Compared with traditional electrode materials, porous nanostructured electrodes can notably advance ion transport and electron conductivity, furthering the overall energy-storage competency. The development of various nanostructures with controlled porosity and their cost-effective synthesis processes can improve the performance of the existing energy-storage systems. Engineering porous nanomaterials with optimized porosity, heteroatom doping, high surface area, and excellent electrochemical performance remains a significant challenge in supercapacitors. Overcoming these challenges will enable the development of high-performance, cost-effective, and scalable energy-storage systems. This chapter discusses fruitful methods and outlines a roadmap for using porous nanostructured electrodes in energy-storage devices. The chapter includes the latest research on 1D, 2D and 3D porous nanostructures of metal oxides and their suitability for electrochemical activity. It also provides an overview of the current state-of-the-art for forming porous core–shell nanomaterials of metal oxides and recommends the future direction for using them in energy-storage applications.

A single-step pyrolysis synthesis method is employed to synthesize AuCo nanoparticles embedded in carbon nanostructures. The eco-friendliness and widespread availability of banana leaf (biomass) make it an ideal precursor to produce carbon-based materials. The natural presence of intrinsic nitrogen in banana leaf enables an environmentally friendly method of N-doping. This naturally N-doped carbon framework incorporating AuCo alloy nanoparticles serves as a bifunctional electrocatalyst. The alloy nanoparticles are encapsulated within graphitic carbon shells, while the outermost carbon layer with defects offers a high surface area and large catalytic active centers. The catalyst demonstrates only 344 mV overpotential at 10 mA/cm2 for the oxygen evolution, while for the oxygen reduction, it reveals an onset potential (Eonset) of 0.947 V and half-wave potential (E1/2) of 0.784 V vs RHE (0.1M KOH). To demonstrate practical applicability, a zinc-air battery was fabricated, exhibiting a high OCV and power density (192.4 mW/cm2), outperforming the commercially available Pt/C + RuO2 (117.5 mW/cm2) catalyst and numerous other reported catalysts. These prepared sample exhibits excellent electrocatalytic performance and has the potential for straightforward, inexpensive fabrication of Zn-air batteries.

Controlling nitrogen functionalities in graphene are crucial for high-performance supercapacitors. In this study, we report the scalable synthesis of nitrogen-doped reduced graphene oxide (N-rGO) via a hydrothermal process using readily available inorganic precursors in an aqueous phase, enabling controlled manipulation of nitrogen functionalities. The optimized N-rGO (GO:aqueous NH3 = 1:6) exhibits a areal capacitance of 0.4 F cm−2 at a scan rate of 5 mV s−1. In a symmetric two-electrode system, N-rGO delivers a high specific capacitance of 532 F g−1, along with an energy density of 41 Wh kg−1 and a power density of 375 W kg−1 at 0.5 A g−1, with a capacitance retention of ∼78% after 10,000 cycles at 1.96 V. These performance metrics are attributed to an optimal defect concentration combined with a high proportion of amino and pyrrolic nitrogen functionalities. Quantum capacitance calculations reveal an enhanced electronic density of states near the Fermi level, thereby improving charge storage performance.

A dual heteroatom (N,P)-doped carbon nanostructure grafted on nickel nanoparticles synergistically enhances the catalytic effect for H2O adsorption during the hydrogen evolution reaction (HER). We developed a one-step pyrolysis route to synthesize N,P-codoped carbon grafted on Ni nanoparticles as an efficient electrocatalyst for H2 generation. The atomic size mismatch of N and P with C disrupts the sp2 carbon lattice, introducing defects. Furthermore, defect coalescence generates pores and abundant active sites, enabling excellent HER performance with an overpotential of only 41 mV at 10 mA cm-2 in 1 M KOH. We developed a custom setup to quantify the generated H2 as a function of applied potential using chronoamperometric measurements. The H2 generation rate increased with applied negative potential, reaching 3.60 mL mg-1 min-1 at -200 mV. The dynamics of ions for a highly efficient catalyst were further investigated using electrochemical impedance spectroscopy (EIS) at the same applied potential. The interconnected pores in the nanostructure facilitated the electroactive species to access internal active sites, consequently lowering the magnitude of impedance (|Z|imp). As the applied negative potential increased, a sharp decrease in |Z|imp indicated an improved HER kinetics, leading to faster reaction rates and enhanced performance.

