Method for enhancing current throughput in an electrochemical system

Abstract

An electrochemical system with reduced limiting-current behavior is disclosed. The electrochemical system is useful for fuel cells and bio-sensors. In part, the invention relates a method of reducing or eliminating limiting-current behavior in the operation electrochemical systems, in particular those with ion-selective membrane or electrochemical electrodes, by spatially reducing the convection near the membrane or the electrode. The invention further relates to electrochemical systems in which micropores, microarrays or pillar arrays are used to reduce convection in comparison to conventional systems without microarrays, micropores or pillar arrays.

Claims

1. An electrochemical system comprising a substrate, a plurality of microchannels fabricated onto said substrate, a plurality of electrodes in fluid communication with said microchannels and distal to the nanojunction, and a nanojunction connecting at least two of said microchannels wherein at least a part of said substrate contains a microarray, micropore array or a pillar array proximal to the nanojunction.

2. The system of claim 1, wherein when said microchannels contain a fluid and a current is applied, convection currents near said nanojunction are reduced when compared to convection currents near a nanojunction of same type electrochemical system without a pillar array or micropore array.

3. The electrochemical system of claim 1, further comprising an ion-selective membrane between said microchannels.

4. The electrochemical system of claim 1, comprising a Nafion membrane.

5. The electrochemical system of claim 1, comprising a pillar array wherein the average diameter of pillar is between about 0.1-1000 m.

6. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 1-1000 m.

7. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 1-500 m.

8. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 1-250 m.

9. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 5-100 m.

10. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 5-50 m.

11. The electrochemical system of claim 5, comprising a pillar array wherein the average pillar height is between about 5-25 m.

12. The electrochemical system of claim 1, comprising a pillar array wherein the average diameter of pillar is between about 1-100 m.

13. The electrochemical system of claim 1, comprising a pillar array wherein the average diameter of pillar is between about 5-50 m.

14. The electrochemical system of claim 1, comprising a pillar array wherein the average diameter of pillar is between about 7-15 m.

15. The electrochemical system of claim 1, comprising a pore array wherein the average diameter of the pores is between about 0.1-500 m.

16. The electrochemical system of claim 15, comprising a pore array wherein the pore depth is between about 1-250 m.

17. The electrochemical system of claim 15, comprising a pore array wherein the pore depth is between about 2-200 m.

18. The electrochemical system of claim 15, comprising a pore array wherein the pore depth is between about 5-100 m.

19. The electrochemical system of claim 1, comprising a pore array wherein the average diameter of the pores is between about 1-50 m.

20. The electrochemical system of claim 1, comprising a pore array wherein the average diameter of the pores is between about 2-25 m.

21. The electrochemical system of claim 1, wherein said substrate is polydimethylsiloxane.

22. The electrochemical system of claim 1, wherein said pillar array is polydimethylsiloxane.

23. A method of reducing limiting current behavior across an ion-selective membrane in an electrochemical system comprising providing a fabricated non-planar structure on at least one side of the membrane wherein convection near said membrane is reduced in comparison to having planar structures on both sides of the membrane.

24. The method according to claim 23, wherein said non-planar structure is a micro-array.

25. The method according to claim 23, wherein said non-planar structure is a pillar array.

26. The method according to claim 23, wherein said non-planar structure is a pillar micro-array.

27. The method according to claim 23, wherein said non-planar structure has a microchannel.

28. The method according to claim 27, wherein said microchannel is curved.

29. The method according to claim 27, wherein said microchannel has a parabolic shape.

30. The method according to claim 29, wherein said microchannel has a locus near a nanochannel.

31. An electrochemical system comprising a substrate, a plurality of fluidic channels fabricated on said substrate, wherein at least two separate fluidic channels are connected by a junction, a plurality of electrodes in fluid communication with said fluidic channels, wherein at least one part of said substrate contains a pillar array.

32. The electrochemical system of claim 31, wherein said fluidic channels are microchannels.

33. The electrochemical system of claim 31, wherein said junction is a nanojunction.

34. The electrochemical system of claim 31, wherein said junction comprises an ion-selective membrane.

35. An electrochemical system comprising an electrode seated in a reservoir, a pillar array placed over and in proximity to, but separate from, said electrode, an electrolyte, a substrate, and a support, wherein said substrate comprises one or more microchannels.

36. The electrochemical system of claim 35 wherein the pillar array is suspended over the electrode.

37. The electrochemical system of claim 36, comprising a pillar array wherein the average diameter of pillar is between about 0.1-1000 m.

38. The electrochemical system of claim 36, comprising a pillar array wherein the average diameter of pillar is between about 1-100 m.

39. The electrochemical system of claim 36, comprising a pillar array wherein the average diameter of pillar is between about 5-50 m.

40. The electrochemical system of claim 36, comprising a pillar array wherein the average diameter of pillar is between about 7-15 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

(2) FIG. 1A: Schematic diagram of microchannel/nanojunction hybrid system with pillars. PDMS substrate having microchannels and vertical nanoporous junctions bonded with a glass support.

(3) FIG. 1B: Sequences of fluorescent images of depletion boundary near nanojunctions for nonpillar and pillar array (pillar-all) system. 30V (left)-15V (right) was applied at the anodic side microchannel.

(4) FIG. 2A: Plots for current drop through nanojunction as a function of time (fixed voltage) in non-pillar system.

