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The strengths and weaknesses of the technology in the context of specific classes of applications, and areas where additional knowledge is required to properly exploit biological fuel cells are focused. Enzymatic biofuel cells are discussed.

[Display omitted] • Co-production of value-added products - biodiesel and triacetin. • Calcined Mg-Al hydrotalcite (Mg:Al – 3:1) as a reusable base catalyst. • Characterization and activity screening of various base catalysts. • Kinetic model of the interesterification reaction. • Efficient and green process. Biodiesel is a renewable fuel with several advantages over petro-based fuels and is conventionally produced by the transesterification of triglycerides with alcohols. Chemical interesterification of triglycerides is a good alternative as compared to transesterification, as it yields a value-added compound, triacetin, as a co-product instead of the already abundant commodity, glycerol. In this work, interesterification of soybean oil was carried out with methyl acetate to co-produce biodiesel and triacetin, using calcined hydrotalcite as a heterogeneous base catalyst. Several base catalysts were tested for this reaction. Calcined Mg-Al hydrotalcite (Mg:Al mole ratio – 3:1) was found as the best catalyst for achieving 95.9 % conversion of soybean oil triglycerides in 4 h and yielding 5.3 % selectivity of triacetin along with the value-added intermediates - monoacetindiglycerides and diacetinmonoglycerides, produced with a selectivity of 46 % and 48.7 %, respectively. The optimum reaction conditions were found at an oil to methyl acetate mole ratio of 1:50, catalyst loading of 0.04 g/cm3 and a temperature of 200 ºC. The catalysts were fully characterized before and after the reaction, and were found to possess high surface area with both basic and acidic sites. The effect of various reaction conditions on the rate of reaction, conversion of triglycerides and selectivity of triacetin were studied. The concentration profiles of the reactants, intermediates and products obtained at different temperatures were used to obtain the kinetic rate constants and the activation energy of each parallel step was evaluated. Reusability studies showed that the catalyst was stable and reusable up to three cycles. [ABSTRACT FROM AUTHOR]

Multi-step reaction cascades can be designed to include channeling mechanisms, which provide electrostatic or steric control over intermediate transport such that intermediates do not escape to the bulk between active sites. Physical confinement of the intermediate pathway between sites retains intermediate from bulk access and thus provides high transport efficiency. In this work, we use molecular dynamics to study the transport of intermediates (charged oxalate and neutral ethanol) inside a nanochannel represented by a single-walled carbon nanotube (SWCNT). This approach reveals that solvent orientation highly impacts intermediate transport. At small nanochannel diameter near 1 nm, highly structured solvent water and Knudsen diffusion decreases effective intermediate diffusivity. Finally, modified SWCNT termini with electrostatically-charged carboxylate groups are shown to increase intermediate retention for both charged and uncharged intermediates by up to five-fold. When catalyst sites are located within the nanochannel, decreased diffusion rate and increased retention time will enhance cascade efficiency. [ABSTRACT FROM AUTHOR]

Mass transport properties of a pair of non-Platinum Group Metal (non-PGM) catalysts in proton exchange membrane fuel cells (PEMFCs) were evaluated through methods developed by Reshetenko et al., demonstrating that the use of different carrier gases can allow for the determination of the mass transport coefficient for oxygen in the gas phase and the electrolyte phase. The gas-phase and non-gas-phase resistances can be elucidated from the slope and intercept, respectively, of the total mass transport coefficient plotted as a function of molecular weight. It was determined through these experiments that the primary sources of mass transfer limitations of the non-PGMs when compared to the PGMs were the catalyst layer (non-gas-phase), rather than the flow fields (gas-phase, primarily Knudsen Diffusion effects), and the gas diffusion layer. This work was combined with a pseudo-2D, isothermal, steady state numerical model to estimate the gas-phase mass transfer coefficient and the fraction of hydrophobic, gas-phase pores in the catalyst layer. Sensitivity studies were also carried out, allowing for more information regarding the influence of several inherent factors on the mass transport limitations, and allow for additional validation of the model beyond simply the quality of the fit. [ABSTRACT FROM AUTHOR]