We report phase pure vertically aligned zinc oxide nanorods (ZnO NRs) using a low-cost hydrothermal process on the pre-deposited RF sputtered ZnO seed layer on SiO2/Si substrate. The variation in seed layer thickness is used to control the diameter and surface porosity of ZnO NRs. The average diameter of NRs is 153, 103, and 90 nm for 2, 4, and 6-min deposited seed layers, respectively, with an average length of 3 mu m irrespective of the seed layer thickness. Further, the Au/Cr interdigitated electrode is deposited on ZnO NRs/SiO2/Si to analyze its H2 sensing performance. It is observed that, among various devices, the 2 min seed layered ZnO NRs (ZNR-2) delivered superior sensing performance with 19.6% relative response, 28 s response, and 104 s recovery time, with exposure to 60 ppm H2 gas at temperature 100 degrees C. The limit of detection and limit of quantification for the ZNR-2 device is 68 ppb and 227 ppb, respectively. This device selectively responds to H2 concerning other gases. Thus, the present findings provide the guiding path for improving the sensing response for Zinc oxide-based H2 sensors.

The industrialization has severely impacted the ecosystem because of intensive use of chemicals and gases, causing the undesired outcomes such as hazardous gases, e.g. carbon monoxide (CO), nitrous oxide (NOx), ammonia (NH3), hydrogen (H2), hydrogen sulfide (H2S) and even volatile organic compounds. These hazardous gases are not only impacting the living beings but also the entire ecosystem. Thus, it becomes essential to monitor these gases for their efficient management. There are continuous efforts to realize such sensors, which rely on the functional materials properties. The widely used such sensors use metal oxide nanomaterials. However, these are not very sensitive and operate at higher temperatures. In contrast, two-dimensional (2D) materials such as Graphene, Borophene, MXenes, and transition metal dichalcogenides (TMDs) including doping, functionalization, and heterostructures offer unique physical, chemical, and optoelectronic properties. The chemical properties with high specific surface area of 2D materials make them suitable for gas sensing applications. The present review covers the recent developments on 2D-layered material, including MoS2, WS2, h-BN, and Graphene, as well as their heterostructures for gas sensing applications. The review article also emphasizes their synthesis and characterization techniques, especially for 2D materials. The electronic properties of these materials are highly sensitive to any chemical changes, resulting in significant changes in their resistance. It led to the development of the highly scalable chemiresistive-based gas sensor. The sensing parameters such as sensitivity, selectivity, gas concentration, limit of detection, temperature, humidity, response, reproducibility, stability, recovery, and response time are discussed in detail to understand the gas sensing characteristics of these 2D materials. This review also includes the past developments, current status, and future scope of these 2D materials as highly efficient gas sensors. Thus, this review article may lead the researchers to design and develop highly sensitive gas sensors based on 2D materials.

