Patent Application
20230366106
Not Applicable.
Not Applicable.
This invention relates to hydrogen generation and chemical energy storage. In particular, it relates to methods, systems, and devices to generate stored hydrogen, thus providing for the storage of energy as increased hydrogen potential energy.
Electrolysis of water into hydrogen and oxygen is a long-established process. As renewable energy mandates continue to be sought, a significant hindrance has been the relatively low availability and reliability associated with renewable energy sources. For example, for solar energy, its availability is limited by access to sunshine in a given locality or in a particular season or under specific weather patterns. Similarly, for wind energy, the conditions to generate energy from wind sources must also be at least adequate in terms of wind activity in a given locality and at given times, if not optimum. Ideally, the ability to “meet demand” also means the ability to vary and introduce more such power, as needed, independent of immediate conditions. Energy storage to vary and meet demand for tapping into such renewable energy sources has thus become important for the long-term viability of renewable energy at levels desired sufficient to achieve targeted renewal energy generation goals and the reduction of carbon emissions associated with public policies.
Energy storage for renewable power continues to be a challenge for larger scale operations that need to accommodate sizable electrical loads and/or for extended time periods.
An efficient system for the enhanced production of hydrogen in connection with renewable and other power generation, which provides for the storage of energy as hydrogen potential energy until there is an electrical energy demand, is currently quite limited, yet highly desirable.
The present disclosure relates to methods and systems for the enhanced electrochemical production of hydrogen to serve as a useful renewable power resource. Such systems and methods provide improved capacity for the storage of energy as hydrogen energy potential until there is an electrical energy demand that needs to be met. The present disclosure provides for the manufacture of hydrogen using electrochemical reactions in specially adapted environments. In embodiments of the disclosure, methods and systems for hydrogen production are provided in which two phases of operation are employed. The systems include an electrolytic cell containing first and second electrodes and a solution. The solution, which can be an aqueous acidic or alkaline solution, may contain salts of various metals, provided that each of these metals can be dissolved with hydrogen evolution as the electron balancing reaction. Therefore, the present invention can be operated with solutions containing metal cations such as iron, nickel, manganese, zinc, tin, and lead. However, the Applicant has found that the better efficiency of the systems is obtained when the metallic salt solution is zinc based. Therefore, for sake of simplicity, in the following part of the document reference will be made only to zinc solutions, while the other metals indicated above can also be used. The first electrode can be made of metal or metal alloy such as zinc, copper, stainless steel, or titanium and the second electrode can be made of one or more of stainless steel, nickel and its alloys, titanium and its alloys, or graphitized carbon sheet or tissue optionally provided with a catalytic coating suitable for lowering the overvoltage for the oxygen evolution reaction (referred to hereinafter as “OER”), hydrogen evolution reaction (referred to hereinafter as “HER”), or both. During the charging phase of the cell containing a solution of a zinc salt (see
In another embodiment of the disclosure (see
In yet a further embodiment of the present disclosure, a system and method of hydrogen production are provided (see
In a still further embodiment of the present disclosure, systems and methods of hydrogen production are provided. In this embodiment, the Applicant has found that the hydrogen production rate can be further increased if, in addition to each of the above processes, the electrolytic solution is heated during the discharging phase (see
Additionally, the hydrogen evolution rate in the short-circuited electrode arrangement of the invention can be influenced or impeded by the electrode distance and the internal resistivity of the solution. It is highly preferable that the electrode distance be reduced to 1.75-3.25 mm, preferably 2-3 mm, ensuring a minimum distance needed to prevent or limit the possibility of premature electrode short circuit due to the known issue of zinc dendrites forming from the first electrode and coming into contact with the second electrode. The internal resistivity of the solution can be substantially decreased, in addition to using a high zinc salt concentration as suggested before, by increasing the temperature of the cell. In fact, the Applicant has found that increasing cell temperature above the 35-55° C., preferably 37.5-52.5° C.; and more preferably 40-50° C. range typical of the charging phase to a 75-105° C.; preferably 77.5-102.5° C.; more preferably 80-100° C. range during the discharging phase allows an important enhancement of the hydrogen evolution rate, which in turn leads to a further decrease of both the required electrode surface area and the associated capital investment. The Applicant has also found that, after the discharging phase has been completed, the cell temperature should preferably be returned to the lower temperature level used during the charging phase to deposit zinc metal with the best efficiency. Heating the solution can represent a minor loss of charging efficiency. However, the loss is preferably minimized by applying a heat exchange arrangement.
These features and other features of the present disclosure will be discussed in greater detail in the following description.
