N-doped ZrO2 nanoparticles embedded in a N-doped carbon matrix as a highly active and durable electrocatalyst for oxygen reduction

Fabricating highly efficient and robust oxygen reduction reaction (ORR) electrocatalysts is challenging but desirable for practical Zn-air batteries. As an early transition-metal oxide, zirconium dioxide (ZrO2) has emerged as an interesting catalyst owing to its unique characteristics of high stability, anti-toxicity, good catalytic activity, and small oxygen adsorption enthalpies. However, its intrinsically poor electrical conductivity makes it difficult to serve as an ORR electrocatalyst. Herein, we report ultrafine N-doped ZrO2 nanoparticles embedded in an N-doped porous carbon matrix as an ORR electrocatalyst (N-ZrO2/NC). The N-ZrO2/NC catalyst displays excellent activity and long-term durability with a half-wave potential (E1/2) of 0.84 V and a selectivity for the four-electron reduction of oxygen in 0.1 M KOH. Upon employment in a Zn-air battery, N-ZrO2/NC presented an intriguing power density of 185.9 mW cm−2 and a high specific capacity of 797.9 mA h gZn−1, exceeding those of commercial Pt/C (122.1 mW cm−2 and 782.5 mA h gZn−1). This excellent performance is mainly attributed to the ultrafine ZrO2 nanoparticles, the conductive carbon substrate, and the modified electronic band structure of ZrO2 after N-doping. Density functional theory calculations demonstrated that N-doping can reduce the band-gap of ZrO2 from 3.96 eV to 3.33 eV through the hybridization of the p state of the N atom with the 2p state of the oxygen atom; this provides enhanced electrical conductivity and results in faster electron-transfer kinetics. This work provides a new approach for the design of other enhanced semiconductor and insulator materials.


Introduction
Global development is facing increasing threats from fossil-fuel energy shortages and environmental pollution [1] .To gradually replace traditional fossil fuels, metal-air batteries and polymer electrolyte membrane fuel cells (PEMFCs) have been developed as next-generation renewable energy conversion devices.However, the sluggish oxygen reduction reaction (ORR) at the cathode hinders the broad application of renewable energy devices [2][3][4].Platinum-group metals (PGMs) are the most efficient ORR catalysts and can address the challenge of sluggish kinetics; however, the limited supply and instability of PGMs severely hinder the industrial application of these catalysts [5][6][7].Therefore, the exploration of efficient and inexpensive electrocatalysts with long-term stability as substitutes for PGM catalysts is crucial.
Group 4 and 5 metal oxides are known as valve metals.From the perspective of stability, they are theoretically superior to nitrides and carbides under the oxidative atmosphere of the ORR and harsh environments [8] .The accessible fabrication of these compounds and their earth-abundant resources further increase their appeal as industrial electrocatalysts.In view of their stability and low price, group 4 and 5 metal oxides are the most promising alternatives for PGMs [9] .The atomic oxygen adsorption properties of metal surfaces indirectly reflect the activation of oxygen in the ORR.For instance, Ota et al. demonstrated that Zr atoms possess smaller atomic oxygen adsorption enthalpies than other group 4 and 5 metals, which indicates that atomic oxygen can be easily adsorbed on the surface of Zr-based materials to increase ORR kinetics [ 10 , 11 ].Thus, Zr-based electrocatalysts for ORR, including ZrN [12] , ZrN x O y [13] , ZrC x N y O z [14] , and ZrO x [ 15 , 16 ], have been extensively studied.Considering the stability of oxides, Zr-based oxides as efficient ORR electrocatalysts are worthy of further study.However, Zrbased oxides as insulators have intrinsically poor electrical conductivity and are considered to be electrocatalytically inactive for ORR involving four-electron transportation [17] .Thus, it is necessary to carefully design Zr-based oxide electrocatalysts with enhanced electrical conductivity to improve their ORR performances.
