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In this study, a nanocomposite of palladium supported graphene oxide (GO)/metal–organic framework (MOF) was prepared using electroless deposition of Pd on GO followed by impregnation method of Pd@GO and MOF. The prepared materials were characterized with various analytical techniques and their applications as HER electrocatalysts were evaluated using cyclic voltammetry (CV), Tafel plots, and turn over frequencies (TOFs). The HER results showed a radical increment of H2 production in the nanocomposite through the Volmer reaction together with Heyrovsky or Tafel mechanism. This disclosed that the addition of Pd@GO/MOF in the electrolytic system possessed better catalytic characteristics with enhanced current density which may open a new way for hydrogen production and storage via HER.

In recent years, ways to modify the thermodynamic and kinetic properties of functional materials for energy storage have gained an immense interest. One way is through formation of composites by combining two or more compatible materials in the hopes of enhancing properties and superior performance. Surface modification of energy storage materials by coating with transition metals, metal oxides, metal halides and carbon materials has also been exploited with great success. Metal oxides have shown great potential as coating candidates due to their high electric conductivity, ability to enhance structural stability and good electrochemical performance when compared to majority of other surface modifications. In this regard, we review recent advances and various aspects in relation to performance enhancement effects of different metal oxides that are used as coatings on materials for hydrogen adsorption/absorption properties. This review further compares the compatibility of metal oxides on porous and non-porous energy storage materials. Fundamental relationships and the state-of-the art in the prediction of properties and experimental observations are outlined and structure–property-relationships are also discussed.

In this study, a sodium alginate poly(acrylic acid) (SA-poly(AA)) hydrogel and sodium alginate poly(acrylic acid)/zinc oxide (SA-poly(AA)/ZnO) hydrogel nanocomposite (HNC) was synthesized by in situ free-radical polymerization for the sequestration of toxic methylene blue (MB) dye from aqueous solution. The structural properties were analyzed using FTIR, XRD, SEM, TEM, TGA, and DMA The swelling analysis revealed that SA-poly(AA)/ZnO HNC exhibited high water uptake capacity. The kinetics, isotherms, and thermodynamics of adsorption were examined, and results showed that equilibrium data fitted the Langmuir isotherm model, and the adsorption kinetics of MB followed pseudo-second-order model. The maximum adsorption capacity was found to be 1129 mg/g for the SA-poly(AA) hydrogel and 1529.6 mg/g for the SA-poly(AA)/ZnO HNC in 0.25 g/L MB solution at pH 6.0 within 40 min. Thermodynamic parameters for SA-poly(AA) hydrogel and SA-poly(AA)/ZnO HNC substantiated the exothermic and endothermic nature of the adsorption processes, respectively. Moreover, SA-poly(AA)/ZnO HNC presented outstanding reusability with relatively better adsorption efficiencies as compared to SA-poly(AA) hydrogel.

Green synthesis of polypyrrole/nanoscale zero valent iron nanocomposite (Ppy/Fe0 NC) fabricated by a simple in-situ oxidative polymerization method was reported. The structure and the properties of Ppy/Fe0 NC were confirmed using various analytical techniques. The as-prepared Ppy/Fe0 NC was used as an adsorbent for the adsorption of hexavalent chromium (Cr(VI)) through batch experiments. The effect of initial pH, adsorbent dose, contact time, equilibrium concentration and co-existing ions were studied. It was deduced that adsorption data followed the pseudo-second-order rate model and Langmuir isotherm model with a maximum adsorption capacity of 202.02 mg/g at 25°C. Furthermore, high selectivity of Ppy/Fe0 nanocomposite for the removal Cr(VI) was observed even in the presence of co-existing ions. These findings suggest that the green synthesized adsorbent can be used for treatment of wastewater containing Cr(VI).

