Hybrid catalyst cascade architecture enhancement for complete ethanol electrochemical oxidation

Bioelectrochemical cascade reaction for energy-saving hydrogen production and innovative Zn-air batteries

The oxygen evolution reaction (OER) is an important half-reaction in electrochemical hydrogen production (EHP) and rechargeable metal-air batteries. However, the sluggish OER kinetics has seriously impeded their performance. Herein, we report a bioelectrochemical cascade system composed of glucose oxidase (GOx)-functionalized N-doped porous carbon nanofibers to replace OER in EHP and rechargeable Zn-air batteries (ZABs) applications. In this cascade system, GOx catalyzes oxidation of glucose to produce value-added gluconic acid accompanied with the generation of H2O2 under aerobic conditions. The subsequent electrocatalytic oxidation of H2O2 replacing the OER results in an onset voltage below 1.10 V for EHP, and a low charging voltage of 1.35 V as well as a small charging/discharging voltage gap of ∼ 280 mV over 170 h for ZABs in neutral aqueous electrolytes. The advantages of employing the innovative bioelectrochemical cascade reaction are demonstrated in EHP and ZABs, achieving the full utilization of biomass energy in energy-saving electrochemical systems for energy storage and conversion.

Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode

The integration of heterogeneous electrocatalysis and molecular catalysis is a promising approach to designing new catalysts for the oxygen evolution reaction (OER) and other processes. We recently showed that the electrostatic potential drop across the double layer contributes to the driving force for electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. Here, we report high current densities and low onset potentials for water oxidation attained using a metal-free voltage-assisted molecular catalyst (TEMPO). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine faradic efficiencies for the generation of H₂O₂ and O₂. The same catalyst was employed for efficient oxidations of butanol, ethanol, glycerol, and H₂O₂. DFT calculations show that the applied voltage alters the electrostatic potential drop between TEMPO and the reactant as well as chemical bonding between them, thereby increasing the reaction rate. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts for OER and alcohol oxidations.

Engineering Self-Powered Electrochemical Sensors Using Analyzed Liquid Sample as the Sole Energy Source

Many healthcare and environmental monitoring devices use electrochemical techniques to detect and quantify analytes. With sensors progressively becoming smaller-particularly in point-of-care (POC) devices and wearable platforms-it creates the opportunity to operate them using less energy than their predecessors. In fact, they may require so little power that can be extracted from the analyzed fluids themselves, for example, blood or sweat in case of physiological sensors and sources like river water in the case of environmental monitoring. Self-powered electrochemical sensors (SPES) can generate a response by utilizing the available chemical species in the analyzed liquid sample. Though SPESs generate relatively low power, capable devices can be engineered by combining suitable reactions, miniaturized cell designs, and effective sensing approaches for deciphering analyte information. This review details various such sensing and engineering approaches adopted in different categories of SPES systems that solely use the power available in liquid sample for their operation. Specifically, the categories discussed in this review cover enzyme-based systems, battery-based systems, and ion-selective electrode-based systems. The review details the benefits and drawbacks with these approaches, as well as prospects of and challenges to accomplishing them.

Key Features of TEMPO-Containing Polymers for Energy Storage and Catalytic Systems

The need for environmentally benign portable energy storage drives research on organic batteries and catalytic systems. These systems are a promising replacement for commonly used energy storage devices that rely on limited resources such as lithium and rare earth metals. The redox-active TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl) fragment is a popular component of organic systems, as its benefits include remarkable electrochemical performance and decent physical properties. TEMPO is also known to be an efficient catalyst for alcohol oxidation, oxygen reduction, and various complex organic reactions. It can be attached to various aliphatic and conductive polymers to form high-loading catalysis systems. The performance and efficiency of TEMPO-containing materials strongly depend on the molecular structure, and thus rational design of such compounds is vital for successful implementation. We discuss synthetic approaches for producing electroactive polymers based on conductive and non-conductive backbones with organic radical substituents, fundamental aspects of electrochemistry of such materials, and their application in energy storage devices, such as batteries, redox-flow cells, and electrocatalytic systems. We compare the performance of the materials with different architectures, providing an overview of diverse charge interactions for hybrid materials, and presenting promising research opportunities for the future of this area.

Hybrid catalyst cascade for enhanced oxidation of glucose in glucose/air biofuel cell

Redox enzymes are capable of harvesting electrical energy from biofuels in high catalytic activity and under mild condition. However, it is difficult to achieve efficient electron transfer and deep oxidation of biofuels simultaneously in a single-enzyme catalytic system. Herein, we report a hybrid catalyst cascade consisting of an organic oxidation catalyst, 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO), and an enzyme, glucose oxidase (GOx), for electrochemical oxidation of glucose. It is found that TEMPO is capable of mediating electron transfer between the redox center of GOx and the electrode surface. While glucose can be oxidized into glucuronic acid under neutral conditions. Thus, combining GOx and TEMPO, we are able to achieve 4e- electrooxidation of glucose using the hybrid enzymatic and organic cascade (HEOC) system. When coupled with an air-breathing Pt cathode, the resulting glucose/air biofuel cell using the proposed HEOC anode exhibits a maximum power density of 38.1 μW cm-2 with a short-circuit current of 651.4 μA cm-2, which can be attributed to the enhanced energetic efficiency, enabling TEMPO a promising catalyst for glucose oxidation in bioelectronics applications.