(5) FIG. 2B: Plots for current drop through nanojunction as a function of time (fixed voltage) in pillar array system showing significant difference in current level.

(6) FIG. 3A: I-V plot for with/without pillars showing Ohmic/limiting/over-limiting current. 1 mM phosphate buffer solution was used. Also the current behavior in the separated pillar system (50 m and 100 m) was shown.

(7) FIG. 3B: Sequence of fluorescent images of growing depletion zone for non-pillar, 50 m separated pillar and pillar array system.

(8) FIG. 4: I-V plot for pillar all and non-pillar systems.

(9) FIG. 5A: Pillar array, narrow-channel and non-pillar systems showing the unstable current behavior in a non-pillar system. I-V curve of narrowed channel had moderate slope compared to pillar and non-pillar system.

(10) FIG. 5B: Snapshots of fluorescent images of depletion zone growth.

(11) FIG. 6A: Schematic diagram of the micro-hole structure that can hold the depletion zone on top of the electrode.

(12) FIG. 6B: Fabricated micro-hole structure on top of the electrodes.

(13) FIG. 6C: Comparison of the current behavior at 5V applying voltage with and without the micro-hole structure.

(14) FIGS. 7A-7C: Difference between planar vs. microelectrode. (A) In the planar electrode, depletion zone is formed in a planar fashion, and the diffusion of charge carriers toward the electrode is inefficient. As a result, the concentration overpotential is significant. (B) and (C), in the line-patterned or dot-patterned microelectrodes, the diffusional pattern is either 2D or 3D, which is more efficient. Depletion zone is limited in length, which leads to smaller concentration overpotentials.

(15) FIGS. 8A and 8B: Exploded schematic view of micro-pore electrode and snapshots of planar (bare) (B) and micro-pore electrodes (A).

(16) FIGS. 9A and 9B: Close-up (5 magnification) snapshots of micro-pore electrode for pore diameter=20 m, depth=15 m and pore center-to-center distance=(A) 30 m & (B) 40 m.

(17) FIG. 10: Schematic of electrode concentration polarization for (a) micro-pore and (b) planar electrodes.

(18) FIG. 11: Normalized steady state current plot with different concentrations of electrolyte. Micropore electrodes specifications are pore diameter=20 m, center-to-center spacing=30 m and depth (a)=15 m denoted as microelectrode 1 and (b)=30 m denoted as microelectrode 2. The top line of the graph corresponds to microelectrode 2, the middle line corresponds to microelectrode 1 and the bottom line corresponds to the planar electrode.

(19) FIG. 12: Normalized average fluorescence intensity plots for different electrodes at a height of z=50 m w.r.t. Au electrode surface. The small insert presents a typical confocal image with size 10241024 pixels and the small red square (region of interest, ROI, is a 270 by 270 m square) is the post-processing windows for average intensity (y-axis) plot. Fluorescent images was recorded by Zeiss LSM 510 laser scanning confocal microscope. All images were captured at steady state current condition (t>600 s) and normalized respect to its initial conditions at t=0. Working electrolyte for depletion observation is 0.3 mM Potassium (IV) Hexachloroiridate in 0.1 M Potassium Nitride with Alexa Fluor 488 fluorescence in volume ratio of 1/1000. Micropore electrodes specifications are pore diameter=20 m, center-to-center spacing=30 m and depth (a)=15 m denoted as microelectrode 1 and (b)=30 m denoted as microelectrode 2.

(20) FIGS. 13A-13C: Micro-pore Electrode Arrays (MEA) (A) Traditional Model: in the diffusive model of MEA, enhanced performance of MEA is understood as a result of dissimilar diffusive transport (1D vs. 3D) at the mouth of the shallow recessed electrode hole. (B) our Model: In reality, there will always be an electrokinetic flow (surface-driven, amplified within the depletion zone), which could lead to a fast, circulating flow within the structure. This could significantly enhance the mass transport, and therefore limit the propagation of the electrode polarization. (C) idealized electrode silencer structure: If a fluid restricting structure is placed (without blocking active electrode surface), one could still achieve the same effect of arresting the propagation of depletion zone (concentration polarization), as in MEAs ((A) & (B)). The key difference is that the active surface area of electrode is maintained, therefore achieving the highest net current flowing through the system.

(21) FIG. 14: Simple electrochemical system schematic and snapshot of testing vehicles. Microhole electrode was created by patterning (via photolithography process) SU8 micro-pore structure over Au planar electrode. The test vehicle was completed by covering the electrode pyrex glass with polydimethysiloxane (PDMS) microfluidic channel with a uniform depth of 100 m. The connecting channel is 500 m in width and the electrode chamber is dia.=2.2 mm.

(22) FIG. 15: Chronoamperotmetry responses of different electrode systems for redox reduction process of Potassium (IV) Hexachloroiridate (IV) in 0.1 M Potassium Nitrite. Three different systems were compared: Planar electrode: bare electrode of 1 mm radius, Microhole electrode 1: micro-pore electrodes (1 mm total radius) with r=10 m, d=30 m and depth=15 m, Microhole electrode 3: micro-pore electrodes with r=10 m, d=40 m and depth=15 m. Upon the application of reduction potential, the system current flow decayed as the electrolysis proceeds to deplete the electroactive species near the electrode surface. The net current measurement show that, even though micropore electrode 1 &3 has significantly smaller active electrode area than planar electrode, the current reaches the steady states faster and maintains the same or higher net current, in the long run (not shown in the diagram).

