Four basic operating modes of friction nanogenerator

Friction nanogenerators are a disruptive technology with unprecedented output performance and benefits. Compared with classical electromagnetic generators, the high efficiency of friction nanogenerators at low frequencies is unmatched by similar technologies. At the same time, it can also be used as a self-actuated sensor to sense information about static and dynamic processes generated by mechanical triggers.

Nanogenerators will be another major application of Maxwell displacement current in energy and sensing after electromagnetic wave theory and technology, which has the potential to lead technological innovation and profoundly change human society.

Today's electronic devices are mostly related to human activities, mainly for health, safety and communication. The most abundant form of energy associated with humans is the mechanical energy produced by human movement. Recently, we discovered that when two different materials come into contact, their surfaces generate positive and negative electrostatic charges as a result of contact. When the two materials are separated due to mechanical force, the positive and negative charges generated by contact electricity also separate, and this charge separation will correspondingly produce an induced potential difference on the upper and lower electrodes of the material. If a load is placed between the two electrodes or if there is a short circuit, this induced potential difference drives electrons through an external circuit to flow between the two electrodes - this is the friction nanogenerator (TENG) first invented by Wang's group in 2012, whose main goal is to collect small-scale mechanical energy. The TENG has the following four basic working modes.

Four basic operating modes of friction nanogenerator. (a) Vertical contact-separation mode; (b) Horizontal sliding mode; (c) Single-electrode mode; (d) Independent layer mode (This drawing has been licensed by the Royal Society of Chemistry)

Vertical contact-separation mode

We take TENG's simplest design as an example (FIG. 1a) in this structure, two dielectric films of different materials are stacked face to face, each with a metal electrode plated on its back surface. When the two dielectric films contact each other, surface charges with opposite symbols are formed on the two contact surfaces. When the two surfaces are separated due to external forces, a small air gap is formed in the middle, and an induced potential difference is formed between the two electrodes. If two electrodes are connected by a load, electrons will flow from one electrode to the other through the load, creating an inverse potential difference to balance the electrostatic field. When the air gap between the two friction layers is closed, the potential difference formed by the friction charge disappears and the electrons return.

Horizontal sliding mode

The initial structure of this mode is the same as that of the vertical contact-separation mode. When two dielectric films come into contact, a relative slip occurs between the two materials along a horizontal direction parallel to the surface, which can also generate friction charges on the two surfaces (Figure 1b). In this way, polarization is created in the horizontal direction, which can drive electrons to flow between the upper and lower electrodes to balance the electrostatic field generated by the friction charge. An AC output can be produced by periodic sliding separation and closure. This is the basic principle of the sliding TENG. This kind of sliding can exist in many forms, including plane sliding, cylindrical sliding and disk sliding. We have conducted relevant studies on these structures to gain a more comprehensive understanding of the sliding patterns and the more complex lattice structures within them.

Single electrode mode

Both modes of operation described earlier have two electrodes connected by a load. In some cases, parts of the TENG are moving parts (such as when a person is walking on the floor), so electrical connections via wires and electrodes are not convenient. To facilitate the collection of mechanical energy in this case, we introduced a single-electrode mode of the TENG, that is, with electrodes only at the bottom and grounded (Figure 1c). If the size of the TENG is limited, the upper charged object approaching or moving away from the lower object will change the local electric field distribution, so that electrons will exchange between the lower electrode and the earth to balance the change in electric potential at the electrode. This basic mode of operation can be used in both contact-separation structures and sliding structures.

Independent layer pattern

In nature, moving objects are often electrically charged due to contact with air or other objects, just as our shoes are electrically charged when walking on the floor. Because the charge density on the surface of the material will reach saturation, and this electrostatic charge will remain on the surface for at least a few hours, continuous contact and friction are not required during this time. If we plated two unconnected symmetrical electrodes on the back of the dielectric layer, and the size and spacing of the electrodes are of the same order as the size of the moving object, then the reciprocating motion of the charged object between the two electrodes will cause the change of potential difference between the two electrodes, and then drive the electrons to flow back and forth between the two electrodes through the external circuit load. To balance the variation of the potential difference (Figure 1d). The reciprocating motion of electrons between the pair of electrodes can form a power output. The moving charged object does not necessarily need to be in direct contact with the upper surface of the dielectric layer. For example, in rotation mode, one disk can rotate freely without direct mechanical contact with the other part, which can greatly reduce the wear of the material surface, which is very beneficial for improving the durability of the TENG.