Control of electro-chemical processes using energy harvesting materials and devices

Energy harvesting is a topic of intense interest that aims to convert ambient forms of energy such as mechanical motion, light and heat, which are otherwise wasted, into useful energy. In many cases the energy harvester or nanogenerator converts motion, heat or light into electrical energy, which is subsequently rectified and stored within capacitors for applications such as wireless and self-powered sensors or low-power electronics. This review covers the new and emerging area that aims to directly couple energy harvesting materials and devices with electro-chemical systems. The harvesting approaches to be covered include pyroelectric, piezoelectric, triboelectric, flexoelectric, thermoelectric and photovoltaic effects. These are used to influence a variety of electro-chemical systems such as applications related to water splitting, catalysis, corrosion protection, degradation of pollutants, disinfection of bacteria and material synthesis. Comparisons are made between the range harvesting approaches and the modes of operation are described. Future directions for the development of electro-chemical harvesting systems are highlighted and the potential for new applications and hybrid approaches are discussed.


Introduction
Energy harvesting of heat, light and mechanical vibrations remains a vibrant topic. In many cases the harvester generates electrical energy, which is subsequently rectified, conditioned and stored within capacitors or batteries for applications such as wireless and self-powered sensors or low-power electronics. This review covers a new and emerging area that aims to directly couple energy harvesting materials and devices with electro-chemical systems. There are excellent reviews available on electro-chemical storage materials 1, 2 which focus on electro-chemical energy storage configurations, such as flexible, fiber and transparent systems 1 and reviews which describe the potential of combining harvesting materials with conventional storage mechanisms to form selfpowered electro-chemical energy storage systems (SEESs), 2 and multi-functional energy devices using supercapacitors and electrochromic based systems 3 . Hybrid systems that to combine a variety of mechanical, thermal and light harvesting approaches have also been recently examined by Lee et al. 4 This review will focus on exploitation of the harvested energy in electro-chemical applications. This will include water splitting, water treatment, catalysis, corrosion protection, degradation of pollutants, disinfection of bacteria and material synthesis. The intention of the review is not to overview the energy generation mechanisms, as these have been covered in detail elsewhere; for example see. [5][6][7][8][9][10][11][12] This review will provide a detailed overview of work to date on how energy harvesting materials and devices have been used to influence electro-chemistry; and aims to inspire new efforts in this emerging area. The coupling of energy generation with electro-chemistry is not entirely new and the intermittent nature of large scale renewable power generation methods, such as solar power 13 and wind energy 14 , has led to interest in using electrical energy to generate hydrogen from water which is then stored. Hybrid wind and light approaches are also under consideration 15 . However for smaller scale harvesting applications, typically in the µW to mW range, this is a more recent topic and has received significant recent attention for a diverse range of applications. Harvesting approaches to be covered include pyroelectric 5 , piezoelectric 6, 7 , triboelectric 7,8 , flexoelectric 10 , thermoelectric 9 and photovoltaic 12 effects, which are shown schematically in Fig. 1. The pyroelectric, piezoelectric and flexoelectric effects generate an electrical charge as a result of a change in polarisation of a material due to the application of a temperature change, applied stress or strain gradient respectively, while the photovoltaic effect arises from electrons being excited to the conduction band by solar energy. We will see in the review that these mechanisms of charge generation can take the form of an external bulk material or harvesting device that is connected to an electro-chemical cell, or it can take the form of electro-active particles that are in direct contact with an electrolyte; as shown by the pyroelectric and piezoelectric examples in Fig. 1. Triboelectric charges are produced as a result of a frictional contact between two materials which become electrically charged; these are typically used as motion Fig. 1 Pyro-electric (temperature cycles), thermo-electric (temperature gradients), piezo-electric (strain), tribo-electric (motion), flexo-electric (strain gradient) and photovoltaic (solar excitation) charge generation mechanisms which are used to control electro-chemical reactions. The thermoelectric and triboelectric approaches typically use external devices, while the pyroelectric, piezoelectric and photovoltaic mechanisms can utilise external devices or particulates that are in direct contact with the electrolyte; as shown in upper images.
harvesting triboelectric nanogenerators (TENGs) which are electrically connected to an electro-chemical cell. The thermoelectric effect generates charge from a thermal gradient between two dissimilar conductors due to the Seebeck effect and is also typically used as a harvesting device which is connected to an electro-chemical cell. We will see that in some cases hybrid approaches are utilised that use more than one of the harvesting mechanisms shown in Fig. 1 or employ additional charge generation mechanisms, such a solar harvesting and photo-generated carriers. Table 1 provides a summary of the electro-chemical applications, harvesting materials and modes of operation, and we will refer to this table throughout the review. The interaction of the harvesting material or device with electro-chemical systems will now be described, and their potential applications. Finally, potential future directions will be explored.

Pyro-electro-chemistry 2.1 Pyro-electro-chemical water splitting
The emission of CO2 has been a significant contributor to global warming and climate change. As a result, renewable and alternative energy sources have become one of the primary topics of interest at a global level. Hydrogen fuel, is one of the most promising energy conversion technologies due to its high energy density and low greenhouse gas emission. Today, over 90% of hydrogen is produced from fossil fuels and biomass which produces greenhouse gases as a byproduct. 16 As an alternative, hydrogen production from clean and renewable resources has gained interest over recent decades. Water splitting, or electrolysis, by electricity or light (solar energy) is one of the most convenient ways of addressing this problem. [17][18][19] In order to trigger electrolysis and produce hydrogen gas, the overall potential difference between the anode and cathode is critical and, thermodynamically, the necessary potential difference is at least 1.23V. Eqn. 1 and Eqn. 2 provide the minimum electrode potential for electrolysis at pH=0. However, an excess energy, termed an overpotential, is required to overcome activation energy barriers during the reaction. Additional factors are that some of the products may catalytically reconvert to water, oxygen may oxidise the anode, and a double layer capacitance may be formed.

Anode:
E=-1. 23V (1) Cathode: E=0.00V (2) According to Faraday's law of electrolysis, the mass of the substance produced by electrolysis is proportional to the quantity of carriers. This is expressed by Eqn. 3, where m is the mass of the substance, Q is the total electric charge passed through the substance, F is the Faraday constant, M is the molar mass of the substance and z is the valence number of ions.
Hence, in addition to the critical potential to initiate the reaction, the available charge plays an important role in determining the amount of hydrogen that can be produced. Pyroelectric energy harvesters, as a potential energy source, have been used in an effort to split water. The pyroelectric charge (Q) generated for a temperature change (ΔT) is given by, (4) where p is the pyroelectric coefficient (C m -2 K -1 ) and A is the surface area. Xie et al. analysed a range of pyroelectric materials and geometries for water electrolysis to determine the minimum material thickness to generate a critical potential (1.23-1.5V) to initialise water decomposition and maximise the charge and mass of hydrogen production (Eqn. 3) 20 . This is shown in Fig. 2 (a) where the potential ΔV developed across a pyroelectric of thickness, h, is given by where "" # is the material permittivity at a constant stress. It is interesting to note that for a given material the charge is proportional to the surface area (Eqn. 4) while the electric potential (Eqn. 5) is proportional to its thickness, indicating a need to tune the geometry of the pyroelectric elements. Using this analysis, Xie et al. demonstrated experimentally that a lead zirconate titanate (PZT) plate, when combined with rectification of the alternating current to provide a unipolar output, was able to harvest heat fluctuations and generate a sufficient electric potential difference for water splitting. In this case the pyroelectric water splitting was achieved by positioning the pyroelectric material outside of the water and the material was used as an external charge source, as in Fig. 2 (b) and further experimental details are summarised in Table 1. A patent on pyroelectric water splitting has also been filed which describes a device that combines a liquid based temperature control device, two gas chambers and a pyroelectric material that is in partial contact with water to undergo Reduction-Oxidations (Red-Ox) reactions when pyroelectric charges are generated on its surface during temperature changes 21  that 300 Vol.-ppb hydrogen was generated after a small number of pyroelectric cycles. The advantage of this approach, compared to using a bulk materials, is that using finely dispersed pyroelectric particulates enables the area of the pyroelectric to be greatly increased, and therefore increase the available charge for hydrogen production (Eqns. 3 and 4). Materials that exhibit high pyroelectric coefficients are typically ferroelectric that have a switchable polarisation, and the direction of polarisation can affect its surface stoichiometry and electronic structure, and therefore adsorption energy. The modification of ferroelectric surfaces to control surface chemistry and enhance catalytic properties has been studied in the areas of adsorption, desorption and photo-catalysis. 24,25 In recent papers 25, 26 , Kakekhani et al. proposed a pyro-catalytic reaction that is activated by cycling a material between two ferroelectric polarisation states (one surface state with a strong adsorption potential and the other surface state with a strong desorption potential). Density functional theory (DFT) indicated that ferroelectric PbTiO3 can effectively convert SO2 to SO3 and can be used to control binding energies and hence decompose NOx into N2 and O2 by using a positive and negative polarisation. In addition, the potential of catalysing the partial oxidation of methane to methanol was reported 27 . DFT was also used examine the potential of pyroelectric materials for water splitting 28 . In the modelling approach, the surface of a ferroelectric lead titanate (PbTiO3) material was cycled between its low temperature ferroelectric state and high temperature para-electric phase by thermally cycling above and below its Curie temperature (Tc). In the lower temperature ferroelectric and polarised state, H2O molecules are thought to dissociate on the negatively poled surface of the lead titanate to produce bound atomic hydrogen (see blue region of Fig. 3 where T<Tc). When the material is heated to the higher temperature non-polarised and para-electric phase, the hydrogen atoms recombine to form weakly bound H2, thereby creating a clean surface that is ready for the next thermal cycle (see red region of Fig. 3 where T>Tc). This provides an intriguing approach to harvest thermal fluctuations to produce hydrogen, although no experimental evidence has been reported to date.