(23) FIG. 16: Two-photon microscopy imaging of fluorescence intensity over a period of 50 seconds upon the application of external applied voltage of 3V. Inserts are the snapshot fluorescent images at t=0 s (top right first three images) and t=50 s (top left last three images). The fluorescence intensity was recorded at a distance of z=50 um w.r.t. Au electrode surface. The two-photon microscopy scanning spot is approx. 2 um in diameter. Small red square on the images represents the post-processing windows for fluorescence intensity analysis. Fluorescence was depleted at around 30 s at planar electrode as it was governed by linear diffusion mechanism, meaning that the depletion layer was expanded without any suppression. Conversely, fluorescence at both micro-pore electrodes was able to sustain over the entire experiment, indicating that the depletion layer has not yet expanded beyond the plane of interest and therefore it could be concluded that depletion layer was well confined. Alexa Fluor 488 fluorescent dye with concentration of 10 uL/mL was used with 1 mM DSP buffer electrolyte. Planar electrode: bare electrode with r=1 mm, Micro-pore electrode 1: r=10 m, spacing=40 m and thickness=15 m, and Micro-pore electrode 3: r=Sum, spacing=30 m and thickness=15 m.

(24) FIGS. 17A and B: (A) Standard electrochemical three electrode measuring system and its electrical connection. This measuring configuration was used in single electrode investigation and (B) Standard two-electrode measuring system connection. This measuring configuration was used in system integration investigations.

DETAILED DESCRIPTION OF THE INVENTION

(25) As used herein, a or an are taken together to mean one or more unless otherwise specified.

(26) In part, the invention relates to a method of reducing or eliminating limiting-current behavior in the operation of electrochemical systems, in particular those with ion-selective membrane or electrochemical electrodes, by spatially reducing the convection near the membrane or electrode. An electrochemical reaction is one that takes place at an interface between the electrodes and the electrolyte. The invention relates to electrochemical systems in which non-planar structures such as micropore arrays, microarrays or pillar arrays are used to reduce convection in comparison to conventional systems without such structures. The reduction in convection results in a reduction in the size or extent of the zone of ion depletion. The terms zone of ion depletion, ion-depleted region, zone of depletion, and depletion zone are used interchangeably herein. The zone of ion depletion is the area in proximity to the electrode surface or the membrane where the concentration of ions is depleted or the region of interest where the electro-active ions have been consumed. In the case of the pillar array located on the anodic side of a nanojunction connecting parallel microchannels such as described in the examples section, the zone of depletion can, for example, be confined to the distance set by the distance between the nanojunction and the pillars. In the case of a micropore array set over an electrode as described in the examples below, the zone of depletion can be confined within the micropore structure.

(27) While the local convection could be promoting over-limiting current, strong circulatory vortices may be defining the extent of ion-depleted region. Without being bound by any particular theory, it is postulated that limiting the size of circulatory flow would lead to a smaller ion depletion region, and the effective reduction or elimination of limiting current behavior. As such, the invention is generally applicable to engineering strategies for electrodes and fuel cells in order to reduce current limitation posed by concentration overpotentials and concentration changes near such electrochemical systems. This concept can be demonstrated by performing experiments in microfluidic systems. In the microfluidic channel, one can control the degree of convection allowed by fabricating designed structures such as pillar arrays or micropore arrays in order to localize the convective flow in the middle of microchannels, such as shown in FIG. 1A. Since the strong convective flow is generated inside the ion depletion zone, the pillars can be patterned at the anodic side of microchannel.

(28) Ion-selective or ion-exchange membrane refers to membranes that allow the passage of the ions, while substantially maintaining the integrity between the contents separated by the membrane. The particular material selected for membrane can be changed for the electrode materials selected and the desired rate of exchange of ions. Examples of ion-selective membranes include high aspect ratio ion-selective membranes made from polytetrafluroethylenes, perfluorosulfonates, polyphosphazenes, polybenzimidazoles, poly-zirconia, polyethyleneimine-poly(acrylic acid), poly(ethylene oxide)-poly(acrylic acid) and non-fluorinated hydrocarbon polymers. A preferred membrane is selected from Nafion, CMI 7000, Membranes International C/R, CMB and CCG-F from Ameridia, AM-1, AM-3 and AM-X and PC-200D.

(29) In one embodiment, a device may contain a plurality of high aspect ratio, ion selective membranes. In one embodiment a plurality of high-aspect ratio ion selective membranes can be fabricated on a chip. In one embodiment fabrication of a plurality of high-aspect ratio ion selective membranes may be done using multi-blade fabrication. In one embodiment multi-blade fabrication can be used for commercialization of a device containing a self-sealed membrane. In one embodiment a self-sealed membrane refers to the high aspect ratio ion selective membrane. In one embodiment self-sealed means that after infiltration, or passage or filling of the trench or the gap, or the scratch made by the blade with a liquid polymer solution, unbending the chip and solidifying the polymer causes a self-sealing process of the scratch or the gap or the trench by the polymer.

