Polycation-Regulated Electrolyte and Interfacial Electric Fields for Stable Zinc Metal Batteries.

Zn metal as one of promising anode materials for aqueous batteries but suffers from disreputable dendrite growth, grievous hydrogen evolution and corrosion. Here, a polycation additive, polydiallyl dimethylammonium chloride (PDD), is introduced to achieve long-term and highly reversible Zn plating/stripping. Specifically, the PDD can simultaneously regulate the electric fields of electrolyte and Zn/electrolyte interface to improve Zn2+ migration behaviors and guide dominant Zn (002) deposition, which is veritably detected by Zeta potential, Kelvin probe force microscopy and scanning electrochemical microscopy. Moreover, PDD also creates a positive charge-rich protective outer layer and a N-rich hybrid inner layer, which accelerates the Zn2+ desolvation during plating process and blocks the direct contact between water molecules and Zn anode. Thereby, the reversibility and long-term stability of Zn anodes are substantially improved, as certified by a higher average coulombic efficiency of 99.7 % for Zn||Cu cells and 22 times longer life for Zn||Zn cells compared with that of PDD-free electrolyte.

Electron-Donors-Acceptors Interaction Enhancing Electrocatalytic Activity of Metal-Organic Polymers for Oxygen Reduction.

Catalysts with metal-Nx sites have long been considered as effective electrocatalysts for oxygen reduction reaction (ORR), yet the accurate structure-property correlations of these active sites remain debatable. Report here is a proof-of-concept method to construct 1,4,8,11-tetraaza[14]annulene (TAA)-based polymer nanocomposites with well-managed electronic microenvironment via electron-donors/acceptors interaction of altering electron-withdrawing ß-site substituents. DFT calculation proves the optimal -Cl substituted catalyst (CoTAA-Cl@GR) tailored the key OH* intermediate interaction with Co-N4 sites under the d-orbital regulation, hence reaching the top of ORR performance with excellent turnover frequency (0.49 e s-1 site-1 ). The combination of in situ scanning electrochemical microscopy and variable-frequency square wave voltammetry techniques contribute the great ORR kinetics of CoTAA-Cl@GR to the relatively high accessible site density (7.71×1019  site g-1 ) and fast electron outbound propagation mechanism. This work provides theoretical guidance for rational design of high-performance catalysts for ORR and beyond.

Longitudinally Grafting of Graphene with Iron Phthalocyanine-based Porous Organic Polymer to Boost Oxygen Electroreduction.

Iron phthalocyanine-based polymers (PFePc) are attractive noble-metal-free candidates for catalyzing oxygen reduction reaction (ORR). However, the low site-exposure degree and poor electrical conductivity of bulk PFePc restricted their practical applications. Herein, laminar PFePc nanosheets covalently and longitudinally linked to graphene (3D-G-PFePc) was prepared. Such structural engineering qualifies 3D-G-PFePc with high site utilization and rapid mass transfer. Thence, 3D-G-PFePc demonstrates efficient ORR performance with a high specific activity of 69.31 µA cm-2 , a high mass activity of 81.88 A g-1 , and a high turnover frequency of 0.93 e s-1 site-1 at 0.90 V vs. reversible hydrogen electrode in O2 -saturated 0.1 M KOH, outperforming the lamellar PFePc wrapped graphene counterpart. Systematic electrochemical analyses integrating variable-frequency square wave voltammetry and in situ scanning electrochemical microscopy further underline the rapid kinetics of 3D-G-PFePc towards ORR.

Deciphering the Precursor-Performance Relationship of Single-Atom Iron Oxygen Electroreduction Catalysts via Isomer Engineering.

