Octahedral spinel electrocatalysts for alkaline fuel cells

Significance

We present a systematic study of a family of 15 types of octahedral spinel electrocatalysts for the oxygen reduction reaction (ORR) in alkaline fuel cells. Three specific candidates, MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄/C, exhibited very promising ORR activity. In particular, CoMn₂O₄/C could rival the ORR activity and selectivity of Pt/C. Such performance was ascribed to a partial charge transfer between Co and Mn. The most active, MnCo₂O₄/C, exhibited a unique Co–Mn core–shell structure with heterogeneous valence states, which were probed at the single-nanoparticle level using scanning transmission and high-energy-resolution electron-loss near-edge structure. We believe that our approach, of combining advanced electron microscopy and spectroscopy, will enable the design of highly active nanocatalysts with unprecedented atomic-scale precision.

Abstract

Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB₂O₄/C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄/C. CoMn₂O₄/C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo₂O₄/C was shown to have a unique Co-Mn core–shell structure. ELNES spectra indicate that the Co in the core is predominantly Co^2.7+ while in the shell, it is mainly Co²⁺. Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells.

As high-efficiency energy-conversion devices, proton exchange membrane fuel cells (PEMFCs) have been recognized as a crucial technology for powering electric vehicles (1–4). However, PEMFCs rely on increasing the amount of expensive Pt-based electrocatalysts to facilitate the sluggish oxygen reduction reaction (ORR) at the cathode. The catalyst cost is projected to be the largest single component (up to 40%) of the total cost of a PEMFC (5). Numerous past studies have been devoted to designing Pt-based alloy catalysts to partially replace the Pt with less-expensive metals (6–9). However, the US Department of Energy target for total Pt loading of ∼0.1 g_Pt/kW has yet to be achieved (10). As an alternative, anion exchange membrane fuel cells (AEMFCs) have drawn increasing attention, because they enable the use of nonprecious metal electrocatalysts, which are stable in alkaline media (11, 12). To facilitate the ORR kinetics in alkaline media, research efforts have been devoted to searching for electrocatalysts, including precious-metal–based alloys (13, 14), nitrogen-doped carbons (15–17), perovskites (18), and transition-metal oxides (19). Due to their high activity, long-term durability, and low cost, 3d transition-metal oxides with the spinel structure are a novel family of ORR electrocatalysts in alkaline fuel cells.

For the ORR and oxygen evolution reaction (OER) in alkaline media, 3d metal oxides have been intensively studied as electrocatalysts. Cobalt oxides and manganese oxides have been reported to be effective electrocatalysts for the ORR (20, 21). Furthermore, Co-based bimetallic oxides have been reported to exhibit enhanced activities for the ORR in alkaline media (22–30). Despite numerous studies of 3d metal oxide electrocatalysts, the electrocatalytic mechanism remains poorly understood (13, 19). One of the larger challenges is that their ORR activities depend on the synthetic methods employed by different research groups, and therefore different particle sizes, morphology as well as surface and crystal structures of the nanoscale electrocatalysts. These factors are formidable to control simultaneously and complicate the understanding of the intrinsic activities of these electrocatalysts. In this work, we prepared a family of 15 different AB₂O₄ spinels as ORR electrocatalysts with well-controlled particle size and morphology. We systematically investigated their ORR activity and selectivity and established correlations with the structure and local chemical environment, using macroscopic-level X-ray absorption spectroscopy (XAS) and microscopic-level scanning transmission electron microscopy (STEM) equipped with electron energy-loss spectroscopy (EELS).

