Valence Engineering Enhancing NH4+ Storage Capacity of Manganese Oxides

Ammonium ions (NH₄⁺), as non-metallic charge carriers, are attracting attention in aqueous batteries due to its low molar mass, element sufficiency, and non-toxicity. However, the host materials for NH₄⁺ storage are still limited. Herein, an oxygen defects-rich manganese oxide (MnO_2–x) for NH₄⁺ storage are reported. The oxygen defects can endow the MnO_2–x sample with improved electric conductivity and low interface activation energy. The electrochemical reaction mechanism is also verified by using ex situ X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FT-IR), demonstrating the insertion and extraction of NH₄⁺ in the MnO_2–x by formation/breaking of a hydrogen bond. As a result, MnO_2–x delivers a high capacity of 109.9 mAh g⁻¹ at the current density of 0.5 A g⁻¹ and retention of 24 mAh g⁻¹ after 1000 cycles at the current density of 4 A g⁻¹, outperforming the pristine MnO₂ sample.     

Self-Supported Pd Nanorod Arrays for High-Efficient Nitrate Electroreduction to Ammonia

Electrochemical nitrate (NO₃⁻) reduction to ammonia (NH₃) offers a promising pathway to recover NO₃⁻ pollutants from industrial wastewater that can balance the nitrogen cycle and sustainable green NH₃ production. However, the efficiency of electrocatalytic NO₃⁻ reduction to NH₃ synthesis remains low for most of electrocatalysts due to complex reaction processes and severe hydrogen precipitation reaction. Herein, high performance of nitrate reduction reaction (NO₃⁻RR) is demonstrated on self-supported Pd nanorod arrays in porous nickel framework foam (Pd/NF). It provides a lot of active sites for H* adsorption and NO₃⁻ activation leading to a remarkable NH₃ yield rate of 1.52 mmol cm⁻² h⁻¹ and a Faradaic efficiency of 78% at −1.4 V versus RHE. Notably, it maintains a high NH₃ yield rate over 50 cycles in 25 h showing good stability. Remarkably, large-area Pd/NF electrode (25 cm²) shows a NH₃ yield of 174.25 mg h⁻¹, be promising candidate for large-area device for industrial application. In situ FTIR spectroscopy and density functional theory calculations analysis confirm that the enrichment effect of Pd nanorods encourages the adsorption of H species for ammonia synthesis following a hydrogenation mechanism. This work brings a useful strategy for designing NO₃⁻RR catalysts of nanorod arrays with customizable compositions.     

Defective TiO2−x for High-Performance Electrocatalytic NO Reduction toward Ambient NH3 Production

Synthesis of green ammonia (NH₃) via electrolysis of nitric oxide (NO) is extraordinarily sustainable, but multielectron/proton-involved hydrogenation steps as well as low concentrations of NO can lead to poor activities and selectivities of electrocatalysts. Herein, it is reported that oxygen-defective TiO₂ nanoarray supported on Ti plate (TiO_2−x/TP) behaves as an efficient catalyst for NO reduction to NH₃. In 0.2 m phosphate-buffered electrolyte, such TiO_2−x/TP shows competitive electrocatalytic NH₃ synthesis activity with a maximum NH₃ yield of 1233.2 µg h⁻¹ cm⁻² and Faradaic efficiency of 92.5%. Density functional theory calculations further thermodynamically faster NO deoxygenation and protonation processes on TiO_2−x (101) compared to perfect TiO₂ (101). And the low energy barrier of 0.7 eV on TiO_2−x (101) for the potential-determining step further highlights the greatly improved intrinsic activity. In addition, a Zn-NO battery is fabricated with TiO_2−x/TP and Zn plate to obtain an NH₃ yield of 241.7 µg h⁻¹ cm⁻² while providing a peak power density of 0.84 mW cm⁻².     

