Au–MIL Nanocomposites with Enhanced Borohydride Oxidation Kinetics for Potential Use in Direct Liquid Fuel Cells

Abstract

A series of metal–organic framework-based materials of the MIL-101 family was prepared for potential application as anodic electrocatalysts in the direct borohydride fuel cell. The MIL-101-based materials were tested for borohydride oxidation reaction using cyclic voltammetry and chronoamperometry in alkaline media, with Au@MIL-101-NH₂ showing high responsiveness. The obtained data allow for the determination of kinetic parameters that characterize the borohydride oxidation on the prepared electrocatalysts. The activation energy for borohydride oxidation using an Au@MIL-101-NH₂ electrocatalyst was as low as 13.6 kJ mol⁻¹ with a reaction order of 0.4. The anodic charge transfer coefficient was 0.85, and the number of transferred electrons was 7.97, matching the theoretical maximum value of 8 electrons transferred during the borohydride oxidation reaction. The promising performance of Au@MIL-101-NH₂ suggests its potential application as an anode for direct borohydride fuel cells.

1. Introduction

The development of sustainable and eco-friendly energy technologies has become increasingly crucial due to growing concerns about environmental degradation and rising global energy demands. Among these technologies, fuel cells represent a promising pathway for clean energy conversion, with various types being explored for different applications. These include proton exchange membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), direct methanol fuel cells (DMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) [1]. SOFCs, for instance, offer high efficiency and fuel flexibility, while more recent developments in single-layer fuel cells simplify device architecture and reduce fabrication costs [2]. Still, their high operating temperature presents challenges in terms of material selection. PEMFCs, on the other hand, operate at lower temperatures and are currently considered the benchmark for most fuel cell applications, including fuel cell electric vehicles. Hydrogen fuel cells, in particular, hold the potential for decarbonizing the transportation and aviation sectors, offering advantages such as lower weight, compactness, and rapid startup times [1]. However, challenges to their widespread adoption remain, including the high cost of hydrogen production and distribution, as well as the need for adequate infrastructure for large-scale production, storage, and supply.

One possible strategy to overcome the drawbacks associated with hydrogen use is the development of direct liquid fuel cells (DLFCs), which work with liquid fuels. These can be organic, such as methanol or ethanol, or inorganic, such as ammonia borane, hydrazine, or sodium borohydride solutions. DLFCs offer several advantages over traditional hydrogen fuel cells, including simplified fuel storage and transport. For example, methanol fuel cells can be utilized in small, portable devices, such as mobile phones and laptops. Among the various DLFCs, the direct borohydride fuel cell (DBFC) has gained attention due to its unique benefits. DBFCs operate at relatively low temperatures and use sodium borohydride (NaBH₄) alkaline solutions as the fuel. Their advantages encompass high energy density, low fuel cost, simplified fuel handling, and enhanced safety. These features make DBFCs particularly attractive for specialized applications, such as in space and underwater vehicles [3].

Two possible reactions occur in a DBFC: the desired borohydride oxidation reaction (BOR) (Equations (1)–(3)) and the detrimental borohydride hydrolysis (Equations (4)–(6)). The latter decreases the Coulombic efficiency, thereby harming the cell’s efficiency, power output, and longevity [4,5].

Anode: BH4− + 8OH− → BO2− + 6H2O + 8e−	E0 = −1.24 V vs. SHE	(1)
Cathode: O2 + 2H2O + 4e− → 4OH−	E0 = 0.40 V vs. SHE	(2)
Overall: BH4− + 2O2 → BO2− + 2H2O	E0 = 1.64 V	(3)
Step 1: BH4− + H2O → BH3OH− + H2		(4)
Step 2: BH3OH− + H2O → BO2− + 3H2		(5)
Overall: BH4− + 2H2O → BO2− + 4H2		(6)

When performed under ideal conditions and without side reactions, the BOR involves transferring eight electrons, with an equilibrium cell voltage of E₀ = 1.64 V [5,6,7]. The unwanted borohydride hydrolysis reaction, which generates a hydroxyborohydride intermediate and gaseous hydrogen [4], results in heterogeneous, non-faradaic hydrogen evolution, leading to a decrease in cell performance, fuel concentration, and Coulombic efficiency [8,9,10]. Thus, considering both concurrent processes, the reaction taking place at the DBFC anode is better described by Equation (7), with n representing the number of exchanged electrons [4].

BH₄⁻ + nOH⁻ → BO₂⁻ + (n − 2)H₂O + (4 − ½ n)H₂ + ne⁻

(7)

To achieve optimal conditions and to prevent reactions that negatively affect DBFC performance, the most critical factors in cell design are the selection of the membrane, the electrolyte, and the electrocatalysts. The latter should be chosen based on high electrocatalytic activity for BOR, high selectivity with minimal activity toward competing hydrolysis reactions, stability (over time, concentration, and temperature), durability, and low cost [11,12,13]. Identifying suitable and promising electrocatalysts for DBFCs is a common goal within the research community. Specifically, this work examines the potential of solid MIL-based materials as anodic electrocatalysts for potential application in DBFCs.

DBFCs are set to play an essential role in developing sustainable energy solutions due to their numerous advantages. Their high energy efficiency, environmental friendliness, and ability to use renewable fuels make them an appealing alternative to traditional power sources. Moreover, their scalability and versatility across various applications further enhance their attractiveness in the transition toward cleaner energy systems. The electrolyte in DBFCs is a highly alkaline NaBH₄ solution. NaBH₄ is an exciting energy carrier due to its high hydrogen content of 10.6 wt.% [3], which enables it to be used as an anodic fuel or hydrogen storage medium.

