A bimetallic-activated MnO₂ self-assembly electrode with a dual heterojunction structure for high-performance rechargeable zinc-air batteries

INTRODUCTION

Zinc-air batteries (ZABs) have attracted significant interest due to their strong competitiveness in terms of cost, safety, and energy density^[1-6]. However, the sluggish dynamics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurring at the cathode severely limit the charge-discharge performance and efficiency of ZABs. Although substantial work has been carried out on the synthesis of noble metal powder catalysts (e.g., Pt/C and IrO₂) for the ORR and OER, the scarcity and unsatisfactory bifunctionality of noble metals severely reduce the possibility of their large-scale applications^[1,7]. Thus, it is particularly necessary to develop low-cost and highly efficient non-precious metal bifunctional catalysts for the ORR and OER.

α-MnO₂-based catalysts remain the most promising non-noble oxygen catalytic reaction candidates in alkaline electrolytes, owing to their unique layered and 2 × 2 tunnel structures^[3,8,9]. Generally, the basic structural lattice framework of α-MnO₂ is a corner-/edge-sharing [MnO₆] octahedral unit and a Mn oxidation state of 4^+[10]. In O₂ catalytic reactions, Mn³⁺, cyclically produced by the Mn⁴⁺reduction, serves as the active site to form the O₂ intermediate by combining with O₂, which is the rate-determining step^[11,12]. However, the formed Mn³⁺ still occupies the octahedral sites of the [MnO₆] units and exists with unpaired single electrons at the high spin electronic energy e_g band (Mn³⁺, t_2g3, e_g1), which shows extremely low thermodynamic stability^[12]. Accordingly, obtaining abundant and stable Mn³⁺ is of great significance for designing high-performance Mn-based bifunctional catalysts. In recent decades, several valuable strategies have been proposed to further enhance the performance of α-MnO₂-based catalysts, such as coupling other transition metal oxides to construct unique heterojunction structures^[13-15]. For example, a Co₃O₄-MnO₂/carbon nanotube (CNT) hybrid exhibited a significant increase in Mn³⁺ density and a concomitant decrease in electron energy due to the bonding of the bottom Co atoms to the surface O on the MnO₂ surface^[13]. As a result, the Co₃O₄-MnO₂/CNT hybrid showed good ORR and OER performance. However, there are still two major issues for Mn-based catalysts manipulated by the unary spinel: (1) the electronic configuration of the Co-Mn heterojunction needs to be further modulated^[16]; and (2) the inherent limitations of powder catalysts still need to be overcome (e.g., significant dead volume, an undesirable three-phase reaction interface and uncontrolled microstructures).

Some studies reported that partially replacing the Co element of Co₃O₄ in Co₃O₄-MnO₂/CNT hybrids with nickel can reduce the particle size and increase the Co-Mn heterojunction density, which contributes to the activity and the long-term durability of the catalyst^[14,17,18]. In particular, the interaction force between the low-spin Co³⁺ in the octahedral site and the oxygen intermediate can be weakened by the partial substitution of Ni atoms^[19,20]. Thus, the bond length of Co-Mn-O can be further modulated by coupling the NiCo₂O₄ binary spinel instead of Co₃O₄^[21]. In contrast, achieving the self-support of active species on conductive substrates can effectively avoid the limitations of powder catalysts^[22,23]. On this basis, it has become more attractive to synthesize high-performance NiCo₂O₄/MnO₂ self-supported bifunctional electrodes. The advantages of such a new self-supported bifunctional electrode include: (1) the construction of a high electron transfer pathway via in situ growth of the conductive substrates with electroactive species. The in situ self-fabrication procedure assures good conductivity and high mechanical stability of the electrode, which does not require the post-costing process and additional organic binders^[24]; (2) a possibly rapid intrinsic kinetics and long-term durability due to the stable double heterojunction structure (NiCoO_x-MnO₂ and MnO₂ substrate) and the activation of Mn sites^[14]; and (3) rapid mass transfer (e.g., the ion diffusion and the emission of the reaction products) at the three-phase reaction interface, which should be attributed to the formation of the open space within a three-dimensional (3D) nanostructured array network^[25]. Notably, a bimetallic-activated MnO₂ self-supported electrode has not yet been explored in ZABs.