Results and Discussion

A family of 15 different spinel metal oxides, AB₂O₄, (A: Mn, Fe, Co, Ni, and Cu, and B: Mn, Fe, and Co), were synthesized using a facile hydrothermal method (SI Appendix, Fig. S1). Briefly, 1 or 2 metal precursors were dissolved in water and reacted with ammonium hydroxide to form coordination compounds with NH₃ ligands (SI Appendix, Fig. S2). Monometallic or bimetallic hydroxides gradually precipitated from the solution through aging processes, at controlled temperatures, with selected solvents. The metal hydroxides subsequently formed metal-oxide nanoparticles, supported on high-surface-area carbon Ketjen black (HSC KB) with a mass loading of 40%, upon reaction in an autoclave at a mild temperature (150 °C) under a modest pressure of about 30 bar. Using powder X-ray diffraction (XRD), the resulting products were found to be single-phase spinels (SI Appendix, Fig. S3). The Co-based spinel family (ACo₂O₄) has a regular cubic spinel structure in which the A atoms occupy the tetrahedral sites while Co atoms occupy the octahedral sites (SI Appendix, Fig. S4). ACo₂O₄/C (A = Mn, Fe, Ni, and Cu) exhibited similar XRD patterns as Co₃O₄/C, with (311) as the strongest peak (SI Appendix, Fig. S3A). The XRD patterns indicated that they all adopt the cubic spinel structure. XRD patterns of MnCo₂O₄/C and FeCo₂O₄/C exhibited a slight shift of the (311) reflections to lower angles, relative to Co₃O₄/C, which is likely due to the lattice expansion upon replacing Co with Fe or Mn, as expected from the larger cationic radii of Mn and Fe relative to Co (31). In contrast to Co₃O₄, the Mn-based spinel family (AMn₂O₄) had a tetragonal spinel structure (SI Appendix, Fig. S3B), due to the Jahn–Teller distortion (32). The XRD patterns of AMn₂O₄/C exhibited more diffraction peaks at around 30° and 60°, when compared to ACo₂O₄/C, indicating a decrease in crystal symmetry (SI Appendix, Fig. S3B). The XRD patterns of the Fe-based spinel family, (AFe₂O₄/C), on the other hand, adopt a cubic inverse spinel structure, where all of the A atoms and half of the Fe atoms occupy octahedral sites, while the other half of the Fe atoms occupy the tetrahedral sites. The XRD patterns of AFe₂O₄/C (A = Mn, Co, Ni, and Cu) suggest a cubic spinel structure, similar to that of Fe₃O₄/C (SI Appendix, Fig. S3C). When compared to Fe₃O₄/C, AFe₂O₄/C exhibited intriguing peak shifts of the (311) reflections: a shift to 2θ values in the case of MnFe₂O₄/C and a shift to higher 2θ values for CoFe₂O₄/C, NiFe₂O₄/C, and CuFe₂O₄/C. These observations likely originate from the fact that relative to Fe, Mn has a larger atomic radius while Co, Ni, and Cu have smaller radii, causing lattice expansions and contractions, respectively. Average domain sizes of AB₂O₄/C nanoparticles were analyzed using the Scherrer equation (SI Appendix, Table S1).

