Benchmarking organic active materials for aqueous redox flow batteries in terms of lifetime and cost.

Flow batteries are one option for future, low-cost stationary energy storage. We present a perspective overview of the potential cost of organic active materials for aqueous flow batteries based on a comprehensive mathematical model. The battery capital costs for 38 different organic active materials, as well as the state-of-the-art vanadium system are elucidated. We reveal that only a small number of organic molecules would result in costs close to the vanadium reference system. We identify the most promising candidate as the phenazine 3,3'-(phenazine-1,6-diylbis(azanediyl))dipropionic acid) [1,6-DPAP], suggesting costs even below that of the vanadium reference. Additional cost-saving potential can be expected by mass production of these active materials; major benefits lie in the reduced electrolyte costs as well as power costs, although plant maintenance is a major challenge when applying organic materials. Moreover, this work is designed to be expandable. The developed calculation tool (ReFlowLab) accompanying this publication is open for updates with new data.

Vanadium Redox Flow Battery Using Activated Carbon Catalyst Produced from Low-Density Polyethylene.

Carbonized and activated low-density polyethylene (LDPE) is suggested as a carbon catalyst for vanadium redox flow battery (VRFB). This carbon catalyst has many surface oxygen functional groups and a large surface area, while such benefits are achieved through activation of carbonized LDPE. According to electrochemical analysis, this carbon catalyst doped graphite felt (GF) enhances the redox reactivity of vanadium ions. More specifically, peak current density and peak potential separation for redox reaction of vanadium ions are 96.0 and 22.1% more improved than those measured by bare GF, while charge transfer resistance for the redox reactions is also improved by use of the catalyst doped GF. When performance of VRFB using this catalyst doped GF is measured, energy efficiency is 39% more improved than that measured without the catalyst. Based on that, this is revealed that new LDPE-based carbon catalyst is effective for performance improvement of VRFB.

Three-Dimensional Hierarchical Core/shell Electrodes Using Highly Conformal TiO2 and Co3O4 Thin Films for High-Performance Supercapattery Devices.

The rational design and development of novel electrode materials with promising nanostructures is an effective technique to improve their supercapacitive performance. This work presents high-performance core/shell electrodes based on three-dimensional hierarchical nanostructures coated with conformal thin transition-metal oxide layers using atomic layer deposition (ALD). This effective interface engineering creates disorder in the electronic structure and coordination environment at the interface of the heteronanostructure, which provides many more reaction sites and rapid ion diffusion. At 3 A g-1, the positive CuCo2O4/Ni4Mo/MoO2@ALD-Co3O4 electrode introduced here exhibits a specific capacity of 1029.1 C g-1, and the fabricated negative Fe3O4@ALD-TiO2 electrode significantly outperforms conventional carbon-based electrodes, with a maximum specific capacity of 372.6 C g-1. The supercapattery cell assembled from these two interface- and surface-tailored electrodes exhibits a very high energy density of 110.4 W h kg-1 with exceptional capacity retention over 20,000 cycles, demonstrating the immense potential of ALD for the next generation of supercapacitors.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices.

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

New Biocatalyst Including a 4-Nitrobenzoic Acid Mediator Embedded by the Cross-Linking of Chitosan and Genipin and Its Use in an Energy Device.

A new anodic catalyst consisting of carbon nanotube, 4-nitrobenzoic acid, chitosan, genipin, and glucose oxidase (GOx) (CNT/4-NBA/[Chit/GOx/GP]) is suggested to promote the glucose oxidation reaction (GOR) and the performance of enzymatic biofuel cell (EBC). In this catalyst, through the cross-linked structure of chitosan and genipin and the proper distribution of amine groups within chitosan, many GOx molecules are maximally captured, their leaching out is suppressed, and the GOR is improved upon. In addition, 4-nitrobenzoic acid plays the role of mediator well. The effect induced by the cross-linked structure is evaluated by ultraviolet-visible (UV-vis) spectroscopy, pH measurements, and electrochemical characterizations. According to the characterizations, the new CNT/4-NBA/[Chit/GOx/GP] catalyst contains a large amount of GOx (17.8 mg/mL) and produces a high anodic current (331 µA/cm2 at 0.3 V vs Ag/AgCl) with a low onset potential (0.05 V vs Ag/AgCl) because its catalytic activity follows the desirable reaction pathway that minimizes creation of a protonated amine group that interferes with GOR. When the performance of EBC using this catalyst as an anodic electrode is measured, the EBC shows a high open-circuit voltage of 0.54 V and a maximum power density of 38 µW/cm2.

