Enzymatically Polymerized Organic Conductors on Model Lipid Membranes

Seamless integration between biological systems and electrical components is essential for enabling a twinned biochemical-electrical recording and therapy approach to understand and combat neurological disorders. Employing bioelectronic systems made up of conjugated polymers, which have an innate ability to transport both electronic and ionic charges, provides the possibility of such integration. In particular, translating enzymatically polymerized conductive wires, recently demonstrated in plants and simple organism systems, into mammalian models, is of particular interest for the development of next-generation devices that can monitor and modulate neural signals. As a first step toward achieving this goal, enzyme-mediated polymerization of two thiophene-based monomers is demonstrated on a synthetic lipid bilayer supported on a Au surface. Microgravimetric studies of conducting films polymerized in situ provide insights into their interactions with a lipid bilayer model that mimics the cell membrane. Moreover, the resulting electrical and viscoelastic properties of these self-organizing conducting polymers suggest their potential as materials to form the basis for novel approaches to in vivo neural therapeutics.

Selectivity Control of Oxygen Reduction Reaction over Mesoporous Transition Metal Oxide Catalysts for Electrified Purification Technologies

Direct electrification of oxygen-associated reactions contributes to large-scale electrical storage and the launch of the green hydrogen economy. The design of the involved catalysts can mitigate the electrical energy losses and improve the control of the reaction products. We evaluate the effect of the interface composition of electrocatalysts on the efficiency and productivity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), both mechanistically and at device levels. The ORR and OER were benchmarked on mesoporous nickel(II) oxide and nickel cobaltite (NiO and NiCo₂O₄, respectively) obtained by a facile template-free hydrothermal synthesis. Physicochemical characterization showed that both NiO and NiCo₂O₄ are mesoporous and have a cubic crystal structure with abundant surface hydroxyl species. NiCo₂O₄ showed higher electrocatalytic activity in OER and selectivity to water as the terminal product of ORR. On the contrary, ORR over NiO yielded hydroxyl radicals as products of a Fenton-like reaction of H₂O₂. The product selectivity in ORR was used to construct two electrolyzers for electrified purification of oxygen and generation of hydroxyl radicals.

Organic-Inorganic Hybrid Films of the Sulfate Dawson Polyoxometalate, [S2W18O62]4-, and Polypyrrole for Iodate Electrocatalysis

The Dawson-type sulfate polyoxometalate (POM) [S₂W₁₈O₆₂]^4- has successfully been entrapped in polypyrrole (PPy) films on glassy carbon electrode (GCE) surfaces through pyrrole electropolymerization. Films of varying POM loadings (i.e., thickness) were grown by chronocoulometry. Film-coated electrodes were then characterized using voltammetry, revealing POM surface coverages ranging from 1.9 to 11.7 × 10^-9 mol·cm^-2, and were stable over 100 redox cycles. Typical film morphology and composition were revealed to be porous using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, and the effects of this porosity on POM redox activity were probed using AC impedance. The hybrid organic-inorganic films exhibited a good electrocatalytic response toward the reduction of iodate with a sensitivity of 0.769 μA·cm^-2·μM^-1.

Understanding of the Electrochemical Behavior of Lithium at Bilayer-Patched Epitaxial Graphene/4H-SiC

Novel two-dimensional materials (2DMs) with balanced electrical conductivity and lithium (Li) storage capacity are desirable for next-generation rechargeable batteries as they may serve as high-performance anodes, improving output battery characteristics. Gaining an advanced understanding of the electrochemical behavior of lithium at the electrode surface and the changes in interior structure of 2DM-based electrodes caused by lithiation is a key component in the long-term process of the implementation of new electrodes into to a realistic device. Here, we showcase the advantages of bilayer-patched epitaxial graphene on 4H-SiC (0001) as a possible anode material in lithium-ion batteries. The presence of bilayer graphene patches is beneficial for the overall lithiation process because it results in enhanced quantum capacitance of the electrode and provides extra intercalation paths. By performing cyclic voltammetry and chronoamperometry measurements, we shed light on the redox behavior of lithium at the bilayer-patched epitaxial graphene electrode and find that the early-stage growth of lithium is governed by the instantaneous nucleation mechanism. The results also demonstrate the fast lithium-ion transport (~4.7–5.6 × 10⁻⁷ cm²∙s⁻¹) to the bilayer-patched epitaxial graphene electrode. Raman measurements complemented by in-depth statistical analysis and density functional theory calculations enable us to comprehend the lithiation effect on the properties of bilayer-patched epitaxial graphene and ascribe the lithium intercalation-induced Raman G peak splitting to the disparity between graphene layers. The current results are helpful for further advancement of the design of graphene-based electrodes with targeted performance.