Finding suitable and promising electrocatalysts for DBFC is crucial for the development of these power sources. Gold (Au) is widely recognized as an effective catalyst for BOR, as Au-based electrocatalysts can prevent borohydride hydrolysis, thus hindering parasitic hydrogen gas generation during borohydride anodic oxidation at the anode. To better understand the BOR mechanism and facilitate the rational design of alloy catalysts, Rostamikia and Janik conducted a thorough investigation using density functional theory (DFT) [14]. Their study aimed to unravel the complexity of the eight-electron BOR process by elucidating the reaction mechanism, including key intermediates, pathways, and rate-determining steps, in order to identify the catalyst materials that can enhance DBFC efficiency. To address the challenges of experimental mechanistic studies, DFT was used to provide atomic-level insights and to guide catalyst optimization. A notable difference was observed between Au and platinum (Pt) surfaces in their activity toward B–H bond cleavage. DFT calculations showed that BH₄⁻ adsorbs molecularly but unfavorably on Au(111), becoming thermodynamically viable only at potentials near 0 V vs. NHE. Conversely, adsorption on Pt(111) is dissociative and energetically favorable at all relevant potentials, but this results in high surface H* coverage and promotes unwanted hydrogen evolution. Therefore, an ideal catalyst should allow molecular BH₄⁻ adsorption with stronger binding than Au, enabling oxidation at lower potentials while suppressing hydrogen generation. These findings underscore the significance of DFT in providing mechanistic insights and facilitating the rational design of efficient and selective BOR catalysts [14].

At the experimental level, numerous studies have been conducted to develop effective Au-based electrocatalysts. For example, Yi et al. tested carbon-supported Au–Fe bimetallic nanocatalysts (Au–Fe/C) as anodic electrocatalysts for DBFCs. Their results showed that Au–Fe/C catalysts showed high catalytic activity for the direct electrooxidation of BH₄⁻, with the Au₅₀Fe₅₀/C catalyst exhibiting the best activity among the tested materials. A DBFC with an Au₅₀Fe₅₀/C anode and an Au/C cathode achieved a peak power density of 34.9 mW cm⁻² at 25 °C [15].

Duan et al. studied carbon-supported Co–Au bimetallic nanoparticles (NPs) with different Co/Au atomic ratios, prepared through a two-step reduction in a reverse microemulsion system, as anode electrocatalysts for BOR. They demonstrated that the Co₄–Au₁/C catalyst exhibited the highest catalytic activity, and a DBFC assembled with a Co₄–Au₁/C anode achieved a maximum power density of 102.4 mW cm⁻² [16].

Xue et al. developed a multisite CoFe&AuC BOR catalyst that combined high activity, selectivity, and a low onset potential. The catalyst demonstrated excellent performance in DBFCs, achieving a peak power density of 580 mW cm⁻² with air as the oxidant and 1371 mW cm⁻² with O₂. These outstanding results were achieved through a tandem catalytic mechanism, where CoFe layered double hydroxides initiate BH₄⁻ oxidation to BH₃, which is then oxidized on Au NPs, thereby enhancing the overall selectivity and efficiency for a BOR [17].

Raul et al. synthesized Au_yNi_100-y NPs supported on multi-walled carbon nanotubes (MWCNTs) via a reverse micelle method and evaluated them as anode electrocatalysts for direct borohydride-hydrogen peroxide fuel cells (DBHPFCs). The Au_yNi_100-y/MWCNT catalysts demonstrated high electrocatalytic performance, with optimal compositions showing enhanced activity, durability, and efficiency for BOR. The improved performance was attributed to synergistic effects between Au and Ni as well as the high dispersion of NPs on the MWCNT support [18]. Raul et al. also synthesized bimetallic Au_xCo_100-x NPs supported on MWCNTs to develop efficient anode electrocatalysts for a BOR in alkaline media. A comprehensive set of structural and electrochemical characterizations was conducted. Among the tested compositions, Au₇₄Co₂₆/MWCNT exhibited the highest current density (24.15 mA cm⁻²), while Au₄₉Co₅₁/MWCNT demonstrated the highest specific current (1127.03 mA mg⁻¹). Au₇₄Co₂₆/MWCNT also displayed the lowest apparent activation energy (8.22 kJ mol⁻¹), the smallest charge transfer resistance (134.9 Ω), and a number of exchanged electrons of 4.70, indicating superior BOR performance [19].

Xue et al. developed an oleylamine-modified, partially oxidized Au catalyst supported on carbon (AuC-OLA) to enhance the BOR in DBFCs. Density functional theory (DFT) calculations revealed that BH₄⁻ adsorbs more strongly on oxidized Au surfaces than on metallic Au, promoting the oxidation of BH₄⁻ to *BH₃. The AuC-OLA catalyst, exhibiting a stable oxidation state, achieved a peak power density of 143 mW cm⁻², twice that of commercial 40 wt.% Au/C (Pretemek). An in situ FTIR analysis confirmed that the improved activity was due to enhanced BH₄⁻ adsorption on the oxidized Au sites [20].

Gaudin et al. employed scanning electrochemical cell microscopy to evaluate the intrinsic electrocatalytic activity of individual Au NPs for BOR. Their single-entity measurements revealed significant heterogeneity: 67.4% of individual Au NPs exhibited no BOR activity, while others with similar size and morphology showed widely varying performances. These findings indicate that not all drop-cast Au NPs are electrocatalytically active, and that catalyst performance is highly dependent on the local environment and interparticle interactions. They emphasized the significance of a particle-by-particle analysis in comprehending and optimizing the BOR electrocatalysts [21].

Yi et al. synthesized a ZIF-derived N-doped carbon-supported Au NP catalyst co-doped with Co and Ni (Au/CoNi@NC/C) as an anode electrocatalyst for BOR. Physical characterization revealed an average particle size of approximately 8.5 nm. Electrochemical tests showed that Au/CoNi@NC/C exhibited significantly higher catalytic activity for the BOR compared to carbon-supported Au NPs (Au/C), highlighting the beneficial effect of Co and Ni co-doping in improving electrocatalytic performance [22].

Saha et al. used targeted electrochemical cell microscopy to examine the BOR kinetics on individual, shape-controlled Au NPs. A high-throughput single-particle analysis showed considerable variability in electrocatalytic activity and stability among the particles. The BOR activity showed a shape dependence, increasing in the order triangles < spheres < octahedra < rods. Moreover, voltammetric features revealed surface deactivation/reactivation dynamics at the NP level, emphasizing the significance of nanoscale heterogeneity in the catalytic performance [23].

Balčiūnaitė et al. optimized the fabrication of AuNi bimetallic catalysts supported on arrays of self-ordered titania nanotubes (AuNi-TiO₂ntb) through anodization, electroless Ni plating, and galvanic displacement. These catalysts, with 1.74 to 15.7 μg Au cm⁻², were tested for a BOR in alkaline media. Fuel cells using AuNi-TiO₂ntb anodes showed increased electrocatalytic activity compared to Ni-TiO₂ntb alone, reaching maximum power densities ranging from 104 to 170 mW cm⁻² at 25–55 °C, with improved performance linked to Ni layer thickness and Au loading [24].

