2D MXenes and their heterostructures for HER,OER and overall water splitting: A review

MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides that have surface termination groups such as –OH, –O, and –F. The presence of transition metal imparts conductivity, surface termination groups induce hydrophilicity and layered structure offers large surface area which makes MXenes a potential candidate to be utilized as an electro-catalyst with enhanced efficiency. The Water Electrolysis (WE) efficiency of an electro-catalysts is dependent on the performance of half-cell reactions i.e. Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). The OER kinetics of most of the bi-functional electrocatalysts are considered sluggish due to which they are tested in alkaline media. However, due to the metallic nature and surface properties of MXenes, they as substrate not only improve HER performance of grown electro-catalyst but also facilitate OER kinetics which is considered sluggish for most bi-functional electrocatalysts. This review presents the significance of MXenes as HER, OER, and bi-functional electrocatalysts by discussing the electrocatalytic properties of a wide range of MXenes and how their hetero-structures affect HER, OER, and bi-functional electrocatalytic performance. In the end, the current challenges, and future perspectives of MXenes and their nanocomposites for water electrolysis have been discussed.     

Titanate-based perovskites for photochemical and photoelectrochemical water splitting applications: A review

Recently, perovskites have been intensively studied for effective hydrogen production through photocatalytic water splitting reactions. The unique properties of perovskite materials are their tunable bandgap and high photocorrosion stability. Titanate-based perovskites are the most widely studied perovskites for activation under visible light and improvement in the hydrogen gas production rate by sunlight. Beneficial modifications were achieved through element doping, catalyst loading, heterojunction formation with other materials and surface modification. This review presents the progress over the last ten years in titanate perovskite modification and the increases achieved in the H₂ production rate.     

A review: Target-oriented transition metal phosphide design and synthesis for water splitting

With the scarcity of fuel energy, non-noble metal compounds assisted water of electrolysis is becoming a potential candidate for cost-effective and high-quality hydrogen production. To date, transition metal phosphide (TMP) has been considered as one of the most promising catalysts for water splitting because of its multitudinous but controlled constituent elements and stoichiometric ratios. In this review, the electronic structure analysis of TMP dialectically reveals the active derivation for catalyzing hydrogen evolution (HER) and oxygen evolution reaction (OER). And then the strategies of rationally designing the structure and composition of electrocatalyst to improve its intrinsic activity are discussed, especially interface engineering. Besides, this review also focuses on the negligible stability issue during the water splitting process. In the end, some key challenges and research orientations of TMPs are pointed out, which is instructive for developing high-efficient and stable electrocatalysts for water splitting.     

Role of perovskites as a bi-functional catalyst for electrochemical water splitting: A review

The electrochemical water splitting by using renewable electricity is being considered as a sustainable, clean and considerable source of hydrogen fuel for future transportation and energy applications. The sluggish kinetics at anode and cathode, thus, require plenty of research work on the development of an efficient and stable electrocatalyst, which would provide the enhanced activity of water splitting reaction as well as stability for long-term operation. This review draws a detailed sketch of the progress in the pursuit of replacing noble metals with non-precious perovskite-based substitutes without compromising the key electrocatalyst characteristics. Herein, we critically analysed the latest research work and progress of perovskite oxides for anodic/oxygen reduction reaction/cathodic, including the mechanism behind perovskite oxide catalytic reactions, controlled composition as well as the role of various design strategies to achieve high catalytic performance. Moreover, the article also provides an insight to the associated density functional theory that can provide profound understanding of mechanism, involved behind these reactions and, the need for computational studies to exploit the active area of catalysts. It is believed that this article will assist researchers to explore key area of research in the current generation perovskites that show enhanced catalytic performance as well as to work on unforeseen challenges.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

A concise review: MXene-based photo catalytic and photo electrochemical water splitting reactions for the production of hydrogen

Since 2015, a number of breakthroughs in the generation of new MAX phases using specified double transition metals have made possible the synthesis of unique MXenes with significant chemical diversity and structural complexity, which are rare in 2D families. MXene and semiconductor hybrids are shown to be effective photocatalysts because to their unique interface features. For photocatalytic purposes, a Schottky heterojunctions may provide faster charge separation and a lower Schottky barrier. When it comes to photocatalytic and photo electrochemical applications, photocatalysts are predicted to be the greatest and most popular new photocatalysts. We discussed some of the semiconductor-based nanocomposites supported by MXenes, including photocatalytic and photo electrochemical water splitting. Next, we discussed some of the difficulties and opportunities that have arisen from working with MXenes to advance semiconductor-based photocatalysts. Scientists in related fields are noticed to prompt the development of novel photocatalysts based on semiconductors.     