With the crystal structures of AB₂O₄/C confirmed by XRD, the ORR activities of AB₂O₄/C were preliminarily evaluated using the rotating-disk electrode (RDE) technique. The ORR polarization profile for Pt/C exhibited a half-wave potential (E_1/2) of 0.890 V vs. the reversible hydrogen electrode (RHE), a benchmark activity value for Pt in 1 M KOH (dashed lines in Fig. 1 A–C). The ORR polarization profiles of ACo₂O₄/C in Fig. 1A suggest that ACo₂O₄/C reaches a transport limited current, I_d, close to that of Pt/C, indicating that the main reaction product is water, as is the case on Pt. It should be noted that, based on the Levich equation, the I_d for the 4e⁻ reduction of oxygen in 0.1 M oxygen-saturated KOH or HClO₄ should be −5.5 mA/cm² at 1,600 rpm. Since the O₂ solubility in 1 M KOH (as used in the present work) at 25 °C and 1 atm is 8.42 × 10⁻⁴ mol/L, which is 70% of its value in 0.1 M KOH (1.21 × 10⁻³ mol/L) (33), the I_d for the 4e⁻ reduction of oxygen in 1 M KOH will be correspondingly lower with a value of −3.8 mA/cm² at 1,600 rpm. The ORR profiles of ACo₂O₄/C were magnified in SI Appendix, Fig. S5A to better illustrate the difference in the early kinetic-controlled region (0.85 to 1.0 V). E_1/2, onset potential (E_onset), and I_d values are summarized in SI Appendix, Table S2 for better quantitative and comparative assessment. Among the 5 types of ACo₂O₄/C catalysts, MnCo₂O₄/C clearly stands out as the most active candidate in the Co-based spinel family with an E_1/2 value of 0.855 V vs. RHE. The mass-specific activity (MA) at 0.85 V was employed as a metric value to partially evaluate the initial electrocatalytic activity. As shown in Fig. 1D, the MA at 0.85 V of MnCo₂O₄ (43 A/g) was about 1.5× as high as that of monometallic Co₃O₄/C (27 A/g), and over 2×, relative to ACo₂O₄/C (A = Fe, Ni, and Cu), respectively. For the Mn-based spinel family, ORR polarization profiles of AMn₂O₄/C demonstrate that CoMn₂O₄/C has significant ORR activity with an E_1/2 value of 0.844 V, when compared to that of the monometallic Mn₃O₄/C (black curve). However, the AMn₂O₄/C materials involving Fe, Ni, and Cu exhibited a decrease in activity (Fig. 1B). The MA at 0.85 V of CoMn₂O₄/C (31 A/g) was 2.5× as high as that of monometallic Mn₃O₄/C (13 A/g), and over 3× higher than other AMn₂O₄/C involving Fe, Ni, and Cu (Fig. 1E). Finally, the ORR polarization profiles of the Fe-based spinel family, AFe₂O₄/C, exhibited an overall lower activity, relative to the Co-based and Mn-based spinel families. I_d values of AFe₂O₄/C were noticeably lower than that of Pt/C, indicating that a significant amount of peroxide formed (Fig. 1C). Among the Fe-based spinels, the most active, CoFe₂O₄/C, had an MA value (18 A/g) twice as large as those of MnFe₂O₄/C (8.7 A/g) and NiFe₂O₄/C (8.7 A/g) and over a 4-fold increase, relative to Fe₃O₄/C and CuFe₂O₄/C (Fig. 1F).

Fig. 1.

ORR activity of AB₂O₄/C spinel electrocatalysts. (A–C) ORR polarization profiles of 15 different AB₂O₄/C in O₂-saturated 1 M KOH at 1,600 rpm and a scan rate of 5 mV/s. Mass loading of metal oxides and Pt: 0.1 mg/cm² and 25 µg/cm², respectively. (D–F) MA of AB₂O₄/C at 0.85 V vs. RHE, which was calculated by normalizing the kinetic current to the mass loading of the metal oxide. MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄/C were found to be the 3 most active ORR electrocatalysts.

In summary, MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄/C have been demonstrated to be the 3 most active candidates among the 15 types of AB₂O₄/C spinels. The impact of particle sizes and types of carbon substrate are discussed in detail in SI Appendix, Figs. S6 and S7. Cyclic voltammetric (CV) profiles of mono/bimetallic oxides were compared to reveal more information about the active redox couples for catalyzing the ORR (SI Appendix, Fig. S8). Among those, MnCo₂O₄/C exhibited a larger redox current than CoMn₂O₄/C and CoFe₂O₄/C, and a pronounced minor redox couple at 1.05/1.04 V. Detailed analysis of the CV profiles suggests that this minor redox couple comes from monolayer species on the surface with a highly reversible reaction process (SI Appendix, Fig. S9), which may partially explain the highest initial activity of MnCo₂O₄ among all AB₂O₄/C materials. MnCo₂O₄/C and CoMn₂O₄/C exhibited smaller Tafel slopes of 46 and 49 mV/dec, respectively, relative to Pt/C (66 mV/dec), suggesting a smaller overpotential to achieve the same kinetic current change (SI Appendix, Fig. S10).

