Glucose Fuel Cells and Membranes: A Brief Overview and Literature Analysis

Abstract

Glucose is a ubiquitous source of energy for nearly all living things, and glucose fuel cells (GFCs) are regarded as a sustainable power source because glucose is renewable, easily available, cheap, abundant, non-toxic and easy-to-store. Numerous efforts have been devoted to developing and improving GFC performance; however, there is still no commercially viable devices on the market. Membranes play an essential role in GFCs for the establishment of a suitable local microenvironment, selective ion conducting and prevention of substrate crossover. However, our knowledge on them is still limited, especially on how to achieve comparable efficacy with that of a biological system. This review article provides the first brief overview on these aspects, particularly keeping in sight the research trends, current challenges, and the future prospects. We aim to bring together literature analysis and technological discussion on GFCs and membranes by using bibliometrics, and provide new ideas for researchers in this field to overcome challenges on developing high-performance GFCs.

1. Introduction

Most life on earth depends on photosynthesis, which captures energy from sunlight and produces chemical energy stored in glucose [1]. Glucose is then used by organisms as an energy carrier to synthesize other substances, such as cellulose and starch. Then herbivores get the energy by eating plants, and carnivores get the energy by eating herbivores. Although most biomass is composed of complex carbohydrates, they can be easily broken down into glucose molecules in organisms [2,3,4]. Therefore, glucose is a ubiquitous and versatile energy source for nearly all living things.

In biological systems, glucose is oxidatively decomposed in mitochondrion and releases energy. Mitochondrion acts as a “glucose fuel cell” and is the powerhouse of the living cell. Mitochondrial membrane plays an important role in cell respiration because it provides the basis for the final production of ATP, which is the main energy currency used by organisms [5,6]. The membrane not only acts as a protective barrier to prevent unwanted material from getting into the matrix, but also fractionates the complex lamellar structures and separates the various membrane protein complexes. The glucose decomposition mainly occurs in the presence of oxygen. As Figure 1 depicted, glucose is used in the first step of cell respiration because it represents the main substrate metabolized during glycolysis. After glucose is decomposed into pyruvate, pyruvate molecules are converted into acetyl coenzyme A(CoA). Acetyl CoA is then used in the citric acid cycle to produce carbon dioxide and transfer electrons used in oxidative phosphorylation. The presence of the membrane creates the trans-membrane electrochemical gradient that drives ATP synthesis. Aerobic respiration eventually results in the production of up to 36 ATP per glucose molecule. Glucose in cell respiration represents the main reactant used to produce ATP. Therefore, the role of the membrane in cell respiration is the key to provide energy for organisms [6].

Inspired by the nature, researchers have long considered developing glucose fuel cells (GFCs) with glucose as fuel [7,8,9,10,11,12,13]. After complete oxidation, glucose can release 2.87 MJ mol⁻¹ energy [2]. Compared with other conventional substrates for fuel cells, such as hydrogen gas and methanol, glucose is easily available, low-cost, and non-toxic [14,15,16,17,18,19]. In addition, renewable biomass resources, including starch and cellulose, can be easily converted into glucose molecules at little energy cost [4,17,20]. Furthermore, there are no explosion hazards or storing problems associated with glucose, while hydrogen and methanol are very flammable and can cause fires and explosions when being handled improperly. These advantages make GFCs a promising source for sustainable energy and has attracted great attention from global researchers and institutions [14,18,19,21,22].

The first abiotic and enzymatic GFCs were reported in 1964 aimed to power closed ecological systems, such as space vehicles [7,8]. However, GFCs have not drawn too much concern until the last 30 years. With the arrival of the fourth industrial revolution, the boundaries between the biological, physical, and chemical world began to intersect, fusing new capabilities for the development of novel energy generation systems and applications. GFCs attracted global concern again as an emerging energy conversion device, which can generate electricity in a sustainable manner [23,24,25,26,27]. GFCs are expected to provide energy for diverse applications, such as self-powered sensors, wearable devices, implantable devices, and Internet-of-Things devices [28,29,30,31].