Zeolitic imidazolate framework-8 (ZIF-8) possesses a well-defined dodecahedral structure and can encapsulate nanoparticles (NPs) in its cavities and prevent the agglomeration of active sites [18] .In addition, the pyrolysis of ZIF-8 gives a N-doped porous carbon (NC) matrix that could serve as a conducting substrate.Furthermore, released CN x species could act as a N source for N-doping to enhance the electrical conduc- tivity of the metal oxide and optimize its electronic structure.However, few studies have focused on the N-doping of Zr-based oxides to develop ORR electrocatalysts.The absence of fundamental research regarding the effect of N-doping on the activity of dominant Zr-based ORR electrocatalysts inhibits the enhancement of these catalysts.In addition, we previously prepared mesoporous thin-walled CuCo 2 O 4 @C nanotubes as efficient bifunctional oxygen electrocatalysts through an electrospinning method [19] , where we found that small catalyst particles with large active specific surface areas, as well as good substrate electrical conductivity are favorable characteristics for excellent electrocatalytic activity.
In this work, ultrafine N-doped ZrO 2 NPs were in situ embeddedinto NC matrix to give a Zr-based ORR electrocatalyst (N-ZrO 2 /NC).Theoretical calculations revealed that the doping of N into ZrO 2 can modify the electronic band structure of the latter to give a decreased band-gap energy, which can be attributed to the p states of the N atom hybridizing with the 2 p states of the oxygen atoms in ZrO 2 .The narrow band gap of the N-ZrO 2 /NC catalyst indicates enhanced electrical conductivity, which provides it with fast electron-transfer kinetics.In addition, N-doping can also increase the electron density around the Zr cations.Facile charge transfer between the catalyst surface and oxygen intermediates can be expected, which would result in reduced reaction barriers.Due to the synergistic effect from the ultrafine size of the particles, the conductive carbon substrate, and the modified electronic band structure of ZrO 2 after N-doping, the N-ZrO 2 /NC electrocatalyst exhibits extraordinary ORR activity with a half-potential of 0.84 V in alkaline media.Thus, our N-ZrO 2 /NC electrocatalyst outperforms the Pt/C catalyst (E 1/ 2 = 0.82 V) and other previously reported catalysts containing group 4 and 5 metals.The power density of Zn-air batteries employing the N-ZrO 2 /NC electrocatalyst was 185.9 mW ‧ cm − 2 , which exceeds that of commercial Pt/C (122.1 mW cm − 2 ).This general strategy can be adapted to design other group 4 and 5 metal-oxide electrocatalysts that exhibit excellent ORR performance.

Results and discussion
The synthesis of N-ZrO 2 /NC is illustrated in Fig. 1 a.First, a mixture of Zn(NO 3 ) 2 ‧ 6H 2 O and ZrCl 4 was added to a methanol solution containing 2-methylimidazole at room temperature to synthesize the ZrCl 4 /ZIF- 8 precursor.Subsequently, the ZrCl 4 /ZIF-8 precursor was thermally annealed at 1000 °C for 3 h under an inert atmosphere.The resultant N-ZrO 2 /NC catalyst with a ZrO 2 loading amount of 12.7 wt% was characterized by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis.For a comparative study, a larger N-ZrO 2 /NC catalyst (N-ZrO 2 /NC-L) was prepared by increasing the Zn(II)-to-methanol ratio [20] .N-ZrO 2 /NC and N-ZrO 2 /NC-L both maintained the welldefined rhombic dodecahedral structures of their precursor, as shown in Fig. S1.The size difference between the two samples can be clearly observed using scanning electron microscopy (SEM) images.The diameter of the N-ZrO 2 /NC catalyst was less than 100 nm, while that of N-ZrO 2 /NC-L was larger than 500 nm ( Figs. 1 b, S3a).Compared with the ZrCl 4 /ZIF-8 precursors, the catalysts exhibit rougher surfaces owing to the evaporation of Zn during heat treatment.Obviously, it is possible to expose more active sites and maximize the ZrO 2 utilization .Transmission electron microscopy (TEM) was used to further characterize the morphologies of both samples.As shown in Fig. 1 c, N-ZrO 2 /NC possesses an overhang-eave structure with stretched edges, which can provide more three-phase (solid-liquid-gas) exchange spots to boost mass transport and achieve the maximum accessibility of active sites [21] .