Iron and nitrogen doped carbon, Fe–N–C, catalysts are synthesized by high pressure pyrolysis of Ketjenblack carbon, melamine and iron acetate precursor mixture in a closed, reusable scale-up stainless steel reactor. The effects of precursor loading with constant precursor ratios on obtained pressure, nitrogen retention and oxygen reduction reaction (ORR) activities are studied. The results indicate that higher precursor loading increases the gas phase pressure and improves nitrogen retention and ORR activity. Furthermore, a relationship is found between active site density, nitrogen retention and pressure that suggests that the limiting reaction may be an adsorption process driven via high pressure of volatile intermediates from the melamine. [ABSTRACT FROM AUTHOR]

The article presents simple modeling approaches for enzymatic fuel cells that could be used in other systems like biosensors and bioreactors. Topics discussed include an overview of Direct Electron Transfer (DET) and Mediated Electron Transfer that are two basic mechanisms for enzyme electrocatalysis and the concept behind the Thin Film Model, the Porous Electrode Model and the Complete Cell Model.

A steady-state, rotating ring disk study of the oxygen reduction reaction (ORR) was conducted in acid environment using a pyrolyzed metal/nitrogen/carbon (MNC) electrocatalyst. Analysis of peroxide generation indicates that ORR proceeds both via a direct fourelectron pathway to water at high potentials and an indirect peroxide pathway at low potentials. Above 0.6 V vs RHE, the direct four-electron pathway to water without a desorbing intermediate dominates oxygen reduction because peroxide generation is inhibited due to site availability. In contrast, at potentials below 0.6 V, oxygen reduction begins to shift to the indirect peroxide pathway due to fast kinetics and higher site availability. The net peroxide generation remains relatively low over the entire range due to reduction of peroxide to water. [ABSTRACT FROM AUTHOR]

Homogeneous, sub-micron thick bioactive films containing redox hydrogels and the enzyme laccase were prepared on gold-coated glass slides via convective self-assembly. Sub-micron film thickness (~10-50 nm) allowed higher mediator and enzyme utilization by lowering transport limitations. This study demonstrates control of redox hydrogel film morphology by manipulating hydrophilicity, spreading and wetting properties of the hydrogel/electrode interface. Use of a non-ionic surfactant (Triton X-100) enabled spreading of enzyme containing precursor solutions with no negative impact on enzyme activity. Ellipsometry was used to measure filmswelling properties of these systems, square wave voltammetry was employed to estimate the amount of electroactive redox polymer, and cyclic voltammetry enabled estimation of apparent electron diffusivity. This study impacts the design of thin, catalytic films for bioelectronic applications. [ABSTRACT FROM AUTHOR]

Iron based nitrogen doped carbon (FeNC) catalysts are synthesized by high-pressure pyrolysis of carbon and melamine with varying amounts of iron acetate in a closed, constant-volume reactor. The optimum nominal amount of Fe (1.2 wt%) in FeNC catalysts is established through oxygen reduction reaction (ORR) polarization. Since the quantity of iron used in FeNCs is very small, the amount of Fe retained in FeNC catalysts after leaching is determined by UV-VIS spectroscopy. As nitrogen is considered to be a component of active sites, the amount of bulk and surface nitrogen retention in FeNC catalysts are measured using elemental analysis and X-ray photoelectron spectroscopy, respectively. It is found that increasing nominal Fe content in FeNC catalysts leads to a decreased level of nitrogen retention. Thermogravimetric analysis demonstrates that increasing nominal Fe content leads to increased weight loss during pyrolysis, particularly at high temperatures. Catalysts are also prepared in the absence of iron source, and with iron removed by washing with hot aqua regia post-pyrolysis. FeNC catalysts prepared with no Fe show high retained nitrogen content but poor ORR activity, and aqua regia washed catalysts demonstrate similar activity to Fe-free catalysts, indicating that Fe is an active site component. [ABSTRACT FROM AUTHOR]