Interface-Engineered Nickel Cobaltite Nanowires through NiO Atomic Layer Deposition and Nitrogen Plasma for High-Energy, Long-Cycle-Life Foldable All-Solid-State Supercapacitors.

The large-scale application of supercapacitors (SCs) for portable electronics is restricted by low energy density and cycling stability. To alleviate the limitations, a unique interface engineering strategy is suggested through atomic layer deposition (ALD) and nitrogen plasma. First, commercial carbon cloth (CC) is treated with nitrogen plasma and later inorganic NiCo2 O4 (NCO)/NiO core-shell nanowire arrays are deposited on nitrogen plasma-treated CC (NCC) to fabricate the ultrahigh stable SC. An ultrathin layer of NiO deposited on the NCO nanowire arrays via conformal ALD plays a vital role in stabilizing the NCO nanowires for thousands of electrochemical cycles. The optimized NCC/NCO/NiO core-shell electrode exhibits a high specific capacitance of 2439 F g-1 with a remarkable cycling stability (94.2% over 20 000 cycles). Benefiting from these integrated merits, the foldable solid-state SCs are fabricated with excellent NCC/NCO/NiO core-shell nanowire array electrodes. The fabricated SC device delivers a high energy density of 72.32 Wh kg-1 at a specific capacitance of 578 F g-1 , with ultrasmall capacitance decline rate of 0.0003% per cycle over 10 000 charge-discharge cycles. Overall, this strategy offers a new avenue for developing a new-generation high-energy, ultrahigh stable supercapacitor for real-life applications.

Effect of Carboxylic Acid-Doped Carbon Nanotube Catalyst on the Performance of Aqueous Organic Redox Flow Battery Using the Modified Alloxazine and Ferrocyanide Redox Couple.

Alloxazine and ferrocyanide are suggested as the redox couple for an aqueous organic redox flow battery (AORFB). Alloxazine is further modified by carboxylic acid (COOH) groups (alloxazine-COOH) to increase the aqueous solubility and to pursue a desirable shift in the redox potential. For obtaining a better AORFB performance, the overall redox reactivity of AORFB should be improved by the enhancement of the rate-determining reaction of the redox couple. A carboxylic acid-doped carbon nanotube (CA-CNT) catalyst is considered for increasing the reactivity. The utilization of CA-CNT allows for the induction of a better redox reactivity of alloxazine-COOH because of the role of COOH within alloxazine-COOH as a proton donor, the fortified hydrophilic attribute of alloxazine-COOH, and the increased number of active sites. With the assistance of these attributes, the mass transfer of aqueous alloxazine-COOH molecules can be promoted. However, CA-CNT does not have an effect on the increase of the redox reactivity of ferrocyanide because the redox reaction is not affected by the same influence of protons that the redox reactivity of alloxazine-COOH is affected by. Such a behavior is proven by measuring the electron transfer rate constant and diffusivity. With regard to AORFB full cell testing, when CA-CNT is used as a catalyst for the negative electrode, the performance of the AORFB increases. Specifically, the charge-discharge overpotential and infrared drop potential are improved. As a result, the voltage efficiency affected by the potentials increases to 64%. Furthermore, the discharging capacity reaches 26.7 A h·L-1, and the state of charge attains 83% even after 30 cycles.

Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses.

15, 25, and 35 µm thick meta-polybenzimidazole (PBI) membranes are doped with H2SO4 and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H2SO4, PBI absorbs about 2 mol of H2SO4 per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, meta-PBI shows a negligible crossover of V3+ and V4+ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBI's nanosized pores. It turns out that charge efficiency (CE) of VRFBs using meta-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick meta-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin meta-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H2SO4-doped meta-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 µm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm-2.