Pyro-electro-catalysis
The demand for clean water continues to rise due to worldwide growth in our population and agriculture, and increased industrial development. When heavy metals, bacteria and organic dyes enter our water, it can be difficult to treat and can lead to health and environmental problems and therefore new technologies for their removal has become desirable. 29 In this regard, the use of pyroelectro-catalysis, which combines the pyroelectric effect and electro-chemical oxidation-reduction reactions, has been studied and used for the disinfection of bacteria and decomposition of a variety of toxic and hazardous organic compounds in aqueous environments.

Pyro-electro-chemical disinfection
Wiesner 30 was first to propose that the pyroelectric effect has the potential to drive a reduction-oxidation reaction, since an electrical charge and a potential difference is developed across a material as a result of a temperature change (Eqn. 4 and 5 solution to create reactive oxygen species (ROS) to disinfect Escherichia Coli (E. coli) bacteria 31 . Nano-and micro-crystalline lithium niobate (LiNbO3) and LiTaO3 powder were placed into an aqueous solution and subjected to a temperature cycle between 20 and 45 °C. The mechanism of disinfection is shown in Fig. 4, where in (a) the material is initially in equilibrium and there are bound surface charges (blue circles) due to the spontaneous polarisation ( &&&&⃗ ) of the ferroelectric LiTaO3 (shown as red circles). When the material is heated in Fig. 4 (b), the polarisation of the pyroelectric is reduced so that some of the bound charge is now free to take part in reduction-oxidation reactions (Red-Ox) until the excess (free) charges are consumed in Fig. 4 (c). When the material is subsequently cooled the polarisation of the material increases and charges move to the surface to balance the uncompensated screening charge carriers, leading to further Red-Ox reactions. It should be noted from Fig. 4 that the polarity of the charges moving to each surface changes depend on whether the material is undergoing a heating cycle (a polarisation decrease) or a cooling cycle (a polarisation increase). To demonstrate the potential of pyroelectric disinfection, Fig. 5 shows vitality staining of bacterial cultures which show the disinfection of the E. coli bacterial cells due to the pyroelectric effect of LiTaO3 powders; the E. coli were killed after thermal treatment of LiTaO3 for three hours. In 2015 Benke, et al. 32 reported an enhanced disinfection process, which was assisted with the use of palladium nanoparticles on the pyroelectric particle surfaces. Barium titanate (BaTiO3) nanoparticles of 100 nm in size where coated with 40 nm palladium (Pd) nanoparticles. The mechanism is shown in Fig. 6(a) where again heating and cooling cycles lead to free surface charges being produced on the polarised material surface, which can contribute to Red-Ox reactions. Due to the small particle size, the surface potential generated by the BaTiO3 nanoparticles from pyroelectric effect is lower than the voltage needed for reactive oxygen species generation (see Eqn. 5). However, in their energy band model, which is shown in Fig. 6(b), the pyroelectric BaTiO3 particle was assumed to be a p-type semiconductor and the Fermi level of BaTiO3 was at the mid-band gap ( Fig. 6(b), upper left). The band of BaTiO3 is thought to tilt due to the internal electric field that is generated by a change in temperature and the pyroelectric effect ( Fig. 6(b), upper right). As the Pd nanoparticles are in contact with the BaTiO3 particles, electrons (e -) are able to transfer into the Pd particles and the holes (h + ) into the valence band of BaTiO3 to become a source of ·OH radicals ( Fig. 6(b), lower image). Hence Benke, et al. assumed that the ·OH generating reaction, which improved disinfection, was strongly dependent on the enhanced charge transfer between the Pd and BaTiO3 nanoparticles.

Pyro-electro-chemical degradation
In a similar approach to pyroelectric disinfection, bismuth ferrite (BiFeO3) nanoparticles have been used to degrade an organic dye solution of Rhodamine B (RhB) using the pyroelectric effect. The process was shown to achieve a 99% degradation efficiency after 85 thermal cycles for a temperature cycle from 27°C to 38°C. 33 The inset of Fig. 7(a) shows the colour change of the RhB dye solution with an increasing number of cold-hot cycles and the degradation efficiency is shown in Fig. 7(b) along with a comparison of the response of a control sample in the dark and at constant temperature. Inspired by this research, the same group investigated the potential of pyroelectric dye degradation using lead-free/nontoxic BaTiO3 nanofibers 34 . The decomposition of RhB was again high, up to 99%, when the material was subjected to 72 cold-hot cycles from 30°C to 47°C. In addition, there was no significant decrease in the pyro-electro-catalytic activity after five dye decompositions; thereby indicating the potential of the material for long-term operation. Additional pyroelectric materials have also been explored for degradation applications; for example ZnO  nanorods exhibited a high pyro-electro-catalytic activity for RhB and attained a 98.15% decomposition after 27 thermal cycles between 22°C and 62°C 35 . A patent for an energy and substance conversion device that uses pyroelectric materials to produce cleavage products from fluids has also been filed. 36

Hybrid pyro-electro-chemical energy cells
For energy harvesters coupled to electro-chemical systems, several hybrid energy generators have been investigated in order to increase the reaction rate and the amount of available charge to contribute to electro-chemical reactions. In particular, the combination of pyroelectric, piezoelectric and triboelectric nanogenerators (TENGs) has been explored and devices based on TENGs will be described in more detail in Section 4. Yang, et al. was first to demonstrate a hybrid energy cell comprised of an anodic aluminium oxide template with a polydimethylsiloxane nanowire array as a triboelectric nanogenerator and a lead zirconate titanate (PZT) film as the pyroelectric energy harvester 37 . In this case the generator was used as an external charge source rather than particulates dispersed in a solution. Energy was harvested from a pyroelectric thermal cycle (heating from 295K to 309K in 100s and then cooling to 309K in 100s) along with mechanical energy from the ambient environment using a TENG to degrade methyl orange. Fig. 8(a) shows a schematic of the self-powered hybrid cell showing the combined pyro-and tribo-electric nanogenerators (NGs), the rectification bridge which acts to maintain the polarity of the alternating current (AC) output and the electrodes connected to the methyl orange solution; a lithium battery is also present to provide additional electrical energy storage. The degree of methyl orange degradation was 80% after 144 hours of operation with an observed colour change; see Fig. 8(b). A hybrid nanogenerator approach is also shown in Fig. 9(a), which used polyvinylidene difluoride (PVDF) as the active pyroelectric material and a patterned PTFE/Al combination as the triboelectric nanogenerator 38 . Since pyroelectric materials are also piezoelectric, the PVDF was also used and a piezoelectric harvester to harvest mechanical vibrations. Since all three mechanisms produce an alternating current under a cyclic temperature or stress, rectification was used to maintain a constant polarity at the steel cathode and carbon anode. In this example the potential produced was used to achieve self-powered cathodic protection and the Nyquist plot in Fig. 9(b) indicates that little or no rust layers are formed on the steel electrode surface compared to steel not connected to the nanogenerators. Pyroelectric materials also have the potential to simply harvest temperature changes associated with exothermic or endothermic reactions. Industrial and lab-scale chemical processes can generate a large amount of waste heat and the low-grade nature of the heat and ease of dissipation makes is difficult to be harvested. Zhao et al. fabricated a flexible pyroelectric energy harvester, which consisted of ferroelectric PVDF sandwiched between two multi-walled carbon nanotubes electrodes. This system was demonstrated to harvest sufficient waste heat from chemical exothermic process for low power electronics 39 .

