Enhancing Compatibility of Two-Step Tandem Catalytic Nitrate Reduction to Ammonia Over P-Cu/Co(OH)2.

Electrochemical nitrate reduction reaction (NO3RR) is a promising approach to realize ammonia generation and wastewater treatment. However, the transformation from NO3 - to NH3 involves multiple proton-coupled electron transfer processes and by-products (NO2 -, H2, etc.), making high ammonia selectivity a challenge. Herein, a two-phase nanoflower P-Cu/Co(OH)2 electrocatalyst consisting of P-Cu clusters and P-Co(OH)2 nanosheets is designed to match the two-step tandem process (NO3 - to NO2 - and NO2 - to NH3) more compatible, avoiding excessive NO2 - accumulation and optimizing the whole tandem reaction. Focusing on the initial 2e- process, the inhibited *NO2 desorption on Cu sites in P-Cu gives rise to the more appropriate NO2 - released in electrolyte. Subsequently, P-Co(OH)2 exhibits a superior capacity for trapping and transforming the desorbed NO2 - during the latter 6e- process due to the thermodynamic advantage and contributions of active hydrogen. In 1 m KOH + 0.1 m NO3 -, P-Cu/Co(OH)2 leads to superior NH3 yield rate of 42.63 mg h- 1 cm- 2 and NH3 Faradaic efficiency of 97.04% at -0.4 V versus the reversible hydrogen electrode. Such a well-matched two-step process achieves remarkable NH3 synthesis performance from the perspective of optimizing the tandem catalytic reaction, offering a novel guideline for the design of NO3RR electrocatalysts.

Next-Generation Ultrathin Lightweight Electrode for pH-Universal Aqueous Flow Batteries.

Aqueous flow batteries (AFBs) are promising long-duration energy storage system owing to intrinsic safety, inherent scalability, and ultralong cycle life. However, due to the thicker (3-5 mm) and heavier (300-600 g m-2) nature, the current used graphite felt (GF) electrodes still limit the volume/weight power density of AFBs. Herein, a lightweight (≈50 g m-2) and ultrathin (≈0.3 mm) carbon microtube electrode (CME) is proposed derived from a scalable one-step carbonization of commercial cotton cloth. The unique loose woven structure composed of carbon microtube endows CME with excellent conductivity, abundant active sites, and enhanced electrolyte transport performance, thereby significantly reducing polarization in working AFBs. As a consequence, CME demonstrates excellent cycling performance in pH-universal AFBs, including acidic vanadium flow battery (maximum power density of 632.2 mW cm-2), neutral Zn-I2 flow battery (750 cycles with average Coulombic efficiency of 99.6%), and alkaline Zn-Fe flow battery (energy efficiency over 70% at 200 mA cm-2). More importantly, the estimated price of CME is only 5% of GF (≈3 vs ≈60 $ m-2). Therefore, it is reasonably anticipated that the lightweight and ultrathin CME may emerge as the next generation electrode for AFBs.

Mo2C-Co heterostructure with carbon nanosheets decorated carbon microtubules: Different means for high-performance lithium-sulfur batteries.

The practical applications of lithium sulfur batteries (LSBs) are hindered by notorious shuttle effect and sluggish conversion kinetics of intermediate polysulfides. Herein, Mo2C-Co heterogeneous particles decorated two-dimensional (2D) carbon nanosheets grown on hollow carbon microtubes (CCC@MCC) are synthesized. Three-dimensional (3D) carbon framework with Mo2C-Co heterogeneous particles combines the conductivity, adsorption and catalysis, effectively trapping and accelerating the conversion of polysulfides. As evidenced experimentally, the hetero-structured Mo2C-Co with high Li+ diffusion coefficient enables uniform precipitation and complete oxidation of Li2S. Meanwhile, CCC@MCC is found to have multiple application possibilities for lithium-sulfur batteries. As an interlayer, the cells deliver an excellent capacity of 881.1 mAh/g at 2C and still retain 438.2 mAh/g after 500 cycles under the low temperature of 0 ℃. As a sulfur carrier, the cell with a sulfur loading of 7.0 mg cm-2 exhibits a high area capacity of 5.3 mAh cm-2. This work provides an effective strategy to prepare heterostructured material and imaginatively exploit the application potential of it.