In the past decades, a lot of work has been done on the development of GFCs, but now it seems to have reached a plateau. The key problem is that the performance of all these GFCs is still too poor for practical applications. Glucose has been used as fuel for organisms for billions of years, and the electrochemical processes of glucose synthesis and decomposition in organisms are mostly carried out on the membrane structure. It is believed that the membrane plays an essential role in the energy conversion process in the GFC systems. Understanding the research progress of these fields can provide new ideas for the efficient utilization of glucose. However, as far as we know, there is no review paper on the GFC membranes. In this review, we described the different GFC types and their working mechanisms, highlighting the important role of membranes. In addition, the latest research progress and hotspots were analyzed by using bibliometrics, and the current challenges and future prospects were also discussed. This review is expected to provide an insight for understanding the trends of the GFC membranes and serve as a reference for researchers in this field.

2. GFC Types

Regarding the catalysts employed, GFCs can be divided into three categories: microbial glucose fuel cells (MGFCs), enzymatic glucose fuel cells (EGFCs) and abiotic glucose fuel cells (AGFCs) (Figure 2).

In MGFC, electroactive microorganisms, namely exoelectrogenes, act as catalysts to catalyze the oxidation of glucose, thus converting chemical energy into electrical energy [32,33]. Generally, these electroactive microorganisms do not easily lose their activity, which improves the stability and durability of the equipment [34,35,36]. The structure of a MGFC generally includes an anode, a cathode, and the membrane between them (Figure 2A). Exoelectrogenes on the electrode in the anode chamber provide electrons to the electrode by oxidizing glucose, while oxygen in the cathode chamber accepts electrons and protons to form harmless water. The electrode is generally made of carbon-based materials, such as carbon fiber, carbon sheet, carbon cloth, carbon graphite and carbon nanotube. Membrane is an important component of MGFC. It can reduce the unnecessary flux between the two electrodes and maintain their ionic and chemical conjugation. If there is no membrane, serious substrate crossover will be generated, which will lead to electrode instability and low-efficiency power generation [34,37,38]. Although MGFC has the advantages of a simple device, convenient maintenance, and relatively low cost, the key disadvantages restricting its wide application and commercialization are the significantly low power output and the extremely difficult control of microbial pollution and microbial activity.

EGFCs use enzymes at the anode and cathode to accelerate the redox reaction [39,40]. Due to the specificity of enzymes towards their substrate, EGFCs can operate without electrode separation, which makes their design relatively simple. Because of this characteristic, EGFCs can be miniaturized and used in implants such as biosensors or pacemakers [41]. Many researchers focused on developing enzymes with high selectivity and electrochemical activity, improving their power density and extending their lifetime. Glucose is ubiquitous in plant, animal, and human physiological fluids. For example, the estimated glucose concentration in human blood is about 2–10 mM, which is sufficient to drive EGFCs. Recently, EGFCs have been implanted into insects, clams, lobsters, rats and even humans to provide power for implanted devices [42]. The structure of an EGFC includes an anode and a cathode, but the membrane may not be involved because of the high substrate specificity of the enzymes on each electrode (Figure 2B). In EGFCs, the enzymes at the anode generally include glucose dehydrogenase (GDH) and GOD, and the enzymes at the cathode generally include laccase, ascorbic acid oxidase, copper containing oxidase, and bilirubin oxidase (BOD) [31,42,43]. For example, when GOD and laccase were employed in the anode and cathode, respectively, glucose is converted into glucolactone at the anode, and two electrons are transferred through the external circuit. The laccase on the cathode then catalyzes the conversion of oxygen into water. Compared with MGFCs, EGFCs can have improved performance because of the high activity of the enzyme and they can operate in physiological fluid to generate electricity [44,45,46,47]. However, EGFCS have many limitations derived from enzymes, such as easy denaturation and instability, high production cost, and difficult electron transfer. In addition, when EGFCs are used in cell culture or in vivo implantation systems, their by-products or generated electrical signals may cause damage or stimulation to surrounding tissues. These biological effects may vary depending on the type, quantity, and current density of the enzyme. For example, the high dose of H₂O₂ produced as the by-product of GOD has led to safety concerns [25].

