Microscale Ionic Diodes: An Overview

Ionic rectifier membranes or devices generate uni-directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Initial electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers but also provides a comparison to different types of nano- and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction. Graphical Abstract


Introduction to Microscale Ionic Rectifiers
Biomimetic single-pore or multi-channel ion transport processes are of considerable interest [1] and linked fundamentally to ionic rectifier behaviour. Fundamentally, a rectifier converts an alternating current (AC) input signal into a constant polarity direct current (DC) output signal. Rectifiers are based on diodes, which restrict current flow into only one direction [2].  Electronic rectifiers are based on semiconductor or molecular electronic junctions with appropriate doping/structure to control flow of charge carriers at the interface [3]. Ionic rectifiers or diodes have been reported based on a range of different experimental approaches.
There are macroscopic rectifiers based on electrolytic or nanopore array junctions [4] or chemical (switchable or analytical) junctions [5], micro-electrolytic interfaces [6] and microfluidic systems [7], and there are nano-electrolytic [8] as well as nanofluidic rectifier systems [9]. In contrast to electronic rectifiers that are based purely on transport of electrons, ionic rectifiers can be based on a wide range of mobile cation or anion species either with or without selectivity. Ionic rectifiers also link ion transport to electro-osmotic transport and are therefore suitable to uni-directionally "pump" neutral molecules [10,11]. It is interesting to explore the underlying fundamental similarities in diodes and the two minimum conditions for diode behavior to be observed: Condition one: the experimental system has to be asymmetric. A symmetric system would not be able to generate an asymmetric output from a symmetric input signal. Asymmetric nanocones, asymmetric ionomer deposits, asymmetric external electrolyte solutions, asymmetric surface modifications, or other asymmetry inducing components are necessary (one type of asymmetry is sufficient). Asymmetry here refers to both absence of a mirror plane and absence of a center of inversion of the rectifier geometry. Note, that at the molecular scale this situation has important implications on enantio-selectivity of ion transport [12].
Condition two: a compositional and resistivity change has to occur as a function of applied voltage. Under steady state conditions, a diode can be represented (in a very simplified way) by a resistor that is variable as a function of the applied external voltage. For a resistor to change resistance, an internal compositional change is required (e.g. a change in charge carrier distribution, a change in electrolyte concentration distribution, a precipitation reaction, or change in orientation of molecular functional groups within ion channels, etc.).
To better illustrate the second condition, Figure 2 shows a schematic drawing of an ionic diode with the charge transport path hypothetically divided into microscopic resistor components (for solution, membranes, interfaces, etc.; assuming a system under steady state). Blue and grey regions are used purely to indicate asymmetry in materials without reference to any particular mechanism or scale. When changing the polarity of the applied voltage from positive to 5 negative, at least one of these microscopic resistors (schematically representing interfaces, bulk materials, and regions in the device) needs to change to result in a net rectification effect. In a symmetric device, there would be no net change. In reality, all diodes (ionic or electronic [13]) exhibit properties that are associated with more complicated equivalent circuits (not just a sequence of resistors) that include capacitive interfaces and components, which are important when describing the transient or frequency-dependent switching behaviour. At higher frequencies diodes lose their ability to switch and to rectify current signals. This can be interpreted as the compositional change requiring time. The time constant for the diode to switch from inactive to active is an important operational characteristic [14] (vide infra).

Figure 2.
Schematic description of the steady state behaviour of a diode as sequence of resistors with at least one resistor showing voltage/polarity dependent resistance caused by compositional changes. A blue and a grey material are indicated here generally to illustrate asymmetry with the grey material only on one side of the device.