Glucose oxidase and polyacrylic acid based water swellable enzyme-polymer conjugates for promoting glucose detection.

Glucose oxidase (GOx) and polyacrylic acid (PAA) based water swellable non-toxic enzyme-polymer conjugate (PAA-GOx) was immobilized on a substrate consisting of graphene oxide (GO) and polyethyleneimine (PEI) (GO-PEI) and the electrochemical performances of the new catalyst were investigated. According to the measurements, although the amount of GOx immobilized on PAA-GOx was lower than that on glutaraldehyde (GA)-GOx, which is a conventionally used conjugate, its catalytic activity was 9.6 times higher than that of GA-GOx. The superior catalytic activity (102.0 µA cm-2, 20 mM of glucose) and glucose sensitivity (6.9 µA cm-2 mM-1) were due to its high swellability in water. Due to this, the PAA-GOx absorbs a large amount of aqueous glucose molecules and rapidly transfers them to the active site of GOx. Desirable hydrogen peroxide and glucose oxidation reactions are accordingly promoted. In addition, since PAA has abundant free carboxylic acid groups, the PAA-GOx forms covalent bonds with the GO-PEI to curtail the leaching-out of GOx molecules.

Co-immobilization of glucose oxidase and catalase for enhancing the performance of a membraneless glucose biofuel cell operated under physiological conditions.

Glucose oxidase (GOx)-catalase co-immobilized catalyst (CNT/PEI/(GOx-Cat)) was synthesized, and its catalytic activity and electrical performance were investigated and compared, whereas the amount of immobilized catalase was optochemically inspected by chemiluminescence (CL) assay. With the characterizations, it was confirmed that the catalase was well immobilized on the CNT/PEI surface, whereas both the GOx and catalase play their roles well in the catalyst. According to the measurements of the current density peak of the flavin adenine dinucleotide (FAD) redox reaction, electron transfer rate, Michaelis-Menten constants and sensitivity, CNT/PEI/(GOx-Cat) shows the best values, and this is attributed to the excellent catalytic activity of GOx and the H2O2 decomposition capability of the catalase. To evaluate the electrical performance, a membraneless glucose biofuel cell (GBFC) adopting the catalyst was operated under physiological conditions and produced a maximum power density (MPD) of 180.8 ± 22.3 µW cm-2, which is the highest value compared to MPDs obtained by adoption of other catalysts. With such results, it was clarified that the CNT/PEI/(GOx-Cat) manufactured by co-immobilization of GOx and catalase leads to enhancements in the catalytic activity and GBFC performance due to the synergetic effects of (i) effective removal of harmful H2O2 moiety by catalase and (ii) superior activation of desirable reactions by GOx.

Yeast and carbon nanotube based biocatalyst developed by synergetic effects of covalent bonding and hydrophobic interaction for performance enhancement of membraneless microbial fuel cell.

Membraneless microbial fuel cell (MFC) employing new microbial catalyst formed as yeast cultivated from Saccharomyces cerevisiae and carbon nanotube (yeast/CNT) is suggested. To analyze its catalytic activity and performance and stability of MFC, several characterizations are performed. According to the characterizations, the catalyst shows excellent catalytic activities by facile transfer of electrons via reactions of NAD, FAD, cytochrome c and cytochrome a3, while it induces high maximum power density (MPD) (344mW·m-2). It implies that adoption of yeast induces increases in catalytic activity and MFC performance. Furthermore, MPD is maintained to 86% of initial value even after eight days, showing excellent MFC stability.

Enzyme precipitate coating of pyranose oxidase on carbon nanotubes and their electrochemical applications.