The crystal structures of the synthesized catalysts were confirmed by powder X-ray diffraction (XRD) and high-resolution TEM (HRTEM).Based on the XRD patterns, adding ZrCl 4 precursor during the synthesized process of ZIF-8 has no influence on the growth of ZIF-8 crystal (Fig. S2).After pyrolysis, the diffraction peaks of the N-ZrO 2 /NC-L catalyst were found to be consistent with those of ZrO 2 (JCPDS No.79-1764) ( Fig. 2 a), in agreement with the observed lattice fringes with spacing of 0.295 nm correspond to the (101) planes of ZrO 2 (inset of Fig. S3b).The XRD pattern of N-ZrO 2 /NC only exhibits a broad shoulder peak located within the range of 20-30°( Fig. 2 a), which is attributed to the (002) facet of graphitic carbon.The signals of ZrO 2 are absent due to their low content and ultrafine size, but lattice fringes with spacing of 0.254 nm were noted in the HRTEM image (inset of Fig. 1 c) and correspond to the (110) plane of ZrO 2 .The contrast of the high-angle annular dark-field TEM (HAADF-TEM) images confirmed that the ZrO 2 NPs of N-ZrO 2 /NC are ultrafine and defined.Corresponding energy-dispersive X-ray (EDX) elemental mapping and line-scan analysis spectroscopy showed that Zr was only dispersed on the brighter core of the matrix, which also illustrates the ZrO 2 NPs encapsulated in the microporous NC matrix (insets of Figs. 1 d, e and S3c, d).In particular, the distribution of N throughout the entire matrix was relatively concentrated over the bright region, which indicates the successful doping of N into the ZrO  possible reason is that the greater number of layers in the larger ZIF-8 crystals hinders Zn evaporation during carbonization, thus generating higher densities of defects in the carbon structure.Structural defects were considered as effective anchoring sites for the ZrO 2 active components.According to the pore-size distribution curves (Fig. S5b), N-ZrO 2 /NC shows more micropores and mesopores due to facile Zn evaporation and its smaller ZrO 2 NPs.These cause less blocking than larger ZrO 2 NPs and are more accessible to the reactants during the ORR than larger NPs are; this is especially true for the active sites embedded inside the carbon matrix.The formation of large ZrO 2 NPs for N-ZrO 2 /NC-L can be ascribed to the agglomeration of Zr atoms, which was due to the increased concentration of ZrCl 4 precursor contained in the cavities.