Carboxymethyl cellulose/poly(acrylic acid) (CMC-cl-pAA) hydrogel and its magnetic hydrogel nanocomposite (CMC-cl-pAA/Fe3O4-C30B) were prepared via a free radical polymerization method and used as adsorbents for adsorption of methylene blue (MB) dye. The samples were characterized using Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectrometer, high-resolution transmission electron microscope, and dynamic mechanical analysis. The adsorption performance of the prepared adsorbents was studied in a batch mode. Adsorption kinetics and isotherm models were applied in the experimental data to evaluate the nature as well as the mechanism of adsorption processes. It was deduced that the adsorption followed the pseudo-second-order rate equation and Langmuir isotherm models. The maximum adsorption capacities were found to be 1109.55 and 1081.60 mg/g for CMC-cl-pAA hydrogel and CMC-cl-pAA/Fe3O4-C30B hydrogel nanocomposite, respectively. The adsorption thermodynamic studies suggested that the adsorption process was spontaneous and endothermic for CMC-cl-pAA/Fe3O4-C30B hydrogel nanocomposite. The homogeneous dispersion of the Fe3O4-C30B nanocomposite in the CMC-cl-pAA hydrogel significantly improved the thermal stability, mechanical strength, and excellent regeneration stability. This study demonstrates the application potential of the fascinating properties of CMC-cl-pAA/Fe3O4-C30B hydrogel nanocomposite as a highly efficient adsorbent in the removal of organic dyes from aqueous solution.

Copper(II)phthalocyanine-incorporated metal organic framework (CuPc/MOF) composite material was synthesized for application as an electrocatalyst for hydrogen evolution reaction (HER). The composite exhibited excellent electroactivity compared to the unmodified MOF, as confirmed by the diffusion coefficients (D) values of 3.89 × 10−7 and 1.57 × 10−6 cm2 s−1 for MOF and CuPc/MOF, respectively. The D values were determined from cyclic voltammetry (CV) experiments performed in 0.1 mol L−1 tetrabutylammonium perchlorate/dimethyl sulfoxide (TBAP/DMSO) electrolyte. The Tafel slope determined from the CV data of CuPc/MOF-catalysed HER for 0.450 mol L−1 H2SO4, was 176.2 mV dec−1, which was higher than that of the unmodified MOF (158.3 mV dec−1). The charge transfer coefficients of MOF and CuPc/MOF were close to 0.5, signifying the occurrence of a Volmer reaction involving either the Heyrovsky or the Tafel mechanism for hydrogen generation. For both MOF and CuPc/MOF, the exchange current density (i0) improved with increase in the concentration of the hydrogen source (i.e. 0.033–0.45 mol L−1 H2SO4) Nonetheless, the CuPc/MOF composite had a higher i0 value compared with the unmodified MOF. Thus CuPc/MOF has promise as an efficient electrocatalyst for HER.

The quest for smart sensors with excellent sensitivity, fast response and high reproducibility for hydrogen gas detection has been ongoing for decades. In this work, polyaniline (PANI) was doped with a cobalt-based zeolitic benzimidazolate framework material (CoZIF), to form PANI-CoZIF metal-organic framework (MOF) composite material, which was applied in the electrochemical hydrogen gas sensing. The synthesis of PANI-CoZIF composite was performed by the chemical oxidative polymerization of aniline in the presence of 3.6 wt% CoZIF. The structural properties of the synthesized materials (i.e., PANI, CoZIF, and PANI-CoZIF) were studied by ultraviolet-visible (UV-vis), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman spectroscopic analyses, as well as by thermogravimetric analysis (TGA). Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and high-resolution transmission electron microscopy/selected area electron diffraction (HR-TEM/SAED) techniques were used for morphological characterization. Both SEM and TEM analyses revealed the grafting of CoZIF on to the surface of PANI in the PANI-CoZIF composite material. XRD patterns and SAED results indicated that PANI and the composite were amorphous in nature. Spectroscopic analyses by FTIR and Raman techniques revealed the coexistence of CoZIF and PANI in the MOF composite material (PANI-CoZIF), which was also confirmed by the high intensities of the characteristic X-ray peaks of ZIF and the polymer. From Tauc plot analysis, it was found that PANI and PANI-CoZIF had band gap energy (Eg) values of 2.5 and 2.3 eV, respectively, indicating that they are both semi-conductors. TGA data confirmed an enhanced thermal stability of PANI-CoZIF compared with pristine PANI. Cyclic voltammetry (CV), chronoamperometry (CA), and Tafel and turn over frequency (TOF) analyses were used to study the electrocatalytic behavior of PANI and PANI-CoZIF in the hydrogen evolution reaction (HER) for gas sensing. There were very significant increases in the TOF value and the magnitude of the catalytic current for PANI-CoZIF compared with pristine PANI as electrocatalysts in the HER. Thus, PANI-CoZIF composite has a greater potential as a suitable electrocatalyst for practical application in the hydrogen fuel technology and hydrogen gas sensing.

