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The redox potential (ERedox) of a ferrocene (Fc) derivative differs, depending on its functional group. In this study, the various Fc derivatives are considered as mediators of anodic catalysts to promote glucose oxidation reaction (GOR) in glucose/oxygen enzymatic biofuel cells (EBFCs). Initially, their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies are calculated using density functional theory to predict their ERedox pattern. According to the calculations, the LUMO and HOMO energies of Fc derivatives combined with electron-donating groups (EDGs) are higher than those of Fc derivatives combined with electron-withdrawing groups (EWGs), including the results that Fc(NH2) has the highest molecular orbital (MO), while Fc(CHO) has the lowest. To verify the prediction for ERedox pattern, electrochemical evaluations are conducted. When glucose is provided, the onset potential (EOnset) of GOR is measured, while the ERedox of Fc derivatives and EOnset of GOR are linearly proportional to each other ( R 2 = 0.98) and DFT calculations. As the energies increase, the above two potentials are shifted more negatively. More specifically, the ERedox of Fc(NH2) and EOnset of GOR show the most negative values at (−0.112 and −0.17) V, respectively, while the ERedox of Fc(CHO) and EOnset of GOR show the most positive values at (0.496 and 0.40) V, respectively. Then, when this correlation is adopted for EBFCs, EBFCs using Fc(NH2) combined with EDG show 3.7 times higher maximum power density than those using Fc(COOH), as representatives combined with EWG. Based on that, it is well established how DFT and electrochemical evaluations should be used to design anodic catalysts including Fc derivatives as mediators for GOR. [ABSTRACT FROM AUTHOR]

In this study, a new cathodic catalyst including hemin catalyzing the H2O2 reduction reaction (HPRR) is suggested to improve the performance of flow-type hydrogen peroxide fuel cells (HPFCs). To develop the hemin-based catalyst showing excellent HPRR reactivity, buckypaper (BP) and polyethyleneimine (PEI) are used, and it is proved that the final [BP/hemin]/PEI/hemin catalyst has excellent catalytic activity in terms of current and onset potential for the HPRR. To evaluate the effect of the catalyst on the performance of HPFC, BP/hemin and BP/PEI/hemin catalysts as the control groups are fabricated and compared. When [BP/hemin]/PEI/hemin is used, the amount of immobilized hemin and the reactivity of the HPRR increase due to the interstitial pores of the carbon nanotube network. Furthermore, its onset potential increases due to the formation of the coordinate bond between the iron core within hemin and the axial ligand within PEI. For preparing membraneless HPFCs, flexible materials such as hemin-modified BP, carbon cloth and polyethylene terephthalate are used, and its open circuit potential and maximum power density are 0.171 ± 0.012 V and 90.7 ± 6.88 μW cm−2 when 0.1 M H2O2 is injected. These results are far better than those of other previous studies. [ABSTRACT FROM AUTHOR]

Summary: Direct liquid fuel cell (DLFC) and membraneless fuel cell using the anode consisting of buckypaper (BP) and vitamin B12 (Vit B12) (BP/Vit B12) is introduced in physiological condition, while vitamin C (Vit C) is used as anodic fuel. Due to the high sensitivity of Vit B12 and the inherent feature of BP, the onset potential for the oxidation reaction of Vit C catalyzed by BP/Vit B12 is −0.1 V (vs Ag/AgCl), and its anodic current density measured at 0.3 V (vs Ag/AgCl) is 1.82 mA cm−2 even in a low Vit C concentration (10 mM). Regarding performance in fuel cells, maximum power density (MPD) of Vit C fuel cell containing membrane and Pt‐based air cathode is 0.62 ± 0.067 mW cm−2 in 0.01 M Vit C, and this is 3.1 times higher than that of fuel cell containing pristine BP. In addition, in a paper‐based flexible membraneless fuel cell using BP/bilirubin oxidase as cathode and the concentration of cerebrospinal fluid within the human body (0.35 mM Vit C), its MPD is 57.7 ± 4.02 μW cm−2. This means this suggested Vit C fuel cell system can replace the conventional primary batteries considered as the power source of implantable sensors and stimulation devices to treat neurological disorders. [ABSTRACT FROM AUTHOR]

