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Article

Sustainably Sourced Mesoporous Carbon Molecular Sieves as Immobilization Matrices for Enzymatic Biofuel Cell Applications

by
Federica Torrigino
1,
Marcel Nagel
2,
Zhujun Peng
1,
Martin Hartmann
2 and
Katharina Herkendell
1,*
1
Institute of Energy Process Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Fürther Str. 244f, 90429 Nuremberg, Germany
2
Erlangen Center for Interface Research and Catalysis, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(11), 1415; https://doi.org/10.3390/catal13111415
Submission received: 23 August 2023 / Revised: 27 October 2023 / Accepted: 1 November 2023 / Published: 4 November 2023
(This article belongs to the Special Issue Biocatalytic Processes: A Multidisciplinary Platform for Future Biorefineries and Energy Conversion Systems)
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Abstract

:
Ordered mesoporous carbon CMK-3 sieves with a hexagonal structure and uniform pore size have recently emerged as promising materials for applications as adsorbents and electrodes. In this study, using sucrose as the sustainable carbon source and SBA-15 as a template, CMK-3 sieves are synthesized to form bioelectrocatalytic immobilization matrices for enzymatic biofuel cell (EFC) electrodes. Their electrochemical performance, capacitive features, and the stability of enzyme immobilization are analyzed and compared to commercially available multi-walled carbon nanotubes (MWCNT) using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The anodic reaction in the presence of glucose oxidase (GOx) and ferrocene methanol (FcMeOH) on the sustainably sourced CMK-3-based electrodes produces bioelectrocatalytic current responses at 0.5 V vs. saturated calomel electrode (SCE) that are twice as high as on the MWCNT-based electrodes under saturated glucose conditions. For the cathodic reaction, the MWCNT-based cathode performs marginally better than the CMK-3-based electrodes in the presence of bilirubin oxidase (BOD) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS2−). The CMK-3-based EFCs assembled from the GOx anode and BOD cathode results in a power output of 93 μW cm−2. In contrast, the output power of MWCNT-based EFCs is approximately 53 μW cm−2. The efficiency of CMK-3 as a support material for biofuel cell applications is effectively demonstrated.

1. Introduction

Enzymatic bioelectrochemistry integrates enzyme technology and electrochemistry to develop sustainable energy conversion technologies. Enzymatic fuel cells (EFCs), a type of bioelectrochemical device, generate electricity from chemical energy through enzyme catalysis by utilizing renewable bio-based fuels, such as glucose, with high efficiency and a low environmental impact. The anode and cathode in EFCs are made from carbon materials (e.g., carbon paper/cloth) coated with enzymes that catalyze substrate oxidation or reduction [1,2,3,4,5,6]. The development of EFCs is still in its early stages and is associated with lower efficiencies and power outputs compared to other fuel cells, and there is a need for more efficient and stable enzymatic electrodes [7,8,9,10,11]. However, significant contributions concerning the advantages of using a wide range of bio-based fuels, such as glucose, fructose, lactate, ethanol, and hydrogen, instead of fossil fuels have been convincingly demonstrated [12,13,14,15,16,17,18,19,20,21,22,23,24].
Efficient electron transfer from enzymes to the electrode’s transducer surface presents a challenge in the development of EFCs. Several methods have been explored, including direct immobilization, cross-linking with bifunctional linker molecules and self-assembled monolayers [5,25]. With direct immobilization strategies, enzymes are directly adsorbed or covalently attached to the transducer surface [26,27,28,29,30,31]. The adsorption method is a straightforward technique that employs enzyme–support material surface interactions that are reversible. On the one hand, the direct adsorption stability of this system is substantially affected by reaction variables such as the solvent pH and ionic strength. Other factors, such as the operability under moderate conditions and the absence of chemical changes, also contribute to the process’s simplification and cost reduction [32,33]. Enzymes are frequently immobilized via the covalent approach. The commercial success of covalently immobilized enzymes is attributed to their ability to maintain stability and reusability under the process settings. However, this strong attachment leads to significant alteration of the enzyme, sometimes resulting in modifications to its hydrophilic/hydrophobic properties [7,9,32,33,34,35]. Cross-linking the enzymes with bifunctional linker molecules is a well-established method of enhancing enzyme immobilization; however, due to the poor mechanical properties of the aggregates, it is typically used to augment other immobilization techniques [4,32,33,36,37].
Also, self-assembled monolayers of enzymes have gained growing attention for their increased electron transfer rates, although the overall current densities are limited by the monolayer architecture of catalytically active materials [38,39,40,41]. In terms of maximizing the wired enzyme density per electrode area, advances have been made in the past decade with entrapment and microfabrication techniques [36,42,43,44,45,46,47]. The latter can also improve enzyme activity and stability by allowing precise control over the enzyme distribution and orientation with complex and costly techniques such as soft lithography, or nanoimprint lithography [48,49,50,51]. Entrapment methods, on the other hand, often offer cost efficient, three-dimensional immobilizing strategies while tolerating a limited share of immobilized enzymes, yielding an optimal orientation and electrical connection [52,53]. Limitations in mass transfer can be a significant disadvantage of this method, and the material must be designed with the correct pore size and matrix properties. However, a material that is excessively porous or has excessively large pores will readily leach, and matrix defects may also contribute to enzyme leaching. The key to successful entrapment is to create an environment within a porous material that permits the substrate and product to diffuse freely while restricting the enzyme’s movement [32,33]. Each of these methods has its own advantages and disadvantages, and the choice of method depends on the specific requirements of the application and the characteristics of the enzymes and transducer surfaces used. The characterization of the wiring method is performed by electrical and electrochemical analyses that characterize the cell’s power outputs, current densities, polarization curves, Faradaic efficiencies and overpotentials, long-term performance, and stability in relevant environments [54,55,56,57,58,59,60,61,62]. Recent research has focused on improving the performance of enzymatic anodes through the use of new materials and structures, such as carbon nanoparticles (e.g., carbon nanotubes, graphene) and renewable bio-based carbon sources (e.g., lignin) [4,10,63,64,65,66,67]. Carbon nanomaterials have gathered significant attention in the scientific community due to their advantageous chemical and physical properties, such as their good electrical conductivity, exceptional chemical stability, well-defined porosity, high mechanical strength, and broad range of applications in material preparation, catalysis, energy storage, biology, and medicine [68,69,70,71,72,73,74,75,76,77]. For these applications to be successful, carbon nanomaterials must have high mesoporosity. This property allows the adsorption of molecules and ions that otherwise could not enter micropores due to their size [30,78]. Several methods exist for incorporating nanoparticles into electrode structures, such as entrapment in matrices like polymers, carbon paste, gels, and covalent bonding, among others [9,26,37,74,79,80,81,82,83,84,85,86]. Biomass, a carbon-rich organic material that is widely available, low in cost, environmentally friendly, and renewable, is a promising source for the preparation of biomass-derived carbon nanomaterials with a large specific surface area, porous structure, and good chemical stability [31,87,88,89].
These materials align with the principles of green and sustainable development [90]. Recently, among the mesoporous carbon nanomaterials, the ordered mesoporous carbons CMK-3, which have a hexagonal pore structure and uniform pore size (3–7 nm), have emerged as promising materials for a wide number of applications [91,92,93,94,95,96,97]. As described in the literature [78], CMK-3 molecular sieves were prepared in this study using SBA-15 material as a template and sucrose as the carbon source. Sucrose can be derived from a variety of renewable resources, including lignocellulosic biomass (sugar content: 50–80% [98]), or they can be derived from waste products produced, for instance, by the food processing sector, for example, from the sugarcane process (sugar content: 67–74% [99]). The waste can undergo a variety of chemical and enzymatic reactions to be converted into sugars.
The CMK-3 sieves synthesized in this study provide a sustainable replacement for conventional carbon nanomaterials synthesized from fossil hydrocarbon gases [100,101]. With their large specific surface areas, large specific pore volumes, and electrical conductivity, these molecular sieves are appealing immobilization materials for enzymatic biofuel cell applications. This study aims to determine the suitability of synthesized CMK-3 as a material for immobilizing enzymes in biofuel cell applications, laying the groundwork for future investigations of the long-term stability. The entrapment of mediator molecules in pores by enzyme capping followed by cross-linking was chosen as the method for immobilization in order to achieve this objective.
To accomplish this, a comparative analysis of its electrochemical performance via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is conducted against a commercially available benchmark material, multi-walled carbon nanotubes (MWCNTs), which are widely utilized in biosensing and EFC research [102,103,104]. Although EIS is frequently utilized for the characterization of fuel cells catalyzed by chemical catalysts and microorganisms, its application in the analysis of enzyme-based systems is currently still limited [58,105,106,107,108,109].However, its potential in their characterization using this method has been demonstrated lately [59,110]. The first section of this paper focuses on material characterization to demonstrate the properties and mesoporosity of CMK-3 sieves. The following section is centered on the capacitive characteristics of the support materials and the stability of enzyme immobilization at the electrode interface. Comparing MWCNTs and CMK-3 sieves as support materials for the bioanode and biocathode and examining the effects of adding a substrate on the electrode current response and enzymatic reaction is the focus of the paper’s middle section. The concluding section evaluates and compares the electrochemical performance of the biofuel cell, which consists of enzyme-modified electrodes immobilized on both of the previously compared support materials.