After studying the ORR activity of AB₂O₄/C and corresponding redox couples, the catalyst selectivity was further investigated using the rotating ring-disk electrode (RRDE) technique. The ORR selectivity (H₂O vs. H₂O₂) of Pt/C was first examined using a glassy carbon (GC)-disk and Pt-ring RRDE, with the ring potential held at 1.3 V vs. RHE to oxidize any peroxide generated at the GC disk (34). As shown in SI Appendix, Fig. S11, Pt/C exhibits a potential-dependent H₂O₂ yield of 2∼5% and an electron transfer number of 3.98 to 3.90. RRDE profiles of CoMn₂O₄/C exhibited a peak ring current of ∼35 µA, larger than that of Pt/C (∼8 µA), indicating a larger fraction of peroxide formation. H₂O₂ yield and electron transfer number were calculated to be around 20% and 3.6, respectively. Inspired by previous studies on the impact of loading of Fe-N-C catalysts on H₂O₂ formation (35–38), RRDE profiles of CoMn₂O₄/C at various loadings were measured. When the metal-oxide loading was increased from 0.1 to 0.5 mg/cm², a significant improvement in ORR activity was observed with the E_1/2 shifting positively by 30 mV from 0.86 V at 0.1 mg/cm² to 0.89 V at 0.5 mg/cm². This last value is essentially the same as the measured benchmark value of Pt/C (0.89 V) (Fig. 2A). The ring current dropped noticeably at higher loadings and the H₂O₂ yield at 0.9 V decreased from 18% at 0.1 mg/cm² to 12.5% at 0.25 mg/cm², and finally to as low as 5% at 0.5 mg/cm² (Fig. 2 B and C). The electron transfer number at 0.9 V increased from 3.65 at 0.1 mg/cm² to as high as 3.9 at 0.5 mg/cm² (Fig. 2D). A similar improvement was also observed for MnCo₂O₄/C (SI Appendix, Fig. S12). The peroxide yield of Co–Mn oxides in alkaline media was comparable to previous reports on similar catalysts (39). At an ultrahigh loading level of 1.0 mg_{metal oxide}/cm², the ORR polarization profile of CoMn₂O₄ in the mass-transport-controlled region (E < 0.8 V) no longer followed the typical diffusion-limited characteristic, indicating significant mass-transport limitations arising from the very thick catalyst layer. Therefore, we propose that metal oxides, at higher loading, can provide a higher number of active sites and a sufficiently thick catalyst layer to decompose a significant portion of the electrogenerated H₂O₂ before it escapes from the catalyst layer into solution_. The high loadings of Co–Mn oxides are critical in practical membrane–electrode assembly measurements for a high peak power density of > 1 W/cm² (SI Appendix, Fig. S13), as we reported, in detail, in our recent work (25, 26).

Fig. 2.

ORR selectivity of CoMn₂O₄/C at various mass loadings of metal oxides. (A and B) RRDE profiles of disk and ring currents measured on a GC-disk and Pt-ring RRDE in O₂-saturated 1 M KOH at 1,600 rpm and a scan rate of 5 mV/s. Ring potential was held at 1.3 V vs. RHE to oxidize the H₂O₂ electrogenerated at the disk. (C and D) H₂O₂ yields and electron numbers calculated based on ring and disk currents. CoMn₂O₄/C at an optimal loading of 0.5 mg/cm² achieved an E_1/2 value of 0.89 V, equal to the benchmark Pt/C, and a H₂O₂ yield of 5% at 0.85 V, comparable to Pt/C.

In order to establish why MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄ stand out as the 3 most active electrocatalysts toward the ORR, thorough investigations of the morphology, crystal structure, and local chemical environment were carried out employing macroscopic-level XAS and microscopic-level STEM-EELS. The X-ray absorption near-edge structure (XANES) of Mn in Fig. 3A indicate that gradually increasing the Co contents in Mn-based spinels, from Mn₃O₄/C to CoMn₂O₄/C and MnCo₂O₄/C, leads to a progressive positive shift of the Mn-edge energy, indicating a higher Mn valence state and charge transfer from Mn to Co. Symmetrically, a gradual increase in the Mn contents in Co-based spinels, from Co₃O₄/C to MnCo₂O₄/C and CoMn₂O₄/C, results in a negative shift in the Co-edge energy, again suggesting a lower Co valence state and charge transfer from Mn to Co (Fig. 3B). This local interaction indicates that Mn and Co have mutually altered their electronic structures, which may explain the fact that both MnCo₂O₄/C and CoMn₂O₄/C exhibit significantly enhanced ORR activity, relative to their monometallic oxide counterparts, Mn₃O₄/C and Co₃O₄/C, respectively. XANES spectra of Fe-based spinels are discussed in SI Appendix, Fig. S14.

