Recent Advances in Transition Metal Carbide Electrocatalysts for Oxygen Evolution Reaction

Nano bowl-like cobalt-cobalt molybdenum carbide coated by N,P co-doped carbon as an advanced bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

Nano bowl-like Co-Co₆Mo₆C₂ coated by N,P co-doped carbon (Co-Co₆Mo₆C₂@NPC) is reported as an electrocatalyst for Zn-air batteries. Co-Co₆Mo₆C₂@NPC only needs an overpotential of 210 mV at 10 mA cm^-2 for the OER, and the half-wave potential for the ORR is 0.81 V. In addition, the Co-Co₆Mo₆C₂@NPC based battery shows a large open-circuit voltage of 1.335 V and a maximum power density of 160.5 mW cm^-2, as well as good stability. The improved catalytic performance can be ascribed to the co-existence of Co₆Mo₆C₂ and Co species to improve the intrinsic catalytic activity, and the bowl-like nanostructure to facilitate the mass transfer.

In Situ Anodic Oxidation Tuning of NiFeV Diselenide to the Core-Shell Heterojunction for Boosting Oxygen Evolution

Developing non-noble metal-based core-shell heterojunction electrocatalysts with high catalytic activity and long-lasting stability is crucial for the oxygen evolution reaction (OER). Here, we prepared novel core-shell Fe,V-NiSe₂@NiFe(OH)_x heterostructured nanoparticles on hydrophilic-treated carbon paper with high electronic transport and large surface area for accelerating the oxygen evolution rate via high-temperature selenization and electrochemical anodic oxidation procedures. Performance testing shows that Fe,V-NiSe₂@NiFe(OH)_x possesses the highest performance for OER compared to as-prepared diselenide core-derived heterojunctions, which only require an overpotential of 243 mV at 10 mA cm^-2 and a low Tafel slope of 91.6 mV decade^-1 under basic conditions. Furthermore, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm the morphology and elementary stabilities of Fe,V-NiSe₂@NiFe(OH)_x after long-term chronopotentiometric testing. These advantages are largely because of the strong synergistic effect between the Fe,V-NiSe₂ core with high conductivity and the amorphous NiFe(OH)_x shell with enriched defects and vacancies. This study also presents a general approach to designing and synthesizing more active core-shell heterojunction electrocatalysts for OER.

Tuning Electronic Structures of Transition Metal Carbides to Boost Oxygen Evolution Reactions in Acidic Medium

Developing low-cost, efficient, and robust nonprecious metal electrocatalysts for oxygen evolution reactions (OER) in acidic medium is the major challenge to realize the application of the proton exchange membrane water electrolyzer (PEM-WE). It is well-known that transition metal carbides (TMCs) have Pt-like electronic structures and catalytic behaviors. However, monometallic carbides in acidic medium show ignored OER activities. Herein, we reported that the catalytic activity of the TMCs can be enhanced by constructing bimetallic carbides (TiTaC₂) fabricated through hydrothermal treatment followed by an annealing process, and further by doping fluorine (F) into the bimetallic carbides (TiTaF_xC₂). The as-prepared reduced graphene oxide (rGO) supported TiTaF_xC₂ nanoparticles (TiTaF_xC₂ NP/rGO) show state-of-the-art OER catalytic activity, which is even superior to Ir/C catalyst (an onset potential of only 1.42 V vs RHE and the overpotential of 490 mV to reach 100 mA cm^-2), fast kinetics (Tafel slope of only 36 mV dec^-1), and high durability (maintaining the current density at 1.60 V vs RHE for 40 h). Detailed structural characterizations together with density functional theory (DFT) calculations reveal that the electronic structures of the bimetallic carbides have been tuned, and their possible mechanism is also discussed.