Pyro-electric ice-formation and other applications
Beyond the reduction-oxidation reactions that have been described above, pyroelectric materials have also been used to control surface ice-formation, indicating potential use of charges generated during heating or cooling to control freezing behaviour. Control of the freezing temperature of super-cooled water is important in areas such as cell and tissue cryo-preservation and prevention of crop freezing. Electro-freezing 40 using electric fields on charged surfaces has been exploited for the formation of ice-like nuclei and enhance the freezing of super-cooled water. The use of electro-freezing on metallic surfaces is difficult since its electrical conductivity isolates the net effect of the electric field. Ehre et al. reported the use of pyroelectric materials to isolate the electric field since they are insulators (dielectrics) and the polarity of the surface can be manipulated by changing the polarisation direction. 41 Positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films were shown to promote ice nucleation, while negatively charged surfaces were shown to inhibit ice nucleation and thereby reduce the freezing temperature. Fig. 10 shows optical microscopy images of water droplets and  condensation on amorphous (top) and quasi-amorphous (bottom) films of SrTiO3 at a variety of temperatures. The water was observed to freeze at a higher temperature of -4°C on the quasiamorphous (pyroelectric) film compared to -12°C on the amorphous (nonpolar) film. In addition, it has been reported that polyvinylidene fluoride (PVDF) can be used as an ice repellent coating 42 and the strength of ice repellency can be increased by increasing the polarisation of the coating. Pyroelectricity has also been recently observed in nonpolar, centrosymmetric crystals of amino acids 43,44 which originates from the thin polar layer of hydrated α-glycine near its surface and this influences its freeing characteristics. With amino acid crystals, Ehre et al. found that ice nucleation can be close to 0°C due to a relatively large pyroelectric effect. 45 In a similar approach to tuning ice nucleation, pyroelectric materials have been used to pattern polymer films; and the reader is referred to a review by Coppola et al 46 . Xi et al. used LiNbO3 (LN) to generate parallel charge patterns, and achieved self-assembly and patterning of a thin polymer film through an electro-hydrodynamic process 47 . By placing a hot polydimethylsiloxane (PDMS) stamp on a LiNbO3 surface, it was possible to induce the formation of local pyroelectric charge by transferring heat from a patterned PDMS stamp to the pyroelectric LiNbO3. The electrostatic stress generated from the patterned surface charge was able to drive the assembly of the thin polymer film into microarrays. A schematic of the process is shown in Fig. 11 where the hot stamp generates surface charges for assembly of solvents (a to b) or immersion in water for patterning (c to e). In addition, periodically poled pyroelectric crystals can lead to electrowetting effect and form liquid lenses on a surface in an electrodes-less and circuit-less manner 48 . This provides scope for thermal scavenging for freezing/wetting, materials assembly and fabrication, and examples of harvesting for materials synthesis is described in the section on tribo-electric harvesting.

Piezo-electro-chemical effect
The direct conversion of mechanical energy into chemical energy, known as the piezo-electro-chemical (PZEC) effect, was first reported in 2010 by Hong et al. 49 . In this work, the piezoelectric properties of the materials were used to achieve water splitting for hydrogen generation. Following this work, the piezo-electro- chemical effect was utilised for a number of other applications, such as water purification and self-charging power cells, either as the sole harvesting mechanism or used in a hybrid system to enhance the primary energy conversion. These approaches will now be described.

Piezo-electro-chemical water splitting
Hong et al. used piezoelectric ZnO microfibres and BaTiO3 microdendrites immersed in water to produce H2 by harvesting ultrasonic vibrations 49 . Fig. 12 shows the piezoelectric charges developed on a ZnO fibre surface as a consequence of being mechanically excited by ultrasonic waves; the conditions are summarised in Table 1. The resulting mechanical strain of the ZnO fibre changes its polarisation to create free surface charges that can contribute to Red-Ox reactions on the positive and negative surfaces of the piezoelectric fibre. This is shown in Fig. 12 where bending from ultrasonic excitation is thought to produce negative charges on the tensile face and positive on the compressive face to contribute to Red-Ox reactions.
To achieve water splitting the developed potential must be greater than the redox potential of water (Eqns. 1 and 2) and potentials lower than 1.23 V will not participate in reactions to form H2 and O2. Starr et al. 50 used a single-crystal ferroelectric Pb(Mg/3Nb2/3)O3-32PbTiO3 (PMN-PT) cantilever in a sealed chamber that was strained using a computer controlled vibrator and linear actuator. Fig. 13 shows the experimental set up where deformation of the piezoelectric cantilever was achieved remotely via two encapsulated magnetic materials placed at the tip of both the cantilever beam and driving lever arm. Following this work, Starr provided general guidance on approaches to exploit the piezoelectric properties of materials to initiate surface reactions 51,52 . Their fundamental analysis indicates that a high piezoelectric coupling coefficient and a low electrical conductivity are desired for enabling high electrochemical activity. Materials that could be used for such applications are PMN-PT, ZnO, BaTiO3 and PbTiO3; and many of the materials are shown in Fig. 2 for the pyroelectric analysis of Xie et al. The correlation between piezoelectric and pyroelectric properties is unsurprising since both originate from a change in polarisation with stress or temperature, respectively 53 ; see Fig.1. In an attempt to achieve a more flexible and inexpensive harvesting design that could utilize low vibration frequencies, rather than high frequency ultrasound, Zhang et al. proposed an indirect piezo-electro-chemical process for water splitting 54 . The device consisted of a piezoelectric bimorph cantilever and a water electrolysis system; the electrical output produced by mechanical vibrations was rectified and connected to an electrolyte to split water into hydrogen and oxygen. This design offered more flexibility and a high output voltage (approximately 12 V, see Table 1). The hydrogen production rate was 10 -8 mol/min and approaches to enhance the production range include using a piezoelectric material with a higher piezoelectric coefficient (dij) and increasing the conductive ion concentration of the NaHSO4 electrolyte solution. Other factors can include increasing area (A) and level of applied stress (s); since Q = dij·A·s. The piezoelectric properties of materials have also been used in combination with the more widely investigated photo-electrochemical (PEC) water splitting to combine vibration and solar harvesting. To overcome the challenges of photo-electro-chemical water splitting, such as the limited absorption of visible light, Tan et al. fabricated a piezoelectric-photo-electro-chemical hybrid device that combined harvesting from both light and vibration using piezoelectric ZnO nanorods on one-dimensional nanowire conductors. This multiple-energy-source powered system was based on a metal-semiconductor branched hetero-structure of Ag/Ag2S-ZnO/ZnS that was partially encapsulated with PDMS for piezoelectric harvesting, while the exposed part acted as a catalyst to enhance the photo-electro-chemical performance of the ZnO nanorods. The system was initially characterised separately under UV-vis irradiation and under ultrasonic vibrations, as in Fig. 14a 55 .
Under simultaneous application of UV-vis light and vibration, as in Fig. 14b, the generation of piezoelectric charge and charge transfer between the active electrode and Pt electrode generated a voltage bias, as in Fig. 14b, which improved the photocurrent density from 8 mA m -2 (no vibration) to 22 mA m -2 (with vibration). The system was also used to enhance photo-electric-chemical degradation of methyl orange for water treatment. Yang et al. have utilised the polarisation of piezoelectric BaTiO3 to enhance photo-electrochemical (PEC) water splitting 56 . An enhanced performance of PEC photo-anodes was reported due to ferroelectric polarisationenhanced band engineering of TiO2/BaTiO3 core/shell nanowires.