An aqueous alkaline zinc-sulfur flow battery.

We demonstrate a rechargeable aqueous alkaline zinc-sulfur flow battery that comprises environmental materials zinc and sulfur as negative and positive active species. Meanwhile, a nickel-based electrode is also obtained by a two-step process to decrease the polarization of the sulfur redox reaction, thus greatly improving the voltage efficiency of the system from 32% to 78% at 10 mA cm-2.

In Situ Mapping of Activity Distribution of V(II)/V(III) and Onset Potential Distribution of Hydrogen Evolution Side Reaction in Vanadium Flow Batteries.

Vanadium flow batteries (VFBs) face a challenge with the low reaction rates of the V(II)/V(III) redox couple, which limits the performance of VFBs. Additionally, the negative electrode in VFBs is often accompanied by the persistent hydrogen evolution reaction (HER), which is difficult to eliminate. Therefore, understanding the spatial distribution of activity on the negative electrode and the HER side reaction on the electrode surface is of critical importance. This study proposes a weak measurement imaging method to characterize the spatial distribution of surface activity and HER onset potential on the negative electrode in VFBs). This method enables the visualization and in situ detection of key parameters such as the absolute values of |ipa |, |ipc |, |∆E|, |ipc /ipa |, and the HER onset potential. By comparing three different types of graphite felts with varying activity levels, it validates the feasibility of this method. Furthermore, electrochemical stability tests are conducted to study the electrodes repeatability, uniformity, and durability. This method holds promise in guiding the design of electrodes with enhanced activity, good reversibility, minimized HER side reactions, and uniform distribution.

A Bi-Co Corridor Construction Effectively Improving the Selectivity of Electrocatalytic Nitrate Reduction toward Ammonia by Nearly 100.

Improving the selective ammonia production capacity of electrocatalytic nitrate reduction reaction (NO3 RR) at ambient conditions is critical to the future development and industrial application of electrosynthesis of ammonia. However, the reaction involves multi-proton and electron transfer as well as the desorption and underutilization of intermediates, posing a challenge to the selectivity of NO3 RR. Here the electrodeposition site of Co is modulated by depositing Bi at the bottom of the catalyst, thus obtaining the Co+Bi@Cu NW catalyst with a Bi-Co corridor structure. In 50 mm NO3 - , Co+Bi@Cu NW exhibits a highest Faraday efficiency of ≈100% (99.51%), an ammonia yield rate of 1858.2 µg h-1  cm-2 and high repeatability at -0.6 V versus the reversible hydrogen electrode. Moreover, the change of NO2 - concentration on the catalyst surface observed by in situ reflection absorption imaging and the intermediates of the NO3 RR process detected by electrochemical in situ Raman spectroscopy together verify the NO2 - trapping effect of the Bi-Co corridor structure. It is believed that the measure of modulating the deposition site of Co by loading Bi element is an easy-to-implement general method for improving the selectivity of NH3 production as well as the corresponding scientific research and applications.

In Situ Detection of Electrochemical Reaction Surface Area by Optical Weak Measurement.

The surface area is key to electrochemical systems, including those in electrocatalysis and energy storage. Studies have shown that the surface area of the electrocatalyst directly affects the electrochemical activity, adsorption performance, and stability of the electrocatalyst. This paper used an optical weak measurement (WM) method, which has little impact on the analyte, to measure the reaction surface area (RSA) that actually participated in the electrochemical reaction. Then compared the RSA obtained by the WM with the total surface area (TSA) obtained by the standard Brunauer-Emmett-Teller (BET) measurement and the active surface area (ASA) obtained by the electrochemical double-layer capacitance (EDLC) method. Their growth trend was consistent, indicating the reliability of the WM method. Compared with the above two methods, the WM method is an in situ detection and easy to operate experimentally, which can help researchers to consider the effect of surface area on electrocatalyst performance more rationally.