The chemical states of the elements and the surface compositions of the synthesized catalysts were characterized by X-ray photoelectron spectroscopy (XPS).As shown in Fig. S6a, Zr, O, C, and N were present in both samples.The N 1s spectra of the two catalysts exhibited five characteristic peaks ( Fig. 2 c), which correspond to oxidized N (402.7 eV), graphitic N (401.1 eV), pyrrolic N (400.1 eV), pyridinic N (398.75eV), and metal N (398.2eV) [21] .The metal N peak verifies the successful doping of N into the ZrO 2 NPs, and shows that metal N makes up approximately 0.7 at% of N-ZrO 2 /NC; it only makes up 0.24 at% of N-ZrO 2 /NC-L (Table S2).Smaller NPs generally possess numerous lowcoordination sites at their edges, corners, and vertices, which are more active than other sites and facilitate the formation of metal-N bonds; this is in marked contrast with the planar face observed for large NPs.A greater amount of metal-N bonds should lead to a pronounced enhancement in the electrical conductivity; this was demonstrated in the present work by subsequent electrochemical impedance spectroscopy (EIS) (Fig. S7).In the Zr 3d XPS spectrum ( Fig. 2 d), the two distinct peaks of N-ZrO 2 /NC located at 182.1 eV and 184.5 eV are attributed to the 3d 5/2 and 3d 3/2 binding energies of Zr 4 + , respectively.This 0.3 eV negative shift in the Zr 3d of N-ZrO 2 /NC relative to that of standard ZrO 2 powder (182.4 eV) indicates an increased charge density for the N-ZrO 2 /NC catalyst and a lower valence state of its Zr cations [22] .This is attributed to the partial substitution of less electronegative N atoms with oxygen atoms, which could offer more electrons and increase the electron density of Zr.Lower amounts of metal-N species also result in a slightly negative shift in the 3d 5/2 binding energy (182.3eV) of N-ZrO 2 /NC-L.The increased electron density facilitates charge transfer between the ZrO 2 active components and reactant intermediates and optimizes the adsorption/desorption of oxygen intermediates, resulting in decreased reaction barriers [23] .The C 1s XPS spectrum of N-ZrO 2 /NC (Fig. S6b) can be resolved into three peaks centered at 284.8 eV, 285.7 eV and 288.6 eV corresponding to C-C, C-N, and C-O bonds, respectively.The C-N bonds confirm the existence of an NC matrix that serves as a conductive substrate to host the active sites of the N-doped ZrO 2 NPs.The above investigation verifies that N-doped ZrO 2 NPs encapsulated in a microporous NC matrix have been successfully synthesized.The partial substitution of N for lattice oxygen and the loading of the ZrO 2 active components on a conductive carbon substrate can modify the electronic band structure of the resultant catalyst and improve its electrical conductivity, thus enhancing its electrocatalytic performance for ORR.
The electrocatalytic ORR performance of the as-prepared N-ZrO 2 /NC catalyst was evaluated by a rotating disk electrode (RDE) method in O 2 -saturated 0.1 M KOH or a 0.5 M H 2 SO 4 solution.For comparison, the electrochemical activities of commercial Pt/C, N-ZrO 2 /NC-L, and NC catalysts were also tested.A linear sweep voltammetry (LSV) survey ( Fig. 3 a) showed that the N-ZrO 2 /NC catalyst has a E 1/2 of 0.84 V relative to the reversible hydrogen electrode (RHE) potential in alkaline media, outperforming the commercial Pt/C (0.82 V), N-ZrO 2 /NC-L (0.81 V), and NC (0.75 V) catalysts.This indicates the excellent ORR performance of N-ZrO 2 /NC.Moreover, the N-ZrO 2 /NC, N-ZrO 2 /NC-L, and NC catalysts exhibited higher limiting current densities of 6.65 mA cm − 2 , 6.6 mA cm − 2 , and 5.70 mA cm − 2 , respectively, than that of commercial Pt/C (5.23 mA cm − 2 ), indicating the remarkable mass transfer performance of the NC matrix.The small Tafel slope value (101 mV dec − 1 ) of N-ZrO 2 /NC further confirms its ORR kinetics are faster than those of commercial Pt/C and its ORR activity is comparable to that of commercial Pt/C ( Fig. 3 b).The kinetic current density (J k ) of N-ZrO 2 /NC at 0.8 V was assessed to be 22.49 mA cm − 2 , exceeding those of Pt/C (10.69 mA cm − 2 ) and NC (1.63 mA cm − 2 ) by 2-and 14.7-fold ( Fig. 3 c).We compared the E 1/2 value of N-ZrO 2 /NC in an alkaline solution with those of other, recently reported ORR electrocatalysts containing group 4 and 5 metals ( Fig. 3 d, Table S3); our catalyst outperformed most of their reported Ti, Ta, and Zr-based counterparts.
