Yttrium Oxide Nanoclusters Boosted Fe‐N4 and Fe4N Electrocatalyst for Future Zinc–Air Battery

Atomically distributed transition metal coordinated with nitrogen is considered as a class of promising oxygen reduction reaction (ORR) catalyst. However, the challenge of ineffective distribution of Fe‐Nx active sites have been long existing, leading to low active site density and unstable performance, which needs be overcome for next generation ORR electrocatalysts. Herein, yttrium (Y) is introduced into atomically dispersed iron (Fe) nitrogen co‐doped carbon materials to integrate nanoparticles, nanoclusters, and atomic sites, which endow the Fe‐N4‐Y2O3 and Fe4N0.94‐Y2O3 (FeY‐NC) with outstanding ORR activity. The FeY‐NC achieves half‐wave potential of 0.926 and 0.809 V in alkaline and acidic condition, respectively. The kinetics current density at 0.9 V in alkaline condition is 31.2 mA cm−2, which is 7.8 times of Fe‐NC and 32.4 times of Pt/C. This outstanding activity of FeY‐NC is enabled by the generated atomic FeN4 and Fe4N nanoparticles dual active‐sites, and further the synergistic effect between the Fe‐Nx/Fe4N0.94 with Y2O3 nanoclusters are loaded on nitrogen‐doped carbon (NC) network. The superior performance of FeY‐NC is demonstrated in a primary Zinc‐air battery, deliver a peak power density of 233 mW cm−2.


Introduction
[3] Behind these devices, the cathode ORR, a sluggish electrochemical process involving coupling of four electrons and four protons, frequently occurred to hinder their applications.The solution to overcome this inert kinetics of DOI: 10.1002/adfm.2023110846] Although Pt-based catalysts is the most effective ORR catalyst, scarce resource hinders the wide commercial application. [7,8]][17] Conventionally, iron and nitrogen co-doped carbon materials contains multi-types of active centres for ORR especially under alkaline conditions, such as P block heteroatoms, atomically dispersed metal active center (M-N x ), metal nitride (MN x ), metal carbide (MC x ), metal sulfide (MS x ) as well as metals or metal oxides NPs with specific structures. [18]][21][22] However, challenges emerge as atomically dispersed active sites are difficult to balance the energy for adsorption-desorption of ORR intermediates, which limit their electrocatalytic performance. [23]The potential solutions are to introduce the rational design of muti-component composite catalysts and utilize the synergistic effect among atomic sites, nanoclusters and nanoparticles. [24][38][39][40][41] Liu et al. utilized Gd to tailor the size of Ni nanoparticles and affect the local electron densities of Ni 3d orbitals, thereby increasing the CO 2 reduction reaction activity. [42]ang et al. co-introduced Fe and Ce into the nitrogen-doped carbon aerogel to optimize the pore structure and significantly increase the pyridinic nitrogen content.The Ce/Fe co-doped materials demonstrated a higher half-wave potential (E 1/2 ) of 0.842 V in contrast to 0.788 V for Fe/NCG. [43]Tang group recently found that the coupling between Nd and Co brings excellent bifunctional electrocatalytic properties with a high E 1/2 of 0.85 V for the ORR [44] as Nd dopant into surface lattice of Co optimizes the binding energy of oxygen-containing intermediates.
Herein, we systematically investigated the composite electrocatalytic materials with Fe and rare-earth species load on NC substrate (FeRE-NC) by a facile metal impregnation strategy.Compared with the solely Fe doped NC materials, the addition of Y induces the formation of a mixture of single atoms, highly dispersed nanoclusters, and nanoparticles.The formation of Fe-N 4 , Fe 4 N 0.94 , and Y 2 O 3 heterostructures uniformly distributed on the NC network are attributed to the high E 1/2 of 0.926 and 0.809 V in alkaline and acidic electrolytes, respectively.

