Electrochemical Characterization of Coordination Complexes

The goal of this collaborative research with the Daly group is to provide fundamental knowledge on how ligand modification impacts substrate binding strength in redox-active metal coordination complexes capable of metal-ligand cooperative binding. A combination of both electrochemical characterization and chemical modeling are used to experimentally determine binding strength for a variety of redox-active ligand systems. This work provides an approach to experimentally quantify subtle changes in binding strength electrochemically, which enables the design and tuning of redox-active coordination complexes for challenging electrocatalytic processes, such as the reduction of carbon dioxide. Future research will utilize metal coordination complexes for this application.

MLC binding
Schematic depicting various ligand modifications and the corresponding connection between binding strength and E1/2 value.
MLC CV
CV showing the equilibrium between reactant and product as substrate is added.

Spielvogel, K. D.; Stumme, N. C.; Fetrow, T. V.; Wang, L.; Luna, J. A.; Keith, J. M.; Shaw, S. K.; Daly, S. R., Quantifying Variations in Metal–Ligand Cooperative Binding Strength with Cyclic Voltammetry and Redox-Active Ligands. Inorganic Chemistry 2022. DOI: 10.1021/acs.inorgchem.1c03014

Spielvogel, K. D.; Luna, J. A.; Loria, S. M.;  Weisburn, L. P.; Stumme, N. C.; Ringenberg, M. R.; Durgaprasad, G.; Keith, J. M.; Shaw, S. K.; Daly, S. R., Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands. Inorg Chem 2020, 59 (15), 10845-10853. DOI: 10.1021/acs.inorgchem.0c01353

 

Redox Flow Batteries

Redox flow batteries are an energy storage technology that stores generated energy in the form of electroactive species contained in liquid electrolytes. They are a versatile energy storage option due to their scalability, decoupled energy and power density, and design tunability. We are part of a multi-institutional collaborative grant for Data-enabled Discovery and Design to Transform Liquid-based Energy Storage (D3TALES). This research is focused on rapid organic electroactive candidate identification via database development, machine learning, and analytical automation, all supported by electrochemical characterization. We perform both electrochemical and spectro-electrochemical measurements on organic redox probes used in redox flow batteries to optimize experimental methodologies, determine quantitative information on these systems, and ultimately inform energy storage technology development.

d3tales
Diagram of the four cross-cutting aims of the D3TaLES project.
d3tales logo
D3Tales logo. 

 

tempo plots
Plot showing conductivity and viscosity trends for various concentrations of TEMPO, an organic molecule used in redox flow batteries.

 

Fluid Flow at Structured Surfaces

This project aims to develop the understanding of complex chemical and physical interactions between fluid molecules surfaces. For many years, empirical evidence has shown that a fluid will flow more slowly (or not at all) as it approaches a solid wall, such as the inside of a garden hose. This research project aims to discover exactly what the relationship is between fluid flow and wall roughness. To do this we will press the limits of nanofabrication techniques to create ultra-smooth surfaces and slowly increase roughness on a sub-nanometer scale while monitoring the fluid behavior. We predict (as do some others) that a critical threshold of roughness exists below which a fluid will flow as freely near the wall as it does far away from the wall. However, we are also cautious to consider the strength of possible intermolecular interactions between the fluid and the solid wall. The outcomes of this work will impact fundamental surface science as well as the foundations of fluid mechanics and the hydro-dynamic theory. The insights to be gained in surface-fluid and fluid-fluid interactions, the role of van der Waals forces, hydrogen bonding, and micro-viscosity, will be immediately useful to all areas involving fluid flow, particularly water filtration, microfluidics, biomedical devices, and countless surface preparation techniques. 