Herein, we report on the in-situ chemical polymerization of polyaniline (PANI) doped with zinc-based zeolitic benzimidazolate framework (ZnZIF) to fabricate the PANI-ZnZIF composite for electrochemical hydrogen gas sensing. Fourier-transform infrared and Raman studies of PANI-ZnZIF composite revealed the presence of functional groups corresponding to both ZnZIF and PANI. Both X-ray diffraction and simultaneous thermal analyses showed the co-existence of both PANI and ZnZIF in the composite supported by the decrease in crystallite size and the improvement in the thermal stability, respectively. The scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction revealed that PANI nanofibers wrapped the cube nanofiber-like structures of ZnZIF. The square wave voltammetry, cyclic voltammetry (CV) and chronoamperometry presented good electrochemical performance indicated by an increase in current response with 3 wt% loading of ZnZIF. The high electrochemical current response is due to extraordinary specific surface area, more accessible active sites available for the electrolyte provided by ZnZIF and high conductivity supplied by PANI. Moreover, Tafel parameters and Turnover frequency values derived from CV showed an improvement in the catalytic hydrogen evolution of PANI-ZnZIF composite. The resulting PANI-ZnZIF composite was found to be highly responsive to hydrogen gas as indicated by higher current response and sensitivity (10.8 μA mM H2), faster steady state response time of 4 s accompanied by lower detection limit of 5.27 μM as compared to pure PANI.

The accumulation of toxic heavy metal ions continues to be a global concern due to their adverse effects on the health of human beings and animals. Adsorption technology has always been a preferred method for the removal of these pollutants from wastewater due to its cost-effectiveness and simplicity. Hence, the development of highly efficient adsorbents as a result of the advent of novel materials with interesting structural properties remains to be the ultimate objective to improve the adsorption efficiencies of this method. As such, advanced materials such as metal–organic frameworks (MOFs) that are highly porous crystalline materials have been explored as potential adsorbents for capturing metal ions. However, due to their diverse structures and tuneable surface functionalities, there is a need to find efficient characterization techniques to study their atomic arrangements for a better understanding of their adsorption capabilities on heavy metal ions. Moreover, the existence of various species of heavy metal ions and their ability to form complexes have triggered the need to qualitatively and quantitatively determine their concentrations in the environment. Hence, it is crucial to employ techniques that can provide insight into the structural arrangements in MOF composites as well as their possible interactions with heavy metal ions, to achieve high removal efficiency and adsorption capacities. Thus, this work provides an extensive review and discussion of various techniques such as X-ray diffraction, Brunauer–Emmett–Teller theory, scanning electron microscopy and transmission electron microscopy coupled with energy dispersive spectroscopy, and X-ray photoelectron spectroscopy employed for the characterization of MOF composites before and after their interaction with toxic metal ions. The review further looks into the analytical methods (i.e., inductively coupled plasma mass spectroscopy, ultraviolet-visible spectroscopy, and atomic absorption spectroscopy) used for the quantification of heavy metal ions present in wastewater treatment.

