An introduction to the fundamentals of electrochemical interfaces.
Key words: Electric Double Layer, Debye-Hückel, Electrolysis, Screening, Electrolyte, Interfaces
by: Sam Johnstone, Seth Anderson
It is likely we have all heard the phrase “opposites attract” when talking about the poles of a magnet or about the love interests in a cheesy romance movie. Well, this concept also applies to the charged ions we find in everyday electrolytes and results in some very interesting behavior that is foundational to electrochemistry. This article will introduce some vocabulary and theories that can be used to describe the fundamental ideas we utilize in researching electrocatalytic reactions.
Completing the Circuit
All electrical systems are fundamentally composed of an electrical circuit. A circuit is a network of components that create a closed loop through which electrical current can flow. This model applies to everything from the human nervous system to batteries in your TV remote and allows for very complex actions to occur. In electrochemistry, our circuit is composed of two electrodes connected together by a wire and submerged in an electrolyte solution. If we supply energy to the system using an external power source, electrons will move from the positively-charged anode to the negatively-charged cathode. We call this entire setup an electrolytic cell (Figure 1). The movement of electrons in the cell will correspond to chemical changes in the electrodes and the electrolyte.
Figure 1: An electrochemical circuit as applied to electrochemistry. Energy is applied to the positive and negative electrodes (cathode and anode respectively). This induces chemical reactions at their surfaces and electrons to flow from the anode to the cathode. The circuit is completed by the movement of positive and negative ions in the electrolyte.
One critically important component of these electrochemical systems is the electrolyte, which is a substance (usually a liquid solution) that contains ions. These ions are dissolved species that carry a positive charge (cations) or a negative charge (anions) and are free to move in solution. We commonly think of electrolytes simply as salt dissolved in a solvent such as water. Table salt, for example, dissociates into sodium cations (Na+) and chlorine anions (Cl–) when it is dissolved. Having these mobile ions is integral to electrochemistry because they make the solution conductive by completing the circuit between the electrodes. We will look into how this works in more detail in a little bit.
The Humble Electrolyte
Electrochemistry has been studied and utilized for decades as a way to generate and store electricity, clean and finish materials, and create new and valuable products. Historically, electrolytes have been thought of simply as a means of completing the electrical circuit. However, electrolytes are highly complex materials that dictate reaction conditions at the electrodes and have the potential to make processes more efficient and safe.
Here in the Gebbie Lab, one of our main objectives is to study electrocatalysis or the catalytic process of creating oxidation and reduction reactions through the transfer of electrons. These processes occur at the all-important interface. Generally, when we think of the term interface, we think of the intersection between two objects or ideas. For instance when performing actions on a computer there is an intersection or “interface” where the user, the hardware of the computer, and the software all communicate with each other. In electrochemistry, the main interface of interest is where the electrolyte meets the electrode. The ability of electrons to move across this interface has many implications on the efficiency and selectivity of electrochemical processes. For another perspective on interfaces and their importance, see Lightning strikes at interfaces!.
In the past, the electrode surface has been the main focus of much of the optimization and research efforts for improving electrocatalysis. This has led to vastly improved electrochemical reactions, but this method ignores half of the interface where electrochemistry occurs. Until recently, the role of the electrolyte in electrochemistry has not gotten as much attention as the electrodes. Therefore, there are many opportunities to research how electrolyte composition influences critically important electrocatalytic reactions such as CO2 remediation, nitrate reduction, and hydrogen production. In the following sections, we will discuss what aspects of the electrolyte that we find interesting and delve into some concepts that frame why the electrolyte can have such a significant impact on electrochemistry.
Maths
Before getting into the structures observed in electrolytic systems, we need a brief refresher of the physics governing charged objects. We mentioned before the effect where “opposites attract.” This electrostatic force is described by Coulomb’s Law where q1 and q2 are the charges of the two particles and r is the distance between them.
Here we see if the charges have a different sign, the force in one dimension will be negative and therefore attractive. We can also see that the force acting on the particles can be very large at small separations, but it quickly decreases as the distance between them increases. This Coulombic force extends outward in all directions from a charged object to create an electric field that will interact with other charges in the region near the original charged object. The electric field exerts a force on other charges that varies in magnitude and direction depending on the other charge’s relative location in space. Figure 2 below shows an example of electric field lines, which point from positive to negative.
Figure 2: Depiction of the electric fields generated by oppositely charged particles and how they interact.
Another term we use is the electrostatic potential (often denoted with ψ), which is the integral of the electric field. The potential difference between two points is the work required to move a charge between them. Charges want to be at the lowest potential possible, so they will move down a potential gradient like a ball rolling down a hill. Figure 3 below shows an example of an electric field and its resulting potential. You could imagine placing a ball somewhere on the potential surface and it naturally rolling towards x = 0 and y = ±2.
Figure 3: Visualization of how an electric field maps to a potential gradient.
Up until now, we have only looked at isolated charges. What would this look like if we instead had multiple interacting charges? Let us return to a 1D example but now with three charged particles. Each object would now experience two electrostatic forces which could enhance or diminish each other. In the diagram below (Figure 4), q1 would experience an attractive force from q3 and a repulsive force from q2. If we assume that q3 is much larger than q2 and the distances are small, then we can say that q1 experiences an overall attractive force to the right. This force, however, is less than if q2 was not present between q1 and q3. This effect where the presence of a charged object decreases the “effective charge” of another is called screening or shielding. Screening will cause the magnitude of the electric field produced by a charged object to decay to zero more quickly. This also causes the potential to decay more quickly, resulting in a larger potential gradient.