Early studies of rectification effects in electrochemistry [15,16] and in electrophysiology were mainly concerned with biological processes [17,18]. Membranes in neuron cells have unidirectional ion channels [19]. These membranes were mounted between two electrolyte-filled compartments, and the membrane rectification properties were measured using a sinusoidal voltage source. Rectification was shown to decrease reversibly under the influence of increased salt concentrations. In the 1950s, Hodgkin, Huxley and Keynes studied sodium and potassium passage through giant axons against a concentration gradient. Their hypothesis was: when the nerve fibres of these species are stimulated, they undergo rapid changes in permeability, which 6 can allow sodium and potassium ions to move down concentration gradients, while co-ions are effectively blocked [20,21,22]. This leads to propagating neve signals. Biological ion-channels provide a wide range of mechanistic cases with complex systems resulting in the movement of nerve pulses and energy conversion [23].
Biological diodes and pioneering experiments. An example of biological pore diodes is the family of inward-rectifying potassium channels [24] or more specifically OmpF (outer membrane protein F), which have been studied for ionic rectification effects [25]. The inner protein channel (which is part of an external membrane protein channel in Escherichia Coli bacteria) has surface charges and may therefore be considered as an "electrostatic" diode, which can be tuned or even inverted using pH (see Figure 3). Surface charges determine the ability of cations and anions to enter the ion channel and this leads to either depletion (high resistance) or accumulation (low resistance) conditions. Inspired by rectification in electronic junctions between p-and n-type semiconductors, electrolytic systems combined with artificial membranes were investigated in the 1950s. Work by Fuller [26], and particularly by Lovrecek et al. [27] showed that a combination of protonconducting and hydroxide-conducting semi-permeable membranes separating two reservoirs filled with identical electrolyte solution (i.e. water) results in ion current rectification (see 7 Figure 4). Similar to a p-n-junction this bipolar membrane (i.e. the combination of a cationand anion-exchange membrane) creates conditions of "depletion" and of "accumulation" of charge carriers (as voltage dependent compositional changes).   [27]. With forward bias, both cations and anions are driven into the cation exchange membrane and anion-exchange membrane, respectively, which forms the semipermeable membrane. Ions accumulate in this junction region. Thus, in all parts of the system the resistance remains low. Note that this accumulation in forward bias lowers the local Debye length to allow not only salt accumulation but also continuous flow of ion current. In contrast in reverse bias, cations and anions are removed from the junction region and depletion of ions results in a zone of high resistance in the ion-exchange membrane. Both conditions are met: there is (one) asymmetry and (two) a compositional change that occurs to generate the typical diode response. Figure 4(B) shows a current-voltage response with "open" state with positive applied potential and "closed" state with negative applied potential.

8
Over the years, the concept of rectification in electrochemistry has been studied and exploited [29]. The concept of ionic rectifiers and diodes has been developed into ionic circuits [30], ionic transistors [31], ionic amplifiers [32], and a range of potential ionic diode applications.
In the following sections, the various types of ionic rectifiers or diodes are compared, and some possible future applications in electrochemistry and in electroanalysis are discussed.

Nanopore Electrolytic and Nanofluidic Ionic Rectifiers
It is possible to distinguish electrolytic diode devices (based on ionomer junctions) and fluidic devices (essentially open channels with electroosmotic flow). However, at the limit of nanopore devices, this distinction becomes obsolete with device sizes similar to the Debye length in the solution phase. Many studies focus on bio-derived nanopores with molecularly defined geometry [33]. In contrast, artificial solid state nanopores can be machined into wafer materials [34] or developed based on "track etch" membranes [35]. In such membranes, solidstate nanopores provide well-defined nanochannels made from thin synthetic films or membranes by heavy ion track formation followed by etching [36]. Compared with biological nanopores, solid-state nanopores are mechanically robust and durable with a tuneable pore size and geometry (but without the molecularly reproducible geometry as seen in biological systems). Synthetic membranes with artificial and well-defined nanopores (solid-state nanopores) that behave similarly to biological membranes, were first reported by Siwy and coworkers in 2002 [37]. These solid-state nanopores are fabricated on track etched polymers (see Figure 5).
