Metal-Organic Frameworks Derived Catalyst for High-Performance Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow battery (VRFB) is one of the most promising technologies for grid-scale energy storage applications because of its numerous attractive features. In this study, metal-organic frameworks (MOF)-derived catalysts (MDC) are fabricated using carbonization techniques at different sintering temperatures. Zirconium-based MOF-derived catalyst annealed at 900 °C exhibits the best electrochemical activity toward VO²⁺/VO₂⁺ redox couple among all samples. Furthermore, the charge-discharge test confirms that the energy efficiency (EE) of the VRFB assembled with MOF-derived catalyst modified graphite felt (MDC-GF-900) is 3.9% more efficient than the VRFB using the pristine graphite felt at 100 mA cm⁻². Moreover, MDC-GF-900 reveals 31% and 107% higher capacity than the pristine GF at 80 and 100 mA cm⁻², respectively. The excellent performance of MDC-GF-900 results from the existence of oxygen-containing groups active sites, graphite structure with high conductivity embedded with zirconium oxide, and high specific surface area, which are critical points for promoting the vanadium redox reactions. Because of these advantages, MDC-GF-900 also possesses superior stability performance, which shows no decline of EE even after 100 cycles at 100 mA cm⁻².

1. Introduction

Because of rapid industrial development, global energy demand is rising quickly. The excessive use of fossil fuels causes environmental pollution and immediate consumption of these non-renewable resources. Sustainable energy sources such as solar and wind have been explored in recent years to alleviate these problems. However, such sustainable energy sources are intermittent and random, inducing low-quality output electricity and influencing power grid stability. Therefore, developing efficient energy storage systems (ESSs) is vital to integrate sustainable energy sources into the electrical grids supply [1]. Among various ESSs, a vanadium redox flow battery (VRFB) is a good choice since it possesses plenty of advantages, such as large-scale design flexibility, long lifetime, and low maintenance costs [2]. Moreover, it can be charged and discharged by transforming the oxidation state of vanadium ions both on the positive side (VO₂⁺/VO²⁺) and the negative side (V²⁺/V³⁺). As reactive molecules on both sides are vanadium ions, the crossover problems between two half-cells can be avoided.

The electrolyte of VRFB contains sulfuric acid. Thus, typical electrode materials in VRFB need to be highly anti-corrosive and highly stable, such as graphite felt (GF), carbon cloth, and carbon felt. Nevertheless, these carbon fiber-based materials have hydrophobic features, poor reversibility in kinetics, and few active sites, resulting in poor performance of VRFB [3,4]. Therefore, different modification methods of electrodes, including thermal treatment, chemical treatment, adding catalysts, have been developed recently. Among them, applying catalysts to modify the electrodes is one of the valuable methods. These catalysts can be roughly divided into metallic, metal oxides, and non-metallic materials. Metallic catalysts like In [5], Ir [6], and Pt [7] have been utilized owing to their high electrochemical activity, outstanding conductivity, and anti-corrosion property toward acidic electrolytes. However, these noble metals have lower elemental abundance and higher costs. To diminish the costs, cheap metal oxides such as Nb₂O₅ [8], Ta₂O₅ [9], W₁₈O₄₉ [10], and CeO₂ [11] have been studied and exhibited good electrocatalytic activity. However, since the transition metal cations present several oxidative states, they make oxidation and reduction reactions carry on poor electronic conductivity and hinder the charge transfer efficiency from the catalysts to electrodes. To solve this problem, some carbonaceous materials with high conductivity, such as carbon nanotubes (CNTs) [12,13], graphene [14,15], and carbon nanorods [16], are used to combine with metal oxides catalysts to further improving the performance of VRFB. These carbonaceous materials can be treated as support of other catalysts and can become superior catalysts by themselves. Generally, they positively affect carbon electrodes due to their high specific surface area, good conductivity, and abundant active sites, which have proved high contents of oxygen-functional groups [17].

