Patent Publication Number: US-2023141687-A1

Title: Method for operating a metal-hydrogen battery

Description:
RELATED APPLICATION 
     This disclosure claims priority to U.S. Provisional Application Serial No. 63/278,046, filed on Nov. 10, 2021, which is herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure is generally related to operating metal-hydrogen batteries, and more particularly to methods that include regenerating decayed metal-hydrogen batteries. 
     BACKGROUND 
     For renewable energy resources such as wind and solar to be competitive with traditional fossil fuels, large-scale energy storage systems are needed to mitigate their intrinsic intermittency. To build a large-scale energy storage, the cost and long-term lifetime are the utmost considerations. Currently, pumped-hydroelectric storage dominates the grid energy storage market because it is an inexpensive way to store large quantities of energy over a long period of time (about 50 years), but it is constrained by the lack of suitable sites and the environmental footprint. Other technologies such as compressed air and flywheel energy storage show some different advantages, but their relatively low efficiency and high cost should be significantly improved for grid storage. Rechargeable batteries offer great opportunities to target low-cost, high capacity and highly reliable systems for large-scale energy storage. Improving reliability of rechargeable batteries has become an important issue to realize a large-scale energy storage. 
     Consequently, there is a need for methods of operating a metal-hydrogen battery. 
     SUMMARY 
     Described herein are methods for operating metal-hydrogen batteries to improve their reliability. Particularly, the method disclosed herein can regenerate decayed metal-hydrogen batteries to improve their performance and prolong their lives. In particular, a method for operating a metal-hydrogen battery includes monitoring operation of the metal-hydrogen battery during normal cycles of discharge and charge; determining whether the anode of the metal-hydrogen battery during normal cycles of discharge and charge is degraded based on an indicator monitored during operation; and in response to determining that the anode is degraded, regenerating the metal-hydrogen battery. In some embodiments, regenerating the metal-hydrogen battery includes applying reconditioning cycles of discharge and charge to the metal-hydrogen battery, wherein the reconditioning cycles of discharge and charge utilize a discharge or charge condition different from normal cycles of discharge and charge. In some embodiments regenerating the metal-hydrogen battery includes increasing a cutoff voltage to a predetermined voltage during discharge operations subsequent to the cycles of discharge and charge. 
     In some embodiments, a metal-hydrogen battery includes a vessel; at least one cell comprising a cathode and an anode, the anode and the cathode each having a catalyst, the at least one cell contained within the vessel, the at least one cell supporting a hydrogen evolution reaction (HER) and a hydrogen oxidation reaction (HOR) during charge and discharge cycles; an electrolyte contained in the vessel; and a controller coupled to the at least one cell, the controller executing instructions to monitor an indicator during operation of the metal-hydrogen battery during normal cycles of discharge and charge; determine whether the anode of the metal-hydrogen battery during normal cycles of discharge and charge is degraded based on the indicator; and in response to determining that anode of the metal-hydrogen battery during normal cycles of discharge and charge is degraded, regenerate the metal-hydrogen battery. 
     Other embodiments are contemplated and explained herein after. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain features of various embodiments of the present technology are set forth with particularity in the appended claims. A better understanding of the features and advantages of the technology will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the disclosure are utilized, and the accompanying drawings of which: 
         FIG.  1 A  depicts a schematic of a metal-hydrogen battery cell according to some embodiments. 
         FIG.  1 B  depicts a schematic of a metal-hydrogen battery according to some embodiments. 
         FIG.  1 C  illustrates a state function for operation of a metal-hydrogen battery as illustrated in  FIG.  1 B . 
         FIG.  2    is a flow chart illustrating a method for operating a metal-hydrogen battery according to the state function illustrated in  FIG.  1 C , according to some embodiments. 
         FIG.  3    is a diagram depicting energy efficiency versus cycle of a metal-hydrogen battery, according to some embodiments. 
         FIG.  4    is a diagram showing chronopotentiometric measurement results on an anode in a 3-electrode metal-hydrogen battery cell, according to some embodiments. 
         FIG.  5    is a diagram showing chronopotentiometric measurement results on an anode in a 3-electrode metal-hydrogen battery cell, according to some embodiments. 
         FIG.  6 A  is a flow chart illustrating a method for operating a metal-hydrogen battery, according to some embodiments. 
         FIG.  6 B  illustrates operation of a metal-hydrogen battery according to some embodiments. 
         FIGS.  7 A- 7 C  are diagrams illustrating improved charge and discharge efficiencies by applying an increased cutoff discharge voltage, according to various embodiments. 
         FIG.  8    is a flow chart illustrating a method for determining whether a metal-hydrogen battery anode is degenerated, according to some embodiments. 
         FIG.  9    is a flow chart illustrating another method for determining whether a metal-hydrogen battery anode is degenerated, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these details. Moreover, while various embodiments of the disclosure are disclosed herein, many adaptations and modifications may be made within the scope of the disclosure in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the disclosure in order to achieve the same result in substantially the same way. 
