A team of researchers at the University Autonoma of Barcelona has created a new atomic force microscopy (AFM) technique that exploits the direct piezoelectric effect to take a measurement of the piezoelectric effect in ferroelectric materials.
Piezoelectric materials have been analyzed for over 100 years, due to their ability to convert mechanical vibrations into electric charge or electric fields into a mechanical strain for sensor, energy harvesting, and actuator applications. A more recent development is the coupling of piezoelectricity and electro-chemistry, termed piezo-electro-chemistry, whereby the piezoelectrically induced electric charge or voltage under a mechanical stress can influence electro-chemical reactions. There is growing interest in such coupled systems, with a corresponding growth in the number of associated publications and patents. This review focuses on recent development of the piezo-electro-chemical coupling multiple systems based on various piezoelectric materials. It provides an overview of the basic characteristics of piezoelectric materials and comparison of operating conditions and their overall electro-chemical performance. The reported piezo-electro-chemical mechanisms are examined in detail. Comparisons are made between the ranges of material morphologies employed, and typical operating conditions are discussed. In addition, potential future directions and applications for the development of piezo-electro-chemical hybrid systems are described. This review provides a comprehensive overview of recent studies on how piezoelectric materials and devices have been applied to control electro-chemical processes, with an aim to inspire and direct future efforts in this emerging research field.
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The origin of piezoelectricity is related to a non-centrosymmetric distribution of positive and negative electric charges in the unit cell of a material [8, 9]. When a piezoelectric material is subjected to an applied stress or mechanical vibration, the induced displacement of ions results in a net electric charge due to a change in the dipole moment of the unit cell, which builds a piezoelectric potential across the material [10, 11]. Generally, among the 21 crystal point groups of non-centrosymmetric crystals, there are 20 point groups of crystals possessing piezoelectricity, where 10 point groups belong to nonpolar crystals which show piezoelectricity and the other 10 point groups of polar crystals exhibit piezoelectricity and ferroelectricity [8, 9]. Piezoelectric materials belonging to nonpolar crystals which are non-ferroelectric can have no electric net dipole in the zero-stress state and only generate an electric dipole under stress due to the separation of electric charge centers and a resulting induced piezoelectric potential; a good example of such a material is quartz [12,13,14,15,16,17,18]. There are also piezoelectric materials belonging to polar crystals that exhibit a spontaneous polarization in the zero-stress state or no electric field state since there is a separation between positive and negative charges [19, 20]. A good example of such a material is zinc oxide. A subclass of piezoelectric materials are ferroelectric materials belonging to polar crystals, whose spontaneous polarization can be changed permanently and switched when exposed to an external strong electric field, for example, in barium titanate [21, 22]. Since the polarization of a ferroelectric changes with stress, all ferroelectric materials exhibit piezoelectricity by default [11, 23, 24].
Typically, an electro-chemical reaction is driven by an external power source [28,29,30], and the coupling of power generation with electro-chemical process remains a vibrant topic. Large-scale renewable and clean power generation approaches are being considered that store solar and wind energy and subsequently convert it into electrical power for driving electro-chemistry [31,32,33,34]. However, smaller-scale and more local energy, such as mechanical energy in the range of microwatt to milliwatt, can be harvested and utilized by systems based on piezoelectric materials [19, 35]. In recent decades, piezoelectrically induced electric fields have been used to control catalytic rates in chemical solutions [36,37,38], the corrosion rate of metals in etchant solutions [39,40,41,42,43], self-charging systems [44,45,46,47,48,49,50], and a variety of other electro-chemical processes [51,52,53]. The coupling of piezoelectricity and electro-chemistry is termed piezo-electro-chemistry, where a piezoelectrically induced electric charge or potential difference generated by a mechanical stress can influence electro-chemical reaction systems [54,55,56].