Besides the different catalysts aforementioned, metal-organic frameworks (MOFs) have attracted lots of attention in catalysts because of their tunable structure, high porosity, and ease of functionalization [18,19]. In addition, MOFs possess a stable structure that is composed of metal-based nodes and organic linkers. However, most MOFs lack good conductivity, so they are seldom used in the electrochemical field. To enhance their conductivity, several methods have been applied, such as building a smooth electron transport route within MOFs [20,21], assembling conductive materials with MOFs [22,23,24], carbonizing MOFs to become porous carbon and metal-containing compounds [25,26]. Among them, the carbonization method is much easier to be conducted and has been tried by few research groups of VRFBs. For example, in 2017, Ge et al. synthesized cobalt phosphide (CoP) nanoparticles from zeolitic imidazole framework (ZIF)-67 by sintering in the air for 5 h at 550 °C, and followed by phosphatization. The as-synthesized CoP catalyst was deposited onto the surface of GF and showed good electrocatalytic activity towards VO₂⁺/VO²⁺ [27]. In 2018, Noh et al. utilized ZIF-8 as seeds to synthesize ZIF-8@ZIF-67 with core-shell structure and carbonization at 900 °C in an argon atmosphere to produce mesoporous N-doped carbon with carbon nanotubes (m-NC@NCNT). Because of its high specific surface area, superior conductivity, and plenty of active sites provided by nitrogen-doping, the single-cell assembled with m-NC@NCNT modified electrodes exhibited 84.2% of energy efficiency (EE) at 120 mA cm⁻², which was much better than that of the control [28]. Recently, Li et al. proposed a new technique to control carbon electrodes’ microstructure and functional doping by carbonizing cobalt-based metal-organic framework (Co-MOF) precursors decorated on GF. The obtained electrode (PCP-800) revealed an excellent VRFB performance [29].

This paper has successfully fabricated ZrO₂ from zirconium-based MOF through a carbonization route at different annealing temperatures and employed it as a catalyst for VRFB (see the supporting information, Scheme S2). The as-synthesized MOF-derived ZrO₂-modified GF electrode exhibits better electrochemical performance towards VRFB due to the higher surface area and porosity of carbon, which are critical points for promoting the vanadium redox reactions.

2. Results and Discussion

2.1. Morphological and Structural Characterization

As shown from the X-ray diffraction (XRD) patterns of Figure 1a, Zr-MOF has strong diffraction peaks at 7.5° and 8.7°, confirming that the successful fabrication of a highly crystalline Zr-MOF structure [30]. However, when carbonization was performed at 500 °C, the original strong diffraction peaks disappear. A low and broad amorphous peak appears at around 30°, indicating that a new crystalline structure is gradually being produced. When sintering at 700 °C, there are four prominent diffraction peaks at 30.2°, 35.3°, 50.2°, and 60.2°, which correspond to the characteristic peaks of tetragonal ZrO₂ (Database: PDF 79-1769). The strongest peak with a sharp shape is formed at 30.2° diffraction from the (101) crystal plane as sintering to 900 °C, signifying that its crystallinity increases with increasing sintering temperature. This result reveals that the main structure of metal-organic frameworks derived from the catalysts at 900 °C (MDC-900) is tetragonal ZrO₂.

Raman spectroscopy uses the Raman scattered light caused by molecular vibration to study the bonding status of materials. It can be seen from Figure 1b that Zr-MOF has Raman scattering peaks at vibration frequencies 633(c), 860(f), 1142(g), 1437(i), 1450(j), and 1615(l) cm⁻¹, which represent “C–C–C benzene ring in-plane bending vibration”, “O–H bending vibration and C–C symmetric ring breathing”, “C–C symmetric ring breathing”, “C–C benzene ring and carboxyl group stretching vibration”, “O–C–O carboxyl group symmetric stretching vibration”, and “C–C benzene ring stretching vibration”, respectively [31]. These signals based on benzene rings prove the presence of terephthalic acid linker molecules in Zr-MOF. After sintering at 500 °C to 1000 °C, the Raman scattering peaks have substantially changed, and the peaks located at 258(a), 466(b), and 636(c) cm⁻¹ can be ascribed to the bending and stretching vibration of tetragonal ZrO₂ [32,33]. Moreover, the peaks which represent disordered carbon and graphitized carbon appear at 1339(h) cm⁻¹ (D-band) and 1588(k) cm⁻¹ (G-band) [34]. The reason for this situation might be the collapse of the benzene ring structure in the Zr-MOF, and the carbon atoms are rearranged to form the graphitized structure. Compared with the D-band and G-band intensity (I_D/I_G) of MDC-500, MDC-700, MDC-900, and MDC-1000, MDC-900 has the lowest I_D/I_G value of 1.05, which shows a higher percentage of graphitic structure with better conductivity. When the sample was treated at a higher sintering temperature (1000 °C), the I_D/I_G ratio of MDC-1000 sample (I_D/I_G = 1.14) is higher than that of MDC-900 (I_D/I_G = 1.05) owing to some breakage of graphitized carbon and the formation of disordered carbon structure at relatively higher temperature [35]. On the other hand, the zirconium and oxygen atoms in the metal-based nodes directly crystallize to form tetragonal ZrO₂. It meant that crystallites of tetragonal ZrO₂ possibly integrated with the carbon structure, and the conductivity might be enhanced compared with pure tetragonal ZrO₂. It is worth mentioning that the scattering peak of 814(e) cm⁻¹ represents “O–H bending vibration” that exists both before and after sintering, confirming that the presence of hydroxyl groups which can serve as active sites on MDC-900.