Corresponding reference numerals are used for corresponding components, etc., as set forth in the description and drawings.
The following detailed description illustrates the claimed disclosure by way of example and not by way of limitation. This description illustrates and enables one skilled in the art to make and use the claimed disclosure, and describes several embodiments, adaptations, variations, alternatives and uses of the claimed disclosure. Additionally, it is to be understood that the claimed disclosure is not limited in its application to the details of the systems, methods and devices specifically set forth in the following description or illustrated by means of the figures. The claimed disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
As used herein, the term “cell” means a vessel, which comprises a first electrode, a second electrode, and optionally more electrodes, said second electrode optionally split into a first and second electrically insulated units, and a solution, preferably an aqueous solution, containing dissolved metal salts, preferably zinc salts, and optionally other dissolved chemical species, such as additives suitable for facilitating deposition of metals and preventing the growth of dendrites.
As used herein the term “electrode” means a conductor through which electrons (electricity), fed to or withdrawn therefrom, participate in reactions at the electrode surface with chemical species present in the solution contained in the cell. For example, during the charging phase of the disclosure electron flow into the first electrode is generated by the external electrical energy source. At the interface between first electrode surface and solution, a reduction reaction takes place in which electrons combine with the zinc ions present in the solution leading to zinc deposition: Zn2++2 electrons→Zn. The external energy source, which works as a kind of electron pump, conveys electrons from the second electrode that are generated from an oxidation reaction which takes place at the interface between the second electrode and the solution. This reaction is represented by the conversion of the OH− ions contained in the solution to oxygen (O2) and water: 2 OH−→0.5 O2+H2O+2 electrons, when the solution is an alkaline solution, or by the conversion of water of the aqueous solution to oxygen (O2) and H+:H2O→0.5 O2+2 H++2 electrons, when the solution is acidic.
As used herein, the term “catalytic” means any compound which is able to facilitate a given reaction. The term catalytic as used in this document represents the ability of the surface of said second electrode to facilitate the oxygen evolution reaction, the hydrogen evolution reaction, or both, which in electrochemical wording means the ability to reduce the overvoltage of a reaction.
As used herein, the terms “large scale hydrogen production” or “large scale chemical storage resource” means the amount of hydrogen or hydrogen potential required to support operation of power systems in the range of many kW power, preferably in the range of at least 1 MW power.
Applicant has devised and discloses herein two-phase methods, systems and devices which greatly enhance the production of hydrogen and enable it to serve as a chemical energy storage resource for large scale renewable power operations. Further, the improved capacity for the storage of such hydrogen energy potential greatly improves the ability for timing the tapping into such energy potential at the point when it is most needed. The present disclosure makes hydrogen using metal deposition/dissolution reactions, and especially zinc deposition/dissolution reactions, and oxygen/hydrogen evolution reactions in specially adapted environments which have been designed to increase the rate of production of hydrogen using a two phase system. Generally speaking, in a charging phase an external electrical energy source is connected to the first and second electrodes of the cell containing a solution of zinc salts. In particular, the first electrode is connected to the negative polarity of the external source and the second electrode to the positive polarity, so that a voltage difference potential is established across the cell. The voltage difference allows electrical current to travel through the cell, the higher the current the higher is the voltage difference, resulting in the deposition of zinc metal contained in the solution as a zinc salt on the first electrode and oxygen evolution on the second electrode. Once the charging phase has been completed, the first and second electrodes are disconnected from the external energy source and the cell is kept in this standby condition until hydrogen production is required. The time within which the cell remains in a standby condition may vary greatly from virtually instantaneously to a substantial duration, such as 12 hours, or even for days, weeks, and longer. There is no practical limitation beyond the demand that would lead to drawing down the stored hydrogen and the economics of how large of a hydrogen storage installation is desired. In a discharging phase, when hydrogen production is required, the zinc metal is oxidatively dissolved back into the solution from the first electrode and hydrogen is produced on the second electrode by short circuiting the first and the second electrodes without withdrawing any, or only insubstantial, electrical power from the system, contrary to what is disclosed in the known prior art. The Applicant has surprisingly discovered that with short circuiting the first and second electrodes the production rate of hydrogen can be substantially increased by directing all of the available energy potential of the cell represented by zinc dissolution reaction to the hydrogen evolution reaction. As an example, the Applicant has been able to calculate evolution of hydrogen at the rate of 400 m3/(hour×m2 of electrode surface), which permits one to meet production at an hourly requirement of hydrogen of a 1 MW power plant with the limited electrode surface of 350 m2. The range of overvoltages required to be applied during the charging phase to electrode systems within the parameters of the present disclosure, for purposes of generating reduced metal deposition layers on the first electrode and evolving oxygen on the second electrode, would be known to those of ordinary skill in this art without undue experimentation.