     Unless the context requires otherwise, throughout the present specification and claims, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” Recitation of numeric ranges of values throughout the specification is intended to serve as a shorthand notation of referring individually to each separate value falling within the range inclusive of the values defining the range, and each separate value is incorporated in the specification as it were individually recited herein. Additionally, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may be in some instances. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     To expedite both hydrogen evolution (HER) and hydrogen oxidation (HOR) reactions, the anode of the metal-hydrogen battery according to aspects of the present disclosure are transition metal anode (TMA) that includes a TMA catalyst that includes both metal sites to bind with hydrogen and metal oxide/metal hydroxide to being with hydroxide anions. The interfaces between metal and metal oxide are highly active for both HER and HOR and an optimal ratio of metal to metal-oxide is maintained to achieve high catalyst activity. Although, with TMA catalysts as described below, a significant self-healing mechanism occurs during normal operational cycling of the battery, oxidation may continue to occur on the TMA that eventually results in degradation of the anode. Aspects of the present invention monitor operation of the metal-hydrogen oxide to detect degradation of the anode and regeneration steps are taken when particular levels of degradation are detected. 
     Consequently, in accordance with some aspects of the present disclosure, an indicator of degradation of the metal-hydrogen battery during normal cycles of discharge and charge is monitored and it is determined whether threshold levels of degradation of the metal-hydrogen battery during the normal cycles of discharge and charge is detected based on the indicator. In response, if it is determined that the battery has degenerated due to anode oxidation, a regeneration procedure for the metal hydrogen battery is performed. In some embodiments, the regeneration procedure includes applying reconditioning cycles of discharge and charge to the metal-hydrogen battery. The reconditioning cycles of discharge and charge utilize a discharge or charge condition different from normal cycles of discharge and charge. In some embodiments, the regeneration procedure includes increasing a cutoff voltage to a predetermined voltage during discharge operations subsequent to the cycles of discharge and charge. Determination of the decay can be performed by monitoring the rates of decays, monitoring an anode voltage, or by using an AI based system. 
       FIG.  1 A  depicts a schematic of a metal-hydrogen battery  100  according to some embodiments. The metal-hydrogen battery  100  includes cell  101  that includes a cathode  102 , an anode  104 , and a separator  106  disposed between the cathode  102  and the anode  104 . The cell  101  is housed in an enclosure  108 . An electrolyte  110  is saturated among cathode, anode and separator. The cathode  102 , the anode  104 , and the separator  106  are porous to allow the electrolyte  110  to communicate between the cathode  102  and the anode  104 . In some embodiments, the separator  106  can be omitted as long as the cathode  102  and the anode  104  can be isolated from each other. For example, the space occupied by the separator  106  may be filled with the electrolyte  110 . The metal-hydrogen battery  100  further includes an inlet  112  configured to introduce certain amount of electrolyte into the enclosure  108 . Inlet  112  may include a valve or may otherwise be sealed after enclosure  108  is filled before operation of battery  100 . As is further discussed below, battery  100  is charged and discharged through a hydrogen evolution reaction (HER) and a hydrogen oxidation reaction (HOR). 
       FIG.  1 B  illustrates a battery arrangement according to some embodiments of this disclosure. As illustrated in  FIG.  1 B , a number of cells  101 - 1  through  101 -N are stacked within enclosure  108 . Cells  101 - 1  through  101 -N can be separated by another separator  106  and may be coupled either in parallel or in series. In this disclosure, the configuration where a number of cells  101  are coupled in parallel are discussed. Output terminals  122  and  124  are provided and coupled to the stack of cells  101 - 1  through  101 -N to provide current in and out of battery enclosure  108  to cells  101 - 1  through  101 -N. 
     Terminals  122  and  124  can be coupled to a charge/discharge controller  126 . Charge/discharge controller  126  can control the current or voltage on terminals  122  and  124  as described in further detail below. Charge/discharge controller  126  can, in some embodiments, include a detection circuit  132  that monitors voltage and current on terminals  122  and  124 . Detection circuit  132  can also be coupled to monitor temperature and pressure in vessel  108 . Detection circuit  132  provides signals to a processor  134 . Processor  134  includes a combination of microcontrollers, microprocessors, digital circuitry, ASICs or other circuitry to process the signals from detection circuit  132  and perform the functions described in further detail below. In some embodiments, processor  134  can include an AI processor that has been trained to perform functions as is described below. Processor  134  can provide signals to a cycle controller  136  that can control the HER/HOR cycling of battery  100  as is further described below. In particular, as described below, processor  134  provides signals to cycle controller  136  to switch from a normal cycle mode to a regeneration cycle mode as discussed further below. 
