Virtual breakdown mechanism

Last updated

The Virtual breakdown mechanism is a concept in the field of electrochemistry. In electrochemical reactions, when the cathode and the anode are close enough to each other (i.e., so-called "nanogap electrochemical cells"), the double layer the regions from the two electrodes is overlapped, forming a large electric field uniformly distributed inside the entire electrode gap. Such high electric fields can significantly enhance the ion migration inside bulk solutions and thus increase the entire reaction rate, akin to the "breakdown" of the reactant(s). However, it is fundamentally different from the traditional "breakdown".

Contents

The Virtual breakdown mechanism was discovered in 2017 when researchers studied pure water electrolysis based on deep-sub-Debye-length nanogap electrochemical cells. Furthermore, researchers found the relation of the gap distance between cathodes and anodes to the performance of electrochemical reactions. [1]

Electric field distribution

Schematic diagram of potential distribution comparison between macrosystem and nanogap cells Schematic diagram of potential distribution comparison between macrosystem and nanogap cells.png
Schematic diagram of potential distribution comparison between macrosystem and nanogap cells

The fundamental difference between traditional cells and nanogap cells is their electric potential distribution. This is the premise of the "virtual breakdown" effect.

For electrochemical reactions with high-concentration electrolyte in the macrosystem, the Debye-length is quite small. Due to the screening effect almost all of the potential drop is confined within the small Debye-length region (or double layer region). The potential in bulk solution (far from the electrodes) does not change too much, meaning that there is nearly zero electric field inside the bulk solution. However, when the counter electrode is within the Debye-length region (i.e., nanogap electrochemical cells), two double layers from anode and cathode overlap with each other. The electrostatic potential inside the entire gap changes dramatically, meaning that the huge electric field is uniformly distributed across the entire gap.

Pure water electrolysis

We shall consider pure water electrolysis as an example to explain the concept of the Virtual breakdown mechanism.

Pure water electrolysis in macrosystem

Pure water in macrosystem cannot be split efficiently due to the lack of rapid ions transport inside bulk solution. Pure water in macrosystem.png
Pure water in macrosystem cannot be split efficiently due to the lack of rapid ions transport inside bulk solution.

For the analysis of water electrolysis, we shall use H3O+ ions (also known as oxonium ions) at the cathode, as an example to explain the traditional reactions.

Water molecules self-ionize to H3O+ and OH ions. Near the cathode surface (within the double layer region), newly generated H3O+ ions become hydrogen gas after obtaining electrons from the cathode; however because there is nearly no electric field inside the bulk solution (see section "Electric field distribution"), OH ions can only transport through the bulk solution very slowly by diffusion. Moreover, in pure water the intrinsic H3O+ concentration is only 10−7 mol/L, not enough to neutralize the newly generated OH ions. In this way OH ions accumulate locally at the cathode surface (turning the solution near cathode into alkaline). Due to Le Chatelier's principle for water self-ionization,

the OH ions accumulation impede further self-ionization of the water, which reduces the hydrogen evolution rate and eventually prevents water electrolysis. In this case water electrolysis becomes very slow or even halts; this manifests as a large equivalent resistance between the two electrodes.

This is why in the macrosystem pure water cannot be electrolyzed efficiently - the fundamental reason is the lack of rapid ion transport inside the bulk solution. [1]

Pure water electrolysis in nanogap cell

In nanogap cell, high electric field in the entire gap can enhance water ionization and mass transport (mainly migration), leading to pure water splitting efficiently limited by electron-transfer. Pure water electrolysis in nanogap cells.png
In nanogap cell, high electric field in the entire gap can enhance water ionization and mass transport (mainly migration), leading to pure water splitting efficiently limited by electron-transfer.

In nanogap cells the high electric field can distribute uniformly across the entire gap (see section "Electric field distribution"). This is different from ion transport in the macrosystem: now newly generated OH ions can immediately migrate from cathode to anode. In the case where the two electrodes are close enough, the mass transport rate can be even larger than the electron-transfer rate. This results in OH ions clustering for electron-transfer at the anode, rather than accumulating at the cathode. In this way the entire reaction can keep going and not self-limit.

Notice that for pure water electrolysis in nanogap cells, the net OH ion accumulation near the anode not only increases the local reactant concentration but also decreases the overpotential requirement (as in the Frumkin effect). [2] According to Butler–Volmer equation, such ion accumulation increases the electrolysis current, i.e. the water splitting throughput and efficiency.

Thus even pure water can be efficiently electrolyzed, when the electrode gap is small enough.

