Redox-active Polymers Research Articles

Two-Dimensional Conductive and Redox-Active Nanostructures Synthesized by Crystal-Controlled Polymerization for Electrochemical Applications

Two-dimensional nanomaterials have attracted much interest for their anisotropic structures and emergent properties. Whereas a variety of inorganic nanosheets are prepared by exfoliation, design and synthetic strategies of organic nanosheets are still developing. Here we report on crystal-controlled synthesis of conductive and redox-active organic polymer nanosheets with tunable thickness and lateral size. Although the nanosheets consist of classical conductive polymers, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), a new crystal-controlled polymerization approach provides 2D nanomaterials with enhanced and characteristic properties. Oxidative polymerization proceeds on the crystal surface of organic oxidants, such as quinone derivatives, with diffusion of the monomer vapor at low temperature under ambient pressure. The present method affords the control of lateral size and thickness of the nanosheets. The resultant nanosheets show the characteristic and enhanced properties origi...

Electrochromic Polymers Processed from Environmentally Benign Solvents

The ability to process conjugated polymers from environmentally benign solvents is essential for making organic electronics commercially viable by reducing costs and enhancing safety in the printing and processing environment. To enhance the solubility of typically alkyl-functionalized redox-active and electrochromic polymers, poly(3,4-propylenedioxythiophenes) with ester-functionalized side chains were synthesized via direct arylation polymerization, resulting in polymers that are processable from 2-methyltetrahydrofuran, ethyl acetate, and propyl acetate. Optical and atomic force microscopy results of spray-processed films indicate that topological features, such as film roughness, can be manipulated via the vapor pressure of the processing solvent. The solvent choice affects the resulting onset of absorbance and relative intensities of vibronic features, which translates into distinctly observable and quantifiable color differences. While the color is sensitive to the casting solvent, the redox prope...

Electrochemical Versatility of Soluble Redox Active Polymers

Organic redox-active materials offer advantages of material tunability while maintaining high performance for next-generation energy storage and conversion solutions. Macromolecular architectures such as polymers, colloids, and oligomers have recently gained interest for their size-exclusion capabilities. Redox Active Polymers (RAPs) have traditionally been electrochemically studied as films or composites [1], with few examples of solution-phase studies until recent years [2]. Soluble RAPs have begun emerging as viable active materials for energy storage solutions such as size-exclusion flow batteries [3-4]. Our laboratories have introduced several redox active motifs used as pendants on these soluble RAPs in recent years [3]. Their introduction into the field of energy storage as fluid energy carriers has elucidated some of their reactivity and applicability, yet detailed characterization of their complex charge transfer mechanisms and their interactions in solution and with catalytic and photocatalytic interfaces has not been fully addressed. In this talk, I will highlight advances in characterizing the redox activity of these polymers and their behavior at electrode interfaces with the aim of using them as versatile redox fluids. I will also describe how their charge transfer mechanisms make them ideal for varied energy storage applications beyond size-exclusion redox flow batteries. The RAPs are evaluated using analytical techniques such as potential-controlled electrolysis, spectro-electrochemistry, and voltammetric methods including transient and steady-state methods. Additionally, comparisons to the small molecule counterparts of the RAP active units aid to understand the differences observed when redox active centers are tethered to a chain containing hundreds of pendants. I will discuss how this new type of soluble RAPs interact with energy conversion electrodes and how charge transfer and transport within RAP particles facilitates catalytic processes associated with energy storage. [1] J. Am. Chem. Soc. 1978, 100, 4248. [2] J. Am. Chem. Soc. 2018, 140, 2093. [3] Acc. Chem. Res. 2016, 49, 2649. [4] J. Am. Chem. Soc. 2014, 136, 16309.