     As illustrated in  FIG.  1 A  and discussed above, each cell  101  includes a cathode  102  and an anode  104  that are separated by a separator  106 . Cell  101  is positioned in a vessel  108  where an electrolyte  110  can flow between cathode  102  and anode  104 . As is discussed further below, cathode  102  is formed of a porous conductive substrate  102   a  coated by a porous active material  102   b . Similarly, anode  104  is formed of a porous conductive substrate  104   a  coated by a porous catalyst  102   b . In some embodiments, the electrolyte  108  is an aqueous electrolyte that is alkaline (with a pH greater than 7). Each of anode  104  and cathode  102  can be formed as electrode assemblies with multiply layered structures. 
     The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) are more mechanistically complex in alkaline conditions than in acidic conditions. Active alkaline HER/HOR catalysts tend to have more dynamic surfaces. In acidic conditions, the reactions proceed via the reduction of H +  to H 2  or the oxidation of H 2  to H + . The activity of a catalyst for these reactions in acidic conditions can be closely correlated to the binding energy of hydrogen to the metal surface. If hydrogen binds too strongly or too weakly, the catalytic process cannot effectively proceed and the kinetic overpotential will be large. Platinum has an ideal binding energy for hydrogen and demonstrates better HER/HOR performance than any other catalyst in low pH solutions. In alkaline conditions, the concentration of free H +  is essentially zero, and thus the HER first proceeds via the cleavage of the H-O bond of a water molecule to generate a surface-adsorbed hydrogen atom and a hydroxide anion according to Eq. 1 below. This step is slow on metal surfaces, resulting in alkaline HER exchange current densities that are two to three orders of magnitude smaller than in acid on the same metal. Hydrogen gas is generated according to Eq. 2 or Eq. 3 below. This step (Eq. 1) occurs in reverse as the last step of HOR and is also rate determining as metal surfaces do not interact strongly with the hydroxide anions required to complete the reaction and form H 2 O. 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     To expedite both HER and HOR on the catalyst, a catalyst material is provided that contains (i) metal sites to bind with hydrogen and (ii) metal oxide/metal hydroxide sites to bind with hydroxide anions. The interfaces where metal and metal oxide meet are highly active for both HER and HOR and an optimal ratio of metal-to-metal oxide is maintained to achieve high catalyst activity. If the catalyst surface becomes too oxidized during prolonged, or high overpotential, HOR, the catalyst surface can become deactivated and the battery performance will decay as a result. 
     Accordingly, the anode  104  as illustrated in  FIG.  1 A  is a catalytic hydrogen electrode. In some embodiments, as discussed above, the anode  104  includes a porous conductive substrate  104   a  and a catalyst layer  104   b  covering the porous conductive substrate  104   a . Although it is illustrated in  FIG.  1 A  that the catalyst layer  104   b  covers the outer surface of the porous conductive substrate  104   a , it is to be understood that since the porous conductive substrate  104   a  has internal pores or interconnected channels (not shown), the catalyst layer  104   b  also covers the surfaces of those pores and channels. The catalyst layer  104   b  includes a bi-functional catalyst to catalyze both HER and HOR at the anode  104 . In some embodiments, the porous conductive substrate  104   a  has a porosity of at least about 10%, at least about 20%, at least about 30%, at least about 40%, or at least about 50%, and up to about 80%, up to about 90%, up to about 95% or greater. In some embodiments, the porous conductive substrate  104   a  is a metal foam, such as a nickel foam, a copper foam, a steel foam, an aluminum foam, or others. In some embodiments, the porous conductive substrate  104   a  is a metal alloy foam, such as a nickel-molybdenum foam, a nickel-copper foam, a nickel-cobalt foam, a nickel-tungsten foam, a nickel-silver foam, a nickel-molybdenum-cobalt foam, or others. Other conductive substrates are encompassed by this disclosure, such as metal foils, metal meshes, and fibrous conductive substrates. In some embodiments, the conductive substrates are carbon-based materials, such as carbon fibrous paper, carbon cloth, carbon felt, carbon mat, carbon nanotube film, graphite foil, graphite foam, graphite mat, graphene foil, graphene fibers, graphene film, and graphene foam. 