The SEM image with high magnification (100,000 times) was used to see the morphology difference before and after Zr-MOF’s carbonization was shown in Figure 2. Zr-MOF exhibits polyhedral particles with rounded corners, in which the particle size is about 100 nm with a smooth surface. On the contrary, MDC-900 shows that the polyhedron morphology has been destroyed, and the boundary between the particles has become blurred. Moreover, in the case of MDC-900, the surface is no longer smooth, and some smaller particles are attached to it. Compared with the previous XRD results, it can be found that the tiny particles appearing on the surface of MDC-900 may be tetragonal ZrO₂ grains produced by high-temperature sintering.

TEM images were utilized to further understand the difference in the particles of Zr-MOF before and after carbonization, as shown in Figure 3. From the Zr-MOF image (Figure 3a), it can be found that the particle size is about 100 nm, which is consistent with the SEM image and without noticeable lattice fringe that implies that the amorphous structure of Zr-MOF. After carbonization at 900 °C, there are several tiny grains in MDC-900 particles, and the original MOF particle boundary almost disappears, which corresponds with the SEM image (Figure 3b). To realize the composition of the new crystallites inside the particles, it can be found in Figure 3c that the diameter of small crystallites produced by sintering is about 7 nm, and the lattice d-spacing is obtained to be 0.296 nm, corresponding to the (101) crystal plane of the tetragonal ZrO₂ grains. It is worth mentioning that tetragonal ZrO₂ grains are integrated with amorphous materials, which might be carbon in the molecules of MDC-900. High-angle annular dark-field-scanning TEM (HAADF-STEM) and the corresponding elemental mapping results (Figure 3d–g) verified the excellent distribution of zirconium, carbon, and oxygen in MDC-900 sample area; these results are consistent with observations from Raman spectra and HRTEM in which ZrO₂ grains were embedded with a carbon structure.

MOFs are highly attracted to numerous applications because of their porous structures and extremely high surface areas. To determine the specific surface area of MOF–derived catalysts, Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption analysis was conducted. It can be found that the adsorption and desorption curves of Zr-MOF present a Langmuir isotherm (Figure 4), indicating that it is a microporous structure with a specific surface area of 1145 m² g⁻¹. The specific surface area of Zr-MOF is remarkably reduced to 130 m² g⁻¹ after 900 °C of carbonization, implying that the porous structure of this MOF might be destroyed at high temperature, which is consistent with the results of Raman spectroscopy. However, the specific surface area of MDC-900 °C is much higher than that of commercial ZrO₂ (32 m² g⁻¹), which might be beneficial to improve catalytic activity towards vanadium redox reactions.