Yi et al. synthesized ZIF-derived Co and N co-doped porous carbon-supported Au NPs (Au/CoNPC) through a straightforward pyrolysis method. A structural and compositional analysis confirmed successful doping and NP dispersion. Electrochemical measurements revealed that the Au/CoNPC-40 catalyst exhibited the highest activity for BOR, achieving a peak current density of 48.4 mA cm⁻² and a number of exchanged electrons of 7.5, indicating effective catalytic performance [25].

Yi et al. synthesized an N-doped, carbon-coated Co₂P-supported Au nanocomposite (Au/Co₂P@NC/C) using a simple pyrolysis method and evaluated its performance for BOR. Characterization confirmed successful synthesis and unique structural features. Among the catalysts tested, Au₍₅₀₎Co₂P@NC₍₅₀₎/C showed the highest electrocatalytic activity, achieving a BOR current density of 48.4 mA cm⁻² at −0.1 V, which is significantly higher than the 24.8 mA cm⁻² observed for an Au/C catalyst [26].

Uzundurukan et al. successfully synthesized Pt, Au, and PtAu bimetallic catalysts supported on a carbon nanotube–graphene hybrid (CNT-G) and examined their electrocatalytic activity for DBPFCs. The PtAu/CNT-G catalyst exhibited synergistic effects, leading to improved catalytic performance and reaching a peak power density of 139 mW cm⁻² at 50 °C, demonstrating its potential as a high-performance anode electrocatalyst [27].

Recently, metal–organic frameworks (MOFs) have shown great promise as anodic electrocatalysts. MOFs have gained popularity as catalyst supports due to their large surface area, non-toxic nature, affordability, tunable morphology, remarkable optical and chemical properties, and their ability to be recycled and reused [28,29,30,31]. MOFs can be utilized as energy acceptors or as catalytic sites to enhance energy-related chemical reactions in alternative energy cycles [5,32,33], thereby garnering significant attention for potential applications in the energy field. They have demonstrated an excellent gas storage capacity at room temperature, which has accelerated their rapid growth, resulting in the development of many novel adsorbents with potential applications in fuel cells, vehicle gas tanks, and stationary power facilities [6].

Given the strong potential of MOF-based materials for electrochemical applications, particularly for BOR, MIL-101(Cr), a chromium-based MOF, where MIL stands for ‘Materials Institute Lavoisier’, was selected for this study. MIL-101(Cr) is known for its high surface area, which rivals or surpasses that of activated carbon, large pores, and exceptional stability [34]. Thus, MIL-101(Cr) is a promising material for application as an electrode material, especially well-suited as a support for silver NPs. Furthermore, amine functionalization can improve both the performance and stability of the electrode [35].

The present work prepared MIL composites by incorporating Au NPs to improve the reaction kinetics and to boost cell performance. The choice of Au was based on previous studies reporting enhanced Au performance as a BOR electrocatalyst due to a Coulombic number close to eight, thereby enhancing the desired direct oxidation of BH₄⁻, while remaining inactive for the competing undesired hydrolysis [36,37,38]. Thus, this work assessed, for the first time, the activity of functionalized Au–MIL composite electrocatalysts (Au@MIL-101-NH₂ and Au@MIL-101-SH) for BOR, to explore their potential application as cost-effective anodes for DBFCs.

2. Materials and Methods

2.1. Chemicals

The following chemicals were used in the synthesis of the materials: 2-aminoterephthalic acid (C₈H₇NO₄, 99 wt.%), 4-aminobenzoic acid (C₇H₇O₂N, 99 wt.%), 4-mercaptobenzoic acid (C₇H₆O₂S, 99 wt.%), chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 99 wt.%), cysteamine (C₂H₇NS, 99 wt.%), dimethylformamide (DMF, 99.99 wt.%), glacial acetic acid (CH₃CO₂H, 100 wt.%), gold(III) chloride solution (HAuCl₄, 99.99 wt.%, 30 wt.% in dilute HCl), hydrofluoric acid (HF, 40–45 wt.%), sodium borohydride (NaBH₄, 99 wt.%), sodium hydroxide (NaOH, 98 wt.%), and terephthalic acid (C₈H₆O₄, 98 wt.), all supplied by Sigma-Aldrich (St. Louis, MO, USA) unless otherwise noted. Additionally, 4-mercaptobenzoic acid was obtained from Acros (Waltham, MA, USA), 4-aminobenzoic acid from Alfa Aesar (Waltham, MA, USA), and sodium hydroxide (98 wt.%) from Fluka (Darmstadt, Germany). For electrochemical studies, Nafion (5 wt.%) was purchased from Sigma-Aldrich, ethanol (C₂H₅OH, 96 vol.%) from Manuel Vieira & Companhia (Irmão) Sucessores, Lda (Lapas, Portugal), sodium borohydride (95–98 wt.%, synthesis grade) from Scharlau (Barcelona, Spain), and sodium hydroxide (99–99.5 wt.%, pellets, analytical reagent grade) from Thermo Fisher Scientific (Waltham, MA, USA). All solutions were prepared using Millipore water (Milli-Q^®, Merck, Jaffrey, NH, USA).

2.2. Synthesis of the Materials

MIL-101(Cr) derived materials were obtained following previously described procedures. To prepare MIL-101, a mixture of Cr(NO₃)₃·9H₂O (2 mmol), terephthalic acid (2 mmol), and HF (0.1 mL) in 10 mL of deionized water was stirred at room temperature for 15 min and then transferred to an autoclave. The reaction mixture was left in an oven at 220 °C for 9 h. The product was collected by centrifugation, washed twice with DMF, and stirred in ethanol at 70 °C under reflux for 6 h. The isolated sample was dried overnight at 60 °C and 120 mbar.

MIL-101-NH₂ was prepared by mixing Cr(NO₃)₃·9H₂O (1 mmol), 2-aminoterephthalic acid (1 mmol), and NaOH (2.5 mmol) in 7.5 mL of deionized water. This mixture was stirred at room temperature for 15 min and then transferred to an autoclave. The reaction mixture was left in an oven at 150 °C for 12 h. The product was collected by centrifugation, washed twice with DMF, and then stirred in ethanol at 70 °C under reflux for 6 h. The isolated sample was dried overnight at 60 °C and 120 mbar.

