR linkage acts as precursor for carbon and nitrogen, and %S doping was varied by changing the concentrations of elemental sulfur during pyrolysis. The optimized concentration of sulfur (5.6%) into the carbon matrix displayed excellent oxygen reduction activity with direct four-electron transfer relative to its pristine counterparts by (1) introducing micro- and mesopores in addition to the already existing macropores by etching the carbon surface (confirmed by N₂ sorption isotherms and microscopic images) with the increase in the external surface area providing more active centers and efficient diffusion of electrolyte ions, (2) providing more – C–S–C– active species than oxidized sulfur species (confirmed by XPS and FT-IR) with more oxygen adsorption sites, and (3) filling the micropores of the carbon as a monolayer, affording increased electronic conductivity to the amorphous carbon. This simple and facile method of incorporating N- and S- together into the porous carbon matrix can be considered as an alternate for nonprecious metal catalysts for oxygen reduction reaction