Fig. 3.

Powder XANES spectra of spinel MnCo₂O₄/C and CoMn₂O₄/C. (A and B) XANES spectra of Mn and Co K edges of Mn₃O₄/C, CoMn₂O₄/C, MnCo₂O₄/C, and Co₃O₄/C, respectively. Black arrows indicate the positive and negative shifts in the edge energy of Mn and Co, respectively. The symmetrical changes in their electronic structures indicate synergistic effects between Co and Mn, which may explain the pronounced ORR activity of MnCo₂O₄/C and CoMn₂O₄/C.

The morphology and microstructure of MnCo₂O₄/C and CoMn₂O₄ were examined using an aberration-corrected STEM. Fig. 4 A and B exhibit the well-defined octahedral morphology of MnCo₂O₄ nanoparticles. The STEM image intensity varies at different regions of the MnCo₂O₄ nanoparticles (Fig. 4A and SI Appendix, Fig. S15), indicating the possible existence of nanometer-scale pores or channels on the surface or inside the metal oxide nanoparticle, which may expose the inner part of the metal oxide to O₂ molecules (39–41). The particle size distribution histogram of ∼200 MnCo₂O₄ nanoparticles indicates an average particle size of 35 nm (SI Appendix, Fig. S16). The majority of the MnCo₂O₄ nanoparticles appear to have multiple subdomains since the domain size, calculated from XRD, was 15 nm. CoMn₂O₄/C nanoparticles were found to have similar octahedral morphology and multiple subdomains (SI Appendix, Fig. S17). Furthermore, atomic-scale high-angle annular dark-field (HAADF)-STEM images of MnCo₂O₄ clearly show the spinel structure on the [110] zone axis (Fig. 4C and SI Appendix, Fig. S18 A and B). MnCo₂O₄ exhibits hexagonal repeating unit cells, as indicated by the dashed boxes with 2 d-spacings of 3.6 and 2.1 Å (Fig. 4C), which match well with the theoretical values (3.64 and 2.12 Å), based on the crystal model of MnCo₂O₄ (powder diffraction file [PDF] 01–084-0482). The atomic columns, labeled b and c, on the side of the MnCo₂O₄ unit cell in Fig. 4C, show a lower intensity than that of atomic columns, labeled a, at the center. The intensity variation comes from the fact that atomic columns, a, have a higher atom density than those of b and c, based on the tilted [110] zone axis (Fig. 4E). Atomic-scale STEM images of CoMn₂O₄ exhibit the characteristic layered structure on the $\left[01\overline{1}\right]$ zone axis of a tetragonal spinel (Fig. 4F and SI Appendix, Fig. S18 C and D). Two d-spacings of 4.9 and 3.0 Å, which were perpendicular to each other, matched well with the theoretical values (4.88 and 3.04 Å), respectively, based on the crystal model of CoMn₂O₄ (PDF 01–077-0471) (Fig. 4 F and G). Atomic-scale electron EELS of the Mn L₂ edge provides evidence of the layered structure on the $\left[01\overline{1}\right]$ zone axis (Fig. 4 F, Inset). It is of particular interest to observe, in Fig. 4F, that atomic columns, b, on the side, show a lower intensity than atomic columns, a, at the corner. This image intensity variation also originates from the fact that atomic columns, b, exhibit a lower atom density than atomic columns, a, based on the tilted $\left[01\overline{1}\right]$ zone axis in Fig. 4H. A similar layered structure was also observed for MnCo₂O₄ though at a different $\left[11\overline{2}\right]$ zone axis (SI Appendix, Fig. S19).

Fig. 4.