(Invited) The Impact of Polyelectrolyte Dynamics on the Electrochemical Reactivity of Soluble Redox-Active Polymers

In this presentation, I will discuss recent findings regarding the impact of polyelectrolyte dynamics on the charge percolation and electrolysis efficiency of soluble Redox-Active Polymers (RAPs). RAPs[1] and their colloidal form (RACs)[2] are a new class of materials for energy storage in fluid dispersions for a novel concept in size-exclusion flow batteries.[3] Despite RAPs displaying excellent charge storage properties, their complex electrochemical reactive pathways are just becoming elucidated. RAPs and RACs rely on intra-particle electron transfer to yield quantitative charge accessibility and high rate in order to achieve full electrolysis.[4] Charge mobility via inter-pendant electron hopping combines with their polyelectrolyte nature to exhibit a rich electrochemical behavior that is strongly modulated by specific and non-specific interactions with the electrolyte. In this talk, I will highlight the impact of several variables on the electrochemical signatures created by RAPs and RACs.The mechanistic characterization of RAPs and RACs requires a spectrum of powerful electrochemical techniques, ranging from bulk methods to single-particle analysis. I will describe how the application of nano-resolved electrochemical microscopy techniques and Raman spectroscopy has helped us understand the mechanisms of individual electrochemical entities.These experiments provide us with unprecedented versatility to identify kinetic bottlenecks, such as charge trapping, and to determine the maximum current densities attainable in flow devices. Altogether, the combination of novel structural motifs, the identification of structure-reactivity relationships, and the use of advanced electrochemical techniques, results in new directions to make better polymers for a new concept in redox flow batteries.[1] J. Am. Chem. Soc. 2014, 136, 16309.[2] J. Am. Chem. Soc. 2016, 138, 13230.[3] Acc. Chem. Res. 2016, 49, 2649.[4] Chem. Mater. 2016, 28, 7362.*This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Single-Particle Evaluation of Nano and Micro-Sized Redox-Active Polymers for Energy Storage

Redox active polymers (RAPs) and redox active colloids (RACs) are new energy storage materials for size-exclusion redox flow batteries (RFBs).1-3 RAPs show beneficial characteristics for RFBs including high capacity, high solubility, electrochemical reversibility and tailorable properties via synthetic methods.1,2,4 Their crosslinked counterparts, RACs, are well defined spheres that can be nano- or micro-sized and have many similar characteristics to RAPs while showing even higher capacities and greater crossover rejection.3 As with other macromolecular systems, much remains unknown about the applicability and practicality of energy storage systems involving organic macromolecules capable of storing 106-109 electrons per molecule. In this presentation, I want to discuss methodologies we have been using to analyze stability and evolution of the RAC charge/discharge process during cycling with a focus on measuring a single or a limited number of particles. As with other non-conjugated RAPs, the mechanism of charge transfer across a RAC involves electron hopping between redox active pendants and simultaneous counterion movement.3,5 We recently investigated the electron transfer process within viologen-based RACs through various analytical methods based on scanning electrochemical microscopy (SECM) and modeling through COMSOL.6 SECM is a scanning probe technique that can assess local redox reactivity at the micro or nanoscale in a liquid electrolyte amenable to real battery environments.7 With SECM, single RACs could be imaged, sized and directly reduced/oxidized through making contact with an SECM probe. We were also able to evaluate monolayer films of nanometer sized RACs with a spectroelectrochemical setup using a coupled Raman-SECM instrument. This study provided some useful fundamental information and analytical tools/methods to guide future development and refinement for long time-scale experiments on RACs. I will present on recent cycling experiments involving SECM and modified electrodes. Both chronoamperometric steps and cyclic voltammetry (CV) have been used to measure the cycling process and to manipulate capacity access during cycling. Our data indicate that RACs undergo some conditioning before higher charge recovery and the proportion of reduced to oxidized viologen, or state-of-charge (SoC), will affect the amount of charge that can be recollected from the RAC. I will also discuss measurements involving Raman coupled to electrochemistry for single and few RACs. Refining and developing effective analytical methods is the key to understanding electron transfer, storage and failure within RACs and all battery materials. Our methods are pushing the limits of electrochemistry on single macromolecules but hopefully will be extended to other energy storage and electron transfer systems. J. Am. Chem. Soc. 2014, 136 (46), 16309-16316. Nature 2015, 527 (7576), 78-81. J. Am. Chem. Soc. 2016, 138 (40), 13230-13237. Chem. Mater. 2016, 28 (20), 7362-7374. J. Phys. Chem. 1991, 95 (16), 6383-6389. Langmuir 2017, 33 (37), 9455-9463. Scanning electrochemical microscopy. CRC Press: 2012.

