Developing efficient catalysts for nitrogen fixation is becoming increasingly important but is still challenging due to the lack of robust design criteria for tackling the activity and selectivity problems, especially for electrochemical nitrogen reduction reaction (NRR). Herein, by means of large-scale density functional theory (DFT) computations, we reported a descriptor-based design principle to explore the large composition space of two-dimensional (2D) biatom catalysts (BACs), namely, metal dimers supported on 2D expanded phthalocyanine (M2-Pc or MM'-Pc), toward the NRR at the acid conditions. We sampled both homonuclear (M2-Pc) and heteronuclear (MM'-Pc) BACs and constructed the activity map of BACs by using N2H* adsorption energy as the activity descriptor, which reduces the number of promising catalyst candidates from over 900 to less than 100. This strategy allowed us to readily identify 3 homonuclear and 28 heteronuclear BACs, which could break the metal-based activity benchmark toward the efficient NRR. Particularly, using the free energy difference of H* and N2H* as a selectivity descriptor, we screened out five systems, including Ti2-Pc, V2-Pc, TiV-Pc, VCr-Pc, and VTa-Pc, which exhibit a strong capability of suppressing the competitive hydrogen evolution reaction (HER) with favorable limiting potential of -0.75, -0.39, -0.74, -0.85, and -0.47 V, respectively. This work not only broadens the possibility of discovering more efficient BACs toward N2 fixation but also provides a feasible strategy for rational design of NRR electrocatalysts and helps pave the way to fast screening and design of efficient BACs for the NRR and other electrochemical reactions.

Tackling the Activity and Selectivity Challenges of Electrocatalysts toward the Nitrogen Reduction Reaction via Atomically Dispersed Biatom Catalysts

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon-free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W2 N3 nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h-1 mgcata-1 (3.80 ± 0.32 × 10-11 mol cm-2 s-1 ) and Faradaic efficiency of 11.67 ± 0.93% at -0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron-based characterizations prove that the nitrogen vacancies on 2D W2 N3 are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W2 N3 can provide an electron-deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.

Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

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Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1-3 nm) 2D Ti3 C2 Tx with large lateral size of about 8 µm possessing ideal viscoelastic properties are formulated for extrusion-based 3D printing of freestanding, high specific surface area architectures to determine the viability of manufacturing energy storage devices. The 3D-printed device achieves a high areal capacitance of 2.1 F cm-2 at 1.7 mA cm-2 and a gravimetric capacitance of 242.5 F g-1 at 0.2 A g-1 with a retention of above 90% capacitance for 10 000 cycles. It also exhibits a high energy density of 0.0244 mWh cm-2 and a power density of 0.64 mW cm-2 at 4.3 mA cm-2 . It is anticipated that the sustainable printing and design approach developed in this work can be applied to fabricate high-performance bespoke multiscale and multidimensional architectures of functional and structural materials for integrated devices in various applications.

3D Printing of Freestanding MXene Architectures for Current-Collector-Free Supercapacitors.

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3D MXene Architectures for Efficient Energy Storage and Conversion

High-Performance Electrocatalytic Conversion of N2 to NH3 Using Oxygen-Vacancy-Rich TiO2 In Situ Grown on Ti3C2Tx MXene

Rational Design of Hydroxyl-Rich Ti3C2Tx MXene Quantum Dots for High-Performance Electrochemical N2 Reduction

Recent advances of MXene as promising catalysts for electrochemical nitrogen reduction reaction

Nanozyme-based colorimetry was suggested to be a rapid method for biomarker (e.g. glutathione) detection, but this method suffers from lack of efficiency and low-toxicity nanozymes till now. Herein, quantum dots of TiO2 loaded on carbon (TiO2/C-QDs) oxidase-like nanozymes were prepared via a hydrothermal treatment of tiny and few-layered Ti3C2Tx MXene nanosheets, which possess abundant thermodynamic metastable Ti atoms on MXene margins as raw materials for the preparation of TiO2/C-QDs. The oxygen vacancy in TiO2 on the surface of the carbon matrix can facilitate O2 adsorption in the solution and generate reactive oxygen species (ROS), thereby quickly oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) to its oxidized form (TMBox) in the absence of H2O2. After adding glutathione (GSH), TMBox was able to be restored to TMB, which resulted in a corresponding decrease in the UV-vis absorbance value at 652 nm. Furthermore, this assay possesses good selectivity, excellent specificity and high sensitivity (limit of detection: 0.2 μM), which made it possible to efficiently detect GSH in complex biological samples such as human serum.

Ti 3 C 2 T x MXene-derived TiO 2 /C-QDs as oxidase mimics for the efficient diagnosis of glutathione in human serum.

The metal-induced FL quenching of two-dimensional material-derived quantum dots (2D-QDs) has been playing vital roles in optical sensing recently; however, the metal type and concentration are usually chosen arbitrarily based on the maximum FL decrease although several metals may offer similar FL quenching effects towards the same 2D-QDs. Thus, a rational strategy was urgently needed to discover more efficient metal-mediated FL system for further applications. Taking boron nitride QDs (BNQDs) as a promising 2D-QD model, it was discovered that both Fe3+ and Cu2+ could quench their FL, which could be further switched on and off upon the addition of pyrophosphate and alkaline phosphatase, respectively. By comparing the percentage of FL quenching and recovery of BNQDs, it was rationally calculated that 300 μM Fe3+-mediated FL quenching of BNQDs paved a more efficient way for the sensitive and selective assay of ALP. Quantitative measurement of the ALP activity can be achieved with a low limit of detection of 0.8 U L−1 (S/N ≥ 3) in a wide range between 2 and 200 U L−1. In addition, the aforementioned ensemble could be exploited to newly construct a 2D-QD-based “INH” logic gate. This approach possessed many superiorities i.e. low cost, simplicity, and high sensitivity.

Yaqian Han

Papers

High-Performance Electrocatalytic Conversion of N2 to NH3 Using Oxygen-Vacancy-Rich TiO2 In Situ Grown on Ti3C2Tx MXene

Rational Design of Hydroxyl-Rich Ti3C2Tx MXene Quantum Dots for High-Performance Electrochemical N2 Reduction

Recent advances of MXene as promising catalysts for electrochemical nitrogen reduction reaction

Ti 3 C 2 T x MXene-derived TiO 2 /C-QDs as oxidase mimics for the efficient diagnosis of glutathione in human serum.

A rational strategy to develop a boron nitride quantum dot-based molecular logic gate and fluorescent assay of alkaline phosphatase activity