Michael B. McDonald

Use of Bipolar Membranes for Maintaining Steady‐State pH Gradients in Membrane‐Supported, Solar‐Driven Water Splitting

A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.

Graphene Oxide as a Water Dissociation Catalyst in the Bipolar Membrane Interfacial Layer

Bipolar membranes are formed by the lamination of an anion- and cation-exchange layer. Upon a sufficient applied reverse bias, water molecules at the layer junction dissociate, generating OH(-) and H(+), which can be useful in electrodialysis and electrosynthesis applications. Graphene oxide has been introduced into bipolar membrane junctions (illustrated in the adjacent graphic) and is shown to be an efficient new water dissociation catalyst, lowering the overpotential by 75% compared to a control membrane. It was found that adjusting deposition conditions changes the nature of the graphene oxide films, leading to tunable membrane performance. Additionally, it is shown that their low overpotentials are stable, making for industrially viable, high-performance bipolar membranes.

Reduced Graphene Oxide Bipolar Membranes for Integrated Solar Water Splitting in Optimal pH

The integration of light absorbers and catalysts for the water splitting process requires a membrane capable of both ion and electron management and product separation to realize efficient solar fuels systems. Bipolar membranes can maintain a pH gradient for optimal reaction conditions by the dissociation of water. Such membranes that contain graphene in the interfacial layer are fabricated by the chemical reduction of a uniformly deposited graphene oxide layer to convert sp(3) catalyst regions to sp(2) conductive regions. The resulting electrical and water dissociation properties are optimized by adjusting the exposure conditions, and treatments of less than 5 min render an interface that exceeds the conductivity requirements for integrated solar water splitting and increases the overpotential by <0.3 V. Integration with photoelectrodes is examined by characterizing the electrical interface formed between graphene and Si microwires, and we found that efficient Ohmic junctions are possible.

Fluid Embeddable Coupled Coil Sensor for Wireless pH Monitoring in a Bioreactor

A passive embeddable coupled coil sensor for remote bioprocess pH monitoring is described. The sensor is sterilizable, able to operate in a fluid medium, and small enough to fit inside a small bioreactor or test tube. It consists of a planar spiral inductor connected parallel to a varactor forming an LC resonant circuit. A pH combination electrode made of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode is connected parallel to the varactor. A potential difference change across the electrodes due to pH variation of the medium changes the voltage-dependent capacitance and shifts the resonant frequency of the sensor. An interrogator coil is inductively coupled to the sensor coil and remotely tracks the resonant frequency of the sensor. For in-fluid monitoring, the sensor is encapsulated in a manner that reduces the influence of the permittivity and conductivity of the medium. The sensor, calibrated over 2-12 pH range, exhibits a rapid response with 2.477-MHz/pH sensitivity. The sensor was used for remote pH monitoring of Yarrowia lipolytica and Saccharomyces cerevisiae fermentation in a shake flask over 63 and 25 h, respectively. The experiments demonstrate that the medium pH can be monitored repeatedly with an accuracy of 0.08 pH.

Wireless passive sensor for pH monitoring inside a small bioreactor

A wireless passive pH sensor for continuous remote bioprocess monitoring is presented. The sensor is small enough to fit inside a small bioreactor or test tube. It consists of a planar spiral inductor connected in parallel to a varactor forming a LC resonant circuit. A pH combination electrode made of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode, is connected in parallel to the varactor. As the medium pH changes, the voltage across the electrodes varies, shifting the resonant frequency of the sensor. For in-fluid monitoring the sensor is hermetically sealed to encapsulate, and reduce parasitic capacitive coupling and eddy current loss. The resonant frequency of the sensor is tracked remotely by an interrogator inductor inductively coupled to the sensor. The sterilizable sensor was used for remote pH monitoring of Yarrowia lipolytica fermentation in a shake flask over 67 hours. Experiment shows that the medium pH can be monitored with 2.46 MHz/pH sensitivity and maximum deviation of 0.07 pH from a commercial pH probe measurement over a 6.55.26 pH range.