Due to the limitations of MGFCs and EGFCs, many efforts also have been devoted to developing AGFCs [9,11,48,49,50,51,52]. AGFCs use abiotic catalysts to catalyze the glucose oxidation, such as metals, metal oxides, metal sulfides and carbon-based nanomaterials [53,54,55,56,57,58]. Environmental conditions such as pH, temperature and nutrients have less impact on AGFCs than on MGFCs and EGFCs. This feature makes AGFCs suitable for large-scale power supply or implantation in the organism body [59]. In addition, the electrochemical oxidation of glucose is easier in strong alkaline medium because it can facilitate the extraction of hydrogen and the formation of active substances. Compared with acid medium, the kinetics of glucose oxidation reaction (GOR) and oxygen reduction reaction (ORR) in alkaline medium were enhanced [60]. Furthermore, the alkaline environment makes it possible to use non-noble metal catalysts. Pt is the most widely reported abiotic catalyst for glucose. However, it has limitations, such as high cost and low catalytic stability caused by catalyst poisoning. Non-platinum metals (including Au, Ag, Ni, PD, Co, Mn, etc.), carbon-based materials and their composites have been developed as alternative catalysts for AGFCs [2]. As to abiotic catalysts are generally not very specific for substrate, the structure of AGFC generally involves a membrane to separate the cathode and anode. According to different reaction conditions, it can be either the proton exchange membrane (PEM, usually Nafion) or the anion exchange membrane (AEM). Compared with MGFCs and EGFCs, AGFCs generally have a higher performance with a maximum power density ≥0.1 mW cm⁻². However, there is still much room for improvement due to the partial oxidation of glucose, which forms gluconic acid by releasing only two electrons.

3. Membranes for GFCs

3.1. Role of Membranes in GFCs

Membranes for GFCs generally have two main functions [61,62,63,64,65]: (1) they can act as electrolyte between cathode and anode for ion conduction; (2) they can be used as a separator to separate the two half-reactions. There are different half-reactions taken place in different types of GFCs.

Reactions 1–3 show the reactions in a MGFC at neutral/acid conditions [32].

$$\mathrm{Anodic}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O}\to 6{\mathrm{CO}}_2+24{\mathrm{H}}^{+}+24{\mathrm{e}}^{-}$$

(1)

$$\mathrm{Cathodic}\mathrm{reaction}:6{\mathrm{O}}_2+24{\mathrm{H}}^{+}+24{\mathrm{e}}^{-}\to 12{\mathrm{H}}_2\mathrm{O}$$

(2)

$$\mathrm{Overall}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}$$

(3)

Reactions 4–6 show the reactions in an EGFC for glucose and oxygen molecules catalyzed by GOD and laccase immobilized on the anode and cathode, respectively [66,67,68].

$$\mathrm{Anodic}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\stackrel{\mathrm{Glucose}\mathrm{oxidase}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_6+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}$$

(4)

$$\mathrm{Cathodic}\mathrm{reaction}:\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\stackrel{\mathrm{Laccase}}{\to }{\mathrm{H}}_2\mathrm{O}$$

(5)

$$\mathrm{Overall}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_6+{\mathrm{H}}_2\mathrm{O}$$

(6)

Reactions 7–9 show the reactions in an AGFC for glucose and oxygen at alkaline conditions (pH 12) [69].

$$\mathrm{Anodic}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+36{\mathrm{OH}}^{-}\to 6{\mathrm{CO}}_3^{2-}+24{\mathrm{H}}_2\mathrm{O}+24{\mathrm{e}}^{-}$$