X-ray photoelectron spectroscopy (XPS) was utilized to analyze the elemental composition and chemical bonding on the surface of materials. To find the possible reasons why MDC-900 possesses superior electrochemical activity, the survey and narrow scan spectra of XPS were performed in Figure 5. As observed in Figure 5a, the zirconium, carbon, oxygen, and gold peaks were identified. Except for gold which was used to enhance the conductivity of samples during XPS characterization, other elements in the survey correspond with the results of TEM elemental mapping. For the narrow scan spectrum of C 1s, the bonds of C–C, Zr–O–C, and C=O appeared at 284.5, 286.3, and 288.5 eV, respectively (Figure 5b) [36]. The peak of C–C bond showed larger full width at half maximum (FWHM), and the reason could be the dangling bonds with different orientations distributed on the edge of graphitic structure [37]. The signal of C=O bond indicated that some oxygen functional groups grafted on the carbon structure of MDC-900, which might be served as the active sites. A new Zr–O–C bond confirms that the ZrO₂ particles are attached strongly to the carbon material, significantly facilitating the vanadium redox couple reaction [15]. In the O 1s spectrum, the lattice O²⁻ could be found because of the formation of tetragonal ZrO₂ crystallites after sintering at 900 °C. At the binding energy of 531.7 and 533.1 eV, the existence of HO-Zr and C–OH bonds displayed that the hydroxyl groups were proved to be more electrochemically active sites for vanadium redox reactions (Figure 5c) [36,38]. For Zr 3d spectrum, two strong peaks of Zr 3d_5/2 and Zr 3d_3/2 were demonstrated at 182.0 and 184.4 eV, respectively, which resulted from the crystallites structure of tetragonal ZrO₂ (Figure 5d) [36].

2.2. Electrochemical Characterization

For realizing the effect of different sintering temperatures on the electrocatalytic activity of Zr-MOF, a rotating disk electrode (RDE) was applied in the half-cell tests on both the positive and negative electrodes separately. On the positive side (Figure 6a), it can be observed that the potential separation between oxidation and reduction peaks for MDC-900 is 222 mV, which is smaller than other treatments except for MDC-1000. However, MDC-1000 has the lowest peak potential separation, but its oxidation peak current density is only 31.258 mA cm⁻², lower than 39.82 mA cm⁻² of MDC-900. Thus, it indicates that MDC-900 has comprehensively high activity. Notably, the peak separation of Zr-MOF without carbonization is slightly smaller than that of catalyst-free RDE. Still, the peak current density is low, which represents its few active sites.

On the other hand, the peak separation of MDC-500 obtained after sintering at 500 °C increases significantly, and the disappearance of the reduction peak indicates insufficient catalytic activity. When the sintering temperature rises to 700 °C, the catalytic activity improves, and the optimum electrocatalytic activity is reached after carbonization at 900 °C. The essential factors for the activity improvement of MDC-900 sample might be the formation of tetragonal ZrO₂ and graphite structure based on the XRD and Raman spectroscopy results, providing more active sites and better conductivity, respectively. In the previous study, Zhou et al. proposed that the hydroxyl groups on ZrO2 can be served as active sites to assist the electrochemical reactions of vanadium ions [39]. In our case, as ZrO₂ is integrated with a graphite structure that has better conductivity, the synergistic effect between them might further improve the electron transfer from active sites to the electrode surface. However, if the sintering temperature further increases to 1000 °C, the peak current density decreased compared with that of 900 °C, and the reason can be the destruction of graphite structure that can bind with oxygen functional groups active sites provide good conductivity. On the negative side (see Figure S1), no matter whether Zr-MOF was carbonized or not, there is no oxidation peak for all samples. This suggests that all samples have poor catalytic activity towards the oxidation reaction of V²⁺ to V³⁺. The charge transfer resistances of all the catalysts were measured by electrochemical impedance spectroscopy (EIS). As shown in Figure 6b, the Nyquist plot often possesses a semicircle part and sloping straight line part in the three-electrode system, which implies the mixed-controlled process by charge transfer and diffusion steps [1,9]. By fitting with the equivalent circuit, some helpful information can be acquired, including the electrical conductivity. R_s represents the sum of the resistances of the electrolyte and the electrode. Since we use the same electrolyte, and the only difference is the catalysts on the electrode. In other words, if the catalyst had the highest electrical conductivity, the R_s resistance becomes the smallest one. Among the tested materials, MDC-900 has the lowest R_s resistance (3.84 ohm) comparing with those of Zr-MOF (4.48 ohm), MDC-500 (3.95 ohm), MDC-700 (5.53 ohm), MDC-1000 (4.23 ohm), which demonstrates that MDC-900 has the highest electrical conductivity relatively. The results almost correspond with that of the Raman spectra. MDC-900 has the lowest I_D/I_G ratio, suggesting more abundant in graphitic structure with better electrical conductivity.