Porous carbon materials with varying structural and compositional properties were studied for their impact on the nitrogen content and activity of metal-nitrogen-carbon (MNC) oxygen reduction catalysts prepared using high-pressure pyrolysis. The carbon materials and resulting catalysts were characterized morphologically using nitrogen physisorption, coupled with non-local density functional theory (NLDFT) analysis to calculate pore size distributions. Graphiticity was assessed via X-ray Diffraction (XRD), bulk nitrogen content was observed using CHN combustion analysis and iron content by Inductively Coupled Plasma (ICP). The catalysts were characterized electrochemically using rotating ring-disk measurements. The results indicate that substrates adsorbing the most nitrogen and iron show the highest activity. Furthermore, a relationship found between mesoporosity and nitrogen adsorption indicate the importance of transport of precursors to potential active sites. [ABSTRACT FROM AUTHOR]

We report a simulation of multi-step enzyme catalysis and cofactor-mediated electron transfer in a bio-anode of a methanol biofuel cell. Three nicotinamide adenine dinucleotide (NAD+) dependent dehydrogenase enzymes, namely alcohol dehydrogenase (ADH), aldehyde dehydrogenase (AlDH), and formate dehydrogenase (FDH) combine to achieve complete oxidation of methanol to carbon dioxide. Oxidation of methanol by the enzyme cascade produces the reduced co-factor, NADH, which is oxidized to regenerate NAD+ at an electrode comprised of a poly(methylene green) catalyst electrodeposited on carbon paper. Kinetic parameters for the enzymatic oxidation reactions were experimentally determined between pH 6.5 and 9 and were employed in the simulation to evaluate the effect of pH. Model results compare closely to previously reported experimental data over a broad pH range. Model-based analysis indicates that cell performance is controlled by NAD+ transport and NADH oxidation kinetics for electrode specific areas lower than 3 m2 cm-3 and shifted to enzyme kinetic control at higher specific area. An optimal electrode design is predicted to achieve current density exceeding 10 mA cm-2. [ABSTRACT FROM AUTHOR]

The impact of methanol on oxygen reduction activity is studied using a mediated biocathode catalyzed by laccase from Trametes versicolor. The sensitivity of oxygen reduction current density to the presence of methanol at such electrodes depends strongly on mediator redox potential. This study demonstrates that the selectivity of laccase cathodes toward oxygen reduction can allow methanol feed concentration up to 5 M in direct methanol fuel cells. Within the 0-5 M concentration range, methanol primarily affects enzyme kinetics and not the electron transport via the mediator. For methanol concentrations of 0-2.5 M, laccase activity toward oxygen was largely maintained; approximately 30% loss of activity occurred in the 2.5-5 M range, and irreversible loss of enzyme activity was observed beyond 7.5 M. The presence of methanol primarily decreases the turnover number of the enzyme rather than altering substrate binding, suggesting a noncompetitive inhibition mechanism. It is proposed that this reduction occurs due to changes in the electron transfer environment near the T1 binding pocket due to the presence of methanol. [ABSTRACT FROM AUTHOR]

Mixed-feed direct methanol fuel cells, which operate on a single feed stream comprised of fuel and oxidant, were constructed using PtRu anode catalysts. Reaction selectivity favoring methanol oxidation at the anode was achieved by application of a barrier layer comprised of reconstituted Nafion, and optionally carbon black, hot pressed between the anode catalyst layer and reactant diffusion layer. The modified anode outperformed an unmodified anode in the presence of mixed feed at 60 and 80°C. This result is attributed to reduced infiltration of oxygen from the mixed feed due to introduction of a barrier that admits mainly aqueous methanol. Inclusion of carbon black in the barrier layer led to ~30% increase in current density at 350 mV/dynamic hydrogen electrode, attributable to reduced contact resistance. On increasing the cell temperature from 60 to 80°C, the activation polarization region vanished for anodes that were either unmodified or modified by a Nafion-carbon barrier layer. This result is attributed to decreased water content in the barrier at elevated temperature, leading to increased oxygen flux and thereby introducing a mixed potential at the anode. [ABSTRACT FROM AUTHOR]

