Metal-Free Bifunctional Ordered Mesoporous Carbon for Reversible Zn-CO2 Batteries

Unlock flow-type reversible aqueous Zn-CO2 batteries

Metal-CO2 batteries, which use CO2 as the active species at cathodes, are particularly promising, but device design for mass-producible CO2 reduction and energetic power supply lag behind, limiting their potential benefits. In this study, an aqueous reversible flow-type Zn-CO2 battery using a Pd/SnO2@C cathode catalyst has been assembled and demonstrates an ultra-high discharge voltage of 1.38 V, a peak power density of 4.29 mW cm-2, high-energy efficiency of 95.64% and remarkable theoretical energy density (827.3 W h kg-1). In the meantime, this optimized system achieves a high formate faradaic efficiency of 95.86% during the discharge process at a high rate of 4.0 mA cm-2. This energy- and chemical-conversion technology could store and provide electricity, eliminate CO2 and produce valuable chemicals, addressing current energy and environment issues simultaneously.

Zn-based batteries for sustainable energy storage: strategies and mechanisms

Batteries play a pivotal role in various electrochemical energy storage systems, functioning as essential components to enhance energy utilization efficiency and expedite the realization of energy and environmental sustainability. Zn-based batteries have attracted increasing attention as a promising alternative to lithium-ion batteries owing to their cost effectiveness, enhanced intrinsic safety, and favorable electrochemical performance. In this context, substantial endeavors have been dedicated to crafting and advancing high-performance Zn-based batteries. However, some challenges, including limited discharging capacity, low operating voltage, low energy density, short cycle life, and complicated energy storage mechanism, need to be addressed in order to render large-scale practical applications. In this review, we comprehensively present recent advances in designing high-performance Zn-based batteries and in elucidating energy storage mechanisms. First, various redox mechanisms in Zn-based batteries are systematically summarized, including insertion-type, conversion-type, coordination-type, and catalysis-type mechanisms. Subsequently, the design strategies aiming at enhancing the electrochemical performance of Zn-based batteries are underscored, focusing on several aspects, including output voltage, capacity, energy density, and cycle life. Finally, challenges and future prospects of Zn-based batteries are discussed.

Co3O4 nanoparticle modified N, P co-doped carbon paper as sodium carrier to construct stable anodes for Na-metal batteries

Sodium (Na) metal batteries such as Na-ion batteries and Na-CO2 batteries are considered to be excellent alternatives to lithium batteries in terms of their potential applications because of their high specific capacity and low cost. However, the sodium anode showed low efficiency and poor cycling in Na-metal battery performance due to the formation of sodium dendrites and serious corrosion. In this work, nitrogen (N), phosphorus (P) co-doped carbon paper (NP-CP) modified with cobalt tetroxide (Co3O4) nanoparticles was prepared as the Na anode carrier (Co3O4@NP-CP), and a sodium-based composite anode (Na-Co@NP-CP) was further prepared by electrodepositing sodium. The experimental results indicate that the N, P and Co3O4 multi-doped carbon paper has good sodiophilicity, which can induce the uniform plating/stripping of Na+ ions and inhibit the growth of Na dendrites. The N, P doped carbon paper provides a high surface area and tremendous three-dimensional (3D) framework to effectively reduce the areal current density, facilitate the transfer of electrons, and enhance battery life. Therefore, Na-Co@NP-CP based symmetric cells exhibit stable cycling of over 1100 hours at current densities of 1 mA cm-2 and fixed capacity of 1 mA h cm-2. When the Na-Co@NP-CP anode couples with CO2, the assembled batteries can deliver a stable cycling of 165 cycles at current densities of 500 mA g-1 and limited capacity of 500 mA h g-1. When Na-Co@NP-CP anode couples with Na3V2(PO4)3 (NVP) cathode, the assembled cells exhibit lower hysteresis and batter cycling performance.

Promoting CO2 Dynamic Activation via Micro-Engineering Technology for Enhancing Electrochemical CO2 Reduction