(7)

$$\mathrm{Cathodic}\mathrm{reaction}:6{\mathrm{O}}_2+12{\mathrm{H}}_2\mathrm{O}+24{\mathrm{e}}^{-}\to 24{\mathrm{OH}}^{-}$$

(8)

$$\mathrm{Overall}\mathrm{reaction}:{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+12{\mathrm{OH}}^{-}\to 6{\mathrm{CO}}_3^{2-}+12{\mathrm{H}}_2\mathrm{O}$$

(9)

An ion-conducting membrane is employed to realize the above dual purposes. It acts both as a separator for substrates and an electrolyte for ions between cathode and anode. Two types of membranes are commonly used in glucose fuel cells: acidic and alkaline membrane. Acidic membranes are also known as cation exchange membranes (CEM) or proton exchange membranes (PEM) (Figure 3). In the acidic environment, H⁺ cations and protons will be able to easily flow through the acidic membrane, whereas electrons or anions will encounter greater resistance. Alkaline membranes are commonly referred to as the anion exchange membrane (AEM) and is made from cationic OH⁻ conducting solid polymers. In the alkaline environment, the AEM allows anions to flow and hinders the flow of cations to a great extent. The AEM has been reported to work effectively in many AGFCs with substantial ionic conductivities. Compared with PEMs which are widely used in neutral/acid fuel cells, AEMs are usually low-cost [70].

Theoretically, the number of electrons to be exchanged in a GFC is 24 [69]. However, the maximum number of extracted electrons per glucose molecule has been shown to be far less than this number. For example, the measurement of reaction intermediates in EGFCs indicated that the oxidation of glucose mainly leads to the production of gluconolactone. Furthermore, in single-chamber fuel cells, the substrate crossover leads to a reduced efficiency at the electrodes and a lower overall performance. The use of ion-conducting membranes in GFCs is expected to alleviate/solve these problems.

GFCs are usually operated with a complex substrate matrix. In these cases, the selectivity for ions of membranes plays an essential role for the normal operation of GFCs, especially those constructed for the purpose of wearable devices and biological implantations. A wide range of biomolecules and cells can precipitate onto the electrode surface and thus interfere with the electron transfer or/and inhibit the catalyst activity. This problem can be overcome by the entrapment of the catalyst into membranes to prevent or minimize biofouling from interfering compounds. Membrane encapsulations and separators are used not only to separate the anolyte and catholyte, but can also provide a good microenvironment for catalyst immobilization, which increases the reaction efficiency and selectivity. The separators inhibit the diffusion of chemical mediators and other chemicals from the cathode chamber to the substrates at the anode chamber, thereby reducing unwanted substrate flux from the anode to cathode and ultimately improving the GFC performance [25].

3.2. Membranes in GFCs Subsection

3.2.1. PEM

PEM is the most commonly used CEM in GFCs. According to the fluorine content, PEM can be divided into perfluorinated PEM, partially fluorinated PEM, non-fluorinated PEM, and composite PEM [70]. Perfluorosulphonic acid polymer (nafion) is the most commonly used material for PEM because of its excellent ionic conductivity (10⁻² S cm⁻¹) [71,72,73]. The main chain of nafion has the structure of polytetrafluoroethylene, which brings excellent chemical stability, thermal stability, and high mechanical strength. The excellent ion conductivity characteristic is attributed to the hydrophilic sulfonate group (–SO₃⁻) attached to the hydrophobic fluorocarbon backbone. Besides the sulfonate group, carboxylate (–COO⁻) and phosphonate (–PO₃²⁻) are also important ionized groups for various ionomers. The polymer structures can be slightly different in the length of the main chain. Partially fluorinated PEM has low cost, but it is not as durable as perfluorinated PEM [70]. Polyimides, such as sulfonated polyimides and polybenzimidazoles, contain a series of aromatic polymers, which have good film-forming properties, excellent chemical and mechanical stability, and high proton conductivity [65,74,75].