Bidirectional Hydrogen Electrocatalysis on Epitaxial Graphene

The climate change due to human activities stimulates the research on new energy resources. Hydrogen has attracted interest as a green carrier of high energy density. The sustainable production of hydrogen is achievable only by water electrolysis based on the hydrogen evolution reaction (HER). Graphitic materials are widely utilized in this technology in the role of conductive catalyst supports. Herein, by performing dynamic and steady-state electrochemical measurements in acidic and alkaline media, we investigated the bidirectional electrocatalysis of the HER and hydrogen oxidation reaction (HOR) on metal- and defect-free epigraphene (EG) grown on 4H silicon carbide (4H-SiC) as a ground level of structural organization of general graphitic materials. The absence of any signal degradation illustrates the high stability of EG. The experimental and theoretical investigations yield the coherent conclusion on the dominant HER pathway following the Volmer-Tafel mechanism. We ascribe the observed reactivity of EG to its interaction with the underlying SiC substrate that induces strain and electronic doping. The computed high activation energy for breaking the O-H bond is linked to the high negative overpotential of the HER. The estimated exchange current of HER/HOR on EG can be used in the evaluation of complex electrocatalytic systems based on graphite as a conducing support.

The effect of crosslinking on ion transport in nanocellulose-based membranes

Ion selective membranes are at the heart of energy conversion and harvesting, water treatment, and biotechnologies. The currently available membranes are mostly based on expensive and non-biodegradable polymers. Here, we report a cation-selective and low-cost membrane prepared from renewable nanocellulose and 1,2,3,4-butanetetracarboxylic acid which simultaneously serves as crosslinker and source of anionic surface groups. Charge density and structure of the membranes are studied. By using different degrees of crosslinking, simultaneous control over both the nanochannel structure and surface charge concentration is achieved, which in turn determines the resulting ion transport properties. Increasing negative charge concentration via higher crosslinker content, the obtained ion conductivity reaches up to 8 mS/cm (0.1 M KCl). Optimal ion selectivity, also influenced by the solution pH, is achieved at 20 wt% crosslinker addition (with ion conductivity of 1.6 mS/cm). As regular ~1.4 nm nanochannels were formed at this composition, nanofluidic contribution to ion transport is likely.

Can Hybrid Na-Air Batteries Outperform Nonaqueous Na-O2 Batteries?

In recent years, there has been an upsurge in the study of novel and alternative energy storage devices beyond lithium-based systems due to the exponential increase in price of lithium. Sodium (Na) metal-based batteries can be a possible alternative to lithium-based batteries due to the similar electrochemical voltage of Na and Li together with the thousand times higher natural abundance of Na compared to Li. Though two different kinds of Na-O2 batteries have been studied specifically based on electrolytes until now, very recently, a hybrid Na-air cell has shown distinctive advantage over nonaqueous cell systems. Hybrid Na-air batteries provide a fundamental advantage due to the formation of highly soluble discharge product (sodium hydroxide) which leads to low overpotentials for charge and discharge processes, high electrical energy efficiency, and good cyclic stability. Herein, the current status and challenges associated with hybrid Na-air batteries are reported. Also, a brief description of nonaqueous Na-O2 batteries and its close competition with hybrid Na-air batteries are provided.

Epitaxial Graphene Sensors Combined with 3D-Printed Microfluidic Chip for Heavy Metals Detection

In this work, we investigated the sensing performance of epitaxial graphene on Si-face 4H-SiC (EG/SiC) for liquid-phase detection of heavy metals (e.g., Pb and Cd), showing fast and stable response and low detection limit. The sensing platform proposed includes 3D-printed microfluidic devices, which incorporate all features required to connect and execute lab-on-chip (LOC) functions. The obtained results indicate that EG exhibits excellent sensing activity towards Pb and Cd ions. Several concentrations of Pb²⁺ solutions, ranging from 125 nM to 500 µM, were analyzed showing Langmuir correlation between signal and Pb²⁺ concentrations, good stability, and reproducibility over time. Upon the simultaneous presence of both metals, sensor response is dominated by Pb²⁺ rather than Cd²⁺ ions. To explain the sensing mechanisms and difference in adsorption behavior of Pb²⁺ and Cd²⁺ ions on EG in water-based solutions, we performed van-der-Waals (vdW)-corrected density functional theory (DFT) calculations and non-covalent interaction (NCI) analysis, extended charge decomposition analysis (ECDA), and topological analysis. We demonstrated that Pb²⁺ and Cd²⁺ ions act as electron-acceptors, enhancing hole conductivity of EG, due to charge transfer from graphene to metal ions, and Pb²⁺ ions have preferential ability to binding with graphene over cadmium. Electrochemical measurements confirmed the conductometric results, which additionally indicate that EG is more sensitive to lead than to cadmium.

