Microfluidic electrical energy harvester

Abstract

Present invention discloses a microfluidic energy harvester for converting solar energy into electrical energy. A preferred embodiment of the present microfluidic energy harvester includes a substrate having an embedded central microchannel, electrolyte configured to reside and/or flow in said central microchannel and electrode assembly having one or more pair of electrodes arranged in a series and integrated with said central microchannel from sides ensuring direct contact between said pair for electrodes with said electrolyte while it reside and/or flow in said central microchannel for ensuing electrochemical photovoltaic effect to convert the solar energy into the electrical energy under direct solar illumination by working under regenerative conditions. The microfluidic energy harvester is capable of producing high density power from three different resources, (a) the solar irradiation produces a photovoltaic potential difference between the metal/metal-oxide electrodes, (b) SPR of the metal nanoparticles suspended in the electrolyte amplifies the photovoltaic potential difference under solar irradiation, and (c) the flow of the nanoparticle laden electrolyte produces a streaming potential between the electrodes by converting the mechanical energy into the electrical one near the electrodes. The transparency of the polymer and adequate absorptivity of the metal/metal-oxide electrodes ensured facile absorption of solar irradiation in the microfluidic energy harvester. The flexibility of the MEH can be tuned by adjusting the cross-linking of the PDMS matrix. The multiplicity of the microchannels and electrodes are expected to increase the total amount of energy harvested.

Claims

1. A microfluidic energy harvester for converting solar energy into electrical energy comprising a polymer substrate having an embedded central microchannel with an inlet and an outlet; an electrolyte in said central microchannel; an electrode assembly having at least one pair of electrodes arranged in series in said polymer substrate and perpendicular to said central microchannel on both sides of the central microchannel, ensuring direct contact between said at least one pair of electrodes and said electrolyte in said central microchannel for ensuing electrochemical photovoltaic effect to convert the solar energy into the electrical energy under solar illumination; wherein the electrolyte is configured to flow in the central microchannel through the inlet by using hydrostatic head or micro-pump at a constant flow rate enabling conversion of mechanical energy of the electrolyte flow into streaming potential across the at least one pair of electrodes; and wherein the electrolyte includes a suspension of aqueous 4.0M NaCl solution with 20 vol. % metallic Au (gold) nanoparticle concentration capable of producing surface plasmon resonance (SPR) under direct solar illumination when the electrolyte flow through the central microchannel which enhance potential difference across the at least one pair of electrodes and generates the potential difference equivalent to combined influences from the solar energy, the streaming potential and the surface plasmon resonance resulting amplification of the electrical energy generation.

2. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 1, wherein the at least one pair of electrodes includes a conducting metal electrode and a semiconducting metal-oxide electrode having bandgap to bind the solar energy under direct solar illumination.

3. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 2, wherein the polymer substrate includes a transparent slab of poly-dimethylsiloxane embedding the central microchannel with the inlet and the outlet; wherein said polymer substrate includes a plurality of tributary microchannels arranged in series and perpendicular to the central microchannel on both sides of the central microchannel, wherein each of the tributary microchannels is connected to the central microchannel and includes provision for inserting the conducting metal electrode and the semiconducting metal-oxide electrode enabling the metal and the semiconducting metal-oxide electrodes to be in contact with the electrolyte residing and/or flowing in the central microchannel without the metal and the semiconducting metal-oxide electrodes touching each other.

4. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 3, wherein the semiconducting metal-oxide electrodes include Cu.sub.2O electrodes inserted in the tributary microchannels in one side of the main microchannel and the conducting metal electrodes include Cu electrodes inserted in the tributary microchannels in other side of the central microchannel ensuring the electrodes from both the sides do not touch each other but touch the electrolyte residing or flowing in the central microchannel.

5. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 1, wherein the electrochemical photovoltaic effect generated electrochemical photovoltaic potential is $U_{ECPV}=\frac{nkT}{q}\ln \left(\frac{I_{Ph}}{I_D}+1\right),$ where n is ideality factor, k is Boltzmann constant, T is absolute temperature, q is charge of electron, I.sub.ph is current in presence of light and I.sub.D is the current in the dark.

6. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 1, wherein the streaming potential generated due to the electrolyte flow is $U_{SP}=\frac{128{\mathrm{.Math.}}_r{\mathrm{.Math.}}_0\zeta L{F}_R}{\pi d^4{K}_L},$ where, d is effective diameter of the central microchannel, L is length of the central microchannel, K.sub.L is specific electrical conductivity of the electrolyte, F.sub.R is flow rate of the electrolyte, ε.sub.r is dielectric constant of water, ε.sub.0 is dielectric permittivity of free space, and ζ is zeta-potential of the electrodes.

7. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 1, wherein the enhancement of potential due to the SPR is U.sub.SPR=n.sub.absU.sub.ECPV, where n.sub.abs is fraction of optical absorption increased due to the SPR.

8. The microfluidic energy harvester for converting solar energy into electrical energy as claimed in claim 3, wherein the conversion of the solar energy into the electrical energy due to the electrochemical photovoltaic effect under direct solar illumination by working under regenerative conditions includes generation of electron-hole pairs or excitons under direct solar illumination wherein electrons from valance band of the semiconductor electrodes jumped to conduction band, leaving a hole behind; transferring of the electron from valence band to cations of redox couples present in the electrolyte; simultaneous transferring of the hole of the electron-hole pairs to the metal electrode through external circuit and oxidizing anions of the redox couple in the electrolyte.

Description

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

(1) FIG. 1 show (A) isometric (B) side and (C) front views of a preferred embodiment of present microfluidic energy harvester.

(2) FIG. 2 shows the different cross sectional views of the present microfluidic energy harvester embodiment.

(3) FIG. 3 shows different views of the present microfluidic energy harvester embodiment's constructional elements.

(4) FIG. 4 shows different cross-sectional views of the present microfluidic energy harvester embodiment's constructional elements.

(5) FIG. 5 shows block representation of the present microfluidic energy harvester embodiment.

(6) FIG. 6 show photographic illustration of the present microfluidic energy harvester (A) and (B); variation of voltage generation involving the present microfluidic energy harvester (C); variations in maximum generated voltage (V.sub.gmax), current density (J.sub.gmax) and power density (P.sub.D) for different operating condition involving the present microfluidic energy harvester (D) and (E).

(7) FIG. 7 show optimization of flow rate of the present microfluidic energy harvester for maximum output, with and without AuNPs: change in maximum generated voltage (V.sub.gmax), current density (J.sub.gmax), and power density (P.sub.D) for 4M NaCl solution at different flow rates with 9 electrodes (A)-(C); change in maximum generated voltage (V.sub.gmax), current density (J.sub.gmax), and power density (P.sub.D) for 4M NaCl solution with 20% of AuNPs concentration at different flow rates with 9-pair electrodes (D)-(F).

(8) FIG. 8 show (A) comparison of theoretical and experimental output and (b) efficiency calculation for optimized flow-rate and AuNP concentration in accordance with the present microfluidic energy harvester.

(9) FIG. 9 (A)-(F) show characterizations of AuNPs and Cu—Cu.sub.2O electrodes in accordance with the present microfluidic energy harvester.

(10) FIG. 10 show agglomeration of AuNPs at different concentration of AuNPs in 4M NaCl in accordance with the present microfluidic energy harvester.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

(11) As stated hereinbefore the present invention discloses a novel microfluidic energy harvester (MEH) which is configured to convert solar energy into electrical energy and amplify voltage generated from the solar energy by involving surface plasmon resonance (SPR) and/or streaming potential.

(12) The microfluidic energy harvester of the present invention includes a polymer substrate in which a microchannel is embedded for the flow of electrolyte. The microchannel is integrated with an electrode assembly from the sides. The electrode assembly includes one or more pair of electrodes arranged in a series wherein each pair for electrodes includes a conducting metal and a semiconducting metal-oxide electrode. The metal/semiconducting metal-oxide electrodes combination is chosen in such a manner that they had just adequate bandgap to bind solar energy under direct solar illumination.

(13) In a preferred embodiment, the electrolyte includes a suspension of salt-water and metal nanoparticles and configured reside and/or flow in the central microchannel with the help of hydrostatic head at a constant flow rate. The metal nanoparticles and their size and the shape are selected in such a manner that they are capable of producing SPR under direct solar illumination.