MIL-101-SH was prepared by post-synthetic modification of previously obtained MIL-101. MIL-101 (0.5 g) and cysteamine (1 mmol) were dispersed in 10 mL of ethanol in a round-bottom flask. This mixture was stirred at 80 °C under reflux for 16 h. The sample was then recovered by centrifugation, washed 3 times with ethanol, and dried overnight at 60 °C and 120 mbar.

To prepare the Au@MIL composite materials, 100 mg of each material was dispersed in 10 mL of an aqueous HAuCl₄ solution (5.1 × 10⁻² mM). The mixture was sonicated for 5 min and then stirred for 5 days. The materials were then recovered by centrifugation and redispersed in 5 mL of deionized water, stirring in an ice bath. To this mixture, 5 mL of an aqueous solution of NaBH₄ (0.044 M) was added, and the mixture was stirred for 1 h. The samples were recovered by centrifugation, washed several times with deionized water, and dried at 75 °C and 120 mbar overnight. Based on their own previous experience with preparing different composite materials using MOFs as supports (e.g., POMs@MOFs and NPs@MOFs, among others), the authors found that the contact time between the guest and host is crucial for efficient incorporation/immobilization, and short periods can be limiting. Therefore, 5 days were chosen to ensure sufficient contact time and to prevent this from being a limiting factor in preparing the Au@MIL-101 composites.

Scheme 1 illustrates the preparation procedures for the MOF materials (MIL-101, MIL-101-NH₂, and MIL-101-SH), as well as the preparation of the Au@MIL-101-SH composite, representative of the Au@MIL-101 and Au@MIL-101-NH₂ composites.

2.3. Physicochemical Characterization

Powder X-ray diffraction (PXRD) patterns were obtained at room temperature on a Rigaku Geigerflex diffractometer operating with a Cu radiation source (λ₁ = 1.540598 Å; λ₂ = 1.544426 Å; λ₁/λ₂ = 0.500) and in a Bragg–Brentano θ/2θ configuration (45 kV, 40 mA). Intensity data were collected using a step-counting method (step size 0.026°) in continuous mode within the 3° ≤ 2θ ≤ 70° range. Fourier-transform Infrared (FT-IR) spectra were obtained using the attenuated total reflectance (ATR) mode of a PerkinElmer FT-IR System Spectrum BX spectrometer (Waltham, MA, USA). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were conducted using a Quanta 400 ESEM FEG (FEI, Hillsboro, OR, USA) high-resolution scanning electron microscope equipped with an EDAX Genesis X4M EDS spectrometer (AMETEK, Inc., Berwyn, PA, USA), operating at 15 kV. Samples were coated with a thin Au/Pd film via sputtering using an SPI-Module Sputter Coater (SPI Supplies, West Chester, PA, USA). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to quantify Au concentrations in various samples using a PerkinElmer Optima 4300 DV (Waltham, MA, USA).

2.4. Electrochemical Characterization

MIL-based working electrodes with a well-defined surface area were fabricated using catalyst inks applied to a polished glassy carbon electrode from ElectraSyn 2.0 (0040002842, IKA, Wilmington, NC, USA). Catalyst inks were made by combining 100 μL of Nafion 0.5 wt.% as the binder, 400 μL of ethanol, 600 μL of H₂O, and 5 mg of the corresponding electrocatalyst material. Each ink was sonicated for 30 min in an Emmi-08ST stainless steel ultrasonic cleaner with heater (EMAG, Mörfelden-Walldorf, Germany) to ensure complete dispersion of the MIL-based catalyst. A total of 15 µL of catalyst ink was carefully drop-cast onto the 0.6 cm² surface of the glassy carbon electrode using a micropipette and then dried in an oven for 15 min at 50 °C.

The electrochemical measurements to examine the BOR kinetics were performed in an 80 mL glass cell with a three-electrode configuration connected to a Squidstat™ potentiostat (Admiral Instruments, Tempe, AZ, USA). Each MIL-based material was used as the working electrode, with a Pt mesh serving as the counter electrode. All potential measurements were conducted versus a saturated calomel electrode (SCE) with saturated KCl, which was then converted to the reversible hydrogen electrode scale for further data analysis. The electrochemical characterization was performed in a solution of 0.03 M NaBH₄ in 2 M NaOH, serving as the supporting electrolyte to provide a strong alkaline environment (pH = 14.3). The materials’ activity for the BOR and suitability for their use as anodic electrocatalysts for DBFC were first screened in general electrochemical characterization via cyclic voltammetry (CV). CV analyses were performed in the potential range of 0.2 to 1.6 V vs. RHE at a scan rate of 50 mV s⁻¹ and an operating temperature of 25 °C, first solely in 2 M NaOH solution and then with the addition of 0.03 M NaBH₄. After this, the dependency of current density on scan rate was examined by performing CV analyses in the same solution with increasing scan rates from 5 to 1000 mV s⁻¹. These studies allowed for the determination of the number of exchanged electrons, n, and the anodic charge transfer coefficient, α. Based on the initial screenings, Au@MIL-101-NH₂ was selected for further electrochemical studies as it showed the highest activity toward BOR. The following studies focused on the effect of temperature on the CV analysis in the 25–65 °C range, which enabled the determination of the activation energy for the BOR at Au@MIL-101-NH₂. The effect of BH₄⁻ concentration in the CV analyses, in the range of 0.01 M to 0.12 M NaBH₄, was also assessed, allowing for the calculation of the reaction order, β.

3. Results and Discussion

3.1. Characterization of the MIL-Based Materials

MIL-101(Cr) is a well-known and highly stable chromium terephthalate MOF built around {Cr₃F(H₂O)₂O[C₆H₄(CO₂)₂]₃·nH₂O} building units to form a mesoporous zeotype architecture with an MTN topology, characterized by its large pore dimensions (29 and 34 Å) and surface area [39]. Figure 1 shows the PXRD patterns recorded for the prepared MIL-101(Cr) based samples.