Hierarchical Nanospheres with Polycrystalline Ir&Cu and Amorphous Cu2O toward Energy-Efficient Nitrate Electrolysis to Ammonia

Electrochemical reduction reaction of nitrate (NITRR) provides a sustainable route toward the green synthesis of ammonia. Nevertheless, it remains challenging to achieve high-performance electrocatalysts for NITRR especially at low overpotentials. In this work, hierarchical nanospheres consisting of polycrystalline Iridium&copper (Ir&Cu) and amorphous Cu₂O (Cu_xIr_yO_z NS) have been fabricated. The optimal species Cu_0.86Ir_0.14O_z delivers excellent catalytic performance with a desirable NH₃ yield rate (YR) up to 0.423 mmol h⁻¹ cm⁻² (or 4.8 mg h⁻¹ mg_cat⁻¹) and a high NH₃ Faradaic efficiency (FE) over 90% at a low overpotential of 0.69 V (or 0 V_RHE), where hydrogen evolution reaction (HER) is almost negligible. The electrolyzer toward NITRR and hydrazine oxidation (HzOR) is constructed for the first time with an electrode pair of Cu_0.86Ir_0.14O_z//Cu_0.86Ir_0.14O_z, yielding a high energy efficiency (EE) up to 87%. Density functional theory (DFT) calculations demonstrate that the dispersed Ir atom provides active site that not only promotes the NO₃⁻ adsorption but also modulates the H adsorption/desorption to facilitate the proton supply for the hydrogenation of *N, hence boosting the NITRR. This work thus points to the importance of both morphological/structural and compositional engineering for achieving the highly efficient catalysts toward NITRR.     

Accelerating Industrial-Level NO3− Electroreduction to Ammonia on Cu Grain Boundary Sites via Heteroatom Doping Strategy

Although the electrocatalytic nitrate reduction reaction (NO₃⁻RR) is an attractive NH₃ synthesis route, it suffers from low yield due to the lack of efficient catalysts. Here, this work reports a novel grain boundary (GB)-rich Sn-Cu catalyst, derived from in situ electroreduction of Sn-doped CuO nanoflower, for effectively electrochemical converting NO₃⁻ to NH₃. The optimized Sn_1%-Cu electrode achieves a high NH₃ yield rate of 1.98 mmol h⁻¹ cm⁻² with an industrial-level current density of −425 mA cm⁻² at −0.55 V versus a reversible hydrogen electrode (RHE) and a maximum Faradaic efficiency of 98.2% at −0.51 V versus RHE, outperforming the pure Cu electrode. In situ Raman and attenuated total reflection Fourier transform infrared spectroscopies reveal the reaction pathway of NO₃⁻RR to NH₃ by monitoring the adsorption property of reaction intermediates. Density functional theory calculations clarify that the high-density GB active sites and the competitive hydrogen evolution reaction (HER) suppression induced by Sn doping synergistically promote highly active and selective NH₃ synthesis from NO₃⁻RR. This work paves an avenue for efficient NH₃ synthesis over Cu catalyst by in situ reconstruction of GB sites with heteroatom doping.     

Coupling Engineering of NH4+ Pre-Intercalation and Rich Oxygen Vacancies in Tunnel WO3 Toward Fast and Stable Rocking Chair Zinc-Ion Battery

Herein, for the first time, a pre-intercalated non-metal ion (NH₄⁺) with rich oxygen vacancies stabilized tunnel WO₃ is proposed as a new intercalation anode to construct Zn-metal-free rocking-chair ZIBs. With the ethylene glycol additive in the aqueous electrolyte, the Zn²⁺ solvation structure can be regulated and the side reaction of hydrogen evolution can also be suppressed. Owing to the integrated synergetic modification, a high-rate and ultra-stable aqueous Zn-(NH₄)_xWO₃ battery can be constructed, which exhibits an improved specific capacity (153 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate performance (when the current density increases to 3 A g⁻¹, the specific capacitance is still 86 mAh g⁻¹), and a high cycle stability with 100% capacity retention after 2,200 cycles under 5 A g⁻¹. Ex situ X-ray diffraction and XPS reveal the reversible insertion/extraction of Zn²⁺ in (NH₄)_xWO₃. The assembled (NH₄)_xWO₃//MnO₂ rocking-chair ZIBs delivers excellent capacity of 82 mAh g⁻¹ at 0.1 A g⁻¹, impressive cyclic stability. Additionally, the flexible (NH₄)_xWO₃//MnO₂ ZIBs can power the electrochromic device-based PANI/WO₃ with high color contrast and fast response time. This study provides new insight for developing high-performance rechargeable aqueous ZIBs.     