Advanced cathodic free-standing interlayers for lithium-sulfur batteries: understanding, fabrication, and modification.

In the past decades, lithium-sulfur batteries (LSBs) have demonstrated huge practical potential due to their ultrahigh theoretical specific capacity, low price, and environmental friendliness. However, LSBs are still faced with the problems of volumetric expansion, slow reaction kinetics, and short working life due to the shuttling of polysulfides. The introduction of a free-standing interlayer is a good way to solve the problems because of the physical confinement, chemical entrapment, and conversion. This review summarizes the common fabrication methods of free-standing interlayers, including the power-originated and film-originated methods. The modification of the as-prepared free-standing interlayers is also accomplished into physical treatment, atomic doping, and compound introduction. Finally, we conclude and compare the different fabrication methods of free-standing interlayers and their modifications and put forward the outlook of the high-performance free-standing interlayers.

Characterizing the Onset Potential Distribution of Pt/C Catalyst Deposition by a Total Internal Reflection Imaging Method.

A catalytic electrode with extraordinary performances for hydrogen evolution reaction (HER) should achieve a low onset potential of the bulk electrode, as well as its uniform distribution. Herein, a total internal reflection imaging (TIRi) method to characterize the onset potential distribution of the catalytic electrode surface is presented. When the potential scans toward negative in a linear sweep voltammetry, the equivalent refractive index of the electrolyte on the electrode surface will decrease due to H2 microbubbles generation, leading to the increase in optical intensity. Analysis of the relationship between the optical intensity and potential in each region results in the onset potential distribution. The TIRi method reveals poor uniformity and repeatability in the catalytic electrodes which are fabricated by depositing Pt/C catalysts on a porous carbon support with polymer binders (e.g., Nafion). Further electrochemical stability test also shows poor durability, whose HER onset potential deteriorates from the edge to the middle of these catalytic electrodes. The present TIRi method realizes direct visualization of the activity distribution on the bulk electrode surface, which provides a powerful tool for better fabrication and evaluation of large-area HER electrodes in industrial energy devices.

In situ detection of electrochemical reaction by weak measurement.

In the field of electrochemical energy storage systems, the use of in situ detection technology helps to study the mechanism of electrochemical reaction. Our group has previously in situ detected the electrochemical reaction in vanadium flow batteries by total internal reflection (TIR) imaging. In order to further improve the detection resolution, in this study, the weak measurement (WM) method was introduced to in situ detect the electrochemical reaction during the linear sweep voltammetry or the cyclic voltammetry tests with quantitative measurement of the absolute current density, which lays a foundation for replacing the TIR for two-dimensional imaging of electrochemical reactions in vanadium flow batteries, oxygen/hydrogen evolution reaction, surface treatments, electrochemical corrosion and so on.

Integrated Design of Interlayer/Current-Collector: Heteronanowires Decorated Carbon Microtube Fabric for High-Loading and Lean-Electrolyte Lithium-Sulfur Batteries.

Low sulfur loading, high electrolyte/sulfur (E/S) ratio, and sluggish sulfur redox reaction are the main challenges that severely impede the practical application of lithium-sulfur batteries (LSBs). To address these problems, a self-standing hollow carbonized cotton cloth (CCC) decorated with TiO2 -TiN heteronanowires (CCC@TiO2 -TiN) is proposed to replace the traditional cathode. Concretely, one side of CCC@TiO2 -TiN serves as a current-collector to load sulfur (CCC@TiO2 -TiN/S), while the other side facing the separator acts as interlayer to inhibit shuttle effect. This advanced intergrated interlayer/current-collector cathode is endowed with excellent 3D electron/ion transportation, a strong confinement barrier, and vast sulfur loading sites. Moreover, the as-developed TiO2 -TiN heteronanowires work as in situ capture and catalysis sites for the reversible and accelerated sulfur redox reaction. Therefore, the intergrated cathode of CCC@TiO2 -TiN/S achieves an ultrahigh sulfur loading of 13 mg cm-2 and delivers a superb areal capacity of 9.09 mAh cm-2 under the ultralow E/S ratio of 4.6 µL mg-1 . This work provides a new model material to achieve high sulfur loading and lean-electrolyte toward the practical LSBs with high specific energy density.