The diffraction profile observed for MIL-101 matches the characteristic and previously reported pattern for this crystalline framework, indicating its successful preparation [40]. MIL-101-SH retains the diffraction pattern recorded for the original sample, which suggests that the framework was maintained after post-synthetic modification. The pattern acquired for MIL-101-NH₂ similarly shows the expected diffraction profile, although it indicates a comparatively low degree of crystallinity. Au@MIL-101 displays all the original diffraction peaks observed for the parent sample, indicating the preservation of its structure after incorporating the Au particles. The same is observed for Au@MIL-101-SH, although two major diffraction peaks characteristic of Au particles can be identified at 38.2 and 44.3° [41]. The fact that these are not observable for Au@MIL-101 indicates the formation of considerably smaller particles in this case. The diffractogram acquired for Au@MIL-101-NH₂ suggests a somewhat diminished degree of crystallinity of the parent framework after the incorporation of the Au particles, the presence of which is also inferred from the diffraction pattern.

Figure 2 shows FT-IR spectra recorded for the various MIL-101-based samples. The spectra display all the absorption bands expected from the MIL-based materials’ molecular structure: a medium-intensity absorption band at 585 nm related to (Cr-O) bond vibrations, along with weak and strong intensity absorption bands attributed to the linkers (C=C) and (O-C-O) bond vibrations in the range between 1700 and 1350 nm [42,43]. Additional bands recorded for MIL-101-NH₂ and its respective composite are due to bond vibrations of the free linker. These bands are detectable because of the low degree of crystallinity and the resulting presence of structural defects. These data support and reinforce the PXRD results. The maintenance of the main vibrational bands of the solid supports, in this case MIL-101, MIL-101-NH₂, and MIL-101-SH, after the incorporation of Au, further indicates that the chemical bonds of the MOFs are preserved (thus supporting that the three-dimensional structure of MOFs is globally conserved).

The SEM micrographs and EDS spectra obtained for the various Au@MIL composites are shown in Figure 3. For Au@MIL-101 (Figure 3A), a uniform distribution of the Au particles between 10 and 25 nm in diameter is observable. Au@MIL-101-SH (Figure 3C) shows a higher size distribution of the Au particles between 15 and 65 nm in diameter. From the MIL-101 sample series, much larger and more dispersed Au particles are observable for Au@MIL-101-NH₂ (Figure 3B) with particle size diameters between 110 and 170 nm.

Overall, it should be emphasized that, even though the powder diffractogram of the Au@MIL-101-NH₂ composite revealed a material with a degree of crystallinity considerably lower than the remaining materials, the global XRD pattern indicates the typical MIL-101 framework, although less crystalline, less organized, and with some structural defects. On the other hand, as the FT-IR spectra were acquired in the ATR mode, this resulted in some vibrational modes, such as -OH, -NH, and -SH (among others), being very weak or practically undetectable. Nevertheless, the EDS spectra of the MIL-101-SH and MIL-101-NH₂ (see Figure S1 of the Supplementary Materials) identify these elements.

3.2. Cyclic Voltammetry Studies

The electrocatalytic activity of the MIL-based working electrodes towards BH₄⁻ oxidation was first assessed through cyclic voltammetry (CV) in a BH₄⁻ solution at 25 °C. Figure 4a illustrates the CV analyses of the Au@MIL-101-NH₂ electrode, recorded both in a plain 2 M NaOH solution and after adding 0.03 M NaBH₄. A distinct response in the BH₄⁻ solution confirms the efficacy of this attempt to use functionalized MOFs combined with Au NPs as a novel approach for improving the BOR electrocatalysis, as an alternative to using carbonized MOFs, thus avoiding high-energy-consuming pyrolysis processes.

The CV analysis in the borohydride solution shows a shape typically obtained when using Au-based electrocatalysts. It exhibits two oxidation peaks on the anodic scan, as well as a third oxidation peak in the backscan. The first anodic peak (a₁) for Au@MIL-101-NH₂ occurs at a potential E_a1 of 1.11 V with j_a1 of 19.9 mA cm⁻², and the smaller, less significant anodic peak (a₂), at E_a2 of 1.36 V with j_a2 of 9.98 mA cm⁻². In the backscan of Au@MIL-101-NH₂ CV_, a sharp oxidation peak (c₁) occurs at E_c1 of 1.30 V and j_c1 of 54.5 mA cm⁻². This c₁ peak results from the oxidation of adsorbed intermediates and is not relevant for the BOR analysis at the tested electrocatalysts; conversely, no oxidation peaks are observed in the CV analysis run in the absence of NaBH₄ (Figure 4a).

When comparing the anodic scan of the two MIL-based materials towards the BOR at 50 mV s⁻¹ (Figure 4b), it is clear that Au@MIL-101-NH₂ achieves a considerably higher a₁ peak current (19.9 mA cm⁻²) compared to Au@MIL-101-SH (1.76 mA cm⁻²), suggesting significantly higher activity towards the electrooxidation of the BH₄⁻ ion. This becomes particularly noteworthy when considering that Au@MIL-101-NH₂ contains less than one quarter of the Au loading of Au@MIL-101-SH (

Table 1. Molality, concentration, mass of Au, peak current density, and specific peak current of the studied Au-containing MIL-based electrocatalysts.

	Au@MIL-101-NH2	Au@MIL-101-SH
bAu (μmolAu gMIL−1)	77.8	330
cAu (mgAu gMIL−1)	15.3	65
mAu (μgAu)	1.74	7.38
jp (mA cm−2)	19.9	1.76
ip (A μgAu−1)	11.4	2.38 × 10−4

).

Table 1 shows the relation between the total gold loading on each MIL electrode and the maximum current density observed. The molality (b_Au, μmol_Au g_MIL⁻¹) was converted to concentration (c_Au, mg_Au g_MIL⁻¹) by multiplying by 0.197 mg_Au μmol⁻¹, based on the molar mass of gold. The gold mass on the electrode (m_Au) was calculated by multiplying the gold concentration (c_Au, mg_Au g_MIL⁻¹) by the applied electrocatalyst mass on the working electrode, which was 0.045 mg.