Design and synthesis of dispersed Ni2P/Co nano heterojunction as bifunctional electrocatalysis for boosting overall water splitting

The design of multi-components nanostructure with interface heterojunction is the cutting-edge research in recent years because the catalytic activity, stability, and durability of catalysts are highly affected by the strong electronic effects, geometric effects, and synergistic effects occurring at the interface. Based on this, an efficient bifunctional electrocatalyst embedding highly dispersed Ni₂P/Co nano heterojunction at the porous hollow-out carbon shell is developed for overall water splitting through evenly epitaxial growth of ultrathin Ni₂P nanosheets on Co-based ZIF-67. The distinct electron interaction between the interfacial Ni₂P (300) and Co (100) effectively lowers the overpotential of OER (316 mV vs. RHE) and HER (149 mV vs. RHE) at the current density of 10 mA cm^?2. Density functional theory (DFT) calculation further identifies that the Ni₂P and Co heterojunctions optimize the adsorption energy of intermediate products and lower the energy barrier of the rate-determining step of OER significantly. This work provides a rational design of a well-defined interface toward overall water splitting electrocatalysts and offers a scientific basis for an in-depth understanding of the mechanism of the catalysts with nano heterojunction.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

Cu2O as an emerging photocathode for solar water splitting - A status review

Recently, cuprous oxide (Cu₂O) based photocathodes have gained research attention for hydrogen (H₂) production through photoelectrochemical (PEC) water splitting reactions due to marginally lower synthesis cost and low energy intensity fabrication processes. Unique properties of Cu₂O, such as tunable bandgap, appropriate band edge potentials with water redox levels and non-toxic nature makes it beneficial for PEC applications. Cuprite is mainly studied under visible light to facilitate enhanced H₂ gas production upon illumination. However, notoriously photocorrosion degrades the PEC performance and restricts the photoactivity of Cu₂O. Moreover, because of the redox potentials lies within the band gap of Cu₂O; self-photocorrosion or self-oxidation upon illumination is unavoidable. Improvement in the Cu₂O photocathodes was achieved by finding elegant solutions such as forming thin heterojunction layers by atomic layer deposition (ALD) as well other methods, co-catalyst deposition, tuning crystal facets and surface modifications with different synthetic methods. In this review, we discuss the improvements in Cu₂O photocathodes achieved over the years for enhanced H₂ production with recently studied photocathodes.     

AgCu/TiO2 hot-electron device for solar water splitting: the mechanisms of LSPR and time-domain characterization

More and more metal/semiconductor nanostructures have been served as a hot-electron device with the localized surface plasmonic resonance (LSPR) effect to boost hydrogen evolution from solar water splitting. In this work, bimetallic AgCu with optimal ratio are deposited onto TiO₂ nanopore/nanotube arrays to construct AgCu/TiO₂ photoanode for photoelectrochemical water splitting, a novel simulation characterization to visualize the LSPR process is proposed. The near electric field enhancement and plasmon resonance energy transfer mechanisms of single Ag and Cu are inferred by time-domain characterization, illustrating the contradictory photocurrent under AM 1.5 illumination with its LSPR effect based on the particle size. The variation of local electric field over time within the interfaces of AgCu bimetals and bimetal/TiO₂ models reveals the migration of hot electrons from Ag into Cu and the synergetic effect of different LSPR mechanisms. The resulting higher photoelectrochemical activities of AgCu/TiO₂ also verifies the positive roles of the coexistence of AgCu on electron generation and energy transfer to interband excitation of TiO₂.     

Eco-designed electrocatalysts for water splitting: A path toward carbon neutrality

Realizing sustainable hydrogen fuel production through water electrolysis is crucial to achieving carbon neutrality. However, the development of cost-effective electrocatalysts continues to be a challenge. Eco-designed electrocatalysts derived from wastes and naturally abundant materials have recently received increasing attention. The development of eco-designed electrocatalysts is of great environmental and economic significance and makes green hydrogen more accessible to the wider community. Here, recent advances in eco-designed electrocatalysts for water splitting are summarized. Eco-design strategies such as pyrolysis, ball milling, wet-chemical methods, and electrochemical treatment are first analyzed. Recent achievements in eco-designed electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS) are then detailed, with an emphasis on analyzing the eco-design strategy-catalyst property-catalytic performance correlation. Perspectives in this blooming field for a greener hydrogen economy are finally outlined.     