Piezo-electro-catalytic degradation of organic pollutants
In addition to harvesting mechanical vibration for water splitting, the piezoelectric effect has been used for degradation of organics in water. After exploiting the piezoelectric effect for water splitting, Hong et al. explored a similar approach for Azo Dye decolourisation in aqueous solutions 57 and for the degradation of Acid Orange 7 (AO7), the origins of the electron-hole pairs were from strained piezoelectric BaTiO3 micro-dendrites. By harvesting ultrasonic vibrations the AO7 dye was degraded by nearly 80% after subjecting the material to 90 min of vibration. Using the same mechanism, Lin et al. used the piezoelectric properties of Pb(Zr0.52Ti0.48)O3 (PZT) fibres to harvest mechanical vibrations and further improve efficiency due to the large amount of electrical charge generated by the highly piezo-active PZT material 58   found in waste water. 59 The fundamentals of the piezo-photochemical mechanism have been described by Z.L. Wang. 60 When the piezoelectric is subjected to a periodically applied strain during vibration the photo-generated electrons and holes migrate to the surface in opposite directions under the influence of the electric field produced by the piezoelectric effect; thereby achieving a higher efficiency due to suppression of electron-hole recombination.
Xue et al. used ZnO nanowires that combine the properties of a piezoelectric and a semiconductor. The photo-catalytic activity of ZnO nanowires was enhanced by the piezoelectric and electric field driven separation of the photo-generated carriers for the degradation of methylene blue (MB). 61 The working mechanism for the piezo-photo-catalytic activity of ZnO nanowires in the work of Xue et al. is shown in Fig. 15 where Fig. 15a shows the woven ZnO nanowires/carbon fibres (CFs) without an applied force or UV irradiation. When subjected to UV light, as in Fig. 15b, there is a transition of electrons from the valence band to the conduction band, leaving an equal number of holes. When a periodic force is simultaneously applied to the ZnO nanowires/CFs, as in Fig. 15c, there is a relative motion between neighbouring ZnO nanowires that results in bending of the ZnO nanowires which produces positive and negative piezoelectric potentials across their width. The generated piezoelectric field then drives electrons and holes to migrate to the surface in opposite directions and the recombination of electrons and holes is therefore reduced. The electrons (e -) react with dissolved oxygen molecules to yield superoxide radical anions (·O 2-), and the holes (h + ) are ultimately trapped by H2O at the surface to yield ·OHradicals, Fig. 15d. The hydroxyl radicals can oxidize MB in aqueous solution, generating non-toxic CO2 and H2O. The photo-catalytic efficiency of ZnO nanowires was thought to be enhanced by the piezoelectric field and reduced recombination of photo-generated carriers.  used Zn1-xSnO3. 63 Fig. 16 shows the increased rate of degradation of methylene blue by applying both stress and UV on piezoelectric ZnSnO3 nanowires; see the blue piezo-photo-catalysis curve. 62 To further improve the piezo-photo-catalytic effect Hong et al. used CuS/ZnO hetero-structure nanowire arrays that were subjected to a combination of UV and ultrasonic irradiation to degrade MB. 64 38,65,66 Again, the higher efficiency and speed of degradation was attributed to the coupling between the built-in electric field of the hetero-structure and the piezoelectric field of ZnO under strain that enhanced electron-hole separation and migration in opposite directions. This is demonstrated in Fig.  17, where the ZnO that is on a stainless steel mesh is mechanically deformed due to the application of ultrasound and this leads to an electric field across the wire. Since the wire is also photo-excited by solar radiation, electrons and holes are separated due to the applied field towards opposite directions and drive the surface reactions, Fig. 17(a). Fig. 17(b) shows the effect that the field has on the energy bands that separate the photo-excited electrons and holes in opposite directions. Further information on the influence of ferroelectricity and piezoelectricity on photo-catalytic activity and surface chemistry, in the absence of vibration, can be found in a recent review 67 and a number of key papers by Dunn et al. [68][69][70][71][72][73][74] Since ferroelectric materials are both piezoelectric and pyroelectric there is significant scope for further efforts to harvest thermal and mechanical energy in combination with light for enhancing electro-chemical reactions.

Piezo-electro-chemical self-charging of power cells
Energy generation and energy storage are two distinct processes and are usually accomplished separately. However, in 2012 Xue et al. introduced a new mechanism where harvested mechanical energy was directly stored as chemical energy. 75 The polyethylene separator of a conventional lithium battery was replaced with a piezoelectric PVDF polymer. When the device was subjected to a compressive strain, the piezoelectric induced electric field of the PVDF separator acted to drive electrical charges from the cathode to the anode, thereby charging the power cell. Later, the same group fabricated a novel integrated self-charging power cell (SCPC) that combined a CuO anode with a PVDF separator in the form of CuO/PVDF nano-arrays. The high efficiency of the cell was attributed to the intimate contact and large interface area between the anode and separator. 76 This is shown in Fig. 18 and when a compressive stress is applied, the PVDF creates a positive piezopotential at the cathode and a negative piezo-potential at the anode ( Fig. 18(b)). Lithium ions then migrate in the electrolyte from the cathode to anode under the generated electric field, leading to charging reactions at the electrodes (Fig. 18(c)) until chemical equilibrium of the two electrodes is re-established and the selfcharging process ceases. When the compressive stress is released, the piezoelectric field of the PVDF disappears, which breaks the electrostatic equilibrium, and residual lithium ions diffuse back to the cathode; see Fig. 18(c). In such a system rectification is not required since it is operated in compression only, so that the polarity of the system is maintained; although care would need to be taken to ensure tensile loads are not experienced by the system.   Fig. 20, where the silicon and BaTiO3 particles are dispersed in a CNT matrix ( Fig. 20(a)). Lithiation of the silicon nanoparticles results in a volume increase that applies pressure to the BaTiO3 nanoparticles to create a piezoelectric potential ( Fig. 20(b)) that enhances the mobility of the lithium ions in the subsequent discharging and charging processes.
A particularly novel approach by Kim et al. does not use the piezoelectric effect, but is based on stress driven electro-chemical effects. 85 In this design different stress states are induced by bending partially lithium-alloyed silicon electrodes which creates a potential difference between the electrodes. Fig. 21 shows the working mechanism. In the initial stress-free condition, the two electrodes are at an iso-potential (Fig. 21 I). Bending of the device generates tension in the lower electrode and compression in the upper electrode in Fig. 21 II. The asymmetric stress state creates a chemical potential difference that drives Li + migration from the compressive electrode to the tensile electrode through the electrolyte (Fig. 21 III). In order to maintain charge neutrality, electrons flow in the outer circuit from the compressive and to the tensile electrodes, thereby generating electrical power. The Li + migration continues until the potential difference vanishes, and a new equilibrium state on the two electrodes with different lithium concentrations are achieved (Fig. 21 IV). When the external stress is removed by unbending the device, the chemical potential shifts on the electrodes and the difference in lithium concentration between the electrodes drives Li + migration in the opposite direction ( Fig. 21 back to I), thereby discharging the device. This provides an interesting approach to harvest mechanical motion for electrochemical based energy generation, which could be enhanced by coupling to piezoelectric effects to generate an additional electric field to enhance ion migration.

Other piezo-electro-chemical applications
Noris-Suarez have reported bone healing due to piezoelectricity. 86 Bone healing and growth are controlled by the rate of deposition of hydroxyapatite (HA) and their work showed that the piezoelectric dipoles produced by deformed collagen can produce the necessary precipitation of HA. Therefore there is scope to harvest biomechanical motion for enhanced bone growth. Lang et al. 87 demonstrated that HA is both piezoelectric and pyroelectric indicating potential for harvesting human motion and temperature changes; the reader is referred to a review on the topic by Baxter et al. 88 A biomimetic approach was used by Soroushian et al. who investigated self-healing structures that were able redistribute their structural mass in response to dynamic loads. 89 In their work the piezoelectric effect was used to convert dynamic mechanical energy applied to a structure into electrical energy that was able to drive an electro-chemical self-healing phenomena within a solid electrolyte. Zhang et al. also used PVDF to use piezoelectric energy to indirectly power cathodic protection 38 and the output of the fabricated device was used to protect metal surfaces from the chemical corrosion, see Fig. 9 . Touach et al. 90 used lithium niobate (LiNbO3) as a ferroelectric and photo-catalyst material as a cathode catalyst for wastewater-fed single-chamber microbial fuel cells (MFCs); microbial cells are also discussed with regard to thermoelectrics (Section 6).