(14) During flow of the electrolyte through the microchannel, the metal/semiconducting metal-oxide electrodes come in direct contact with the electrolyte suspended with nanoparticles. The contact of the electrolyte with the electrodes not only ensure easy flow of current because of the voltage generated by the solar energy and SPR but also the arrangement facilitates conversion of mechanical energy of the electrolyte flow into the streaming potential across the electrodes. Thus, under direct solar illumination, when the electrolyte suspended with metal nanoparticles is flown inside the microchannel of the present microfluidic energy harvester, the electrodes show a potential difference equivalent to the combined influences from the solar energy, SPR, and streaming potential.

(15) Reference is first invited from the accompanying FIGS. 1(A)-(C) and 2 which show a preferred embodiment of the present microfluidic energy harvester and its different cross sectional views respectively. As shown in the referred figures, the polymer substrate 101 which preferably includes a central microchannel 103 embedded inside a transparent slab of poly-dimethylsiloxane. One ends of the central microchannel 103 is used as inlet and other end of the central microchannel 103 is used as outlet. A series of tributary microchannels 102 are also fabricated in perpendicular to the central microchannel 103 on both the sides. Each of the tributary microchannels 102 is connected to the central microchannel 103 and includes provision for inserting the metal and/or metal-oxide electrode.

(16) Reference next invited from the accompanying FIG. 3, which shows different views of the present microfluidic energy harvester embodiment's constructional elements. The microfluidic energy harvester's substrate with integrated electrodes is demonstrated in the FIG. 3 in which multiple metal/semiconducting metal-oxide electrodes 104 are inserted within the tributary microchannels 102 from the sides. The accompanying FIG. 4 shows the cross-sectional image of the microfluidic energy harvester's substrate with integrated electrodes as shown in the FIG. 3.

(17) The electrodes 104 of the present microfluidic energy harvester preferably include a selective combination of metal and semiconducting metal-oxide electrodes having adequate band-gap to bind the solar energy under direct solar irradiation. In a preferred embodiment, the semiconducting metal-oxide electrodes include Cu.sub.2O electrode whereas, the metal electrodes include Cu electrodes.

(18) The arrangement of the electrodes include disposition of the Cu.sub.2O electrodes inserted in the tributary microchannels 102 in one side of the main microchannel 103 and cooperative disposition of the Cu electrodes inserted in the tributary microchannels 102 in other side of the central microchannel 103 ensuring electrodes from both the sides do not touch each other and each of the electrodes touches flow of the electrolyte through the main microchannel 103. The arrangement is described in detail in the FIG. 4.

(19) Reference is next invited from the accompanying FIG. 5 which shows a block representation of the present microfluidic energy harvester where the electrolyte is flow through the central microchannel of effective diameter of d and length L. The electrodes are inserted from the sides of the channel. The streaming potential for such an energy harvester is

(20) $\begin{array}{cc}{U}_{SP}=\frac{128{\mathrm{.Math.}}_r{\mathrm{.Math.}}_0\zeta L{F}_R}{\pi d^4{K}_L},& \left(1\right)\end{array}$
where, K.sub.L is specific electrical conductivity of the electrolyte, F.sub.R is the flow rate of the electrolyte, ε.sub.r is dielectric constant of water, ε.sub.0 is dielectric permittivity of free space, and ζ is zeta-potential of the electrodes.

(21) The electrochemical photovoltaic potential is calculated employing the expression,

(22) $\begin{array}{cc}{U}_{ECPV}=\frac{nkT}{q}\ln \left(\frac{I_{Ph}}{I_D}+1\right),& \left(2\right)\end{array}$
where n is ideality factor, k is Boltzmann constant, T is absolute temperature, q is charge of electron, I.sub.ph is current in presence of light and I.sub.D is the current in the dark.

(23) The enhancement of potential due to SPR is evaluated from the expression U.sub.SPR=n.sub.absU.sub.ECPV . . . (3), where n.sub.abs is fraction of optical absorption increased due to SPR.

(24) Combining all the aforementioned potentials as expressed in the Eqn, (1), (2) and (3), the total voltage which could be generated by the microfluidic energy harvester will be, U.sub.T.sup.th=U.sub.ECPV+U.sub.SP+U.sub.SPR.