A Single Source, Scalable Route for Direct Isolation of Earth-Abundant Nanometal Carbide Water-Splitting Electrocatalysts

Single-phase M_xCs (M = Fe, Co, and Ni) were prepared by solvothermal conversion of Prussian blue single source precursors. The single source precursor is prepared in water, and the conversion process is carried out in alkylamines at reaction temperatures above 200 °C. The reaction is scalable using a commercial source of Fe-PB. High-resolution transmission electron microscopy, X-ray photoelectron microscopy, and powder X-ray diffraction confirm that carbides have thin oxide termination but lack graphitic surfaces. Electrocatalytic activity reveals that Fe₃C and Co₂C are oxygen evolution reaction electrocatalysts, while Ni₃C is a bifunctional [OER and hydrogen evolution reaction (HER)] electrocatalyst.

Boosting the Electrocatalytic Activity of Nickel-Iron Layered Double Hydroxide for the Oxygen Evolution Reaction byTerephthalic Acid

The development of a new type of oxygen evolution reaction (OER) catalyst to reduce the energy loss in the process of water electrolysis is of great significance to the realization of the industrialization of hydrogen energy storage. Herein, we report the catalysts of NiFe double-layer hydroxide (NiFe-LDH) mixed with different equivalent terephthalic acid (TPA), synthesized by the hydrothermal method. The catalyst synthesized with the use of the precursor solution containing one equivalent of TPA shows the best performance with the current density of 2 mA cm−2 at an overpotential of 270 mV, the Tafel slope of 40 mV dec−1, and excellent stable electrocatalytic performance for OER. These catalysts were characterized in a variety of methods. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), and Raman spectrum proved the presence of TPA in the catalysts. The lamellar structure and the uniform distribution of Ni and Fe in the catalysts were observed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In X-ray photoelectron spectroscopy (XPS) of NiFe-LDH with and without TPA, the changes in the peak positions of Ni and Fe spectra indicate strong electronic interactions between TPA and Ni and Fe atoms. These results suggest that a certain amount of TPA can boost catalytic activity.

Bifunctional rare metal-free electrocatalysts synthesized entirely from biomass resources

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are important processes for various energy devices, including polymer electrolyte fuel cells, rechargeable metal-air batteries, and water electrolyzers. We herein report the preparation of a rare metal-free and highly efficient ORR/OER electrocatalyst by calcination of a mixture of blood meal and ascidian-derived cellulose nanofibers. The obtained carbon alloys showed high ORR/OER performances and proved to be promising electrocatalysts. The carbon alloys synthesized entirely from biomass resources not only lead to a new electrocatalyst fabrication process but also contribute to CO2 reduction and the realization of a good life-cycle assessment value in fabrication of a sustainable energy device.

Advancements in Solar Desalination of Seawater by Various Ti3C2 MXene Based Morphologies for Freshwater Generation: A Review

For a few years, we have been witnessing ubiquitous fresh and drinking water scarcity in various countries. To mitigate these problematic situations, many countries relied on non-conventional freshwater generation technologies through solar desalination of seawater. In this manner, we excel the ability of new class 2D Ti3C2 MXenes as a photothermal material (solar absorber) for freshwater generation via the solar desalination technique. In this review, the air–water interfacial interaction is highlighted for improving the evaporation efficiency. To provide the dependence of the desalination efficiency on the microstructure of the solar absorbers, we summarized various forms of 2D Ti3C2 MXenes (aerosol, films, foam, hydrogel, membrane, monolith and porous structure) and their characteristics. These microstructures prevailed ultrahigh photoconversion efficiency. In this aspect, we further explained key features such as light absorption, reflection, multiple internal reflection, hydrophilicity, lower thermal conduction, light-to-heat generation, and salt rejection for achieving efficient desalination output throughout the visible and broadband region. Specifically, we targeted to explore the self-floating and salt rejection nature of various state-of-the-art 2D Ti3C2 MXene structures. Further, we highlighted the long-term stability. Among the above morphologies, Ti3C2 MXene in the form of a membrane is believed to be a promising morphology which effectively desalinates seawater into freshwater. Finally, we highlighted the challenges and future perspectives, which can pave a potential path for advancing the sustainable solar desalination of seawater into freshwater.

