A neutral Mn(II)-BIF for electrocatalytic oxygen reduction reaction

Abstract

A neutral Mn(II) boron imidazolate framework (BIF-251) containing an unexpected bridging formate linker was synthesized in situ via a solvothermal reaction without directly using formic acid. This newly developed Mn-BIF ORR electrocatalyst offers valuable insights into the design of Mn-based BIF materials for renewable energy applications.

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The oxygen reduction reaction (ORR) plays a pivotal role in the performance of energy conversion devices, such as fuel cells and metal–air batteries.^1–3 However, the sluggish kinetics of ORR significantly limit the efficiency of these technologies. Although platinum-based catalysts currently offer the highest activity, their high cost, scarcity, and poor long-term stability have driven intense research toward earth-abundant, non-noble-metal catalysts as viable alternatives.^4,5 For example, on the one hand, carbon-supported non-noble-metal nanoparticles, such as Fe and Mn, have been demonstrated to be efficient ORR catalysts.^6,7 On the other hand, metal-free ORR catalysts, including covalent organic frameworks (COFs) and N-doped/hard carbon materials, have shown amazing catalytic behavior.^8,9 Notably, manganese is an attractive transition metal due to its natural abundance, environmental friendliness, and variable oxidation states (Mn²⁺/Mn³⁺/Mn⁴⁺), which can facilitate multi-electron transfer processes essential for efficient ORR catalysis.^10–12

Over the past decade, metal–organic frameworks (MOFs) have been extensively studied as promising electrocatalytic materials. MOFs have emerged as highly attractive crystalline materials, widely recognized across interdisciplinary applications due to their excellent crystallinity, well-ordered internal pores, and large surface areas.^13–17 For instance, certain MOFs have garnered significant attention in energy storage applications, including fuel cells and water oxidation.^18,19 However, most porous MOFs suffer from poor stability, with their frameworks readily degraded in the presence of water, or acidic or alkaline environments—an issue that severely limits their broader application in electrocatalytic ORR. To overcome these limitations, crystalline-material-derived nanoparticles, metal oxides, and even single-atom (SA) catalysts have been explored, showing promising ORR performance.^20–23 Nonetheless, their synthesis often involves high cost, complex procedures, time-consuming processes, and low yields. Therefore, considerable challenges remain before these materials can be practically scaled for industrial energy conversion applications.

To address the urgent energy crisis, new and affordable materials need to be explored. Boron imidazolate frameworks (BIFs), a subclass of MOFs, feature unique tetrahedral ligands and exhibit remarkable chemical stability under alkaline conditions. The first monovalent zeolitic BIF was reported by Jian Zhang et al. in 2009.²⁴ Subsequently, neutral divalent metal-based BIFs were developed using boron imidazolate ligands in combination with auxiliary carboxylate linkers, such as 2,5-thiophenedicarboxylic acid (BIF-90) and 4,4′-oxybis(benzoic acid) (BIF-20).^25,26 Early studies on BIFs focused primarily on gas adsorption and separation. More recently, BIFs have demonstrated promising electrocatalytic properties; for instance, Co-based BIFs have shown excellent activity in the oxygen evolution reaction (OER),²⁶ while Cu-based BIFs have enhanced CO₂ reduction to value-added products such as ethylene.²⁷ Incorporating Mn into BIF architectures is expected to generate atomically dispersed active sites, improve electrical conductivity, and enhance catalytic performance. This study aims to synthesize Mn-based BIFs, tune their physicochemical properties, and assess their potential as non-precious-metal ORR electrocatalysts in alkaline media.

Herein, we report a neutral boron imidazolate framework, Mn[B(im)₄]CHO₂ (BIF-251; im = imidazolate, CHO₂⁻ = formate), where the unexpected formate serves as an auxiliary linker to balance the valence of the framework (Scheme 1). BIF-251 was synthesized under alkaline conditions using DMF and H₂O, demonstrating its stability in alkaline media. Notably, the dense structure of BIF-251 protected against attack by guest species. In addition, Mn ions were known to serve as active sites for ORR electrocatalysis. Unlike traditional porous MOFs, these features of BIF-251 prevented structural collapse and contributed to enhanced electrocatalytic activity and stability.

Scheme 1 Unexpected assembly of neutral Mn(II)-BIFs from B(im)₄⁻ and CHO₂⁻ ligands via an in situ solvothermal reaction.

Phase purity was confirmed by powder X-ray diffraction (PXRD). BIF-251 exhibited excellent chemical and thermal stability, retaining crystallinity in alkaline solution (pH = 13) for at least 6 h (Fig. S1) and showing decomposition above 300 °C, as indicated by thermogravimetric analysis (TGA) (Fig. S2). Notably, BIF-251 demonstrates electrocatalytic activity for the ORR under basic conditions, with a Tafel slope of 139.1 mV dec⁻¹. The structure–activity relationship highlights the influence of framework and Mn coordination on ORR performance.