We synthesized CoRu bimetallic nanoparticles with varying metal contents (Co : Ru: 3 : 1, 1 : 1, and 1 : 3) embedded in a graphitized carbon framework and used them as electrocatalysts. Benefiting from the porous nature of the carbon structure, which exposes a large number of active centers and the synergy with bimetallic nanoparticles, the catalysts exhibit excellent performance for both oxygen evolution and reduction reactions (OER/ORR). The catalyst CoRu@C (with a 1 : 1 metallic ratio) introduces a strong synergistic effect between alloy nanoparticles and the carbon layer, enhancing its catalytic activity compared to the other catalysts. This CoRu@C sample exhibits a low overpotential (eta 10) of 310 mV surpassing that of RuO2 (360 mV) for the OER and a half-wave potential (E1/2) of 0.838 V vs. the reversible hydrogen electrode (RHE), which is comparable to that of Pt/C (0.845 V) for the ORR in 0.1 M KOH. For practical feasibility, the fabricated Zn-air battery using this CoRu@C catalyst exhibits a higher open circuit voltage (OCV) of 1.461 V, surpassing that of Pt/C + RuO2 (1.443 V) and achieves a peak power density of 201.3 mW cm-2, outperforming Pt/C + RuO2 (138.1 mW cm-2). The ex situ characterization conducted at each 50 hour interval over the 250 hour cycling period demonstrates the robust nature of CoRu@C. This study provides a new avenue for designing efficient nanostructures for energy storage and conversion.

Spectrally selective absorbers advance rapidly, yet achieving lower emittance at higher temperatures is still a challenge. The present study examined the absorptance and emittance performance of thermally treated stainless steel (Annealed/SS) and stainless steel coated with barium titanate oxide (BTO/SS) as efficient solar selective absorbers and their robustness. The absorptance values for Annealed/SS and BTO/SS in the UV-Vis range (0.2 to 0.8 mu m) are 90 and 88 %, respectively, whereas the emittance values in the infrared range (2.5 to 25 mu m) are 3 % and 9 %. The solar selective coating maintains its absorptance of similar to 88 - 90 % and emittance of 2 - 9 % for BTO/SS and annealed/SS for 60 h after annealing at 500 degrees C. In addition, at a scan rate of 50 mV/s, the corrosion measurement indicates that the pristine SS substrate has a greater rate of corrosion (0.097 mm/yr) compared to BTO/SS (similar to 0.057 mm/yr) and Annealed/SS (similar to 0.0001 mm/yr) suggesting the long span stability of Annealed/SS in a harsh environment, as also evident from the electrochemical impedance spectroscopy (EIS) measurements. The annealed stainless steel (SS) exhibits higher charge transfer resistance (1000 k Omega) compared to BTO/SS and bare SS. Thus, the thermally annealed SS can be used an efficient solar selective absorber with enhanced durability, thermal stability, and corrosion resistance. It, in turn, allows to realize the spectrally selective absorber coatings without any additional processing, offering a low-cost, scalable alternative to traditional methods for creating solar selective coatings on a large scale.

We synthesized CoRu bimetallic nanoparticles with varying metal content (Co:Ru :: 3:1, 1:1, and 1:3) embedded in a graphitized carbon framework and used as electrocatalysts. Benefiting from the porous nature...

Hydrogen, with the highest gravimetric energy density, is recognized as one of the most promising alternatives to rapidly depleting fossil fuels. However, high flammability and undetectability due to odor and color-less characteristics pose a severe threat to its safe use. Thus, the efficient monitoring of hydrogen becomes essential, starting from its generation/storage sites, transportation, and even hydrogen-fuel vehicles. It relies on developing efficient hydrogen sensing materials capable of sensing even ultra-low concentrations. Carbon nanomaterials are the materials of choice because of their tuneable physical/chemical and electronic properties by controlling the surface morphologies/modifications of their various allotropes, making them the ideal platform for efficient hydrogen detection. Considering the continuously growing demand for hydrogen and its uses, this article presents recent developments in carbon nanomaterials such as single/multiwalled carbon nanotubes (S/M-WCNT), graphene oxide (GO), reduced graphene oxide (rGO), graphene, and graphitic carbon nitride (g-C3N4) for hydrogen sensor. The 3S (i.e., sensitivity, selectivity, and stability) sensor parameters are discussed for these materials in conjunction with respective response/recovery times. Further, the article emphasizes possible hydrogen sensing mechanisms, which will assist in designing room temperature hydrogen sensors with enhanced sensitivity even at parts per billion (ppb) level and fast response and recovery times. Thus, this article will be helpful for both researchers and industrialists working on hydrogen sensors to mitigate the safety issues of the ever-growing hydrogen energy sector.