Carbon Microtube Textile with MoS2 Nanosheets Grown on Both Outer and Inner Walls as Multifunctional Interlayer for Lithium-Sulfur Batteries.

The shuttle effect of soluble lithium polysulfides during the charge/discharge process is the key bottleneck hindering the practical application of lithium-sulfur batteries. Herein, a multifunctional interlayer is developed by growing metallic molybdenum disulfide nanosheets on both outer and inner walls of cotton cloth derived carbon microtube textile (MoS2@CMT). The hollow structure of CMT provides channels to favor electrolyte penetration, Li+ diffusion and restrains polysulfides via physical confinement. The hydrophilic and conductive 1T-MoS2 nanosheets facilitate chemisorption and kinetic behavior of polysulfides. The synergic effect of 1T-MoS2 nanosheets and CMT affords the MoS2@CMT interlayer with an efficient trapping-diffusion-conversion ability toward polysulfides. Therefore, the cell with the MoS2@CMT interlayer exhibits enhanced cycling life (765 mAh g-1 after 500 cycles at 0.5 C) and rate performance (974 mAh g-1 at 2 C and 740 mAh g-1 at 5 C). This study presents a pathway to develop low-cost multifunctional interlayers for advanced lithium-sulfur batteries.

In situ mapping of activity distribution and oxygen evolution reaction in vanadium flow batteries.

Understanding spatial distribution difference and reaction kinetics of the electrode is vital for enhancing the electrochemical reaction efficiency. Here, we report a total internal reflection imaging sensor without background current interference to map local current distribution of the electrode in a vanadium redox flow battery during cyclic voltammetry (CV), enabling mapping of the activity and reversibility distribution with the spatial resolution of a single fiber. Three graphite felts with different activity are compared to verify its feasibility. In long-term cyclic voltammetry, the oxygen evolution reaction is proved to enhance activity distribution, and homogeneity of the electrode and its bubble kinetics with periodic fluctuation is consistent with the cyclic voltammetry curve, enabling the onset oxygen evolution/reduction potential determination. Higher activity and irreversibility distribution of the electrode is found in favor of the oxygen evolution reaction. This sensor has potential to detect in situ, among other processes, electrochemical reactions in flow batteries, water splitting, electrocatalysis and electrochemical corrosion.

Simultaneously Providing Iron Source toward Electro-Fenton Process and Enhancing Hydrogen Peroxide Production via a Fe3O4 Nanoparticles Embedded Graphite Felt Electrode.

Electro-reduction of O2 to generate H2O2 is an attractive alternative to the current anthraquinone process and quite necessary for chemical industries and environmental remediation. In general, sufficient porous structure contributes to expose more catalytic active sites and shorten diffusion paths for the heterogeneous catalysis of O2. In this work, initially the Fe3O4 nanoparticles embedded graphite felt (Fe3O4@GF) is prepared through a mild hydrothermal following with thermal reduction method. This special combination not only provides iron source for the electro-Fenton reaction but also supplies rich active sites from the Fe3O4 embedded structure with abundant cracks, which are beneficial to increase the reaction rate. Compared with raw graphite felt (RGF), fresh Fe3O4@GF exhibits superior pollutant degradation kinetics with more than 400% increase and approximately 37.8% improvement to the removal of total organic carbon. A 98% decolorization of rhodamine B (RhB) can be achieved in just 5 min and quickly completes 100% removal of RhB in the next few seconds. As the electro-Fenton reaction progresses, Fe3O4 dissolves in the electrolyte, leaving a porous structure on the surface of the GF to form a porous GF (PGF), and the rapid radical reaction activates the GF surface. Both the chemical etching of Fe3O4 and the electro-Fenton process can further increase the specific surface area, defects, and actives sites of the electrode. As expected, the active PGF exhibits favorable performance of H2O2 production in electrolytes of different pHs: 1 (320.0 ± 36.5 mg L-1), 3 (301.9 ± 13.2 mg L-1), and 7 (320.4 ± 21.2 mg L-1). The degradation performance of PGF does not significantly decay even after 20 cycles of repeated use, indicating the good structural stability and long-term durability. The superiority of the in situ Fe source and fast reaction kinetics for electro-Fenton of Fe3O4@GF is confirmed, and this holey engineered strategy also provides the possibility to achieve swift water purification and open up a new way for developing efficient carbon-based electrodes.