Hence, inspired by the aforementioned considerations, we design a useful strategy to construct a bimetallic-activated MnO₂-based electrode by a self-assembly process. During this process, NiCo₂O₄-bound CNTs (a nanosheet) along the self-assembled MnO₂ arrays on the surface of the nickel foam form a unique tremella-like 3D hierarchical structure with an abundant heterojunction (NiCo₂O₄@MnO₂/CNTs-Ni foam). The unique 3D hierarchical structure effectively promotes electrolyte penetration, reactant (O₂ and OH^-) diffusion, and product emission, consequently facilitating the ORR and OER kinetics. In particular, the intrinsic activity of oxygen catalysis is further enhanced by the strategy of introducing nickel foam to form abundant Mn³⁺ active sites and utilizing bimetallic activation of the active sites to reduce the surface activity of MnO₂ for the O, OH, and OOH species. As expected, the as-prepared NiCo₂O₄@MnO₂/CNTs-Ni foam self-supporting electrode exhibits good OER and ORR performance with an ultralow total overpotential of 0.69 V. The NiCo₂O₄@MnO₂/CNTs-Ni foam-based ZABs show a high peak power density of 226 mW cm^-2 at 0.74 V, which is superior to Pt/C-RuO₂ (93 mW cm^-2 at 0.47 V). Additionally, the NiCo₂O₄@MnO₂/CNTs-Ni foam electrode also displays long-term charge-discharge cycling stability, which should be attributed to the stable 3D hierarchical hollow hybrid structure and the active sites. This study provides a simple strategy to construct 3D hierarchical hollow hybrids and affords implications for the interface engineering field of high-performance catalysts.

RESULTS AND DISCUSSION

As shown in Figure 1A, the NiCo₂O₄@MnO₂/CNTs-Ni foam electrode was synthesized via a facile hydrothermal coupled calcination approach. The SEM image in Figure 1B illustrates that the NiCo₂O₄@MnO₂/CNTs-Ni foam synthesized at 160 °C for 6 h has a tremella-like 3D hierarchical structure with a pore size of ~0.4 μm. This hollow porous structure is believed to not only be beneficial for buffering the volume change induced by charge-discharge cycling but also to accelerate the penetration of the electrolyte, the diffusion of OH to the electrode surface, and the rapid dissociation of the reaction products. However, when the reaction time is less than 6 h, it is difficult to form this tremella-like 3D hierarchical structure, and when the reaction time is higher than 6 h, the structure is destroyed [Supplementary Figure 1]. This is mainly due to the thermodynamic conditions during the synthesis. When the reaction time is too short, its thermodynamic environment cannot perform the process of MnO₂ self-assembly, and when the reaction time is too long, the excess energy will dissolve to form nanosheets.

Figure 1. (A) Schematic of NiCo₂O₄@MnO₂/CNTs-Ni foam synthesis. (B) Scanning electron microscope (SEM) image of NiCo₂O₄@MnO₂/CNTs-Ni foam. (C) Transmission electron microscope (TEM) image of NiCo₂O₄@MnO₂/CNTs-Ni foam. (D) and (E) High-resolution transmission electron microscopy (HRTEM) images of NiCo₂O₄@MnO₂/CNTs-Ni foam. (F) Selected area electron diffraction (SAED) image of NiCo₂O₄@MnO₂/CNTs-Ni foam. (G) Elemental (Co, Ni, Mn and O) mapping of the area within the red dotted box in Figure 1C.