Catalyst Synthesis and Characterization
The FeY-NC electrocatalyst was prepared by a facile impregnation method, as illustrated in Figure 1a.Briefly, the iron and yttrium precursors were in situ introduced during the synthesis of ZIF-8 by adding Fe(NO 3 ) 3 •9H 2 O, Y(NO 3 ) 3 •6H 2 O, and Zn(NO 3 ) 2 •6H 2 O together with 2-methylimidazole. [45]The resulting FeY-ZIF-8 powder was mixed with dicyandiamide and then heated at 950 °C for 2 h to obtain FeY-NC.For comparison, Fe-NC and Y-NC samples were also synthesized following the same method with Fe(NO 3 ) 3 •9H 2 O or Y(NO 3 ) 3 •6H 2 O (see Supporting Information).
The crystal structure of as prepared electrocatalysts is characterized by X-ray diffraction (XRD, Figure 1b).The introduction of N dopants increases the layer space of graphitic carbon, leading to the graphitic carbon peak at (002) shifts from 26.0°to near 22.0°.The metal addition slightly increases the graphitization and leads to the peaks shift back to 22.6°for Y-NC, 23.0°for Fe-NC, and 24.5°for FeY-NC.Apart from the broad peak for carbon, a set of diffraction peaks located at 28.6, 33.1, 47.8, and 56.6°are observed for Y-NC, corresponding to (222), (400), (440), and (622) planes of Y 2 O 3 phase (JCPDS Card No. 43-1036).For Fe-NC, no metal-related diffraction peaks observed, which can be attributed to the good dispersity of Fe in the carbon matrices, consistent with the result of atomically dispersed Fe-NC. [63]9][50][51][52][53][54][55][56][57][58] had similar defect structures.FeY-NC exhibits a higher BET surface area (1304 m 2 g −1 ) than that of Fe-NC (1023 m 2 g −1 ) and Y-NC (1197 m 2 g −1 ), which proved the ion diffusion and entanglement of Fe and Y at high temperature brings more pores (Table S1, Supporting Information).The distribution of pore diameter is summarized in Figure S2a,b (Supporting Information), we find that the pore dimension of all samples are mostly below 5 nm.These micro-/nano-pores not only create numerous active sites, but also fulfil the material a huge specific surface area.The scanning electron microscopy (SEM) observation unveil that the NC retain the original dodecahedron morphology of ZIF-8 particles with a diameter of ≈100 nm (Figure S3, Supporting Information).And the introduction of metal ions increases the size of ZIF particles, the average diameter of FeY-NC, Fe-NC, and Y-NC particles is about 650, 150, and 500 nm, respectively, due to the additional Y species accelerating growth rate of ZIF-8.
Transmission electron microscopy (TEM) was employed to characterize the microstructure of catalyst.In Figure 1c, the Fe-NC catalysts show a regular dodecahedral shape with apparent mesoporous structure and no particles are observed in the region.We further utilize the scanning transmission electron microscope coupled with energy dispersive spectrometry (STEM-EDS) to map the distribution of Fe element all over the ZIFderived carbon material.It is found that the Fe and N are evenly distributed in the polyhedron.AC-STEM is applied to characterize the atomic structure of Fe-NC and many isolated bright dots are observed (Figure 1e), indicating an atomic dispersion of Fe in Fe-NC with good agreement to the report. [46]The coexistence of Fe and Y precursor change the distribution of Fe elements.
The FeY-NC catalyst also has a uniform rhomboid dodecahedron morphology structure, with an average size of 50 nm observed for most nanoparticles (Figure 1f  enhanced immobilization for N in carbon substrate through M-N x by Y introduction (Table S2, Supporting Information).Moreover, the N 1s spectrum in Figure 2b can be deconvoluted into five peaks, pyridinic N (Pyri-N), metal N (M-N x ), pyrrolic N (Pyrr-N), graphitic N (Grap-N), and oxidized N (Oxi-N), located around 398.6, 399.1, 400.1, 401.0, and 404.2 eV, respectively. [59]It can be seen from the M-N x and Grap-N content of FeY-NC significantly higher than Fe-NC, while Pyri-N content is similar to that of Fe-NC (Figure S8b, Supporting Information).61] The Fe 2p spectrum provides the classical splitting bands of Fe 2p 3/2 and Fe 2p 1/2 , with four groups of peaks corresponding to Fe 0 , Fe 2+ , Fe 3+ and satellite peaks (Figure 2c).The Y element in the sample only deconvolved a pair of 3d 5/2 and 3d 3/2 peaks (Figure 2d), corresponding to Y 3+ , indicating the formation of Y 2 O 3 .Notably, the binding energy of both Fe 2p and Y 3d states exhibit down-shifting trends when Fe and Y co-exist.The peak position of Fe 2+ in FeY-NC is negatively shifted from 710.40 to 710.05 eV compared with Fe-NC, leading to the increase of Fe 0 and Fe 2+ , proving the formation of Fe 4 N nanoclusters.Moreover, a downshifting of binding energy of 0.7 eV for Y is observed in FeY-NC relative to the Y-NC, indicating the Y 2 O 3 tend to pull the electrons from the surrounding coordinated environment, providing a strong synergistic interaction to tune the electronic structure of the components adjacent.
To unravel the ORR kinetics and electronic transmission pathway of FeY-NC, the LSV curves are recorded at different sweep rates from 400 to 2500 rpm (Figure 3d).By calculating the electron transfer number based on the K-L plots under 0.   intermediates, to reduce the energy berries and improve the catalytic performance.
We then explore the influence of other rare earth elements, such as Pr, Sm, Eu, Sc, Gd, Tb, Dy, Er, toward ORR in 0.1 m KOH. Figure 4d summarize the E 1/2 of FeRE-NC with the Fe-NC as reference.Gd, Eu, Tb, Er, Pr, and Sm bring negative effects to Fe, where the E 1/2 decrease compared with Fe-NC.Whereas, the E 1/2 of the composite catalysts positively shift when Sc, Dy, and Y are co-doped with Fe, and the E 1/2 of FeDy-NC and FeY-NC is 0.911 and 0.926 V, respectively, indicating the excellent performance.It is found that the strong interaction between Y and Fe species gives the composite catalyst the best ORR performance than most other rare earth composite catalysts.