Selective Electro-Oxidation of Glycerol to Dihydroxyacetone by PtAg Skeletons.

Developing high-performance electrocatalysts for the selective conversion of glycerol into value-added chemicals is of great significance. Herein, three-dimensional nanoporous PtAg skeletons were studied as catalysts for the electro-oxidation of glycerol. The structural features of the PtAg skeletons were revealed by electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. The electrochemical activity of the catalysts was examined by cyclic voltammetry, linear sweeping voltammetry, and chronoamperometry. The resulting PtAg skeletons exhibit a peak current density of 7.57 mA cm-2, which is 15.4-fold higher than that of Pt/C, making the PtAg skeletons one of the best electrocatalysts for glycerol oxidation. High-performance liquid chromatography results show that the PtAg skeletons yield a remarkable dihydroxyacetone selectivity of 82.6%, which has so far been the second largest value reported in the literature. The superior activity and selectivity of the PtAg skeletons are ascribed to the large surface area and abundant Pt(111) facets. Additionally, the effects of glycerol and KOH concentrations and reaction time on product selectivity were investigated.

Bilayer Designed Hydrocarbon Membranes for All-Climate Vanadium Flow Batteries To Shield Catholyte Degradation and Mitigate Electrolyte Crossover.

The use of low-cost hydrocarbon membranes in vanadium flow batteries (VFBs) still remains a great challenge because of the strong oxidation of VO2+ catholyte and rapid capacity fading. Here, we report a bilayer design strategy using an antioxidant and dense cross-linked sulfonated polyimide (cSPI) layer as a protective layer for a sulfonated poly(ether ether ketone) (SPEEK) membrane to shield catholyte degradation and mitigate electrolyte crossover. A scalable process is developed to fabricate an integrated bilayer SPEEK/cSPI membrane without delamination by spraying a SPEEK transition layer between the two polymers. The tightly bridged cSPI layer not only protects the SPEEK membrane from degradation but also enhances its mechanical strength, puncture resistance, and proton/vanadium-ion selectivity. When assembled in a VFB, the bilayer SPEEK/cSPI membrane demonstrates excellent rate performance under current densities of 40-200 mA cm-2, high adaptability at a wide temperature range of -15 to 60 °C, very slow capacity decay rate of 0.054% per cycle at 160 mA cm-2, and a maximum power density of 480 mW cm-2. These merits make the bilayer SPEEK/cSPI membrane a promising candidate for the next-generation VFB to achieve low-cost, high-rate, and all-climate energy storage.

Waste cotton cloth derived carbon microtube textile: a robust and scalable interlayer for lithium-sulfur batteries.

A robust carbonized cotton cloth interlayer composed of numerous knitted hollow carbon microtubes is simply derived from waste cotton cloth by scalable carbonization. The interlayer acts as an upper current collector and a lithium polysulfide barrier simultaneously, thus greatly improving the electrochemical performances of the lithium-sulfur batteries.

Sandwiching h-BN Monolayer Films between Sulfonated Poly(ether ether ketone) and Nafion for Proton Exchange Membranes with Improved Ion Selectivity.

