Recent advances in the role of MXene based hybrid architectures as electrocatalysts for water splitting

The development of non-noble metal based and cost-effective electrocatalysts for water splitting has attracted significant attention due to their potential in production of clean and green hydrogen fuel. Discovered in 2011, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, have demonstrated promising performance as electro catalysts in the water splitting process due to their high electrical conductivity, very large surface area and abundant catalytic active sites. However, their-long term stability and recyclability are limited due to restacking and agglomeration of MXene flakes. This problem can be solved by combining MXene with other materials to create their hybrid architectures which have demonstrated higher electrocatalytic performance than pristine MXenes. Electrolysis of water encompasses two half-cell reactions, hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. Firstly, this concise review explains the mechanism of water splitting. Then it provides an overview of the recent advances about applications of MXenes and their hybrid architectures as HER, OER and bifunctional electrocatalysts for overall water splitting. Finally, the recent challenges and potential outlook in the field have been presented. This concise review may provide further understanding about the role of MXene-based hybrid architectures to develop efficient electrocatalysts for water splitting.

Metal Electrocatalysts for Hydrogen Production in Water Splitting

The rising demand for fossil fuels and the resulting pollution have raised environmental concerns about energy production. Undoubtedly, hydrogen is the best candidate for producing clean and sustainable energy now and in the future. Water splitting is a promising and efficient process for hydrogen production, where catalysts play a key role in the hydrogen evolution reaction (HER). HER electrocatalysis can be well performed by Pt with a low overpotential close to zero and a Tafel slope of about 30 mV dec-1. However, the main challenge in expanding the hydrogen production process is using efficient and inexpensive catalysts. Due to electrocatalytic activity and electrochemical stability, transition metal compounds are the best options for HER electrocatalysts. This study will focus on analyzing the current situation and recent advances in the design and development of nanostructured electrocatalysts for noble and non-noble metals in HER electrocatalysis. In general, strategies including doping, crystallization control, structural engineering, carbon nanomaterials, and increasing active sites by changing morphology are helpful to improve HER performance. Finally, the challenges and future perspectives in designing functional and stable electrocatalysts for HER in efficient hydrogen production from water-splitting electrolysis will be described.

Oxygen-Bridged Cobalt-Chromium Atomic Pair in MOF-Derived Cobalt Phosphide Networks as Efficient Active Sites Enabling Synergistic Electrocatalytic Water Splitting in Alkaline Media

Electrochemical water splitting offers a most promising pathway for "green hydrogen" generation. Even so, it remains a struggle to improve the electrocatalytic performance of non-noble metal catalysts, especially bifunctional electrocatalysts. Herein, aiming to accelerate the hydrogen and oxygen evolution reactions, an oxygen-bridged cobalt-chromium (Co-O-Cr) dual-sites catalyst anchored on cobalt phosphide synthesized through MOF-mediation are proposed. By utilizing the filling characteristics of 3d orbitals and modulated local electronic structure of the catalytic active site, the well-designed catalyst requires only an external voltage of 1.53 V to deliver the current density of 20 mA cm-2 during the process of water splitting apart from the superb HER and OER activity with a low overpotential of 87 and 203 mV at a current density of 10 mA cm-2 , respectively. Moreover, density functional theory (DFT) calculations are utilized to unravel mechanistic investigations, including the accelerated adsorption and dissociation process of H2 O on the Co-O-Cr moiety surface, the down-shifted d-band center, a lowered energy barrier for the OER and so on. This work offers a design direction for optimizing catalytic activity toward energy conversion.

Molybdenum Phosphide Quantum Dots Encapsulated by P/N-Doped Carbon for Hydrogen Evolution Reaction in Acid and Alkaline Electrolytes

A facile and eco-friendly strategy is presented for synthesizing novel nanocomposites, with MoP quantum dots (QDs) as cores and graphitic carbon as shells, these nanoparticles are dispersed in a nitrogen and phosphorus-doped porous carbon and carbon nanotubes (CNTs) substrates (MoP@NPC/CNT). The synthesis involves self-assembling reactions to form single-source precursors (SSPs), followed by pyrolysis at 900 °C in an inert atmosphere to obtain MoP@NPC/CNT-900. The presence of carbon layers on the MoP QDs effectively prevents particle aggregation, enhancing the utilization of active MoP species. The optimized sample, MoP@NPC/CNT-900, exhibits remarkable electrocatalytic activity and durability for the hydrogen evolution reaction (HER). It demonstrates a low overpotential of 155 mV at 10 mA cm-2 , a small Tafel slope of 76 mV dec-1 , and sustained performance over 20 hours in 0.5 M H2 SO4 . Furthermore, the catalyst shows excellent activity in 1 M KOH, with a relatively low overpotential of 131 mV and long-term durability under constant current input. The exceptional HER activity can be attributed to several factors: the superior performance of MoP QDs, the large surface area and good conductivity of the carbon substrates, and the synergistic effect between MoP and carbon species.

