Boron-Doped Ti3C2Tx MXene for Effective and Durable High-Current-Density Ammonia Synthesis.

Ammonia (NH3) synthesis via the nitrate reduction reaction (NO3RR) offers a competitive strategy for nitrogen cycling and carbon neutrality; however, this is hindered by the poor NO3RR performance under high current density. Herein, it is shown that boron-doped Ti3C2Tx MXene nanosheets can highly efficiently catalyze the conversion of NO3RR-to-NH3 at ambient conditions, showing a maximal NH3 Faradic efficiency of 91% with a peak yield rate of 26.2 mgh-1 mgcat. -1, and robust durability over ten consecutive cycles, all of them are comparable to the best-reported results and exceed those of pristine Ti3C2Tx MXene. More importantly, when tested in a flow cell, the designed catalyst delivers a current density of ‒1000 mA cm-2 at a low potential of ‒1.18 V versus the reversible hydrogen electrode and maintains a high NH3 selectivity over a wide current density range. Besides, a Zn-nitrate battery with the catalyst as the cathode is assembled, which achieves a power density of 5.24 mW cm-2 and a yield rate of 1.15 mgh-1 mgcat. -1. Theoretical simulations further demonstrate that the boron dopants can optimize the adsorption and activation of NO3RR intermediates, and reduce the potential-determining step barrier, thus leading to an enhanced NH3 selectivity.

Co Nanoparticles Confined in Mesoporous Mo/N Co-Doped Polyhedral Carbon Frameworks towards High-Efficiency Oxygen Reduction.

Exploiting effective non-noble metal electrocatalysts for oxygen reduction reaction (ORR) is crucial for fuel cells and metal-air batteries. Herein, we designed and fabricated Co nanoparticles confined in Mo/N co-doped polyhedral carbon frameworks (Co-NP/MNCF) derived from polyoxometalate-encapsuled metal-organic framework, which showed comparable ORR performance with commercial Pt/C and a larger diffusion-limited current density. Moreover, the Co-NP/MNCF also exhibited excellent ORR stability and methanol tolerance. These appealing performances can be attributed to the porosity regulation and heteroatom doping of metal-organic framework derived polyhedral carbon frameworks, which could be beneficial for the exposure of more active sites, the optimization of electronic structure and the mass transfer of electrolyte/electron/ion.

Advances in the Electrocatalytic Hydrogen Evolution Reaction by Metal Nanoclusters-based Materials.

With the development of renewable energy systems, clean hydrogen is burgeoning as an optimal alternative to fossil fuels, in which its application is promising to retarding the global energy and environmental crisis. The hydrogen evolution reaction (HER), capable of producing high-purity hydrogen rapidly in electrocatalytic water splitting, has received much attention. Abundant research about HER has been done, focusing on advanced electrocatalyst design with high efficiency and robust stability. As potential HER catalysts, metal nanoclusters (MNCs) have been studied extensively. They are composed of several to a hundred metal atoms, with sizes being comparable to the Fermi wavelength of electrons, that is, < 2.0 nm. Different from metal atoms/nanoparticles, they exhibit unique catalytic properties due to their quantum size effect and low-coordination environment. In this review, the activity-enhancing approaches of MNCs applied in HER electrocatalysis are mainly summarized. Furthermore, recent progress in MNCs classified with different stabilization strategies, that is, the freestanding MNCs, MNCs with organic, metal and carbon supports, are introduced. Finally, the current challenges and deficiencies of these MNCs for HER are prospected.

MOF-Derived Pt/ZrO2 Carbon Electrocatalyst for Efficient Hydrogen Evolution.

One type of porous carbon nanomaterial decorated by abundant Pt/ZrO2 nanoparticles can be conveniently prepared, which is pyrolyzed from flower-shaped Zr-based UiO-67 precursor with a small amount of H2PtCl6 molecules in its large pores. In addition, the obtained Pt/ZrO2 carbon electrocatalyst can bring efficient electrocatalytic hydrogen evolution performance and long-term stability.

Sulfur-Induced Growth of Coordination Polymer Derived-Straight Carbon Nanotubes on Carbon Nanofiber Network for Zn-Air Batteries.