O2 was electroreduced to water at 0.6 V (SHE) near neutral pH on the ‘wired’ Pleurotus ostreatus laccase cathode. We previously reported high-current density (5 mA cm−2), four-electron electroreduction of O2 to water on a ‘wired’ Coriolus hirsutus laccase electrode at +0.7 V (SHE) in pH 5 in citrate buffer. Since the enzyme was inhibited by chloride and because its activity declined steeply when the pH was raised to neutral, the rate of O2 electroreduction in a physiological buffer solution was only ∼1% of that at pH 5 in absence of chloride. Here we show that substitution of the C. hirsutus laccase by laccase from P. ostreatus allows the upward extension of the pH range of O2 electroreduction. The current density of the electrode made with laccase from P. ostreatus in pH 7 citrate buffer was ∼100 μA cm−2 and at pH 7 and in phosphate buffered NaCl (PBS, 20 mM phosphate, 0.1 M NaCl) it still retained 6% of its maximal (1 mA cm−2) current density at pH 5 in citrate buffer. The electrocatalyst consisted of the crosslinked P. ostreatus laccase and the electron conducting redox polymer PVI-Os(dmebpy)(tpy)2+/3+ [PVI=poly(N-vinyl imidazole) with about 1/5th of the rings complexed with (Os-dmebpy-tpy)2+/3+; dmebpy=4,4′-dimethyl-2,2′-bipyridine; tpy=2,2′,6′,2″-terpyridine]. [Copyright &y& Elsevier]

Offers a study that assessed the relative effect of general corrosion, corrosion cracking and hydrogen embrittlement on the deterioration in material properties of high-strength steel bridge wire. Experimental procedure; Results; Conclusion.

An introduction is presented in which the editor discusses various reports within the issue which recognizes the contributions of inventor and professor Adam Heller in biochemistry and photoelectrochemistry including the interactions of enzymes, solar energy conversion, and microbial fuel cells.

The high efficiency of cascade reactions in supramolecular enzyme nanoassemblies, known as metabolons, has attracted substantial attention in various fields ranging from fundamental biochemistry and molecular biology to recent applications in biofuel cells, biosensors, and chemical synthesis. One reason for the high efficiency of metabolons is the structures formed by sequential enzymes that allow the direct transport of intermediates between consecutive active sites. The supercomplex of malate dehydrogenase (MDH) and citrate synthase (CS) is an ideal example of the controlled transport of intermediates via electrostatic channeling. Here, using a combination of molecular dynamics (MD) simulations and a Markov state model (MSM), we examined the transport process of the intermediate oxaloacetate (OAA) from MDH to CS. The MSM enables the identification of the dominant transport pathways of OAA from MDH to CS. Analysis of all pathways using a hub score approach reveals a small set of residues that control OAA transport. This set includes an arginine residue previously identified experimentally. MSM analysis of a mutated complex, where the identified arginine is replaced by alanine, led to a 2-fold decrease in transfer efficiency, also consistent with experimental results. This work provides a molecular-level understanding of the electrostatic channeling mechanism and will enable the further design of catalytic nanostructures utilizing electrostatic channeling., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The electrochemical reduction of CO 2 is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte's bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO 2 reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO 2 reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO 2 reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO 2 atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO 2 atmosphere due to the formation of HCO 3 - buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Millions of years of evolution have produced biological systems capable of efficient one-pot multi-step catalysis. The underlying mechanisms that facilitate these reaction processes are increasingly providing inspiration in synthetic chemistry. Substrate channelling, where intermediates between enzymatic steps are not in equilibrium with the bulk solution, enables increased efficiencies and yields in reaction and diffusion processes. Here, we review different mechanisms of substrate channelling found in nature and provide an overview of the analytical methods used to quantify these effects. The incorporation of substrate channelling into synthetic cascades is a rapidly developing concept, and recent examples of the fabrication of cascades with controlled diffusion and flux of intermediates are presented.