Zinc-Air Battery Performance
We next assembly the obtained electrode materials into zincair batteries to demonstrate the electrocatalytic performance of catalysts (Figure 5a).The FeY-NC-based battery possesses a higher open-circuit potential of 1.483 V than that of Fe-NC-based (1.458 V) and Pt-C-based (1.452 V) due to its higher ORR activity (Figure 5b).Meanwhile, the discharge polarization curves suggest that the FeY-NC-based battery exhibits a peak power density of 233 mW cm −2 , which is much higher than that of Fe-NC (176 mW cm −2 ) and Pt-C (167 mW cm −2 ) (Figure 5c).The stabilities of three catalyst-based batteries are evaluated by chronopotentiometry at 5 and 20 mA cm −2 (Figure 5d; Figure S14, Supporting Information).The FeY-NC-based battery achieve a nearly horizontal discharge curve with an output voltage of ≈1.37 V at 5 mA cm −2 , which is better than that of Fe-NC-based and Pt-C-based batteries.The FeY-NC-based battery display a higher discharge voltage of 1.31 V within 25 h at 20 mA cm −2 than that of Fe-NC-based and Pt-C-based batteries (Figure S14, Supporting Information), revealing the FeY-NC has superior activity and durability.
The specific capacity of FeY-NC-based battery is calculated to be 772 mAh g −1 Zn based on the consumed mass of Zn anode at 20 mA cm −2 , much higher than that of Fe-NC (704 mAh g −1 Zn ) and Pt-C (639 mAh g −1 Zn ) (Figure 5e).Moreover, the FeY-NC-based battery exhibits the impressive rate performance with stable output voltage platforms at all current densities (Figure 5f).Particularly, FeY-NC-based battery still has a high discharge voltage of 1.21 V at a high current density of 50 mA cm −2 , which is better than Fe-NC and Pt-C.By comparing the peak power density from our FeY-NC based Zn-air battery with other recently reported catalysts in Figure 5g [62][63][64][65][66][67][68][69][70][71][72] , it is found that a peak power density of 233 mW cm −2 from FeY-NC is the unprecedent value that have never been reached.
The electrocatalytic performance of materials are evaluated in acidic state.Figure 6a shows the LSV curves of catalysts in O 2saturated 0.1 m HClO 4 , the FeY-NC presented satisfactory ORR performance with a high E 1/2 of 0.809 V, which was 16 mV higher than that of Fe-NC (0.793 V) and significantly higher Y-NC (0.495 V), and even comparable to commercial Pt-C (0.839 V).The calculated Tafel slope of FeY-NC is as low as 63.2 mV dec −1 , which is slightly inferior to Pt-C (61.6 mV dec −1 ), demonstrating a fast ORR kinetics (Figure S15a, Supporting Information).
In addition, the FeY-NC possess a large kinetic current density of 8.934 mA cm −2 than that of Fe-NC (3.889 mA cm −2 ) and Y-NC electrocatalysts (Figure 6b).The electron transfer number calculated based on the K-L plots under 0.2-0.5 V reveals that the FeY-NC had an electron transfer number of 3.98 in comparison to that of 3.51 for Fe-NC (Figure 6c and Figure S16b, Supporting Information).Moreover, the cyclic ADT test (5000 cycles) reveal that the E 1/2 of FeY-NC only negatively shift by 25 mV after 5000 cycles, which is much lower than that of Fe-NC (39 mV) and Pt-C (58 mV, Figure 6d; Figure S15b, Supporting Information).The loss of activity likely due to the leaching of Y 2 O 3 in acid condition.From above, the FeY-NC possess better ORR performance than Fe-NC at acidic environment with excellent catalytic activity and stability, which are attributed to the strong synergistic interaction between Fe and Y species.