3.2.2. AEM

Although PEM is widely used in the design of biotic GFCs, sometimes it is ineffective in improving catalyst stability because of its acidic microenvironment. AEM may be a suitable alternative in these cases. AEM contains positively charged cation groups, allowing for the passage of negatively charged ions through the membrane [70]. Polyvinyl alcohol (PVA) and polysulfone (PS) based membranes are well researched AEMs for fuel cell applications [76,77,78,79,80,81]. PVA has good hydrophilicity and highly active functional groups, and it can be crosslinked by radiation and thermochemical treatment. The side chain hydroxyl of PVA provides an opportunity to change its structure. The modification of PVA by quaternization groups can significantly improve the stability of these kinds of membranes. AEM is more prone to degradation or loss of mechanical properties under hydration conditions than PEM [70]. Therefore, the mechanical properties of the membrane play a key role in the operation process because the integrity of the membrane will affect the life of the fuel cell. Compared with other polymers, PS has the highest temperature performance. In addition, its high chemical inertness makes it suitable for fuel cell membranes.

3.2.3. Other Membranes

Despite the advantages of polymer-based membranes, GFCs with a polymer membrane exhibit several drawbacks related to the polymer nature of the membrane: polymer membranes require a minimum thickness to avoid substrate cross-over; challenges surrounding integration with microfabrication design; and the inability for thermal sterilization [82,83]. These challenges may be overcome by developing GFCs that operate on ceramic proton-conducting electrolytes [84]. Ceramic membranes serve as a cost-effective substitute to polymer membranes and have the capability to lower the diffusion rate of the glucose and increase the surface loading for the catalyst. In addition, the metal–organic framework and other composite nanomaterials are also explored for catalyst encapsulation and membrane fabrication [85].

4. Literature Analysis

4.1. Data Gathering and Data Analysis

The literature analysis used data from Web of Science Core Collection database. In total, 497 documents were extracted by using (TS = glucose) AND ((TS = “fuel cell”) OR (TS = “fuel cells”)) AND (TS = membrane) as the query string for a period from 1992 to 2021. Bibliometric tools, such as CiteSpace [86] and Biblioshiny [87], were used for visualizing and analyzing trends and patterns in scientific literature. Gephi was used for visualizing bibliometric networks [88].

4.2. Literature Analysis of Research Progress

4.2.1. Output of the Research Publications

The publications related to the GFCs and the membrane were summarized in Figure 4. Before 2005, there were fewer publications, and the annual publications were less than 10. In 2006, attention was focused on this field, and the number of publications grew rapidly. From 2006 to 2017, the number of articles kept increasing, with an annual number of 10–40 publications. In 2017, the number reached its top (43 publications). After 2017, the number decreased for two years (2018–2019), then the growth resumed in 2020 and 2021. The possible reason for this phenomenon is that the global research trend may be easily affected by the energy policy of different countries. It also implies that the development of this area may have reached a plateau stage, and that further development would need a bottleneck breakthrough. Figure S1 shows the three-fields plot of countries, affiliations, and keywords of the publications on GFCs and membranes, demonstrating the top 20 affiliations in different countries.

4.2.2. Co-Occurrence Analysis of Keywords

Top 50 most frequent keywords are shown in Figure S2, and word growth dynamics of top 10 most frequent keywords during 1992 to 2021 are depicted in Figure S3. Figure 5 shows the co-occurrence analysis result of the top 35 keywords. Among them, microbial fuel cell has the highest connection density, while fuel cell has the largest betweenness centrality, indicating its role as a bridge center in this field. We can also see that the nodes of performance, membrane and biofuel cell were relatively large. There were more than 100 articles included in these keywords, implying that they represented research hotspots.