There are a variety of excellent reviews on electro-chemistry [19, 57,58,59,60,61,62,63,64], but those that specifically focus on piezoelectrically influenced electro-chemical reactions have received limited attention to date. This review places a focus on the range of piezoelectric materials used for controlling electro-chemical processes. It will provide an overview of the basic characteristics of piezoelectric materials and comparison of the operating conditions and electro-chemical performance. The reported piezo-electro-chemical mechanisms will then be examined in detail. Within this review, we have collected virtually all published research work to date on the use of piezoelectric materials for controlling electro-chemistry; this body of work is summarized in Table 1 which contains detailed information regarding the specific piezoelectric materials, along with the electro-chemical processes and performance. In addition, the piezo-electro-chemical reaction systems to be covered within this review include materials that are in bulk [65, 66], fiber [67,68,69], sheet [70, 71], flower [37, 72, 73], particle [74, 75], and irregular [32, 76] form. The piezoelectric materials include ferroelectric perovskites [77, 78], wurtzite zinc oxide [79, 80], two-dimensional layered transition metal dichalcogenide-based materials [81, 82], organic piezoelectric materials [44, 83, 84], and biological materials [85] that are used for a variety of applications such as selective deposition [38, 77, 86], hydrogen production [32, 65, 69], dye degradation [73, 76, 87,88,89], self-charging power cells [44, 45, 49, 83], and others [47, 90]. The above-mentioned piezo-electro-chemical reactions are shown schematically in Fig. 1, and the intention of this review is to overview recent studies on piezoelectric materials and devices that have been applied to control electro-chemical processes and inspire increasing efforts in this new and emerging research field.
For piezo-electro-chemistry, the magnitude of Q is primarily a response to the charge output of the specific piezoelectric materials as a result of a change in polarization under a mechanical load. The material factors for output will be discussed in the following section.
Piezoelectric material factors that influence the value of Q can be firstly related to aspects of the most suitable structure, since materials of the same nature and different structures have a far-reaching effect on the transfer of electric charges or ions. A range of structures have been suggested as a piezo-separator for self-charging power cells. For example, the migration rate of lithium ions can be evaluated by an important parameter, the ionic conductivity, and this materials parameter in the solid state refers to the ease of ion motion in a crystal lattice. Porous nanostructured PVDF films have shown higher ionic conductivity compared to a quasi-bulk film, and the reported explanation of this phenomenon is that the pores can act as a pathway for Li ions to move across the piezo-separator solid [83, 84]. Additionally, porous structures are beneficial for a higher intake of electrolyte solution to facilitate the migration of lithium ions. Therefore, the design of piezoelectric material structures should take account of the influence in the transfer of ions and electric charges.
In this section, we have discussed the dependence of piezoelectric materials characteristics on the output performance, where the shape, size, and mechanical properties have been taken into detailed consideration. We now discuss the charge transfer mechanism for piezo-electro-chemical and piezo-photo-electro-chemical processes.
In addition to the geometry, size, and mechanical properties of the piezoelectric materials affecting the piezoelectric output, the applied experimental conditions are also of importance for piezo-electro-chemical processes. To induce piezoelectricity, mechanical vibrations with a specific orientation and amplitude can affect the emergence of electric carriers [19, 108]. In addition, illumination by light leads to excited photo-electro-chemical reactions in piezoelectric materials that are affected by the built-in piezoelectric potential of the material [38, 109].
A schematic of this piezoelectrically induced charge transfer mechanism is shown in Fig. 3a, which is a system that is stimulated by mechanical vibration only [37]. The variation of the orientation and magnitude of polarization electric field across the material depends on the material type (ferroelectric or piezoelectric), and the force as a function of time since both varying the direction of vibration and mechanical intensity can influence the polarization field. Starr and Wang have pointed out the difference between the three subcategories of materials in terms of their polarization and electric dipoles [3]. In the absence of strain, ferroelectric materials exhibit a spontaneous polarization, where positive and negative electric charge centers exhibit no superposition, giving rise to resultant electric dipoles along the material, while piezoelectric materials without ferroelectricity such as quartz exhibit a zero internal dipole. However, upon the application of a strain both polarization orientation and magnitude can be varied for piezoelectric materials, since there is a separation between the positive and negative electric charge centers, where the polarization orientation is related to the direction of applied force in general. 2ff7e9595c
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