Tribo-electro-chemical effects
Compared with the piezo-and pyro-electric based energy harvesters, the utilisation of the triboelectric effect in a triboelectric nanogenerator (TENG) to scavenge mechanical energy is a relatively new approach in the field of energy harvesting, and was first reported in 2012 by Z.L. Wang's group. 91 The mechanism of charge generation exploits the intrinsic ability of materials in different positions of the triboelectric series to gain or lose charge when they make contact with each other. The triboelectric charges that are generated on the surface of the two materials can be regarded as a combination of tribo-electrification and electrostatic induction. TENGs typically have four basic working modes, which are shown in Fig. 22. 8 The generation of charge relies on contact and separation cycles between two different materials and the modes of operation can involve rubbing or contact between two materials under vertical (Fig. 22A) or lateral sliding (Fig. 22B), with two electrodes to collect the charge. Single-electrode configurations can also be employed, as seen in Fig. 22C, and freestanding moving objects without an electric connection can be utilised; see Fig. 22D. In all of the potential modes of operation, the movement of the free electrons enables the generation of a current that flows in an external circuit, which can be an electro-chemical system, storage capacitor or an electrical load. TENG devices are being rapidly developed with increasing efficiency in terms of their ability to convert mechanical energy into electricity. The power densities and instantaneous energy conversion efficiency for a single device has been reported to be as high as 500 W m −2 and 70%. 92,93 Owing to their high power density, high efficiency, small volume, light-weight nature and low fabrication cost, TENGs are attracting potential in harvesting 'blue energy' from wave power. 94 The energy that is harvested from ambient environments by TENGs varies significantly and can be periodic, random and timedependent, which results in an alternating current (AC). Moreover, the magnitude of the pulsed voltage in TENGs is relatively high (typically 2 V to 1.3 kV, see Table 1) while the current remains relatively low (85 nA to 13 mA, see Table 1) which often requires the use of a power management circuit. As a result, when TENGs are coupled to electro-chemical processes, they can act as the power source either by directly powering the electro-chemical system with a pulsed output that is rectified to maintain the polarity of the output or a combination of transformer (to step down the voltage) and rectifier that is used to charge an integrated capacitor/battery before supplying the electro-chemical system with a direct current (DC) electrical output. As a result, integrated TENG-controlled electro-chemical systems often consist of a triboelectric generator, an AC/unipolar signal converter with a rectifier and/or transformer and the functional electro-chemical unit. Cao et al. have provided an excellent review on triboelectric nano-generators and electro-chemical systems. 95 Here we will concentrate on the introduction to the mechanisms of triboelectric generation for controlling electro-chemical processes and describe recent achievements in electro-chemical processes controlled by TENGs.

Tribo-electro-chemical water splitting
As discussed in Section 2.1, water splitting can be electrically triggered when the potential difference between the anode and cathode is greater than 1.23V (Eqns. 1 and 2). Triboelectric nanogenerators have been used to harvest ambient sources of mechanical energy and supply electrical power to achieve electrochemical water splitting without the need for an external power source. [96][97][98] Yu et al. 99 fabricated a wind-driven TENG to supply a bias voltage after rectification between a platinum counter electrode and a working electrode with a 3-D structure based on TiO2 nanowires and graphite fibres. The combined system provided enhanced photo-catalytic activity and generated 4.87 mmol/(h.g) H2 with the aid of solar illumination and wind, this was almost three times higher than a TiO2@MoS2 catalyst under a visible light source. 100 Fig. 23(a) shows the system with the wind powered TENG, and rectification bridge to maintain electron flow in one direction. Under the application of wind and UV light, hydrogen generation was observed on the counter electrode. However, due to the relatively low current density on the TiO2 nanowire/graphite fibre electrode and the resistance mismatch between the TENG, electrical load and electrochemical system, Li et al. 101 combined both electrolysis and photo-electro-chemical (PEC) effects to improve H2 production and increase solar efficiency. Fig. 23(b) shows the system employed, that used a TENG driven by a linear motor to supply a potential across a photo-anode and counter electrode. A lithium-ion battery, not shown in Fig. 23(b), was used to provide a stable DC output (1.5V) after being charged by a TENG for 42 min using rectification and transformation, where a series of loads of different resistance were chosen to match the electrical impedance in the external circuit. A similar photo-anode approach was used by Pu et al. to achieve enhanced photo-activity by a surface plasmonic resonant effect. 102 This anode consisted of Au nanoparticle decorated TiO2 arrays on fluorine doped in tin oxide (FTO) conductive glass, as shown in Fig. 23(c). Au nanoparticles were used to localize the optical energy due to the surface plasmonic resonance and the interaction between this electric-field amplification and TiO2 promoted photon absorption. In addition, the gold nanoparticles also act as a photosensitiser when the wavelength of the surface plasmonic resonant matches the optical band gap of TiO2, therefore hot electron injection from Au to the TiO2 conduction band was enhanced under visible light. 102 This coupled TENG-photo-electro-chemical (PEC) hybrid cell provided a route to split water by converting mechanical and solar energy into chemical energy. Interestingly, the generated H2 and O2 on the anode and cathode from water splitting has also been employed to pressurise a drug solution in a reservoir for an implantable drugdelivery system (iDDS) 103 ; see Fig. 23(d) which shows the TENG, rectification, and the implantable drug delivery system. Yang et al. 97 developed a hybrid energy cell that integrated a TENG, a thermoelectric cell and a solar cell that could simultaneously harvest mechanical, thermal and/or solar energies for water splitting; the specific application of thermoelectrics to electrochemical systems will be described in more detail (Section 6).