Electrochemical activation of glassy carbon, carbon paper and functionalized carbon nanotubes via high-applied-potential cyclic voltammetry leads to the formation of adsorbed, redox active functional groups and increased active surface area. Electrochemically activated carbon electrodes display enhanced activity toward nicotinamide adenine dinucleotide (NADH) oxidation, and more importantly, dramatically improved adsorption of bioelectrochemically active azine dyes. Adsorption of methylene green on an electroactivated carbon electrode yields a catalyst layer that is 1.8-fold more active toward NADH oxidation than an electrode prepared using electropolymerized methylene green. Stability studies using cyclic voltammetry indicate 70% activity retention after 4000 cycles. This work further facilitates the electrocatalysis of NADH oxidation for bioconversion, biosensor and bioenergy processes.

We report on the synthesis of thin, transparent, and highly catalytic carbon nanotube films. Nanotubes catalyze the reduction of triiodide, a reaction that is important for the dye-sensitized solar cell, with a charge-transfer resistance as measured by electrochemical impedance spectroscopy that decreases with increasing film thickness. Moreover, the catalytic activity can be significantly enhanced by exposing the nanotubes to ozone in order to introduce defects. Ozone-treated, defective nanotube films could serve as catalytic, transparent, and conducting electrodes for the dye-sensitized solar cell. Other possible applications include batteries, fuel cells, and electroanalytical devices.

Stimulus-responsive, or "smart" protein-based hydrogels are of interest for many bioengineering applications, but have yet to include biological activity independent of structural functionality. We have genetically engineered bifunctional building blocks incorporating fluorescent proteins that self-assemble into robust and active hydrogels. Gelation occurs when protein building blocks are cross-linked through native protein-protein interactions and the aggregation of alpha-helical hydrogel-forming appendages. Building blocks constructed from different fluorescent proteins can be mixed to enable tuning of fluorescence loading and hydrogel strength with a high degree of independence. FRET experiments suggest a macro-homogeneous structure and that intragel and interprotein reactions can be engineered. This design approach will enable the facile construction of complex hydrogels with broad applicability.

O(2) was electroreduced to water at 0.6 V (SHE) near neutral pH on the "wired" Pleurotus ostreatus laccase cathode. We previously reported high-current density (5 mA cm(-2)), four-electron electroreduction of O(2) to water on a "wired" Coriolus hirsutus laccase electrode at +0.7 V (SHE) in pH 5 in citrate buffer. Since the enzyme was inhibited by chloride and because its activity declined steeply when the pH was raised to neutral, the rate of O(2) electroreduction in a physiological buffer solution was only approximately 1% of that at pH 5 in absence of chloride. Here we show that substitution of the C. hirsutus laccase by laccase from P. ostreatus allows the upward extension of the pH range of O(2) electroreduction. The current density of the electrode made with laccase from P. ostreatus in pH 7 citrate buffer was approximately 100 microA cm(-2) and at pH 7 and in phosphate buffered NaCl (PBS, 20 mM phosphate, 0.1 M NaCl) it still retained 6% of its maximal (1 mA cm(-2)) current density at pH 5 in citrate buffer. The electrocatalyst consisted of the crosslinked P. ostreatus laccase and the electron conducting redox polymer PVI-Os(dmebpy)(tpy)(2+/3+) [PVI=poly(N-vinyl imidazole) with about 1/5th of the rings complexed with (Os-dmebpy-tpy)(2+/3+); dmebpy=4,4'-dimethyl-2,2'-bipyridine; tpy=2,2',6',2"-terpyridine].