Optimizing the coordination structure and microscopic reaction environment of isolated metal sites is promising for boosting catalytic activity for electrocatalytic CO₂ reduction reaction (CO₂ RR) but is still challenging to achieve. Herein, a newly electrostatic induced self-assembly strategy for encapsulating isolated Ni-C₃ N₁ moiety into hollow nano-reactor as I-Ni SA/NHCRs is developed, which achieves FE_CO  of 94.91% at -0.80 V, the CO partial current density of ≈-15.35 mA cm^-2 , superior to that with outer Ni-C₂ N₂ moiety (94.47%, ≈-12.06 mA cm^-2 ), or without hollow structure (92.30%, ≈-5.39 mA cm^-2 ), and high FE_CO of ≈98.41% at 100 mA cm^-2 in flow cell. COMSOL multiphysics finite-element method and density functional theory (DFT) calculation illustrate that the excellent activity for I-Ni SA/NHCRs should be attributed to the structure-enhanced kinetics process caused by its hollow nano-reactor structure and unique Ni-C₃ N₁ moiety, which can enrich electron on Ni sites and positively shift d-band center to the Fermi level to accelerate the adsorption and activation of CO₂ molecule and *COOH formation. Meanwhile, this strategy also successfully steers the design of encapsulating isolated iron and cobalt sites into nano-reactor, while I-Ni SA/NHCRs-based zinc-CO₂ battery assembled with a peak power density of 2.54 mW cm-^-2 is achieved.

Facile Preparation of Cobalt Nanoparticles Encapsulated Nitrogen-Doped Carbon Sponge for Efficient Oxygen Reduction Reaction

The facile preparation of non-noble metal nanoparticle loaded carbon nanomaterials is promising for efficient oxygen reduction reaction (ORR) electrocatalysis. Herein, a facile preparation strategy is proposed to prepare nitrogen-doped carbon sponge loaded with fine cobalt nanoparticles by the direct pyrolysis of the cobalt ions adsorbed polymeric precursor. The polymeric sponge precursor with continuous framework and high porosity is formed by the self-assembly of a poly(amic acid). Taking advantage of the negatively charged surface and porous structure, cobalt ions can be efficiently adsorbed into the polymeric sponge. After pyrolysis, fine cobalt nanoparticles covered by carbon layers are formed, while the sponge-like structure of the precursor is also well-preserved in order to give cobalt nanoparticles loaded nitrogen-doped carbon sponges (Co/CoO@NCS) with a high loading content of 44%. The Co/CoO@NCS exhibits promising catalytic activity toward ORR with a half-wave potential of 0.830 V and a limiting current density of 4.71 mA cm^-2. Overall, we propose a facile polymer self-assembly strategy to encapsulate transition metal nanoparticles with high loading content on a nitrogen-doped carbon sponge for efficient ORR catalysis.

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.

Si Doping-Induced Electronic Structure Regulation of Single-Atom Fe Sites for Boosted CO2 Electroreduction at Low Overpotentials

Transition metal-based single-atom catalysts (TM-SACs) are promising alternatives to Au- and Ag-based electrocatalysts for CO production through CO₂ reduction reaction. However, developing TM-SACs with high activity and selectivity at low overpotentials is challenging. Herein, a novel Fe-based SAC with Si doping (Fe-N-C-Si) was prepared, which shows a record-high electrocatalytic performance toward the CO₂-to-CO conversion with exceptional current density (>350.0 mA cm^-2) and ~100% Faradaic efficiency (FE) at the overpotential of <400 mV, far superior to the reported Fe-based SACs. Further assembling Fe-N-C-Si as the cathode in a rechargeable Zn-CO₂ battery delivers an outstanding performance with a maximal power density of 2.44 mW cm^-2 at an output voltage of 0.30 V, as well as high cycling stability and FE (>90%) for CO production. Experimental combined with theoretical analysis unraveled that the nearby Si dopants in the form of Si-C/N bonds modulate the electronic structure of the atomic Fe sites in Fe-N-C-Si to markedly accelerate the key pathway involving *CO intermediate desorption, inhibiting the poisoning of the Fe sites under high CO coverage and thus boosting the CO₂RR performance. This work provides an efficient strategy to tune the adsorption/desorption behaviors of intermediates on single-atom sites to improve their electrocatalytic performance.

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.

Electrochemically Activated CNT Sheet as a Cathode for Zn-CO2 Batteries

High demand for electrochemical storage devices is increasing the need for high-performance batteries. A Zn-CO₂ battery offers a promising solution for CO₂ reduction as well as energy storage applications. For this study, a Zn-CO₂ battery was fabricated using a Carbon Nanotube (CNT) sheet as a cathode and a Zn plate as an anode. The electrochemical activation technique was used to increase the surface area of the CNT electrode by roughly 4.5 times. Copper (Cu) as a catalyst was then deposited onto the activated CNT electrode using electrodeposition method and different Cu loadings were investigated to optimize CO₂ reduction. The final assembled Zn-CO₂ battery has a 1.6 V output voltage at a current density of 0.063 mA/cm², which is higher than most devices reported in the literature. This study demonstrates the importance of activation process which enabled more catalyst loading on the cathode resulted in additional active sites for electroreduction process. This paper presents the activated CNT sheet as a promising cathode material for Zn-CO₂ batteries.