Low-cost heteroatom-doped carbon nanomaterials have been widely studied for efficient oxygen reduction reaction and energy storage and conversion in metal-air batteries. A Masson pine twigs-like 3-dimensional network construction of carbon nanofibers (CNFs) with abundant straight long Co, N, and S-doped carbon nanotubes (CNTs) is developed by thermal treatment of Co-based polymer coated onto polyacrylonitrile nanofiber network together with thiourea at 900 °C, denoted as CNFT-Co9 S8 -900. It is interesting to note that the introduction of a high concentration of sulfur does not lead to the complete toxicity of catalysts, but promotes the axial growth to selectively form straight CNTs instead of curly bamboo-like CNTs. The highly graphitized in-situ grown Co, N, S-doped CNTs and the 3-dimensional N-doped CNF network provide both active catalytic sites and highly conductive paths, which are beneficial for oxygen reduction reaction (ORR). Thus, the optimal CNFT-Co9 S8 -900 performs the excellent ORR catalytic activity with a half-wave potential of 0.84 V and a diffusion-limited current density of 5.49 mA cm-2 . Furthermore, the CNFT-Co9 S8 -900-based Zn-air devices also possess a high power density of 136.9 mW cm-2 better than commercial Pt/C.

Surfactant-Mediated Morphological Evolution of MnCo Prussian Blue Structures.

In the preparation of nanomaterials, the kinetics and thermodynamics in the reaction can significantly affect the structures and phases of nanocrystals. Therefore, people are keen to adopt various synthetic strategies to accurately assemble the target nanocrystals, and reveal the underlying mechanism of the formation of specific structures. In this work, the total reaction time is adjusted to let the prepared MnCo Prussian blue analogous (MnCoPBA) crystals show four evolving morphological changes at different stages with the assistance of sodium dodecyl sulfate. Furthermore, it is clearly observed that the epitaxial growth along the (100) plane on the shell of MnCoPBA nanocrystals is favored, and the thermodynamics and kinetics in the morphology change process are analyzed in detail. Through the simple pyrolysis, MnCoPBA crystals can be successfully converted into the corresponding carbon composites, of which Mn2 Co2 C nanoparticles are evenly distributed in highly graphitized carbon matrix. Among them, PBA-III-700 performs good oxygen reduction reaction performance in alkaline solution with the half-wave potential of 0.801 V and diffusion-limited current density of 5.36 mA cm-2 , and its zinc-air battery exhibits the peak power density of 103.4 mW cm-2 competitive with commercial Pt/C.

Structural and Morphological Conversion between Two Co-Based MOFs for Enhanced Water Oxidation.

The rapid development of suitable and cheap water oxidation catalysts is of great significance in energy conversion and storage. In this context, herein we have synthesized two different types of metal-organic frameworks (MOFs, denoted as BMM-11 and BMM-12) constructed from the same metal salts (cobalt nitrate) and organic linkers (H4BPTC) at the similar solvothermal conditions. Interestingly, we learned that both crystalline materials can be conveniently converted into each other by a single-crystal-to-single-crystal transformation method at their corresponding synthetic conditions. Meanwhile, we applied them directly as electrocatalysts into OER application where the pure BMM-11 and BMM-12 can achieve a stable current density of 10 mA cm-2 with an overpotential of 0.362 and 0.393 V, respectively, during which MOF degradation unexpectedly occurs. After electrolysis, the following microscopic, spectroscopic, as well as electrochemical measurements confirm that these initial MOF precursors are rapidly transformed into the mixed phases of CoOxHy species consisting of CoOOH and Co(OH)2, which are essentially active components for OER performance. Finally, we have also considered other strategies to improve MOF-derived composites in oxygen evolution activity, including bimetallic doping and physical grinding strategy. The approach described here can further be extended to other cobalt-based MOFs-derived electrocatalysts for water splitting.

FeCu bimetallic clusters for efficient urea production via coupling reduction of carbon dioxide and nitrate.

Urea electrosynthesis has appeared to meet the nitrogen cycle and carbon neutrality with energy-saving features. Copper can co-electrocatalyze among CO2 and nitrogen species to generate urea, however developing effective electrocatalysts is still an obstacle. Here, we developed a nitrogen-doped porous carbon loaded with FeCu clusters that convert CO2 and NO3- into urea, with the highest Faradaic efficiency of 39.8 % and yield rate of 1024.6 µg h-1 mgcat.-1, under optimized ambient conditions, exceeding that at the Fe or Cu homogeneous sites. Furthermore, a favorable CN coupling pathway originates from *NHCO and *NHCONO two intermediates with lower free energy barriers on FeCu dual active sites are verified through in-situ Fourier transform infrared spectroscopy and theoretical calculations. This research might provide deep insights into coupling mechanisms and investigation of efficient catalysts for green urea production.

Easily constructed porous silver films for efficient catalytic CO2 reduction and Zn-CO2 batteries.