Among the tested materials, Au@MIL-101-NH₂ exhibits the highest current output per loading. The comparison between Au@MIL-101-NH₂ and Au@MIL-101-SH reveals notable differences in gold loading and electrocatalytic performance for the BOR. Au@MIL-101-SH has a significantly higher gold molality, b_Au, of 330 μmol_Au g_MIL⁻¹ compared to 77.8 μmol_Au g_MIL⁻¹ for Au@MIL-101-NH₂, which is also reflected in the gold concentration, c_Au, of 65 mg_Au g_MIL⁻¹ for Au@MIL-101-SH, more than four times the 15.3 mg_Au g_MIL⁻¹ in Au@MIL-101-NH₂. Accordingly, the total mass of gold (m_Au) deposited on the working electrode follows the same trend, with Au@MIL-101-SH containing 4.43 μg_Au of gold, compared to 1.04 μg_Au for Au@MIL-101-NH₂. However, despite the lower gold content, Au@MIL-101-NH₂ exhibits superior electrocatalytic performance, with a peak current density, j_p, of 19.9 mA cm⁻², significantly higher than the 1.76 mA cm⁻² observed for Au@MIL-101-SH. This indicates that Au@MIL-101-NH₂ is much more efficient in the BOR catalysis. Furthermore, the specific peak current, which measures current per unit gold mass, is 11.4 A μg_Au⁻¹ for Au@MIL-101-NH₂, about five orders of magnitude higher than the 2.38 × 10⁻⁴ A μg_Au⁻¹ for Au@MIL-101-SH. This stark difference highlights the much higher catalytic efficiency of Au@MIL-101-NH₂. The results suggest that the functionalization of the MIL-101 structure plays a crucial role, with the amine-functionalized material, Au@MIL-101-NH₂, outperforming the thiol-functionalized version, Au@MIL-101-SH. The enhanced activity of Au@MIL-101-NH₂ is likely due to improved electron transfer, better Au NP dispersion, or more favorable interactions between the [NH₂] groups and Au, which promote superior electrocatalytic performance for BOR. In fact, the enhancement of MIL catalytic activity upon adding the [NH₂] functional group has been previously reported by Eom et al. [35]. Likely, the less crystalline (i.e., more disordered) support in Au@MIL-101-NH₂ has some effect on the electrocatalytic activity. There are many cases in catalysis where less crystalline MOFs, or MOFs with defects, have a positive impact on catalytic activity. Furthermore, the advantages of Au–MIL composites are also attributed to the unique 3D framework and stability of MIL-101. It has two distinct mesoporous cages with internal diameters of 29 Å and 34 Å, respectively. The smaller cages feature pentagonal windows with an opening of 12 Å, while the larger cages possess both pentagonal and larger hexagonal windows with an aperture of 14.5 Å × 16 Å, enabling the efficient incorporation and dispersion of Au NPs. Thus, the prepared materials’ structural properties, high surface area, tunable porosity, and functional group modification effectively contribute to their enhanced catalytic activity.

The BOR activity of Au@MIL101-NH₂ was further studied by running CV analyses at increasing scan rates from 5 to 1000 mV s⁻¹, as shown in Figure 5. As expected, significantly higher currents were obtained for higher scan rates. Additionally, the peak potentials shifted to more positive values with an increase in the scan rate due to the inherently slow BOR kinetics on the examined electrodes.

Based on the peak potential and peak current density data from Figure 5, respectively, the values of the anodic charge transfer coefficient, α, and the number of exchanged electrons, n, could be determined. The α values were calculated from the equation that relates the peak potential to the natural logarithm of the polarization rate, E_p vs. ln(ν), for irreversible processes (Figure 6a). Assuming the number of electrons involved in the rate-determining step, n_a, is 1, the α value was calculated to be 0.85.

A linear increase in the BOR peak current density with the square root of the scan rate shows that a diffusion-controlled process is occurring at the electrode surface. The n value was calculated from the slope of the j_p vs. ν^1/2 plots (Figure 6b) using the modified Randles–Sevcik equation for irreversible processes, considering a diffusion coefficient, D_BH4⁻, of 1.21 × 10⁻⁵ cm² s⁻¹ [38]. The obtained n value of 7.97 for Au@MIL-101-NH₂ matches the theoretical value of 8 e⁻ exchanged during BOR.

Considering the two different possible oxidation reactions taking place for BH₄⁻ oxidation, the direct one with a total number of transferred electrons of eight (Equation (1)) and an indirect one with typically four transferred electrons (Equation (7)), this n value shows that, for Au@MIL-101-NH₂, the oxidation reaction is direct, not being significantly impacted by detrimental side processes, such as the undesired hydrolysis reaction.

Based on the CV analysis, the BOR kinetics for Au@MIL-101-NH₂ were further examined, starting with the temperature dependence. In practical applications, increasing the operating temperature of a DBFC is expected to increase current density by promoting faster charge transfer at the electrodes’ surface and to enhance mass transfer of the fuel and oxidant within the electrolyte [44]. The latter is driven by a higher diffusion coefficient resulting from reduced electrolyte viscosity [45]. However, it is essential to note that the DBFC operating temperature must not exceed the membrane’s tolerance, as this can lead to increased cell resistance due to partial membrane drying. Additionally, the undesired hydrolysis reaction becomes more favored at higher temperatures, which can decrease current density not only by lowering fuel concentration but by forming H₂ gas bubbles that attach to the electrode surface, thereby reducing the effective surface area [44].

Figure 7a shows the effect of temperature on the CV analysis of Au@MIL-101-NH₂ at 50 mV s⁻¹ in 2 M NaOH + 0.03 M NaBH₄ electrolyte solution. The increase in temperature led to an increase in current densities and a slight shift of the oxidation peaks to more positive potentials. The increase in current density with the operating temperature can be used to determine the respective apparent activation energy, E_a^app, for BOR. By plotting the natural logarithm of the current densities at 1 V vs. RHE against the reciprocal temperature, it was possible to calculate E_a^app from the slope of the Arrhenius plot (Figure 7b) and by using the Arrhenius equation. The E_a^app for Au@MIL-101-NH₂ was determined to be 13.6 kJ mol⁻¹, a notably low value given the low Au loading.

The effect of NaBH₄ concentration on the current density of Au@MIL-101-NH₂ was also examined. Based on the Nernst equation, an increase in the fuel concentration should lead to more negative anode potentials, thereby increasing the open-circuit voltage of a DBFC [46]. Besides this thermodynamic effect, a higher NaBH₄ concentration enhances mass transport, accelerating the BOR kinetics and, consequently, resulting in increased current density. Figure 8a confirms this assumption, as a steady increase in peak current densities is observed with the increasing NaBH₄ concentration from 0.01 M to 0.12 M.

