Dr Andrew P. Doherty

Research

Electrochemical, Ionic Liquid and Green Chemistry Research

Background: "Dr Doherty started his career at Queen's as a University Research Fellow of the Royal Society, one of the world's foremost Academies of Sciences". Our research is predominantly in the broad field of electrochemistry which is a cornerstone discipline within the physical sciences. For example, electrochemistry may be used to synthesis molecules, analyse diverse trace species such as heavy metals or neuro-transmitters in vivo, as well as for technological applications including fuel cells, batteries and photovoltaic devices. Electrochemical events are also important biologically e.g. in photosynthesis and respiration which involve concerted electron transfer reactions via redox enzymes. Electrochemical research has contributed profoundly to both the development of our understanding of such complex reactions and the design of synthetic redox materials to mimic such natural processes. Our interest lies in a number of areas; primarily in the fundamental and applied aspects of direct and catalytic electrochemistry in organised self-assembled nanofluids, ionic liquids, electrosynthesis of small organic molecules and redox catalysis. Much of our research is collaborative with colleagues within QUB (Aeronautical Engineering and Chemical Engineering) and in France, Italy, Portugal, Germany and Ukraine.

Redox catalysis and green electro-synthesis in ionic liquid media: Redox catalysts are small molecules which undergo reversible oxidation (or reduction) at electrodes whereupon they, in turn, oxidise (or reduce) solution species. Recently, we have investigated tetramethylpiperidinyl-1-oxy (TEMPO) as a redox catalyst for the oxidation of alcohols in ionic liquids and other non-aqueous solvents. The general electrochemistry is shown in the Figure below.

 

We have established that the mechanism of the reaction involves a pre-equilibrium (K_eq) between the alcohol and a BrØnsted base (B) giving an equilibrium concentration of alcoholate anion (RCH₂CO-) which is the oxidisable form of the alcohol. We have shown that the rate of reaction (as K_eqk_ex) in ionic liquid and non-aqueous solvents increases logarithmically with the pK_b of the base. We have also demonstrated that this catalysis can be performed at preparative scale in ionic liquid media using a proton-conducting solid polymer membranes as cell dividers. Close to 100% stoichiometric and coulometric are easily achievable.

Current research on redox catalysis in ionic liquid media focuses on the TEMPO system described above (generating an aldehyde or ketone product) but coupled with electrochemical reduction of the protons liberated from the alcohol oxidation reaction to produce H₂ (g). Conceptually, the H₂ (g) generated can be fed into a fuel cell to reuse much of the electrical energy expended in the initial oxidation. We are also investigating redox catalysts in ionic liquid media for the indirect reduction of CO₂ to produce products such as formic acid and oxalic acid.

Redox-active ionic liquids: Ionic liquids have "moved on" from being considered as mere solvent-replacements to being truly functional materials in their own right. One of our contributions to the field (Patent WO 2006003395) is redox-active ionic liquids. In this scenario, a redox couple is deliberately built into the ionic liquid structure such that the ionic liquid acts as the reaction medium, the electrolyte and the redox catalyst. For example, over recent years, we have synthesised and investigated libraries of quinone/hydroquinone based ionic liquids. Quinones/hydroquinones are extremely useful molecules since they participate in a large number of chemical reactions. For example, quinone/hydroquinones can be used for the desulfurisation of crude and refined oil/gas products but the most important reaction is the reduction of oxygen to hydrogen peroxide which is a multi-million dollar a year global industry. The Figure below shows the electrochemistry and follow-up chemistry associated with electrolytic generation of hydrogen peroxide. Building quinone/hydroquinone functionality into ionic liquids allows for the total solvent-free synthesis of peroxide in and safe and benign reaction medium.