2. Results and Discussion

Commercially available carbon nanotubes (MWCNTs, diameter Ø 6–13 nm and length 2.5–20 μm) and carbon molecular sieves synthetized from the SBA-15 template (CMK-3, diameter Ø 310–510 nm and length 0.64–0.92 µm) are utilized in combination with two different redox mediator solutions as support materials for enzyme immobilization to create integrated bioelectrocatalytic electrodes. Ferrocene methanol (FcMeOH) is used as a mediator at the anode, while 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS2−) is used at the cathode, each entrapped in the mesoporous matrices. The assemblies are applied to a glassy carbon electrode and subsequently dried using a vacuum desiccation procedure. The oxygen-reducing enzyme bilirubin oxidase (BOD) is utilized at the cathode, while glucose oxidase (GOx), which catalyzes the oxidation of glucose, is used for the anode. The surface is coated with the respective enzymes for mediated electron transfer and, therefore, bioelectrochemical communication with the transducer surface, while preventing the mediator’s release from the pores. The resulting enzyme-modified surfaces are cross-linked using bis(sulfosuccinimidyl) suberate, BS3, and the integrated bioelectrocatalytic electrode is finished by coating the electrode surface with a Nafion solution, which serves as an ion exchange membrane for the long-term stabilization of the enzymes on the electrode. Using this technique, two relay-loaded assemblies, each capped by a distinct enzyme, are prepared for each support material.
For material characterization, the elemental composition and surface properties are analyzed by the elemental analysis, scanning electron microscopy (SEM), and N2 sorption techniques. In order to compare the electrochemical performance of CMK-3 sieves to that of a commercial MWCNTs, the CV and EIS methods are used. Using a potentiostat, the GOx-modified anode and BOD-modified cathode enzymatic bio-fuel cells are subjected to various external resistances in order to evaluate and compare their performances and to generate polarization curves.

2.1. Material Characterization

2.1.1. Elemental Analysis

The CHNS analysis (raw data in Table 1) with a correction for possible residual water gives an elemental composition of 96.19 wt.% carbon in CMK-3 and 94.81 wt.% in MWCNT. As anticipated, the CHNS measurements confirm that the predominant component for both examined samples is carbon. The remaining amount is considered to be oxygen due to the oxygenated compounds.

2.1.2. Scanning Electron Microscopy (SEM)

Scanning electron microscopy images of the synthesized CMK-3 mesoporous sieves at two magnifications are shown in Figure 1.
The morphology of the materials, as depicted in Figure 1B, exhibits a rod-like structure, in alignment with the expectations derived from the existing literature [31,78]. As shown in Figure 1A, macropores with a diameter of about 2–3 µm are formed if a large number of particles collide and leave voids.