(30) In one embodiment, multi-blade fabrication can be used for massive parallelization of the membrane or the device fabrication process. In one embodiment, multi-blade fabrication can be used to make a plurality of membranes in parallel. In one embodiment, multi-blade fabrication renders the fabrication process fast. In one embodiment, multi-blade fabrication renders the device a low-cost device. In one embodiment multi-blade fabrication is combined with multi-syringe or multi-dispenser system that enables parallel injection of liquid polymer to all trenches or cuts made by the multiple blades. In one embodiment, the multi-blade fabrication technique is part of an automated fabrication technique, in which all steps of forming the high aspect ratio ion selective membranes are automated, and all steps are performed in parallel on many channels or on many device parts or on many devices. In one embodiment, such automation enables mass production of devices, low cost, high yield and reproducibility of device properties. In one embodiment parallel multi-blade fabrication facilitates quality control and reliability measurements to be done on selected devices. In one embodiment, multi-blade fabrication and/or automation of the process is achieved using computers, computer programs, robotics or a combination thereof. In one embodiment, the number of high aspect ratio ion selective membranes produced is equal to the number of channels described herein above. In one embodiment the number of high aspect ratio ion selective membranes produced is greater than the number of channels described herein above. In one embodiment, the number of high aspect ratio ion selective membranes produced is smaller than the number of channels described herein above. In one embodiment, the number of high aspect ratio ion selective membranes produced is more than 5, or, in other embodiments, more than 10, 96, 100, 384, 1,000, 1,536, 10,000, 100,000 or 1,000,000 channels, or in any number desired to suit a particular purpose.

(31) A pillar array has a plurality of pillars. In some embodiments, the pillar diameter is between about 0.1-1000 m, preferably between about 1-100 m, more preferably between about 5-50 m, more preferably between about 5-25 m, more preferably between about 7-15 m. In some embodiments, the average pillar size was between about 1-1000 m. In some embodiments, the average pillar size was between about 1-500 m. In some embodiments, the average pillar size was between about 1-250 m. In some embodiments, the average pillar size was between about 5-100 m. In some embodiments, the average pillar size was between about 5-50 m. In some embodiments, the average pillar size was between about 5-25 m. In some embodiments the average distance between the pillars in the pillar array are between about 0.1-500 m. In some embodiments the average distance between the pillars in the pillar array are between about 1-100 m. In some embodiments the average distance between the pillars in the pillar array are between about 5-50 m. In some embodiments the average distance between the pillars in the pillar array are between about 5-25 m.

(32) A micropore array has a plurality of micropores. In some embodiments, the pore diameter is between about 0.1-500 m. In certain embodiments, the pore diameter is between about 10-100 m. In some embodiments, the pore diameter is between about 1-50 m. In additional aspects, the pore diameter is between about 10-40 m. In some embodiments, the pore diameter is between about 2-25 m. In some embodiments, the pore diameter is between about 5-20 m. In some embodiments, the pore depth is between about 0.1-500 m. In some embodiments, the pore depth is between about 1-250 m. In some embodiments, the pore depth is between about 2-200 m. In some embodiments, the pore depth is between about 5-100 m. In additional aspects, the pore depth is between about 10-100 m. In some embodiments the average distance between the pores in the micropore array are between about 1-500 m. In some embodiments, the distance between the pores in the micropore array are between about 2-250 m. In some embodiments, the distance between the pores in the micropore array are between about 2-100 m. In some embodiments, the distance between the pores in the micropore array are between about 5-75 m. In some embodiments, the distance between the pores in the micropore array are between about 10-50 m. In additional embodiments, the distance between the pores is between about 20-50 m.

(33) In some embodiments, more than one set of micropores/pillar arrays are present, where the gap between the sets of arrays is at least about 50% greater than the average distance between the individual pillars/pores in the array. In some embodiments, the gap between the arrays is between about 10-1000 m. In some embodiments, the gap between the arrays is between about 25-500 m. In some embodiments, the gap between the arrays is between about 25-200 m.

(34) In one embodiment, the width of the microchannel is between about 0.1-500 m, and in one embodiment, the width of the channel is between about 5-200 m. In some embodiments, the width of the channel is between about 20-1200 m. In some embodiments the width of the channel is between about 50 and 500 m. In some embodiments the width of the channel is between about 50 and 250 m.

(35) In some embodiments, the depth of the microchannel is between about 0.5-200 m, and in some embodiments, the depth of the channel is between about 5-150 m. In some embodiments, the depth of the channel is between about 5-100 m. In some embodiments, the depth of the channel is between about 5-50 m. In some embodiments, the depth of the channel is between about 5-25 m. In some embodiments, the depth of the channel is between about 10-25 m. In some embodiments, the depth of the channel is between about 10-20 m.

(36) In some embodiments, the ion-selective membrane has a width of between about 0.01-100 m, and in some embodiments, the width of the ion-selective membrane is between about 1-10 m. In some embodiments, the ion-selective membrane has a width of between about 100-500 nm. In some embodiments, the ion-selective membrane has a depth of between about 0.01-3000 m, and in some embodiments, the depth of the ion-selective membrane is between about 10-500 m and in some embodiments, the depth of the ion-selective membrane is between about 100-1000 m. In some embodiments, the ion-selective membrane has a depth of between about 500-1100 m. In some embodiments, the membrane is cation selective. In some embodiments, the membrane is anion selective.

(37) In some embodiments, the fluidic chip comprises a silicon polymer, preferably, polydimethylsiloxane (PDMS). In some embodiments, the fluidic chip has a hydrophobic surface. In some embodiments, the fluidic chip comprises an elastomeric polymer. The elastomeric polymer can be a silicone elastomeric polymer. The elastomeric polymer can be solidified by curing. In some embodiments, the elastomeric polymer can be treated with high intensity oxygen or air plasma to permit bonding to the compatible polymeric or non-polymeric media. The polymeric and non-polymeric media can be glass, silicon, silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, glassy carbon, or epoxy polymers.