Some fundamental and practical aspects of electrocatalysis for water electrolysis

This paper gives a brief account of the fundamental work on electrocatalysis for alkaline water electrolysers at The City University as well as presenting some results on the design of Teflon bonded and porous electrodes supported on nickel mesh of varying sizes.     

Bandgap engineering in MnPS3 and ZnPS3 for photocatalytic water splitting: A first-principles study

By virtue of their wide band gaps, the photocatalytic ability of MnPS₃ and ZnPS₃ for water splitting is subject to narrow visible light adsorption. In this work, the first-principle calculations are adopted to study the impact of biaxial strain and electric field on photocatalytic performance of MnPS₃ and ZnPS₃. Our theoretical calculations demonstrate that MnPS₃ under ?10% biaxial strain, ?5% biaxial strain and a 0.05 V/Å electric field possess appropriate band gaps and band edge positions required for photocatalytic water splitting as well as pronounced absorbance in the visible and ultraviolet spectrum. Whereas, the band structures for ZnPS₃ can not be altered significantly either by applying biaxial strain or electric field. Through the free energy change analysis, it is found that water splitting reactions on MnPS₃ under ?10% biaxial strain as well as ?5% biaxial strain can be thermodynamically spontaneous at pH 7–10 using natural sunlight.     

A review on recent advances and progress in Mo2C@C: A suitable and stable electrocatalyst for HER

In this review, the environmental issues and the energy crisis caused due to overuse of diminishing fossil fuels have been introduced and discussed. The need for new renewable, non-carbon emission energy sources and the importance of hydrogen (H₂) as clean and efficient energy generated via hydrogen evolution reaction (HER) through electrochemical water splitting is discussed. The role of an efficient electrocatalyst for HER has been highlighted. The requirement of low-cost, efficient, and durable electrocatalysts for HER to replace the costly platinum (Pt) electrocatalysts is introduced. The importance of nano transition metal carbides (TMCs), especially molybdenum carbide (Mo₂C), as an efficient and stable electrocatalyst has been discussed in detail. The progress and advances to improve the electrocatalytic performance of nano Mo₂C on the basis of synthesis, size, shape, and carbon support in different forms have been presented. The role of different carbon species over Mo₂C on HER activity has been discussed. The importance of nitrogen (N) doping within the carbon structure encapsulating Mo₂C particles to improve the HER activity has been highlighted. The importance of in-situ developed surface carbon over Mo₂C has been discussed in detail. The utilization of waste carbon materials to address the environmental issues caused due to non-biodegradable wastes is also discussed. In the end, the future scope has been highlighted to commercialize Mo₂C as a low-cost electrocatalyst for hydrogen production through HER.     

Graphitic-polytriaminopyrimidine (g-PTAP): A novel bifunctional catalyst for photoelectrochemical water splitting

In this study, an electrode of g-PTAP, a novel bifunctional catalyst for photoelectrochemical was fabricated and utilized for water splitting. The graphitic-poly (2,4,6-triaminopyrimidine (g-PTAP) was synthesized by the thermal vapor condensation polymerization (TVCP) method on FTO glass. The structure, morphology, and optical characteristics of the resultant g-PTAP were analyzed using analytical techniques such as FT-IR, Raman, XRD, XPS, CHNS, FE-SEM, EDS, and DRS. The synthesized g-PTAP was graphitic with sheet-like morphology and revealed maximum light absorbance capacity in the visible range. The DFT calculation showed an appropriate HOMO-LUMO band position for overall water splitting which was verified experimentally for H₂ and O₂ generation at photocathode and photoanode, respectively. Moreover, the g-PTAP sample exhibited good photo-stability as a photocathode as compared to a photoanode. This work can provide a pathway for fabricating highly efficient semiconductor photocatalyst for overall water splitting and solar energy such as conversion.     

NaFeTiO4: A novel visible light active photocatalyst for water splitting and environmental remediation

Single phase, crystalline NaFeTiO₄ with tunnel structure is prepared by a solid state method and explored as a novel photocatalyst for the first time. Structural, optical and morphological properties of NaFeTiO₄ are investigated by various characterization techniques such as X-ray diffraction (XRD), scanning & transmission electron microscopy (SEM & TEM), Energy dispersive X-ray spectroscopy (EDS), N₂ adsorption-desorption study (BET), UV-vis, X-ray photoelectron, X-ray absorption (UV-vis DRS, XPS and XANES) and photoluminescence (PL) spectroscopy. The interfacial charge transfer ability of the prepared n-type NaFeTiO₄ was also investigated by transient photocurrent response and electrochemical impedence spectroscopy which proved to be an efficient tool for better understanding of electronic properties of NaFeTiO₄. The photocatalytic efficiency of NaFeTiO₄ is evaluated for decomposition of methylene blue (MB) and Rhodamine B (RhB) dyes as well as for H₂ evolution through water splitting reaction under visible light. NaFeTiO₄ exhibits efficient charge separation properties, excellent photocatalytic activities and reusability.     