Electrochemical performance and redox mechanism of naphthalene-hydrazine diimide polymer as a cathode in magnesium battery

A magnesium (Mg) battery in the combination with the redox active polymers possesses attractive electrochemical properties as a future battery technology. This concept has been demonstrated by using quinone based polymers and the non-nucleophilic electrolytes, while polyimides were rarely investigated. In this work, a naphthalene-hydrazine diimide polymer (NP) has been tested as a cathode material for Mg organic battery in two different electrolytes. Average discharge voltage of the Mg-NP battery is 1.70 V with Coulombic efficiency above 99% in the TEGDME/DOL based electrolyte. The electrochemical mechanism was studied by in operando ATR IR spectroscopy and confirmed by DFT calculation of the IR spectra. We confirmed redox activity of carbonyl moieties, which undergo reversible reduction to enolate anion. Moreover, in operando IR spectroscopy shows charge delocalization in the reduced form since none of the bonds has pure single/double bond character.

Electrochemistry of membrane proteins and protein–lipid assemblies

The electrochemistry of membrane proteins allows us to examine the association of proteins with membranes and low-molecular ligands, such as drugs, toxins and others. In addition, electrochemical methods are applicable for the quantification of transport phenomena, for monitoring the change in the structure of the protein, and determining its catalytic activity if the protein is an enzyme. In addition to approaches based on the concept of protein film voltammetry, methods based on the direct redox transformations or electrocatalytic reactions of the amino acid residues in the protein structure could be used. The above areas of interest fall under basic research, which is the main subject of this paper. However, the utilization of new immobilization strategies for proteins and the application of redox active polymers is an important part of the development of new biosensors and the construction of devices employing the principles of bioelectrocatalysis.

Effect of the Backbone Tether on the Electrochemical Properties of Soluble Cyclopropenium Redox-Active Polymers

Few reports to date have focused on the chemical and electrochemical reversibility of redox pendants assembled into soluble redox-active polymers (RAPs). Here we report a series of soluble RAPs for flow battery applications designed with cyclopropenium (CP) pendants. The tether length between CP and a polystyrene backbone was varied and found to influence electrochemical activity and stability. Different tether lengths of x methylene groups (x = 1–7) were simulated, and x = 1, 5, and 7 were synthesized to evaluate experimentally. This study illustrates that polymers with extended tether groups display an improved reversibility in cyclic voltammetry. The behavior is mirrored in the stability of the charged state tested in galvanostatic half-cells. When paired with a viologen polymer, these CP-based polymers produce a 1.55 V nonaqueous flow battery. The capacity decays for the polymers were structure-dependent, which provides empirical insight into materials design for high potential catholyte polymers.

A Mechanistic Investigation of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

Thermally Activated Redox Conduction in Shewanella Oneidensis MR-1 Biofilms

Shewanella oneidensis MR-1 can gain energy by performing extracellular electron transport (EET) to external insoluble electron acceptors ranging from natural minerals to synthetic electrode surfaces when poised at proper potentials. Outer membrane c-type multi-heme cytochromes play a critical role in mediating this electron transport. The abundance of these cytochromes on the cell surface has also been hypothesized to facilitate long-distance electron transport along multicellular biofilms, in a manner similar to redox active polymers containing discrete redox moieties. Here we report measurements of electron transport in living S. oneidensis biofilms that bridged interdigitated array (IDA) microelectrodes. Electrochemical gating reveals a peak in the conduction current at the expected formal potential of the cytochrome conduits, consistent with redox conductivity where transport is driven by a multistep electron hopping mechanism through the heme network. The temperature dependency of the biofilm conduction is also consistent with such a thermally activated process, where conductivity increases with increasing temperature. In addition, the measured activation energy (0.315 eV) is consistent with the computationally-determined activation energy of the S. oneidensis Mtr decaheme cytochromes.

Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light-Driven Hydrogen Evolution and Oxidative Dehydrogenation.

Dehydrogenation of the tetrahydroisoquinoline derivatives coupled with hydrogen production is important for hydrogen storage applications. Herein, we formulated a new system that embedded Dawson-type polyoxometalates as efficient photosensitizers into the pores of redox-active coordination polymers for the light-driven photocatalytic oxidative Mannich reaction and hydrogen evolution. In the designed Co-POM polymer, UV light excitation gives the excited state of the Dawson-type polyoxometalate first to oxidize electron donors or substrates; the reduced form (i.e., heteropolyblue) adsorbs visible light to achieve a new excited state, which reduced the cobalt redox sites and facilitates hydrogen evolution reaction. The photosensitizer recovered to the ground state, completing the catalytic cycle. Under the optimized conditions, Co-POM enabled the hydrogen evolution and dehydrogenation of tetrahydroisoquinoline without the presence of any other additives. The high catalytic efficiency and robustness indicated the advantages of the combining functional polyoxometalate-based catalysts and porous characters of the coordination polymers for the development of highly active heterogeneous catalysts.