During this track-etch process, an asymmetric pore is readily formed, which can be subsequently altered to different shapes of different diameters and surface charge using chemical means [38,39,40,41]. The ester functional groups normally present in polyester substrates such as polyethylene terephthalate (PET) can easily be converted to carboxylates, which under alkaline pH conditions produce negative surface charges that can assist in the conduction of positively charged species while blocking voltage driven anion transport. The asymmetry in shape of the nanopore, together with the carboxylate surface charges after chemical etching, have been found (by experiment and by theory) responsible for the diodelike behaviour of these nanopores [42]. Microfabricated arrays of longer (>100 µm) nanochannels with an asymmetry of fixed surface charges deposited along the channel walls have been demonstrated to form efficient nanofluidic diodes [43].
Recent studies include for example nanofluidic diodes [44] in which CO2 and N2 are used to "open" and "close" the diode state of a conically shaped nanopore in a PET membrane. CO2 is used to protonate the imidazole group (see Figure 6). The pH is regulated by a buffer mechanism based on CO2 dissolution. Purging with nitrogen can be used to reverse the pH and the diode state. This work is related to olfactory sensory neurons that are specialized for sensing CO2 [45,46] biologically. The switch of the diode is reversible if N2 is bubbled through the nanopore.

11
A new strategy for generating ionic current rectification through larger microscale pores was introduced in 2010 by Yusko et al. [47]. Ion current rectification was coupled to electroosmotically driven flow of liquids of varying viscosity into/out of pores with diameters ranging from 0.01 m to 2.2 µm on borosilicate glass of 150 µm thickness. This method does not require spatially patterned surface charges, ionic gradients, or pores with a dimension on the order of the Debye screening length to generate ion current rectification. Instead, the asymmetry necessary for rectification is achieved by relying on different ion conductivities in liquids of different viscosities at both ends of a cylindrical micropore [45].
Extending the concept of fluidic diodes to the micropore range could be of interest for applications, such as desalination and energy conversion. Today, a variety of ionic diode concepts based on micro-interfaces are available and have been shown to work in larger microhole devices with diameters up to 40 µm, with sometimes better rectification performance than observed for devices based on nanopores [48]. Nanofluidic crystals and close-packed nanoparticle arrays have been developed [49]. Diode phenomena in confined nano-spaces have been exploited for bioanalysis and label-free sensing [50].
An ionic diode that rectifies or allows passage of cations while blocking anions may be called a "cationic diode". A diode that rectifies anion flow instead of cation flow may be called an "anionic diode". These two types of cases are observed in different types of devices. Figure 7 shows typical I-V curves for microfluidic ion channel measurements in organic solvent systems in the presence of electroosmotic flow [51]. For a channel of 700 nm diameter, it was shown that the surface charge could be switched from negative (native PET) to positive (with adsorbed poly-allylamine). Due to the dimensions of the ion channel, it is apparent that ion currents in this case will contain contributions from both anion transport and cation transport, as well as contributions from electroosmosis (flow of neutral electrolyte solution). However, the net process appears to be dominated either by cations (cationic diode) or by anions (anionic diode).
The region with high current can be called the "open" state of the diode, while the region with suppressed currents can be called the "closed" state. To evaluate the rectification ratio of the device (the diode performance), a ratio of ionic currents is taken at opposing voltages of the same magnitude (+/-1 V), and the higher the ratio is, the better the diode performance or rectification ability. The distinction between "cationic diode" and "anionic diode" is very useful as a diagnostic tool and often the switch from one type to the other type has been shown to occur when the surface charge is changed (e.g. when changing the pH of the electrolyte solution). However, it is important to remember that the transport of protons and the transport of hydroxide are chemically equivalent. Therefore, a cationic proton diode and an anionic hydroxide diode cannot be easily distinguished simply from steady state I-V curves and identifying the mechanism in this case would require further, more detailed experiments. Therefore, the I-V curve is not necessarily reliable as a diagnostic mechanistic tool.