The structural information for the catalysts was obtained by TEM. As shown in Figure 1C, the NiCo₂O₄/CNT nanolayered structure can be well synthesized using the second hydrothermal self-assembly process at 160 °C for 6 h. Considerable NiCo₂O₄/CNT nanolayered structures with an average thickness of 30 nm wrap around the MnO₂ nanotubes. MnO₂ nanotubes were introduced into the NiCo₂O₄/CNT hybrid catalytic system and served as its skeleton. The distribution of the individual phases was studied by HRTEM analysis. The lattice spacings in Figure 1D and E are 0.69, 0.22, and 0.24 nm, corresponding to the (110) plane of a-MnO₂ and the (222) and (311) planes of NiCo₂O₄, respectively. The SAED pattern [Figure 1F] results are consistent with those of the XRD analysis [Supplementary Figure 2], indicating a nickel-cobalt bimetallic composite of oxides and α-MnO₂. The XRD characteristic peaks in Supplementary Figure 2 are contributed by α-MnO₂ and NiCo₂O₄. This result proves that the Ni/Co bimetallic oxide synthesized by this method is NiCo₂O₄. Co, Ni, O, and Mn are uniformly distributed, as confirmed by the TEM elemental mappings shown in Figure 1G.

In order to gain a deeper understanding of the self-assembly process of the NiCo₂O₄@MnO₂/CNTs-Ni foam, the SEM images of NiCo₂O₄, the Co₃O₄@MnO₂-CNT@Ni foam, Co₃O₄/MnO₂-CNTs, and MnO₂ were obtained. As shown in Supplementary Figure 3, the length of the pure MnO₂ tubes approaches 500 nm with a width of 50 to 100 nm. For the NiCo₂O₄ composites prepared at the same synthesis conditions without MnO₂ and Ni foam, the diameter of the NiCo₂O₄ spheres is ~500 nm and they are huddled together tightly. When doped only with Co ions (Co₃O₄/MnO₂-CNTs) without Ni foam, the Co₃O₄ particles are sparsely arranged on the surface of MnO₂. In contrast, for Co and Ni ions (NiCo₂O₄/MnO₂-CNTs) without Ni foam, the NiCo₂O₄ nanoparticles wrap on the surface of the MnO₂ nanotubes densely and uniformly. The size of the NiCo₂O₄ particles (25 nm) on the MnO₂ nanotubes is 25 times smaller than that of pure NiCo₂O₄ (500 nm). Smaller particle sizes tend to provide better structural stability and larger surface areas, thereby further facilitating the electrode activity^[26]. Although the catalyst has been improved with regards to size and dispersibility, deficiency of the active sites still exists caused by random gatherings of the nanoparticles. When the Ni foam is introduced to the catalyst synthesis process, these nanoparticles self-assemble with CNTs along the MnO₂ nanotubes to build a unique 3D network structure. In addition, the Co₃O₄@MnO₂/CNTs-Ni foam [Supplementary Figure 3] also shows a similar tremella-like 3D hierarchical hollow hybrid structure as the nickel-cobalt bimetallic oxide composite.

The above findings reveal the importance of the Ni substrate, which can significantly reduce the size of the metal oxides by forming a steric hindrance effect during the self-assembling process, as well as avoiding the agglomeration of metal oxides and forming an abundant metal oxide-MnO₂ heterojunction structure^[27,28]. Ni foam can also provide growth sites for the self-assembly of MnO₂, avoid the agglomeration of MnO₂ and form the Ni foam-MnO₂ heterojunction structure. Furthermore, it can induce the evolution of the nanoparticles on the surface of MnO₂ into two-dimensional nanosheets, thereby realizing the rapid transformation of the metal oxide-MnO₂ heterojunction in structure and density. This dense heterojunction structure forms an abundant three-phase reaction interface.

The XPS survey scan of the NiCo₂O₄@MnO₂/CNTs-Ni foam confirms the presence of Mn, Co, Ni, C, and O elements [Figure 2A]. As shown in Figure 2B, for the spectrum of Mn 2p, the characteristic peaks of 640-650 eV and 650-660 eV belong to Mn 2p3/2 and Mn 2p1/2, respectively. The Mn 2p3/2 spectrum is deconvoluted into three peaks. The peaks located at 645.2, 641.4, and 638.2 eV are attributed to Mn⁴⁺, Mn³⁺, and Mn²⁺, respectively. Mn³⁺ is widely regarded as the active center for the oxygen-catalyzed reaction. Notably, the proportion of Mn³⁺ in the NiCo₂O₄@MnO₂/CNTs-Ni foam is 55.8%, which is higher than for pure MnO₂ (0%, Supplementary Figure 4A), the NiCo₂O₄@MnO₂/CNTs-Ni foam catalyst (0%, Supplementary Figure 4B) and the Co₃O₄@MnO₂-CNT@Ni foam (36.4%, Supplementary Figure 4C). Therefore, for the Ni-based self-supporting electrode, the introduction of Ni foam is beneficial to the existence of Mn ions in the form of Mn³⁺, which will effectively increase the active site density of the catalytic system.