HAADF-STEM images of spinel MnCo₂O₄/C and CoMn₂O₄/C. (A and B) Octahedral MnCo₂O₄ nanoparticles with an average particle size of 35 nm supported on HSC KB carbon. (C–E) Atomic-scale STEM image of MnCo₂O₄ and the corresponding crystal model projected on the [110] zone axis. Atom columns a exhibit brighter intensity than b and c due to the higher atom density, as shown in the tilted crystal model in E. (F) Atomic-scale STEM image of CoMn₂O₄ on the $\left[01\overline{1}\right]$ zone axis. (Inset) Atomic-scale EELS elemental mapping of Mn showing the characteristic layered structure in spinels. (G and H) Crystal model of CoMn₂O₄ corresponding to the image in F with 2 perpendicular lattice planes of (011) and (200). Atom columns a exhibit higher intensity than b due to the higher atom density, as shown in the tilted crystal model in H.

The chemical composition of selected metal oxides was then investigated using EELS. EELS elemental maps were extracted from the sharp Mn, Co, and Fe L₃ edges of the core-loss EELS spectra (SI Appendix, Fig. S20). Fig. 5 A–C present an octahedral MnCo₂O₄ nanoparticle with the corresponding EELS elemental maps of Mn (red) and Co (green). The composite map of Mn vs. Co of MnCo₂O₄ exhibits a core–shell structure with 2 to 4-nm Mn shell on the surface (Fig. 5D). We observed that CoMn₂O₄/C (Fig. 5 E–H) and CoFe₂O₄/C (Fig. 5 I–L) have a relatively homogeneous elemental distribution of Co vs. Mn and Co vs. Fe, respectively. Atomic-scale STEM-EELS of MnCo₂O₄ suggests smooth elemental gradients from a Co-rich core to an Mn-rich shell without a distinct boundary (SI Appendix, Fig. S21). This helps corroborate the previous CV analysis of the minor redox couple of MnCo₂O₄/C (SI Appendix, Fig. S9), as the STEM-EELS mapping of MnCo₂O₄ further suggests that the monolayer species on the surface is Mn-rich, which may serve as a key structural factor for the pronounced ORR activity of MnCo₂O₄.

Fig. 5.

EELS elemental mapping of MnCo₂O₄/C, CoMn₂O₄/C, and CoFe₂O₄/C. (A–D) STEM image of MnCo₂O₄/C and the corresponding EELS elemental maps of Mn and Co. Composite map in D exhibits a Co–Mn core–shell structure with a 2- to 4-nm Mn shell. (E–L) STEM images of CoMn₂O₄/C (E) and CoFe₂O₄ (I), respectively, and the corresponding EELS elemental maps of Mn–Co (F–H) and Fe–Co (J–L), respectively. Composite maps of Mn vs. Co in H and Fe vs. Co in L suggest relatively homogeneous distributions of Mn with Co and Fe with Co, respectively.

To gain a detailed description of the local electronic structure of MnCo₂O₄/C, we performed fine-structure EELS investigations with high spatial resolution. We employed the energy-loss near-edge structure (ELNES) that reflects the density of unfilled states (unfilled DOS) above the Fermi level (E_F), which is particularly sensitive to the local atomic environment, such as valence state, chemical bonding, and coordination (predominantly nearest-neighbor) (42). Measurements of the fine structure can help us understand the electronic structure of the catalysts and ultimately establish its correlation with electrocatalytic performance. Normally 3d transitional metals have sharp L edges in core-loss spectra (SI Appendix, Fig. S20), which can be further divided into an L₂ edge (2p^1/2 to 3d^3/2 transition) and a stronger L₃ edge (2p^3/2 to 3d^3/23d^5/2 transition). In this work, ELNES spectra with an energy resolution of 0.5 eV (SI Appendix, Fig. S22) were first calibrated against the simultaneously acquired zero loss peak and then employed to fingerprint the catalyst structure.