Constructing Co@TiO2 Nanoarray Heterostructure with Schottky Contact for Selective Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical nitrate (NO₃⁻) reduction reaction (NO₃⁻RR) is a potential sustainable route for large-scale ambient ammonia (NH₃) synthesis and regulating the nitrogen cycle. However, as this reaction involves multi-electron transfer steps, it urgently needs efficient electrocatalysts on promoting NH₃ selectivity. Herein, a rational design of Co nanoparticles anchored on TiO₂ nanobelt array on titanium plate (Co@TiO₂/TP) is presented as a high-efficiency electrocatalyst for NO₃⁻RR. Density theory calculations demonstrate that the constructed Schottky heterostructures coupling metallic Co with semiconductor TiO₂ develop a built-in electric field, which can accelerate the rate determining step and facilitate NO₃⁻ adsorption, ensuring the selective conversion to NH₃. Expectantly, the Co@TiO₂/TP electrocatalyst attains an excellent Faradaic efficiency of 96.7% and a high NH₃ yield of 800.0 µmol h⁻¹ cm⁻² under neutral solution. More importantly, Co@TiO₂/TP heterostructure catalyst also presents a remarkable stability in 50-h electrolysis test.     

Light Field-Enhanced Single-Site Cu Electrocatalyst for Nitrogen Fixation

Direct electrocatalytic reduction of N₂ to NH₃ under mild conditions is attracting considerable interests but still remains enormous challenges in terms of respect of intrinsic catalytic activity and limited electrocatalytic efficiency. Herein, a photo-enhanced strategy is developed to improve the NRR activity on Cu single atoms catalysts. The atomically dispersed Cu single atoms supported TiO₂ nanosheets (Cu SAs/TiO₂) achieve a Faradaic Efficiency (12.88%) and NH₃ yield rate (6.26 µg h⁻¹ mg_cat⁻¹) at −0.05 V versus RHE under the light irradiation field, in which NH₃ yield rate is fivefold higher than that under pure electrocatalytic nitrogen reduction reaction (NRR) process and is remarkably superior in comparison to most of the similar type electrocatalysts. The existence of external light field improves electron transfer ability between Cu O and Ti O, and thus optimizes the accumulation of surface charges on Cu sites, endowing more electrons involved in nitrogen fixation. This work reveals an atomic-scale mechanistic understanding of field effect-enhanced electrochemical performance of catalysts and it provides predictive guidelines for the rational design of photo-enhanced electrochemical N₂ reduction catalysts.     

Constructing Ru@C3N4/Cu Tandem Electrocatalyst with Dual-Active Sites for Enhanced Nitrate Electroreduction to Ammonia

Electroreduction of nitrate to ammonia reaction (NO₃⁻RR) is considered as a promising carbon-free energy technique, which can eliminate nitrate from waste-water also produce value-added ammonia. However, it remains a challenge for achieving satisfied ammonia selectivity and Faraday efficiency (FE) due to the complex multiple-electron reduction process. Herein, a novel Tandem electrocatalyst that Ru dispersed on the porous graphitized C₃N₄ (g-C₃N₄) encapsulated with self-supported Cu nanowires (denoted as Ru@C₃N₄/Cu) for NO₃⁻RR is presented. As expected, a high ammonia yield of 0.249 mmol h⁻¹ cm⁻² at −0.9 V and high FE_NH3 of 91.3% at −0.8 V versus RHE can be obtained, while achieving excellent nitrate conversion (96.1%) and ammonia selectivity (91.4%) in neutral solution. In addition, density functional theory (DFT) calculations further demonstrate that the superior NO₃⁻RR performance is mainly resulted from the synergistic effect between the Ru and Cu dual-active sites, which can significantly enhance the adsorption of NO₃⁻ and facilitate hydrogenation, as well as suppress the hydrogen evolution reaction, thus lead to highly improved NO₃⁻RR performances. This novel design strategy would pave a feasible avenue for the development of advanced NO₃⁻RR electrocatalysts.     