(38) Construction of the microchannels may be accomplished according to, or based upon any method known in the art, for example, as described in Z. N. Yu, P. Deshpande, W. Wu, J. Wang and S. Y. Chou, Appl. Phys. Lett., 77 (7), 927 (2000); S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl. Phys. Lett., 67 (21), 3114 (1995); Stephen Y. Chou, Peter R. Krauss and Preston J. Renstrom, Science, 272, 85 (1996), U.S. Patent Publication 20090242406, and U.S. Pat. No. 5,772,905, which are incorporated herein by reference. In one embodiment, the microchannels can be formed by imprint lithography, interference lithography, self-assembled copolymer pattern transfer, spin coating, electron beam lithography, focused ion beam milling, photolithography, reactive ion-etching, wet-etching, plasma-enhanced chemical vapor deposition, electron beam evaporation, sputter deposition, stamping, molding scanning probe techniques and combinations thereof. In some embodiments, the methods for preparation of the devices of this invention may comprise or be modifications of Astorga-Wells J., et al, Analytical Chemistry, 75: 5207-5212 (2003); or Joensson, M. et al, Proceedings of the MicroTAS 2006 Symposium, Tokyo Japan, Vol. 1, pp. 606-608. Alternatively, other conventional methods can be used to form the microchannels. In one embodiment, the microchannels are formed as described in J. Han, H. G. Craighead, J. Vac. Sci. Technol., A 17, 2142-2147 (1999) and J. Han, H. G. Craighead, Science, 288, 1026-1029 (2000), hereby incorporated fully herein by reference.

(39) In one embodiment, a series of reactive ion etchings are conducted, after which nano- or micro-channels are patterned with standard lithography tools. In one embodiment, the etchings are conducted with a particular geometry, which, in another embodiment, determines the interface between the microchannels, and/or nanochannels. In one embodiment, etchings, which create the microchannels, are performed parallel to the plane in which etchings for the nanochannels are created. In another embodiment, additional etching, such as, for example, and in one embodiment, KOH etching is used, to produce additional structures in the device, such as, for example, for creating loading holes.

(40) In one embodiment, an interface region is constructed which connects the channels on the chip, for example two microchannels. In one embodiment, diffraction gradient lithography (DGL) is used to form a gradient interface between the channels of this invention, where desired. In one embodiment, the gradient interface region may regulate flow through the concentrator, or in another embodiment, regulate the space charge layer formed in the microchannel, which, in another embodiment, may be reflected in the strength of electric field, or in another embodiment, the voltage needed to generate the space charge layer in the microchannel. In some embodiments, the ion-selective membrane is positioned at such an interface.

(41) In another embodiment, the device may contain at least two pairs of electrodes, each providing an electric field in different directions. In one embodiment, field contacts can be used to independently modulate the direction and amplitudes of the electric fields to, in one embodiment, orient the space charge layer, or a combination thereof.

(42) In some examples, the electrochemical systems and methods described herein can be used in devices and applications which are associated with electrode polarization, such as concentration polarization. Non-limiting examples of such devices and applications include solid oxide fuel cells (SOFCs), lithium ion batteries, biosensors and dielectric spectroscopy-based sensors.

(43) By limiting the lateral dimension of the local circulating flow near a membrane or electrode, a reduction in the limiting current behavior can be achieved resulting in enhanced ion transport. In some embodiments, the current behavior can be limited by addition of the structural feature described herein without modifying the chemistry or physics of the electrode or membrane operation.

EXAMPLES

Example 1: Electrochemical System Composed of Two Parallel Microchannels Connected by Nanochannels (or a Nanoporous Membrane)

(44) Compared to the classical membrane geometry that blocked a straight channel, fluid flow in one embodiment is not blocked by the membrane but rather flows along it. The polydimethylsiloxane (PDMS) microfluidic chips were fabricated with perm-selective nanojunctions using the previously published methods. (S. J. Kim, and J. Han, Anal. Chem. 80, 3507 (2008)). The anodic and cathodic microchannel had the dimension of about 100 m width15 m depth. Pillar arrays were fabricated at the anodic side of microchannel and they had the size of about 10 m diameter (about 15 m height). The gap between each pillar was about 10 m. Separated pillar systems had either about 100 m or about 50 m distance between two groups of pillar arrays. A Nafion (sulfonated tetrafluoroethylene based fluoropolymer-copolymer) nanojunction was infiltrated at the center of pillar structures.

(45) 1 mM of potassium phosphate dibasic solution (pH=8.4) was used as main buffer solution and it contained 1 g/ml of FITC for fluorescent tracking. All the flow patterns were imaged with an inverted fluorescence microscope (Olympus, IX-51) and a CCD camera (SensiCam, Cooke corp.). Sequences of images were analyzed by Image Pro Plus 5.0 (Media Cybernetics Inc.). A DC power supply (Keithley 236 source measure unit (SMU)) was used to apply electrical potential to each reservoir through a voltage divider. As shown in FIG. 1A, Ag/AgCl electrodes (A-M Systems, Inc.) were placed into each reservoir for proper electrical connections. At the same time, the ionic current through nanojunctions was also measured by Keithley 236 SMU. The sequence of current was analyzed by commercial software, LabView 8.2 (National Instrument). For the ionic current with constant applied voltages, the same voltages were applied at the anodic sides, while the cathodic sides were electrically grounded. The currents were measured at every 0.1 second. For measuring Ohmic/limiting/over-limiting current, voltages at the anodic sides were ramped up from 0V to 80V at the rate of 0.2V/30 sec.