Modulation of the Electrochemical Reactivity of Solubilized Redox Active Polymers via Polyelectrolyte Dynamics.

Redox active polymers (RAPs) are electrochemically versatile materials that find key applications in energy storage, sensing, and surface modification. In spite of the ubiquity of RAP-modified electrodes, a critical knowledge gap exists in the understanding of the electrochemistry of soluble RAPs and their relation to polyelectrolyte dynamics. Here, we explore for the first time the intersection between polyelectrolyte behavior and the electrochemical response that highly soluble and highly substituted RAPs with viologen, ferrocene, and nitrostyrene moieties elicit at electrodes. This comprehensive study of RAP electrolytes over several orders of magnitude in concentration and ionic strength reveals distinct signatures consistent with surface confined, colloidal, and bulk-like electrochemical behavior. These differences emerge across polyelectrolyte packing regimes and are strongly modulated by changes in RAP coil size and electrostatic interactions with the electrode. We found that, unlike monomer motifs, simple changes in the ionic strength caused variations over 1 order of magnitude in the current measured at the electrode. In addition, the thermodynamics of adsorbed RAP films were also affected, giving rise to standard reduction potential shifts leading to redox kinetic effects as a result of the mediating nature of the RAP film in equilibrium with the solution. Full electrochemical characterization via transient and steady-state techniques, including the use of ultramicroelectrodes and the rotating disk electrode, were correlated to dynamic light scattering, ellipsometry, and viscometric analysis. These methods helped elucidate the relationship between electrochemical behavior and RAP coil size, film thickness, and polyelectrolyte packing regime. This study underscores the role of electrostatics in modulating the reactivity of redox polyelectrolytes.

Design of a novel redox-active gel polymer electrolyte with a dual-role ionic liquid for flexible supercapacitors

By choosing an ionic liquid 1-butyl-3-methylimidazolium iodide (BMIMI) as plasticizer and redox additive, along with a neutral Li2SO4 aqueous solution to achieve wide operating voltage, a redox-active poly(vinyl alcohol) (PVA)-Li2SO4-BMIMI gel polymer electrolyte (GPE) was prepared and used for the assemble of an activated carbon-based flexible supercapacitor. The electrochemical performances of the flexible supercapacitor with an optimized PVA-Li2SO4-BMIMI GPE were evaluated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy techniques. The mechanical performance of the flexible supercapacitor under different bending angles was also discussed. The obtained PVA-Li2SO4-BMIMI GPE exhibits the rarely found elongation at break of up to about 1200%. The flexible supercapacitor can reach a high energy density of 29.3 Wh kg−1, which is attributed to the wide voltage (1.5 V) resulted from the strong solvation of Li+ cations and SO42− anions, and the pseudocapacitance contributed by Faradaic reactions of the I-based ions in the GPE. Moreover, the flexible supercapacitor shows good cyclic durability and superior mechanical performance. For comparison, the PVA-Li2SO4-BMIMCl GPE was also synthesized and then its performance was studied.

Diffusion-Cooperative Model for Charge Transport by Redox-Active Nonconjugated Polymers.

Charge transport processes in nonconjugated redox-active polymers with electrolytes were studied using a diffusion-cooperative model. For the first time, we quantitatively rationalized that the limited Brownian motion of the redox centers bound to the polymers resulted in the 103-4-fold decline of the bimolecular and heterogeneous charge transfer rate constants, which had been unexplained for half a century. As a next-generation design, a redox-active supramolecular system with high physical mobility was proposed to achieve the rate constant as high as in free solution system (>107 M-1 s-1) and populated site density (>1 mol/L).