According to the above results of half-cell tests, MDC-900 has the best catalytic activity toward VO²⁺/VO₂⁺. Thus it is chosen to modify graphite felt (GF). The electrocatalytic activity MDC-900 was further confirmed after depositing on the GF electrode via CV measurements, conducted at a scan rate of 5 mV s⁻¹. In Figure 7, it can be found that when GF is modified by MDC-900, the peak separation of 404 mV is much smaller than 579 mV performed by pristine GF. Moreover, the oxidation and reduction peak current density ratios of MDC-900 modified GF and pristine GF are 0.914 and 1.516, respectively. Thus, the former value is closer to 1 than that of the latter, indicating that MDC-900 modified GF electrode has better reversibility toward the redox reaction of vanadium ions which corresponded with the comparison results of peak separation. Based on the material analysis results, there might be three reasons for such electrochemical activity enhancement of this catalyst. Firstly, MDC-900 is composed of tetragonal ZrO₂, which is active in catalyzing VO²⁺/VO₂⁺ redox couple. Secondly, Zr-MOF has been annealed at high temperatures, forming a graphitized carbon structure between tetragonal ZrO₂ grains, which can help to improve the poor conductivity of metal oxide. This inlaid structure has a more effective reaction area than the mixture of metal oxides and carbon materials acquired by physical methods. Thirdly, the carbon-supported tetragonal ZrO₂ grains from the metal-organic framework are porous structures with higher specific surface areas than those synthesized by traditional methods. In other words, the efficiency of the redox reaction would be higher due to more reaction sites.

2.3. VRFB Single Cell Performance

Graphite felts modified by MDC-900 possessed superior electrochemical activity toward VO²⁺/VO₂⁺, and the charge-discharge tests were conducted to realize VRFB performance at various current densities, including 80, 100, 120, and 140 mA cm⁻². The VRFB assembled with MDC-GF-900 electrode is applied on the positive electrode, while the pristine GF electrode was used on the negative electrode. For comparison, the VRFB assembled with the same pristine GF electrode is employed on both sides. The cell performance was evaluated by using voltage efficiency (VE), coulombic efficiency (CE), and energy efficiency (EE). In Figure 8a, the onset potential of the charging stage of MDC-GF-900 is 36.69 mV lower than that of the pristine GF. On the other hand, in the discharging stage, the onset potential of MDC-GF-900 is 19.44 mV higher than that of the pristine GF. The phenomenon originated from reducing overpotential and internal resistance because MDC-900 possesses more active sites and the synergistic effect between ZrO₂ and graphite structure. Specifically, the graphite structure with higher conductivity can help conduct the electrons coming from the active sites to the surface of electrodes. Moreover, MDC-900 has a high specific surface area that could facilitate the redox reaction rate and improve the accessibility of electrolytes. Thus, the process of charge transfer would carry on more smoothly.

Figure 8b shows the EEs of VRFBs using MDC-GF-900 and pristine GF as cycle numbers at various current densities.

Table 1. The VE, CE, and EE of VRFB single-cell tests with MDC-GF-900 and GF electrodes at 80, 100, 120, 140 mA cm⁻².

Electrode	Current Density (mA cm−2)	5 Cycles Average Efficiency (%)			Capacity (mAh)
		VE	CE	EE
MDC-GF-900	80	75.7	97.2	73.6	1127.1
	100	69.3	98.0	67.9	704.8
	120	62.7	98.2	61.6	356.9
	140	53.3	97.3	51.8	70.2
GF	80	74.4	97.2	72.3	860.9
	100	65.6	97.6	64.0	340.5
	120	-	-	-	-
	140	-	-	-	-