(46) From fluorescent images in FIG. 1B, the fluid vortex generated in the pillar-array system was shown to be mostly confined between pillars, while the vortex (related to the size of depletion zone) continuously expanded their size in non-pillar system. In the pillar-array system where laterally limited vortex was observed, the following effects were observed; 1) the concentration gradient was maintained at higher level so that diffusive transport of ion is promoted, while it quickly reaches near zero with rapidly expanding depletion region in a nonpillar system; and 2) shorter diffusion length (lower overall electrical resistance) results in higher electric field, which is the driving force of drift ion transport, inside the depletion zone. These attributions were confirmed by measuring ionic currents through the nanojunctions as functions of both time and applied voltage as shown in FIGS. 2A, 2B and FIG. 3A. In the first case (FIG. 2A), the constant voltages were suddenly applied at both anodic side microchannels, while the cathodic microchannels were electrically grounded. At low voltages (e.g., 1 V in FIGS. 2A and 2B), time-independent currents were observed in both systems because the ion current did not reach the limiting current regime yet at such low voltages. While the non-pillar channel shows precipitous drop in ionic current due to the formation of ion depletion zone (i.e., continuously increasing electrical resistance) at higher applied voltages as shown in FIG. 2A, ion currents in the pillar-all system recover from the initial drop and increased up to 70% of the original level as shown in FIG. 2B, demonstrating an effective elimination of current suppression caused by depletion. The initial current drop in pillar-array system was due to the formation of ion depletion zone within the interpillar distance.

Example 2: Limiting/Over-Limiting Current Behavior

(47) The current-voltage characteristics across the nanojunction were measured to characterize their detailed limiting current behaviors. In addition to the pillar and non-pillar systems shown in FIG. 1B, two additional systems were fabricated with a defined gap between pillar array and the nanojunction (50 m and 100 m) as shown in the microscopic insets of FIG. 3A. The I-V traces for all systems largely overlapped in the sub-limiting current regime (less than 2V). The fluorescent images less than 2V did not have any changes in all systems as shown in FIG. 3B. However, critical differences were observed between pillar and non-pillar systems, in that (1) the limiting current behavior was almost eliminated in the pillar-array system, (2) overall current level was higher in any kind of pillar system than in the non-pillar system. Confined vortex leads to much shorter diffusion lengths in any pillar systems along over-limiting current regimes as confirmed by the fluorescent images (FIG. 3B), leading to higher diffusive and drift ion-flux. These effects were seen more clearly in the two separated-pillar systems. Due to the space between the pillar arrays, the depletion zone initially formed just as in non-pillar systems. At this moment, the limiting current behavior was observed as usual. This can also be seen by the fact that the I-V traces for distanced-pillar and non-pillar systems are overlapping in the limiting current regime (at 5 nA in the voltage range of 2V-4.5V). As the voltage increased above the limiting regime, however, the lateral size of the fluid vortex are essentially determined by the distance between the pillar array and the nanojunction, since the depletion boundary is pinned at the pillar array and cannot expand any more as shown in FIG. 3B. In such conditions, the resulting (over-limiting) current levels were much higher than that of non-pillar system. For example, the lengths of depletion boundary were approximately 250 m (non-pillar system) and 60 m (pillar-all system) at 17V (FIG. 3B) and their current values were 25 nA (non-pillar system) and 100 nA (pillar-array system) (FIG. 3A). Importantly, the current level of each system was highly correlated with the distance between pillar structures as observed in two separated pillar systems. Since the amplified electrokinetic vortical flows were dominant inside the depletion zone, shorter gap (50 m) enforced relatively higher electric field and higher concentration gradient than wider gap (100 m). As a result of this, the over-limiting current of 50 m-separated pillar system had increased faster than 100 m separated system. The inset plot in FIG. 3A shows the distinct difference of over-limiting current behaviors between each system (wider x- and y-axis range).

(48) From the experimental results of FIG. 3A, one can conclude that the extent of over-limiting current behavior is related to the lateral size of the depletion zone, which in turn is determined by the convection profile. In a pillar system, depletion region does not expand much more than the distance from the nanojunction to the nearest pillar, leading to a near-complete elimination of limiting current behavior. In the non-pillar and 50 m/100 m separated pillar systems, depletion region can start to expand when the bias reaches the critical point (2V, near the onset of the limiting current). In the 50 m and 100 m separated pillar systems, depletion region was ultimately confined to the distance set by the distance between the nanojunction and the pillars. The shorter this distance is, the lower overall resistance in the overlimiting current regime. This is reflected in the different slopes in FIG. 3A. However, doubled distance did not exactly impose doubled resistance (slope of 1-V curve) because the concentration distribution inside the depletion zone could be altered by addition parameters such as the degree of convection and an EOF through the microchannel, etc.