For the electroreduction of carbon dioxide into high value-added chemicals, highly active and selective catalysts are crucial, and metallic silver is one of the most intriguing candidate materials available at a reasonable cost. Herein, through a novel two-step operation of Ag paste/SBA-15 coating and HF etching, porous silver films on a commercial carbon paper with a waterproofer (p-Ag/CP) could be easily fabricated on a large scale as highly efficient carbon dioxide reduction reaction (CO2RR) electrocatalysts with a CO Faraday efficiency (FECO) as high as 96.7% at -1.0 V vs. the reversible hydrogen electrode (RHE), and it still reaches up to 90% FECO over applied potentials ranging from -0.8 to -1.1 V vs. the RHE. Meanwhile, the membrane electrode assembly (MEA) utilizing the p-Ag/CP catalyst has achieved a current density, FECO, and stability of ∼60 mA cm-2, >91%, and 11 h, respectively. Furthermore, the assembled aqueous Zn-CO2 battery using p-Ag/CP cathode yielded a peak power density of 0.34 mW cm-2, 75 charge-discharge cycles for 25 h, and 64% FECO at 2.5 mA cm-2. Compared with flat Ag/CP, the significant enhancement in the CO2RR activity of p-Ag/CP was mainly attributed to the distinctive porous structure and an improved three-phase boundary, which is capable of inducing the stabilization of *COOH intermediates, increased active specific surface areas, fast electron transfer kinetic and mass transportation. Further, theoretical calculations revealed that p-Ag/CP possessed an optimized energy barrier for *COOH intermediates.

Copper Phosphide Nanowires as High-Performance Catalysts for Urea-Assisted Hydrogen Evolution in Alkaline Medium.

Water electrolysis represented a promising avenue for the large-scale production of high-purity hydrogen. However, the high overpotential and sluggish reaction rates associated with the anodic oxygen evolution reaction (OER) posed significant obstacles to efficient water splitting. To tackle these challenges, the urea oxidation reaction (UOR) emerged as a more favorable thermodynamic alternative to OER, offering both the energy-efficient hydrogen evolution reaction (HER) and the potential for the treating of urea-rich wastewater. In this work, a two-step methodology comprising nanowire growth and phosphating treatment was employed to fabricate Cu3P nanowires on Cu foam (Cu3P-NW/CF) catalysts. These novel catalytic architectures exhibited notable efficiencies in facilitating both the UOR and HER in alkaline solutions. Specifically, within urea-containing electrolytes, the UOR manifested desirable operational potentials of 1.43 V and 1.65 V versus the reversible hydrogen electrode (vs. RHE) to reach the current densities of 10 and 100 mA cm-2, respectively. Concurrently, the catalyst displayed a meager overpotential of 60 mV for the HER at a current density of 10 mA cm-2. Remarkably, the two-electrode urea electrolysis system, exploiting the designed catalyst as both the cathode and anode, demonstrated an outstanding performance, attaining a low cell voltage of 1.79 V to achieve a current density of 100 mA cm-2. Importantly, this voltage is preferable to the conventional water electrolysis threshold in the absence of urea molecules. Moreover, our study shed light on the potential of innovative Cu-based materials for the scalable fabrication of electrocatalysts, energy-efficient hydrogen generation, and the treatment of urea-rich wastewater.

Cathode materials for halide-based aqueous redox flow batteries: recent progress and future perspectives.

As the population increases sharply around the globe, huge shortages are occurring in energy resources. Renewable resources are urgently required to be developed to satisfy human demands. Unlike the lithium-ion batteries with safety and cost issues, the redox flow battery (RFB) is economical, stable, and convenient for the development of large-scale stationary electrical energy storage applications. Especially, the aqueous redox flow battery (ARFB) further exhibits a promising potential in larger power grids owing to its unique structural features of storing energy by filling the tank with electrolytes. The ARFB is capable of modulating battery parameters by controlling the volume and concentration of the electro-active species (EAS). Further, halogens show excellent properties, such as low cost and appropriate potential as an EAS for ARFB, further showing an efficient, safe, and affordable energy storage system (ESS). Moreover, to attain the demands of strong activity, high sensitivity, convenience as well as practicality, further attention needs to be paid to material (electrode) design and adjustment. In this mini-review, novel electrode materials, including their potential internal mechanisms and effective regulatory means, are summarized and applied in the zinc-halogen, hydrogen-halogen, and polysulfide-halogen ARFB systems, promoting the development of valuable material systems and the innovation of the energy storage/conversion technologies.

Asymmetric Coordination Environment Engineering of Atomic Catalysts for CO2 Reduction.

Single-atom catalysts (SACs) have emerged as well-known catalysts in renewable energy storage and conversion systems. Several supports have been developed for stabilizing single-atom catalytic sites, e.g., organic-, metal-, and carbonaceous matrices. Noticeably, the metal species and their local atomic coordination environments have a strong influence on the electrocatalytic capabilities of metal atom active centers. In particular, asymmetric atom electrocatalysts exhibit unique properties and an unexpected carbon dioxide reduction reaction (CO2RR) performance different from those of traditional metal-N4 sites. This review summarizes the recent development of asymmetric atom sites for the CO2RR with emphasis on the coordination structure regulation strategies and their effects on CO2RR performance. Ultimately, several scientific possibilities are proffered with the aim of further expanding and deepening the advancement of asymmetric atom electrocatalysts for the CO2RR.