4.2.3. Burst Detection Analysis

Figure 6 shows 15 burst words based on the keywords from 1992 to 2021 (red parts), they reflect the research frontier in different times. Glucose, mediator, Fe(iii) reducing bacterium and electron transfer were the most lasting burst words throughout this whole period. However, they all ended before 2010, demonstrating that they may be no longer representing the cutting-edge research topics in this field. Reduction was the most lasting burst word after 2010, being a hotspot from 2010 to 2014. New burst words emerged every two years after 2010, reflecting the fast-developing speed in this field. Glucose oxidase, immobilization and membrane fuel cell were the most recent burst words. This result is interesting and may represent the succession process of GFC research. The development of GFCs has experienced three stages: MGFC developed rapidly before 2010; Then AGFC took its place. After 2015, the development of EGFC attracted more attention. Glucose oxidase has long been used for glucose sensing. The concept of a glucose sensor by using glucose oxidase was firstly proposed in 1962 by Clark and Lyons [89], and the GOD-based glucose-sensing technology has reached the commercial phase for many years [42]. Therefore, the combination of GFC technology and glucose-sensing technology can bring a new product for self-powered continuous glucose monitoring to market [90].

4.2.4. Analysis of Leading Countries and International Cooperation

Figure 7 shows the collaborations between countries. China and USA were the most active countries. As the center of global cooperation, they work closely with each other and many countries around the world. China has cooperated with 17 countries, and the United States has cooperated with 9 countries. A total of 98 articles were completed through cooperation between countries, of which China and the United States cooperated most frequently. It can be seen that there is still a lot of room for global cooperation on GFC-related topics in the future.

5. Research Challenges

Nearly half a century has passed since the first GFC was reported. It is a fact that significant progress has been made in the improvement of electrochemical performance and the increase of lifetime. However, the availability of these systems as commercial equipment is still far behind. GFCs involve electrochemical reactions and ion transport, in which electrolyte and electrode characteristics play a major role in cell performance. Compared with the complex biological membrane system in living cells, GFCs lack a similar mechanism to ensure the highly efficient glucose oxidation and selective ion transfer. Normally, only two electrons are released from partial oxidation of glucose to form gluconolactone [69]. The actual performance of GFC is low because the subsequent oxidation steps are much slower and do not contribute significantly to the total electron yield. In addition, the service life of the membrane is limited and needs to reach thousands of hours without performance degradation.

Electrode and membrane materials are needed in order to have the electrochemical properties required for fast electrode dynamics and the high conductivity required for charge transfer. Membrane electrode assembly (MEA) structure is commonly used because it can provide an enhanced membrane/electrode interface for improved fuel cell performance and durability [91,92]. Since substrates and reaction products must be transferred to/from the membrane/electrode interface, electrode and membrane porosity must be designed to ensure adequate mass and charge transfer, as well as the maximum number of active reaction sites per unit surface area. The matrix porosity and pore size distribution must be properly designed to ensure proper pore filling and ideal contact with electrode materials and substrates.

In addition, each type of GFC system has its unique challenges. MGFCs are currently facing problems such as short life, high cost, low productivity, and limited efficiency. These shortcomings are generally related to membrane pollution and instability, the high cost of the membrane, and insufficient length of the membrane life [32]. EGFCs have limited power density, low voltage output and poor operation stability. This is because the complex protein structure of enzymes will degrade over time, so the life of the EFC is generally short [93,94]. AGFCs are of great advantage due to their higher volumetric and gravimetric energy densities [2]. Alkaline conditions are particularly interesting because they are compatible with the use of non-noble metal catalysts at low temperatures. In the alkaline environment, the electrooxidation rate of glucose is higher than that in acidic and neutral media. This was because the catalytic activity of glucose oxidation and oxygen reduction in alkaline medium was better than that in acidic medium. However, there are some remaining problems to be solved: low hydroxide conductivity, the effect of carbonation, and insufficient long-term chemical stability of the membrane under alkaline environment [2,69].

Furthermore, the success of GFC is not only about the technology but also about the cost. Despite all the achievements in the development of the GFC and its promising potential as a sustainable power source, commercialization is still a major concern. Present GFCs devices normally use noble metals as a catalyst and nafion as a membrane, which form the largest cost components in the GFC devices. The overall performance of alternative membranes is still low, and more work needs to be done to improve performance and reduce costs.