     In some embodiments, the bi-functional catalyst of the catalyst layer  104   b  is a nickel-molybdenum-cobalt (NiMoCo) alloy. Other transition metal or metal alloys as bi-functional catalysts are encompassed by this disclosure, such as nickel, nickel-molybdenum, nickel-tungsten, nickel-tungsten-cobalt, nickel-carbon, nickel-chromium, based composites. In some embodiments, bi-functional catalyst is a transition metal alloy that includes two or more of Ni, Co, Cr, Mo, Fe, Mn and W. Other precious metals and their alloys as bi-functional catalysts are encompassed by this disclosure, such as platinum, palladium, iridium, gold, rhodium, ruthenium, rhenium, osmium, silver, and their alloys with precious and non-precious transition metals such as platinum, palladium, iridium, gold, rhodium, ruthenium, rhenium, osmium, silver, nickel, cobalt, manganese, iron, molybdenum, tungsten, chromium and so forth. In some embodiments, bi-functional catalysts are a combination of hydrogen evolution reaction and hydrogen oxidation reaction catalysts. In some embodiments, the bi-functional catalysts of the metal-hydrogen battery  100  include a mixture of different materials, such as transition metals and their oxides/hydroxides, which contribute to hydrogen evolution and oxidation reactions as a whole. In some embodiments, the catalyst layer  104   b  includes nanostructures of the bi-functional catalyst having sizes (or an average size) in a range of, for example, about 1 nm to about 100 nm, about 1 nm to about 80 nm, or about 1 nm to about 50 nm. In some embodiments, the catalyst layer  104   b  includes microstructures of the bi-functional catalyst having sizes (or an average size) in a range of, for example, about 100 nm to about 500 nm, about 500 nm to about 1000 nm. 
     In some embodiments, to create different affinities with respect to the electrolyte (e.g., electrolyte  108 ) on the anode  104 , the catalyst layer  104   b  may be partially coated with a surface-affinity modification material. For example, when the catalyst layer  104   b  on the porous substrate  104   a  are hydrophilic to the electrolyte, the catalyst layer  104   b  may be partially or entirely coated with a material that is hydrophobic to the electrolyte. On the contrary, when the catalyst layer  104   b  on the porous substrate  104   a  are hydrophobic to the electrolyte, the catalyst layer  104   b  may be partially or entirely coated with a material that is hydrophilic to the electrolyte. This structure can facilitate movement of hydrogen gas in the pores of the anode  104  and improve HOR during discharge. 
     The cathode  102  may include a conductive substrate  102   a  and a coating  102   b  covering the conductive substrate  102   a . The coating  102   b  includes a redox-active material that includes a transition metal. In some embodiments, the conductive substrate  102   a  is porous, such as having a porosity of at least about 10%, at least about 20%, at least about 30%, at least about 40%, or at least about 50%, and up to about 80%, up to about 90%, or greater. In some embodiments, the conductive substrate  102   a  is a metal foam, such as a nickel foam, or a metal alloy foam. Other conductive substrates are encompassed by this disclosure, such as metal foils, metal meshes, and fibrous conductive substrates. In some embodiments, the transition metal included in the redox-active material is nickel. In some embodiments, nickel is included as nickel hydroxide or nickel oxyhydroxide. In some embodiments, the transition metal included in the redox-reactive material is cobalt. In some embodiments, cobalt is included as cobalt oxide or zinc cobalt oxide. In some embodiments, the transition metal included in the redox-active material is manganese. In some embodiments, manganese is included as manganese oxide or doped manganese oxide (e.g., doped with nickel, copper, bismuth, yttrium, cobalt or other transition or post-transition metals). Other transition metals are encompassed by this disclosure, such as silver. In some embodiments, the cathode  102  is a cathode, and the anode  104  is an anode. In some embodiments, the coating microstructures of the redox-active material, may have sizes (or an average size) in a range of, for example, about 1 µm to about 100 µm, about 1 µm to about 50 µm, or about 1 µm to about 10 µm. 
     In some embodiments, the electrolyte  108  is an aqueous electrolyte. The aqueous electrolyte is alkaline and has a pH greater than 7, such as about 7.5 or greater, about 8 or greater, about 8.5 or greater, or about 9 or greater, or about 11 or greater, or about 13 or greater. As a non-limiting example, the electrolyte  108  may include KOH or NaOH or LiOH or a mixture of LiOH, NaOH and/or KOH. 
     Although hydrogen oxidation catalysts such as inexpensive transition metals are suitable for metal-hydrogen batteries, they may be passivated during prolonged HOR and significantly hindered their use in practical devices. According to some embodiments of the present disclosure, catalyst  104   b  can be a bi-functional TMA (transition metal alloy). In some embodiments, combinations of Ni, Co, Cr, Mo, Fe and W can be used as an alternative to the bi-functional TMA catalyst. For example, a catalyst composed of Ni with CrOx particles decorating the surface can be used. A small amount of Pt can be added to further improve the activity. One such TMA catalyst is described in U.S. Pat. Application 16/373,247, which is herein incorporated by reference in its entirety. 
     Batteries formed with the TMA catalyst described here show a slow decay in the energy efficiency, primarily caused by voltage losses during discharge. Tests with a 3-electrode configuration shows a potential on anode  104  that generally increases beyond 0.1 V when the overall battery voltage drops below 0.95 V during degradation. It has also been discovered that the TMA catalyst shows remarkable improvements towards the discharge reaction (HOR) without damaging the catalyst. This catalyst further demonstrates high T performance gains and can operate efficiently at high temperature. Regeneration efforts can be enhanced at higher operating temperatures, and in particular at temperatures higher than room temperature. 