(25) Fill-factor (FF) and power efficiency ( ) can also be evaluated for the present microfluidic energy harvester by employing the expressions,

(26) $FF=\left(\frac{V_m\times J_m}{V_{\mathrm{OC}}\times J_{\mathrm{SC}}}\right)\times 100\%\mathrm{and}$$\eta =\frac{P_{\mathrm{out}}}{P_{\mathrm{in}}}=\frac{V_{\mathrm{OC}}\times I_{SC}\times FF}{P_{\mathrm{in}}}=\frac{P_{\mathrm{Dout}}}{P_{Din}},$
where V.sub.OC, J.sub.OC, V.sub.m, J.sub.m, P.sub.out, P.sub.in, P.sub.Dout, P.sub.Din denote open-circuit voltage, open-circuit current density, voltage corresponds to maximum power, current density corresponds to maximum power, output power, input power, output power density, and input power density, respectively.

(27) In a preferred embodiment of the present invention, the metal nanoparticles of the electrolyte which preferably include Au nanoparticles (AuNPs) is synthesized by sodium borohydride (NaBH.sub.4) reduction method of auric chloride (HAuCl.sub.4.3H.sub.2O) salt [refer to, Martin et al., Langmuir 2010, 26, 7410-7417]. Initially, a 50 mM aqueous solution of AuCl.sup.4− (solution I) is mixed with hydrochloric acid (HCl) in equal molar ratio. Following this, a 50 mM of BH.sup.4− solution (solution II) is prepared by dissolving NaBH.sub.4 in aqueous 1 mM sodium hydroxide (NaOH) in equal molar ratio. Thereafter, 100 μl of solution I is mixed with water to obtain a solution where the concentration of gold ions is 0.5 mM. Then, 300 μI solution II is mixed with solution I with continuous stirring. The AuNPs formed as the change in color took place from yellow to orange and then to red. The AuNP suspensions are mixed with 4.0 M NaCl solution (v/v) at different proportion to prepare the electrolyte suspensions having different amount of nanoparticles.

(28) Reference is next invited from the accompanying FIG. 6(A)-(E), wherein the FIGS. 6(A) and 6(B) shows photographic illustration of the present microfluidic energy harvester and the FIG. 6(C)-6(E) shows the combined effect of electrochemical photovoltaics, streaming potential and surface plasmon resonance in the present microfluidic energy harvester in respect to convert solar energy into electrical energy.

(29) The electrolyte used in the present microfluidic energy harvester is a 4.0 M NaCl (234 gm/L) aqueous solution with suspended Au nanoparticle which is pumped into the inlet of the central microchannel (103) with the help of hydrostatic head or micro-pumps. During the flow through the central microchannel, the electrolyte comes in contact with the semiconducting and metal electrodes (104) inside the microfluidic energy harvester and generates voltage under the exposure of the sunlight.

(30) The combination of semiconductor (e.g. Cu.sub.2O) electrodes, electrolyte (NaCl solution) and metal counter electrodes (e.g. Cu) within the microfluidic energy harvester constitute electrochemical photovoltaic (ECPV) units across the microfluidic energy harvester. These ECPV units generate electron-hole pairs or excitons under direct solar illumination in which electrons from valance band of the semiconductor electrodes jump to the conduction band, leaving a hole behind. The created electron is then transferred to the cations of the redox couples present in the electrolyte (e.g. Na+). Consequently, the holes reached the Cu counter electrode through the external circuit and oxidized the anions (Cl.sup.−) of the redox couple in the electrolyte. Since there is no net consumption of material in the electrolyte due to this process, the ECPV units work under regenerative conditions.

(31) The voltage generation inside the present microfluidic energy harvester can be categorized in three parts depending on their origins. The plots as shown in the FIG. 6(C) illustrates the voltage generation for a 9-pair electrodes based microfluidic energy harvester due to (i) only electrochemical photovoltaic effect where a stationary electrolyte which include 4.0M NaCl solution is exposed to the sunlight, (ii) electrochemical photovoltaic effect along with streaming potential for a flowrate of F.sub.R=0.01 ml/min of the electrolyte which include 4.0M aqueous NaCl solution through the central microchannel under the exposure of the sunlight and (iii) combined effect of electrochemical photovoltaic effect, streaming potential, and surface plasmon resonance for a flowrate F.sub.R=0.01 ml/min of the electrolyte which include a suspension of 4.0M aqueous NaCl solution and 20% (v/v) metallic nanoparticle (AuNP) concentration through the central microchannel.