lists the VEs, the CEs, the EEs, and capacities of the VRFBs using MDC-GF-900 and pristine GF at different current densities. Firstly, for both VRFBs, the EE becomes lower as the current density increases because the polarization increases with increasing current density. Secondly, at 80 mA cm⁻², the difference of EE between MDC-GF-900 and the pristine GF is only 1.3%, and MDC-GF-900 exhibits a more remarkable improvement of EE by 3.9% higher than that of GF at 100 mA cm⁻², which is also comparable with previously reported ZrO₂-based catalyst [39]. Furthermore, at 120 and 140 mA cm⁻², the EEs of MDC-GF-900 still keeps 61.6% and 51.8%, respectively. However, the VRFB using the pristine GF is not able to show any results. Since the polarization of pristine GF is too high, it is challenging to conduct the charge-discharge process. Even in the last five cycles, as the current density returning to 80 mA cm⁻², the EE of the pristine GF is about 1.8% lower than that of the first five cycles. Interestingly, on the other hand, MDC-GF-900 could perfectly maintain its EE as the current density returning from 140 to 80 mA cm⁻². The superiority of MDC-GF-900 becomes even more substantial when the current density is further increased. The CE is affected by the battery or vanadium ions’ leakage through the membrane during the charge-discharge process. Therefore, the capacity can be considered the utilization of electrolytes for the VRFB. The capacities of MDC-GF-900 reveal 31% and 107% higher than those of the pristine GF at 80 and 100 mA cm⁻², respectively. Even at 120 and 140 mA cm⁻², MDC-GF-900 shows 356.9 and 70.2 mAh capacity, respectively, but the pristine GF cannot get any energy from the electrolytes during this situation. In other words, MDC-GF-900 improves the utilization rate of electrolytes and thus increases the capacity of VRFB. It represents the cost down of purchasing electrolytes for constructing and maintaining VRFB systems.

To evaluate whether the performance of MDC-GF-900 can preserve or not after several charge-discharge cycles, the stability tests were conducted in Figure 9. From the results, the VE of MDC-GF-900 has no decline after 100 charge-discharge cycles at 100 mA cm⁻², but it is declined from 66.7% to 64.2% for GF. Similarly, the CE and the EE of MDC-GF-900 also maintain well after several cycles, suggesting that the improved electrochemical activity of MDC-GF-900 can be retained for a long time. Figure 9d displays the discharge capacity of the MDC-GF-900 and GF measured at 100 mA cm⁻² for 100 cycles. As shown in the figure, the VRFB delivered more than 71% total capacity retention instead of stable cycling performance after 100 cycles, which is much higher than GF (42%) and comparable to previously reported literature with minimum decay rates for VRFBs [40,41]. The slow capacity decay and stable EE demonstrate the outstanding cycling stability of the nanocomposite material that remained on the GF surface over succeeding cycles in the flowing electrolyte. Furthermore, to understand the stability of the catalyst on the GF, we have taken the SEM images before and after 100 cycles. As shown in Figure S2, after 100 cycles of charging and discharging operation under flowing electrolyte, the MDC-900 nanoparticles are still adhered to and uniformly distributed on the felt surface, which signifies better electrode stability. To sum up, MDC-GF-900 possesses favorable and stable electrochemical activity and can promote the capacity of VRFB. Therefore, by applying MDC-GF-900 in VRFB systems, the targets of increasing efficiency of operation, raising utilization rate of electrolyte, and maintaining high performance for a long time can all be achieved.

To explain the electrocatalytic activity in VRFB, we intended to focus on the redox reaction of the VO²⁺/VO₂⁺ couple on the surface of the MDC-GF-900 electrode. The detailed electrocatalytic process and the proposed mechanism on the electrode towards VO²⁺/VO₂⁺ redox reaction are shown in the supporting information (Scheme S1). The catalytic mechanism can be explained in the following way. Like other transition metal oxides, the presence of oxygen bonding within ZrO₂ might be the critical factor for the notable improvement in electrochemical activities of the MDC-GF-900 electrode [42]. As previously reported, the oxygen-containing functional groups (–OH, –COOH, –C=O, and –C–O) on the electrode surface probably behave as active sites, catalyzing the VO²⁺/VO₂⁺ redox reactions [43]. It can be seen from this reaction mechanism that, for example, the charging and discharging process at the positive electrode involves the transfer of an oxygen atom, which is likely to be the rate-determining step in the overall mechanism [44]. First, VO²⁺ ions diffuse from the bulk electrolyte and ion exchange with H⁺ of the oxygen-containing functional groups on the surface, and then they bonded onto the electrode surface. Second, the electron transfer occurs from the VO²⁺ to the electrode along the ZrO₂–V– bond and the transfer of one of the oxygen atoms on the functional group to the VO²⁺ forming VO₂⁺. Finally, the VO₂⁺ exchanges with H⁺ from the solution and diffuses back into the bulk solution. For the discharge process, reverse reactions would occur.

