The piezoelectrochemical transducer effect (PECT) is a coupling between the
electrochemical potential and the mechanical strain in ion-insertion-based electrode materials. It is similar to the
piezoelectric effect – with both exhibiting a voltage-strain coupling - although the PECT effect relies on movement of ions within a material microstructure, rather than charge accumulation from the
polarization of
electric dipole moments.
These materials all exhibit a voltage-strain coupling, whereby the material expands when it is charged with ions, and contracts when it is discharged. The reverse is also true: when applying a mechanical strain the electrical potential changes.
This has led to various proposals of applications for the PECT effect with research focusing on
actuators, strain-sensors, and
energy harvesters.
Origins
The PECT effect was first reported by Dr. F Lincoln Vogel in 1981 when studying how
intercalation voltages could be used to provide an actuation force in graphitized carbon fibres.[12] The research used
sulphate (SO4) ions from
sulfuric acid to intercalate into the microstructure of
carbon fibers, forming
graphite intercalation compounds (GICs). It was hypothesized that an axial strain of up to 2% should be possible, however only 0.2% was observed due to experimental limitations.[13]
The effect is often explained by the theories of Larché and Cahn[14][15][16] who derived mathematical formulations for the equilibrium relationships between the
electric potential,
chemical potential, and mechanical stress in solid materials. In summary the theory states that solid materials under mechanical stress undergo a change in
chemical potential, which in turn affects their
electrical potential.[17]
Different materials exhibit different amounts of expansion/contraction, with a response that is dependent on the type of ion, as well as the amount of charge. For example, silicon expands by more than 300% when inserted with lithium,[19] whereas graphite expands by around 13%.[19] Carbon fibres expand by up to 1% when inserted with lithium,[2] but only around 0.2% when inserted with potassium.[6]
Strain-sensing
As PECT materials exhibit a change in voltage upon application of strain, it is possible to calibrate this change in voltage to the level of strain in a material. This has been proposed for applications in battery health monitoring,[22] as well as
structural health monitoring.[6][18][17]
Electricity production
When mechanical strain is applied to a PECT material it changes the
chemical potential, and therefore the
electric potential of that material.[14][15][16][23] Since current flows from more negative materials to more positive materials, it is possible to induce a current flow between two ionically connected materials by simply applying a mechanical strain. It is therefore possible to harness and convert mechanical energy into electrical energy.
A number of materials have been demonstrated to be capable of PECT-based energy harvesting, including:
carbon fibers inserted with
lithium,[3][24][18] sodiated
black phosphorus;[7] lithiated
aluminium;[8] and lithiated
silicon.[11] A structural carbon fibre composite has also been shown to be capable of harvesting energy using the PECT effect.[17] Conventional
lithium-ion batteries have also been shown to be capable of PECT-based energy harvesting.[25][23]
This effect has most often been demonstrated using a two-electrode bending setup:[7][8][11][18][17][24]
Two electrodes of the same material are connected ionically through an electrolyte, and electrically via an outer circuit.
A bending deformation is applied causing tension in one electrode and compression in the other.
The resulting change in chemical potential results in current flow in the outer circuit, which can be used to power an external device.
PECT energy harvesting is limited by the rate of ionic diffusion, and therefore is only efficient at low frequency (typically below around 1 Hz).[8]
Figures of merit for comparing different PECT-based energy harvesters were formulated by Preimesberger et al.[26]
Implications for batteries
The PECT effect is also present in typical ion-insertion-based battery electrodes (e.g. Li-ion).[25][27][28] The electrodes expand and contract when inserted with ions, which is one of the issues that leads to battery ageing and capacity loss over time.[29] The PECT effect in battery electrodes could be an issue in situations where battery electrodes are mechanically stressed (e.g. in
structural batteries), causing a change in electrical potential when the stress-state changes.
It has been proposed that the PECT effect in
Li-ion batteries could be exploited to measure battery health.,[22] and to harvest mechanical energy.[25]