Novel Conducting Polymer-Heteropoly Acid Hybrid Material for Artificial Photosynthetic Membranes

Artificial photosynthetic (AP) approaches to convert and store solar energy will require membranes capable of conducting both ions and electrons while remaining relatively transparent and chemically stable. A new approach is applied herein involving previously described in situ chemical polymerization of electronically conducting poly(3,4-ethylenedioxythiophene) (PEDOT) in the presence of proton conducting heteropoly acid (HPA) phosphomolybdic acid (PMA). The electrochemical behaviour of the PEDOT/PMA hybrid material was investigated and it was found that the conducting polymer (CP) is susceptible to irreversible oxidative processes at potentials where water is oxidized. This will be problematic in AP devices should the process occur in very close proximity to a conducting polymer-based membrane. It was found that PEDOT grants the system good electrical performance in terms of conductivity and stability over a large pH window; however, the presence of PMA was not found to provide sufficient proton conductivity. This was addressed in an additional study by tuning the ionic (and in turn, electronic) conductivity in creating composites with the proton-permselective polymer Nafion. It was found that a material of this nature with near-equal conductivity for optimal chemical conversion efficiency will consist of roughly three parts Nafion and one part PEDOT/PMA.

Catalytic, Conductive Bipolar Membrane Interfaces through Layer‐by‐Layer Deposition for the Design of Membrane‐Integrated Artificial Photosynthesis Systems

In the presence of an electric field, bipolar membranes (BPMs) are capable of initiating water disassociation (WD) within the interfacial region, which can make water splitting for renewable energy in the presence of a pH gradient possible. In addition to WD catalytic efficiency, there is also the need for electronic conductivity in this region for membrane-integrated artificial photosynthesis (AP) systems. Graphene oxide (GO) was shown to catalyze WD and to be controllably reduced, which resulted in electronic conductivity. Layer-by-layer (LbL) film deposition was employed to improve GO film uniformity in the interfacial region to enhance WD catalysis and, through the addition of a conducting polymer in the process, add electronic conductivity in a hybrid film. Three different deposition methods were tested to optimize conducting polymer synthesis with the oxidant in a metastable solution and to yield the best film properties. It was found that an approach that included substrate dipping in a solution containing the expected final monomer/oxidant ratio provided the most predictable film growth and smoothest films (by UV/Vis spectroscopy and atomic force microscopy/scanning electron microscopy, respectively), whereas dipping in excess oxidant or co-spraying the oxidant and monomer produced heterogeneous films. Optimized films were found to be electronically conductive and produced a membrane ohmic drop that was acceptable for AP applications. Films were integrated into the interfacial region of BPMs and revealed superior WD efficiency (≥1.4 V at 10 mA cm-2 ) for thinner films (<10 bilayers≈100 nm) than for either the pure GO catalyst or conducting polymer individually, which indicated that there was a synergistic effect between these materials in the structure configured by the LbL method.

An inductively coupled passive tag for remote basic volatile sensing

We present a passive tag capable of detecting basic volatiles in the surrounding environment. The tag is based on a voltage dependant frequency shift approach using a LC resonator comprised of a spiral inductor in parallel with a varactor. The volatile detector, comprised of a pair of pH-sensitive electrodes coated with a very thin layer of hydrogel, is connected in parallel with the varactor. The hydrogel is utilized as an absorptive medium for the basic volatiles and acts to contain the electrolyte. Due to the absorption of the basic volatile, the hydrogel pH changes which in turn changes the voltage across the pH-sensitive electrode pair shifting the resonant frequency of the tag. An interrogator coil is inductively coupled to the tag inductor to remotely track the resonant frequency of tag at distances ranging from 5 to 11 cm. Tests with ammonia show that the tag has a detection limit of 1.5 ppm. With less than a 20 min response time, the tag has potential for application in food freshness monitoring where detecting basic volatiles is important.

Efficient Transport Networks in a Dual Electron/Lithium-Conducting Polymeric Composite for Electrochemical Applications

In this work, an all-functional polymer material composed of the electrically conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) and lithium-conducting poly(ethylene oxide) (PEO) was developed to form a dual conductor for three-dimensional electrodes in electrochemical applications. The composite exhibits enhanced ionic conductivity (∼10-4 S cm-1) and, counterintuitively, electronic conductivity (∼45 S cm-1) with increasing PEO proportion, optimal at a monomer ratio of 20:1 PEO:PEDOT. Microscopy reveals a unique morphology, where PSS interacts favorably with PEO, destabilizing PEDOT to associate into highly branched, interconnected networks that allow for more efficient electronic transport despite relatively low concentrations. Thermal and X-ray techniques affirm that the PSS-PEO domain suppresses crystallinity, explaining the high ionic conductivity. Electrochemical experiments in lithium cell environments indicate stability as a function of cycling and improved overpotential due to dual transport characteristics despite known issues with both individual components.