We synthesized sustainable, biomass-derived electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In particular, cobalt nanoparticle-embedded, defect-rich, and N-doped porous carbon framework samples are synthesized through pyrolysis of natural banana leaves dispersed in cobalt nitrate solution. Dispersing the fixed amount of banana leaf powder in different concentrations (10, 25, and 50 mM) of cobalt nitrate solutions, we achieved varying nitrogen contents in the samples, which maintained the C/N ratio across samples. According to the theory of nucleation and growth, the average size of the nanoparticles decreases as the amount of homogeneously dispersed nanoparticles increases gradually from the 10 mM (Co@C10) to the 50 mM (Co@C50) sample. The samples were utilized for studying the rates of hydrogen and oxygen gas evolution through the HER and the OER, respectively. We observed a high gas evolution rate of 0.22 mL/min for H2 (at -0.8 V vs RHE) and 0.078 mL/min for O2 (at 1.95 V vs RHE) in aqueous 0.1 M KOH electrolyte for the Co@C50 sample in comparison to the other samples. As the concentration of the electrolyte was increased from 0.1 to 1.0 M KOH, the H2 and O2 gas production rate also increased, even though OH- ions of the electrolyte were not involved in HER. We provided a detailed explanation of the mechanism of the enhanced rate of gas production. Hence, the banana leaf-derived N-doped, defect-rich porous carbon materials embedded with cobalt nanoparticles are excellent catalysts for the production of green hydrogen energy to combat the global green fuel demand.

The present study provides a way to modify the structural, morphological, and optical properties of tungsten trioxide (WO3) thin films through high-energy ion beam irradiation. The WO3 thin films fabricated by the spin-coating technique were irradiated with 120 MeV Ni7+ at fluences 5 x 1012, 1 x 1013, and 3 x 1013 ions/cm2. Characterizations such as XRD, Raman, FTIR, AFM, FESEM, and UV-visible spectroscopy have been carried out to identify structural, spectroscopic, morphological, and optical properties. The XRD spectra of the pre-and post-irradiated thin films showed the monoclinic structure for all the films with variations in the intensity of the peaks. The Raman peak intensity decreased due to the onset of defects, and reduction in crystallinity after the ion beam irradiation. The AFM analysis revealed the effect of irradiation on the crystallinity and surface roughness of films compared to the pristine thin films. The optical properties studied by absorption spectra revealed a reduction in the optical band gap after irradiations. The tuned characteristics of the thin films by irradiation of the high-energy ion beams in the present study are suitable for technological applications in various fields.

A facile methodology to synthesize the nitrogen-doped reduced graphene oxide (NG) via a solvothermal process exhibiting a substantial content of either pyrrolic-N or pyridinic N, is demonstrated. In the synthesis of NG, different amounts of dicyandiamide (DCDA) with a constant (1 g) amount of graphene oxide have been taken in DMF (solvent). This approach produced the nanosheets of N-doped graphene (NG-2) which possess optimum defect density and 10.58 % total nitrogen. Nitrogen within this NG material predominantly manifests in four distinct bonding configurations, with the most prevalent being pyrrolic-N. The solvothermally synthesized N-doped reduced graphene oxide exhibited good charge storage characteristics. Three electrode testing of NG-2 shows high specific capacitance (865.82 F/g at a particular current density of 0.5 A/g) and a high energy density (76.96 Wh/kg) in acidic conditions. This sample showed poor electrocatalytic activity for oxygen evo-lution reaction (OER). However, the material prepared with very high amount of DCDA (NG-4) showed a total N-content of 29 % with high contribution of pyridinic N, demonstrated good electrocatalytic activity for OER following the four-electron path. The Tafel slope of NG-4 sample is around 140 mV/dec which implies the facile conversion of OH- into O2, at relatively low overpotential within an alkaline system. Consequently, the syn-thesized dual-functional N-doped reduced graphene oxide material holds characteristics for the advancement of carbon-based energy storage materials, and simultaneously be utilized as catalyst for OER.