Coupling heterostructured CoP-NiCoP nanopin arrays with MXene (Ti3C2Tx) as an efficient bifunctional electrocatalyst for overall water splitting

Developing a highly effective bifunctional electrocatalyst for alkaline-condition electrochemical water splitting is both essential and challenging. The work presented here successfully synthesizes and employs a heterostructured CoP-NiCoP ultra-long nanopin array in situ growing on MXene (Ti₃C₂T_x) as a stable bifunctional electrocatalyst for electrochemical water-splitting. The heterogeneous structure formed by CoP nanoparticles and NiCoP nanopins provides extra active sites for water-splitting. Also, Ti₃C₂T_x works as a support substrate during electrochemical operations, accelerating mass transfer, ion transport, and rapid gas product diffusion. Meanwhile, throughout the catalytic process, the dense nanopin arrays shield Ti₃C₂T_x from further oxidation. At a result, the CoP-NiCoP-Ti₃C₂T_x (denoted as CP-NCP-T) demonstrated excellent catalytic activity, with overpotentials of just 46 mV for hydrogen evolution at 10 mA cm^-2 and 281 mV for oxygen evolution at 50 mA cm^-2. Furthermore, in 1.0 M KOH solution, the outstanding bifunctional electrode (CP-NCP-T || CP-NCP-T) exhibits efficient electrochemical water splitting activity (1.54 V@10 mA cm^-2) and outperforms the comparable device Pt/C || IrO₂ (1.62 V@10 mA cm^-2).

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Bifunctional WC-Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media

We report the strong catalyst-support interaction in WC-supported RuO₂ nanoparticles (RuO₂ -WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η₁₀ of 347 mV and a mass activity of 1430 A g_Ru ^-1 , eight-fold higher than that of commercial RuO₂ (176 A g_Ru ^-1 ). Theoretical calculations demonstrate that the strong catalyst-support interaction between RuO₂ and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO₂ -WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm^-2 . The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

Recent Advances in Inorganic Electrochromic Materials from Synthesis to Applications: Critical Review on Functional Chemistry and Structure Engineering

For the assembly of electrochromic devices (ECDs) generally with multilayer structures, supportive components usually are needed to be incorporated with EC materials. The reasonable project and development of ECDs will achieve broad expected applications. In this study, we reviewed several impressive methods to design and fabricate ECDs with high-performance and versatility based on recent frontier research. The first part of the review is centered on the desirability and strengthening mechanism of nanostructured inorganic EC materials. The second part illustrates the recent advances in transparent conductors. We then summarize the demands and means to modify the formation of electrolytes for practicable ECDs. Moreover, efforts to increase the compatibility with the EC layer and ion capacity are delineated. In the end, the application prospects of inorganic ECDs are further explored, which offers a guideline for the industrialization process of ECDs.

Tuning the Electronic Structure of the CoP/Ni2P Nanostructure by Nitrogen Doping for an Efficient Hydrogen Evolution Reaction in Alkaline Media

As one of the most sustainable, efficient, and cleanest ways for hydrogen production, electrochemical water splitting relies heavily on cost-efficient and stable electrocatalysts. Herein, a self-supported and nitrogen-doped hybrid CoP/Ni₂P was synthesized through a simple two-step hydrothermal process followed by low-temperature phosphorization and nitridation (N-CoP/Ni₂P@NF). Both experimental and density functional theory calculation results suggest that nitrogen doping can tune the electrical structure of the CoP/Ni₂P heterostructure and thus optimize the free energy of adsorbed H on the surface of N-CoP/Ni₂P@NF and accelerate the electronic transport activity. The prepared N-CoP/Ni₂P@NF exhibits excellent electrocatalytic hydrogen evolution reaction (HER) performance, which merely requires an overpotential of -46 mV at -10 mA cm^-2 and shows a negligible decay after a long durability test for 72 h in alkaline (1.0 M KOH) media. Consequently, this work supplies a novel strategy with great potential for designing transition metal phosphate-based catalysts with high HER performance.