Example 3: Confining Convection Using Narrow Microchannel

(49) In another scenario, we narrowed down the microchannel width of the non-pillar system only around the nanojunction as shown in inset of FIG. 5A. The narrow region has only 10 m width (200 m length). Due to high flow resistance, the current through the groove decreased such that a direct comparison to pillar system could not be made. Thus, in the Ohmic current region<2V), I-V curve of narrowed channel had moderate slope compared to pillar and non-pillar system as shown in FIG. 5A. However, the groove was able to limit the convection and hold the depletion zone into the narrowed region as shown in FIG. 5B. The density of electric field inside the narrowed region is geometrically 10 times larger than outside the region. Furthermore, the electric field inside the depletion zone can be amplified more than 30 times greater than one outside the depletion zone. These combined effects gave an extremely high gradient of electric field at the narrowed region interfaces and would play a significant role of holding the depletion zone inside the narrowed region. With aforementioned consideration, the limiting current behavior almost disappeared and the current level was much higher than non-pillar system as shown in FIG. 5A.

Example 4: Enhancing the Stability of Electrochemical Membrane Systems

(50) The pinned depletion boundary also significantly affects the stabilities of measured current behavior (current fluctuation) which is the deterministic factor for the performance of the ICP related applications. As shown in FIG. 4, the current plot of non-pillar system had large standard deviation, while most of the values were overlapped in the case of pillar-all system. In a non-pillar system the depletion region is constantly expanding, changing the system resistance, concentration profile and (electroosmotic) flow dynamically. While this could be the source of the instability and run-to-run variability, the confinement of convection (and depletion zone) in pillar systems eliminates dynamical changes in resistance/current. Therefore, the defined depletion zone would induce more stable performance of the membrane system.

Example 5: Limiting Convection Near Electrodes by Microstructures

(51) Single Electrode Systems

(52) Enhancing the performance of an electrochemical system can be achieved by optimizing various factors such as electrode/membrane characteristics, used electrolyte and catalysts. For example, during microfluidic fuel cell operation, a concentration boundary layer (lCP layer) that depends on channel geometry and flow rate will develop in the channel, starting at the leading edge of the electrode. Thus, the maximum current density of the fuel cell is determined by the rate of the convective/diffusive mass transport from the bulk to the surface of the electrode under the assumption of rapid electrochemical reaction. For limiting case, the oxidant/fuel concentration is zero at the entire surface of the electrode, meaning high electrical resistance which lowers the total performance of fuel cell. In order to enhancing diffusive transport, one can use line electrodes, to which the diffusive ion transport occurs in two directions (2D diffusion-drift), or use planar electrodes (1D diffusion-drift). In addition, point electrodes (3D diffusion-drift) can also be used. Planar type electrochemical membranes and electrodes generally increase the overall (membrane/electrode) surface area.

(53) In one example, micro-pore structures on top of planar electrode with non-conducting materials were fabricated, as shown in FIG. 6A. Simple photoresist polymer such as SU8 (MicroChem Inc.) can be used for fabrication. This electrode system was similar to point electrodes system. As shown in FIG. 6A, the micro-pore structure, which has the dimension of about 5-100 m depth and about 5-20 m diameter, can hold the depletion zone thickness down to distance determined by the thickness (height) of the pore, by similar mechanism to the narrow microchannel case. FIG. 6B shows the fabrication of the micro-hole structure on top of the electrode. In some examples, the micro-holes are about 20 m in diameter, about 20 m in distance between each hole and about 50 m in height. With this pattern on the electrodes, the current behaviors at 5V constantly applying voltage were measured by Keithley 6514 electrometer as shown in FIG. 6C. With the structure, the initial dropped current, which was caused by initial formation of depletion zone, recovered quickly, while the current dropped and fluctuated without the structure. This result showed that the current (power density) in the system would be greatly enhanced because the high electrical resistance due to the depletion zone could be confined within the micro-pore structure. This strategy can be applicable both to normal electrodes and perm-selective membranes (such as electrolytes in fuel cell).

(54) In another embodiment of this invention, non-conducting micro-pore structures were fabricated on top of a planar electrode, as shown in FIG. 8. Commercial photoresist polymer such as SU8 (MicroChem Inc.) with standard lithography process can be used for micro-pore structure fabrication. FIG. 9A-9C shows the micro-pore electrode systems with a pore diameter of 20 m and center-to-center spacing of 30 m and 40 m. This electrode system was similar to a point electrode system in an array formation. As shown in FIG. 10, the micro-pore structure which has a range of size from 10-100 m for depth, 5-20 m for pore diameter and 20-50 m for pore spacing can suppress the growth of ion depletion (or also known as electrode concentration polarization) and contain this depletion thickness relatively much thinner than planar electrode (electrode without structure). The degree of suppression is strongly dependent on the thickness (height) and spacing of the pore structure. The quantitative characterization of steady-state current response for planar and two micro-pore electrodes is presented in FIG. 11. The planar electrode was out-performed by the micro-pore electrodes as shown in FIG. 11 and these results were in good agreement with our earlier hypothesis. For a direct visualization, confocal imaging of fluorescence intensity for different electrodes was recorded and illustrated in FIG. 12. The fluorescence intensity was correlated to the concentration of electro-active species (i.e. ions) for the required electrical current flow in an electrochemical system. The fluorescent images were captured at a height of 50 m with respect to the electrode surface after 600 s upon the application of voltage for a steady state current flow. In FIG. 12, it was clearly shown that for planar electrode without structure, the fluorescence was depleted to half of its initial concentration at steady state current indicating that the ion depletion thickness was grown beyond 50 m thick. However, with micro-pore structure the fluorescence concentration remained showing that the ion depletion was successfully suppressed, and hence resulted in a higher current response as shown in FIG. 11.