The outstanding performance of the MDC-GF-900 electrode can be ascribed to (1) the synergistic effects, which are related to the high electronic conductivity of the carbon material from MOF-derived material and interfacial properties created between the ZrO₂ nanoparticles distributed on it, (2) the existence of oxygen-containing functional groups which acts as the active sites, and (3) the porous structures and an increase in the surface area, which can contribute to increasing the active sites on the surface to enhance the electrochemical performance of the cell.

3. Experiment Methods

3.1. Materials and Preparation

Zirconium-based MOF was synthesized referring to a previously reported procedure by Moghaddam et al. [45]. First, 125 mg of ZrCl₄ (Acros Organic, Geel, Belgium) was dissolved in 5 mL dimethylformamide (DMF) (Acros Organic, Geel, Belgium) and 1 mL HCl (Scharlau, Barcelona, Spain) through ultrasonication for 10 min. Simultaneously, 125 mg of terephthalic acid (Acros Organic, Geel, Belgium) was dissolved in another 10 mL DMF under sonication for 10 min. Subsequently, these two solutions were mixed by using magnetic stirring and ultrasonication until well-distribution. Next, the mixture was heated at 80 °C for 12 h in a muffle furnace. When the reaction was completed, the MOF was separated from the solution by vacuum filtration, rinsed thoroughly with DMF and absolute ethanol, and dried overnight in a vacuum oven at 80 °C. To optimize MOF-derived catalyst activity, different sintering temperatures such as 500, 700, 900, and 1000 °C were applied on the as-prepared MOFs for 2 h in an argon atmosphere at a heating rate of 10 °C per minute. After the process, MDC (MOF–derived catalyst)-X (X = 500, 700, 900, 1000) were acquired and conducted for further measurements. On the other hand, pristine GF was preheated at 400 °C for 15 min in the microwave oven before decorating MOF-derived catalyst on GF. Then, heat-treated graphite felt (HGF) was placed inside the precursor solution of MOF and following the aforementioned synthetic steps.

3.2. VRFB Single-Cell Tests

The VRFB single cell tests were conducted in 1.6 M VOSO₄ plus 4.6 M H₂SO₄ solution. The graphite felts (5 cm × 5 cm × 0.65 cm) with MOF-derived catalyst were applied on the positive electrode, and the pristine GF was applied on the negative electrode. Nafion 212 membrane was used as an ion-exchange membrane (DuPont) sandwiched in the middle of VRFB by the cell frames. The storage chambers of both side electrolytes were 60 mL which were circulated by two FMI pumps (Fluid Metering, Inc., Syosset, NY, USA) separately at the flow rate of 80 mL min⁻¹. Before the charge-discharge tests began, nitrogen gas was purged to remove oxygen from the electrolyte. Moreover, the potential range of charge-discharge tests was between 0.7 and 1.6 V. Different current densities, such as 80, 100, 120, 140, and 160 mA cm⁻², were applied in the measurements to evaluate their columbic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) (equation 1 to3). To compare cell performance with modified GFs, the pristine GFs were used as both electrodes for single-cell tests.

$$\mathrm{CE}=\frac{Q_{discharge}}{Q_{charge}}=\frac{{{\displaystyle \int }}_0^{t_d}{I}_{d}dt}{{{\displaystyle \int }}_0^{t_c}{I}_{c}dt}=\frac{I_d{t}_d}{I_c{t}_c}$$

(1)

$$\mathrm{EE}=\frac{{{\displaystyle \int }}_0^{t_d}{V}_d{I}_{d}dt}{{{\displaystyle \int }}_0^{t_c}{V}_c{I}_{c}dt}=\frac{I_d{{\displaystyle \int }}_0^{t_d}{V}_{d}dt}{I_c{{\displaystyle \int }}_0^{t_c}{V}_{c}dt}=\mathrm{CE}·\mathrm{VE}$$

(2)

$$\mathrm{VE}=\frac{\mathrm{EE}}{\mathrm{CE}}$$

(3)

t_c is the charging time, t_d is the discharging time, I is the current, V_c is the charging voltage, V_d is the discharging voltage, and V_d and V_c are time functions.