Figure 2. (A) Overall XPS spectrum of NiCo₂O₄@MnO₂/CNTs-Ni foam. High-resolution curves of (B) Mn 2p, (C) Co 2p, (D) Ni 2p, (E) C 1 s and (F) O 1 s regions.

In addition, compared with the normal Mn 2p peaks (641.8 eV for Mn 2p3/2 and 653.5 eV for Mn 2p1/2), the main Mn 2p peaks (641.4 eV and 652.9 eV) in the NiCo₂O₄@MnO₂/CNTs-Ni foam shift to a lower binding energy. The binding energy shift should be caused by the transfer of electrons from NiCo₂O₄ to the MnO₂ surface and the heterojunction formation of NiCo₂O₄-MnO₂. The electrons obtained from NiCo₂O₄ activate the active site on the surface of MnO₂ by moving the Mn electron cloud to a lower energy, thereby further enhancing the ORR and OER activities^[29,30]. Specifically, in comparison to the Co₃O₄@MnO₂-CNT@Ni foam, the NiCo₂O₄@MnO₂/CNTs-Ni foam modified by Ni ions showed a stronger active center activation effect on MnO₂ by changing the strength of the Ni-Co-O-Mn bond^[14]. The Ni-Co-O-Mn bond can be significantly controlled by substituting Co²⁺ at tetrahedral sites with Ni ions^[18]. Overall, the change in the Mn state may be due to the abundant Ni ions provided by the Ni foam and nitrate (especially Ni foam).

Figure 2C shows the Co 2p spectrum, with two peaks observed for Co 2p1/2 and Co 2p3/2 at ~790-800 eV and ~775-785 eV, respectively^[31]. For the unfolded spectra, the peaks located at 798.9 and 795.8 eV are considered as Co³⁺ and Co²⁺, respectively, with the remaining two peaks at 783.5 and 780.1 eV also corresponding to Co³⁺ and Co²⁺. In addition, the Ni 2p spectra can be deconvoluted into 2 paired peaks of Ni²⁺ (874.0 and 855.9 eV)/Ni³⁺ (872.1 and 854.0 eV) with an energy separation of 18.1 eV, as well as two shakeup satellites (879.3 and 861.2 eV) [Figure 2D]^[32]. In addition, the intense peak of the C 1s spectrum [Figure 2E] at 284.6 eV is assigned to the sp² C-C bond^[33,34]. The other two weak peaks at 286.1 and 288.4 eV correspond to the C-O-C bonds and O-C=O functionalities, respectively^[35]. The O 1s spectra of the NiCo₂O₄/MnO₂-CNT@Ni foam can be deconvoluted into three main peaks, which correspond to surface lattice oxygen (M-O (M = NiCo, Mn) bond at 529.2 eV), surface hydroxyls and adsorbed H₂O, respectively [Figure 2F]^[36]. Similar to Mn 2p, the ratios of hydroxyl oxygen in the NiCo₂O₄@MnO₂/CNTs-Ni and Co₃O₄@MnO₂/CNTs-Ni foams are significantly increased after the introduction of nickel foam, which is higher than for MnO₂ and NiCo₂O₄/MnO₂-CNTs [Supplementary Figure 4D-F]. Notably, owing to the H₂O decomposition ability of the surface metal species, numerous surface hydroxyl groups can be formed on the surface of the catalyst, which is essential for increasing the active sites and promoting the OER performance.

For the ORR and OER, the bimetallic activation effect caused by the construction of the abundant heterojunction of the self-supported electrode significantly decreases the hydrogen bonding with OH and OOH. In addition, the rich Mn³⁺ ions as the active sites further accelerate its intrinsic dynamics. In particular, the tremella-like 3D hierarchical structure builds a rich three-phase reaction interface, which improves the mass transfer. Finally, the coupling of CNTs and Ni foam form a unique "meridian" conductive structure that enables rapid electron transfer.