Microenvironment Engineering to Promote Selective Ammonia Electrosynthesis from Nitrate over a PdCu Hollow Catalyst

The electrosynthesis of recyclable ammonia (NH₃) from nitrate under ambient conditions is of great importance but still full of challenges for practical application. Herein, an efficient catalyst design strategy is developed that can engineer the surface microenvironment of a PdCu hollow (PdCu-H) catalyst to confine the intermediates and thus promote selective NH₃ electrosynthesis from nitrate. The hollow nanoparticles are synthesized by in situ reduction and nucleation of PdCu nanocrystals along a self-assembled micelle of a well-designed surfactant. The PdCu-H catalyst shows a structure-dependent selectivity toward the NH₃ product during the nitrate reduction reaction (NO₃⁻RR) electrocatalysis, enabling a high NH₃ Faradaic efficiency of 87.3% and a remarkable NH₃ yield rate of 0.551 mmol h⁻¹ mg⁻¹ at -0.30 V (vs reversible hydrogen electrode). Moreover, this PdCu-H catalyst delivers high electrochemical performance in the rechargeable zinc-NO₃⁻ battery. These results provide a promising design strategy to tune catalytic selectivity for efficient electrosynthesis of renewable NH₃ and feedstocks.     

High-Performance Bifunctional Porous Iron-Rich Phosphide/Nickel Nitride Heterostructures for Alkaline Seawater Splitting

Seawater is the most abundant natural water resource in the world, which is an inexhaustible and low-cost feedstock for hydrogen production by alkaline water electrolysis. It is appearling to develop robust and stable electrocatalysts for alkaline seawater electrolysis. However, the development of seawater electrolysis is seriously impeded by anodic chloride corrosion and chlorine evolution reaction, and few non-noble electrocatalysts show prominent catalytic performance and excellent durability. Here, a heterogeneous electrocatalyst constructed by in situ growing highly dispersed iron-rich bimetallic phosphide nanoparticles on metallic Ni₃N (Fe_2−2xCo_2xP/Ni₃N), which exhibits outstanding bifunctional catalytic activities for alkaline seawater splitting, is reported. The optimal (Fe_0.74Co_0.26)₂P/Ni₃N and Fe₂P/Ni₃N electrocatalysts demand only 113 and 212 mV to afford 100 mA cm⁻² for hydrogen and oxygen evolution reactions (HER and OER) in 1 m KOH, respectively, thus substantially expediting overall water/seawater electrolysis at 100 mA cm⁻² with 1.592/1.645 V. Particularly, Fe₂P/Ni₃N displays an unprecedented overpotential of 302 mV at 500 mA cm⁻², which represents the best alkaline seawater oxygen evolution activity among the ever-reported non-noble electrocatalysts; and thus substantially expedites overall water/seawater splitting at 500 mA cm⁻² with 1.701/1.768 V, surpassing most of the reported non-noble lectrocatalysts. This work provides a new approach for developing high-performance electrocatalysts for seawater splitting.     

Porous and Partially Dehydrogenated Fe2+-Containing Iron Oxyhydroxide Nanosheets for Efficient Electrochemical Nitrogen Reduction Reaction (ENRR)

The design and development of efficient catalysts for electrochemical nitrogen reduction reaction (ENRR) under ambient conditions are critical for the alternative ammonia (NH₃) synthesis from N₂ and H₂O, wherein iron-based electrocatalysts exhibit outstanding NH₃ formation rate and Faradaic efficiency (FE). Here, the synthesis of porous and positively charged iron oxyhydroxide nanosheets by using layered ferrous hydroxide as a starting precursor, which undergoes topochemical oxidation, partial dehydrogenated reaction, and final delamination, is reported. As the electrocatalyst of ENRR, the obtained nanosheets with a monolayer thickness and 10-nm mesopores display exceptional NH₃ yield rate (28.5 µg h⁻¹ mg_cat.⁻¹) and FE (13.2%) at a potential of −0.4 V versus RHE in a phosphate buffered saline (PBS) electrolyte. The values are much higher than those of the undelaminated bulk iron oxyhydroxide. The larger specific surface area and positive charge of the nanosheets are beneficial for providing more exposed reactive sites as well as retarding hydrogen evolution reaction. This study highlights the rational control on the electronic structure and morphology of porous iron oxyhydroxide nanosheets, expanding the scope of developing non-precious iron-based highly efficient ENRR electrocatalysts.     

A Robust n-n Heterojunction: Cu?N and B?N Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with Cu N and B N dual active sites is synthesized via in situ growth of a conductive metal–organic framework (MOF) [Cu₃(HITP)₂] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu₃(HITP)₂@h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu₃(HITP)₂@h-BN shows the outstanding eNRR performance with the NH₃ production of 146.2 µg h⁻¹ mg_cat⁻¹ and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and Cu N/B N dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH₃ production catalyzed by the Cu₃(HITP)₂@h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.     