2.1.3. Nitrogen Sorption

The nitrogen adsorption and desorption isotherms for the support materials are shown in Figure 2. According to the IUPAC classification, a type IV(a) isotherm is observed for both carbons [111,112,113]. The MWCNTs (Figure 2A) exhibit a narrow type H4 hysteresis loop at high relative pressures (p/p0 = 0.87–0.99), indicating the capillary condensation of nitrogen in larger mesopores (20–40 nm). In agreement with the literature, it can be assumed that the mesoporosity of MWCNTs is caused by the assembly of nanotubes of different diameters and orientations into aggregate structures held together by intermolecular forces [111,112,113]. However, the CMK-3 molecular sieves (Figure 2B) show a characteristic combination of type H1 and H2 hysteresis loops, which is typical for well-ordered, mostly non-restricted mesoporous materials [111,112,114,115,116].
In Figure 3, the BJH pore size distributions of the two support materials are presented. For the MWCNTs, a pore diameter of approximately 1 to 5 nm is observed (Figure 3A) in accordance with the literature [102], In detail, maxima in the micropore (diameter < 2 nm) and mesopore (2.5–5 nm) ranges can be identified for the material. The BHJ pore size distribution curve of CMK-3 sieves (Figure 3B) shows that the carbon is predominantly mesoporous with a narrow pore size distribution that is primarily dominant at 4–5 nm. This result is also in agreement with previous research and fulfills the pore size requirements for mesoporous carbon [30,31,78].
The BHJ total pore volume (Vp), the BHJ average pore diameter (dp), and the BET surface area (SBET) for both support materials resulting from the nitrogen adsorption data are depicted in Table 2. Compared to the CMK-3 sieves, which have a dp of 4.82 nm, the MWCNTs have a larger value of 7.44 nm, which is consistent with the manufacturer’s particle size specifications. Also, both values indicate a mesoporous structure. However, multi-walled MWCNTs are typically not classified as mesoporous materials in the literature, as they exhibit a lower secondary mesoporosity due to the formation of mesopores between aggregated multi-walled nanotubes in close proximity [112].
The nitrogen adsorption data for MWCNTs provides an SBET of 89 m2 g−1 and a Vp of 0.17 cm3 g−1. This low values could be explained by the interlayer spacing of the concentric tubes, which prevents N2 molecules from penetrating the material [114]. In contrast, CMK-3 has a SBET of 1133 m2 g−1 and a Vp of 1.36 cm3 g−1. The calculated SBET and Vp values for both support materials are in agreement with the literature [78,111,112,113,114,116,117,118].

2.2. Material Capacitance

The electrical conductivity of the material is a crucial factor to consider when determining the material’s potential for use in bioelectrocatalytic applications [104]. The capacitance of both the MWCNT and CMK-3 support materials is determined using cyclic voltammetry by applying a solution of each material devoid of enzymes, mediators, and Nafion membranes to a glassy carbon electrode. The capacitance of the adsorbed substance on the electrode is represented by the area of its cyclic voltammetry curve [104,119]. Figure 4 depicts the cyclic voltammograms of MWCNTs and CMK-3 sieves.
At the same voltage, the synthetized CMK-3 sieves have approximately triple the capacitive current of the MWCNTs. Evidently, the surface structures of the materials have a significant impact on the capacitive current. CMK-3 sieves, as mesoporous materials, have a large surface area compared to commercial MWCNTs. The specific capacitance (and the corresponding capacitive current) grows with an increasing surface area. Other factors, such as the pore volume, affect its growth, making it nonlinear [120].

2.3. Enzyme Immobilization

The time-dependent cyclic voltammograms in Figure 5 show the effectiveness of enzyme immobilization on both support materials at the anode. Figure 5A and Figure 5B depict MWCNTs and CMK-3 sieves, respectively, loaded with FcMeOH in the absence of GOx. The rapid release of the relay unit from the materials is indicated by the significant decrease in the peak current, which is also evident by the current decrease curve in Figure 5C. However, when comparing the CMK-3 and MWCNT particles, it is evident that CMK-3 particles release mediator molecules at a slower rate, which relates to the morphological characteristics of the particles. The fine pore structure of the mesoporous materials CMK-3 (dp = 4.8 nm) permits mediator molecules (average hydrodynamic diameter 0.62 nm [121]) to adhere to them more effectively, thereby slowing the diffusion of the mediator into the electrolyte. The lower mesoporosity of MWCNTs compared to CMK-3 sieves results in weaker entrapment of the FcMeOH mediator molecule on the surface and a quicker release into the electrolyte.
Figure 5D and Figure 5E show the CVs of the MWCNTs and CMK-3 sieves, respectively, loaded with FcMeOH in the presence of the enzyme GOx immobilized on the electrode surface. The enzyme inhibits the rate of decrease in the peak current in both instances, as can be seen in Figure 5F, and the effect is amplified in the mesoporous material CMK-3. This can be explained by comparing the dimensions of the pores on the CMK-3 sieves to the diameter of GOx. The average pore size of CMK-3 is 4.82 nm, while the enzyme GOx has a hydrodynamic diameter ranging from 7.6 nm to around 9 nm [122,123]. It can be assumed that the pores of these particles allow for the immobilization of the enzyme GOx at the pore entrance and the entrapment of the mediator molecules within the pores, suggesting that CMK-3 is a promising anode material.
Similar experiments were conducted on the cathode to evaluate the efficacy of mediator entrapment, mirroring the anode’s methodology, as depicted in Figure 6.
Figure 6A,B depict cyclic voltammograms of MWCNTs and CMK-3 loaded with ABTS2− in the absence of BOD. Similar to the anode, the sharp drop in the peak current in the current decrease curve in Figure 6C indicates that the mediator relay unit has been quickly released from both materials. The ABTS2− decrease over time from both materials is slower than that observed at the anode for FcMeOH. Analyzing the structural properties of the mediator ABTS2− can provide insight into a possible explanation. In particular, ABTS2− possesses a delocalized π-electron system, which facilitates the temporary adhesion (π-π-interaction) of the molecule to the carbonaceous surface, delaying its release into the electrolyte. The cyclic voltammograms of MWCNT and CMK-3 loaded with ABTS2− in the presence of BOD on the electrode surface are depicted in Figure 6D and Figure 6E, respectively. Clearly, the addition of BOD affects the release of the mediator into the electrolyte for both support materials.
The enzyme significantly contributes to the inhibition of the rate of peak current decrease, as shown in Figure 6F. Here, it is important to note that the MWCNT particles appear to have a slower current decrease than the CMK-3 materials. A possible cause can be identified by examining the molecular dimensions and material structure of the mediators. The hydrodynamic diameter of the ABTS2− molecules is approximately 100 nm [124]. This value is notably larger than that of the anodic mediator. The use of CMK-3 sieves as the cathodic support materials hinders the entrapment of the mediator within the relatively smaller mesopores. However, the measurements suggest a mediator entrapment within the pores that are formed due to the disordered contact of multiple particles, which have diameters of ~2 μm. Overall, the ABTS2− molecule may be more efficiently entrapped in the disordered MWCNT structure among the tubes compared to the CMK-3 sieves at the cathode.