Electrosynthesis of redox-active and electrochromic polymer films from triphenylamine-cored star-shaped molecules end-capped with arylamine groups

Four starburst molecules with a triphenylamine (TPA) core through the amide group linked to the terminal triphenylamine or (N-phenyl)carbazole (NPC) unit, coded as T-3(NHCO-TPA), T-3(NHCO-NPC), T-3(CONH-TPA), and T-3(CONH-NPC), were synthesized and their electrochemistry and electropolymerization were investigated. The oxidation potential and the stability of the oxidized forms of the starbursts are affected by the orientation of amide linkage and the structure of terminal triarylamine unit. Robust polymeric films could be built onto ITO/glass surface by repetitive cyclic voltammetry (CV) of the T-3(NHCO-TPA) solution containing an electrolyte. The electrodeposited films exhibited reversible electrochemical processes and strong color changes upon electro-oxidation, which can be switched by potential modulation. The electrochromic behavior of the film was clearly interpreted on the basis of spectroelectrochemical studies. T-3(CONH-TPA) did not undergo electropolymerization effectively. T-3(CONH-NPC) also could electrochemically polymerize to polymer films upon repeated CV scans. However, their electrochromic switching stability is less than that of the polymer film from T-3(NHCO-TPA).

Metallated azo-naphthalene diimide based redox-active porous organic polymer as an efficient water oxidation electrocatalyst

Metallated redox active porous organic polymer for water oxidation.

Redox Active Polymer as a pH Actuator on a Re-Sealable Microfluidic Platform

The electrochemical control of molecular concentrations in liquids is quite challenging because of the small ratio of interacting sites between the electrode surface and the number of molecules in the volume. Here we review our recent works aiming the control of proton concentration combining redox self-assembled monolayers with a modified platform. We used Aminothiolphenol, to obtain coatings with quasi-reversible redox states able to exchange protons with the electrolyte at low voltages using different polymerization methods. We studied their charge exchange during different cycles, which would provide us the possibility to use different cycles of reactions controlled by acidity. We achieved the control of proton concentration over the liquid with an efficient design of the microfluidic device to control the diffusion of protons avoiding the reduction of hydrogen at the counter electrode and providing long lasting stability to carry control of chemical reactions during several tenths of minutes. The experiments were carried out in aqueous environment using a pH fluorescence marker to track the pH that allowed us to monitor the proton concentration down to pH 5, where the fluorescence molecule lost its sensitivity, while calculations indicate that the pH can be below 1.

Coronene Diimide Containing Redox Active Electrochromic Polymer Film via Electropolymerization of Diphenylamine End Groups

A new monomer containing coronene diimide with diphenylamine end groups (DCTD) has been synthesized and polymer thin films were electropolymerized onto gold, glassy carbon and ITO electrode surfaces. The resulting polymers show a reversible 2e− oxidation of the diphenylbenzidine unit and reversible reductions of the aromatic diimide. AFM was used to characterize the polymer film morphology. The electronic and electrochromic properties of these films were evaluated for use in organic electronic applications.

Controllable fabrication of redox-active conjugated microporous polymers on reduced graphene oxide for high performance faradaic energy storage

Two-dimensional (2D) composites composed of redox active conjugated microporous polymer (CMP) and reduced graphene oxide (rGO) enable efficient faradaic energy storage.

Encapsulation of Microorganisms, Enzymes, and Redox Mediators in Graphene Oxide and Reduced Graphene Oxide.

Graphene oxide (GO) and reduced graphene oxide (rGO) were demonstrated in the past decade as biocompatible carbon-based materials that could be efficiently used in bioelectrochemical systems (BESs). Specifically, for redox enzyme encapsulation in order to improve electron communication between enzymes and electrodes. The addition of GO to different solvents was shown to cause gelation while still allowing small molecule diffusion through its gel-like matrix. Taking the combination of these traits together, we decided to use GO hydrogels for the encapsulation of enzymes displayed on the surface of yeast in anodes of microbial fuel cells. During our studies we have followed the changes in the physical characteristics of GO upon encapsulation of yeast cells displaying glucose oxidase in the presence of glucose and noted that GO is being rapidly reduced to rGO as a function of glucose concentrations. GO reduction under these conditions served as a proof of electron communication between the surface-displayed enzymes and GO. Hence, we set out to study this phenomenon by the encapsulation of a purified glucose dehydrogenase (in the absence of microbial cells) in rGO where improved electron transfer to the electrode could be observed in the presence of phenothiazone. In this chapter, we describe how these systems were technically constructed and characterized and how a very affordable matrix such as GO could be used to electrically wire enzymes as a good replacement for expensive mediator containing redox active polymers commonly used in BESs.