(32) From the plot (C), it would be apparent that there is a productive effect of flow and SPR in energy harvesting utilizing the sunlight. Further experiments are performed to measure the change in maximum generated voltage (V.sub.gmax) and current density (J.sub.gmax) with different concentration (C.sub.Au) of AuNPs for the 9-pair electrodes system with a flow rate F.sub.R=0.01 ml/min and effective light intensity of 1.1 kLux. The results of V.sub.gmax and J.sub.gmax were plotted in FIG. 6(D) while the FIG. 6(E) shows the change in power density (P.sub.D) for this device with different concentration of AuNPs (C.sub.Au). It is clear from plots under 6(D) and 6(E), that 20% (v/v) AuNPs is the optimum concentration for SPR to get the best performance, under the aforementioned conditions. After optimization of AuNP concentration, experiments are performed to optimize the flow rate (F.sub.R) of the electrolyte. For this purpose, the maximum generated voltage (V.sub.gmax), current density (J.sub.gmax), and power density (P.sub.D) are measured at different light intensities for different flow rates with and without AuNPs as shown in the accompanying FIGS. 7(A)-7(F).

(33) The plots in FIGS. 7(A)-7(C) and 7(D)-7(F) show the maximum generated voltage (V.sub.gmax), current density (J.sub.gmax) and power density (P.sub.D) of the present microfluidic energy harvester for different flow rates (F.sub.R) of 4.0M NaCl solution at different intensities of the light without and with 20% (v/v) AuNPs, respectively. In the situations with and without AuNPs, the optimum flow rate is found to be F.sub.R˜0.025 ml/min.

(34) The plots in FIGS. 7(A)-7(C) and 7(D)-7(F) clearly show that the generated voltage (V.sub.gmax), current density (J.sub.gmax) and power density (P.sub.D) of the present microfluidic energy harvester are significantly enhanced due to the introduction of the AuNPs in the electrolyte solution. The figures also suggest that at higher flow rates of the electrolyte, the generated voltage is less because the partial destruction of electrical double layer (EDL) due to the flow near the electrodes. Moreover, mobility of electrons and holes should be too high to support the recombination of excitons. However, in case of higher flow rates the retention time of the redox species near the semiconductor surface abruptly decreases leading to a higher recombination in the system as the excitons do not get enough time to perform the redox reaction which in turn reduce the maximum generated voltage.

(35) Reference is next invited from the accompanying FIGS. 8(A) and 8(B) which show comparison between theoretical and experimental studies and efficiencies of the present microfluidic energy harvester with different electrolyte viz. (I) stationary NaCl solution, (II) NaCl solution with flow, and (III) AuNPs in NaCl suspension flowing at optimized conditions (i.e. F.sub.R=0.025 ml/min and C.sub.Au=20% (v/v)), respectively.

(36) For theoretical calculation of the streaming potential, it is assumed that width of the central microchannel (L) is 0.05 m, effective diameter of the central microchannel (d) is 10.sup.−4 m, the specific electrical conductivity (K.sub.L) of the electrolyte is 0.015 S m.sup.−1, the flow rate of the electrolyte (F.sub.R) is (4.17×10.sup.−10) m.sup.3 s.sup.−1, dielectric constant of water (εE.sub.r) is 80, dielectric permittivity of free space (ε.sub.0) is 8.854×10.sup.−12 F m.sup.−1, and the zeta-potential of the electrodes (ζ) are 0.03 V. Plugging the values into the eqn. 1 the theoretical value for the U.sub.SP can be obtain equal to 12.04 mV. The experimental value of the U.sub.SP=15 mV, is obtained by simply flowing the electrolyte inside the channel and then measuring the potential difference between the electrodes through a digital multi-meter.

(37) Similarly, for theoretical calculation of the electrochemical photovoltaic potential, it is assumed that, the ideality factor (n) is 1, Boltzmann constant (k) is 1.38×10.sup.−23 m.sup.2 kg s.sup.−2 K.sup.−1, the absolute temperature (T) is 300 K, charge of electron (q) is 1.6×10.sup.−9 C, the current in presence of light (I.sub.ph) is 5.4 A, and the current in the dark (I.sub.D) is 0.3 A. Plugging the values into the eqn 2, the theoretical value for the U.sub.ECPV can be obtained equal to 76.5 mV. The experimental value of the U.sub.ECPV=73 mV, is obtained by illuminating the stationary salt-water electrolyte and then measuring the potential difference between the electrodes through a digital multi-meter.

