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 of 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". 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.
Electric field distribution
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
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,
H 3 O + + OH − ↽ − − ⇀ 2 H 2 O {\displaystyle {\ce {H3O+ + OH- <=> 2H2O}}}
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.
Pure water electrolysis in nanogap cell
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). 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.
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