To reveal the vital role of ZrO 2 NPs during the ORR process, we evaluated the ORR activity of a series of N-ZrO 2 /NC catalysts with various loading amounts in O 2 -saturated 0.1 M KOH, which were synthesized by altering the content of ZrCl 4 added to the precursor (denoted as N-ZrO 2 /NC-15, − 35, − 40, − 50, and − 70, where the number denotes the amount of ZrCl 4 (mg)) (Figs.S8, S9).The electrochemically active surface area (ECSA) is considered to be compatible with catalytic activity and is usually investigated by double-layer capacitance (C dl ).LSV and cyclic voltammetry (CV) surveys demonstrated that the N-ZrO 2 /NC catalyst exhibited higher E 1/2 and C dl values with increasing ZrO 2 NP content (Figs.S10-12), which can be attributed to the increasing number of N-doped ZrO 2 active components.Among the catalysts, as-prepared N-ZrO 2 /NC-50 delivered superior ORR activity with the highest E 1/2 of 0.84 V and the largest C dl of 115.1 mF cm − 2 .However, further increasing the ZrCl 4 content (e.g., N-ZrO 2 /NC-70) resulted in structural collapse, the degradation of E 1/2 , and a lower C dl of 57.85 mF cm − 2 ; this further proves that the rhombic dodecahedral structures play an important role in boosting the ORR performance.
The selectivity and stability of the as-prepared N-ZrO 2 /NC and Pt/C catalysts were assessed using a rotating ring-disk electrode (RRDE), accelerated durability tests, and a chronoamperometry method.As shown in Fig. 3 e, in contrast to the commercial Pt/C catalyst, the N-ZrO 2 /NC catalyst exhibits better four-electron selectivity with an electron transfer number (n) of approximately 4. Furthermore, the peroxide yields of the latter were less than ∼4% in the range of 0.25 to 0.8 V. Stability is a mandatory factor for measuring the potential of practical applications, especially for ORR-based devices operating in harsh environments.According to the accelerated durability test of N-ZrO 2 /NC ( Fig. 3 f), hardly any degradation in E 1/2 or the limiting current density was observed after 10,000 cycles.Chronoamperometry measurements were carried out to evaluate the stability of the as-prepared catalysts in a 0.1 M KOH solution ( Fig. 3 g), where 92.0% of the original current density of N-ZrO 2 /NC was retained after 10 h of chronoamperometric tests.Contrastingly, only 77.0% of the original current density of Pt/C was retained after less than 6 h of testing.In terms of direct methanol (alcohol) fuel cells, methanol partly permeates the cathode from the anode through the polymer elec- trolyte membrane and is oxidized at the cathode, resulting in a lower electrical efficiency.Thus, methanol tolerance is necessary for ORR catalysts.As shown in Fig. 3 h, the current density of the N-ZrO 2 /NC catalyst was not affected by methanol being injected into the O 2 -saturated 0.1 M KOH solution at 400 s of the chronoamperometry test, whereas the current density of the Pt/C catalyst decreased significantly, indicating the excellent methanol tolerance of N-ZrO 2 /NC catalyst.Additionally, its electrocatalytic ORR performance are also considerable in O 2 -saturated 0.5 M H 2 SO 4 .It has been reported that pristine NC is electrochemically inactive in acid media due to corrosion, whereas loading ZrO 2 NPs onto pristine NC exhibits higher E 1/2 , indicating the key role of ZrO 2 NPs during the acidic ORR process (Fig. S13a).As shown in Fig. S13b, the N-ZrO 2 /NC catalyst exhibits four-electron selectivity with an electron transfer number (n) of approximately 4 and low peroxide yields of below ∼2%, which are even better than those observed in an alkaline solution.Benefiting from the excellent stability of ZrO 2 , the N-ZrO 2 /NC catalyst showed only an 18% degradation in its original current density after 10 h of chronoamperometric testing in O 2 -saturated 0.5 M H 2 SO 4 ; this is in sharp contrast to the more than 50% degradation observed for the current density of Pt/C (Fig. S13c).We have summarized the ORR potentials of group 4 and 5 metal catalysts in acid solutions at 10 μA cm − 2 reported in the literature and compared them to that of our catalyst to demonstrate its outstanding ORR performance in acidic media (Fig. S13d, Table S4).