Furthermore, as the concentration increases, the peak potential shifts to more positive values. This is expected in the case of a slow electron transfer, related to an irreversible process, where more time is needed for total BH₄⁻ depletion at the electrode surface. By using the current density values for each BH₄⁻ concentration at a fixed potential of 1.1 V vs. RHE, one could calculate the reaction order, β, from the slope of the ln(j) vs. ln(c) plots, as seen in Figure 8b. The calculated β value for Au@MIL-101-NH₂ was determined to be 0.4, showing a fractional-order reaction for BOR; this suggests the possibility of undesired BH₄⁻ hydrolysis, particularly at higher NaBH₄ concentrations.

To compare the performance of the synthesized Au@MIL-101-NH₂ electrocatalyst with previously reported Au-based electrocatalysts for BOR, it is important to analyze parameters such as the number of exchanged electrons, n, the charge transfer coefficient, α, and the apparent activation energy, E^a_app (

Table 2. Comparison of the kinetic parameters (n, α, β, and E_a^app) for the BOR of Au-based electrocatalysts reported in the literature.

Electrocatalyst	n	α	β	Eaapp/kJ mol−1	Source
Au@MIL-101-NH2	7.97	0.85	0.4	13.6	This work
CoFe&AuC	5.4	-	-	-	[17]
Au50Ni50/MWCNT	5.8	-	-	7.4	[18]
Au74Co26/MWCNT	4.7	-	-	8.2	[19]
AuPPy	4.4	-	-	-	[47]
Au/FeNPC	7	-	-	-	[48]
Au/CoNPC	7.5	-	-	-	[25]
Au/c-IL	2.4	-	1.0	13.8	[49]
Au–RE alloys (RE = Sm, Dy, Ho, Y)	2.4–4.4	0.60–0.83	1.0	16.4–20.2	[4]

).

Most Au-based catalysts report significantly lower n values, ranging from 2.4 to 7.5, indicating partial to nearly complete oxidation of BH₄⁻. For example, CoFe&AuC [17] and Au₅₀Ni₅₀/MWCNT [18] exhibit moderate n values of 5.4 and 5.8, respectively, indicating incomplete borohydride oxidation and suboptimal fuel efficiency. Catalysts such as Au₇₄Co₂₆/MWCNT [19] and gold polypyrrole (AuPPy) [47] exhibit even lower n values (4.4~4.7), underscoring their limitations in driving the complete eight-electron oxidation pathway, which is essential for maximizing energy output. Charge transfer coefficients, when reported, further reveal the kinetics of electron transfer at the electrode interface. The α value varies widely, ranging from 0.60 to 0.83 for Au–rare earth (RE) alloys [4], somewhat below the value of 0.85 for Au@MIL-101-NH₂. As for the reaction order, the β value of 0.4 determined for Au@MIL-101-NH₂ was significantly lower than that reported for Au/c-IL and Au–RE alloys [4,49], close to unity. Apparent activation energies among these catalysts also vary substantially, from as low as 7.4 kJ mol⁻¹ for Au₅₀Ni₅₀/MWCNT to upwards of 20.2 kJ mol⁻¹ for Au–RE alloys, indicating different energy barriers for BH₄⁻ oxidation that directly influence reaction rates. Thus, despite some previously reported catalysts exhibiting low activation energies or a moderate number of exchanged electrons, none fully optimize both kinetics and the completeness of oxidation, indicating the need for improved catalyst designs that effectively balance these factors.

Another factor that has to be taken into account is the catalyst cost. While the incorporation of Au into MOF-based catalysts has shown promise in enhancing electrocatalytic performance for BOR, its economic feasibility cannot be overlooked. Au is a noble metal with limited availability and high cost. However, although Au has currently doubled the price of Pt, the Au content in Au@MIL-101-NH₂ (1.5 wt.%) is notably lower than the traditional 20 wt.% used in commercial Pt/C, compensating for the difference in the two metals’ prices and leading to a final lower cost for the developed electrocatalyst. Furthermore, the strategic use of Au at low loadings within MOFs offers a favorable activity-to-gold ratio, capitalizing on its unique catalytic properties while minimizing resource usage. The porous and tunable nature of MOFs provides a highly accessible and stable environment for Au NPs, contributing to improved catalytic efficiency, stability, and durability compared to conventional supports. Further development of cost-effective and sustainable BOR electrocatalysts should focus on reducing the Au content through alloying with earth-abundant metals, and optimizing the MOF composition and structure to enhance electronic conductivity and active site exposure.

Moreover, since this is the first study reporting the use of MIL-based materials as electrocatalysts for BOR, it is not possible to make a direct comparison of the performance of the herein-tested electrocatalysts with previous reports using the same class of materials. However, the motivation for the present work stems from earlier works reporting the successful use of MIL-101-based materials in other (electro)catalytic processes, particularly for photocatalysis. For example, MIL-101(Cr) was synthesized via an acid-free method and employed as a photocatalyst for CO₂ reduction to methanol under visible light. The catalyst’s structural features were confirmed, and kinetic parameters were evaluated. This catalyst achieved a methanol production rate of 62.4 mmol h⁻¹ g⁻¹ using 100 µL of triethylamine as an electron donor, demonstrating the material’s high catalytic activity and its potential for sustainable CO₂ conversion to methanol [50].

In another study, MIL-101(Cr) was combined with phosphotungstic acid (PTA) to create S-scheme heterojunction photocatalysts (PTA@MIL-101(Cr)-x, x = 50–200) for degrading rhodamine B (RhB) and tetracycline (TC) in wastewater. The optimal composite, P@M-100, achieved 97.81% RhB and 99.9% TC degradation under visible light in 180 min at pH 7, with excellent stability over repeated cycles. DFT calculations confirmed that electrons were transferred to MIL-101(Cr) while holes accumulated on PTA, consistent with an S-scheme mechanism. The synergy between MIL-101(Cr) and PTA enhanced light absorption and charge separation, identifying •O₂⁻ and h⁺ as key reactive species. PTA@MIL-101(Cr)-x catalysts were demonstrated to offer an effective approach for removing organic pollutants from water [51].

The TiO₂@salicylaldehyde–NH₂–MIL-101(Cr) (TS-MIL) photocatalyst was evaluated for atrazine removal from water under visible light. Owing to its high surface area, tunable porosity, and exposed metal sites, TS-MIL exhibited a strong adsorptive performance. At an initial atrazine concentration of 30 mg L⁻¹ and a catalyst loading of 2 g L⁻¹, approximately 90% total removal was achieved within 30 min. Comparative photocatalytic experiments under visible light showed that TS-MIL degraded 78% of atrazine in 60 min, outperforming MIL-101 (37%) and S-MIL (69%). These findings suggest that adsorption was the primary removal mechanism, with photocatalysis contributing to a lesser extent under the tested conditions [52].