The activity and stability of the oxygen electrocatalyst were tested via cyclic voltammetry and linear sweep voltammetry (LSV) in O₂ saturated 0.1 M KOH. As expected, the LSV curves of the OER indicate that the NiCo₂O₄@MnO₂/CNTs-Ni foam exhibits a markedly decreased overpotential (η, η = E_{j = 10} -1.23 V, E_{j = 10}: the OER potential at 10 mA cm^-2) for the OER of 270 mV in comparison to the Co₃O₄@MnO₂/CNTs-Ni foam (480 mV), NiCo₂O₄/MnO₂-CNTs (510 mV) and NiCo₂O₄ (590 mV) [Figure 3A]. This overpotential is also 300 mV lower than that of commercial Pt/C-RuO₂ catalysts [Supplementary Figure 5A]. This finding demonstrates that the self-supporting electrodes exhibit better OER activity than the powder catalyst, which can be attributed to the 3D hierarchical hollow hybrid structure created by the Ni foam base and the binder-free advantage for the self-supporting electrode^[37]. Furthermore, the bimetallic-activated MnO₂-based sample also shows a higher OER activity than the single metal-activated MnO₂-based catalyst and pure MnO₂, indicating that a metal oxide anchored on the MnO₂ surface is an effective method to improve the catalytic activity.

Figure 3. (A) OER curves, (B) Tafel slopes, (C) ORR curves, (D) RDE curves at 400-1600 rpm (insert: K-L plots), € EIS curves, and (F) potential gap (ΔE) of ORR and OER for Pt/C-RuO₂, NiCo₂O₄-CNTs, NiCo₂O₄/MnO₂-CNTs, Co₃O₄@MnO₂/CNTs-Ni foam, and NiCo₂O₄@MnO₂/CNTs-Ni foam. (G) Schematic reaction mechanism of ORR and OER electrocatalyzed by NiCo₂O₄@MnO₂/CNTs-Ni foam.

The Tafel slope was also calculated to reveal the reaction kinetics for the OER. As shown in Figure 3B, the small slope (105 mV dec^-1) further indicates the rapid OER kinetics of the NiCo₂O₄@MnO₂/CNTs-Ni foam, which is faster than the Co₃O₄@MnO₂/CNTs-Ni foam (235 mV dec^-1), NiCo₂O₄/MnO₂-CNTs (158 mV dec^-1), NiCo₂O₄ (147 mV dec^-1) and MnO₂ ((215 mV dec^-1). In addition to the good OER activity, the NiCo₂O₄@MnO₂/CNTs-Ni foam also shows promising ORR performance [Figure 3C]. Notably, the NiCo₂O₄@MnO₂/CNTs-Ni foam shows a larger limiting current density of 8 mA cm^-2 than NiCo₂O₄/MnO₂-CNTs (4.4 mA cm^-2), NiCo₂O₄ (4.8 mA cm^-2), MnO₂ (3.1 mA cm^-2) and Pt/C-RuO₂ (6 mA cm^-2) [Supplementary Figure 5B], which further illustrates that the novel hierarchical structure constructed by the self-assembly process of the metal oxide facilitates not only the gas diffusion but also the mass transfer of the ORR^[34]. In addition, the onset potential (E_onset) and half-wave potential (E_1/2) of NiCo₂O₄/MnO₂-CNTs are 0.87 and 0.81 V, respectively, which are better than for MnO₂ (0.82 and 0.67 V) and NiCo₂O₄ (0.86 and 0.77 V). The high ORR activity of the NiCo₂O₄@MnO₂/CNTs-Ni foam demonstrates that the introduction of bimetallic ions and Ni foam can effectively regulates the electronic structure of the Mn ions and increase the density of the active site (Mn³⁺) in order to form a suitable chemical environment for the oxygen catalytic reaction^[10-12,38]. The large limiting current density should be attributed to the 3D hierarchical hollow hybrid structure, which boosts the mass transfer during the ORR.