     Significant surface passivation begins to occur when the anode overpotential reaches about 0.1 or 0.1 V vs. H 2 /H 2 O. At some point, the passivation begins to outpace HOR and the potential rapidly ramps as more oxide grows on the catalysts such that the catalyst activity degrades. Techniques disclosed herein may reverse the passivation and restore degraded batteries back to their energy efficient state. 
       FIG.  1 C  illustrates a state function  150  of an example operation of processor  134  illustrated in  FIG.  1 B . As is illustrated in  FIG.  1 C , state function  150  includes a normal operation state  152  in which charge and discharge cycles are performed by battery  100 . During normal operation  152 , a monitoring state  154  is also operating. Monitoring state  154  monitors voltages and/or currents in battery  100  to determine when degradation has occurred. Monitoring state  154  is continuously or cyclically engaged during normal operation state  152 . 
     If a degradation is detected through monitoring state  154 , then state function  150  transitions from normal operation  152  to a regeneration state  158 . In regeneration state  158 , battery  100  is cycled in such a way as to recover, at least partially, the degradation that was detected. As is further illustrated, a monitoring step  156  monitors the charge and current of battery  100  during cycling in state  158  to determine when battery  100  has recovered. Once recovered, then state  150  returns to normal monitoring state  152 . These states are discussed in further detail below. As is further described, higher temperature operation may be used in regeneration state  158  to enhance recovery of the battery  100 . 
       FIG.  2    is a flow chart illustrating a method  200  for operating a metal-hydrogen battery consistent with state function  150  illustrated in  FIG.  1 C , according to one example embodiment. In some embodiments, the method  200  can regenerate a degraded metal-hydrogen battery by reversing oxide passivation on the electrodes of the metal-hydrogen battery. At  202 , an indicator of degradation of the anode of the metal-hydrogen battery during cycles of discharge and charge is monitored during monitoring state  154 . For example, when the metal-hydrogen battery is subject to repeated cycles of discharge and charge operations during normal operation  152 , an indicator of degradation of those operations may be obtained or calculated according to techniques disclosed herein. In some embodiments, the indicator of degradation may also indicate energy efficiency. An example of the energy efficiency during cycles of discharge and charge in a battery such as battery  100  is illustrated in  FIG.  3   .  FIG.  3    is a diagram depicting energy efficiency versus cycle of a metal-hydrogen battery, according to one example embodiment. As shown in  FIG.  3   , the energy efficiency of the metal-hydrogen battery decreases as more cycles of discharge and charge are performed, indicating that the performance of the metal-hydrogen battery is degrading/decayed. The energy efficiency can be determined, for example, by monitoring the current, duration, and cell voltage during charge in comparison with the current, duration, and cell voltage during discharge of the battery in each cycle. In some embodiments, the energy efficiency can be monitored by monitoring the voltage on anode  104 , which can also be used as an indicator of anode degradation. Other methods may also be used to monitor an indicator and determine whether or not degradation has occurred sufficiently to trigger entry to regeneration state  158 . 
     Referring back to  FIG.  2   , at  204 , it is determined whether the monitored indicator of the metal-hydrogen battery during the monitored cycles of discharge and charge indicates decay beyond a threshold value. Whether the monitored indicator of degradation of the metal-hydrogen battery during the cycles of discharge and charge indicates a decay can be determined in a number of ways as is described further below. 
     If it is determined from the monitored indicator of the metal-hydrogen battery during the monitored cycles of discharge and charge is not decayed (i.e. the monitored indicator has not reached the threshold value), the method returns to operation  202  to continue monitoring the indicator of the metal-hydrogen battery. But if it is determined that the monitored indicator of the metal-hydrogen battery during the monitored cycles of discharge and charge indicates analog decay below a threshold value, reconditioning cycles of discharge and charge are applied to the metal-hydrogen battery in  206 . In  206 , the reconditioning cycles of discharge and charge utilize a different discharge or charge condition, different from the normal cycles of discharge and charge performed in normal operation state  152 , to regenerate the metal-hydrogen battery such that discharge and charge performance of the metal-hydrogen battery at the end of the reconditioning cycles of discharge and charge is better than that of metal-hydrogen battery at the end of the monitored cycles of discharge and charge or at the beginning of the reconditioning cycles of discharge and charge. In some embodiments, the monitored cycles of discharge and charge may be in the normal operation of the metal-hydrogen battery or a specific session of testing the performance of the metal-hydrogen battery, while the reconditioning cycles of discharge and charge may be referred to as remedial/self-healing cycles (or regeneration cycles) of discharge and charge that are configured to regenerate the degraded metal-hydrogen battery. 