Conclusion
In summary, we develop a FeY-NC composite consisting of atomic dispersed Fe-N x , Fe 4 N 0.94, and Y 2 O 3 nanoclusters as well as Fe-N x -/Fe 4 N 0.94 -Y 2 O 3 heterostructures by combining the excellent ORR catalytic performance of transition metal Fe and the unique electron orbital configuration of rare earth element Y.The strong intrinsic synergistic effect of Fe-N x and Fe 4 N with Y 2 O 3 significantly decrease the binding energy of * OH, therefore reduce the overpotential for ORR.The FeY-NC material possessed excellent electrocatalytic performance than Fe-NC, reaching a high E 1/2 of 0.926 and 0.809 V in alkaline and acidic media, respectively.The FeY-NC demonstrate a large power density (233 mW cm −2 ), a high specific capacity (772 mAh g −1 Zn ) and a superior rate discharge performance in Zn-air primary battery.Furthermore, other rare earth elements, such as Sc and Dy, also demonstrate high synergistic effect, revealing the co-doping rareearth elements with iron in NC could provide a promising strategy to enhance the ORR performance.

Figure 1 .
Figure 1.a) Schematic illustration of the synthesis of the FeY-NC.b) XRD patterns of FeY-NC, Fe-NC, Y-NC, and NC.c) TEM, d) HAADF-STEM elemental mapping images and e) AC-STEM images of Fe-NC.f) SEM, g) TEM, h) high-resolution TEM images i,j) AC-STEM images k) AC-STEM-EDS images of FeY-NC.

Figure 2 .
Figure 2. a) XPS C 1s spectra and b) N 1s spectra of FeY-NC, Fe-NC, Y-NC and NC.c) Fe 2p spectra of FeY-NC and Fe-NC.d) Y 3d spectra of FeY-NC and Y-NC.
2, 47.9°corresponded to Fe 4 N 0.94 (JCPDS Card No. 71-1294) are identified, in addition to a set of small Y 2 O 3 diffraction peaks.The fairly low intensively of these Fe 4 N 0.94 peaks indicates the small crystalline in the carbon matrix, clearly revealing that the addition of Y could induce the formation of Fe 4 N 0.94 and Y 2 O 3 nanostructures.The addition of Y(NO 3 ) 3 •6H 2 O leads to the formation of Y 2 O 3 is resulted from the high affinity of Y with oxygen, due to the low reduction potential of these rare earth elements.We next investigate the carbon structures via the Raman spectra and calculate the I D /I G to understand the defects inside of samples (Figure S1, Supporting Information).The I D /I G of all samples are around 1.0, indicating that all as-prepared samples