Virtual breakdown mechanism

In reality water molecule dissociation (the splitting into H3O+ and OH ions) occurs only at the electrode region (because of the ions continuously consumed at the two electrodes); however it effectively appears that the molecules split in the middle of the gap, with H3O+ ions migrating towards the cathode and OH ions migrating towards the anode, respectively. The assistance of the huge electric field in the nano-gap (see section "Electric field distribution") not only increases the transport rate but also the water molecules' ionization has been enhanced (i.e. local concentration has been enhanced). Looking from a microscopic perspective, the total effect appears like the breakdown of water molecules.

However this effect is not traditional breakdown, which in fact requires a much larger electric field around 1 V/Å. [3] In the nanogap cells the huge electric field is still not large enough to split water molecules directly. However it can take advantage of the self-ionization of water, facilitating the equilibrium reaction to shift in the ionization direction. [1]

Such field-assisted ionization, with the fast ion transport (mainly migration), performs very in a similar way to the breakdown of water molecules; that is why this field-assisted effect is named the "virtual breakdown mechanism".

Consider the equation of conductivity,

Here the ion charges are not changed. The ion concentration is enhanced but only contributes to the conductivity partially. The fundamental change here is that "apparent mobility" has been significantly enhanced, as the "breakdown" effect. (In traditional electrochemical cells, although the ion intrinsic mobility is high, since there is nearly zero electric field inside bulk solution, it cannot contribute to the conductivity.) Consider the equivalent resistance between the two electrodes, as given by:

When we decrease the gap distance between the two electrodes, not only does the value of L decrease but also the value of resistivity decreases as well; this in fact contributes more to the decrease of the total resistance. [1]

This "virtual breakdown mechanism" can be applied to almost all kinds of weakly-ionized materials; in fact, such weaker ionization can lead to larger Debye-length inside the solution. At the same size scale it actually helps to achieve the virtual breakdown effect.

Gap size effect

Phase diagram of electrochemical performance vs. gap distance Phase diagram of electrochemical performance vs. gap distance.png
Phase diagram of electrochemical performance vs. gap distance

The phase diagram shows the importance of the electrode gap distance to the performance of electrochemical reactions. For traditional macrosystems, where the electrode gap distance is much larger than the Debye-length, two half-reactions are decoupled and cannot influence each other. Normally the electrochemical current is limited by a slow diffusion step. When the gap distance is reduced to around the Debye-length, a large electric field can form between the two electrodes (due to double layers and the two regions overlapping with each other); this enhances the mass transport rate. In this region the electrolysis current is very sensitive to the gap distance and the reactions are migration-rate limited. When the gap distance is further reduced to the deep-sub-Debye-length region, the mass transport can be enhanced further to a level even faster than the electron-transfer step. In this region, even when we shrink the gap distance further, the current cannot be enlarged any more, meaning that the current has reached saturation. Here the two half-reactions are coupled together and the reactions are limited by the electron-transfer steps.

Therefore, by just adjusting the gap distance, the fundamental performance of the electrochemical reactions can be significantly changed.

Related Research Articles

<span class="mw-page-title-main">Cathode</span> An electrode where reduction take place

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit. For example, the end of a household battery marked with a + (plus) is the cathode.

<span class="mw-page-title-main">Electrochemistry</span> Branch of chemistry

Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outcome of a particular chemical change, or vice versa. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.

<span class="mw-page-title-main">Electrode</span> Electrical conductor used to make contact with nonmetallic parts of a circuit

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. Electrodes are essential parts of batteries that can consist of a variety of materials depending on the type of battery.

<span class="mw-page-title-main">Electrochemical cell</span> Electro-chemical device

An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic or galvanic cells and those that generate chemical reactions, via electrolysis for example, are called electrolytic cells. A common example of a galvanic cell is a standard 1.5 volt cell meant for consumer use. A battery consists of one or more cells, connected in parallel, series or series-and-parallel pattern.

<span class="mw-page-title-main">Electrolysis</span> Technique in chemistry and manufacturing

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".

<span class="mw-page-title-main">Galvanic cell</span> Electrochemical device

A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus generally consists of two different metals, each immersed in separate beakers containing their respective metal ions in solution that are connected by a salt bridge or separated by a porous membrane.

<span class="mw-page-title-main">Electrolytic cell</span> Cell that uses electrical energy to drive a non-spontaneous redox reaction

An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to drive a chemical reaction that would not otherwise occur. This is in contrast to a galvanic cell, which itself is a source of electrical energy and the foundation of a battery. The net reaction taking place in a galvanic cell is a spontaneous reaction, i.e, the Gibbs free energy remains negative, while the net reaction taking place in an electrolytic cell is the reverse of this spontaneous reaction, i.e, the Gibbs free energy is positive.