     For example, a degraded metal-hydrogen battery may be regenerated by subsequent symmetric cycling where a same current for a same duration is applied to the metal-hydrogen battery during each of the subsequent cycles (e.g., following the monitored cycles) of discharge and charge. A non-limiting example is shown in  FIG.  4   .  FIG.  4    is a diagram showing chronopotentiometric measurement results on an anode  104  in a 3-electrode metal-hydrogen battery cell, according to one example embodiment. The metal-hydrogen battery cell employs a 26% KOH electrolyte  110 . A constant current of 0.5 A is applied at each cycle of discharge and charge for 2 minutes. As shown in  FIG.  4   , at the beginning of the subsequent/reconditioning cycles, the potential at the anode is about 170 mV vs. H 2 /H 2 O, indicating the surface of the anode is covered by oxides that hinder the HOR. The potential at the anode is brought down quickly after initial 10 cycles to less than 100 mV vs. H 2 /H 2 O. And after 250 cycles, the HOR and HER potentials are stable at about 37 mV vs. H 2 /H 2 O and 34 mV vs. H 2 /H 2 O, respectively, indicating a significant performance improvement of the metal-hydrogen battery. Based on these techniques, a degraded metal-hydrogen battery can be regenerated such that the performance of the metal-hydrogen battery at the end of the reconditioning (e.g., remedial/self-healing) cycles of discharge and charge is better than that at the beginning of the reconditioning cycles of discharge and charge. 
     In some embodiments, a degraded metal-hydrogen battery may be regenerated by subsequent asymmetric cycling where different currents are applied to the metal-hydrogen battery in the discharge and charge operations. In some embodiments, the time periods of the discharge and charge operations in the subsequent asymmetric cycling may be different. A non-limiting example is shown in  FIG.  5   .  FIG.  5    is a diagram showing chronopotentiometric measurement results on an anode in a 3-electrode metal-hydrogen battery cell, according to one example embodiment. The metal-hydrogen battery cell employs a 26% KOH electrolyte. 
     As shown in  FIG.  5   , at the beginning of the subsequent/reconditioning cycles, the HOR potential at the anode is about 800 mV vs. H 2 /H 2 O, indicating the surface of the anode is covered by oxides that hinder the HOR. In the subsequent/reconditioning cycles, a current of 5 A is applied at the initial charge operation (HER) for 30 minutes, followed by a discharge operation (HOR) in which a current of 1 A is applied for 5 minutes. After the initial charge and discharge operations, an iterated of charge and discharge operations are applied to the metal-hydrogen battery, where a current of 10 A is applied at a charge operation (HER) for 5 minutes, followed by a discharge operation (HOR) in which a current of 1 A is applied for 5 minutes. After about 6 reconditioning (e.g., remedial/self-healing) cycles of discharge and charge, the potential at the anode is brought down quickly to less than 100 mV vs. H 2 /H 2 O, indicating a significant performance improvement of the metal-hydrogen battery. Based on these techniques, a degraded metal-hydrogen battery can be regenerated such that the performance of the metal-hydrogen battery at the end of the reconditioning (e.g., remedial/self-healing) cycles of discharge and charge is better than that at the beginning of the reconditioning cycles of discharge and charge. 
       FIG.  6 A  is a flow chart illustrating another method  600  for operating a metal-hydrogen battery, according to one example embodiment. In some embodiments, the method  600  can regenerate a degraded metal-hydrogen battery by reversing oxide passivation on the electrodes of the metal-hydrogen battery. As discussed above, at  202 , the metal-hydrogen battery during cycles of discharge and charge is monitored. For example, when the metal-hydrogen battery is subject to repeated cycles of discharge and charge operations, an indicator of degradation during those operations may be obtained or calculated based on the monitoring. At  204 , it is determined whether the monitored indicator of the metal-hydrogen battery during the monitored cycles of discharge and charge indicates a degenerated anode. 
     If it is determined in step  204  that conditions indicating degradation of the metal-hydrogen battery during the monitored cycles of discharge and charge have not been met, the method returns to operation  202  to continue monitoring the metal-hydrogen battery. But if it is determined that conditions indicate degradation of the metal-hydrogen battery during the monitored cycles of discharge and charge at step  204 , method  600  increases a cutoff voltage to a predetermined voltage during discharge operations subsequent to the monitored cycles of discharge and charge in step  602  during state  158 . This operation will regenerate the metal-hydrogen battery such that discharge and charge performance of the metal-hydrogen battery is improved. In some embodiments, the predetermined cutoff voltage may be about 0.6, 0.8, 1, 1.2, or 1.5 V. In some embodiments, the monitored cycles of discharge and charge may be in the normal operation of the metal-hydrogen battery or a specific session of testing the performance of the metal-hydrogen battery, while the discharge operations subsequent to the monitored cycles of discharge and charge may be parts of remedial/self-healing cycles of discharge and charge subsequent to the monitored cycles of discharge and charge. 