Synergistic Bimetallic CoCu-Codecorated Carbon Nanosheet Arrays as Integrated Bifunctional Cathodes for High-Performance Rechargeable/Flexible Zinc-Air Batteries

The unremitting exploration of well-architectured and high-efficiency oxygen electrocatalysts is promising to speed up the surface-mediated oxygen reduction/evolution reaction (ORR/OER) kinetics of rechargeable zinc–air batteries (ZABs). Herein, bimetallic CoCu-codecorated carbon nanosheet arrays (CoCu/N-CNS) are proposed as self-supported bifunctional oxygen catalysts. The integrated catalysts are in situ constructed via a simple sacrificial-templated strategy, imparting CoCu/N-CNS with 3D interconnected conductive pathways, abundant mesopores for electrolyte penetration and ion diffusion, as well as Cu-synergized Co – N_x/O reactive sites for improved catalytic activities. By incorporating a moderate amount of Cu into CoCu/N-CNS, the bifunctional activities can be further increased due to synergistic oxygen electrocatalysis. Consequently, the optimized CoCu/N-CNS realizes a low overall overpotential of 0.64 V for OER and ORR and leads to high-performance liquid ZABs with high gravimetric energy (879.7 Wh kg⁻¹), high peak power density (104.3 mW cm⁻²), and remarkable cyclic stability upon 400 h/1000 cycles at 10 mA cm⁻². More impressively, all-solid-state flexible ZABs assembled with the CoCu/N-CNS cathode exhibit superior rate performance and exceptional mechanical flexibility under arbitrary bending conditions. This CoCu/N-CNS monolith holds significant potential in advancing cation-modulated multimetallic electrocatalysts and multifunctional nanocatalysts.     

N-Coordinated Cu–Ni Dual-Single-Atom Catalyst for Highly Selective Electrocatalytic Reduction of Nitrate to Ammonia

As a traditional method of ammonia (NH₃) synthesis, Haber–Bosch method expends a vast amount of energy. An alternative route for NH₃ synthesis is proposed from nitrate (NO₃⁻) via electrocatalysis. However, the structure–activity relationship remains challenging and requires in-depth research both experimentally and theoretically. Here an N-coordinated Cu–Ni dual-single-atom catalyst anchored in N-doped carbon (Cu/Ni–NC) is reported, which has competitive activity with a maximal NH₃ Faradaic efficiency of 97.28%. Detailed characterizations demonstrate that the high activity of Cu/Ni–NC mainly comes from the contribution of Cu–Ni dual active sites. That is, (1) the electron transfer (Ni → Cu) reveals the strong electron interaction of Cu–Ni dual-single-atom; (2) the strong hybridizations of Cu 3d—and Ni 3d—O 2p orbitals of NO₃⁻ can accelerate electron transfer from Cu–Ni dual-site to NO₃⁻; (3) Cu/Ni–NC can effectively decrease the rate-limiting step barriers, suppress N–N coupling for N₂O and N₂ formation and hydrogen production.     

Biomimetic Photocatalytic System Designed by Spatially Separated Cocatalysts on Z-scheme Heterojunction with Identified Charge-transfer Processes for Boosting Removal of U(VI)

Designing highly efficient photocatalysts with rapid migration of photogenerated charges and surface reaction kinetics for the photocatalytic removal of uranium (U(VI)) from uranium mine wastewater remains a significant challenge. Inspired by natural photosynthesis, a biomimetic photocatalytic system is assembled by designing a novel hollow nanosphere MnO_x@TiO₂@CdS@Au (MTCA) with loading MnO_x and Au nano particles (Au NPs) cocatalysts on the inner and outer surfaces of the TiO₂@CdS. The spatially separated cocatalysts efficiently drive the photogenerated charges to migrate in opposite directions, while the Z-scheme heterogeneous shell further separates the interfacial charges. Theoretical calculation identifies multiple consecutive forward charge transfers without charge recombination within MTCA. Thus, MTCA could efficiently remove 99.61% of U(VI) after 15 min of simulated sunlight irradiation within 3 mmol L⁻¹ NaHCO₃ with 0.231 min⁻¹ of the reduction rate constant, outperforming most previously reported photocatalysts. MTCA further significantly removes 91.83% of U(VI) from the natural uranium mining wastewater under sunlight irradiation. This study provides a novel approach to designing an ideal biomimetic photocatalyst for remediating environmental pollution.     