The electron transfer number of the NiCo₂O₄@MnO₂/CNTs-Ni foam was then calculated using Koutecky-Levich (K-L) plots. The LSV measurement of the sample powder was also carried out at 400-2025 rpm [Figure 3D], and the electron transfer number of NiCo₂O₄/MnO₂-CNTs is in the range of 3.92-3.99, indicating a four-electron dominant transfer ORR pathway (O₂ directly forms OH^-). In addition, the semicircle of the Nyquist plot at low frequency corresponds to the Rct, i.e., the charge transfer resistance^[39]. The Rct of the NiCo₂O₄@MnO₂/CNTs-Ni foam is closer to the Co₃O₄/MnO₂-CNTs@NF and lower than NiCo₂O₄/MnO₂-CNTs, NiCo₂O₄, and MnO₂, suggesting that the tremella-like 3D hierarchical structure and dual heterojunction structure of the NiCo₂O₄@MnO₂/CNTs-Ni foam are beneficial to the rapid electron transfer, ion diffusion and product emission [Figure 3E]. Accordingly, the high ORR and OER activities should be due to the unique 3D hierarchical structure effectively promoting electrolyte penetration, O₂/OH^- diffusion, and product emission. Furthermore, the abundant Mn³⁺ active sites are activated by bimetallic ions, which shorten the oxygen catalytic reactions distance between the active sites and reactant and reduce the surface activity of MnO₂ for the O, OH, and OOH species^[13-14,26]. Finally, the CNTs combined with Ni foam form a novel “meridian” conductive structure that enables rapid electron conduction. As a result, the NiCo₂O₄@MnO₂/CNTs-Ni foam electrode exhibits the lowest potential gap of ORR and OER (ΔE = E_{(j = 10)}-E_1/2, 0.69 V) among all samples [Figure 3F]. In addition, the ΔE of the NiCo₂O₄@MnO₂/CNTs-Ni foam is also smaller than the recently reported work^[7,40-46], revealing its high bifunctional oxygen catalytic activity. For better interpreting the influence of the heterojunction structure, ion activation, and Ni substrate of the self-supported electrode, the electrocatalytic mechanism of the ORR and OER occurring in the NiCo₂O₄/MnO₂-CNTs@Ni foam electrode is illustrated in Figure 3G.

To examine the practical applications of the NiCo₂O₄@MnO₂/CNTs-Ni foam electrode, ZABs were assembled with a zinc plate as the anode and the NiCo₂O₄@MnO₂/CNTs-Ni foam self-supporting electrode as the air cathode. A ZAB with Pt/C-RuO₂ was tested as a contrast. The open-circuit voltage (OCV) of the NiCo₂O₄@MnO₂/CNTs-Ni foam battery was 1.51 V, which is superior to the Pt/C-RuO₂ counterpart (1.48 V) [Figure 4A]. The power density curves and the charge/discharge polarization curves [Figure 4B] indicate that NiCo₂O₄/MnO₂-CNTs@NF owned higher peak power density (0.74 V in 303 mA cm^-2, 226 mW cm^-2) than Pt/C-RuO₂ (0.47 V in 290 mA cm^-2, 93 mW cm^-2), thus implied good potential in high-power working conditions. The results of the total area specific resistance (ASR, dV/di) further reveal that NiCo₂O₄@MnO₂/CNTs-Ni foam as the cathode exhibits smaller activation polarization, ohmic polarization, and concentration

Figure 4. ZAB using Pt/C-RuO₂ and NiCo₂O₄@MnO₂/CNTs-Ni foam. (A) OCV curves. (B) Polarization curves and corresponding power density plots. (C) Total ASR curves. (d) Different rate discharge cycling curves at different current densities (5-30 mA cm^-2). (E) Charge and discharge polarization curves. (F) Charge-discharge cycles at 5 mA cm^-2 (insert: a small bulb powered by a ZAB in series).