Investigating the active sites in molybdenum anchored nitrogen-doped carbon for alkaline oxygen evolution reaction

Developing durable and efficient non-precious-metal based catalysts for oxygen evolution reaction (OER) is highly desirable in the field of electrocatalysis. In this work, a series of novel Mo anchored N-doped carbon catalysts (denoted as Mo/NC-T) were prepared starting from the zeolitic imidazolate framework 8 (ZIF-8) precursor. Firstly, Mo doped ZIF-8 precursor (Mo/ZIF-8) with a regular polyhedron structure was formed through a simple ion-exchange method process between molybdenum pentachloride (MoCl₅) and ZIF-8. Afterward, Mo/ZIF-8 was converted to Mo/NC-T through a two-step calcination process in nitrogen (N₂) and ammonia (NH₃). The as-synthesized Mo/NC-T samples exhibited superior electrocatalytic OER properties. The optimal sample at 650 °C (Mo/NC-650) presented a low overpotential of 320 mV at 10 mA cm^-2, a Tafel slope of 71 mV dec^-1, and an outstanding long-term stability for 30 h in 1 M KOH solution. The remarkable OER activity of Mo/NC-T could be ascribed to the structural stability of carbon matrix and the synergistic effect between Mo³⁺ (derived from Mo-C bonds) and pyridinic N. This work provides a novel perspective on the roles of Mo species in the N-doped carbon electrocatalysts for OER.

2D-Layered Non-Precious Electrocatalysts for Hydrogen Evolution Reaction: Fundamentals to Applications

The production of hydrogen via the water splitting process is one of the most promising technologies for future clean energy requirements, and one of the best related challenges is the choice of the most highly efficient and cost effective electrocatalyst. Conventional electrocatalysts based on precious metals are rare and very-expensive for large-scale production of hydrogen, demanding the exploration for low-cost earth abundant alternatives. In this context, extensive works from both theoretical and experimental investigations have shown that two-dimensional (2D) layered materials have gained considerable attention as highly effective electrocatalytic materials for electrical-driven hydrogen production because of their unique layered structure and exciting electrical properties. This review highlights recent advancements on 2D layered materials, including graphene, transitional metal dichalcogenides (TMDs), layered double hydroxides (LDHs), MXene, and graphitic carbon nitride (g-C3N4) as cost-effective and highly efficient electrocatalysts for hydrogen production. In addition, some fundamental aspects of the hydrogen evolution reaction (HER) process and a wide ranging overview on several strategies to design and synthesize 2D layered material as HER electrocatalysts for commercial applications are introduced. Finally, the conclusion and futuristic prospects and challenges of the advancement of 2D layered materials as non-precious HER electrocatalysts are briefly discussed.

Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

Electrochemical oxygen evolution reaction (OER) activity was studied on nickel foam-based electrodes. The OER was investigated in 0.1 M NaOH solution at room temperature on as-received and Co- or Mo-modified Ni foam anodes. Corresponding values of charge-transfer resistance, exchange current-density for the OER and other electrochemical parameters for the examined Ni foam composites were recorded. The electrodeposition of Co or Mo on Ni foam base-materials resulted in a significant enhancement of the OER electrocatalytic activity. The quality and extent of Co, and Mo electrodeposition on Ni foam were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis.

A covalent organic polymer-TiO2/Ti3C2 heterostructure as nonenzymatic biosensor for voltammetric detection of dopamine and uric acid

Heterostructures have potential to blend the advantages of each material, even exhibiting the evolutionary performance due to synergistic effects. Herein, covalent organic polymers (NUF) are integrated with a TiO₂/Ti₃C₂T_x nanocomposite (TiO₂/TiCT) to form TiO₂/TiCT-NUF heterojunctions as an enlarged nonenzymatic biosensor for dopamine (DA) and uric acid (UA). Detection is performed by differential pulse voltammetry (DPV). The TiO₂/TiCT/NUF exhibits high sensing activity with low detection limits of 0.2 and 0.18 nM (S/N = 3) in the concentration ranges from 0.002 to 100 μM and 0.001 to 60 μM for simultaneous determination of DA and UA, respectively. In addition, the TiO₂/TiCT/NUF provides good selectivity and reproducibility for DA and UA detection in urine and serum samples with recoveries of 98.4 to 100.9%. The proposed heterojunctions manifest an intriguing potential as a candidate of an electrochemical sensor for sole and simultaneous detection of DA and UA.