2.4. Anodic Reaction

As shown in Figure 7, the bioelectrocatalytic substrate oxidation is investigated by varying the glucose concentration at the GOx-modified electrodes immobilized on both support materials. The cyclic voltammograms for MWCNTs and CMK-3 sieves at varying glucose concentrations are illustrated in Figure 7A and Figure 7C, respectively. In both cases, the current increases with an increasing substrate concentration, suggesting that glucose is undergoing enzymatically catalyzed oxidation.
As shown in Figure 7B for MWCNTs and Figure 7D for CMK-3 sieves, the current responses stabilize at a specific concentration, indicating that enzyme saturation has been reached and that all enzyme active sites are continuously occupied. In either scenario, the concentration amounts to approximately 175 mM. At the specified glucose concentration, the MWCNT anode exhibits a saturation current of 535 µA cm−2, while the CMK-3 anode exhibits the highest current, with a saturation current of around 1014 µA cm−2.
Electrochemical impedance spectroscopy was used alongside cyclic voltammetry to analyze the anodic reaction and assess the feasibility of sustainably derived CMK-3 sieves, as shown in Figure 8. The most challenging aspect of this technique is interpreting the results displayed on the EIS spectrum, also known as the Nyquist plot. According to the existing literature, the total polarization resistance can be calculated by locating the intersection of the Nyquist and the x-axis during the anodic reaction [107,125].
As depicted in Figure 8A, B for both MWCNT and CMK-3, in the absence of a substrate, the Nyquist plots intersect the x-axis in the region between 0.70 kΩ and 0.88 kΩ, as reported in Table 3. This value, which is comparable for both systems, is an estimation of the total resistance of the electrolyte solution, given that no reaction takes place in the absence of a substrate. Notable are the different shapes of the Nyquist plot for CMK-3 and MWCNTs in the medium–low frequency range. The literature suggests that the observed difference is a consequence of the differences in the closed pore geometry [126,127,128]. The observed variation in impedance deviates from the expected Warburg behavior, particularly in the case of the mesoporous rod-like CMK-3 material, where a noticeable curvature is observed in the Nyquist plot.
When the substrate concentration increases, both MWCNT- and CMK-3-based electrodes exhibit a shift to the left in their Nyquist plots, which indicates an increase in their total internal resistance. This can be explained by the fact that, as more glucose molecules are added to the solution, the reaction kinetics accelerate, the concentration of ions in the electrolyte solution increases, and the transfer charge resistance also increases, leading to a polarization resistance increase. Another noteworthy observation is that the variation in glucose concentration induces a more significant alteration in the overall internal resistance of electrodes modified with MWCNTs compared to those modified with CMK-3. Due to the mesopores of CMK-3, the mediator units entering the pores are seemingly entrapped by the enzyme, and due to the close proximity with the enzyme, even at a lower glucose concentration, electrons seem to be transferred more efficiently. The aforementioned phenomenon results in a decrease in mediator diffusion within the electrolyte and a concomitant increase in its interaction with the active site of the capping enzyme. This observation corresponds with the increased current responses detected in the cyclic voltammograms (Figure 7) for CMK-3-based systems, further indicating the beneficial properties for bioelectrocatalytic anode material applications.

2.5. Cathodic Reaction

The investigation of the bioelectrocatalytic substrate reduction is conducted by changing the purging gas at the electrodes for the BOD-modified electrodes immobilized on both support materials, as illustrated in Figure 9. The cyclic voltammograms for MWCNTs and CMK-3 sieves under nitrogen and oxygen are illustrated in Figure 9A and Figure 9B, respectively. After purging the electrolyte with oxygen, the reduction current for both enzyme-modified electrodes decreases, indicating that oxygen undergoes, in both cases, enzymatically catalyzed reduction. In the oxygen-saturated electrolyte, the MWCNT-based electrode reaches its maximum reducing current at E ~0.32 V vs. SCE with an approximate saturation current of ca. −347 µA cm−2. At the same potential and saturation for oxygen, the CMK-3 sieves-based cathode exhibits a current of −253 µA cm−2.
As anticipated in Section 2.1, the minor difference in current between the two materials compared to the anodic case is attributed to the substantial size difference between the mediators immobilized at the cathode and anode, with the molecule ABTS2− being significantly larger in size than the molecules of FcMeOH. Nonetheless, the electrode immobilized with BOD and utilizing MWCNTs as a support material demonstrates a marginally enhanced performance in comparison to the electrodes that have been modified with CMK-3 sieves.
The influence of the substrate on the cathodic performance is examined using electrochemical impedance spectroscopy, as illustrated in Figure 10, similar to the methodology employed for the anode. The Nyquist plots presented in Figure 10A, B indicate that the absence of oxygen results in an intersection of the EIS spectra with the x-axis within the range of 1.26 kΩ to 1.21 kΩ for both modified electrodes, as shown in Table 4. Assuming no reaction is occurring during the nitrogen purge, this value constitutes an approximation of the total resistance of the electrolyte solution for both systems, independent of the bioelectrocatalytic reduction.
Under saturated oxygen conditions, the total polarization resistance decreases, and the Nyquist plots shift to the left. According to the results, there is a strong correlation between the oxygen partial pressure and the total electrolyte resistance. Oxygen exerts a greater effect on the MWCNT cathode than on the CMK-3 cathode, resulting in a higher resistance that is indicative of its enhanced performance efficiency. Further research is necessary to thoroughly explore this correlation.