Coupling Atomically Dispersed Fe-N5 Sites with Defective N-Doped Carbon Boosts CO2 Electroreduction

Atomically dispersed iron immobilized on nitrogen-doped carbon catalyst has attracted enormous attention for CO₂ electroreduction, but still suffers from low current density and poor selectivity. Herein, atomically dispersed FeN₅ active sites supported on defective N-doped carbon successfully formed by a multistep thermal treatment strategy with the aid of dicyandiamide are reported. This dual-functional strategy can not only construct intrinsic carbon defects by selectively etching pyridinic-N and pyrrolic-N, but also introduces an additional N from the neighboring carbon layer coordinating to the commonly observed FeN₄ , thus creating an FeN₅ active site supported on defective porous carbon nanofibers (FeN₅ /DPCF) with a local 3D configuration. The optimized FeN₅ /DPCF achieves a high CO Faradaic efficiency (>90%) over a wide potential range of -0.4 to -0.6 V versus RHE with a maximal FE_CO of 93.1%, a high CO partial current density of 9.4 mA cm^-2 at the low overpotential of 490 mV, and a remarkable turnover frequency of 2965 h^-1 . Density functional theory calculations reveal that the synergistic effect between the FeN₅ sites and carbon defects can enhance electronic localization, thus reducing the energy barrier for the CO₂ reduction reaction and suppressing the hydrogen evolution reaction, giving rise to the superior activity and selectivity.

Atomic- and Molecular-Level Modulation of Dispersed Active Sites for Electrocatalytic CO2 Reduction

Global climate changes have been impacted by the excessive CO 2 emission, which exacerbates the environmental problems. Electrochemical CO 2 reduction (CO 2 RR) offers the solution for utilizing CO 2 as feedstocks for value-added products while potentially mitigating the negative effects. Owing to the extreme stability of CO 2 , selectivity and efficiency are crucial factors in the development of CO 2 RR electrocatalysts. Recently, single-atom catalysts have emerged as potential electrocatalysts for CO 2 reduction. They generally comprise of atomically- and molecularly dispersed active sites over conductive supports, which enable atomic-level and molecular-level modulations. In this minireview, catalyst preparations, principle of modulations, and reaction mechanisms are summarised together with related recent advances. The atomic-level modulations are first discussed, followed by the molecular-level modulations. Finally, the current challenges and future opportunities are provided as guidance for further developments regarding the discussed topics.

Nitrogen-Doped Carbon Polyhedrons Confined Fe-P Nanocrystals as High-Efficiency Bifunctional Catalysts for Aqueous Zn-CO2 Batteries

Emerging Fe bonded with heteroatom P in carbon matrix (FePC) holds great promise for electrochemical catalysis, but the design of highly active and cost-efficient FePC structure for the electrocatalytic CO₂ reduction reaction (CO₂ RR) and aqueous ZnCO₂ batteries (ZCBs) is still challenging. Herein, polyhedron-shaped bifunctional electrocatalysts, FeP nanocrystals anchored in N-doped carbon polyhedrons (Fe-P@NCPs), toward a reversible aqueous ZnCO₂ battery, are reported. The Fe-P@NCPs are synthesized through a facile strategy by using self-templated zeolitic imidazolate frameworks (ZIFs), followed by an in situ high-temperature calcination. The resultant catalysts exhibit aqueous CO₂ RR activity with a CO Faradaic efficiency up to 95% at -0.55 V versus reversible hydrogen electrode (RHE), comparable to the previously best-reported values of FeNC structure. The as-constructed ZCBs with designed Fe-P@NCPs cathode, show the peak power density of 0.85 mW cm^-2 and energy density of 231.8 Wh kg^-1 with a cycling durability over 500 cycles, and outstanding stability in terms of discharge voltage for 7 days. The high selectivity and efficiency of the battery are attributed to the presence of highly catalytic FeP nanocrystals in N-doped carbon matrix, which can effectively increase the number of catalytically active sites and interfacial charge-transfer conductivity, thereby improving the CO₂ RR activity.

Nitrogen-doped mesoporous carbon supported CuSb for electroreduction of CO2

The construction of an efficient catalyst for electrocatalytic reduction of CO₂ to high value-added fuels has received extensive attention. Herein, nitrogen-doped mesoporous carbon (NMC) was used to support CuSb to prepare a series of materials for electrocatalytic reduction of CO₂ to CH₄. The catalytic activity of the composites was significantly improved compared with that of Cu/NMC. In addition, the Cu content also influenced the activity of electrocatalytic CO₂ reduction reaction. Among the materials used, the CuSb/NMC-2 (Cu: 5.9 wt%, Sb: 0.49 wt%) catalyst exhibited the best performance for electrocatalytic CO₂ reduction, and the faradaic efficiency of CH₄ reached 35%, and the total faradaic efficiency of C1–C2 products reached 67%.