6. Some Latest Solutions for GFCs

Various solutions have been developed by the global researchers to enhance the performance, reduce the fabrication cost, and practicalize GFCs. Some solutions are listed as follows:

Developing highly efficient and robust catalysts for the glucose oxidation reaction has been a long-standing pursuit. The electro-oxidation of glucose was studied on various metals, particular Au, Pt and Pd, and different metal oxides (Ni, Co, Mn and Ru oxides). The latter have been proposed as low-cost catalysts for glucose electro-oxidation, especially in alkaline solutions [10,12,16]. Gao et al. prepared Ni-Co composite catalysts by the NaBH₄ reduction method, and the GFC performance was greatly improved with the addition of the Ni-Co composite catalyst in the anode [16]. Viologens can also oxidize glucose in alkaline solutions, making them potential catalysts in GFCs [10,17]. Liu et al. reported a one-compartment GFC that use methyl viologen as an electron mediator and a nickel foam as the anode. The rudimentary fuel cell generates a peak power density of 0.62 mW cm⁻², and deep oxidation of glucose was achieved [10]. Polyoxometalate (POM) is also a powerful abiotic catalyst for glucose oxidation. Liu et al. reported a solar-induced hybrid fuel cell using polyoxometalates as the photocatalyst and charge carrier [95]. Its principle is that H₃PMo₁₂O₄₀ can oxidize biomass under solar irradiation while being reduced from Mo⁶⁺ to Mo⁵⁺, and then Mo⁵⁺ can be oxidized to Mo⁶⁺ again by oxygen through a catalytic electrochemical reaction. Li et al. presented an abiotic GFC with excellent performance by using a Lewis-acidic POM (Co²⁺-P(W₃O₁₀)₄³⁻) modified activated carbon as an anode catalyst (Figure 8A) [96]. The catalytic activities of POMs for glucose oxidation can be remarkably improved by the introduction of Lewis acidity, and the enhanced anode performance can be attributed to the synergic effect of two reversible redox systems.

Implantable GFCs generally need a high degree of miniaturization, good electrochemical conversion efficiency and long-term stability. Although most GFCs are polymer-based, they have many technical challenges, including miniaturization, silicon on-chip integration, and thermal sterilization. Simons et al. presented an ultrathin GFC that is fabricated entirely from ceramics and noble metals (Figure 8B) [84]. The entire GFC is fabricated on a silicon chip using standard semiconductor microfabrication techniques. The device is also resilient and remains stable through the high-temperature sterilization process required for all implantable devices. Kenya et al. reported a self-powered continuous glucose monitoring system contact lens by combining a low-current transmitter with a GFC that functions as both the power source and sensing transducer (Figure 8C) [97]. The prototype demonstrated the potential of GFCs for self-powered operation. Fabrication of an outer membrane is crucial for an implantable device to maintain the long-term stability. Jin reported an adaptable, controllable, porous outer membrane for an implantable biosensor, allowing fine control of the membrane microstructure and maximum retention of the enzyme activity (Figure 8D) [98]. The glucose biosensor coated with the membrane exhibited a more stable output current than bare sensors.

Wearable electronics require high-power and sustainable energy sources in order to realize continuous, multiple, and complex tasks, such as the Internet of Things for human body data or human–machine interactions [90,99]. However, conventional power systems, including lithium ion, alkaline batteries and solar cells, are generally heavyweight and their performance is hard to maintain. Wang et al. proposed a wearable and flexible textile-based GFC using moisture management fabric (Figure 8E). The biofuel cell utilizes glucose as a fuel and carbon fabrics as an enzyme substrate [100]. The fabric consists of polyester with modified structures for quick water absorption, wicking, and evaporation. This design enables long-term, high-speed, continuous flow in the fabric, and it ensures the sufficient fuel supply and efficient molecule transport for the redox reactions. Therefore, sustainable power generation can be more effectively realized in this sportswear-based GFC than in other textile-based GFCs.