In electroanalysis, biological nanopore sensors have been exploited for detection of nucleic acids and for DNA quantification. For artificial solid state nanopores, it has been shown that surface modification and charge are important for electroanalytical detection. For example, with a positively charged ion channel, increasing the DNA concentration in solution may cause adsorption, which is associated with a dramatic shift in the I-V curve from a dominating positive current to a dominating negative [52] (see Figure 8). When working with a glass nanopipette, an overall negative wall charge repels the DNA strands. A poly-L-lysin (PLL) layer needs to first be adsorbed onto the nanopipette to produce a net positive charge, with which the DNA can be attracted and detected. The change in surface charge due to DNA 13 binding leads to switching of the diode effect (see Figure 8). The effect has been demonstrated with multiple consecutive layers of PLL and DNA. In contrast to the case of electronic diodes, specificity between ions is important for the case of ionic diodes. There have been attempts to employ molecular dynamics simulations to mimic the selectivity in biological pores, namely K + /Na + channels in artificial graphene-type structures. Thus, this work is addressing one of the most important challenges in ion separation.
These in silico functionalised nanopores are proposed to be made from graphene sheets that contain pores. These pores then have the potential to be "ionic sieves" with selectivity and, by chemical modification the capability of being either cation or anion selective [53]. The calculated selectivity depended on the terminating group arrangements, for example the carboxyl arrangement being selective between Na + and K + ions. The introduction of 4 carboxylate and 4 carbonyl groups increased the selectivity towards K + [54]. The functionalised pore with 3 carboxylate groups has been proposed to exhibit Na + selectivity [55,56] (see Figure   9). The corresponding asymmetric ion channels would provide ionic diode phenomena. (B) carbon nanochannel modified by carbonyl groups in a membrane studied by MD simulations; (C) graphene nanopores etched and terminated for selectivity towards K + / Na + . Reprinted with permission from [55]. Copyright (2015) Elsevier.

Macro-Electrolytic and Chemical Ionic Rectifiers
Classic work by Lovrecek and Bockris has shown that hydrogel | hydrogel interfaces create macrosocpic rectifying junctions [27]. This effect has been confirmed for different types of ionomer junctions [57] and it is not limited to small or nano-scale devices. Both hydrogel junctions and arrays of nanocones can be employed to prepare ionic diode devices for higher currents. Nevertheless, hydrogels have been employed also for ionic rectification in combination with conical nanopores [58]. The hydrogel has the effect of increasing the diode effect only for "tip coatings" but decreasing the rectification effect for "base coating" and coating on both sides (see Figure 10). With this particular nanopore having an overall negative charge, it was showing rectification effects of cation selectivity without the hydrogel. One 15 intriguing aspect of this research is that improved rectification was only achieved when the hydrogel was introduced at the tip of the cone, and not for the other arrangements (illustrated in Figure 10). "Chemical diodes" are ionic diodes that rely on chemical processes, instead of relying on purely physical phenomena. These chemical processes, such as precipitation are utilized to induce ionic rectification effects. Chemical diodes can be very novel in the way that they have been found in non-water-based liquid electrolytes [59]. Usually, for ionic diode behavior to be observed the electrolyte medium is water-based, where the ionic solutes can dissociate easily and conduct upon flow of a current. In this example ( Figure 11) poly-ethylene-oxide (PEO 400) was used as the electrolyte medium and AgI was the precipitate created at the interface of two compartments. The AgI precipitation caused a "chemical blocking" of the ionic current in the diode.

Microscale Ionic Rectifiers: A Materials Approach
Rectification phenomena have been investigated predominantly with a focus on nanoscale channels. Based on this, it may seem that effects are difficult to achieve when the pore diameter is more than 10 times larger than the Debye screening length. However, rectification effects are also commonly observed for microhole devices [60] and for larger electrolytic junction diodes. Yossifon and coworkers developed microfluidic diodes [61] as well as poly-electrolytebased microfluidic ionic rectifiers [62]. In larger open channels (e.g. in micropipettes) the flow of ions is accompanied by electroosmotic flow and both ion current and electroosmotic transport can be rectified [10,63,64,65]. A hybrid system can be introduced (to suppress electroosmotic flow) by combining a microhole with ionomer materials. on inert microhole substrates that are coated asymmetrically with ionomer materials. Semipermeable ionomer materials, when deposited onto an inert poly-ethylene-terephthalate (PET) substrate with a laser-drilled microhole, were shown to give ionic diode effects [66]. The use of laser-drilled PET substrates for studying liquid | liquid phase boundary processes was introduced by Girault and coworkers [67] as a tool to create stable interfaces, to restrict current flow, and to ensure steady state conditions similar to those observed at microelectrodes [68].