The first and second electrodes of the cell can have a gap of 1.75-4.25, preferably 2-3, and the solution can contain 50-70, preferably 55-65, more preferably 60 gram/liter of metal cation in the preferred case of zinc metal. The Applicant has also surprisingly discovered that it is particularly advantageous combining said short circuit operation with both the increase of the concentration of zinc metal salt in the solution to reduce the electric resistivity and improve mass transport, for example up to 100 gram/liter of zinc cation, and the reduction of the electrode gap, for example to as low as a 2-3 mm gap, which still allow a reasonably trouble free operation without any internal short circuit danger between the first and the second electrodes due to metal dendrite growth. Such combination leads to producing hydrogen at an even higher rate than the simple short circuit. As an example, by using the arrangement including short circuiting, high metal salt concentration and reduced electrode gap, the Applicant has found that a hydrogen production rate can be raised to about 550 m3/(hour×m2 of electrode surface), which corresponds to the hourly production of hydrogen required by a 1 MW power plant with the limited electrode surface of about 280 m2. As further disclosed below, Applicant has developed additional mechanisms to enhance the production of hydrogen by various alternatives and combinations on the disclosed theme of two phase hydrogen production. Thus, Applicant has provided for greatly increased hydrogen potential energy for use as a chemical storage resource. In all of the two-phase systems disclosed herein, the system can be configured so that the system's cell connects to a separate, external electric power source, or the system can be configured to fold in the external electric power source as an integral part of the system.
Referring to
The solution 30 in the electrolytic cell 200 can be either alkaline or acidic. When the solution is alkaline, the zinc metal salt 40 comprises zinc as a complex compound, such as, for example, but not exclusively, a zincate ZnO22− or zinc hydroxyl complex Zn(OH)42−, while when the solution is acidic the zinc salt can be zinc sulfate. The use of acidic or alkaline solutions is not a matter of secondary importance. When an acidic solution is used with zinc sulfate as the zinc salt, this may cause both a partially loose zinc deposit and easier dendrite growth, which may reduce the system efficiency and lead to the danger of an internal short circuit between first and second electrodes. In an alkaline solution, the metal deposition is more compact and less prone to dendrite growth, both effects allowing operation with reduced electrode gap and ensuring better efficiency. The alkaline solution also serves to stabilize the system such that the use of a single second electrode can feasibly be considered (see below a discussion of an alternative embodiment employing a second electrode split into two electrically insulated first and second units). In fact, when the solution employed is alkaline, the second electrode is preferably made from plain nickel or a nickel alloy. These materials are suitable for operation in both the charging phase, where oxygen is evolved with lower overvoltage, and the discharging phase, where hydrogen is evolved also with lower overvoltage. In addition, these materials are characterized by appropriate chemical stability to corrosion attack. In said embodiment employing an alkaline solution, minimum corrosion can also be expected when the second electrode is made of high surface-area nickel such as sandblasted nickel or thermally sprayed nickel which are particularly efficient in further lowering overvoltages.
When the solution is acidic, in addition to the loose deposit and dendrite growth effects already discussed, the operation can be severely penalized by the poor performance of the second electrode. In fact, the resistance to corrosion attack during the charging phase with oxygen evolution appears to only be attainable if the second electrode is made of titanium or titanium alloy provided with a catalytic coating as is taught by the available prior art. However, such second electrode would not be operable in the discharging phase where hydrogen is evolved. In fact, titanium becomes brittle and unsuitable for safe hydrogen production in an acidic solution due to hydrogen penetration inside the metal lattice. Stainless steel and nickel and nickel alloys could be suitable for use in the second electrode during the discharge phase with hydrogen evolution but would be subjected to heavy corrosion during the charging phase with oxygen evolution. The Applicant has found that this negative feature can be overcome by adopting a second electrode split into two electrically insulated units, as discussed below.
Referring to
Another alternative embodiment of the disclosure also involves a two phase system and method for the enhanced production of hydrogen. Referring to
In a further embodiment, hydrogen evolution can be significantly increased by heating the solution during the discharging phase, as the elevated solution temperature works to greatly reduce both the internal resistance of the solution and the overvoltage for hydrogen generation on the second electrode (see
In view of the above, it will be seen that the several objects and advantages of the present disclosure have been achieved and other advantageous results have been obtained.
As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