Summary: A facile and inexpensive method of fabricating a myoglobin‐mimic nanostructure is introduced by evaluating the influence of temperature conditions on the axial coordination between the Fe core of hemin and amine of polyethyleneimine (PEI). Through the high‐temperature (100°C) synthesis condition, more hemin molecules are strongly attached to the carbon nanotube and PEI composite owing to the amide bond formation, whereas the energy distribution of hemin is deformed, and the electrical connection is improved by the coordination of axial ligands when the catalyst is synthesized on a lower temperature (25°C). Benefiting from the high concentration of axial ligands, the onset potential is positively shifted by 0.258 V, and the highest current density (155.43 μA cm−2) is observed with 10 mM H2O2 under physiological conditions. These phenomena occur because of the different hydrogen peroxide reduction reaction (HPRR) mechanisms and the overpotential stemming from the effect of the axial ligand, which induces the lowest catalytic and charge transfer resistance for HPRR at 51 and 820 Ω cm−2, respectively. In the polarization curves measured using a 3D printed membraneless flow‐type fuel cell, the maximum power density reaches 129.0 μW cm−2 with 0.340 V of open‐circuit voltage, respectively, which offers the best performance among the reported studies for the membraneless hydrogen peroxide fuel cells driving under physiological conditions so far. [ABSTRACT FROM AUTHOR]

Summary: Hollow cobalt nitrogen‐doped carbon (H‐CoNC) is suggested for use in the membraneless hydrogen peroxide fuel cell (HPFC) and enzymatic biofuel cell (EBC) as anodic catalyst boosting hydrogen peroxide oxidation reaction (HPOR). For fabricating H‐CoNCs, a facile spray pyrolysis‐assisted process is used, and such produced H‐CoNCs show a porous and hollow‐shell structure, while they include a large amount of isolated Co atoms and coordinate bonds with Co and nitrogen (CoN4). This structure promotes mass transfer to the active site and excellent catalytic activity for HPOR. With these benefits of H‐CoNCs, the current density of the bioanode consisting of H‐CoNC, carbon nanotube, and glucose oxidase (H‐CoNC/CNT/GOx) observed at 0.3 V under 150 mM glucose is 315.5 μA cm−2, which is 2.1 times higher than that of a conventionally synthesized catalyst using Co‐doped carbon nanoparticles (CoNC‐NPs) (CoNC‐NPs/CNT/GOx, 146.2 μA cm−2). With this superior catalytic activity for HPOR, maximum power density (MPD) of membraneless EBC using H‐CoNC/CNT/GOx is 33.8 ± 4.52 μW cm−2, which is 52% higher than that of CoNP‐NPs/CNT/GOx. In addition, a membraneless HPFC using H‐CoNC/CNT demonstrates 4.87 times higher MPD (231.3 ± 11.3 μW cm−2) than that using CoNC‐NPs/CNT, proving that H‐CoNC improves the performance of fuel cells by its excellent catalytic activity. [ABSTRACT FROM AUTHOR]

A buckypaper (BP) and cobalt phthalocyanine (CoPc) composite anode (BP/CoPc) is suggested for alkaline sugarcane extract fuel cells (SCEFCs). With the use of the BP/CoPc catalyst, the amount of immobilized CoPc increases because many CoPc molecules are entrapped in the interstitial pores of the carbon nanotube (CNT) network, which is the main component of BP. As a result, the catalytic activity is improved. More specifically, CoPc loaded onto the CNT surface promotes the glucose oxidation reaction (GOR), showing a low onset potential (GOR-A), while CoPc placed in the interstitial pores of BP produces a high current density (GOR-B). In contrast, the sucrose oxidation reaction (SOR) shows only GOR-B-like behavior, demonstrating that the mass transfer of sucrose is sluggish. The open-circuit voltage and maximum power density of the fuel cells (FCs) using BP/CoPc are 0.93 ± 0.005 V and 17.6 ± 2.83 W m−2 with 0.5 M glucose, while they are 0.82 ± 0.008 V and 3.9 ± 0.23 W m−2 in FCs using sugarcane extract, respectively. These are significantly better than those of FCs using noble metal catalysts and microbial FCs. Additionally, BP/CoPc preserves 93.1% of its initial current density for 2 h, whereas that of conventional platinum black (Pt/C) drops to 22.5%, showing that BP/CoPc has strong resistance against poisoning. [ABSTRACT FROM AUTHOR]