In the recent years, lithium-ion batteries have prevailed and dominated as the primary power sources for mobile electronic applications. Equally, their use in electric resources of transportation and other high-level applications is hindered to some certain extent. As a result, innovative fabrication of lithium-ion batteries based on best performing cathode materials should be developed as electrochemical performances of batteries depends largely on the electrode materials. Elemental doping and coating of cathode materials as a way of upgrading Li-ion batteries have gained interest and have modified most of the commonly used cathode materials. This has resulted in enhanced penetration of Li-ions, ionic mobility, electric conductivity and cyclability, with lesser capacity fading compared to traditional parent materials. The current paper reviews the role and effect of metal oxides as coatings for improvement of cathode materials in Li-ion batteries. For layered cathode materials, a clear evaluation of how metal oxide coatings sweep of metal ion dissolution, phase transitions and hydrofluoric acid attacks is detailed. Whereas the effective ways in which metal oxides suppress metal ion dissolution and capacity fading related to spinel cathode materials are explained. Lastly, challenges faced by olivine-type cathode materials, namely; low electronic conductivity and diffusion coefficient of Li+ ion, are discussed and recent findings on how metal oxide coatings could curb such limitations are outlined.

The deterioration of hydrogen charging performances resulting from the surface chemical action of electrophilic gases such as CO2 is one of the prevailing drawbacks of TiMn1.52 materials. In this study, we report the effect of autocatalytic Pd deposition on the morphology, structure, and hydrogenation kinetics of TiMn1.52 alloy. Both the uncoated and Pd-coated materials were characterized using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). XRD analyses indicated that TiMn1.52 alloy contains C14-type Laves phase without any second phase, while the SEM images, together with a particle size distribution histogram, showed a smooth non-porous surface with irregular-shaped particles ranging in size from 1 to 8 µm. The XRD pattern of Pd-coated alloy revealed that C14-type Laves phase was still maintained upon Pd deposition. This was further supported by calculated crystallite size of 29 nm for both materials. Furthermore, a Sieverts-type apparatus was used to study the kinetics of the alloys after pre-exposure to air and upon vacuum heating at 300 °C. The Pd-coated AB2 alloy exhibited good coating quality as confirmed by EDS with enhanced hydrogen absorption kinetics, even without activation. This is attributed to improved surface tolerance and a hydrogen spillover mechanism, facilitated by Pd nanoparticles. Vacuum heating at 300 °C resulted in removal of surface barriers and showed improved hydrogen absorption performances for both coated and uncoated alloys.

With growth in civilisation and industrialisation, there is an increase in the release of toxic heavy metal ions and dyes into water system, which is of public concern. As a result, appropriate treatment methods have to be implemented in order to mitigate and prevent water pollution. The discovery of nanotechnology has led to the development and utilisation of various nanoadsorbent for the removal of pollutants from water. PANI nanostructures and nanocomposites are noble adsorbents that have gained popularity in addressing water pollution issues and have been reported in literature. In this chapter, the main focus is on the synthesis of PANI nanocomposites and nanostructures and their application as efficient adsorbents for water treatment. Detailed discussions on different synthetic routes and characterisation have been dedicated to applications of these materials and are compared for the adsorptive removal of heavy metal ions and dyes from water.

The current major global crisis is in the sector of water, energy and food security. Due to the growth in civilisation, industrialisation and environmental fluctuations water scarcity and freshwater availability poses major concern to the world, especially to the developing countries. Nearly, one-third of drinkable water is obtained from surface sources such as rivers, dams, lakes, and canals, and some of this water is exposed to contamination. Therefore, appropriate treatment methodologies must be implemented in order to mitigate and prevent water contamination. Photocatalysis is an approach to curb the challenge of water contamination by introducing the use various materials for the degradation of pollutants from wastewater. Metal organic frameworks (MOFs) and their composites as materials of interests have gained popularity in addressing water pollution in photocatalysis applications by acting as absorbents of light, prominent to improve performance of photocatalytic activity. This chapter focuses on the synthesis and characterization of MOFs and their composite materials as efficient photocatalysts for degradation of dyes from wastewater.