Graphene oxide (GO) sheets were prepared using the improved Hummers’ method, followed by simultaneous reduction and nitrogen doping using an easy solvothermal route to obtain Nitrogen-doped reduced graphene oxide (N-rGO) sheets. The amount of N-doping was controlled using the different amounts of GO and Urea, keeping the ratio of the two (1:3) fixed. Here, we examined several variations in synthesis parameters, viz. change in precursor content, variation in synthesis time, and volume of dispersive solvent. The N-rGO sample prepared with 250mg GO and 750 mg urea at 24-hour synthesis time in 80 ml dispersive solvent exhibits a superlative specific capacitance of 1244 F/g at 0.5 A/g using 3-electrode measurements in an acidic medium. The high value can be understood via the synergistic roles of optimum defects, degree of reduction, and types of N-environments. The symmetric 2-electrode device prepared by depositing the synthesized material onto carbon cloth exhibits supercapacitance of 635 F/g at 1 A/g current density and high cyclic constancy with capacitive retention of ~ 96% after 10000 charge-discharge cycles. Our work provides critical mechanistic insights on the complex intermingling of an optimum N-content, the relative amount of the different N-environments, degree of reduction, and disorder to generate a high specific capacitance, remarkably better than previously reported works on similar materials. Notably, a relatively high graphitic-N, with moderate surface N-content (4.64%) and a moderate degree of reduction, leads to this higher specific capacitance.

This study evaluates the critical roles of the dispersion medium and temperature during the solvothermal synthesis of nitrogen-doped reduced graphene oxide (NG) for enhancing its performance as an active material in supercapacitor electrodes. Using a fixed volume of a solvent (THF, ethanol, acetonitrile, water, N,N-Dimethylformamide, ethylene glycol, or N-Methyl-2-pyrrolidone) as the dispersive medium, a series of samples at different temperatures (60, 75, 95, 120, 150, 180, and 195 °C) are synthesized and investigated. A proper removal of the oxygen moieties from their surface and an optimum number of N-based defects are essential for a better reduction of graphene oxide and better stacking of the NG sheets. The origin of the supercapacitance of NG sheets can be correlated to the inherent properties such as the boiling point, viscosity, dipole moment, and dielectric constant of all the studied solvents, along with the synthesis temperature. Due to the achievement of a suitable synthesis environment, NG synthesized using N,N-Dimethylformamide at 150 °C displays an excellent supercapacitance value of 514 F/g at 0.5 A/g, which is the highest among all our samples and also competitive among several state-of-the-art lightweight carbon materials. Our work not only helps in understanding the origin of the supercapacitance exhibited by graphene-based materials but also tuning them through a suitable choice of synthesis conditions.

The Brown gas (HHO), i.e., hydrogen as well as oxygen, is considered as one of the assuring fuels for the future. It can be generated by renewable process such as (i) photoelectrochemical water splitting, (ii) thermochemical water splitting, (iii) electrochemical water splitting, etc., with zero carbon emission. We designed an HHO generator using different electrode materials and investigated the impact of electrolyte molarity, and plate counts, which determine the efficiency and gas generation capacity of the device. We observed that the combination of stainless steel with KOH results in the maximum yield in a hydrolytic reaction among other electrode materials. On the other hand, aluminium was failing to provide a minimum power to initiate the reaction at a feasible rate, aside of that stainless steel provided power of 90 W attributed to the higher electrolysis rate. They will endure for a long period so that we can escalate the production at the industrial scale. While working with 3 biased and 2 neutral electrodes combination, the device was able to continuously generate HHO at a flow rate of 175 ml/min for 30 min with power of 100.8 W. And with 3 biased electrodes and 4 neutral electrodes, highest production of HHO gas was 180 ml/min at 118 W power. Thus, the present device with stainless steel as electrode and KOH as electrolyte can be a better option for a cost-efficient large-scale production of the HHO gas.