Tribo-electro-chemical degradation and wastewater treatment
The pyroelectric effect, which harvests temperature fluctuations, and the piezoelectric effect, that harvests vibrations, has been used for water treatment, as discussed in Section 2.1.2 and 3.2 respectively. TENGs that harvest motion have also attracted interest as a form of self-powered electro-chemical wastewater treatment and SO2 removal as it is environmentally friendly and does not create toxic waste. 37, 104-109 Zheng et al. 110 deployed a TENG to remove toxic organics in water; this included aniline (ANI) and N(p-C6H4Br)3 which was removed via an oxidation process; the performance of the self-powered treatment by a TENG was reported to be as effective as an electro-chemical workstation which requires a galvanostat as an external power source. Gao et al. fabricated a TENGs with either a multilayer-linkage configuration (rd-TENG in Fig. 24(a) 111 or a rotary disc-structure 112 which were used as a self-powered unit to drive a water treatment process. In both cases rectification of the TENG output was used to ensure a unipolar supply to the anode and cathode along with a transformer to reduce the operating voltage as the potential produced by a TENG is typically higher than other harvesting systems, see Table 1. The TENGs were used for the removal of highly toxic and carcinogenic organics by conversion to CO2; this included oligomers, azobenzene dye and 4-aminoazobenzene. The electric oxidation potentials were controlled via either the resistance of the external load or the numbers of the friction layers in the TENG device. In later work, the metal electrode was replaced with a carbon material manufactured from bean curd that was used to degrade methyl red. 113 In addition to the above toxic chemical compounds, heavy metal ions (such as Cu, Ni and Cr) are another main component in wastewater, which can cause severe illness when they enter our food chain. One of the most promising ways for reducing heavy metal ions is electro-deposition where the heavy metal can be reduced and collected from the surface of the electrode. 114 Chen et al. 115 employed a TENG as a power source connected to a sewage treatment system, shown in the left of the Fig. 24(b). Using a NaCl solution as the electrolyte, toxic Rhodamine B was degraded into CO2, H2O, and N2 at the anode, while the heavy metal copper ions were electrodeposited at the cathode, see right of the Fig. 24(b). It was demonstrated that the removal efficiency of Rhodamine B and Cu 2+ from a pulsed electrical output from a TENG harvester was better than that of a DC supply. Similar research on Cu 2+ reduction was also reported by Yeh et al. 116 who used a TENG operating in a freestanding mode (Fig. 22(D)) which was coupled to the anode and cathode to drive the electro-chemical reduction of Cu 2+ ions on the surface of the cathode in an aqueous based solution. Other wastewater electro-chemical treatments, such as electrocoagulation, can be driven by TENGs as demonstrated by Jeon et al. 117 . In this case, wind energy was utilised to generate electrical power via a TENG to power an electro-coagulation unit. In their system, shown in Fig. 24(c), metal ions (M n+ , which are Al 3+ in the work) were generated at the anode while OHions were generated at the cathode. Colloidal pollutants, such as algae and dyes, reacted with the insoluble metal hydroxide and produce flocs which form coagulated pollutants so that clean water could be collected from the top of the water treatment system. Li et al. 118 recently built an electro-chemical ramie fiber degumming system and wastewater treatment with a water-driven TENG. Raw ramie fibers are a common textile material and exist as fiber bundles where the individual fibers are bonded to each other In order to extract the individual cellulose fibers the harvesting system shown in Fig. 25(A) was constructed; the system consisted of (i) a TENG, power management (transformer/rectification circuit and capacitors) and reaction pond, (ii) a Ti/PbO2 anode and titanium cathode immersed in the reaction pond. Due to the potential difference produced by TENG, hydroxyl ions move towards the anode which make the gummy materials break away from the cellulose part of the fiber which are then readily dissolved in a hot alkaline solution; see Fig. 25 (A, iii). The surface of the treated fiber with the TENG exhibited the most clean and smooth surface, as shown in (c) and (f) in Fig. 25 (B). Furthermore, the degumming wastewater was also electro-chemically degraded using a TENG after finishing the fiber treatment.

Tribo-electro-chemical corrosion protection
Cathodic protection is a common technique for protecting metals from corrosion to increase service life of metallic components and reducing maintenance costs; this can be achieved by applying organic coatings or using an electro-chemical cell. In a typical electro-chemical protection procedure, the metal to be protected will have an ohmic-contact to the negative polarity of the external power source and an inert electrode will be connected to the positive pole. Both electrodes are then placed in the corrosion electrolyte to form an electrolytic cell so that the metal will be protected as the cathode. Inspired by preliminary work 119-123 on the realisation of cathodic protection powered by TENG, recent efforts have been undertaken on optimising the triboelectric material 124 , using a conductive polymer PPy (polypyrrole) 125 or combining TENGs with the piezo-and pyro-electric effects to form hybrid systems 38 to ensure the cathode is below the corrosion potential to prevent corrosion. Fig. 26 shows an example of wind driven TENG whose output is rectified and then connected to platinum counter electrode and carbon steel for cathodic protection. 125 In addition, the triboelectric effect has also been adopted for anti-fouling applications, where the pulsed energy generated by a TENG is used to remove or prevent the accumulation and growth of organisms on both anode and cathode, as demonstrated by Feng et al. 124

Tribo-electro-chemical synthesis (electro-deposition and electro-oxidation)
Energy harvesting by TENGs has also been used for materials processing. Electro-deposition is a process that uses an electrical current to reduce cations of a desired material from a solution and coat the material as a thin film onto a conductive substrate surface. TENGs have been used by Zhu et al. to harvest ambient mechanical energy for a self-powered electro-deposition process 126 . Due to the risk of oxidation or corrosion of the metallic TENG electrodes when exposed to a wet environment, a conducting polymer polypyrrole (PPy) was chosen as the electrode to fabricate an all-plastic based TENG that included an integrated PPy-based supercapacitor, thereby providing sustainable power 127 . Wang 128 has recently exploited TENGs to provide a pulsed rectified output to electrochemically polymerise PPy for up to nine chemical reactors; see Fig.  27. Similarly, TENGs can also facilitate the oxidation process for preparation of mesoporous Al2O3. 129 A cross-linked TENG was designed by Zheng et al. 110 which could supply power to chemically synthesise polyaniline (PANI) via either oxidisation of PANI-OH or ANI in an H2SO4 aqueous solution.

Additional tribo-electro-chemical applications
Additional electro-chemical research based on the triboelectric effect has been undertaken in the application of the electrochemical sensing 107, 130-135 and electro-chromic applications [136][137][138][139][140][141][142] . Due to the different triboelectric polarity between Hg 2+ and Au nanoparticles (Au NPs), Hg 2+ can selectively bond to the surface of the Au NPs. Lin et al. 132 exploited this effect to detect and measure the concentration of Hg 2+ using a self-powered triboelectric nanosensor. TENG powered electro-chemical sensors have been reported that could detect humidity 143 , melamine 144 and ultraviolet light 145,146 . Furthermore, electro-chromic materials were able to interact with the electrical energy produced by a TENG to achieve a colour change or to fluoresce continuously and reversibly during electro-chemical oxidation and reduction. A number of research efforts 136,[139][140][141]147 have shown that TENGs can supply a sufficient electrical output to stimulate electro-chromic behaviour either in a cathode (colour change under a negative potential) or anode (colour change under a positive potential).

Flexo-electric and electro-chemical effects
While the piezoelectric effect relates to the generation of charge in response to an applied strain, the flexoelectric effect involves the generation of charge due to the presence of a strain-gradient. Biomembranes exhibit flexoelectric effects and are the basic building units of many cells and cellular structures 148 . The phospholipid molecules in most biomembranes are organised in a bilayer format, as shown in Fig. 28, and both artificial and natural cell membranes have been shown to exhibit direct flexoelectricity (current generation from curvature) and converse flexoelectricity (voltage-induced curvature changes) [149][150][151] . Phospho-lipid molecules include a hydrophilic phosphate head and a hydrophobic C-H chain, as shown in Fig. 28(a) 152 , which resembles a cone and has a dipole directed toward the apex. Under the application of electric field the bilayer membrane that has proteins with an intrinsic polarisation, pp, undergoes a splay deformation via the converse flexoelectric effect. Direct flexoelectricity in biomembranes results from a curvatureinduced polarisation of the liquid crystal membrane, in which the molecules (lipids, proteins) of the membrane are initially uniaxially orientated as shown in Fig. 28(b) 153 . In this flat bilayer membrane, the polarised cones are randomly directed, with no net polarisation. However, when subjected to bending, as in Fig. 28(c) 153 , a conformational change occurs that imposes a polar symmetry, so that on one side of the membrane the molecules move apart whereas on the other side they move closer together 154,155 . The resulting polarisation (P) is: Where f is the flexoelectric coefficient, k is the mean curvature and n is the normal vector to the membrane. Petrov discussed experimental evidence of flexoelectric phenomenon 148, 155-157 and Ahmadpoor et al. has presented a thorough review on soft biological membranes. 158 Flexoelectricity has been found to have implications for mechanosensitivity and mechano-transduction in living systems, including ion transport 155,159 and the hearing mechanism in mammals 160-163 . Petrov et al. 159 proposed a model for ion transport in biological systems, as shown in Fig. 29, which demonstrates an ion pumping through a membrane. The driving force for ion transport is a flexoelectric electric field from a change in curvature of the bilayer membrane induced by Adenosine triphosphate (ATP) and ionic concentration gradients. The resulting curvature induces polarisation and creates a depolarising electric field that acts as a driving force for ion pumping. Similar flexoelectric effects are responsible for the main transduction component for sensing sound in mammals as inner hair cells consist of flexoelectric stereocilia that transform mechanical vibrations into electrical potentials that are sensed by the nervous system. [160][161][162][163] The ability of biological membranes and cells to exhibit flexoelectric effects to achieve ion pumping and sensing could inspire new methods to harvest mechanical motion to drive ions in electro-chemical systems at the nano-scale.