2.6. Biofuel Cell Performance

In order to conduct biofuel cell testing, anodes for glucose oxidation (GOx-immobilized, FcMeOH-loaded) and cathodes for O2 reduction (BOD-immobilized, ABTS2−-loaded), using both sustainably sourced CMK-3 sieves and commercial MWCNTs as immobilization material, were electrically wired to construct glucose/oxygen enzymatic biofuel cells. Utilizing sustainably sourced CMK-3 as the immobilization material resulted in an anodic potential of −0.12 V vs. SCE for the FcMeOH-mediated bioelectrocatalytic oxidation of glucose. In contrast, the ABTS2− -mediated cathode exhibited a reduction potential of oxygen of approximately 0.46 V vs. SCE, resulting in an open circuit potential (OCP) of 0.58 V.
In the case of MWCNT, the OCP potential was approximately 0.49 V. The discharges (polarization curves) for MWCNTs and CMK-3 sieves were measured at variable external resistances in 175 mM glucose/oxygen-saturated electrolyte (HEPES buffer, pH = 7), and the relative power density curves are depicted in Figure 11. The OCP and the maximum power point (MPP) are listed in Table 5.
As can be seen in Figure 11A, an activation loss of approximately 0.1 V can be observed from the OCP potential up to a current density of 40 µA cm−2 in both cases. The linear decrease in voltage observed in the CMK-3 EFC (down to 0.26 V) and in the MWCNT EFC (down to 0.12 V) can be attributed to ohmic losses resulting from the electrode and buffer resistances. Mass transport losses account for the additional voltage drop at high current densities. The observed MPP for the CMK-3 biofuel cell and MWCNT biofuel cell are 93 μW cm−2 and 53 μW cm−2, respectively, as shown in Figure 11B. Consistent with the CV findings, the CMK-3-based fuel cell shows superior electrical catalyst wiring, resulting in an enhanced extractable current density compared to the MWCNT-based fuel cell.

3. Materials and Methods

3.1. Chemicals

Multi-walled carbon nanotubes (>90% carbon basis), polyvinylidene fluoride (PVDF, average Mw~275,000), deionized water, ferrocene methanol, 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid), HEPES sodium salt, Nafion perfluorinated ion-exchange powder solution, and suberic acid bis (3-sulfo-N-hydroxysuccinimide ester) sodium salt were purchased from Sigma Aldrich (St. Louis, MO, USA). D (+)-Glucose was purchased from Carl Roth GmbH (Karlsuhe, Germany). Glucose oxidase (from Aspergillus niger, min. 220 U/mg, EC 1.1.3.4) was purchased from SERVA (Heidelberg, Germany). Bilirubin oxidase (from Myrothecium verrucaria, 44 U/mg, EC 1.3.3.5) was purchased from Sigma Aldrich. N-Methyl-2-pyrrolidon was purchased from Thermo Fisher Scientific (Waltham, MA, USA).

3.2. Preparation of the Electrodes

3.2.1. CMK-3 Synthesis

The used SBA-15 material, with a pore diameter in the range of 8 nm, was prepared according to the method reported by Hartmann and Vinu [129] with a synthesis temperature of 130 °C. Also, following the CMK-3 method of Vinu et al. [116], 1 g of the synthesized SBA-15 template was added to a solution obtained by dissolving 1.25 g of sucrose and 0.14 g of H2SO4 in 5 g of water, and the mixture was kept in a drying oven at 99.5 °C for 6 h. The drying oven temperature was raised to 160 °C for a further 6 h. To obtain fully polymerized and carbonized sucrose in the pores of the silica template, again, 0.8 g of sucrose, 0.09 g of H2SO4 and 5 g of water were added to the pretreated sample, and the mixture was subjected to the thermal treatment described above. The template–polymer composites were then pyrolyzed in a nitrogen stream at a heating rate of 5 K min−1 up to 877 °C using a compact module tube furnace (Carbolite Gero, Sheffield, UK) and kept under these conditions for 6 h to carbonize the polymer. The obtained powder was suspended in 100 mL of 2 M NaOH solution at 85 °C for 16 h, filtered, and the treatment was repeated to completely dissolve the silica framework. The mesoporous carbon was recovered by filtration, washed several times with water and ethanol, and dried at 85 °C in a vacuum of 100 mbar. The commercial MWCNTs were subjected to heat treatment in a muffle oven (Carbolite AAF 1100, Carbolite Gero, Sheffield, UK) at 600 °C for 15 min. This treatment aimed to activate the carbon surface area of the MWCNT, thereby improving its porosity and surface adsorption.

3.2.2. Immobilization Slurry

The immobilization slurry was prepared according to the literature [52] by combining 5 mg of MWCNTs or CMK-3 sieves with 0.5 mL of NMP in an ultrasonic bath for 90 min to produce a slurry. After 90 min in the ultrasonic bath, 120 µL of either 0.1 M FcMeOH solution (dissolved in water/ethanol, 1:1 v/v) or 0.1 M aqueous ABTS2− solution was added to the slurry, which was then ultrasonically treated for an additional 90 min. The final step involved the addition of 50 µL of a solution containing 10% (w/w) of PVDF (polyvinylidene fluoride) dissolved in NMP (N-Methyl-2-pyrrolidon) to the resulting slurry, followed by ultrasonication for 30 min according to Trifonov et al. [52].

3.2.3. Modification of the Electrodes

A total of 4 µL of the prepared slurry was pipetted onto the electrode. The electrode was then dried for 120 min (75 min for CMK-3) at 60 °C in a vacuum desiccator (heated vacuum desiccator “Vacuo-Temp”, JP Selecta, Barcelona, Spain). After removing the dried electrode, 5 µL of 0.5 mg ml−1 enzyme solution (dissolved in HEPES buffer, pH = 7.0) was applied to the surface and cross-linked for 15 min using 1 µL of BS3 aqueous solution with a concentration of 1 mg ml−1. The measured enzymatic activity of GOx was 84% of the solubilized enzyme solution, while the measured activity of BOD was 89% using a colorimetric approach. The percentage value of the solubilized solution was obtained by comparing the calculated value with that specified in the manufacturer’s certificate of analysis. After one hour of drying at room temperature, 5 µL of 0.5% (w/w) Nafion (dissolved in water) was added, and after 30 min, the working electrode was ready for use.