Figure 3 .
Figure 3. a) IR corrected LSV curves of five catalysts in 0.1 m KOH (rotation rate: 1600 rpm; sweep rate: 10 mV s −1 ).b) the summarized E 1/2 and the kinetic current density at 0.9 V, c) Tafel plots of the catalysts investigated.d) LSV curves of FeY-NC at various rotation rates, the inset shows the Koutecky-Levich plots at different potentials.e) LSV curves before and after 15 000 CV cycles of FeY-NC and Fe-NC in 0.1 m KOH.f) Comparison of E 1/2for various non-precious metal catalysts reported.[47][48][49][50][51][52][53][54][55][56][57][58] ,g).Further analyse reveal that these nanoparticles are Fe 4 N 0.94 -Y 2 O 3 heterostructures, where the cubic Y 2 O 3 shows an interplanar spacings of 0.262 and 0.299 nm and cubic Fe 4 N 0.94 displays an interplanar spacing of about 0.218 nm, consistent with the XRD results.The STEM-EDS mapping suggest that the iron elements is not perfectly overlapped with Y elements, further demonstrating the formation of Fe 4 N 0.94 and Y 2 O 3 heterostructures (Figure S4, Supporting Information).The high-resolution TEM and further AC-STEM images of FeY-NC (Figure 1i,j) confirmed that the uniformly distribution of atomic sites, nanoclusters that embedded in the carbon structures with size less than 5 nm, and mostly uniformly formed Y and Fe nanoclusters with a few to dozens of atoms.The detailed STEM-EDS results confirm the accumulation of Y in the case of FeY-NC, most iron are atomically dispersed with minor accumulation likely attribution around the Fe 4 N nanoclusters.The AC-STEM images disclose that the yttrium oxide nanoclusters (Y 2 O 3 ) are embedded in the carbon matrices with the atomic Fe-N x and Fe 4 N nanoclusters on the surface.The surface chemical states of FeY-NC, Fe-NC, Y-NC, and NC are further studied by X-ray photoelectron spectroscopy (XPS).The XPS survey scan spectrum confirms the presence of C, N, O, Fe, and Y elements (Figure S8a.Supporting Information).The high-resolution C 1s XPS spectrum (Figure 2a) are fitted into four peaks corresponding to C-sp 2 (284.8eV), C-sp 3 (285.4eV), C─N (286.3 eV), and O─C─O (290.1 eV), respectively.Higher content of C-N in FeY-NC (16%) compared with Fe-NC (9%) arise from
4-0.7 V, it is found that FeY-NC has the larger electron transfer number about 3.93 than that of Fe-NC (3.73), Y-NC (3.67), and NC (3.66) (Figure S9, Supporting Information), with that of the Pt/C determined to 4.00 as the references.This result suggest that FeY-NC catalyst has a more complete 4-electron pathway to reduce O 2 to H 2 O directly.To further confirm the contribution of Y 2 O 3 in the FeY-NC, we conduct the LSV test

Figure 5 .
Figure 5. a) Schematic illustration of the liquid Zn-air battery.b) Open circuit voltages plots.c) The polarization curves of discharging with power density curves of FeY-NC, Fe-NC, and Pt-C catalysts.d) Discharge curves at a constant current density of 5 mA cm −2 .e) Comparison of specific capacities plots at 20 mA cm −2 .f) Discharge voltage and time diagram of catalysts under different current densities of 2, 5, 10, 20, 30, 40, and 50 mA cm −2 .g) Comparison of peak power density of the Zinc-air battery for FeY-NC with the recently reported catalysts.

Figure 6 .
Figure 6.a) LSV curves in 0.1 m HClO 4 (rotation rate: 1600 rpm; sweep rate: 10 mV s −1 ).b) E 1/2 and j k at 0.8 V of five catalysts.c) LSV curves of FeY-NC at various rotation rates in 0.1 m HClO 4 , the inset shows the Koutecky-Levich plots at different potentials.d) LSV curves before and after 5000 CV cycles of FeY-NC and Fe-NC in 0.1m HClO 4 .