Progress in the development of heteroatom-doped nickel phosphates for electrocatalytic water splitting

Hydrogen energy is expected to replace fossil fuels as a mainstream energy source in the future. Currently, hydrogen production via water electrolysis yields high hydrogen purity with easy operation and without producing polluting side products. Presently, platinum group metals and their oxides are the most effective catalysts for water splitting; however, their low abundance and high cost hinder large-scale hydrogen production, especially in alkaline and neutral media. Therefore, the development of high-efficiency, durable, and low-cost electrocatalysts is crucial to improving the overpotential and lowering the electrical energy consumption. As a solution, Ni₂P has attracted particular attention, owing to its desirable electrical conductivity, high corrosion resistance, and remarkable catalytic activity for overall water splitting, and thus, is a promising substitute for platinum-group catalysts. However, the catalytic performance and durability of raw Ni₂P are still inferior to those of noble metal-based catalysts. Heteroatom doping is a universal strategy for enhancing the performance of Ni₂P for water electrolysis over a wide pH range, because the electronic structure and crystal structure of the catalyst can be modulated, and the adsorption energy of the reaction intermediates can be adjusted via doping, thus optimizing the reaction performance. In this review, first, the reaction mechanisms of water electrolysis, including the cathodic hydrogen evolution reaction and anodic oxygen evolution reaction, are briefly introduced. Then, progress into heteroatom-doped nickel phosphide research in recent years is assessed, and a discussion of each representative work is given. Finally, the opportunities and challenges for developing advanced Ni₂P based electrocatalysts are proposed and discussed.

Induction of Co2P Growth on a MXene (Ti3C2Tx)-Modified Self-Supporting Electrode for Efficient Overall Water Splitting

It still is a challenge to create a superior and easily coupled bifunctional electrocatalyst for water splitting impelled by a low voltage. In this work, the controlled growth of Co₂P NAs on the surface of a MXene (Ti₃C₂T_x)-modified self-supporting electrode is demonstrated as a competent and reliable bifunctional electrocatalyst for efficient water splitting. The heterointerface in Co₂P@Ti₃C₂T_x with an optimized adsorption free energy of H*, H₂O, and better conductivity can give enhanced HER (hydrogen evolution reaction) activity, with a low overpotential (42 mV) at 10 mA cm^-2. Additionally, the OER (oxygen evolution reaction) activity has also been similarly strengthened by the synergy of Co₂P and MXene with an overpotential of 267 mV to arrive at 10 mA cm^-2. Furthermore, the excellent bifunctional electrode (Co₂P@Ti₃C₂T_x∥Co₂P@Ti₃C₂T_x) exhibits efficient engineering water-splitting performance (1.46 V@10 mA cm^-2) in alkaline solution. This simple design can propose a promising approach to exploit precious-metal-free catalysts for energy conversion.

Synergetic Effect of Ni2P and MXene Enhances Catalytic Activity in the Hydrogen Evolution Reaction

Developing highly efficient non-precious electrocatalytic materials for H₂ production in an alkaline medium is attractive on the front of green energy production. Herein, we successfully designed an electrocatalyst with superb hydrophilicity, high conductivity, and a kinetically beneficial structure using Ni₂P/MXene over a 3D Ni foam (NF) for the alkaline hydrogen evolution reaction (HER) based on the laboratory and computational research works. The designed self-supported and highly effective electrocatalyst achieves a huge boost in the HER activity compared with that of pristine Ni₂P nanosheets owing to the distinctive structure and synergy of coupling Ti₃C₂T_x and Ni₂P. More specifically, Ni₂P/Ti₃C₂T_x/NF produces an electric current density of 10 mA·cm^-2 under a low overpotential (135 mV) and shows excellent durability under alkaline (1 M KOH) conditions, and the observed performance degradation is negligible. The outstanding HER activity makes the synthetic strategy of Ni₂P/Ti₃C₂T_x/NF a potential approach to be extended to other transition-metal-based electrocatalysts for enhanced catalytic performance.