3.3. Measurements and Instrumentation

3.3.1. Material Characterization

A Euro EA 3000 from Euro Vector (Pavia, Italy) was used for the CHNS elemental analysis. Each measurement was performed on approximately 0.5 mg of the sample. All carbon materials were dried at 150 °C for 16 h. The SEM images were taken at 1 kV using a Carl Zeiss Gemini Ultra 55 equipped with an SE2 detector. Nitrogen measurement samples were measured using a Micrometrics ASAP 2000 (Norcross, GA, USA). Approximately 0.1 g of each sample was used and degassed at 200 °C under vacuum for at least 16 h. Nitrogen isotherms were recorded at −196.5 °C using liquid nitrogen. The Roquerol condition was taken into account in the Brunauer–Emmet–Teller (BET) area calculations (SBET). The pore size distributions were calculated using the desorption branch and applying the Barret, Joyner, and Halenda (BJH) method with the carbon black STSA approach and a standard correction. The total pore volume (Vp) and the average BHJ pore diameter (4·Vp/SBET) were determined at a relative pressure of p/p0 = 0.97 for the adsorption branch.

3.3.2. Electrochemical Characterization

The electrochemical experiments were carried out using a PalmSens 4 (PalmSens, Houten, The Netherlands) potentiostat that was configured with a three-electrode setup. Utilizing PalmSens PS Trace software version 5.9, the measurement plots were created. Glass carbon electrodes (diameter = 3 mm, geometric area = 7.07 mm2), a KCl saturated calomel electrode (SCE), and a graphite rod were provided from PalmSens and used as working, reference, and counter electrodes, respectively. Experiments were conducted at room temperature in a standard electrochemical cell (PalmSens) containing 8 mL of 0.1 M HEPES buffer (pH 7.0). Impedance measurements were carried out by applying a sinusoidal voltage of 20 mV to the sample and measuring the current response in the range of frequencies from 0.1 Hz to 100 kHz. Before each EIS measurement, a standard cyclic voltammetry measurement was performed to activate the electrode surface. To build glucose/oxygen enzymatic biofuel cells, GOx-based anodes were connected to BOD cathodes via electrical wiring. The polarization curves (discharge) were obtained using PalmSens 4 potentiostat and applying variable external resistances to the fuel cell system. All measurements presented in this study were repeated a minimum of three times to ensure their reliability. The accuracy of each of the three independent measurements was determined to be within a margin of error of less than 5%, so all results of the measurements presented in this manuscript are deemed reliable.

4. Conclusions

The synthesis of ordered mesoporous carbon (CMK-3) was achieved via a hydrothermal route by utilizing sucrose as the carbon source and an SBA-15 silica template. In order to determine the mesoporosity of the synthesized CMK-3 sieves, their material properties were initially investigated (Vp = 1.36 cm3 g−1, dp = 4.82 nm, SBET = 1133 m2 g−1). Cyclic voltammetry and electrochemical impedance spectroscopy were utilized to gain a deeper understanding of the electrochemical properties of the materials. At the same voltage, the synthesized CMK-3 mesoporous particles exhibited roughly three times the capacitive current as the commercially available MWCNT reference material. This can be explained by the mesoporous structure and the resulting higher BET surface area of the synthetized sieves. Effective immobilization of the enzymes GOx and BOD on the CMK-3 material indicates its potential as an anodic and cathodic support material for enzymatic biofuel cell applications, for which the anodic and cathodic behavior were studied. Regarding the anodic reaction, it was observed that the CMK-3-based electrodes delivered a bioelectrocatalytic current response under saturated glucose conditions that was roughly double at 0.5 V (1014 μA cm−2) than that of the MWCNT-based electrodes (535 µA cm−2) with FcMeOH mediated electron transfer. Concerning the cathodic reaction, however, the electrode utilizing MWCNTs as a support material exhibited a marginally superior performance under saturated oxygen levels and ABTS2− mediation at 0.45 V (−347 µA cm−2) compared to the electrodes modified with CMK-3 sieves (−253 µA cm−2). The aforementioned result can be attributed to the size difference between the cathodic and anodic mediators corresponding to the different pore sizes of the respective support materials. Upon analyzing the anodic and cathodic EIS spectra, the total electrolyte resistance was successfully identified in both instances. The outcomes of the fuel cell experiments indicate that the fuel cell based on CMK-3 sieves exhibits a better performance (OCP = 0.58 V and MPP = 93 μW cm−2) with the immobilized enzymes in comparison to the one based on MWCNTs (OCP = 0.49 V and MPP = 53 μW cm−2). The results can be attributed to the increased BET surface area of the CMK-3 material, which enables more efficient loading of enzymes. Furthermore, the utilization of CMK-3 demonstrates an enhancement in the stability of the anode, leading to a reduced level of degradation and an enhanced overall efficiency in the fuel cell. The present study demonstrates the efficacy of sucrose-derived mesoporous carbon molecular sieves as a support material for enzymatic cell applications, which is particularly interesting for the increasing market of recycled carbonaceous electrode matrices from waste materials. The successful demonstration of sustainably sourced mesoporous support structures for enzyme immobilization and electrical contacting could contribute towards the effort to design more environmentally friendly fuel cells.

Author Contributions

F.T.: administered the study, performed and supervised the electrochemical measurements, carried out the data curation, analysis, and validation, drafted the manuscript, and visualized the data. M.N. was responsible for the synthesis of CMK-3 sieves and the characterization measurements of both materials. Z.P. contributed to the investigation by performing the electrochemical measurements. M.H. supervised the work and provided resources and funding. K.H. conceptualized the study, provided resources and funding, contributed to the drafting of the manuscript, and supervised the work. All authors discussed the results and reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