Carbon Nanotube PtSn Nanoparticles for Enhanced Complete Biocatalytic Oxidation of Ethylene Glycol in Biofuel Cells

We report a hybrid catalytic system containing metallic PtSn nanoparticles deposited on multiwalled carbon nanotubes (Pt₆₅Sn₃₅/MWCNTs), prepared by the microwave-assisted method, coupled to the enzyme oxalate oxidase (OxOx) for complete ethylene glycol (EG) electrooxidation. Pt₆₅Sn₃₅/MWCNTs, without OxOx, showed good electrochemical activity toward EG oxidation and all the byproducts. Pt₆₅Sn₃₅/MWCNTs cleaved the glyoxilic acid C-C bond, producing CO₂ and formic acid, which was further oxidized at the electrode. Concerning EG oxidation, the catalytic activity of the hybrid system (Pt₆₅Sn₃₅/MWCNTs+OxOx) was twice the catalytic activity of Pt₆₅Sn₃₅/MWCNTs. Long-term electrolysis revealed that Pt₆₅Sn₃₅/MWCNTs+OxOx was much more active for EG oxidation than Pt₆₅Sn₃₅/MWCNTs: the charge increased by 65%. The chromatographic results proved that Pt₆₅Sn₃₅/MWCNTs+OxOx collected all of the 10 electrons per molecule of the fuel and was able to catalyze EG oxidation to CO₂ due to the associative oxidation between the metallic nanoparticles and the enzymatic pathway. Overall, Pt₆₅Sn₃₅/MWCNTs+OxOx proved to be a promising system to enhance the development of enzymatic biofuel cells for further application in the bioelectrochemistry field.

Cascaded Biocatalysis and Bioelectrocatalysis: Overview and Recent Advances

Enzyme cascades are plentiful in nature, but they also have potential in artificial applications due to the possibility of using the target substrate in biofuel cells, electrosynthesis, and biosensors. Cascade reactions from enzymes or hybrid bioorganic catalyst systems exhibit extended substrate range, reaction depth, and increased overall performance. This review addresses the strategies of cascade biocatalysis and bioelectrocatalysis for ( a) CO2 fixation, ( b) high value-added product formation, ( c) sustainable energy sources via deep oxidation, and ( d) cascaded electrochemical enzymatic biosensors. These recent updates in the field provide fundamental concepts, designs of artificial electrocatalytic oxidation-reduction pathways (using a flexible setup involving organic catalysts and engineered enzymes), and advances in hybrid cascaded sensors for sensitive analyte detection.

Ethanol Biofuel Cells: Hybrid Catalytic Cascades as a Tool for Biosensor Devices

Biofuel cells use chemical reactions and biological catalysts (enzymes or microorganisms) to produce electrical energy, providing clean and renewable energy. Enzymatic biofuel cells (EBFCs) have promising characteristics and potential applications as an alternative energy source for low-power electronic devices. Over the last decade, researchers have focused on enhancing the electrocatalytic activity of biosystems and on increasing energy generation and electronic conductivity. Self-powered biosensors can use EBFCs while eliminating the need for an external power source. This review details improvements in EBFC and catalyst arrangements that will help to achieve complete substrate oxidation and to increase the number of collected electrons. It also describes how analytical techniques can be employed to follow the intermediates between the enzymes within the enzymatic cascade. We aim to demonstrate how a high-performance self-powered sensor design based on EBFCs developed for ethanol detection can be adapted and implemented in power devices for biosensing applications.

Enhanced electrochemical oxidation of ethanol using a hybrid catalyst cascade architecture containing pyrene-TEMPO, oxalate decarboxylase and carboxylated multi-walled carbon nanotube

The work presented herein demonstrates a hybrid bi-catalytic architecture for the complete electrochemical oxidation of ethanol. The new catalytic system contains pyrene-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-N-oxyl) immobilized on the surface of carboxylated multi-walled carbon nanotubes (MWCNT-COOH), and oxalate decarboxylase enzyme (OxDc) immobilized onto a carbon cloth electrode. Electrolysis revealed a stable amperometric curve and an excellent current density value over a duration of 10 h. In addition, the hybrid system immobilized on the carbon electrode exhibits outstanding stability after electrolysis. Nuclear magnetic resonance (NMR) and gas chromatography (GC) demonstrate that the hybrid electrode system is able to oxidize ethanol to CO₂ after 10 h of electrolysis. Overall, this study illustrates the enhancement of an enzymatic biofuel cell through the hybrid multi-catalytic systems, which exhibit high oxidation rates for all substrates involved in complete ethanol oxidation, enabling the collection of up to 12 electrons per molecule of ethanol.

Bioinspired architecture of a hybrid bifunctional enzymatic/organic electrocatalyst for complete ethanol oxidation

Electrochemical ethanol oxidation was performed at an innovative hybrid architecture electrode containing TEMPO-modified linear poly(ethylenimine) (LPEI) and oxalate oxidase (OxOx) immobilized on carboxylated multi-walled carbon nanotubes (MWCNT-COOH). On the basis of chromatographic results, the catalytic hybrid electrode system completely oxidized ethanol to CO₂ after 12 h of electrolysis. The fact that the developed system can catalyze ethanol electrooxidation at a carbon electrode confirms that organic oxidation catalysts combined with enzymatic catalysts allow up to 12 electrons to be collected per fuel molecule. The Faradaic efficiency of the hybrid system investigated herein lies above 87%. The combination of OxOx with TEMPO-LPEI to obtain a novel hybrid anode to oxidize ethanol to carbon dioxide constitutes a simple methodology with useful application in the development of enzymatic biofuel cells.