Pyranose oxidase (POx), which doesn't have electrically non-conductive glycosylation moiety, was immobilized on carbon nanotubes (CNTs) via three different preparation methods: covalent attachment (CA), enzyme coating (EC) and enzyme precipitate coating (EPC). CA, EC and EPC of POx on CNTs were used to fabricate enzymatic electrodes for enzyme-based biosensors and biofuel cells. Improved enzyme loading of EPC resulted in 6.5 and 4.5 times higher activity per weight of CNTs than those of CA and EC, respectively. After 34 days at room temperature, EPC retained 65% of initial activity, while CA and EC maintained 9.2% and 26% of their initial activities, respectively. These results indicate that precipitation and crosslinking steps of EPC have an important role in maintaining enzyme activity. To demonstrate the feasibility of POx-based biosensors and biofuel cells, the enzyme electrodes were prepared using CA, EC, and EPC samples. In the case of biosensor, the sensitivities of the CA, EC, and EPC electrodes without BQ were measured to be 0.27, 0.76 and 3.7mA/M/cm2, while CA, EC and EPC electrode with BQ showed 25, 25, and 60mA/M/cm2 of sensitivities, respectively. The maximum power densities of biofuel cells using CA, EC and EPC electrodes without BQ were 41, 47 and 53µW/cm2, while CA, EC and EPC electrodes with BQ showed 260, 330 and 500µW/cm2, respectively. The POx immobilization and stabilization via the EPC approach can lead us to develop continuous glucose monitoring biosensors and high performing biofuel cells.

Fabrication of Mediatorless/Membraneless Glucose/Oxygen Based Biofuel Cell using Biocatalysts Including Glucose Oxidase and Laccase Enzymes.

Mediatorless and membraneless enzymatic biofuel cells (EBCs) employing new catalytic structure are fabricated. Regarding anodic catalyst, structure consisting of glucose oxidase (GOx), poly(ethylenimine) (PEI) and carbon nanotube (CNT) is considered, while three cathodic catalysts consist of glutaraldehyde (GA), laccase (Lac), PEI and CNT that are stacked together in different ways. Catalytic activities of the catalysts for glucose oxidation and oxygen reduction reactions (GOR and ORR) are evaluated. As a result, it is confirmed that the catalysts work well for promotion of GOR and ORR. In EBC tests, performances of EBCs including 150 µm-thick membrane are measured as references, while those of membraneless EBCs are measured depending on parameters like glucose flow rate, glucose concentration, distance between two electrodes and electrolyte pH. With the measurements, how the parameters affect EBC performance and their optimal conditions are determined. Based on that, best maximum power density (MPD) of membraneless EBC is 102 ± 5.1 µW · cm(-2) with values of 0.5 cc · min(-1) (glucose flow rate), 40 mM (glucose concentration), 1 mm (distance between electrodes) and pH 3. When membrane and membraneless EBCs are compared, MPD of the membraneless EBC that is run at the similar operating condition to EBC including membrane is speculated as about 134 µW · cm(-2).

Development of a glucose oxidase-based biocatalyst adopting both physical entrapment and crosslinking, and its use in biofuel cells.

New enzymatic catalysts prepared using physical entrapment and chemical bonding were used as anodic catalysts to enhance the performance of enzymatic biofuel cells (EBCs). For estimating the physical entrapment effect, the best glucose oxidase (GOx) concentration immobilized on polyethyleneimine (PEI) and carbon nanotube (CNT) (GOx/PEI/CNT) was determined, while for inspecting the chemical bonding effect, terephthalaldehyde (TPA) and glutaraldehyde (GA) crosslinkers were employed. According to the enzyme activity and XPS measurements, when the GOx concentration is 4 mg mL(-1), they are most effectively immobilized (via the physical entrapment effect) and TPA-crosslinked GOx/PEI/CNT(TPA/[GOx/PEI/CNT]) forms π conjugated bonds via chemical bonding, inducing the promotion of electron transfer by delocalization of electrons. Due to the optimized GOx concentration and π conjugated bonds, TPA/[GOx/PEI/CNT], including 4 mg mL(-1) GOx displays a high electron transfer rate, followed by excellent catalytic activity and EBC performance.

Fabrication of a biofuel cell improved by the π-conjugated electron pathway effect induced from a new enzyme catalyst employing terephthalaldehyde.