CuSb anchored onto nitrogen-doped mesoporous carbon (CuSb/NMC) were prepared for electroreduction of CO₂ to CH₄, C₂H₄ and CO.

Interface engineering triggered by carbon nanotube-supported multiple sulfides for boosting oxygen evolution

Finding an efficient, stable and cheap oxygen evolution reaction (OER) catalyst is very important for renewable energy conversion systems. There are relatively few related research reports due to the thermodynamic instability of transition metal sulfides (TMSs) at the oxidation potential and these are usually focused on single metal sulfides or bimetal sulfides. Metal sulfide mixture systems are rarely studied. The fabrication of a TMS/TMS interface is a feasible method to improve the kinetics of the OER. Here, we constructed TMS hybrid electrocatalysts with multiple phase interfaces for the oxygen evolution reaction, named S-CoFe/CNTs. The results show that the S-CoFe/CNT catalyst exhibits a low overpotential of 258 mV to achieve a current density of 10 mA cm^-2, and has high activity in the OER process. Meanwhile, the catalyst also shows a low Tafel slope (69 mV dec^-1) and good stability. This can be attributed to the synergistic catalysis of the multiphase interface in the catalyst and the rapid electron transfer pathway brought by CNTs. The new strategy for the synthesis of catalysts containing the TMS/TMS interface provides a new idea and method for the development of efficient and practical water splitting catalysts.

Sulfur dopant-enhanced neutral hydrogen evolution performance in MoO3nanosheets

Developing nonprecious-metal based catalysts with highly active and stable performance for hydrogen evolution reaction (HER) in neutral media is crucial points for realizing low-carbon economy because their practical use typically suffers from the slow kinetics. Herein, we developed S-doped MoO₃nanosheets toward neutral HER, fabricated by a versatile solvothermal and subsequently sulfuration processes. The obtained catalyst exhibits a small overpotential of 106 mV to reach 10 mA cm^-2in 1.0 M phosphate buffered saline, overwhelming most of recently reported catalysts. Meantime, it shows no notable deactivation after more than 60 h continuous electrolysis and 50 000 cycling tests. More importantly, the catalyst also can be applied in buffered seawater for electrocatalyzing HER, requiring 262 mV at 10 mA cm^-2and maintaining over 60 h. These findings open a new route for designing MoO₃-based catalysts for neutral hydrogen production.

Highly Efficient Oxidative Cyanation of Aldehydes to Nitriles over Se,S,N-tri-Doped Hierarchically Porous Carbon Nanosheets

Oxidative cyanation of aldehydes provides a promising strategy for the cyanide-free synthesis of organic nitriles. Design of robust and cost-effective catalysts is the key for this route. Herein, we designed a series of Se,S,N-tri-doped carbon nanosheets with a hierarchical porous structure (denoted as Se,S,N-CNs-x, x represents the pyrolysis temperature). It was found that the obtained Se,S,N-CNs-1000 was very selective and efficient for oxidative cyanation of various aldehydes including those containing other oxidizable groups into the corresponding nitriles using ammonia as the nitrogen resource below 100 °C. Detailed investigations revealed that the excellent performance of Se,S,N-CNs-1000 originated mainly from the graphitic-N species with lower electron density and synergistic effect between the Se, S, N, and C in the catalyst. Besides, the hierarchically porous structure could also promote the reaction. Notably, the unique feature of this metal-free catalyst is that it tolerated other oxidizable groups, and showed no activity on further reaction of the products, thereby resulting in high selectivity. As far as we know, this is the first work for the synthesis of nitriles via oxidative cyanation of aldehydes over heterogeneous metal-free catalysts.

Fe-doped MoS2nanosheets array for high-current-density seawater electrolysis

Designing highly active and cost-effective electrocatalysts for seawater-splitting with large current densities is compelling for developing hydrogen energy. Great advancements in hydrogen evolution reaction (HER) have been achieved, but the progress on driving HER in seawater is still limited. Herein, Fe-doped MoS₂nanoshseets array supported by 3D carbon fibers was explored to be an efficient HER electrocatalyst operating in seawater. Strikingly, it exhibited small overpotentials of 119 and 300 mV to reach the current densities of 10 and 250 mA cm^-2in buffered seawater, respectively, both of them are comparable to the best-reported values under similar conditions. Meantime, the catalyst could keep the stable HER activity for 30 h without notable loss. Theoretical calculations revealed that Fe doping increases the S-edge activity. Our work provides a new avenue for designing MoS₂-based HER electrocatalysts for industry application.