The crystal structure of BIF-251 was determined by single-crystal X-ray diffraction, revealing that it crystallizes in the tetragonal space group P4/n (Table S1). In the BIF-39-Mn framework,²⁸ the coordinated Mn²⁺ center adopts a six-coordinated, five-connected geometry through bonding with tetradentate B(im)₄⁻ and oxalate ligands (Fig. 1a), exhibiting a distorted octahedral geometry due to the Jahn–Teller effect. In contrast, BIF-251 features a six-coordinated, six-connected Mn²⁺ center coordinated to boron imidazolate ligands and formic acid groups (Fig. 1b), forming a nearly perfect octahedral geometry. This highly symmetric coordination environment corresponds to higher crystallographic symmetry, as explained by crystal field theory. Notably, the in situ formation of formic acid during synthesis plays a critical role in modulating the structural geometry. Each Mn²⁺ center is coordinated by four nitrogen atoms from boron imidazolate ligands, with an Mn–N bond length of 2.134 Å. The remaining two axial positions are occupied by disordered formate groups, with Mn–O bond lengths of 2.157 and 2.148 Å, respectively. In the one-dimensional –Mn–O–CH–O–Mn– chains (Fig. S3), these Mn–O distances fall within the typical range reported for Mn(II) compounds containing formate ligands (2.138–2.321 Å).²⁹ The B(im)₄⁻ ligands link Mn²⁺ centers into layered structures (Fig. 1c), with the formate groups acting as auxiliary linkers within the framework. The four-coordinate boron imidazolate ligands (B(im)₄⁻, −1 charge), together with the Mn²⁺ centers and formate groups, assemble into a neutral three-dimensional framework (Fig. 1d). PLATON analysis indicated a total solvent-accessible volume of 0.0 ų per unit cell (0.0%). N₂ adsorption–desorption measurements further confirmed the dense structure, as the Brunauer–Emmett–Teller (BET) results showed extremely low surface area and pore size (Fig. S4).

Fig. 1 Coordination comparison of BIF-39-Mn (a) and BIF-251 (b); layered structure of Mn-BIF (c); 3D structure of BIF-251 (d). Color code: N (blue), C (gray), O (red), B (pink), Mn (cyan).

To investigate the unexpected presence of the formic group, we employed Raman and infrared (IR) spectroscopy. In the Raman spectrum of BIF-251, the HCOO⁻ ion exhibited similar vibrational modes to manganese(II) formate (Fig. 2a).^29,30 A prominent band at 2835 cm⁻¹ was assigned to the C–H stretching vibration, while the C=O stretching vibration appeared around 1720 cm⁻¹. The C–O stretching and torsional modes were observed in the range 1032–1436 cm⁻¹. Apart from the formate Raman spectra, Fig. 2a also shows the Raman spectra of B(im)₄⁻, highlighting key imidazole bands: ring breathing (∼673 cm⁻¹), C–N stretching (∼1177 cm⁻¹), C–N/C=C stretching (∼1536–1593 cm⁻¹), and high-frequency C–H stretching (∼2982–3208 cm⁻¹). The weaker out-of-plane bending mode (∼818 cm⁻¹) confirmed B–N bonding. The IR spectrum of the formic acid group also revealed two characteristic bands associated with its vibrational modes (Fig. 2b), excluding contributions from the imidazole ring, which appeared around 1625 cm⁻¹. Specifically, bands near 1208 cm⁻¹ and 620 cm⁻¹ corresponded to C–O stretching and O–C=O bending modes, respectively. Furthermore, energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) elemental mapping images confirmed the uniform distribution of Ni, C, N, and O in BIF-251 (Fig. 2c and Fig. S5). These features indicated retention of CHO₂⁻ coordination and incorporation of B(im)₄⁻ into the neutral hybrid framework.

Fig. 2 (a) Raman spectrum and (b) IR spectrum of BIF-251. (c) SEM image and corresponding elemental mapping of BIF-251.