Flexo-electric energy harvesting
The potential of flexoelectricity in energy harvesting has been examined by several researchers. Significant enhancement of the piezoelectric coefficient of a piezoelectric nano-beam/ribbon due to flexoelectric effect has been reported. 164,165 Pre-stretched buckled PZT ribbons show high strain gradients that provides charge in addition to the conventional piezoelectric effect. 165 Majdoub et al. used a continuum model to show that flexoelectricity can lead to an enhancement of electro-mechanical coupling of non-uniformly strained piezoelectric and nonpiezoelectric nanostructures. 164 For example, BaTiO3 nanobeams with a thickness of 5 nm experienced inhomogeneous strains almost five times larger than macro-scale BaTiO3 beams. Therefore in a narrow range of geometric dimensions, piezoelectric nanostructures can exhibit dramatically enhanced energy harvesting capability due to flexoelectric effects. For a PZT cantilever beam, the total harvested power increased by 100% for a 21 nm beam thickness under short circuit conditions and ~ 200% increase could be achieved for tailored cross-sections. 166 Deng et al. developed a continuum model for flexoelectric nano-scale energy harvesting for cantilever beams, as shown in Fig. 30(a) where the polarisation due to the resulting strain gradient is shown in Fig. 30 (b). On bending an AC potential difference is generated across the electrodes on its upper and lower surfaces. The output power density and conversion efficiency increased significantly when the beam thickness was reduced from the micro-to nano-scale and conversion efficiency at sub-micron thickness levels increases by two orders of magnitude as the thickness was reduced by an order of magnitude. Wang et al. 167,168 has also presented analytical model for nano-scale energy harvesters using flexoelectric effects.
Han et al. have reported a flexoelectric nanogenerator consisting of direct-grown piezoelectrics on multi-walled carbon nanotubes (mwCNT) in a PDMS matrix. 169 Nano-generators based on lead zirconate titanate (PZT)-mwCNTs generated a voltage of 8.6 V and a current of 47 nA from a mechanical load of 20 N; see Table 1. The high performance was reported to originate from the strong connection between the PZT and mwCNTs with an enhanced flexoelectric effect due to the strain gradient in the material. The epitaxial PZT nanogenerators with an internal strain distribution are shown in Fig. 31 where the change in lattice spacing indicates a strain gradient, which are thought to contribute to the   Fig. 28, where bending of the membrane leads to an electric polarisation or an electric field leads to bending. This is shown in Fig. 32 where the open circuit (Displacement, D=0) and closed circuit (electric field, E=0) states are shown and bending is imposed by an externally imposed pressure drop from the contacting fluid phases (P1 and P2) and charge separation in the membrane is due to the flexoelectric effect.
Flexoelectric materials can therefore provide new approaches to create nanodevices for sensing and electromechanical energy harvesting that may be combined with, or even replace, piezoelectric systems. The size effect in this class of materials could provide self-powered integrated nano-systems that make use of mechanical forces in cells to manipulate their biological behaviour. In addition, flexoelectric based energy harvesting techniques offer efficient solutions in electrochemical recovery of ions such as calcium and lithium. For example, Trocoli et al. recently proposed a new method for lithium recovery from brines using an electrochemical ion-pumping process 171 .

Flexo-electric effects in electro-chemical strain microscopy
Electro-chemical strain microscopy (ESM) is a new scanning probe microscopy (SPM) technique that enables local ionic flows and electrochemical reactions in solids to be probed and is based on flexoelectric and bias-strain coupling. 172,173 Fig. 33 shows an ESM measurement where the film being analysed is composed of mobile charged defects and electrons. The upper SPM tip produces an inhomogeneous electric field that leads to elastic strains that are proportional to the electric field gradient due to flexoelectric coupling. The redistribution of mobile charged particles influences the electric potential distribution in the film and induces local strains that are detected by the SPM tip; thereby providing electrochemical information about the specimen. 174 The models developed that couple the electric-chemical-mechanical-thermal processes in this technique can also take into account bulk defect electrochemical reactions -these new models may provide new insights where ion flow in a mechanical strain or electric field gradient can be used for nano-scale harvesting and storage.

Photo-flexo-electric effects
A photo-induced flexoelectric response was initially reported by Spassova for bilayer lipid membranes 175 and, recently, flexoelectricity has been reported to be responsible for lightinduced current generation at structural boundaries between two phases of ferroelectric bismuth ferrite due to a coupling between strain gradient and photo-electric activity, see Fig. 34. 176 A localised light beam creates electron-hole pairs in the material and at grain boundary interfaces the electric fields from a strain gradient due to flexoelectric effect separates the two types of charge, and results in current flow normal to the interface 177 . Since the material is also piezoelectric there is potential to combine flexoelectric effects with the piezoelectric and photo-electro-chemical aspects described in Section 3.

Thermoelectric-chemical effects
Thermoelectrics are able to generate electrical energy from spatial thermal gradients, unlike pyroelectric materials that require temperatures that vary with time. Work on using thermoelectric power for water splitting has been reported as early as 1976 178 , where a hybrid thermo-chemical water-splitting cycle using solar  energy was proposed which consisted of three subsystems. The first subsystem was a photochemical reaction (2Fe 2+ + I3 -+ light → 2Fe 3+ + 3I -) that was achieved in a flat cell through which the liquid reactant flowed. The cell was placed in direct sunlight above a Fresnel lens, as in Fig. 35 (upper right). The second subsystem was below the Fresnel lens where the remaining solar energy was concentrated onto a thermoelectric element. Cooling fins were used on the other side of the thermoelectric generator to maintain a temperature gradient. The final subsystem consisted of two electrolysers that combine the products of the photo-chemical reactions and electrolysis (Fig. 35, left). The overall efficiency was estimated as 15-25% with an initial production rate of one liter per hour of hydrogen was achieved 178 ; further work 179 indicated that while the efficiency of the thermoelectric device is as low as 5%, the overall efficiency of hydrogen production can be raised to 20% by the additional electric power from the thermoelectric. Photoelectro-chemical (PEC) conversion of solar energy for water splitting has also used with thermoelectric devices. Self-biased water splitting was achieved by Jung et al. since the PEC system operated Fig. 35 Schematic of hybrid thermo-chemical water splitting, as described by Ref. 178. Subsystem I is the photochemical cell. Subsystem II is the Fresnel lens and thermoelectric. Subsystem III is the electrolysis system. spontaneously on application of an overpotential generated by a thermoelectric. The system therefore allowed capture of both photons and waste heat and achieved a hydrogen power of 55mW/cm 2 181 . Chen et al. 180 reported a microbial electrolysis cell (MEC) to produce H2 from acetate where an additional voltage was supplied by a thermoelectric to overcome the energy barrier, see Fig. 36. They showed that the thermoelectric micro-converter could convert waste heat energy to electricity, even at relatively low temperature differences of 5°C, and the hydrogen yield was increased from 1.05 to 2.7 mol/mol acetate and the coulombic efficiency increased from 27 to 83%. Rectification in not required for thermoelectrics since a unipolar output is achieved, provided the temperature gradient does not change sign. However, some conditioning of the voltage is necessary to ensure the voltage is at an appropriate output level since output voltages are typically low compared to the other harvesting approaches; see Table 1. Liu et al. have recently reported the coupling of both thermo-electricity and electrocatalysis for hydrogen production via a PbTe -PbS/TiO2 heterojunction. 182 The triboelectric-thermoelectric-photovoltaic water splitting cell of Yang et al. 97 inspired an overview by Andrei et al. 183 on the potential for creating hybrid thermoelectric systems and the reader is referred to this excellent opinion article for additional details on thermoelectric devices for water splitting. Electro-chemical effects in thermoelectric polymers are also attracting attention. Ion conducting materials have large Seebeck coefficients, and an advantage of polymers over inorganic materials is their high ionic conductivity at ambient temperature. As an example, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), namely PEDOT:PSS, has demonstrated a large, but shortterm increase, in Seebeck coefficient and Chang et al. has shown that the duration of the ionic Seebeck enhancement can be improved by controlling whether electro-chemistry occurs at the (PEDOT:PSS) /electrode interface 184 .