Figure 8. Some latest solutions for GFCs. (A) Polyoxometalate-based catalyst for glucose oxidation reactions [96]. (B) Optical photograph of an ultrathin fuel-cell chip containing 30 individual GFC devices (I) and optical microscopy image of an individual freestanding ceria membrane (II) [84]. (C) Combination of a low-power transmitter and a glucose fuel cell allows for the development of glasses-free contact lens for continuous glucose monitoring [97]. (D) An adaptable, controllable, porous outer membrane for an implantable glucose sensor allows maximum retention of enzyme activity and fine control over membrane microstructure [98]. (E)A sustainable and high-power wearable GFC using long-term and high-speed flow in sportswear fabrics [100].

Figure 8. Some latest solutions for GFCs. (A) Polyoxometalate-based catalyst for glucose oxidation reactions [96]. (B) Optical photograph of an ultrathin fuel-cell chip containing 30 individual GFC devices (I) and optical microscopy image of an individual freestanding ceria membrane (II) [84]. (C) Combination of a low-power transmitter and a glucose fuel cell allows for the development of glasses-free contact lens for continuous glucose monitoring [97]. (D) An adaptable, controllable, porous outer membrane for an implantable glucose sensor allows maximum retention of enzyme activity and fine control over membrane microstructure [98]. (E)A sustainable and high-power wearable GFC using long-term and high-speed flow in sportswear fabrics [100].

7. Future Perspectives

It is believed that further work and research in the following area will help to promote the development of GFCs and membranes:

(1)

Researchers can draw inspiration from biological systems to find solutions. Researchers have always been fascinated by biological systems because of their complexity and efficiency in accomplishing the tasks required to thrive. Biomimetic membranes are expected to break through existing bottlenecks by using the strategies that nature has evolved over billions of years for in order to improve transport efficiency and specificity.

(2)

Fundamental and extensive analytical studies must be conducted to understand the processes that occur in GFC membranes in operando. Ion exchange dynamics in the membranes, the effect of carbonation, and the swelling of the polymer upon hydration are some of the fundamental investigations needed to be cultivated in this field. These investigations also need new methods for ex-situ and in-situ characterization.

(3)

Composite membrane materials can combine the required characteristic and are promising to address most of the outstanding issues and challenges. A wide range of properties can be available through appropriate doping, structural characteristics adjustment, core-shell formation, and even composite microstructures. For example, heterostructures can be precisely assembled to provide unique ion and electron transport properties. The composite membrane can also provide an appropriate balance among various features.

(4)

GFCs are expected to incorporate with various other commercialized technology to realize multiple applications, such as continuous glucose monitoring, drug smart delivery, self-sustained sensing, and targeted therapy for cancer. For implantation devices, the incorporation with GFCs make it possible to remove external power sources, which drastically simplifies the electronics required and allows for miniature designs.

(5)

GFC research and its successful commercialization must be achieved through extensive global cooperation among researchers with different expertise. The global cooperation can speed up the resolution of bottleneck issues and promote the innovation ability of the international community.

8. Conclusions and Outlooks

This work presented a brief summary of GFC membranes and conducted a literature investigation into the research progress, with emphasis on the research trends and current challenges. GFCs are indeed the future of the renewable energy sector; however, cheap membranes are needed to reduce the overall cost of GFCs without performance compromise. For GFCs operated with a complex substrate matrix, membrane encapsulations and separators are useful to separate the substrates and provide a microenvironment conducive to electrochemical reactions. The global research trend analysis indicated that GFC studies may have reached a plateau stage, and the further development of GFCs calls for a technical breakthrough. The keywords of GFC performance, membrane, and biofuel cell represented current research hotspots. Burst words analysis indicated that glucose oxidase, immobilization, and membrane fuel cell were the most recent burst words, implying the possible combination of GFC technology with glucose-sensing technology. China and USA were the most active countries, and they were also the countries with the closest cooperation. However, there is still a lot of room for global cooperation on GFC research in the future.