SYSTEM AND METHOD FOR GENERATION OF GASES

Abstract

Provided are electrochemical cells and methods for generating hydrogen gas and oxygen gas.

Claims

1.-43. (canceled)

44. An electrochemical system for generating hydrogen gas and/or oxygen gas, the system comprising: at least one stack of two or more electrochemical thermally activated chemical (ETAC) cells, each cell configured for holding an aqueous solution and comprising at least one electrode assembly, a control unit configured to operate the two or more cells in each stack in a continuous hydrogen or oxygen generation mode, wherein the continuous mode comprises two or more operation cycles, each independently comprising alternating non-parallel hydrogen gas and oxygen gas generation periods.

45. The system according to claim 44, the system comprising: at least one stack of two or more ETAC cells, each of said at least one stack being configured for holding an aqueous solution and comprising at least one electrode assembly, each having a cathode electrode and an anode electrode, said cathode electrode being configured to affect reduction of water in said aqueous solution in response to an applied electrical bias, to thereby generate hydrogen gas.

46. The system according to claim 44, comprising a heat source or a heat exchanger.

47. The system according to claim 44, wherein the electrode assembly is provided in a form of a roll in said cells.

48. The system according to claim 44, wherein the control unit is further configured to operate the two or more cells in accordance with a predetermined operational pattern, wherein the operational pattern provides for each cell in the two or more cells an output in a form of at least one of mode selector and operational parameter selector.

49. The system according to claim 44, wherein the control unit is further configured to operate the two or more cells in accordance with a predetermined operational pattern, wherein the operational pattern provides an output comprising at least one of (i) applied bias, (ii) timing for operation, and (iii) duration of operation of each of the two or more cells.

50. The system according to claim 49, wherein the operational pattern further provides a temperature value for each of the two or more cells.

51. A method of generating hydrogen gas and/or oxygen gas, the method comprising: in a system comprising at least one stack of two or more electrochemical thermally activated chemical (E-TAC) cells, each cell containing an aqueous solution, and comprising an electrode assembly having a cathode electrode and an anode electrode and at least two of said two or more cells are non-partitioned cells, wherein the method is operable in a continuous mode comprising two or more operation cycles, such that the continuous mode comprises two or more operation cycles, each independently comprising alternating non-parallel generation of hydrogen gas and oxygen gas generation periods.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0195] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0196] FIG. 1 is a schematic representation of a control unit of the system described herein comprising an input data, output data, a memory and a processor.

[0197] FIGS. 2A to 2F are schematic representations of exemplary embodiments of a system described herein (FIGS. 2A and 2B) and various components of the system; E-TAC (FIG. 2C) and electrode assembly (FIG. 2D to 2F)

[0198] FIG. 3 is a graph showing the low energy consumption of the E-TAC system disclosed herein.

DETAILED DESCRIPTION OF EMBODIMENTS

Non-Limiting Examples

Example 1—Electric Power Consumption

[0199] This experiment was aimed at measuring the energy consumption of the E-TAC system.

[0200] The experiment included a sequence of four E-TAC cycles. During each cycle hydrogen is produced electrochemically while consuming electrical energy supplied by the potentiostat, as presented in FIG. 3.

[0201] During four E-TAC cycles 43.4 ml of pure hydrogen was produced while consuming only 154.3 mWh (equivalent to 39.5 kWh/kg (3.55 kWh/Nm.sup.3)). The electrical power consumption is the product of the applied voltage and current. The hydrogen produced is calculated according to the charge transferred during each cycle and to the hydrogen electrochemical reaction (HER):

4H.sub.2O+4e.sup.−.fwdarw.2H.sub.2+4OH.sup.−

[0202] This experiment demonstrate the low energy (by electric power) consumption of a E-TAC based electrolysis system.

Example 2—Heat Balance of the E-TAC Process

[0203] The table below summarizes the thermodynamic properties of the E-TAC process (Macdonald & Challingsworth, n.d.; Silverman, 1981):

TABLE-US-00001 ΔG (kJ/ ΔH (kJ/ mol.sub.Hydrogen) mol.sub.Hydrogen) [voltage [voltage Reaction (V)] (V)] Step 1: 2Ni(OH).sub.2 .fwdarw. 274.7 301.1 electro- 2NiOOH + H.sub.2 [1.42] [1.56] chemical hydrogen evolution Step 2: 2NiOOH + H.sub.2O .fwdarw. −37.6 −15.3 chemical 2Ni(OH).sub.2 + ½ O.sub.2 oxygen evolution Total: water H.sub.2O .fwdarw. H.sub.2 ½ O.sub.2 237.2 285.8 splitting [1.23] [1.48]

[0204] According to the thermodynamic data presented in table, the electrochemical hydrogen evolution reaction is non-spontaneous, and absorbs heat from the environment for cell voltage below 1.56 V. On the other hand, the chemical oxygen evolution reaction is spontaneous and exothermic, releasing heat to its environment. Thus (considering only the reaction thermodynamics), for the low-temperature hydrogen evolution phase, the cell temperature will decrease with operation if the cell voltage does not surpass 1.56 V, and for the high-temperature oxygen evolution phase, the cell temperature will increase due to the exothermic reaction.