Schematic of no slip, partial slip, and complete slip boundary conditions present at the solid liquid interface. In the no slip boundary condition, the velocity of the fluid at the solid interface matches the velocity of the surface. In a partial or complete slip boundary condition, the fluid’s velocity at the solid interface can oppose the velocity of the surface. Bottom: AFM images of 1 x 1 um sections of rough and smooth silver substrates with 7.1 ± 2.7 nm and 2.3 ± 0.3 nm average RMS roughness. These surfaces correspond to the rough and smooth (right most) figures in the schematic.

 Nania, Samantha L.; Shaw, Scott K. Analysis of fluid film behaviour using dynamic wetting at a smooth and roughened surface. Analytical Methods Advance Article. DOI: 10.1039/C5AY00574D

 

Ionic Liquid Interfaces

ION ORIENTATION 
 
Ionic liquids exhibit low vapor pressure, high thermal stability and electrical conductivity. They are of interest for applications in electrochemistry, catalysis, lubrication, photovoltaics etc. Their macroscopic properties in the above mentioned application environments are governed by their orientation and behavior at the solid surface. This study aims at studying IL films on silver substrate using spectroscopic techniques. 
 
Previous studies on IL interface show ordering at the solid -liquid interface which extends up to few tens of nanometers and is characterized by oscillatory density profiles as shown in the above figure. Beyond this region bulk isotropic characteristics of the fluid are observed. Our results indicate high molecular ordering throughout the film which extends up to few microns. This is found to be a result of rearrangement of bis(trifluoromethylsulfonyl)imide anion as evidenced by the change in the IRRAS spectra. 

IRRAS spectra of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide while the film is being formed compared to spectra of 60 min old static film. It shows the anion reorients over the span of 60 min.
Oscillatory density profile at the interface compared to the bulk.

 

CAPACITANCE STUDIES ON IONIC LIQUIDS 
 
This project aims to study the relationship between an ionic liquid's capacitance and its' other properties such as viscosity, density, and ion orientation at an electrode surface. These studies are accomplished using a variety of techniques such as vibrational sum frequency generation spectroscopy (vSFG) as well as various rheological and electrochemical impedance analyses.

 

Urban Film Studies

Urban films are found on virtually every surface in a city’s landscape and act as sponges for atmospheric molecules and particulates, including persistent organic pollutants (POP). The films are generally composed of soot, water, and ‘grime’ that is emitted from various natural and man-made sources. 
 
Our interest in these films is two-fold: to investigate the molecular level interactions that makes them efficient as chemical sponges, and to learn how these films interact with their environments to accumulate, mature, and release problematic chemicals into the environment. Our expertise in surface analysis will allow for groundbreaking research in this area relevant to atmospheric science, environmental chemistry, and corrosion science.

Beijing smog contributes to the accumulation of urban films on every solid surface in a one-hundred mile radius. 
Schematic illustrating urban film formation and maturation. 

Grant, Jacob S.; Shaw, Scott K. A model system to mimic environmentally active surface film roughness and hydrophobicity. Chemosphere 2017, 185, 772-779. DOI:  10.1016/j.chemosphere.2017.07.068

 

Carbon Dioxide Recycling

The goal of this work is to capture atmospheric carbon dioxide and develop an environmentally responsible technology to recycle it back into valuable materials or fuels. The electrochemical reduction of CO2 in ionic liquids holds several distinct advantages, including a high solubility of CO2, an extended range of electrochemical stability, and nearly infinite tunability of the ionic liquid as a solvent / charge carrier. This work will involve a multi-pronged approach including advanced spectroscopic methods, electrochemical analysis, theoretical simulations, and synthesis of new electrodes and ionic liquid materials.

Cyclic voltammograms showing reduction current enhancement of pyridinium in CO2 environment.

 Lucio, Anthony J.; Shaw, Scott K. Pyridine and Pyridinium Electrochemistry on Polycrystalline Gold Electrodes and Implications for CO2 Reduction. Journal of Physical Chemistry C. 2015, 119 (22), 12523-12530. DOI: 10.1021/acs.jpcc.5b03355