Figure 4: A 1D demonstration of electrostatic screening. The small charge q2 partially blocks the electric field of q3 from reaching q1. As a result, q1 experiences a weaker, “screened” influence from q3.
Even overall neutral particles can contribute to charge screening since they have their own electrons that can interact with electric fields. So, if the particles in the examples above were placed in a solution (i.e. water) instead of vacuum, the forces they experience would now be more complicated to calculate. The forces on an individual ion at a real electrochemical interface are generally too complicated to determine, but we have methods to describe the overall behavior of ions in solution.
Electric Double Layer Boogaloo
Now that we have the basic forces, we can apply them on the larger scale of real electrochemical interfaces. If we introduce a charged surface into an electrolyte, such as in our electrolytic cell from earlier, the electric field produced by the surface will act on the charged ions in solution and cause them to rearrange their positions. Ions that have the same charge as the surface are referred to as co-ions, while those of opposite charge are called counterions.
The movement of ions will result in different concentrations at the interface than what they are in the bulk solution, with an abundance of counterions and a deficiency of co-ions near the surface. This gives rise to a structuring of ions called the electric double layer (EDL), a model which describes how ions arrange themselves in solution under applied potential (Figure 5). The EDL is essentially a wall of ions that surrounds the surface in a way that balances its charge and screens the potential from the surface. The EDL as a whole is generally modeled as having two regions, a compact layer (also called the Stern or Helmholtz layer) and a diffuse layer (also called the Gouy-Chapman layer).
Figure 5: A simplified diagram of the structure of an electric double layer. The magnitude of the potential decays quickly and linearly in the compact layer where counterions closely crowd the surface. The potential then gradually decays to zero in the diffuse layer. Note that solvent molecules would be filling the empty spaces, but are omitted from this graphic for clarity.
The compact layer is made up of ions that crowd the surface and essentially become stuck to it. Ions in this region highly screen the surface potential, but are unable to completely shield the charge. The diffuse layer comes after the compact layer and is less dense and structured than the compact layer. Any remaining unscreened potential decays gradually to zero in the diffuse layer. The distance over which the potential decays in the diffuse layer can be estimated using Debye-Hückel theory. The derivation of this theory is too complicated for this discussion, so we will just jump to the solution for the potential near a flat electrode surface.
In the above equation, ψ0 is the potential at the electrode surface, x is the distance from the electrode, and κ-1 is the Debye length. This equation shows us that we expect the potential to decay exponentially as we move away from the electrode. How quickly the potential decays is measured by the Debye length. This value is a property of the electrolyte solution and is dependent on things such as ion concentration and temperature. A smaller Debye length corresponds to a shorter diffuse layer, which results in stronger screening and a larger potential gradient.
Solving the Debye-Hückel equation requires knowledge of the geometry of the system you are studying and some simplifying assumptions, such as having a small surface charge and ionic concentration. These assumptions are limiting for many applications relevant to study, but the Debye length is regardless a valuable metric to generally describe the EDL.
The EDL: A Promising Topic
When electrochemical reactions are happening, the environment surrounding the reactants is dictated by the structure of the EDL. This has major implications on reactivity, as the EDL can influence the movement of target molecules and the stability of key intermediates. This is why we study electrolytes in the Gebbie lab. Different electrolytes form EDLs with different qualities that may positively or negatively affect a reaction of interest. Currently, the EDL is not rigorously understood and there are still many unanswered questions about it. How do they form? How do we describe the chemical and electrical gradients within them? How can we modify them to best serve our purposes?
Answering these questions is no easy task, but there are researchers from all different fields working towards solutions. Understanding the EDL is a critical step to creating new and better technologies with impacts ranging from energy storage to membrane separation to CO2 reduction and more, while allowing them to be powered using renewable energy sources. All of these opportunities make electric double layers an exciting topic of research!
Additional Resources
https://demos.smu.ca/demos/e-n-m/172-electrochemical-circuit – A great demonstration of an electrochemical circuit produced by Saint Mary’s University.
Catalyzing Interest in Catalysis: What is Catalysis? – More general information about what catalysis is.
https://www.khanacademy.org/test-prep/mcat/physical-processes/electrostatics-1/a/electric-potential – Another discussion of electric fields and how they relate to potential.
https://www.anl.gov/cse/interfacial-chemistry – Studying the interface in electrocatalysis falls under a broader class of study called interfacial science. Check out how Argonne national lab studies interfacial science and some of their initiatives.
https://www.youtube.com/watch?v=Nzm9fLNwJTI – A great video on what an EDL is and its formation produced by NPTEL.
Works Cited
- Hademenos, G. J. et al. McGraw-Hill Education MCAT: Chemical and Physical Foundations of Biological Systems; McGraw Hill, 2015 https://www.mhpracticeplus.com/CPFBS_MCAT/mca88378_V2_Ch03.pdf
- Pletcher, D. A First Course in Electrode Processes; The Royal Society of Chemistry, 2009.
- Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons Inc., 2000.
- Israelachvili, J. N. Intermolecular and Surface Forces; Elsevier, 2011.