     In some embodiments, a third platinum reference electrode can be included. Such a reference electrode allows for the independent monitoring of the anode and cathode voltages in a cell.  FIG.  6 B  demonstrates an example 160 Ah battery during operation  610 . In particular, the battery illustrated in operation  610  can be characterized by a C rate of C/5, charge capacity (CCAP) of 153 Ah, CE of 83%, EE of 73%, and VE of 88%.  FIG.  6 B  illustrates an anode voltage vs the reference in curve  612 , a cathode voltage vs the reference at curve  614 , and the output voltage in curve  616 . As is illustrated, the anode voltage  612  shows rapid voltage increase at area  618 , near the end of discharge. As is illustrated at the same time, the battery performance, as indicated by the cathode voltage vs reference curve  614  and the battery output voltage  616 , is hindered as a result. As illustrated, an anode voltage rising above 0.06 V vs. H 2 /H 2 O limits cell performance may trigger step  204  of algorithm  600  to proceed to step  602  as indicated below. 
       FIGS.  7 A- 7 C  are diagrams illustrating improved charge and discharge efficiencies by applying an increased cutoff discharge voltage, according to various embodiments.  FIG.  7 A  depicts columbic efficiency (CE) for a 4 Ah battery cell cycling at C/2.  FIG.  7 B  depicts voltaic efficiency (VE) for a 4 Ah battery cell cycling at C/2.  FIG.  7 C  depicts energy efficiency (EE) for a 4 Ah battery cell cycling at C/2. 
     In  FIGS.  7 A- 7 C , the discharge cutoff voltage is 0.8 V during the monitored cycles of discharge and charge in normal operation state  152 , and is increased to 1 V during the discharge operations subsequent to the monitored cycles of discharge and charge in  602  of method  600  during regeneration state  158 .  FIG.  7 A  depicts columbic efficiency (CE), defined as the ratio of the total charge extracted from the battery to the total charge put into the battery over a full charge/discharge cycle, for a 4 Ah battery cell cycling at C/2. As shown in  FIG.  7 A , after about 40 cycles of discharge and charge, the CE for 0.8 V discharge cutoff voltage is about 98.2% while the CE 1 V discharge cutoff voltage is about 99%.  FIG.  7 B  depicts voltaic efficiency (VE), defined as the difference between the average voltage during charging and during discharging over a charge/discharge cycle, for a 4 Ah battery cell cycling at C/2. After about 45 cycles of discharge and charge, the VE for 0.8 V discharge cutoff voltage is about 85.6% while the VE 1 V discharge cutoff voltage is about 87.8%.  FIG.  7 C  depicts energy efficiency (EE), defined as the ratio of the current provided to the battery to the current retrieved from the battery during a full cycle, for a 4 Ah battery cell cycling at C/2. After about 45 cycles of discharge and charge, the EE for cycles with 0.8 V discharge cutoff voltage is about 83.7% while the EE for cycles with 1 V discharge cutoff voltage is about 86.5%. These results demonstrate that increasing discharge cutoff voltage has an effect to regenerate the battery such that a degraded metal-hydrogen battery can have an increase energy efficiency. 
     Techniques disclosed herein can also be used to determine whether a metal-hydrogen battery is decayed in step  204  of  FIGS.  2  and  6   . As a nonlimiting example, a potential on an anode of the metal-hydrogen battery may be used as the monitored indicator to determine whether the metal-hydrogen battery is decayed. In some embodiments, the metal-hydrogen battery during the cycles of discharge and charge is determined to have been decayed when the potential on the anode of the metal-hydrogen battery is greater than about 0.1 V vs. H 2 /H 2 O. In some embodiments, the metal-hydrogen battery during the cycles of discharge and charge is determined to have been decayed when the potential on the anode of the metal-hydrogen battery is greater than about 75, 80, 85, 90, 95, 105, 110, 115, 120, or 125 mV vs. H 2 /H 2 O. 
     Another nonlimiting example method of determining whether the anode is degraded (likely from oxidation) in  204  is shown in  FIG.  8   .  FIG.  8    is a flow chart illustrating a method  800  for determining whether a metal-hydrogen battery is degraded, according to one example embodiment. At  802 , a first decaying rate of a discharge voltage during discharge operations of cycles of discharge and charge is calculated. At  804 , a second decaying rate of a charge voltage during charge operations of the cycles of discharge and charge is calculated. At  806  it is determined whether the first decaying rate is greater than the second decaying rate by a threshold. If the first decaying rate is greater than the second decaying rate, at  808  it is determined that the energy efficiency of the metal-hydrogen battery during the cycles of discharge and charge is degraded. If the first decaying rate is not greater than the second decaying rate, at  810  it is determined that the energy efficiency of the metal-hydrogen battery during the cycles of discharge and charge is not degraded sufficiently. In the illustrated embodiment, the decaying rates of discharge and charge voltages are employed as an indicator for determining whether a metal-hydrogen battery is decayed. It is discovered that generally, the charge voltage decays slower with respect to the decay of the discharge voltage, but the decay rate of discharge increases substantially when sufficient degradation has occurred. This is because over a prolong use, oxides may be accumulated on the anode during HOR, passivating the catalyst sites for further HOR. 