Photovoltaic-chemical coupling
Solar energy is an important renewable and efficient clean energy source, and significant effort has been made to couple this form of energy to electro-chemical systems. Since Fujishima et al. first reported photo-electro-chemical water splitting 185 and photoreduction of CO2 186 on TiO2, in 1972 and 1979, respectively, numerous studies have focused on photo-catalytic materials and mechanisms, which have been summarised in a number of detailed reviews on the topic [187][188][189][190][191][192][193][194] . The impact of mechanical loads on the generation, separation, and recombination of photon-induced carriers has also been recently reviewed in the context of piezophoto-tronic effects 195 . As this review has a focus on the coupling of energy harvesting devices to electro-chemical systems, we will overview efforts on the use of photovoltaics as a power source for applications related to water splitting, CO2 reduction and water treatment. Due to the relative maturity of solar cell technologies, power levels can be larger than the µW to mW range that is typical of 'energy harvesting' and can be in the W to MW range 196 , which is typical of 'energy generation'. Recent examples of emerging applications are now described. As already highlighted for the piezo-, pyro-and tribo-electric systems, water splitting is a widely explored application to couple with solar harvesters 185,[197][198][199][200][201][202][203][204][205][206][207] . In terms of the potential configurations for coupling a photovoltaic to an electro-chemical cell, Bonke et al. 197 illustrated three main approaches. This included a wireless, wired and modular photo-electro-chemical (PEC) system, as shown in Fig. 37 to achieve water splitting via Eqns 1 and 2. The developed system employed the simple modular configuration in Fig. 37c, using nickel foam electrodes and a commercial GaInP/GaAs/Ge multi-junction photovoltaic module with a solar-toelectrical power conversion efficiency of 37% under concentrated solar conditions. By optimizing the electrode material, electrode size, electrolyser conditions and using concentrated solar power the solar-energy to fuel-energy conversion efficiency was 22.4%, which was higher than previously reported results that were typically 10-18%. Solar cells have also been coupled to a proton-exchange membrane (PEM) electrolyser to generate hydrogen, with a maximum efficiency of 16.8 % 204, 205, 208 .  Another promising approach is the electro-chemical reduction of CO2 into fuels 186,[209][210][211][212][213] . Schreier et al. 209 used atomic layer deposition of SnO2 on CuO nanowires to produce a catalyst for CO2 reduction to CO, which was then combined with a triple junction GaInP/GaInAs/Ge photovoltaic cell (PV cell). This is shown in Fig. 38, which also shows a bipolar membrane as the separator to allow for operation using a different catholyte and anolyte. Surface modification of the CuO nanowire electrodes with SnO2 provided improved selectivity of the catalyst, and the solar cell was used to drive the electro-chemical reaction between the anode and cathode. Photolysis of CO2 with a peak solar-to-CO free-energy conversion efficiency of 13.4% was achieved in a system that used far more abundant and lower-cost materials, as compared to other approaches that employed noble metals. An additional application is the use of photovoltaic cells for water treatment 214-219 . Wang, et al. 219 have recently reported a hybrid photovoltaic-solar water disinfection system with a dual-axis tracking system to provide drinkable water and renewable electricity. Fig. 39 shows a schematic of the hybrid system, where the use of a V-trough concentrator was found to significantly improve the sterilisation efficiency compared to a nonconcentrating system; the addition of H2O2 to the water also aided disinfection. Two types of bacteria, Salmonella and Escherichia coli, were evaluated and it was demonstrated that they were completely inactivated in 2.5 h and 1.25 h respectively. As with piezo-and tribo-electric systems, photovolatics have also be used for corrosion protection, such as electro-chemical chloride extraction in concrete 220 ; electro-chemical refrigeration 221 has also been explored.

Conclusions and perspectives
This review has covered in detail the progress to date on energy harvesting based on piezoelectric, pyroelectric triboelectric, flexoelectric, thermoelectric and photovoltaic effects that are coupled to electro-chemical systems. Table 1 provides a summary of the harvesting approaches and potential applications which cover a wide range of electro-chemical processes including water splitting, water treatment such as disinfection and degradation of pollutants, corrosion protection, materials synthesis including electro-deposition and oxidation, sensing and electro-chromic systems. The energy sources are varied and include thermal, solar/light, wave/water flow, wind, vibrations and mechanical loads; hybrid systems are also being developed to overcome the intermittent nature of some ambient sources of energy. Since a number of harvesting mechanisms generate AC electrical outputs in response to compressive/tensile loads or heating/cooling cycles some form of rectification is often required to maintain a unipolar operation when the harvester is coupled to cathodes and anodes. Voltage transformation, typically to reduce the high voltages generated by tribo-& piezo-electric (see Table 1), is also required; although if the generated potential is too low to drive a specific electro-chemical reaction there is scope to increase the operating voltage. While the voltages are high (typically 0.02 V to 1.3 kV) to drive the electro-chemical reaction, currents are often low (14.5 µA to 13 mA) which limits the charge and extent of the electrochemical reaction (Eqn. 3); although there is growing interest in potential for scale up 222 . To date, rectification methods applied to electro-chemical systems have been relatively simple, such as simple diode and full wave rectification, while in conventional harvesting applications a range power management circuits have been developed to minimise losses and optimise impedance matching of the harvester to the electrical load; this provides scope for further improvement in the electro-chemical applications 223 . Greater information regarding overall efficiencies of systems would also be of interest. For solar harvesting, the system efficiencies are more widely reported, and the other harvesting approaches would benefit from similar information. Solar is also typically associated with larger harvesting systems. Tribo-electric devices have been typically used as an external charge source to drive an electrochemical reaction since a complete TENG device is typically required to achieve charge generation. However, for pyro-and piezo-electric systems the inherent spontaneous polarisation of the material enables them to be either used as an external thermal or vibration harvesting device or as fine particulates or porous material dispersed in an electrolyte. This is of interest since it enables the high surface areas (and therefore surface charge) associated with nano-sized particulates and architectures. This provides an opportunity to overcome the low electric current/charge limitations in Table 1. The potential to exploit the high surface area of 2D materials, and their ability to also exhibit flexoelectric effects, is also of interest. A closer integration of TENGs with electrolytes may provide new directions for research and triboelectric effects associated with electrical double layers has recently been reported in novel sensing tribo-devices 224 .
There is also potential to combine the active harvesting material with storage, as an example the understanding of the pyro-and piezo-electric properties of metal-organic frameworks (MOFs) is growing 225 , and these materials also have potential for gas storage applications so that combined harvesting for hydrogen production and storage may be feasible. The potential of pyroelectric effects to achieve water splitting by thermal cycling above and below the Curie temperature is a new and exciting prospect for hydrogen generation, but experimental data is needed to validation model predictions and mechanisms. Flexo-electric systems are clearly at an early stage, but their scope for nano-scale systems and ability to exploit membrane effects provides intriguing potential for membrane based nano-generators combined with ion pumping for charge storage applications. There is also scope for using strain gradients and flexoelectric effects at interfaces to influence and enhance photo-flexo-electro-chemical effects and subjecting the materials to mechanical loads or thermal expansion. Ferro-electrets or electro-active polymers could also be explored, and there has been limited work on electro-magnetic harvesting. Hybrid systems that use the DC output of thermoelectric modules are attractive due to their ability to supply overpotentials to electro-chemical reactions and new thermoelectric polymers and nanocomposites also provide new directions 226,227 . There are also additional electro-chemical harvesting mechanisms to couple with the harvesting approaches described here. For example, there are sensitised thermal cells, thermo-galvanic and thermo-electro-chemical cells (thermocells) [228][229][230][231][232][233][234][235] , where the dependence of electrode potential on temperature is used to construct harvesting thermal cycles and this has recently been reviewed 236 . Piezo-galvanic effects have even been observed where applying an asymmetric force to an electrolyte cell produces an electrical response 237 and thermo-magnetic effects for water splitting 238 . Finally a number of new electro-chemical processes could be explored in attempt to exploit the high voltages of the energy harvesting mechanism described in this review; such as material synthesis based on electro-chemical exfoliation 239 . The potential of local energy harvesting to supply bioreactors to produce protein as a food source has recently been described 240 . Finally, while there is a growing number of academic publications, and patents, there is also a need to transfer such ideas and potential applications into commercial activities to fully exploit the potential of the applications described here.

Conflicts of interest
There are no conflicts to declare.