Co doping promotes the alkaline overall seawater electrolysis performance over MnPSe3 nanosheets.

Herein, two-dimensional cobalt-doped MnPSe3 nanosheets (CMPS) were constructed, which served as an outstanding bifunctional catalyst for alkaline seawater splitting, i.e., offering the current density of 10 mA cm-2 with applied overpotentials of 59 and 300 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The assembled two-electrode system of CMPS//CMPS also demonstrated excellent catalytic activity (10 mA cm-2, 1.59 V) and can remain stable for more than 100 h. Moreover, the theoretical calculations showed that CMPS features a suitable H* adsorption beneficial for the HER, as well as a lower free energy barrier favorable for the OER.

An efficient glucose sensor thermally calcined from copper-organic coordination cages.

Due to the harmfulness of diabetes, a fast and efficient glucose detector is particularly important. Metal-organic polyhedron (MOP) provides a porous framework and a special matrix, which makes it an excellent precursor for electrochemical detection. Herein, we report a novel MOP as a precursor for the preparation of an electrocatalytic detector for glucose. The new metal-organic polyhedron of Cu4(TPDC)4 can be solvothermally obtained and characterized by X-ray crystallography, which can be thermally converted into nanosized copper oxides embedded into graphitic carbon layers (MOP-CO). The as-prepared MOP-CO electrode is further applied to glucose detection, which shows a fast response time (<1 s) in a wide linear range of 0-4000 µM and high sensitivity of 2720 µA mM-1 cm-2, as well as low detection limit (26 nM (S/N = 3)), good anti-interference, repeatability and stability (>3600 s).

Differentiated Oxygen Evolution Behavior in MOF-Derived Oxide Nanomaterials Induced by Phase Transition.

Oxygen evolution reaction (OER) on the anode has become one of the most widely studied electrochemical processes, which poses an important role in several energy generation technologies. In this work, we have designed and synthesized a series of metal-organic framework (MOF)-derived oxides pyrolyzed at different temperatures for efficient water oxidation in alkaline solutions. First, the barrel-shaped BMM-10 microcrystals can be conveniently synthesized under solvothermal conditions, and the hollow morphology of BMM-10-Fe with low crystallinity can be obtained through the fierce hydrolysis of Fe(III) ions. After being oxidized in air, there are only two typical phases of oxides including BMM-10-Fe-L and BMM-10-Fe-H. During electrolysis, BMM-10-Fe-L turns out to be immediately degraded into active Ni/FeOOH nanosheets with improved OER performance, while there is almost no structural and morphological change in BMM-10-Fe-H due to the structural rigidity and robust stability. Furthermore, the optimal BMM-10-Fe-H exhibits a promising electrocatalytic OER performance with a low Tafel slope of 137.4 mV dec-1, a small overpotential of 260 mV at 10 mA cm-2, and a high current retention of 93.8% after the stability test. The present work would motivate the scientific community to construct various MOF-derived nanomaterials for efficient energy storage and conversion applications.

In-MOF-derived ultrathin heteroatom-doped carbon nanosheets for improving oxygen reduction.

For electrocatalysis, the development of highly active and low-cost stable electrocatalysts, which would be directly applied in cathodes for fuel cells that are regarded as the most promising candidates for clean energy conversion in the quest for alternatives to conventional fossil fuel technology, remains a massive challenge. In this context, oxygen reduction reaction (ORR) is a critical process under intense research for the direct conversion of chemical energy into electricity. Herein, a facile synthetic method is proposed for the preparation of hierarchically porous 2-dimensional nanosheets consisting of Fe4C and FeCo nanoparticles incorporated in N/S-doped carbon materials at 900 °C, denoted as InFeCo@CNS900. This composite can be conveniently prepared by directly calcining the crystalline indium-organic framework of InOF-24, which is impregnated with the ferric thiocyanate and cobalt ammonium complexes under Ar atmosphere, in which Fe4C and FeCo nanoparticles were in situ formed and embedded into the well-developed carbon materials, which display the hierarchically porous nanosheets with microporous and mesoporous structures. Due to the synergistic effects between different active substances, high specific surface area, suitable graphitization degree, and rich active sites, the as-obtained InFeCo@CNS900 electrocatalyst exhibits an excellent ORR activity, which shows a lower Tafel slope of 59.5 mV dec-1, higher diffusion limit current of 5.15 mA cm-2, and better stability than the commercial 20 wt% Pt/C catalyst. This study provides a facile approach for the design and synthesis of highly efficient non-noble metal-doped carbon materials with a unique 2-dimensional morphology, which are potentially applied in energy science and technology.