Note, that all liquid | liquid phase boundaries are inherently asymmetric and could be considered as ionic current rectifier systems [69]. However, here the practically relevant case of aqueous solution phase on both sides of the microhole is considered. Figure 12(A) shows an optical micrograph for a single 20 m diameter laser-drilled microhole and for an array of 100 microholes (employed to increase the net current through the rectifier). Figure 12  A single microhole (or several) is drilled in PET films using a laser device, then an ionconducting microporous material is deposited onto one side of the microhole to introduce asymmetry and semi-permeability. Practically, this can be achieved by placing the PET film onto an agarose gel that blocks the back and that allows asymmetric ionomer casting onto the front side only. The microhole, in the absence of any deposited microporous material results in 18 linear Ohmic current-voltage plots consistent with the bulk resistivity of the aqueous electrolyte [70]. When coating both sides equally, for example with the ionomer Nafion [71] (see Figure   13(A)), again ohmic current-voltage behaviour (without rectification) is observed, although with a lower resistance due to a higher concentration of charge carriers in the microhole region.
Only when coated asymmetrically (see the red line in Figure 13(B)) is an ionic diode effect observed. Nafion has fixed sulphonate functional groups in 1 to 4 nm diameter channels and therefore allows semipermeable cation transport. Data in Figure 13(C) demonstrate the cases of a Na + diode and a H + diode. Current-voltage curves (scan rate 0.05 V s -1 ) in aqueous 10 mM HCl comparing an empty microhole, a symmetrically coated microhole, and an asymmetrically coated microhole. (C) Cationic diode effects for aqueous 10 mM HCl and 10 mM NaOH. (D-F) Electrochemical impedance data (frequency range 100 kHz to 1 Hz; amplitude) for a 20 m diameter microhole in PET immersed into 10 mM HCl on both sides compared to data obtained with symmetric and with asymmetric Nafion deposits. Data are shown for (D) the PET microhole without Nafion, (E) the microhole with Nafion applied to both sides, (F) the microhole with Nafion applied asymmetrically to only one side (reproduced with permission [71]). Reprinted with permission from [71]. Copyright (2017) American Chemical Society. aspects of the ionic diode behavior. Figure 13(D) shows a Nyquist semi-circle consistent with the interfacial charging of the PET film and the high frequency resistance of the diode. Only when going to lower frequencies and for asymmetric devices (Figure 13(F)) can a second semicircle be observed linked to the time constant for diode opening/closing.
In order to gain further insight into the diode mechanism, approximate computational models can be employed. Figure 14 shows and illustrates the case of Nafion deposited asymmetrically onto a 20 m diameter microhole and immersed into aqueous NaCl [72].  A more general but simplified analytical model for ionomer coated microholes was developed by Aoki and Chen based on experimental data obtained with an ionic diode based on graphene oxide [73]. Figure 15(A) shows the experimental arrangement with graphene oxide asymmetrically deposited onto the PET substrate. Figure 15   Even very thin films of only 300 nm thickness of PIM-EA-TB (on a PET substrate) still resulted in considerable ionic diode effects [77] and a switch from cationic diode for aqueous HCl to anionic diode for aqueous NaOH (Figure 16(D)). Ionic diode current data from PIM-EA-TB diodes clearly show a different type of current-voltage characteristic with a plateau region (see Figure 16(B)). This is believed to be linked to interfacial polarization (at the PIM-EA-TB | aqueous electrolyte interface; see Figure 16(E) and 16(F)), which occurs in addition to the electrolyte accumulation and depletion within the microhole region.