An efficient bifunctional electrocatalyst based on a nickel iron layered double hydroxide functionalized Co3O4 core shell structure in alkaline media

An electrocatalyst based on a nickel iron layered double hydroxide functionalized Co₃O₄ core shell structure.

Understanding Graphene Response to Neutral and Charged Lead Species: Theory and Experiment

Deep understanding of binding of toxic Lead (Pb) species on the surface of two-dimensional materials is a required prerequisite for the development of next-generation sensors that can provide fast and real-time detection of critically low concentrations. Here we report atomistic insights into the Lead behavior on epitaxial graphene (Gr) on silicon carbide substrates by thorough complementary study of voltammetry, electrical characterization, Raman spectroscopy, and Density Functional Theory (DFT). It is verified that the epitaxial graphene exhibits quasi-reversible anode reactions in aqueous solutions, providing a well-defined redox peak for Pb species and good linearity over a concentration range from 1 nM to 1 µM. The conductometric approach offers another way to investigate Lead adsorption, which is based on the formations of stable charge-transfer complexes affecting the p-type conductivity of epitaxial graphene. Our results suggest the adsorption ability of the epitaxial graphene towards divalent Lead ions is concentration-dependent and tends to saturate at higher concentrations. To elucidate the mechanisms responsible for Pb adsorption, we performed DFT calculations and estimated the solvent-mediated interaction between Lead species in different oxidative forms and graphene. Our results provide central information regarding the energetics and structure of Pb-graphene interacting complexes that underlay the adsorption mechanisms of neutral and divalent Lead species. Such a holistic understanding favors design and synthesis of new sensitive materials for water quality monitoring.

Lead (Pb) interfacing with epitaxial graphene

Here, we report the electrochemical deposition of lead (Pb) as a model metal on epitaxial graphene fabricated on silicon carbide (Gr/SiC). The kinetics of electrodeposition and morphological characteristics of the deposits were evaluated by complementary electrochemical, physical and computational methods. The use of Gr/SiC as an electrode allowed the tracking of lead-associated redox conversions. The analysis of current transients passed during the deposition revealed an instantaneous nucleation mechanism controlled by convergent mass transport on the nuclei locally randomly distributed on epitaxial graphene. This key observation of the deposit topology was confirmed by low values of the experimentally-estimated apparent diffusion coefficient, Raman spectroscopy and scanning electron microscopy (SEM) studies. First principles calculations showed that the nucleation of Pb clusters on the graphene surface leads to weakening of the interaction strength of the metal-graphene complex, and only spatially separated Pb adatoms adsorbed on bridge and/or edge-plane sites can affect the vibrational properties of graphene. We expect that the lead adatoms can merge in large metallic clusters only at defect sites that reinforce the metal-graphene interactions. Our findings provide valuable insights into both heavy metal ion electrochemical analysis and metal electroplating on graphene interfaces that are important for designing effective detectors of toxic heavy metals.

Complementary Logic Circuits Based on High-Performance n-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have been the subject of intense research in recent years. To date, however, most of the reported OECTs rely entirely on p-type (hole transport) operation, while electron transporting (n-type) OECTs are rare. The combination of efficient and stable p-type and n-type OECTs would allow for the development of complementary circuits, dramatically advancing the sophistication of OECT-based technologies. Poor stability in air and aqueous electrolyte media, low electron mobility, and/or a lack of electrochemical reversibility, of available high-electron affinity conjugated polymers, has made the development of n-type OECTs troublesome. Here, it is shown that ladder-type polymers such as poly(benzimidazobenzophenanthroline) (BBL) can successfully work as stable and efficient n-channel material for OECTs. These devices can be easily fabricated by means of facile spray-coating techniques. BBL-based OECTs show high transconductance (up to 9.7 mS) and excellent stability in ambient and aqueous media. It is demonstrated that BBL-based n-type OECTs can be successfully integrated with p-type OECTs to form electrochemical complementary inverters. The latter show high gains and large worst-case noise margin at a supply voltage below 0.6 V.

Poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) electrodes in thermogalvanic cells

The interest in thermogalvanic cells (TGCs) has grown because it is a candidate technology for harvesting electricity from natural and waste heat. However, the cost of TGCs has a major component due to the use of the platinum electrode. Here, we investigate new alternative electrode material based on conducting polymers, more especially poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) together with the ferro/ferricyanide redox electrolyte. The power generated by the PEDOT-Tos based TGCs increases with the conducting polymer thickness/multilayer and reaches values similar to the flat platinum electrode based TGCs. The physics and chemistry behind this exciting result as well as the identification of the limiting phenomena are investigated by various electrochemical techniques. Furthermore, a preliminary study is provided for the stability of the PEDOT-Tos based TGCs.