Single atom Fe-nitrogen-carbon (Fe-N-C) catalysts have high catalytic activity and selectivity for the oxygen reduction reaction (ORR), and are possible alternatives for Pt-based materials. However, the reasonable design and selection of precursors to establish their relationship with Fe-N-C catalyst performance is still a formidable task. Herein, precursors with controllable structures are easily achieved through isomer engineering, with the purpose of regulating the active site density and microscopic morphology of the final electrocatalyst. As-proof-of-concept, phenylenediamine isomers-based polymers are used as precursors to fabricate Fe-N-C catalysts. The Fe-PpPD-800 derived from p-phenylenediamine shows that the best ORR activity with a half-wave potential (E1/2 ) reaches 0.892 V vs reversible hydrogen electrode (RHE), which is better than the counterparts derived from o-phenylenediamine (Fe-PoPD-800) and m-phenylenediamine (Fe-PmPD-800), even surpassing commercial Pt/C (E1/2  = 0.881 V vs RHE). Furthermore, the self-made zinc-air battery based on Fe-PpPD-800 achieves high power density and specific capacity up to 242 mW cm-2 and 873 mA h gZn -1 respectively, a stable open circuit voltage of 1.45 V, and excellent cycling stability. This work not only proves the practicability of adjusting the catalytic activity of single-atom catalysts through isomer engineering, but also provides an approach to understand the relationship between precursors and target catalysts performance.

Simultaneously Integrate Iron Single Atom and Nanocluster Triggered Tandem Effect for Boosting Oxygen Electroreduction.

Atomically nitrogen-coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious-metal-based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co-doped porous carbons are rationally prepared and are verified with rich Fe-active sites, including atomically dispersed FeN4 and Fe nanoclusters (FeSA-FeNC@NSC), according to systematically synchrotron X-ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN4 and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA-FeNC @NSC delivers an impressive ORR activity with half-wave-potential of 0.90 V, which exceeds that of state-of-the-art Pt/C (0.87 V). Furthermore, FeSA-FeNC @NSC-based Zn-air batteries deliver excellent power densities of 259.88 and 55.86 mW cm-2 in liquid and all-solid-state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single-atom catalysts by tandem effect.

Optimizing Microenvironment of Asymmetric N,S-Coordinated Single-Atom Fe via Axial Fifth Coordination toward Efficient Oxygen Electroreduction.

Single-atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure-activity relationship is critical, which remains challenging. Herein, a self-sacrificing strategy is developed to synthesize asymmetric N,S-coordinated single-atom Fe with axial fifth hydroxy (OH) coordination (Fe-N3 S1 OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta-coordination microenvironment is determined by cutting-edge techonologies aiding of systematic simulations. The as-obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half-wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN3 S1 OH can optimize 3d orbitals of Fe center to strengthen O2 adsorption and enhance O2 activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H2 O2 fuel cell performs a high peak power density of 512 mW cm-2 , and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm-2 in liquid and flexible all-solid-state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single-atom metal sites for electrocatalysis.

Simultaneously Integrating Single Atomic Cobalt Sites and Co9 S8 Nanoparticles into Hollow Carbon Nanotubes as Trifunctional Electrocatalysts for Zn-Air Batteries to Drive Water Splitting.

The development of rechargeable metal-air batteries and water electrolyzers are highly constrained by electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the construction of efficient trifunctional electrocatalysts for ORR/OER/HER are highly desirable yet challenging. Herein, hollow carbon nanotubes integrated single cobalt atoms with Co9 S8 nanoparticles (CoSA + Co9 S8 /HCNT) are fabricated by a straightforward in situ self-sacrificing strategy. The structure of the CoSA + Co9 S8 /HCNT are verified by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Theoretical calculations and experimental results embrace the synergistic effects between Co9 S8 nanoparticles and single cobalt atoms through optimizing the electronic configuration of the CoN4 active sites to lower the reaction barrier and facilitating the ORR, OER, and HER simultaneously. Consequently, rechargeable liquid and all-solid-state flexible Zn-air batteries based on CoSA + Co9 S8 /HCNT exhibit remarkable stability and excellent power density of 177.33 and 51.85 mW cm-2 , respectively, better than Pt/C + RuO2 counterparts. Moreover, the as-fabricated Zn-air batteries can drive an overall water splitting device assembled with CoSA + Co9 S8 /HCNT and achieve a current density of 10 mA cm-2 at a low voltage of 1.59 V, also superior to Pt/C + RuO2 . Therefore, this work presents a promising approach to an efficient trifunctional electrocatalyst toward practical applications.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells.