(38) The theoretical value of the SPR potential is obtained as U.sub.SPR=30.6 mV. In order to check the effect of SPR, an absorption spectroscopy is performed keeping the volume of the AuNPs constant, which provided the amount of absorption in the electrolyte suspension with different AuNP loading. The average absorption is calculated for a full spectral range and is found to be about 40% when the AuNP loading in the electrolyte is about 20% (v/v). The experimental value of the U.sub.SPR=27 mV, is obtained by illuminating the stationary AuNP loaded salt-water electrolyte, then measuring the potential difference between the electrodes through a digital multi-meter, and thereafter subtracting the experimental value of the U.sub.ECPV from the multi-meter reading.

(39) Combining all the aforementioned theoretical potentials, the total theoretical value of the voltage, which could be generated by the MEH as, U.sub.T.sup.th=U.sub.ECPV+U.sub.SP+U.sub.SPR=119.1 mV. Combining all the aforementioned experimental potentials, the total experimental value of the voltage, which could be generated by the MEH as, U.sub.T.sup.Ex=U.sub.ECPV+U.sub.SP+U.sub.SPR=115 mV. The FIG. 8(A) shows a typical comparison between theoretical values evaluated and experimental values measured for the reported MEH.

(40) The fill-factor (FF) and power efficiency ( ) are also evaluated for the MEH employing the expressions,

(41) $\mathrm{FF}=\left(\frac{V_m\times J_m}{V_{\mathrm{OC}}\times J_{\mathrm{SC}}}\right)\times 100\%\mathrm{and}$$\eta =\frac{P_{\mathrm{out}}}{P_{\mathrm{in}}}=\frac{V_{\mathrm{OC}}\times I_{SC}\times FF}{P_{\mathrm{in}}}=\frac{P_{\mathrm{Dout}}}{P_{Din}},$
where V.sub.OC, J.sub.OC, V.sub.m, J.sub.m, P.sub.out, P.sub.in, P.sub.Dout, P.sub.Din denote open-circuit voltage, open-circuit current density, voltage corresponds to maximum power, current density corresponds to maximum power, output power, input power, output power density, and input power density, respectively. The P.sub.Din is measured using a lux meter where 1 lux is 1 lumen/m.sup.2. The luminous efficacy of a 100 W incandescent bulb is ˜15 lumen/W, which is used to calculate the optical flux ˜0.067 W/m.sup.2. The light source is placed at a distance of 35 cm, which led to an intensity of ˜1100 lux=73.7 W/m.sup.2. Thus, the input power density is P.sub.Din=73.7 W/m.sup.2. The output power density, P.sub.Dout V.sub.m×J.sub.m where, V.sub.m and J.sub.m is the voltage and current density corresponds to maximum power which can be achieved from the J-V curve. FIG. 8(C) shows the typical variations in the power efficiency ( ) for the MEH (I) salt-water electrolyte is stationary, (II) when the salt-water electrolyte is flown at an optimum flow rate, and (III) when the salt-water electrolyte with suspended AuNPs (20% v/v) is flown at an optimum flow rate.

(42) The following Table 1 shows the typical theoretical and experimental values evaluated during these calculations for the MEHs (I)-(III).

(43) TABLE-US-00001 TABLE I V.sub.oc J.sub.SC V.sub.m J.sub.m System (mV) (mA/cm2) (mV) (mA/cm2) FF % η % I. Stationary Salt- 68.6 0.91 42.4 0.6 41.06 ~0.35 water Electrolyte II. Flowing Salt- 79.4 1.17 48 0.86 44.29 ~0.56 water Electrolyte III. Flowing 106.3 1.78 83.7 1.49 65.91 ~1.71 Electrolyte with AuNPs

(44) Reference is now invited from the accompanying FIGS. 9 and 10 which show the typical characterizations of the AuNPs and Cu—Cu.sub.2O electrodes. The FIGS. 9(A) and 9(B) show the TEM image and SAED pattern of the synthesized AuNPs and FIG. 9(C) shows the EDX of AuNPs. The FIG. 9(D) shows the FESEM image of grown Cu.sub.2O film.

(45) The cross-sectional FESEM image [FIG. 9(E)] of the electrode shows the thickness of the Cu.sub.2O layer and FIG. 9 (F) shows the EDX of the Cu.sub.2O film. The TEM images in the FIGS. 10(A)-10(C) show the AuNP in 4M NaCl at the proportions of, 1:4 (v/v), 1:1 (v/v), and 4:1 (v/v), respectively.