Given the presence of potentially active Mn(II) sites and its good alkaline stability, BIF-251 prompted us to tentatively explore its electrocatalytic activity toward the ORR. The ORR performance of the BIF-251 catalyst was first evaluated to explore its potential application as an Mn-based boron imidazolate framework (BIF) electrocatalyst. Cyclic voltammetry (CV) measurements were first performed in 0.1 M KOH electrolyte saturated with either N₂ or O₂. As expected, the CV curves obtained under N₂ saturation showed no distinct redox features, indicating an absence of electrochemical reactions in the inert atmosphere. In contrast, the CV curves under O₂ saturation exhibited clear oxygen reduction peaks, confirming the electrocatalytic activity of BIF-251 (Fig. 3a). To further investigate its performance, linear sweep voltammetry (LSV) was conducted using a rotating disk electrode (RDE) at 1600 rpm with a scan rate of 10 mV s⁻¹ in O₂-saturated 0.1 M KOH. The LSV results showed characteristic ORR behavior. Measurements were also conducted across a range of rotation speeds (400–2500 rpm) to study mass transport effects (Fig. 3b). From the LSV data, the half-wave potential (E_1/2) of BIF-251 was determined to be 0.6 V, which showed performance comparable to some reported MOFs, such as the porphyrinic zirconium MOF (PCN-226, E_1/2 ≈ 0.60 V) and ZIF-67 (E_1/2 ≈ 0.60 V).^31,32 The corresponding Tafel slope was 139.7 mV dec⁻¹ (Fig. 3c), indicating moderate reaction kinetics. Although the ORR performance of BIF-251 does not yet surpass that of the benchmark Pt/C catalyst (Fig. S6), it significantly outperforms isolated boron imidazolate ligands (Tafel slope: 148.0 mV dec⁻¹; Fig. S7). Thus, upon completion of the self-assembled framework, the coordinated Mn(II) centers play an important role during the ORR process. This highlights the potential of structurally engineered BIFs in next-generation non-precious-metal ORR catalysts. Finally, the stability and durability of BIF-251 were assessed. After 3000 continuous cycles, the LSV curves showed minor changes (ΔE_1/2 = 0.01 V; Fig. S8 and S9), likely due to slight material loss. Moreover, the catalyst demonstrated excellent methanol tolerance, with only minimal changes in CV after methanol crossover tests (Fig. 3d), underscoring its promise for use in direct methanol fuel cells.

Fig. 3 (a) Cyclic voltammetry (CV) curves of BIF-251 in O₂- and N₂-saturated 0.1 M KOH electrolyte. (b) Linear sweep voltammetry (LSV) curves of BIF-251 electrocatalyst at different rotation rates. (c) Tafel slope of the BIF-251 electrocatalyst. (d) Methanol tolerance test of the BIF-251 electrocatalyst.

To evaluate the stability of BIF-251, XPS, SEM, and TEM analyses were conducted after electrocatalytic activity tests. The X-ray photoelectron spectroscopy (XPS) survey spectrum confirmed the presence of Mn, O, N, and B elements in BIF-251 (Fig. 4a). In the Mn 2p region (Fig. 4b), two prominent peaks at 642.1 and 653.7 eV correspond to Mn²⁺ 2p_3/2 and Mn²⁺ 2p_1/2, respectively, indicating that Mn remains in the divalent state. Additionally, peaks observed at 191.7, 284.8, 400.0, and 531.7 eV are attributed to B 1s, C 1s, N 1s, and O 1s regions, respectively (Fig. S10–S13). SEM images revealed that the crystal retained its original block-like morphology after catalysis (Fig. 4c and d). In addition, EDS and elemental mapping confirmed that all elements still retained a uniform distribution throughout the sample (Fig. S14 and S15). Finally, TEM analyses indicated that the morphology and composition remained largely unchanged after testing (Fig. S16). These results suggested that the framework of BIF-251 remained unchanged after catalysis, supporting its feasibility as a potential ORR electrocatalyst.

Fig. 4 (a) XPS survey spectrum of BIF-251. (b) High-resolution Mn 2p XPS spectrum. SEM images of BIF-251 catalyst (c) before and (d) after electrocatalysis.

In conclusion, a neutral, alkaline-stable Mn(II)-based boron imidazolate framework (BIF-251) was synthesized, featuring an unexpected formic acid group as the charge-balancing anion. Interestingly, infinite 1D –Mn–O–C–O–Mn– chains link the layered Mn-BIF frameworks into a 3D structure. Leveraging the Mn(II) coordination nodes with potential catalytic activity, Mn-BIF was directly employed as an ORR catalyst for the first time. This work opens up a new avenue for designing chemically stable Mn-based MOF electrocatalysts for energy-related applications.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The authors confirm that the related data supporting the findings of this research will be available in the article and its supplementary information (SI). Supplementary information such as TGA, PXRD, SEM, TEM, XPS, CCDC number, and Crystal data, is available. See DOI: https://doi.org/10.1039/d5dt01965f.

CCDC 2476471 contains the supplementary crystallographic data for this paper.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Grants 22272086) and Natural Science Foundation of Sichuan Province (2023NSFSC0009).

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