To further understand the fundamental mechanism behind the effect of N-doping on the enhanced ORR activity, the density of states (DOS) and partial density of states (PDOS) of ZrO 2 and N-doped ZrO 2 samples were studied by density functional theory (DFT) calculations ( Fig. 4 a,b).According to Fig. 4 c, the energy bandgap for ZrO 2 is calculated to be 3.96 eV, whereas this value decreases to 3.33 eV for N-doped ZrO 2 , indicating the enhanced electrical conductivity of N-doped ZrO 2 .The valence band of ZrO 2 mainly consists of the 2 p orbitals of oxygen mixed with the 3 d orbitals of Zr.N-doping can alter the valence band edge and narrow the band gap through the hybridization of the N p states and oxygen 2 p states [24] , resulting in enhanced electrical conductivity compared to that of pristine ZrO 2 .The improved conductivity was also verified by EIS (Fig. S7).Theoretical calculations and experiments both demonstrated that introducing N can narrow the energy bandgap and Inspired by the excellent ORR performance of the N-ZrO 2 /NC catalyst, we assembled a primary Zn-air battery device to test its practical applicability.The Zn-air batteries were fabricated using the N-ZrO 2 /NC or Pt/C catalysts as the cathode, Zn foil as the anode, and 6 M KOH/0.2M Zn(OAc) 2 as the electrolyte ( Fig. 5 a).As shown in Fig. 5 b, the open circuit voltage with N-ZrO 2 /NC as cathode is 1.40 V. Thus, the N-ZrO 2 /NC-based battery exhibited a higher power density (185.9 mW cm − 2 ) than the Pt/C-based Zn-air battery (122.1 mW cm − 2 ) ( Fig. 5 c).As shown in Fig. 5 d, in contrast to the theoretical capacity of Zn-air batteries ( ∼820 mA h g Zn − 1 ), the experimental specific capacity at 10 mA cm − 2 is 797.9 mA h g Zn − 1 , indicating that the N-ZrO 2 /NC-based battery achieves approximately 97% utilization efficiency; this is higher than that of the Pt/C-based Zn-air battery (782.5 mA h g Zn − 1 , 95% utilization efficiency).Overall, the excellent performance of N-ZrO 2 /NC demonstrates the potential utilization of N-ZrO 2 /NC electrodes in Zn-air batteries, as well as their application in other powering devices, such as Na, Mg, and Al-air batteries and fuel cells.
It is clear that the outstanding ORR performance of N-ZrO 2 /NC and its durability may be attributed to the following aspects ( Scheme 1 ): (1) Serving as the substrate, the porous structure and high surface area of the NC matrix endowed the catalyst with facile electron transfer and mass diffusion.latter, which maximized the accessibility of its active sites.(3) Introducing N addressed the poor electrical conductivity of ZrO 2 .DFT calculations verified that the doping of N into ZrO 2 can effectively narrow its bandgap, which is attributed to the hybridization of the N p states and oxygen 2 p states, thereby resulting in enhanced electrical conductivity and fast electron-transfer kinetics.Furthermore, XPS confirmed that N-doping increases the electron density around Zr cations, which facilitates charge transfer between the catalyst surface and oxygen inter-mediates, consequently resulting in a reduced reaction barrier.(4) Oxides are theoretically more stable than nitrides and carbides under the oxygen-abundant operating environment of metal-air batteries.Thus, the N-ZrO 2 /NC catalysts delivered satisfactory stability and methanol tolerance.Based on these four aspects, the as-prepared N-ZrO 2 /NC catalyst exhibits excellent performance for the ORR, which demonstrates its potential as an efficient catalyst for metal-air batteries.