Support from Energiecampus Nürnberg (EnCN) is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. SEM images of synthetized CMK-3 particles used in experiments with different magnitude scales: (A) 2000×; (B) 10,000×. The SEM images were obtained with an operation voltage of 1 kV.
Figure 1. SEM images of synthetized CMK-3 particles used in experiments with different magnitude scales: (A) 2000×; (B) 10,000×. The SEM images were obtained with an operation voltage of 1 kV.
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Figure 2. Nitrogen adsorption and desorption isotherms at −196.5 °C of MWCNT (A) and CMK-3 (B). The adsorbed nitrogen volume is plotted against the relative pressure p/p0.
Figure 2. Nitrogen adsorption and desorption isotherms at −196.5 °C of MWCNT (A) and CMK-3 (B). The adsorbed nitrogen volume is plotted against the relative pressure p/p0.
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Figure 3. BJH pore size distribution of MWCNTs (A) and CMK-3 sieves (B).
Figure 3. BJH pore size distribution of MWCNTs (A) and CMK-3 sieves (B).
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Figure 4. Cyclic voltammograms of MWCNTs and CMK-3 sieves without enzymes, mediators, and Nafion membrane. Scan rate: 20 mV s−1.
Figure 4. Cyclic voltammograms of MWCNTs and CMK-3 sieves without enzymes, mediators, and Nafion membrane. Scan rate: 20 mV s−1.
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Figure 5. Cyclic voltammograms depicting the time-dependent release of FcMeOH from the MWCNT and CMK-3 surfaces in the presence and absence of the enzyme GOx: (A) the FcMeOH-loaded MWCNT-modified electrode, (B) the FcMeOH-loaded CMK-3-modified electrode, and (C) comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 20 mV s−1. (D) The GOx-immobilized, FcMeOH-loaded MWCNT-modified electrode, (E) the GOx-immobilized, FcMeOH-loaded CMK-3-modified electrode, and (F) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 20 mV s−1.
Figure 5. Cyclic voltammograms depicting the time-dependent release of FcMeOH from the MWCNT and CMK-3 surfaces in the presence and absence of the enzyme GOx: (A) the FcMeOH-loaded MWCNT-modified electrode, (B) the FcMeOH-loaded CMK-3-modified electrode, and (C) comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 20 mV s−1. (D) The GOx-immobilized, FcMeOH-loaded MWCNT-modified electrode, (E) the GOx-immobilized, FcMeOH-loaded CMK-3-modified electrode, and (F) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 20 mV s−1.
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Figure 6. Cyclic voltammograms depicting the time-dependent release of ABTS2− from the MWCNT and CMK-3 surfaces in the presence and absence of BOD: (A) the ABTS2−-loaded MWCNT-modified electrode, (B) the ABTS2−-loaded CMK-3-modified electrode, and (C) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 5 mV s−1. (D) the BOD-immobilized, ABTS2−-loaded MWCNT-modified electrode, (E) the BOD-immobilized, ABTS2−-loaded CMK-3-modified electrode, and (F) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 5 mV s−1.
Figure 6. Cyclic voltammograms depicting the time-dependent release of ABTS2− from the MWCNT and CMK-3 surfaces in the presence and absence of BOD: (A) the ABTS2−-loaded MWCNT-modified electrode, (B) the ABTS2−-loaded CMK-3-modified electrode, and (C) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 5 mV s−1. (D) the BOD-immobilized, ABTS2−-loaded MWCNT-modified electrode, (E) the BOD-immobilized, ABTS2−-loaded CMK-3-modified electrode, and (F) a comparison of the time-dependent relative decrease in amperometric responses following immersion in the HEPES buffer (0.1 M, pH = 7.0). Immersion time intervals (a–j): 0–30 min. Scan rate: 5 mV s−1.
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Figure 7. Cyclic voltammograms showing the bioelectrocatalytic glucose oxidation at varying concentrations on the MWCNT and CMK-3 modified electrodes. (A): The GOx-immobilized, FcMeOH-loaded MWCNT-modified electrode at different glucose concentrations: (a) 0, (b) 50, (c) 90, (d) 130, (e) 175, (f) 250 mM glucose in HEPES buffer (0.1 M, pH = 7.0). Scan rate 20 mV s−1. (B): Current responses for the various glucose concentrations at E = 0.5 V versus SCE. (C): The GOx-immobilized, FcMeOH-loaded CMK-3-modified electrode at different glucose concentrations: (a) 0, (b) 50, (c) 90, (d) 130, (e) 175, (f) 250 mM glucose. (D): Current responses for the various glucose concentrations at E = 0.5 V versus SCE. Scan rate 20 mV s−1.
Figure 7. Cyclic voltammograms showing the bioelectrocatalytic glucose oxidation at varying concentrations on the MWCNT and CMK-3 modified electrodes. (A): The GOx-immobilized, FcMeOH-loaded MWCNT-modified electrode at different glucose concentrations: (a) 0, (b) 50, (c) 90, (d) 130, (e) 175, (f) 250 mM glucose in HEPES buffer (0.1 M, pH = 7.0). Scan rate 20 mV s−1. (B): Current responses for the various glucose concentrations at E = 0.5 V versus SCE. (C): The GOx-immobilized, FcMeOH-loaded CMK-3-modified electrode at different glucose concentrations: (a) 0, (b) 50, (c) 90, (d) 130, (e) 175, (f) 250 mM glucose. (D): Current responses for the various glucose concentrations at E = 0.5 V versus SCE. Scan rate 20 mV s−1.
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Figure 8. Nyquist plots of bioelectrocatalytic glucose oxidation at varying substrate concentrations: (A) GOx-FcMeOH-loaded MWCNT-modified electrode at 0 mM, 50 mM, 130 mM, and 175 mM glucose in HEPES buffer (0.1 M, pH = 7.0). (B) GOx-FcMeOH-loaded CMK-3 modified electrode at 0 mM, 50 mM, 130 mM, and 175 mM glucose in HEPES buffer (0.1 M, pH = 7.0). Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Figure 8. Nyquist plots of bioelectrocatalytic glucose oxidation at varying substrate concentrations: (A) GOx-FcMeOH-loaded MWCNT-modified electrode at 0 mM, 50 mM, 130 mM, and 175 mM glucose in HEPES buffer (0.1 M, pH = 7.0). (B) GOx-FcMeOH-loaded CMK-3 modified electrode at 0 mM, 50 mM, 130 mM, and 175 mM glucose in HEPES buffer (0.1 M, pH = 7.0). Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