The removal of persistent heavy metal ions and recovery of platinum group metals from industrial wastewater continues to be an essential step in safeguarding the available water resources to ensure the safety of humans, animals, and the environment. With adsorption technology being widely implemented due to its cost-effectiveness, metal-organic frameworks (MOFs) have gained popularity owing to their high surface area, porosity, and tunable functionality. MOF-based materials have since been developed for various applications such as catalysis, energy storage, sensing, drug delivery, and recently in water purification. This review gives a background of different synthetic routes available for the preparation of MOFs and also their application in various fields. With more emphasis on the removal of pollutants, MOFs have shown to be efficient in removing organic pollutants and heavy metal ions. Furthermore, potential improvement in the adsorption of heavy metals by MOFs through ion-imprinting technology has emerged as a future field to be explored.

A dye-sensitized solar cell (DSSC) is an unconventional opportunity for silicon solar cells, owing to their flexibility, transparency, and low cost. The necessity for DSSCs to capture more of the sunlight radiation was the motivating force for the fabrication of semiconductor materials based on high surface area. The constituents of DSSC syndicate form a solar cell device. The main components in DSSCs are the conducting substrate, dye, photoanode, catalyst, and electrolyte. The photoanode is a well-known element that generates the energy conversion efficiency. There are different photoanode materials that have been studied to date. Amongst photoanode materials, titanium oxide (TiO2) materials are extensively documented, explored, and considered. In this chapter, effort will be made to survey the strategies to advance the energy efficiency of TiO2-based photoanodes with the introduction of metal-organic frameworks (MOFs) in DSSCs to suppress the recombination rate and electron loss. It was seen that to attain a high conversion yield in DSSCs, it is for very fast collection of produced ion carriers before electron re-combination. This chapter is devoted to the TiO2 and MOF photoanode, their properties, and various improvement approaches.

Exploiting low-cost and efficient electrocatalysts for hydrogen evolution reaction (HER) is an important route to solve the energy crisis and environmental pollution. HER process plays a vital role in many energy storage and conversion systems including water splitting, rechargeable metal–air batteries and the unitised regenerative fuel cells. The platinum-based catalysts are regarded as best electrocatalysts for the HER; nevertheless, they are very exorbitant and scarce. Therefore, it is necessary to develop efficient electrocatalysts based on carbon materials. Graphene oxide (GO), for instance, is a monolayer structure with a high contribution of sp2 hybridised carbon atoms, and various oxygen-containing surface functional groups have attracted the attention of a worldwide research community in the last decades due to its various potential applications linked to the unique combination of properties, such as hardness, physical and chemical stability, high specific surface area, electron mobility and heat transfer. It can be easily manufactured by simple and scalable chemical oxidation approaches from graphite, the reduction of GO into reduced GO (rGO) is a widely used method to obtain graphene. In HER, GO can also be incorporated with metals and porous materials for synergetic effect as co-catalysts for enhancement of electrocatalytic activities. On the other hand, metal–organic frameworks (MOFs) are crystalline materials with porous network structure. They possess various compositions, large surface area, tunable pore structures and are easily functionalised. Recently, MOF‐based electrocatalysts have been rapidly developed with excellent catalytic performance, demonstrating a promising application prospect in HER. In this chapter, the background on hydrogen energy and HER structure, category and synthesis of GO and MOFs are discussed. The application of GO- and MOF-based electrocatalysts for HER is discussed in detail. Their HER parameters such as Tafel slope (b) and exchange current density (i0) are emphatically discussed and followed by the synergetic effect of HER studies of GO/MOF composites as an alternative electrocatalyst for future hydrogen production and storage via HER mechanism.