Conclusion
N-doped ZrO 2 NPs embedded in an NC matrix with a rhombic dodecahedral structure were prepared using a host-guest strategy of ZIF-8.The as-synthesized N-ZrO 2 /NC catalyst exhibited high performance with an E 1/2 of 0.84 V and excellent selectivity for the four-electron reduction of oxygen in 0.1 M KOH; thus, this catalyst outperforms most electrocatalysts containing group 4 and 5 metals.In addition, due to the stable characteristics of ZrO 2 , the N-ZrO 2 /NC catalyst possesses outstanding stability compared to Pt/C in both alkaline and acidic media.The superior ORR catalytic performance of N-ZrO 2 /NC can be attributed to its ultrafine size and conductive carbon substrate.N-doping also effectively narrowed the band gap of ZrO 2 , which enhanced the electrical conductivity of the catalyst and accelerated its reaction kinetics, consequently boosting its ORR activity.The strategy presented provides a new method for improving other semiconductor and insulator metal-oxide materials (such as TiO 2 and HfO 2 ) towards excellent electrochemical applications.

Declaration of Competing Interest
The authors declare that they have no conflicts of interest in this work.
2 core and carbon matrix shell.The ZrO 2 NPs encapsulated in the NC matrix could improve electrical conductivity, which is of great significance for ORR involving four-electron transportation.More importantly, based on TEM and HAADF-TEM, the ZrO 2 NPs in N-ZrO 2 /NC are smaller than those in N-ZrO 2 /NC-L; smaller ZrO 2 active components are beneficial to the exposure of active sites, which improves activity.This was confirmed by subsequent ORR tests.The I D /I G value of the Raman spectra is an important factor for measuring the defects and graphitic level of carbon materials.As shown in Figs. 2 b and S4, the larger CN matrix possesses numerous defects.One

Fig. 3 .
Fig. 3. ORR activity and durability.(a) Linear sweep voltammetry (LSV) curves collected at a sweep rate of 10 mV ‧ s − 1 , (b) the corresponding Tafel slopes, and (c) the J k values at 0.80 V and E 1/2 values of N-ZrO 2 /NC, N-ZrO 2 /NC-L, NC and Pt/C.(d) Comparisons of the experimental E 1/2 of N-ZrO 2 /NC in an alkaline medium with the literature values of the reported representative catalysts containing group 4 and 5 metals (corresponding to Table S3).(e) The H 2 O 2 yield and the electron transfer numbers (n) of N-ZrO 2 /NC and Pt/C.(f) Polarization curves of N-ZrO 2 /NC before and after accelerated durability tests of 10,000 cycles from 0.85 to 1.05 V (vs.RHE) at a sweep rate of 100 mV s − 1 .(g) Chronoamperometry tests of N-ZrO 2 /NC and Pt/C at a constant potential of 0.7 V (vs.RHE) and 400 rpm for 10 h.(h) Methanol tolerance tests of N-ZrO 2 /NC and Pt/C at 0.7 V (vs.RHE), where 3 mL of methanol was injected into a 70 mL electrolyte solution at 400 s.All tests were measured in O 2 -saturated 0.1 M aqueous KOH.

Fig. 4 .
Fig. 4. Electronic structure studies.The optimized structures of (a) ZrO 2 and (b) N-ZrO 2 models.(c) The density of states (DOS) and partial DOS (PDOS) for ZrO 2 and N-doped ZrO 2 ; the Fermi level is defined as zero.

Fig. 5 .
Fig. 5. Zn-air battery demonstration.(a) Schematic diagram of a Zn-air battery.(b) Open circuit voltage measurement of a Zn-air battery with N-ZrO 2 /NC as the cathode catalyst (inset shows a photograph of the assembled zinc-air battery).(c) Discharge polarization curves and power density plots and (d) long-time galvanostatic discharge curves (at 10 mA • cm − 2 ) of Zn-air batteries with N-ZrO 2 /NC or Pt/C as the cathode catalyst.

( 2 )
Scheme 1. Schematic diagram of the synergy that enhanced the oxygen reduction reaction (ORR) activity of the N-ZrO 2 /NC catalyst.