polarization than Pt/C-RuO₂ [Figure 4C]. In addition, the zinc-air battery with NiCo₂O₄@MnO₂/CNTs-Ni foam cathode shows a stable discharge performance with a high specific capacity of 814 mAh g^-1 [Supplementary Figure 6]. Therefore, the high discharge performance of NiCo₂O₄@MnO₂/CNTs-Ni foam should be attributed to the high ORR activity (bimetallic-activated MnO₂ and dual heterojunction), good conduction (Ni foam and binder-free), and rapid mass transfer (3D tremella-like hierarchical structure). Compared with Pt/C-RuO₂, the voltage drop of the battery with the NiCo₂O₄@MnO₂/CNTs-Ni foam air electrode was relatively small during constant current discharge measurements, with the current densities varying from 5 to 30 mA cm^-2 [Figure 4D], indicating its excellent rate capability. Discharge voltage would recover entirely when the current density was looped to 5 mA cm^-2, which verified its satisfactory reversibility.

To further evaluate the charge-discharge potential and cycling stability, the galvanodynamic method and discharge-charge cycling testing were performed, respectively. As shown in Figure 4E, the NiCo₂O₄@MnO₂/CNTs-Ni foam shows a lower charge-discharge potential at a larger current density than Pt/C-IrO₂. The ZAB based on the NiCo₂O₄@MnO₂/CNTs-Ni foam also displayed better cycling stability (165 h) with a durable and small charge-discharge voltage gap of 0.72 V than Pt/C-RuO₂ (6 h, 0.82 V) [Figure 4F]. For the NiCo₂O₄@MnO₂/CNTs-Ni foam, the voltage gap of the ZAB based on the NiCo₂O₄@MnO₂/CNTs-Ni foam increased from 0.72 to 0.77 V after a continuous cycling operation for 150 h, showing that good long-term charge-discharge cycling performance. In comparison, the Pt/C-RuO₂ electrode showed a fast performance degradation after 7 h. Finally, the morphology and electron structure of the NiCo₂O₄@MnO₂/CNTs-Ni foam were obtained by using SEM and XPS after the long-term stability testing; the result proves the self-supporting 3D hierarchical hollow hybrid structure and the stable active sites greatly improve the catalyst lifetime [Supplementary Figure 7]. The ZAB based on the NiCo₂O₄@MnO₂/CNTs-Ni foam air electrode shows better primary/rechargeable battery performance with higher power density and smaller charge-discharge voltage gap than the most recently reported NiCo₂O₄/MnO₂-based bifunctional electrodes [Supplementary Table 1]. Simultaneously, the ZAB in series can power multiple small bulbs, showing good practical application prospects (insert of Figure 4F).

CONCLUSIONS

In summary, a NiCo₂O₄@MnO₂/CNTs-Ni foam self-supported electrode was successfully designed and synthesized by a facial hydrothermal self-assembly process. The as-synthesized NiCo₂O₄@MnO₂/CNTs-Ni foam self-supported electrode has a unique tremella-like 3D hierarchical structure with an abundant heterojunction and evolves rich Mn³⁺ active sites. The unique hierarchical structure effectively promotes electrolyte penetration, reactant (O₂ and OH^-) diffusion, and product emission. In addition, the abundant Mn³⁺ active sites activated by bimetallic ions effectively improve the oxygen catalytic intrinsic performance. Furthermore, the steric hindrance effect and providing self-assembly site mechanism of the Ni substrate were studied. The introduction of Ni foam can significantly avoid the agglomeration of metal oxides and induce the evolution of the nanoparticles on the surface of MnO₂ into two-dimensional nanosheets, which realize the rapid formation of the abundant three-phase reaction interfaces. As expected, the NiCo₂O₄@MnO₂/CNTs-Ni foam demonstrates a promising ORR and OER performance with high catalytic activity (total overpotential of 0.67 V) and the rapid intrinsic kinetics (a small Tafel slope of 105 mV dec^-1). In addition, the ZABs with the NiCo₂O₄@MnO₂/CNTs-Ni foam cathode also show a high peak power density of 226 mW cm^-2, a large specific capacity of 814 mAh g^-1 and a stable charge-discharge cycles with low voltage gap of 0.72 V for 165 h at 5 mA cm^-2. These findings provide inspiration for the design of novel performance-oriented Mn-based self-supported electrodes for wider applications in metal-air batteries.