A review of PEM hydrogen fuel cell contamination: Impacts,mechanisms, and mitigation

This paper reviewed over 150 articles on the subject of the effect of contamination on PEM fuel cell. The contaminants included were fuel impurities (CO, CO₂, H₂S, and NH₃); air pollutants (NO_x, SO_x, CO, and CO₂); and cationic ions Fe³⁺ and Cu²⁺ resulting from the corrosion of fuel cell stack system components. It was found that even trace amounts of impurities present in either fuel or air streams or fuel cell system components could severely poison the anode, membrane, and cathode, particularly at low-temperature operation, which resulted in dramatic performance drop. Significant progress has been made in identifying fuel cell contamination sources and understanding the effect of contaminants on performance through experimental, theoretical/modeling, and methodological approaches. Contamination affects three major elements of fuel cell performance: electrode kinetics, conductivity, and mass transfer.     

MOFs in photoelectrochemical water splitting: New horizons and challenges

Growing energy consumption with the augmentation in universal population to more than nine billion by 2050 and exhausting fossil fuel reserves necessitates a harsh revolution from non-renewable energy reservoirs to renewable energy reservoirs with zero carbon emission. In the present scenario, solar energy prompted photoelectrochemical (PEC) water splitting or “Artificial Photosynthesis” via light gripping semiconductor material, originates out as the most promising methodology in accomplishing the global energy crisis. Recent studies have amply demonstrated the potential of metal-organic frameworks (MOF) towards PEC applications. They are porous crystalline coordination polymers assembled through an appropriate choice of metal ions and multidentate organic ligands. Owing to their structural regularity and synthetic tunability, MOFs integration with PEC is considered in terms of enhancing and broadening light absorption, providing active sites and directing charge transfer dynamics. Here, we have explored MOFs role in PEC and classified them into different categories such as photosensitizers, co-catalysts, counter electrode, template and also for imparting additional stability to the electrode system. MOFs mediated PEC water splitting is promising but is still rare and in its infancy. Therefore, it is pertinent and timely to take stock of the advancements made and develop insight on the use of MOFs, as an emerging solution for the problems encountered in PEC. This review covers the basics of MOF & mainly describes various case studies done during last 10 years and providing adequate impetus to researchers for critically assessing the recent advances and challenges that are faced by scientists and researchers at large.     

Improvement of TiO2 nanotubes for photoelectrochemical water splitting: Review

Photoelectrochemical (PEC) water splitting is an ideal method to produce clean hydrogen. Developing photoelectrodes that fulfill the PEC water-splitting criteria has become the greatest challenge for commercialization of this technology. Titanium dioxide, the first material used for this application, remain appealing due to its one-dimensional nanotube structure. However, the bandgap of TiO₂ nanotubes, ~3.0 eV, is relatively wide, leading to problems such as limited utilization of light energy and easy recombination of the photogenerated products, i.e., electrons and holes. Several approaches have been developed to overcome this problem, including (i) modification of surface morphology to enhance the active catalytic area, (ii) band structure modification to reduce photogenerated charge recombination, and (iii) surface sensitization to improve light absorption ability. This review reports the improvements achieved by all of these approaches for TiO₂ nanotubes, including the basic principles of the photocatalytic water-splitting process and the preparation and polymorphs of TiO₂ nanotubes. This review also discusses combinations of several methods that enable high photocurrent density with fabulous stability.     

0D, 1D and 2D nanomaterials for visible photoelectrochemical water splitting. A Review

Global energy problems of the 21st century have led to the search for alternative energy sources, among which is hydrogen produced via photoelectrochemical solar water splitting. Photo-electrochemical water splitting using semiconductor nanostructured materials is a progressive method for producing hydrogen. The unique electronic, mechanical, surface and optical properties of nanomaterials make it possible to create photocatalysts with complex structures of energy zones, allowing the use of a wide range of sunlight and exerting a positive effect on absorption and scattering of sunlight. This review contains a detailed analysis of current studies aimed at improving the efficiency of photocatalytic systems by using 0D, 1D and 2D nanostructures. Special attention is paid to the mechanisms of photocatalytic water splitting to produce hydrogen with the help of various nanostructures.