We introduce an environmentally sustainable approach to create uniformly nitrogen-incapacitated carbon nanotubes nucleated through cobalt nanoparticles. A unique architecture involving carbon shells on Co nanoparticles exists along with the surface morphology prompted by the existence of various nitrogen moieties. The Co nanoparticles are covered by graphitic carbon shells and confined within a defective porous carbon framework, enhancing the surface area and porosity and exposing large active centers for catalytic activities. Our approach uses a single-step in situ synthesis process, which yields these exceptionally good bifunctional electrocatalysts for oxygen evolution reaction, oxygen reduction reaction, and zinc-air battery applications. These massive active centers and pores provide the advantage of exceptional oxygen reduction reaction (ORR) performance. Our samples with Co-embedded carbon nanotubes with optimal N-doping exhibit a higher half-wave potential (E-1/2) of similar to 0.882 V vs a reversible hydrogen electrode (RHE) compared to a standard Pt/C electrode (similar to 0.874 V vs RHE) in aqueous 0.1 M KOH solution, at the same catalyst-mass loading. For zinc-air batteries, our samples, as air-reducing catalysts, exhibit a high open-circuit voltage of 1.512 V and an impressive peak power density (150.6 mW cm(-2)), which surpasses the commercial Pt/C + RuO2 catalysts (92.4 mW cm(-2)), as well as many reported bifunctional electrocatalysts. Hence, our samples are potential candidates for the coherent depiction of multifunctional, efficient, and durable electrocatalysts for the straightforward, inexpensive, and scalable fabrication of rechargeable zinc-air batteries.

We demonstrated the resistive random-access memory (RRAM) characteristics in cost-efficient, single-step, and in-situ grown nanostructured mixed-phase Cu x O (x = 1, 2) thin films, on a commercially available Cu sheet, fabricated using thermal treatment under ambient conditions. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements explain the surface morphology of grown mixed-phase Cu x O thin film with substrate imprints. Fourier-transform Infrared spectroscopy confirmed the mixed phase of Cu 2 O and CuO of synthesized thin films. The fabricated Al/Cu x O/Cu RRAM device showed bipolar digital resistive switching followed by analog resistive switching. The device showed a fast-switching speed of 250 ms, high endurance durability for 2500 cycles, and better retention of 10 3 s, respectively. The interconnected Cu x O with Cu sheet provides efficient charge carrier migration from the bottom electrode to the active layer. The formed defect and trapped site are responsible for the resistive switching behavior. Thus, the present study provides a way to harness the potential of RRAM on either thermally treated Cu sheets or realizing a Cu thin film followed by low-temperature annealing, which can easily be integrated into existing electronic devices for data storage applications.

Two types of nitrogen-doped carbon nanomaterials embedded with novel transition metal-based medium entropy alloy nanoparticles are synthesized via a simple, scalable, and cost-effective single-step pyrolysis route, by varying the stoichiometric ratio of iron and cobalt. The samples predominantly have bamboo-shaped nanotube morphology with some globular structures. The globules are core-shell type nanoparticles, i.e., containing the metallic-alloy nanoparticles encapsulated by the graphitic shell and embedded within defective amorphous-type carbon matrix. The electrochemical studies using these samples are conducted in basic medium to explore their potential for Zn-air batteries. Electrochemical measurements are carried out in 1 M and 0.1 M KOH, respectively, on a rotating disk electrode setup. The sample with higher cobalt content performs better for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) with regards to lower onset potential, lower Tafel slope and lower activation energy. However, the stability towards OER is better for the sample with higher iron content. The sample with higher cobalt content shows a better overall oxygen electrochemistry compared to the one with higher iron content, is at par with several state-of art electrocatalysts and is utilized for fabrication of a prototype Zn-air battery demonstrating its practical applicability.