3.3. Physicochemical Characterization

The MOF-derived catalysts’ morphology was characterized through field emission scanning electron microscope (FE-SEM, JSM-6500F) (JEOL Ltd., Tokyo, Japan). The crystallite information such as d-spacing can be performed by X-ray field emission gun transmission electron microscope (FEG-TEM, FEI Tecnai™ G2 F-20 S-TWIN) (FEI Co., Northeast Dawson Creek Dr Hillsboro, OR, USA) through its high resolution and transmission ability. The phase and crystallographic structure of the samples were determined by X-ray diffraction (XRD, Bruker, D2-PHASER X-ray Diffractometer) (Bruker Corp., Billerica, MA, USA) with a Cu Kα radiation source of λ = 1.54 Å. To identify the bonding situations and structural defects of materials, a Raman spectrometer (iHR550) (Horiba Ltd., Kyoto, Japan) with 532 nm wavelength of the laser beam was applied by detecting the vibration states in molecules. The binding energy between surface atoms of materials can be measured using X-ray photoelectron spectroscopy (XPS, Thermo, K-Alpha) (Thermo Fisher Scientific Inc., Waltham, MA, USA) analysis, providing strong evidence of bonding and chemical compositions of the samples. Brunauer–Emmett–Teller (BET, Quantachrome Instruments, NOVA touch LX²) (Quantachrome Instruments, Boynton Beach, FL, USA) analysis was used to check materials’ specific surface area and porosity by adsorption-desorption of nitrogen gas.

3.4. Electrochemical Characterization

The cyclic voltammetry (CV) was performed through a typical three-electrode measurement using an electrochemical workstation (Bio-Logic, VSP-300) (Bio-Logic Science Instruments, Seyssinet-Pariset, France). A glassy carbon ring disk electrode (RDE) was used as a working electrode. Counter and reference electrodes were platinum wire and Ag/AgCl, respectively. The ink dropped onto RDE was composed of a 7.5 mg catalyst, 2.8 mL isopropanol, 2.8 mL DI water, and 0.04 mL of 5 wt% Nafion solution, and the electrolyte of RDE tests contained 1.6 M VOSO₄ plus 4.6 M H₂SO₄ solution. The scan range was from 0 to 1.5 V, and the scan rate was 10 mV s⁻¹. The electrochemical impedance spectroscopy (EIS) test was conducted at room temperature in the frequency range from 100 kHz to 10 MHz at applied potential of 1.0 V.

On the other hand, in the CV tests of the GFs electrode, the GFs were cut with the circular shape (geometric area of 1.327 cm²) and employed as the working electrodes. 0.05 M VOSO₄ in a 2 M H₂SO₄ was prepared and utilized as an electrolyte in this case. The potential range and the scan rate were 0 to 1.5 V and 5 mV s⁻¹, respectively. Nitrogen gas was purged to prevent side reactions like oxygen evolution reaction during the CV tests.

4. Conclusions

In summary, metal-organic frameworks–derived catalysts (MDC) have been prepared through carbonization techniques at different sintering temperatures. The prepared samples were characterized by several techniques, such as XRD, Raman spectra, SEM, TEM, BET, and XPS, confirming the successful fabrication of zirconium-based MOF–derived catalysts. Furthermore, the cyclic voltammetry results show that the optimum zirconium-based MOF–derived catalyst (MDC-900) reveals the highest electrochemical performance for VO²⁺/VO₂⁺ redox reaction. Furthermore, the MDC-GF-900 electrode delivers high EE, which is 3.9% more efficient than the pristine GF at 100 mA cm⁻², and shows good stability. The excellent performance of MDC-GF-900 arises from the existence of oxygen-containing functional groups active sites, graphite structure with high conductivity embedded with ZrO₂, and high surface area, which play an excellent role in promoting the vanadium redox reactions.