Similar cases of semi-permeable ion conducting materials leading to ionic diode behavior were observed with metal-organic framework (MOF) materials [78], with cellulosic materials [79], with TiO2 nanosheet films [80], and with biological materials such as M13 bacteriophage aggregate deposits [81]. Figure 17 shows the shape of M13 phage with surface protein defining a positive surface charge when immersed in dilute aqueous acid. When deposited 24 asymmetrically onto a PET support containing a microhole, channels through the deposit are formed from the protein shell of the M13 phage. In acidic solution, protonation of the protein leads to anionic diode behavior (see Figure 17(E)). Other molecules such as indigo carmine have been proposed to bind into the ion channels. A dip in ionic diode performance was observed for 5 M indigo carmine and explained with the adsorption of indigo carmine into channels. Other ionomer materials have been investigated for ionic rectifier effects. Fumasep anion conductor has been shown to give anionic diode phenomena [82] and partially hydrolysed polyacrylonitrile [83] was shown to give a switchable ionic diode with cationic diode behavior in alkaline solution and with anionic ionic diode behavior in acidic solution. Carbon nanofiber based ionic diodes are formed as a result of asymmetric deposition of carbon nanofiber dispersions (in dimethylformamide) to form a nano-mat onto micro-interfaced substrates with varying microhole sizes. Essentially, the nano-mats provide an ion conducting membrane material, while the micro-interface allows ionic current rectification to occur. The carbon nanofiber backbone or pore wall is oxidized to introduce surface charges, which are able to attract or repel mobile ions in solution. Surface oxidised carbon nanofibers allow cationic diode behaviour (see Figure 18 [84]), while amine surface modified carbon nanofibers allow anionic diode behaviour [85]. An interesting case of additional bipolar reactivity is observed particularly for iodide interaction with oxidised carbon nanofibers and localised iodine formation changing the diode behaviour. Diodes made in this way and with additional surface modification are envisaged to be useful in electroanalysis or in ionic diode desalination systems.

Potential for Applications of Ionic Rectifiers in Electrochemistry
Electrochemical Desalination. The separation of salt from water poses a technical challenge that requires different types of solutions, depending on the operational scale (plant size scale to supply cities or survival kits for individuals) and depending on the input/output streams and their impact on the environment [86]. Desalination concepts based on ionic rectifiers have been proposed for example using chemically modified nanopore arrays [87]. Ionic diode phenomena could be employed for example for smaller scale water desalination as a means of replacing energy-intensive reverse osmosis [88]. There have been new types of desalination concepts put forward based on new materials such as polymers of intrinsic microporosity (PIM-EA-TB) [89]. When coupling AC driving voltages and a cationic diode and an anionic diode, a twocompartment system would be sufficient (one compartment where the salt concentration decreases and the second compartment where the salt concentration increases). Figure 19 illustrates a possible device based on only one type of rectifier, based on an anionic diode (and cationic resistors, e.g. Nafion membranes). In this case a multi-chambered system is required.
As drawn, chambers I and VI would retain salt levels and contain the driver electrodes.
Chambers II and V will be desalination compartments and chambers III and IV are salination compartments. A similar system based on 4 chambers and based on Nafion cationic diodes has recently been demonstrated [66]. A two-chamber system would be possible when combining a cationic and an anionic diode into a single membrane. Appling AC currents reduces unwanted electrolysis processes (to avoid energy losses) and therefore eliminates problems with hydrogen and chlorine production. This could also offer benefits in areas such as electrodialysis [90], electro-deionisation [91], and in capacitive deionization [92]. The challenge of adding ion selectivity into the desalination process will result in future devices that pump specific cations or anions. Figure 19. Schematic drawing of a six-chamber desalination process based on two anionic diodes coupled to four cationic resistors. When AC electricity is applied with external driver electrodes, two chambers undergo desalination whereas two other chambers accumulate salt (reproduced with permission [89]). Reprinted with permission from [89]. Copyright (2015) Royal Society of Chemistry.