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Figure 9. Cyclic voltammograms corresponding to the bioelectrocatalytic reduction of O2. (A) BOD-immobilized, ABTS2−-loaded MWCNT-modified electrode; (B) BOD-immobilized, ABTS2−-loaded CMK-modified electrode. The HEPES buffer electrolyte (0.1 M, pH = 7.0) was purged with either N2 or O2. Scan rate: 5 mV s−1.
Figure 9. Cyclic voltammograms corresponding to the bioelectrocatalytic reduction of O2. (A) BOD-immobilized, ABTS2−-loaded MWCNT-modified electrode; (B) BOD-immobilized, ABTS2−-loaded CMK-modified electrode. The HEPES buffer electrolyte (0.1 M, pH = 7.0) was purged with either N2 or O2. Scan rate: 5 mV s−1.
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Figure 10. Nyquist plots of the bioelectrocatalytic oxygen reduction: (A) BOD-immobilized, ABTS2−-loaded CMK-modified electrode. (B) BOD-immobilized, ABTS2−-loaded CMK-3 modified electrode. The HEPES buffer electrolyte (0.1 M, pH = 7.0) was purged with either N2 or O2. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Figure 10. Nyquist plots of the bioelectrocatalytic oxygen reduction: (A) BOD-immobilized, ABTS2−-loaded CMK-modified electrode. (B) BOD-immobilized, ABTS2−-loaded CMK-3 modified electrode. The HEPES buffer electrolyte (0.1 M, pH = 7.0) was purged with either N2 or O2. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
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Figure 11. Polarization (discharge) curves (A) and power density curves (B) obtained by applying different external resistances of GOx/BOD-FcMeOH/ABTS2−-loaded MWCNT- and sustainably sourced CMK-3-modified EFCs with 175 mM glucose concentrated HEPES buffer electrolyte (0.1 M, pH = 7.0) purged with oxygen and measured against variable resistances.
Figure 11. Polarization (discharge) curves (A) and power density curves (B) obtained by applying different external resistances of GOx/BOD-FcMeOH/ABTS2−-loaded MWCNT- and sustainably sourced CMK-3-modified EFCs with 175 mM glucose concentrated HEPES buffer electrolyte (0.1 M, pH = 7.0) purged with oxygen and measured against variable resistances.
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Table 1. Elemental composition CHN of the two support materials, MWCNT and CMK-3.
Table 1. Elemental composition CHN of the two support materials, MWCNT and CMK-3.
SampleC (wt.%)H (wt.%)N (wt.%)S (wt.%)
MWCNT94.730.01--
CMK-391.990.49--
Table 2. Textural properties of the MWCNT and CMK-3 support materials. Total BHJ pore volume calculated at p/p0 = 0.97. Average BHJ pore diameter calculated from the adsorption branch: dp = 4·Vp/SBET.
Table 2. Textural properties of the MWCNT and CMK-3 support materials. Total BHJ pore volume calculated at p/p0 = 0.97. Average BHJ pore diameter calculated from the adsorption branch: dp = 4·Vp/SBET.
Support
Material		BHJ Total
Pore Volume, Vp		BHJ Average
Pore Diameter, dp		BET
Surface Area, SBET
MWCNT0.17 cm3 g−17.44 nm89 m2 g−1
CMK-31.36 cm3 g−14.82 nm1133 m2 g−1
Table 3. Total polarization resistances resulting from Nyquist plots of the bioelectrocatalytic glucose oxidation at varying substrate concentrations for the GOx-FcMeOH-loaded MWCNT-modified electrode and the GOx-FcMeOH-loaded CMK-3 modified electrode. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Table 3. Total polarization resistances resulting from Nyquist plots of the bioelectrocatalytic glucose oxidation at varying substrate concentrations for the GOx-FcMeOH-loaded MWCNT-modified electrode and the GOx-FcMeOH-loaded CMK-3 modified electrode. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Support Material0 mM50 mM110 mM175 mM
MWCNT0.70 kΩ1.05 kΩ1.20 kΩ1.23 kΩ
CMK-30.88 kΩ1.20 kΩ1.22 kΩ1.30 kΩ
Table 4. Total polarization resistance resulting from Nyquist plots of bioelectrocatalytic oxygen reduction for BOD-immobilized, ABTS2−-loaded MWCNT- or CMK-3-modified electrodes. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Table 4. Total polarization resistance resulting from Nyquist plots of bioelectrocatalytic oxygen reduction for BOD-immobilized, ABTS2−-loaded MWCNT- or CMK-3-modified electrodes. Sinusoidal excitation voltage: 20 mV, frequency range: 0.1 Hz–100 kHz.
Support MaterialN2O2
MWCNT1.26 kΩ1.19 kΩ
CMK-31.21 kΩ1.17 kΩ
Table 5. Principal values for the polarization (discharge) curves and power density curves presented in Figure 11, which were obtained by applying different external resistances of GOx/BOD-FcMeOH/ABTS2−-loaded MWCNT- and sustainably sourced CMK-3-modified EFCs with 175 mM glucose concentrated HEPES buffer electrolyte (0.1 M, pH = 7.0) purged with oxygen and measured at different variable external resistances.
Table 5. Principal values for the polarization (discharge) curves and power density curves presented in Figure 11, which were obtained by applying different external resistances of GOx/BOD-FcMeOH/ABTS2−-loaded MWCNT- and sustainably sourced CMK-3-modified EFCs with 175 mM glucose concentrated HEPES buffer electrolyte (0.1 M, pH = 7.0) purged with oxygen and measured at different variable external resistances.
Support MaterialOCPMMP
MWCNT0.49 V53 µW cm−2
CMK-30.58 V93 µW cm−2
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Torrigino, F.; Nagel, M.; Peng, Z.; Hartmann, M.; Herkendell, K. Sustainably Sourced Mesoporous Carbon Molecular Sieves as Immobilization Matrices for Enzymatic Biofuel Cell Applications. Catalysts 2023, 13, 1415. https://doi.org/10.3390/catal13111415

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Torrigino F, Nagel M, Peng Z, Hartmann M, Herkendell K. Sustainably Sourced Mesoporous Carbon Molecular Sieves as Immobilization Matrices for Enzymatic Biofuel Cell Applications. Catalysts. 2023; 13(11):1415. https://doi.org/10.3390/catal13111415

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Torrigino, Federica, Marcel Nagel, Zhujun Peng, Martin Hartmann, and Katharina Herkendell. 2023. "Sustainably Sourced Mesoporous Carbon Molecular Sieves as Immobilization Matrices for Enzymatic Biofuel Cell Applications" Catalysts 13, no. 11: 1415. https://doi.org/10.3390/catal13111415

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