We further investigated the core–shell structure of MnCo₂O₄ to see how the compositional changes across the nanoparticle affect the local binding of both Mn and Co using ELNES (Fig. 6). We first obtained reference ELNES spectra for Co and Mn in 2+, 2.67+, and 3+ valence states (SI Appendix, Figs. S23 and S24) using CoO (MnO), Co₃O₄ (Mn₃O₄), and CoOOH (Mn₂O₃), respectively. The edges progressively shift to higher energy values for higher valence states, due to the larger energy loss for ejecting electrons from metal p orbitals at higher metal valence states. Larger L₃ to L₂ intensity ratios at higher metal valence states also indicate more covalent metal–oxygen bonds (43). Co₃O₄ and Mn₃O₄ (II,III) appear to have ELNES spectra between +2 and +3 valence states, and the spectra can be reproduced from linear combinations of CoO, MnO (II) and CoOOH, Mn₂O₃ (III), respectively (43). Once the reference spectra were obtained, we compared them to the ELNES of the MnCo₂O₄ core–shell structure. Shell sections at the 4 corners were labeled as S-1, S-2, S-3, and S-4 (Fig. 6 A and B). ELNES spectra of Co and Mn L edges were processed to remove some of the noise (SI Appendix, Fig. S25). ELNES spectra of Co and Mn in the shell exhibited similar features among the 4 different shell locations (S-1, S-2, S-3, and S-4), suggesting that all shell sections likely have a uniform chemical bonding environment of Co (Fig. 6E) and Mn (SI Appendix, Fig. S26). The ELNES spectrum of the Co L₃ edge in the core resembles the features of Co₃O₄ (II,III), indicating Co in the core is predominantly cubic spinel, and Co²⁺ in the core occupies the tetrahedral sites while Co³⁺ in the core occupies half of the octahedral sites (Fig. 6C). In contrast, Co in the shell appears (convincingly) to be Co²⁺, resembling the feature of the CoO (II) reference (Fig. 6C). Furthermore, ELNES spectra of the Mn L₃ edge indicate that Mn in the core has a larger contribution from higher Mn valence, when compared to Mn in the shell (Fig. 6D). Mn L₃ edges, at both core and shell, are much broader than the Mn oxide references, indicating a less-ordered nearest-neighbor oxygen coordination environment, and a lower crystal symmetry, likely caused by a Jahn–Teller distortion of Mn³⁺[3d⁴] (44). In summary, both Co and Mn have lower metal valences in the shell, suggesting that the shell may have more oxygen vacancies. Compared with the core, where Co occupies both tetrahedral and octahedral sites, Co in the shell prefers to stay in the tetrahedral sites as it is present mainly as Co²⁺. As a comparison, Mn in the shell occupies both tetrahedral and octahedral sites.

Fig. 6.

ELNES of the core–shell MnCo₂O₄/C with an energy resolution of 0.5 eV. (A and B) STEM image and EELS elemental map of 1 MnCo₂O₄ nanoparticle with an Mn-rich shell. Four different regions on the shell for further analysis were labeled as S-1, S-2, S-3, and S-4, respectively. (C) Comparison of average ELNES spectra of Co in the core and shell with standard Co oxide references. Co at the core clearly has a major contribution from Co₃O₄ (II,III) while Co at the shell is predominantly Co²⁺, resembling the feature of CoO (II) reference. (D) Comparison of average ELNES spectra of Mn in the core and shell with standard Mn oxide references. Mn ELNES spectra, with a higher valence in the core than in the shell, exhibit broader features than Co, indicating a less-ordered nearest-neighbor environment. (E) ELNES spectra of Co in the core and shell show the similar features of Co L_2,3 edges among S-1∼4, which indicates the relatively uniform local chemical environment of Co at the shell. (F) ELNES average spectra of oxygen K edge in the core and shell corresponding to the nanoparticle. The higher first-to-second peak ratio of ELNES spectrum in the core indicates a higher metal valence and metal-oxygen bond covalency. Similar ELNES features of the core and shell suggests they possibly share the same spinel structure.