Oxygen-induced doping on reduced PEDOT

The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) has shown promise as air electrode in renewable energy technologies like metal-air batteries and fuel cells. PEDOT is based on atomic elements of high abundance and is synthesized at low temperature from solution. The mechanism of oxygen reduction reaction (ORR) over chemically polymerized PEDOT:Cl still remains controversial with eventual role of transition metal impurities. However, regardless of the mechanistic route, we here demonstrate yet another key active role of PEDOT in the ORR mechanism. Our study demonstrates the decoupling of conductivity (intrinsic property) from electrocatalysis (as an extrinsic phenomenon) yielding the evidence of doping of the polymer by oxygen during ORR. Hence, the PEDOT electrode is electrochemically reduced (undoped) in the voltage range of ORR regime, but O₂ keeps it conducting; ensuring PEDOT to act as an electrode for the ORR. The interaction of oxygen with the polymer electrode is investigated with a battery of spectroscopic techniques.

Thermoelectric Properties of Solution-Processed n-Doped Ladder-Type Conducting Polymers

Ladder-type "torsion-free" conducting polymers (e.g., polybenzimidazobenzophenanthroline (BBL)) can outperform "structurally distorted" donor-acceptor polymers (e.g., P(NDI2OD-T2)), in terms of conductivity and thermoelectric power factor. The polaron delocalization length is larger in BBL than in P(NDI2OD-T2), resulting in a higher measured polaron mobility. Structure-function relationships are drawn, setting material-design guidelines for the next generation of conducting thermoelectric polymers.

Electroactive biomimetic collagen-silver nanowire composite scaffolds

Electroactive biomaterials are widely explored as bioelectrodes and as scaffolds for neural and cardiac regeneration. Most electrodes and conductive scaffolds for tissue regeneration are based on synthetic materials that have limited biocompatibility and often display large discrepancies in mechanical properties with the surrounding tissue causing problems during tissue integration and regeneration. This work shows the development of a biomimetic nanocomposite material prepared from self-assembled collagen fibrils and silver nanowires (AgNW). Despite consisting of mostly type I collagen fibrils, the homogeneously embedded AgNWs provide these materials with a charge storage capacity of about 2.3 mC cm(-2) and a charge injection capacity of 0.3 mC cm(-2), which is on par with bioelectrodes used in the clinic. The mechanical properties of the materials are similar to soft tissues with a dynamic elastic modulus within the lower kPa range. The nanocomposites also support proliferation of embryonic cardiomyocytes while inhibiting the growth of both Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis. The developed collagen/AgNW composites thus represent a highly attractive bioelectrode and scaffold material for a wide range of biomedical applications.

Enhancement of nitrite and nitrate electrocatalytic reduction through the employment of self-assembled layers of nickel- and copper-substituted crown-type heteropolyanions

Multilayer assemblies of two crown-type type heteropolyanions (HPA), [Cu20Cl(OH)24(H2O)12(P8W48O184)](25-) and Ni4(P8W48O148)(WO2)](28-), have been immobilized onto glassy carbon electrode surfaces via the layer-by-layer (LBL) technique employing polycathion-stabilized silver nanoparticles (AgNP) as the cationic layer within the resulting thin films characterized by electrochemical and physical methods. The redox behaviors of both HPA monitored during LBL assembly with cyclic voltammetry and impedance spectroscopy revealed significant changes by immobilization. The presence of AgNPs led to the retention of film porosity and electronic conductivity, which has been shown with impedance and voltammeric studies of film permeabilities toward reversible redox probes. The resulting films have been characterized by physical methods. Finally, the electrocatalytic performance of obtained films with respect to nitrite and nitrate electrocatalytic reduction has been comparatively studied for both catalysts. Nickel atoms trapped inside HPA exhibited a higher specific activity for reduction.

Redox switching of polyoxometalate-methylene blue-based layer-by-layer films

Iron-substituted crown-type polyoxometalate (POM) [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) has been successfully immobilized onto glassy carbon electrode surfaces by means of the layer-by-layer (LBL) technique employing the cationic redox active dye, methylene blue (MB). The constructed multilayers exhibit pH-dependent redox activity for both the anionic POM and the cationic dye moieties, which is in good agreement with their solution behavior. The films have been characterized by alternating current impedance, atomic force microscopy, and X-ray photoelectron spectroscopy, whereby the nature of the outer layer within the assemblies was found to have an effect upon the film's behavior. Preliminary investigations show that the POM dye-based films show electrocatalytic ability toward the reduction of hydrogen peroxide, however, only when there is an outer anionic POM layer.