A model explaining the π-conjugated electron pathway effect induced by a novel cross-linker adopted enzyme catalyst is suggested and the performance and stability of an enzymatic biofuel cell (EBC) adopting the new catalyst are evaluated. For this purpose, new terephthalaldehyde (TPA) and conventional glutaraldehyde (GA) cross-linkers are adopted on a glucose oxidase (GOx), polyethyleneimine (PEI) and carbon nanotube (CNT)(GOx/PEI/CNT) structure. GOx/PEI/CNT cross-linked by TPA (TPA/[GOx/PEI/CNT]) results in a superior EBC performance and stability to other catalysts. It is attributed to the π bonds conjugated between the aldehyde of TPA and amine of the GOx/PEI molecules. By π conjugation, electrons bonded with carbon and nitrogen are delocalized, promoting the electron transfer and catalytic activity with an excellent EBC performance. The maximum power density (MPD) of an EBC adopting TPA/[GOx/PEI/CNT] (0.66 mW cm(-2)) is far better than that of the other EBCs (the MPD of EBC adopting GOx/PEI/CNT is 0.40 mW cm(-2)). Regarding stability, the covalent bonding formed between TPA and GOx/PEI plays a critical role in preventing the denaturation of GOx molecules, leading to an excellent stability. By repeated measurements of the catalytic activity, TPA/[GOx/PEI/CNT] maintains its activity to 92% of its initial value even after five weeks.

Enhanced electrochemical sensitivity of enzyme precipitate coating (EPC)-based glucose oxidase biosensors with increased free CNT loadings.

Enzymatic electrodes were fabricated by using three different immobilizations of glucose oxidase (GOx): covalent enzyme attachment (CA), enzyme coating (EC), and enzyme precipitate coating (EPC), here referred to as CA-E, EC-E, and EPC-E, respectively. When additional carbon nanotubes (CNTs) were introduced from 0 to 75wt% for the EPC-E design, its initial biosensor sensitivity was improved from 2.40×10(-3) to 16.26×10(-3) A∙M(-1)∙cm(-2), while its electron charge transfer rate constant was increased from 0.33 to 1.47s(-1). When a fixed ratio of CNTs was added for three different electrode systems, EPC-E showed the best glucose sensitivity and long-term thermal stability. For example, when 75wt% of additional CNTs was added, the initial sensitivity of EPC-E was 16.26×10(-3) A∙M(-1)∙cm(-2), while those of EC-E and CA-E were only 6.42×10(-3) and 1.18×10(-3) A∙M(-1)∙cm(-2), respectively. Furthermore, EPC-E retained 63% of its initial sensitivity after thermal treatment at 40°C over 41days, while EC-E and CA-E showed only 12% and 1% of initial sensitivities, respectively. Consequently, the EPC approach with additional CNTs achieved both high sensitivity and long-term stability, which are required for continuous and accurate glucose monitoring.

Enzyme precipitate coatings of glucose oxidase onto carbon paper for biofuel cell applications.

Enzymatic biofuel cells (BFC) have a great potential as a small power source, but their practical applications are being hampered by short lifetime and low power density. This study describes the direct immobilization of glucose oxidase (GOx) onto the carbon paper in the form of highly stable and active enzyme precipitation coatings (EPCs), which can improve the lifetime and power density of BFCs. EPCs were fabricated directly onto the carbon paper via a three-step process: covalent attachment (CA), enzyme precipitation, and chemical crosslinking. GOx-immobilized carbon papers via the CA and EPC approaches were used as an enzyme anode and their electrochemical activities were tested under the BFC-operating mode. The BFCs with CA and EPC enzyme anodes produced the maximum power densities of 50 and 250 µW/cm(2) , respectively. The BFC with the EPC enzyme anode showed a stable current density output of >700 µA/cm(2) at 0.18 V under continuous operation for over 45 h. When a maple syrup was used as a fuel under ambient conditions, it also produced a stable current density of >10 µA/cm(2) at 0.18 V for over 25 h. It is anticipated that the direct immobilization of EPC on hierarchical-structured electrodes with a large surface area would further improve the power density of BFCs that can make their applications more feasible.