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

Carbon Nanocage with Maximum Utilization of Atomically Dispersed Iron as Efficient Oxygen Electroreduction Nanoreactor.

As key parameters of electrocatalysts, the density and utilization of active sites determine the electrocatalytic performance toward oxygen reduction reaction. Unfortunately, prevalent oxygen electrocatalysts fail to maximize the utilization of active sites due to inappropriate nanostructural design. Herein, a nano-emulsion induced polymerization self-assembly strategy is employed to prepare hierarchical meso-/microporous N/S co-doped carbon nanocage with atomically dispersed FeN4 (denoted as Meso/Micro-FeNSC). In situ scanning electrochemical microscopy technology reveals the density of available active sites for Meso/Micro-FeNSC reach to 3.57 × 1014 sites cm-2 , representing more than threefold improvement compared to micropore-dominant Micro-FeNSC counterpart (1.07 × 1014 sites cm-2 ). Additionally, the turnover frequency of Meso/Micro-FeNSC is also improved to 0.69 from 0.50 e- site-1 s-1 for Micro-FeNSC. These properties motivate Meso/Micro-FeNSC as efficient oxygen electroreduction electrocatalyst, in terms of outstanding half-wave potential (0.91 V), remarkable kinetic mass specific activity (68.65 A g-1 ), and excellent robustness. The assembled Zn-air batteries with Meso/Micro-FeNSC deliver high peak power density (264.34 mW cm-2 ), large specific capacity (814.09 mA h g-1 ), and long cycle life (>200 h). This work sheds lights on quantifying active site density and the significance of maximum utilization of active sites for rational design of advanced catalysts.

High Energy and Power Zinc Ion Capacitors: A Dual-Ion Adsorption and Reversible Chemical Adsorption Coupling Mechanism.

Zinc ion capacitors (ZICs) hold great promise in large-scale energy storage by inheriting the superiorities of zinc ion batteries and supercapacitors. However, the mismatch of kinetics and capacity between a Zn anode and a capacitive-type cathode is still the Achilles' heel of this technology. Herein, porous carbons are fabricated by using tetra-alkali metal pyromellitic acid salts as precursors through a carbonization/self-activation procedure for enhancing zinc ion storage. The optimized rubidium-activated porous carbon (RbPC) is verified to hold immense surface area, suitable porosity structure, massive lattice defects, and luxuriant oxygen functional groups. These structural and compositional merits endow RbPC with the promoted zinc ion storage capability and more matchable kinetics and capacity with a Zn anode. Consequently, RbPC-based ZIC delivers a high specific energy of 178.2 W h kg-1 and a peak power density of 72.3 kW kg-1. A systematic ex situ characterization analysis coupled with in situ electrochemical quartz crystal microbalance tests reveal that the preeminent zinc ion storage properties are ascribed to the synergistic effect of the dual-ion adsorption and reversible chemical adsorption of RbPC. This work provides an efficient strategy to the rational design and construction of high-performance electrodes for ZICs and furthers the fundamental understanding of their charge storage mechanisms or extends the understanding toward other electrochemical energy storage devices.

Fe3O4-Encapsulating N-doped porous carbon materials as efficient oxygen reduction reaction electrocatalysts for Zn-air batteries.

Fe3O4 nanoparticle-encapsulating N-doped porous carbon was synthesized. Owing to the large specific surface area, hierarchical porous structure, and sufficient number of active sites from the graphitic carbon wrapped Fe3O4 NPs as well as the joint effect with Fe-Nx moieties, the as-prepared 2D-Fe3O4@FeNC-700 electrocatalyst exhibits exceptional performance in Zn-air batteries.