A new anodic catalyst containing carbon nanotubes (CNTs) and vitamin B12 (CNT/VitB12) and its electrochemical activation are introduced for promoting the hydrogen peroxide oxidation reaction (HPOR) and the performance of a membraneless hydrogen peroxide fuel cell (HPFC). Through activation, the anodic current density of the activated catalyst (CNT/VitB12-Co2+) (271.0 μA cm−2) is 2.28 times better than that of the non-activated catalyst (CNT/VitB12-Co3+) (118.8 μA cm−2) at a low potential (0.4 V) when only 3 mM H2O2 is injected. This is due to the two-step HPOR mechanism of CNT/VitB12 where Co2+ ions are initially converted into Co3+ ions by the reaction with H2O2 (step 1) and electrons are generated when Co3+ ions return to Co2+ ions (step 2). Such produced electrons play a role in increasing the anodic current density. Based on that, it is confirmed that electrochemical activation controlling the oxidation state of Co ions is a key factor to enhance the catalytic activity of CNT/VitB12. Regarding stability, desirable CNT/VitB12-Co2+ preserves its initial current for 2 h in an amperometric response test. The onset potentials for CNT/Vit B12 and CNT/hemin, which are used as cathodic catalysts, are 0.1 and 0.35 V vs. Ag/AgCl, respectively, indicating that when the two catalysts and H2O2 fuel are used, the open circuit voltage (OCV) is 0.25 V. When the membraneless HPFC is operated, the OCV and maximum power density are 0.233 ± 0.005 V and 53.8 ± 0.4 μW cm−2 with 0.1 M H2O2 fuel. The similarity between the difference in onset potential of the catalysts and the OCV of the membraneless HPFC is evidence that the catalytic activity of the catalysts is the dominant parameter to determine the performance of membraneless HPFCs. The suggested membraneless HPFC shows an excellent performance despite a low fuel concentration and membraneless structure. [ABSTRACT FROM AUTHOR]

A new cathodic catalyst is introduced for enzymatic biofuel cells (EBCs) by enhancing the oxygen reduction reaction (ORR) consisting of the glucose oxidation reaction (GOR and partial ORR) and H2O2 reduction reaction (HRR). In this catalyst, glucose oxidase (GOx) and hemin are used for promoting both the partial ORR and HRR, respectively. The hemin is attached onto polyethyleneimine (PEI) and carbon nanotubes (CNTs) (CNT/[PEI/hemin]), while GOx is conjugated with poly acrylic acid (PAA) (PAA/GOx). The two structures are then connected to each other ({CNT/[PEI/hemin]}/{PAA/GOx}). According to electrochemical evaluations of this catalyst, the onset potential is positively shifted (0.4 V vs. Ag/AgCl) and the current density increases considerably (522 μA cm−2) by the use of the PEI/hemin composite and PAA/GOx conjugate. The maximum power density of an EBC using this catalyst is 255 ± 2.8 μW cm−2, which is a comparable level to previously reported excellent results. This is due to the facile relay between the partial ORR and HRR by the (i) proper linkage between GOx and hemin through a cascade type preparation, (ii) four step redox reaction of Fe ions within hemin and (iii) use of water swellable PAA facilitating glucose mass transfer. It is observed that the upper water swellable PAA/GOx layer promotes smooth glucose mass transfer of glucose fuel and produces a sufficient amount of H2O2 by the partial ORR. The produced H2O2 is further reduced by the HRR at positive potential. The chemical structure and the degree of enzyme immobilization of the best catalyst are examined by EDS, XPS and glucose colorimetric assay. [ABSTRACT FROM AUTHOR]

Glucose, which is a primary energy source of living organisms, can induce diabetes or hypoglycemia if its concentration in blood is irregular. It is therefore important to develop glucose biosensor that reads the concentration of glucose in blood precisely. In the present work, we suggest new glucose oxidase (GOx) based catalysts that can improve the sensitivity of the glucose biosensor and make glucose measurements over a wide concentration ranges possible. For synthesizing such catalysts, a composite including pyrenecarboxaldehyde (PCA) and GOx is attached to substrate including carbon nanotube (CNT) and polyethyleneimine (PEI) (CNT/PEI/[PCA/GOx]). Catalytic activity and stability of the catalyst are then evaluated. According to the investigation, the catalyst shows excellent glucose sensitivity of 47.83 μAcmmM, low Michaelis-Menten constant of 2.2 mM, and wide glucose concentration detection, while it has good glucose selectivity against inhibitors, such as uric acid and ascorbic acid. Also, its activity is maintained to 95.7% of its initial value even after four weeks, confirming the catalyst is stable enough. The excellence of the catalyst is attributed to hydrophobic interaction, C=N bonds, and π-hydrogen interaction among GOx, PCA and PEI/CNT. The bindings play a role in facilitating electron transport between GOx and electrode. [ABSTRACT FROM AUTHOR]