(55) Overall, these sets of experimental observations and prior studies suggest an emerging picture: Electrochemical efficiency of any electrode (or ion-selective membranes) would critically depend on the spatial extent of the depletion region, which forms a high-resistance barrier to the system.

(56) Adding a structural feature (which may be a partially blocking structure (as in FIGS. 13A and B)) would affect the local electrokinetic fluid flow could bring about significant reduction in concentration overpotential (suppression of ion depletion thickness) and enhancement in membrane/electrode performance. In some embodiments, to further boost the performance, some embodiments could be a suspended structural feature (FIG. 13C) without blocking the active electrode surfaces as in micro-pore systems. This could lead to a significant boost in electrode efficiency in various electrochemical energy devices, by addressing the key limitation of the microelectrode arrays (MEA) application to energy applications. The net result of the proposed structure in FIG. 13C would be an additional mechanism to arrest the propagation of depletion region, which will lead to lower concentration overpotential and higher current density.

(57) System Integration

(58) In one example for a system-based performance evaluation, a complete electrochemical system was set-up as shown in FIG. 14. Micro-pore electrodes were fabricated using photolithography process as earlier mentioned and the polymer-based micro-lengthscale channel network structure was fabricated with standard polydimethylsiloxane (PDMS) process. Micro-pore electrode was seated in a cylindrical reservoir located at each side of the system-of-interest (i.e. a microscale connecting channel in FIG. 14). Essentially, it is two disk-shaped micro-pore electrodes connected by a microfluidic channel filled with electrolyte, used for the impedance characterization. In FIGS. 15 & 16, we compared three cases; Planar electrode with no structural features, Microelectrode 1 with a denser array of micro-pores (dia.=10 m, spacing=30 m & depth=15 m), Microelectrode 3 with an array of micro-pores (dia.=10 m, spacing=40 m & depth=15 m). In all three cases, the original size of circular electrodes were the same, but in microelectrodes 1 & 3 an insulating film (SU-8) covers the majority of the electrode surface, creating a micro-pore array systems with relatively high density (low spacing/dia. values). However, the active (open) area of electrode were diminished (to 46% for electrode 1 and 24% for electrode 3, respectively, of the planar electrode) from that of planar electrode system. When we measured the current in the systems in response to a DC bias, we found that the later, quasi-steady-state net current values are quite similar, and at one time point the Microelectrode 3 carries the largest current (FIG. 15). This result can also be correlated to the propagation of diffusion layer (depletion zone) near the electrode, which were directly measured using the two photon microscopy systems (FIG. 16), where it was shown that the depletion region (monitored by dye tracer) were seen to grow more than 50 m in planar electrode within a minute from the biasing, but not in micro-pore electrode systems.

(59) All chemicals were purchased from Sigma Aldrich and used as received unless otherwise noted.

(60) 0.1 M Potassium Nitride (KNO.sub.3) supporting electrolyte was prepared by dissolving 5.02 g KNO.sub.3 salt in 50 mL de-ionized (DI) water. While the stock solutions 10 mM Potassium (IV) Hexachloroiridate (K.sub.2IrCl.sub.6) in 0.1 M Potassium Nitride (KNO.sub.3) were prepared by dissolving 483.1 mg K.sub.2IrCl.sub.6 in 100 mL 0.1 M KNO.sub.3. Several different dilute K.sub.2IrCl.sub.6 (0.1, 0.3, 0.5, 1, 3, 5 mM) solutions were prepared by diluting 10 mM K.sub.2IrCl.sub.6 with 0.1 M KNO.sub.3 at appropriate ratio.

(61) Cyclic Voltammetry, and Chronoamperometry measurements were performed using a Versastat 3 Potentiostat (V3 Studio Software, Princeton Applied Research) connected with a standard three-electrode cell for single electrode study, see FIG. 1. While a standard two-electrode measuring configuration was used in system integration study, see FIG. 2.

(62) The three measuring electrodes are working electrode, counter electrode and reference electrode (R. S. Rodgers, Stalking the Wild Potentiostat, Today's Chemist at Work, June, 1995, p. 30. (V4#6)). Four different working electrodes were put on test and their surface areas were 3.1416, 1.39, and 0.74 mm.sup.2 for planar electrode, microhole electrode 1 & 2 and microhole electrode 3 respectively. [Note: Planar electrode: bare electrode of 1 mm radius, Microhole electrode 1: micro-pore electrodes (1 mm total radius) with r=10 m, d=30 m and depth=15 m, Microhole electrode 2: micro-pore electrodes (1 mm total radius) with r=10 m, d=30 m and depth=30 m and Microhole electrode 3: micro-pore electrodes with r=10 m, d=40 m and depth=15 m.]

(63) A platinum electrode with a large surface area (2.00.2 mm in diameter and 7 cm long) was employed as the counter electrode while the reference was an Ag/AgCl reference electrode (in 3 M NaCl/Saturated AgCl filling solution).

(64) Cyclic voltametry (CV) measurements were recorded between 0.3 and 1 V/Ag/AgCl at several scan rates 10, 50, 100, 500, 1000, 6000 mVs.sup.1, while all chronoamperometry (CA) measurements were recorded over a time period of 600 s for a potential step from +0.9 to 0.3 V/Ag/AgCl. The steady current values reported in the text were an average current response of the last 200 s of each CA test. All experiments were carried out in quiescent solutions unless otherwise stated.

(65) While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.