Electroanalytical sensors based on ionic diodes. Ionic diodes have much in common with
Coulter counters [93,94] and are therefore suitable as amplified ion current detector systems [95]. In particular, nanopore devices are extremely useful as sensors at the single molecule level [96]. A method of DNA base read-out for sequencing DNA could be used as a way of speeding up sequencing, passing aqueous DNA molecules through a nanopore of the same charge has been proven as a quantitative measurement depending on the size and salt concentration of the aqueous solution [97]. A nanopore sequencing technique could also greatly reduce the cost of sequencing [98]. This type of technology is today a commercial reality.
New biosensors made from ionic diode devices [99,100]    Photodiodes and energy conversion with ionic diodes. Ionic mechanisms to generate electricity from sunlight have been investigated [103,104], although the photo-excitation in these systems remains fundamentally an electronic (rather than ionic) process. A Ru(bpy)3 2+based design is illustrated in Figure 22. A sandwich of agarose gels is employed [105].
Illumination with light has been proposed to lead to ion transport and voltage/current output, although efficiencies are low. The desalination process is endergonic to give pure water and enriched salt water as the two main products. The reversal of this process can be employed for energy production (see "blue energy" from river water and seawater [106]). The appeal for energy production from "hidden solar energy" in river water is considerable [107]. Salinity gradient techniques have been proposed for example based on ionic diodes in heterojunctions of mesoporous carbon (~7 nm, negatively charged) and macro-porous alumina (~80 nm, positively charged). Power output has been predicted but only at an early proof-of-principle level. The ion selectivity of the heterojunctions is dominated by the mesoporous carbon (see Figure 23). If the salinity gradient is reversed from mesoporous carbon to macro-porous alumina, then the resistance is reduced to 53%, which supports the idea of the preferential direction for cation transport [108]. Similar to the case of desalination where both DC or AC methods can be developed, for blue energy harvesting AC driven ionic circuits exploiting the rectification effects could be developed to avoid external driver electrode reactions and to make the process continuous. Figure 23. Schematic representation of how the mesoporous carbon and macroporous alumina selectively allow ion transport to produce the diode effect. Reprinted with permission from [106]. Copyright (2012) American Chemical Society.
Another related form of energy conversion could be based on driven chemical processes in which salinity gradients allow production of chemical products (e.g. producing hydrogen and oxygen; compare the conversion of ion gradient energy to chemical energy in the ATPase mechanism in nature [109]). Ion transfer at liquid | liquid interfaces has been employed to trigger redox chemical reactions [110], but also bipolar reactions that occur when electrically conducting materials are embedded into the ionic diode will produce chemical products from redox reactions. There are many more opportunities for energy conversion processes in ionic diode and ionic rectifier systems and for applications that mimic natural membrane reactions.

Conclusions and Outlook
This overview summarises some recent developments based on microscale ionic diodes and it compares various types of ionic diodes, their mechanisms, and possible applications in electrochemical technologies. It has been shown that ionic diode phenomena occur in both nature and in artificial devices. The ion current "rectification effect" can be generated in semipermeable systems based on two prerequisites: (i) asymmetry in the device and (ii) a compositional change linked to the applied external potential.
When using a microhole device, it is possible to employ a "materials approach" to the ionic diode design in which the properties of an ionomer material deposited asymmetrically onto an inert substrate are used to create conditions for high rectification effects. Good ionomer coatings are (i) mechanically robust, (ii) deposited as thin film to give a low ionic resistance, (iii) possess a high charge carrier concentration and mobility, and (iv) do not require extensive 32 and expensive microfabrication compared to solid state single nanopore or track-etched nanopore approaches. In applications based on alternating current (AC) driven processes, the requirement for durability of ionomer materials is high. Fast switching ionic diodes with high rectification ratio will be desirable in order to operate at higher frequencies. Molecular mechanisms for ionic diode switching e.g. valve-like processes within nano-channels could be a possible way towards higher frequencies and higher performance. There are many more opportunities for the development of new materials/architectures for ionic diodes to provide fast switching devices with strong rectification effects. This will allow new desalination technology and energy conversion technology to emerge as well as new electroanalytical sensing devices with high amplification effect or chemoreceptive sense/pump systems [95] to be realised.