<span class="mw-page-title-main">Paschen's law</span> Physical law about electrical discharge in gases

Paschen's law is an equation that gives the breakdown voltage, that is, the voltage necessary to start a discharge or electric arc, between two electrodes in a gas as a function of pressure and gap length. It is named after Friedrich Paschen who discovered it empirically in 1889.

The chloralkali process is an industrial process for the electrolysis of sodium chloride solutions. It is the technology used to produce chlorine and sodium hydroxide, which are commodity chemicals required by industry. 35 million tons of chlorine were prepared by this process in 1987. The chlorine and sodium hydroxide produced in this process are widely used in the chemical industry.

<span class="mw-page-title-main">Alkaline fuel cell</span> Type of fuel cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, Francis Thomas Bacon, is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.

A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.

<span class="mw-page-title-main">Electrophoretic deposition</span>

Electrophoretic deposition (EPD), is a term for a broad range of industrial processes which includes electrocoating, cathodic electrodeposition, anodic electrodeposition, and electrophoretic coating, or electrophoretic painting. A characteristic feature of this process is that colloidal particles suspended in a liquid medium migrate under the influence of an electric field (electrophoresis) and are deposited onto an electrode. All colloidal particles that can be used to form stable suspensions and that can carry a charge can be used in electrophoretic deposition. This includes materials such as polymers, pigments, dyes, ceramics and metals.

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Electrolysis of water, also known as electrochemical water splitting, is the process of using electricity to decompose water into oxygen and hydrogen gas by a process called electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, or remixed with the oxygen to create oxyhydrogen gas, which is used in welding and other applications.

Electrosynthesis in chemistry is the synthesis of chemical compounds in an electrochemical cell. Compared to ordinary redox reaction, electrosynthesis sometimes offers improved selectivity and yields. Electrosynthesis is actively studied as a science and also has industrial applications. Electrooxidation has potential for wastewater treatment as well.

<span class="mw-page-title-main">Double layer (surface science)</span> Condensed matter physics

A double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.

The electrochemical regeneration of activated carbon based adsorbents involves the removal of molecules adsorbed onto the surface of the adsorbent with the use of an electric current in an electrochemical cell restoring the carbon's adsorptive capacity. Electrochemical regeneration represents an alternative to thermal regeneration commonly used in waste water treatment applications. Common adsorbents include powdered activated carbon (PAC), granular activated carbon (GAC) and activated carbon fibre.

Electrodeionization (EDI) is a water treatment technology that utilizes electricity, ion exchange membranes, and resin to deionize water and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a polishing treatment to reverse osmosis (RO). There are also EDI units that are often referred to as continuous electrodeionization (CEDI) since the electric current regenerates the resin mass continuously. CEDI technique can achieve very high purity, with a conductivity below 0.1 μS/cm. Electrodeionization (EDI) can be differentiated into three stages, so the basics of EDI reside in the simultaneity of the following processes.

Ion transport number, also called the transference number, is the fraction of the total electrical current carried in an electrolyte by a given ionic species ,

Mixed oxidant solution is a type of disinfectant which is used for disinfecting, sterilization and eliminating pathogenic microorganisms in water and in many other applications. Using a mixed oxidant solution for water disinfection, compared to other methods, may have various benefits such as higher disinfecting power, stable residual chlorine in water, improved taste and odour, elimination of biofilm, and safety. Mixed-oxidant solution is produced by electrolysis of sodium chloride and is a mixture of disinfecting compounds. The main component of this product is chlorine and its derivatives (ClO, HClO and Cl2 solution). It may also contain high amounts of chlorine dioxide solution, dissolved ozone, hydrogen peroxide, and oxygen, from which the name "mixed oxidant", is derived.

References

  1. 1 2 3 4 Wang, Yifei; Narayanan, S. R.; Wu, Wei (2017-07-11). "Field-Assisted Splitting of Pure Water Based on Deep-Sub-Debye-Length Nanogap Electrochemical Cells". ACS Nano. 11 (8): 8421–8428. doi:10.1021/acsnano.7b04038. ISSN   1936-0851. PMID   28686412.
  2. De Kreuk, C.W.; Sluyters-Rehbach, M.; Sluyters, J.H. (December 1970). "Electrode kinetics and double-layer structure". Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 28 (2): 391–407. doi:10.1016/s0022-0728(70)80133-4. hdl: 1874/15071 . ISSN   0022-0728.
  3. Stuve, Eric M. (January 2012). "Ionization of water in interfacial electric fields: An electrochemical view". Chemical Physics Letters. 519–520: 1–17. Bibcode:2012CPL...519....1S. doi:10.1016/j.cplett.2011.09.040. ISSN   0009-2614.