Immobilization of glucose oxidase into polyaniline nanofiber matrix for biofuel cell applications.

Glucose oxidase (GOx) was immobilized into the porous matrix of polyaniline nanofibers in a three-step process, consisting of enzyme adsorption, precipitation, and crosslinking (EAPC). EAPC was highly active and stable when compared to the control samples of enzyme adsorption (EA) and enzyme adsorption and crosslinking (EAC) with no step of enzyme precipitation. The GOx activity of EAPC was 9.6 and 4.2 times higher than those of EA and EAC, respectively. Under rigorous shaking at room temperature for 56 days, the relative activities of EA, EAC and EAPC, defined as the percentage of residual activity to the initial activity, were 22%, 19% and 91%, respectively. When incubated at 50°C under shaking for 4h, EAPC showed a negligible decrease of GOx activity while the relative activities of EA and EAC were 45% and 48%, respectively. To demonstrate the feasible application of EAPC in biofuel cells, the enzyme anodes were prepared and used for home-built air-breathing biofuel cells. The maximum power densities of biofuel cells with EA and EAPC anodes were 57 and 292 µW/cm(2), respectively. After thermal treatment at 60°C for 4h, the maximum power density of EA and EAPC anodes were 32 and 315 µW/cm(2), representing 56% and 108% of initially obtained maximum power densities, respectively. Because the lower power densities and short lifetime of biofuel cells are serious problems against their practical applications, the present results with EAPC anode has opened up a new potential for the realization of practical biofuel cell applications.

Highly stable enzyme precipitate coatings and their electrochemical applications.

This paper describes highly stable enzyme precipitate coatings (EPCs) on electrospun polymer nanofibers and carbon nanotubes (CNTs), and their potential applications in the development of highly sensitive biosensors and high-powered biofuel cells. EPCs of glucose oxidase (GOx) were prepared by precipitating GOx molecules in the presence of ammonium sulfate, then cross-linking the precipitated GOx aggregates on covalently attached enzyme molecules on the surface of nanomaterials. EPCs-GOx not only improved enzyme loading, but also retained high enzyme stability. For example, EPC-GOx on CNTs showed a 50 times higher activity per unit weight of CNTs than the conventional approach of covalent attachment, and its initial activity was maintained with negligible loss for 200 days. EPC-GOx on CNTs was entrapped by Nafion to prepare enzyme electrodes for glucose sensors and biofuel cells. The EPC-GOx electrode showed a higher sensitivity and a lower detection limit than an electrode prepared with covalently attached GOx (CA-GOx). The CA-GOx electrode showed an 80% drop in sensitivity after thermal treatment at 50°C for 4 h, while the EPC-GOx electrode maintained its high sensitivity with negligible decrease under the same conditions. The use of EPC-GOx as the anode of a biofuel cell improved the power density, which was also stable even after thermal treatment of the enzyme anode at 50°C. The excellent stability of the EPC-GOx electrode together with its high current output create new potential for the practical applications of enzyme-based glucose sensors and biofuel cells.

Nanoscale enzyme reactors in mesoporous carbon for improved performance and lifetime of biosensors and biofuel cells.

Nanoscale enzyme reactors (NERs) of glucose oxidase in conductive mesoporous carbons were prepared in a two-step process of enzyme adsorption and follow-up enzyme crosslinking. MSU-F-C, a mesoprous carbon, has a bottleneck pore structure with mesocellular pores of 26 nm connected with window mesopores of 17 nm. This structure enables the ship-in-a-bottle mechanism of NERs, which effectively prevents the crosslinked enzymes in mesocellular pores from leaching through the smaller window mesopores. This NER approach not only stabilized the enzyme but also expedited electron transfer between the enzyme and the conductive MSU-F-C by maintaining a short distance between them. In a comparative study with GOx that was simply adsorbed without crosslinking, the NER approach was proven to be effective in improving the sensitivity of glucose biosensors and the power density of biofuel cells. The power density of biofuel cells could be further improved by manipulating several factors, such as by adding a mediator, changing the order of adsorption and crosslinking, and inserting a gold mesh as an electron collector.