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Helmholtz's discovery: What are the hidden interactions between electrodes and electrolytes?
In surface science, a double layer (also called an electrical double layer, EDL) is a structure that appears when an object is exposed to a fluid. Such objects may be solid particles, bubbles, droplets, or porous bodies. A double layer refers to two parallel layers of charge surrounding an object. The first layer is surface charge, where ions stick to an object due to chemical interactions. The second layer consists of ions that are attracted to the surface charge by Coulomb forces. This layer of charges is loosely associated with the object and moves freely through the fluid rather than being firmly attached. This layer is therefore called the "diffusion layer". Interfacial double layers are most obvious in systems with large surface area to volume ratios, such as colloids or porous materials, but they are also important in other phenomena, such as the electrochemical behavior of electrodes.
Double layers play a fundamental role in many everyday substances, for example, homogenized milk exists precisely because fat droplets are covered with an electrical double layer, which prevents them from solidifying into butter.
Helmholtz was the first to recognize that when an electronic conductor comes into contact with a solid or liquid ionic conductor (electrolyte), a common boundary occurs between the two phases. He showed in 1853 that the electric double layer was essentially a molecular dielectric and could store charge electrostatically. There is a linear dependence between the stored charge and the applied voltage. Prior to this, many relied on earlier models, however these models did not take into account the diffusion/mixing process of ions in the solution, nor did they resolve the possibility of surface adsorption.
The improved "Gouy-Chapman model" was proposed by Louis-Georges Gouy in 1910 and David Chapman in 1913 respectively. Their observations demonstrated that the capacitance is not a constant and depends on the applied voltage and ion concentration. The emergence of this model allows us to use Maxwell-Boltzmann statistics to describe the charge distribution of ions. The electric potential decreases exponentially with the increase of the fluid, which is particularly relevant for bioelectrochemistry.
The insufficient number of ions between this diffusion layer results in a reduced shielding effect, resulting in an electric field extending several nanometers.
In 1924, Otto Stern proposed a more specific model that combined the Helmholtz model and the Guy-Chapman model. His model took into account the finite size of the ions and suggested an internal Stern layer and a diffusion layer. While this model offers new insights, it also faces its own limitations.
As these theories evolved, many researchers made suggestions and observations. In 1947, D.C. Graham put forward his own opinions based on the Stern model, suggesting that certain ions or uncharged substances can penetrate the Stern layer and form "specifically adsorbed ions". Subsequently, many important studies discovered the influence of the solvent interface on the double-layer charge. Brian Evans Conway conducted an in-depth study of nickel-based electrochemical capacitors between 1975 and 1980 and further distinguished between supercapacitors and batteries, which helped to understand the behavior of the double layer.
With the deepening of research, from the perspective of electron transfer, the formation process of the electric double layer is assumed to consist of two steps.
Further research shows that the electrification of contact with solids is dominated by electron transfer. These models suggest that electron transfer and ion transfer coexist at the liquid-solid interface, providing a deeper understanding of the inner workings of the electrical double layer.
To summarize, the formation and behavior of the electric double layer must not only take into account the interface charge generation mechanism of the substance, but also require an in-depth understanding of the interaction between electrons and ions. What potential breakthroughs and discoveries will there be in future research in this field?