Super-Hydrophilic Microporous Ni(OH)x/Ni3S2 Heterostructure Electrocatalyst for Large-Current-Density Hydrogen Evolution

Exploiting active and stable non-precious metal electrocatalysts for alkaline hydrogen evolution reaction (HER) at large current density plays a key role in realizing large-scale industrial hydrogen generation. Herein, a self-supported microporous Ni(OH)x/Ni₃S₂ heterostructure electrocatalyst on nickel foam (Ni(OH)x/Ni₃S₂/NF) that possesses super-hydrophilic property through an electrochemical process is rationally designed and fabricated. Benefiting from the super-hydrophilic property, microporous feature, and self-supported structure, the electrocatalyst exhibits an exceptional HER performance at large current density in 1.0 M KOH, only requiring low overpotential of 126, 193, and 238 mV to reach a current density of 100, 500, and 1000 mA cm⁻², respectively, and displaying a long-term durability up to 1000 h, which is among the state-of-the-art non-precious metal electrocatalysts. Combining hard X-rays absorption spectroscopy and first-principles calculation, it also reveals that the strong electronic coupling at the interface of the heterostructure facilitates the dissociation of H₂O molecular, accelerating the HER kinetics in alkaline electrolyte. This work sheds a light on developing advanced non-precious metal electrocatalysts for industrial hydrogen production by means of constructing a super-hydrophilic microporous heterostructure.     

Advanced In Situ Electrochemical Induced Dual-Mechanism Heterointerface toward High-Energy Aqueous Zinc-Ion Batteries

In situ electrochemical activation brings unexpected electrochemical performance improvements to electrode materials, while the mechanism behind is still needed to study deeply. Herein, an in situ electrochemically approach is developed for the activation of heterointerface MnO_x/Co₃O₄ by inducing Mn-defect, wherein the Mn defects are formed through a charge process that converts the MnO_x with poor electrochemical activities toward Zn²⁺ into high electrochemically active cathode for aqueous zinc-ion batteries (ZIBs). Guided by the coupling engineering strategy, the heterointerface cathode exhibits an intercalation/conversion dual-mechanism without structural collapse during storage/release of Zn²⁺. The heterointerfaces between different phases can generate built-in electric fields, reducing the energy barrier for ion migration and facilitating electron/ion diffusion. As a consequence, the dual-mechanism MnO_x/Co₃O₄ shows an outstanding fast charging performance and maintains a capacity of 401.03 mAh g⁻¹ at 0.1 A g⁻¹. More importantly, a ZIB based on MnO_x/Co₃O₄ delivered an energy density of 166.09 Wh kg⁻¹ at an ultrahigh power density of 694.64 W kg⁻¹, which outperforms those of fast charging supercapacitors. This work provides insights for using defect chemistry to introduce novel properties in active materials for highly for high-performance aqueous ZIBs.     

Spontaneous Internal Electric Field in Heterojunction Boosts Bifunctional Oxygen Electrocatalysts for Zinc–Air Batteries: Theory,Experiment, and Application

Heterojunctions are a promising class of materials for high-efficiency bifunctional oxygen electrocatalysts in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the conventional theories fail to explain why many catalysts behave differently in ORR and OER, despite a reversible path (^*O₂⇋^*OOH⇋^*O⇋^*OH). This study proposes the electron-/hole-rich catalytic center theory (e/h-CCT) to supplement the existing theories, it suggests that the Fermi level of catalysts determines the direction of electron transfer, which affects the direction of the oxidation/reduction reaction, and the density of states (DOS) near the Fermi level determines the accessibility for injecting electrons and holes. Additionally, heterojunctions with different Fermi levels form electron-/hole-rich catalytic centers near the Fermi levels to promote ORR/OER, respectively. To verify the universality of the e/h-CCT theory, this study reveals the randomly synthesized heterostructural Fe₃N-FeN_0.0324 (Fe_xN@PC with DFT calculations and electrochemical tests. The results show that the heterostructural F₃N-FeN_0.0324 facilitates the catalytic activities for ORR and OER simultaneously by forming an internal electron-/hole-rich interface. The rechargeable ZABs with Fe_xN@PC cathode display a high open circuit potential of 1.504 V, high power density of 223.67 mW cm⁻², high specific capacity of 766.20 mAh g⁻¹ at 5 mA cm⁻², and excellent stability for over 300 h.