 

In these ionic liquid redox-active materials either the anion (as above) or cation can possess the quinone/hydroquinone functionality; they can be tuned synthetically to be either hydrophobic (facilitate aqueous recovery) or hydrophilic; and their density can be tuned synthetically to be more or less dense than water. Although bi-phasic extraction with water is frequently cited as a key advantage of ionic liquids, membrane extraction of electrolytically generated peroxide from aqueous / hydrophilic quinone ionic liquid mixtures is currently under investigation as a means of harvesting peroxide product. Other areas of research that were immediately apparent at the early stages of this work and which are on-going include desulfurisation of petroleum products and the direct catalytic synthesis of propylene oxide in situ in the quinone ionic liquid/peroxide mixture. The approach under investigation is schematically represented in the Figure below where it should be noted that the process is entirely electrolytically driven.

 

Ionic liquid batteries: Electrical energy storage is one of the key technological areas that requires a step-change in performance to meet future demand. Building reversible electrochemistry into ionic liquids opens new opportunities in electrical energy storage, in particular for large-scale (MW-GW) storage capacity redox flow batteries. We have already developed prototype batteries (as shown in the Figure below) based on the aforementioned quinone/hydroquinone ionic liquids. These innovative devices are the subject of an International patent application (International Patent Number PCT/GB2011/050659.)

 

Again, the advantages of these liquid materials include synthetic tuneability of physical and electrochemical properties and the fact the liquids act mult-functionally insofar as the play the role of the solvent, the electrolyte and the energy-storing chemistry. Also, the fact that they can never "dry out", and are relatively benign, are major advantages.

Other electrochemical/ionic liquid programmes: We are currently developing and researching an array of functional ionic liquids for application in electrochemical sensors including metal-based redox-active ionic liquids which act as electron transfer mediators for enzyme-based biosensors. Also, given the nature of much of the chemistry (coupled proton-transfer) that we are interested in, we are also investigating the concept of "acidity and basicity" in ionic liquid using the platinised platinum H₂ (Pt-Pt/H₂) electrodes. For example, the Figure below shows the 1st derivative of a Pt-Pt/H₂ monitored (potentiometric) acid/base titration curve in ionic liquid.

 

 

Rheology Research

Rheology is an important, but much ignored, area of physical chemistry. We have published a significant number of papers over recent years dealing with multi-phase and bifurcated flow in pipes. We have also investigated electrolyte flow under turbulent conditions and developed a mathematical model to explain excess pressure loss (P) in terms of an electrokinetic retardation force (drag) operating in the laminar sub-layer at the pipe's surface. The liquid volume flowing in the laminar layer (Q) is given by the expression;

where, h is viscosity, e is the dielectric constant of the electrolyte solution, x is distance from the pipe's wall to the middle of the tube, l is the Debye screening length, d is the diameter of the pipe, and y_o is the surface electrical potential on the pipe. We are currently working on CFD calculations relating to the flow of polymer melts and resins.

 

Research facilities

We have a fully equipped electrochemical research laboratory including the following instrumentation;

• x2 Radiometer Voltalab 40 Potentiostats
• x2 Radiometer PGZ 301 FRAs (galvanostatic/potentiostatic impedance spectroscopies)
• BAS EPSILON Potentiostat / Bi-potentiostat
• BAS CV W 50 Potentiostat
• x2 BAS Controlled Growth Hg Drop electrode apparatus
• CH Instruments 600B 4-electrode Potentiostat (KV/s voltammetry, L-L electrochemistry)
• Sycopel AWE2 Potentiostat
• Sycopel AWE3 4-electrode Potentiostat (L-L electrochemistry)
• Digi Ivy 2100 and 2300 Potentiostats
• PAR EG&G 173 Potentiostat
• Sycopel Rotating Ring Disk Electrode apparatus/controllers
• Brookfield DV cone/plate Rheometer
• Easyscan Scanning Tunnelling Microscope (STM) / Easyscan Atomic Force Microscope (AFM)
• EG&G Seiko QCA922 Electrochemical Quartz Crystal Microbalance
• 4-Electrode Conductivity apparatus
• Plus various current amplifiers, potentiometers, temperature controllers, recirculating water baths, Linkam heating stage, wave form generators (HP), oscilloscopes (HP), data acquisition hardware, power supplies, vacuum pumps , and Schlenk-line facilities.