Two-dimensional (2D) hexagonal boron nitride (h-BN) has attracted great interest due to its excellent chemical and thermal stability, electrical insulating property, high proton conductivity, and good flexibility. Integration of 2D h-BN into commercial proton exchange membranes (PEMs) has the potential to improve ion selectivity while maintaining the proton conductivity of PEMs simultaneously, which has been a longstanding challenge in membrane separation technology. Until now, such attempts are only limited in mechanically exfoliated small area h-BN and in proof-of-concept devices, due to the difficulty of growing and transferring large area uniform h-BN monolayers. Here, we develop a space-confined chemical vapor deposition approach and achieve the growth of wafer-scale uniform h-BN monolayer films on Cu rolls. We further develop a Nafion functional layer assisted transfer method which effectively transfers as-grown h-BN monolayer films from the Cu roll to sulfonated poly(ether ether ketone) (SPEEK) membrane. The as-fabricated Nafion/h-BN/SPEEK sandwich structure is used as the membrane and compared with the pure SPEEK membrane for flow batteries. Results show that the sandwich membrane exhibits ion selectivity 3-fold greater than that of a pure SPEEK membrane ( i.e., 32.1 × 104 vs 9.7 × 104 S min cm-3). In addition, we fabricate vanadium flow batteries using the Nafion/h-BN/SPEEK sandwich membrane and find that the sandwich structure does not affect the proton transport but inhibits vanadium crossover at low current density (<120 mA cm-2) due to the selective blocking of vanadium ions by 2D h-BN. As a result, the sandwich membrane exhibits a significantly improved Coulombic efficiency and energy efficiency, ∼95% and ∼91%, respectively. Our results suggest that a functional layer/2D film/target substrate-based sandwich structure shows clear potential for future 2D material-based membranes in separation technologies.

Exceptional Performance of Hierarchical Ni-Fe (hydr)oxide@NiCu Electrocatalysts for Water Splitting.

Developing low-cost bifunctional electrocatalysts with superior activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for the widespread application of the water splitting technique. In this work, using earth-abundant transition metals (i.e., nickel, iron, and copper), 3D hierarchical nanoarchitectures, consisting of ultrathin Ni-Fe layered-double-hydroxide (Ni-Fe LDH) nanosheets or porous Ni-Fe oxides (NiFeOx ) assembled to a metallic NiCu alloy, are delicately constructed. In alkaline solution, the as-prepared Ni-Fe LDH@NiCu possesses outstanding OER activity, achieving a current density of 10 mA cm-2 at an overpotential of 218 mV, which is smaller than that of RuO2 catalyst (249 mV). In contrast, the resulting NiFeOx @NiCu exhibits better HER activity, yielding a current density of 10 mA cm-2 at an overpotential of 66 mV, which is slightly higher than that of Pt catalyst (53 mV) but superior to all other transition metal (hydr)oxide-based electrocatalysts. The remarkable activity of the Ni-Fe LDH@NiCu and NiFeOx @NiCu is further demonstrated by a 1.5 V solar-panel-powered electrolyzer, resulting in current densities of 10 and 50 mA cm-2 at overpotentials of 293 and 506 mV, respectively. Such performance renders the as-prepared materials as the best bifunctional electrocatalysts so far.

Asymmetric vanadium flow batteries: long lifespan via an anolyte overhang strategy.

Fast capacity decay is a serious problem in vanadium flow batteries (VFBs). How to eliminate or slow down capacity fading has become a critical issue for the practical application of VFBs. Herein, the concept of an asymmetric vanadium flow battery (aVFB) is introduced, in which the asymmetric design of a catholyte and an anolyte is used to suppress the capacity decay of the VFB. Based on the comprehensive analysis of the capacity decay and electrolyte imbalance process of the traditional symmetric VFB, it was found that the capacity fading is mainly owing to the loss of the anolyte in the long-term cycling test. Therefore, this work attempts to use excess anolyte (i.e. 10%, 20% and 30%) to mitigate the capacity decay during the long-term operation of the VFB. To gain deeper insights into the capacity retention mechanism of these novel anolyte overhang aVFBs, long-term cycle performance of the corresponding symmetric overhang VFBs and catholyte overhang aVFBs is investigated for comparison. The optimal excess ratio of anolyte and how to add the excess anolyte are also suggested for future study.