Liu et al. reported a Pd@MIL-101/P25 composite featuring a core-shell structure that enables the efficient removal of coexisting Cr(VI) and RhB pollutants, utilizing a promising design that allows for the treatment of complex wastewater. Pd NPs inside MIL-101 facilitated electron accumulation for Cr(VI) reduction, while P25 on the surface promoted hole-driven RhB oxidation. This spatial separation enhanced charge carrier utilization and boosted photocatalytic activity, resulting in 3.4 times and 4.2 times higher Cr(VI) and RhB removal rates, respectively, compared to MIL-101 alone [53].

In another study, a Ni–Fe quasi-metal–organic framework (quasi-MOF) was synthesized by pyrolyzing MIL-101(Fe) and incorporating Ni²⁺ via ion exchange. Partial electron transfer from Ni²⁺ to Fe³⁺ enhanced the formation of active species, improving oxygen evolution reaction (OER) performance. The optimized MIL-101(Fe)₃₅₀-Ni catalyst showed a low overpotential (290 mV at 10 mA cm⁻²), a low Tafel slope (89 mV dec⁻¹), and good stability (5% loss over 20 h) for OER. The strong Fe–Ni interaction enhanced electron transfer efficiency, highlighting quasi-MOFs as promising candidates for efficient and scalable OER electrocatalysis [54].

Another example is the development of a heterojunction composite of Ag₃PO₄ encapsulated in MIL-101(Fe) to efficiently degrade tetracycline (TC) via photocatalysis, Fenton catalysis, and photo-Fenton catalysis under natural sunlight, thereby eliminating the need for artificial energy. The optimal TC degradation rate (2.5730 min⁻¹) was achieved using a 5:1 mass ratio of MIL-101(Fe) to Ag₃PO₄, significantly outperforming other catalyst combinations. The composite also showed reduced photocorrosion, improved reusability, and lower toxicity of degradation products compared to Ag₃PO₄ alone. Both radical and non-radical species were involved in the degradation mechanism. The Ag₃PO₄/MIL-101(Fe) composite was demonstrated to be a sustainable and effective photocatalyst for treating organic pollutants in wastewater [55].

Fe single-atom catalysts (Fe SACs) with FeN_x active sites were developed using NH₂-MIL-101(Al) as a mesoporous precursor. After pyrolysis and Fe(II)-phenanthroline complex impregnation, the resulting Fe SAC-MIL101 catalysts showed enhanced mass transfer and more accessible active sites. The Fe SAC-MIL101-1000 variant demonstrated excellent oxygen reduction reaction performance in alkaline media, with a half-wave potential of 0.94 V, and showed high efficiency in both aqueous and solid-state zinc–air batteries [56].

MIL-101-based composites functionalized with hydrogen bond donor poly(ionic liquids) (PILs), specifically PIL-R@MIL-101 (R = COOH, OH, NH₂), were developed for efficient CO₂-epoxide cycloaddition. Among the variants, MIL-101 incorporated with PIL-COOH exhibited superior catalytic activity, achieving 92.7% conversion of epichlorohydrin under mild, solvent-free, and cocatalyst-free conditions (70 °C, 1.0 MPa, 2.5 h). This performance is attributed to the combined effects of MIL-101’s Lewis acidic Cr centers, carboxyl groups, nucleophilic Br⁻ ions, high surface area (1178 m² g⁻¹), and strong CO₂ affinity. Additionally, MIL-101–PIL-COOH demonstrated simple recoverability via centrifugation and excellent reusability, showing strong potential for CO₂ capture and conversion technologies [57].

These comparative studies underline the exceptional versatility of MIL-101 as a multifunctional platform for both photocatalysis and electrocatalysis. The adaptability of the framework, whether through doping, post-synthetic modification, heterojunction formation, or single-atom incorporation, offers vast potential for tailored catalytic systems. MIL-101-based materials not only demonstrate high activity and selectivity in diverse reaction environments but provide recyclability, structural tunability, and scalability. The continued development of MIL-101 composites with rationally designed architecture is likely to yield even more efficient solutions for various applications in environmental remediation and electrochemical energy conversion, particularly for DBFCs.

4. Conclusions

MIL-based materials have been synthesized and characterized in terms of their morphology, phase composition, physicochemical properties, and crystal structure. The electrocatalytic activity for the BOR of the prepared MIL-based working electrodes was investigated using voltammetric measurements in a three-electrode arrangement. Au@MIL-101-NH₂ revealed outstanding BOR activity, indicated by notable anodic current densities and a well-defined oxidation peak. By contrast, Au@MIL-101-SH showed minimal activity for a BOR. The determined BOR kinetic parameters confirm the exceptional features exhibited by Au@MIL-101-NH₂. As mentioned, the BOR efficiency depends on the electron transfer rate, catalytically active sites, and stability of the electrocatalyst. The incorporation of -NH₂ functional groups into MOFs lowers the MOF’s band gap, enhancing electrical conductivity and accelerating electron transfer, which improves the current output. Their lone-pair electrons facilitate interactions with the BH₄^- ions and their intermediates, promoting favorable adsorption and reaction kinetics. Additionally, the NH₂ groups improve metal NP dispersion and active site formation, while their hydrophilic nature enhances electrolyte access and mass transport. Together, these effects result in higher catalytic activity and improved selectivity, making NH₂-functionalized MOFs highly promising electrocatalyst supports for the BOR applications.

Thus, the enhancement of the BOR kinetics cannot be attributed solely to the presence of Au in the electrocatalyst, but also to the incorporation of the NH₂ functional group. It is therefore of significant interest to determine whether MILs incorporating these complexes exhibit beneficial behavior independently, or if the promising performance observed in this study only occurs in the presence of Au or other noble metals. This investigation is critical not only for improving the technical performance of the DBFCs but for simplifying the synthesis process and for reducing manufacturing costs. These factors are crucial for the commercial competitiveness of DBFCs employing MIL-based anodic electrocatalysts. Future work should focus on evaluating the catalytic activity and long-term stability of functionalized MILs in the absence of noble metals, as this could streamline synthesis and lower production costs, key considerations for the practical deployment of DBFCs.