Step 1: Electrochemical Hydrogen Production

[0205] The total energy requirement for the hydrogen generation phase is given by ΔH=ΔG+TΔS, where ΔH is the enthalpy of reaction, ΔG is the Gibbs free energy or electricity requirement, and TΔS=ΔQin is the heat requirement at a constant temperature T. The heat requirement equal to TAS may be supplied electrically, in which case the cell operation is adiabatic. Therefore, adiabatic conditions for the hydrogen generation phase are achieved at Vcell=1.56V. Below 1.56V the system is endothermic, absorbing heat from the environment, and effectively cooling the system. To keep the cell at room temperature, heat must be supplied according to:

[00005]$\Delta Q=\Delta H-P_{\mathrm{electricity}}=\left(1.56-V_{\mathrm{cell}}\right)\left(V\right)\times 2\left(\frac{\mathrm{mol}{e}^{-}}{\mathrm{mol}{H}_2}\right)\times 96485\left(\frac{C}{\mathrm{mol}}\right)\times 1\left(\frac{J}{C\times V}\right)\times {10}^{-3}\left(\frac{\mathrm{kJ}}{J}\right)$$\Delta Q=\left(1.56-V_{\mathrm{cell}}\left(V\right)\right)\times 192.97\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$$\Delta Q@1.48V=\left(1.56-1.48\right)\times 192.97\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)=15.4\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

Step 2: Chemical Oxygen Production

[0206] The oxygen generation phase is exothermic, releasing heat to the environment.

[00006]$\Delta Q=\Delta H-15.3\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

[0207] Although, the water and nickel oxyhydroxide must be heated from room temperature to the process temperature (at least 60° C.) the heat released is equal to the calculated at 25° C.

[0208] Following the regeneration reaction, oxygen is released from the cell, cooling back to room temperature, and the nickel hydroxide is placed back in a cold electrolyte for hydrogen production. Therefore, the sum of the enthalpy changes equals the regeneration reaction enthalpy change at 25° C., ΔH° r×n=−15.3 kJ/mol H.sub.2.

[0209] The released heat by the exothermic oxygen generation is equal to the heat needed by the endothermic hydrogen production at V.sub.cell=1.48V. This result is that 1.48V is the thermo-neutral voltage for water splitting.

[0210] In addition to the chemical reaction heating one should also consider the heating of the water supply (water is consumed during oxygen production). To estimate the heat required for heating the water supply to 90° C. (to promote oxygen generation) the following measures were taken:

[0211] 1. The water were heated from room temperature (25 C) to 90 C.

[0212] 2. The E-TAC cell was isolated—adiabatic process.

[0213] It was assumed that all the heat stored in the water consumed by the reaction is lost as it escapes with the O.sub.2. This lost heat can be reused by adding an heat exchanger to capture heat from the escaping oxygen gas.

[0214] In order to produce 1 mole of hydrogen, 1 mole of water is heated from 25 C to 90° C.

[00007]$C_p\left(\mathrm{water},K\right)=52.928+47.614.Math.{10}^{-3}.Math.T-7.238.Math.{10}^5.Math.T^2\left(\frac{J}{\mathrm{mol}}\right)$$Q=m\underset{298}{\overset{373}{\int }}{C}_p\mathrm{dT}=4.03\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

[0215] All C.sub.p data used in this calculation were described by Macdonald, Digby D (1993).

[0216] Macdonald, Digby D., and Mark L. Challingsworth. “Thermodynamics of Nickel-Cadmium and Nickel-Hydrogen Batteries.” Journal of the Electrochemical Society 140.3 (1993): 606-609.

Switching Between Oxygen and Hydrogen Production Steps

[0217] In addition to the heat consumed and released by the chemical reactions, the heating and cooling of the E-TAC cell, should also be considered.

[0218] To estimate the heating and cooling of the E-TAC cell during switching between hydrogen and oxygen production step the following measures were taken: [0219] 1. the cell was cooled or heated from room temperature (25° C.) to 90° C. [0220] 2. The E-TAC cell was isolated—adiabatic process. Therefore, only the electrodes change their temperature. [0221] 3. The anode and cathode substrate was nickel foam. [0222] 4. The anode is 1:2 molar ration of nickel foam to NiOH.sub.2. The electrodes fabricated so far has as 1:1 ratio, but 1:2 ratio is preferred and within reach.

[0223] In order to produce 1 mole of hydrogen, 2 mole of Ni(OH).sub.2 is needed. Therefore in the anode and cathode together 2 mole of nickel and 2 mole of Ni(OH).sub.2 were heated and cooled during switching.

[00008]$C_p\left(\mathrm{Ni},K\right)=26.07\left(\frac{J}{\mathrm{mol}}\right);C_p\left({\mathrm{Ni}\left(\mathrm{OH}\right)}_2,K\right)=92.33\left(\frac{J}{\mathrm{mol}}\right)$$Q=m\underset{298}{\overset{373}{\int }}{C}_p\mathrm{dt}=2.Math.26.07.Math.65-2.Math.92.33.Math.65=15.4\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

[0224] All C.sub.p data used in this calculation were described by Macdonald, Digby D (1993).

Overall E-TAC Cycle

[0225] Considering both the reactions thermodynamics and the water and electrodes heating and cooling, it have been found that:

[0226] During hydrogen production the reaction consumes

[00009]$15.4\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

and cooling down the electrodes from 90° C. to 25° C. release about

[00010]$15.4\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

Therefore, hydrogen production is expected to be thermo-neutral. Nevertheless, under practical conditions this step might require some heating or cooling which can be easily achieved by slight changes in the operating voltage or some air-cooling.

[0227] During oxygen production the reaction releases

[00011]$15.3\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

and heating up the electrodes and water supply from 90° C. to 25° C. consumes

[00012]$19.4\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

Therefore, some heating is required to provide the excess

[00013]$4.1\left(\frac{\mathrm{kJ}}{\mathrm{mol}\mathrm{Hydrogen}}\right)$

which is only 1.4% of the 284 kJ stored in the mole hydrogen produced.