The current interest in energy storage systems (ESSs) is primarily due to the environmental concerns of the harmful carbon print emissions from the fossil fuels used presently. The development of energy storage systems (ESSs) comes as a crucial factor when it comes to addressing the energy problems arising due to the rapid development of the global economy and the depletion of fossil fuels that brings the increase of environmental pollution. The innovation brought about by the discoveries in nanotechnology on the production of nanostructured materials has driven the rapid growth in the research of novel carbon nanomaterials towards various applications. Through these research efforts, various carbon-based nanomaterials have been discovered to possess unique properties such as high surface area, high electrical conductivity, as well as a range of shapes, sizes and pore-size distributions that are exploited extensively for electrochemical energy storage applications. Supercapacitors (SCs), also called ultracapacitors or electrochemical capacitors, are electrochemical energy storage (EES) devices that store electrical charge on high-surface-area conducting materials. This chapter explores the fundamental electrochemical properties of various nanostructured carbon materials with a great interest based on the synthesis and supercapacitive behaviour of carbon-based nanomaterials such as carbon nano-onions (CNO), carbon nanotubes (CNTs), activated carbon (AC), carbon nanofibres (CNFs), graphene and graphene oxide (GO) and their composites.

There are concerns regarding the depletion of the fossil-fuel resources and the destruction of the environment accompanied with drastic climate changes. Hence, researchers have derived attention on looking for an alternative clean, sustainable renewable, highly efficient energy conversion and storage technologies/systems. The fuel cell is one of the most promising renewable energy source that can provide a stable and constant energy output as long as fuel is supplied continuously. Currently, platinum-based metals are the best electrocatalysts for fuel cells applications. However, due to high cost of platinum, the large-scale synthesis and commercialisation of these electrocatalysts is challenging. Apart from its high cost, the Pt-based electrode also suffers from its susceptibility to time-dependent drift and CO deactivation, and it is unselective. For these reasons, many research groups are developing non-platinum electrocatalysts that are more active, stable and more economical. Carbon-based nanostructured and nanosized materials are widely applied to tackle these demanding challenges associated with energy conversion. Nanostructured carbon-based materials have received interest due to their fascinating physical and chemical properties. They have electronic behaviour ranging from metallic to semiconducting that depends on their structure, composition and chirality. In this chapter, numerous types of carbon nanomaterials such as carbon black, carbon nanofibers, carbon nanotubes and graphenes applied in the electrochemical activity of fuel cells are conversed.

A novel composite of poly(3-aminobenzoic acid) (PABA) and a cobalt zeolitic benzimidazolate framework (CoZIF) has been studied for the production of hydrogen through the hydrogen evolution reaction (HER). The structural characteristics and successful synthesis of PABA, CoZIF and the PABA/CoZIF composite were confirmed and investigated using different techniques. Probing-ray diffraction for phase analysis revealed that the composite showed a decrease and shift in peak intensities, confirming the incorporation of CoZIF on the PABA backbone via in situ polymerization, with an improvement in the crystalline phase of the polymer. The thermal stability of PABA was enhanced upon composite formation. Both scanning electron microscopy and transmission electron microscopy showed that the composite had a rough surface, owing to an interaction between the CoZIF and the external surface of the PABA. The electrochemical hydrogen evolution reaction (HER) performance of the synthesized samples was evaluated using cyclic voltammetry and Tafel analysis. The composite possessed a Tafel slope value of 156 mV/dec and an &alpha, of 0.38, suggesting that the Volmer reaction coupled with either the Heyrovsky or Tafel reaction as the rate determining step. The fabricated composite showed high thermal stability and excellent tolerance as well as high electroactivity towards the HER, showing it to be a promising non-noble electrocatalyst to replace Pt-based catalysts for hydrogen generation.