Besides the metal L edges, the oxygen K edge can also provide valuable complementary information about the chemical bonding of the metal center (45). The first and second peaks in the ELNES spectra of the O K edge represent the joint density of states between the O-2p and the partially filled transition-metal 3d bands and empty 4s,p bands, respectively (Fig. 6F). Oxygen in the core exhibits a higher-energy second peak, relative to O in the shell, which normally indicates that the metal center in the core has a higher valence (45). This positive shift agrees with the ELNES spectra of Mn oxide references where the second peak gradually shifts to higher energies as the Mn valence increases from MnO (+2) to Mn₃O₄ (+2.67) and Mn₂O₃ (+3) (SI Appendix, Fig. S27A). Additionally, a larger first-to-second peak ratio in the core indicates that the metal center has a higher metal valence, meaning a higher DOS of unfilled 3d bands available for mixing with the O 2p bands and typically a higher metal covalency. This can also be clearly observed in the ELNES spectra of Co oxide references where CoOOH (+3) and Co₃O₄(+2.67) have larger first-to-second peak ratios than CoO (+2) (SI Appendix, Fig. S27B). The critical spectroscopic evidence of Co, Mn, and O in the core/shell was further confirmed on another MnCo₂O₄ nanoparticle (SI Appendix, Figs. S28 and S29) and examined on 5 other particles for reproducibility. In summary, the overall similarity in ELNES features of the O K edges between core and shell indicates that the core and shell likely share the same spinel structure, which is consistent with XRD as well as the STEM image of a single-crystal spinel MnCo₂O₄ NP (SI Appendix, Fig. S21).

Finally, with the intent of developing a Pt-free cathode for AEMFCs, nonprecious ORR electrocatalysts need to not only exhibit high initial ORR activity, but also achieve long-term durability (4, 5). As shown in SI Appendix, Fig. S30, MnCo₂O₄/C and CoMn₂O₄/C exhibited an activity decay comparable to Pt/C, in terms of the negative shift in the E_1/2. The ORR activity decrease was partially ascribed to the loss of electrochemical surface area, as evidenced by the lower redox peak current after 10,000 CV cycles. Particle aggregation of both MnCo₂O₄/C and CoMn₂O₄/C was observed from STEM images of catalysts after 10,000 cycles (SI Appendix, Figs. S31 and S32), which was consistent with the surface area loss in CV profiles. Interestingly, from the energy dispersive X-ray (EDX) spectroscopy quantitative analysis, the relative contents of Mn and Co were virtually the same before and after CV cycles, indicating a relatively stable elemental distribution through long-term cycling (SI Appendix, Fig. S33). Furthermore, EELS maps of catalysts after durability testing suggest that MnCo₂O₄/C still preserves the Co–Mn core–shell structure although with a thinner Mn shell (SI Appendix, Fig. S34), while CoMn₂O₄/C after 10,000 cycles exhibits the similar uniform distribution of Co and Mn as the initial state (SI Appendix, Fig. S35). The aforementioned structural investigation provides us with a microscopic picture of the catalyst evolution during durability tests.

In summary, a class of 15 compositions of AB₂O₄/C spinel electrocatalysts for the ORR has been rationally designed with a well-controlled particle size and morphology. CoMn₂O₄ could reach an ORR activity and selectivity (H₂O vs. H₂O₂) comparable to Pt/C. Crystal structures of electrocatalysts were thoroughly examined, using both macroscopic-level XAS and microscopic-level STEM-EELS techniques. ELNES spectra provided fine details on the local chemical environment of single MnCo₂O₄ nanoparticles with a unique Co–Mn core–shell structure. The methodology of using analytical electron microscopy and spectroscopy in this work can enable researchers in the broad catalyst community to understand and design electrocatalysts with atomic-scale precision. This in-depth structural investigation will offer insightful strategies for material design and developments in the renewable energy community, in general, and in fuel cells, in particular.

Methods

A class of 15 types of AB₂O₄/C, A = Mn, Fe, Co, Ni, Cu, and B = Mn, Fe, Co, has been synthesized using a facile hydrothermal method. In general, the pH and ethanol/H₂O volume ratio in the solvent controlled the precipitation rate of metal hydroxides, and therefore influenced the final metal-oxide particle size, morphology, and distribution on the carbon substrate. The crystal structures were examined by XRD and XANES at Cornell High Energy Synchrotron Source (CHESS). The microstructures and chemical compositions were investigated by STEM imaging and EELS spectroscopy (Cornell Nion UltraSTEM). The electrochemical activity and selectivity were evaluated in RDE and RRDE measurements in O₂-saturated 1 M KOH at 1,600 rpm and 5 mV/s. More experimental details can be found in SI Appendix.