     In some embodiments, a ratio of the first decaying rate and the second decaying rate may be determined and compared to a threshold to determine whether a metal-hydrogen battery is decayed. For example, the threshold may be set to a larger value, such as 2.0, 2.25, or 2.5 to catch significant decay of the metal-hydrogen battery. This can reduce the frequency to regenerate the metal-hydrogen battery. In some embodiments, the threshold may be set to a smaller value, such as between 1.5 and 2.0, inclusive, to identify an early stage decay of the metal-hydrogen battery. 
       FIG.  9    is a flow chart illustrating another method  900  for determining whether a metal-hydrogen battery is decayed in  204 , according to one example embodiment. The method  900  employs a machine learning model to automatically determine whether a metal-hydrogen battery is decayed. For example, in a battery system, each battery is associated with a communication interface configured to forward battery information to a server that manages the batteries in the battery system. The server may include a machine learning model to analyze the battery information of each battery and to identify if a particular batter is in need of regeneration in regeneration state  158  to recover the performance of the battery. If so, the server may employ the techniques disclosed herein such as those explained in connection with  FIGS.  2  and  6    to regenerate the degraded battery. 
     With reference to  FIG.  9   , at  902 , sample cycles of discharge and charge of a plurality of metal-hydrogen batteries are obtained. For example, a communication interface of each of the metal-hydrogen batteries may be configured to periodically or on demand forward battery information to the server. The battery information may include battery identifiers and the historical usage information of the battery such as how many cycles of discharge and charge the battery has undertaken, potentials at the anode and cathode during those cycles of discharge and charge, the state-of-charge, temperature, the battery voltage, coulombic efficiency vs. cycle, voltaic efficiency vs. cycle, energy efficiency vs. cycle, impedance data, pressure data, the derivative of charge vs voltage (dQ/dV), and other data parameters. The battery information is exemplary and non-limiting. Other battery performance indexes are contemplated. The battery information may be sent to the server along with a determination as to whether each of the plurality of metal-hydrogen batteries is determined to be decayed. 
     At  904 , parameters of the sample cycles of discharge and charge indicating that the plurality of metal-hydrogen batteries are decayed are extracted. After the battery information is obtained, the server may be configured to analyze the battery information and extract parameters of the sample cycles of discharge and charge indicating that the plurality of metal-hydrogen batteries are decayed. In some embodiments, parameters of the sample cycles of discharge and charge indicating that the plurality of metal-hydrogen batteries are not decayed can also be extracted for training a machine-learning model. 
     At  906 , a machine-learning model is trained with the extracted parameters to generate a trained machine-learning model. In some embodiments, the machine-learning model may undergo a supervised learning in which the extracted parameters are input to the machine-learning model with labels indicating whether a metal-hydrogen battery is decayed. Various models may be employed for the machine learning process. For example, neural networks, decision trees, regression analyses, support-vector machines, etc. may be adopted in operation  906  to generate a trained machine-learning model. 
     At  908 , parameters of the cycles of discharge and charge of a metal-hydrogen battery in question during normal operation are extracted. When a new battery or any existing batter in the battery system is due for a determination of whether the battery is decayed, its battery information in obtained. Similar to operation  904 , the parameters of the battery in question are extracted. 
     At  910 , the extracted parameters of the battery in question are fed to the trained machine-learning model to determining whether the metal-hydrogen battery in question during cycles of discharge and charge is decayed. If the metal-hydrogen battery in question during cycles of discharge and charge is decayed, the server may initiate the techniques disclosed herein to regenerate the battery in question. 
     Based on these techniques, a battery management software system (BMS) may be established that allows metal-hydrogen batteries to last longer than NASA’s battery cells at a cost of less than $150/kWh. The techniques allow the batteries to achieve decades of stability without using significant precious metal catalysts by controlling the surface oxidation state of the catalysts during cycling. 
     Extensive battery cycling data of metal-hydrogen batteries are collected under various conditions. The data is then used to train a cycling algorithm that becomes the basis of the battery management system. Field data of individual batteries in the battery system can be continuously sent back to the server and can be used to further improve the BMS algorithm. The individual batteries in the battery system can be periodically or on demand provided with updated BMS software, which can be automatically installed at the a controller for a battery to ensure each of the batteries last for decades with minimum physical maintenance. 
     The health of an anode of a metal-hydrogen battery depends on a plurality of factors including the state-of-charge, temperature, the battery voltage, etc. 
     In some embodiments, the cycles of discharge and charge as disclosed herein may be conducted at a temperature above the room temperature, such as at about 30° C., 40° C., 50° C., 60° C., 70° C., or 80° C., or between any two of the above values. Increasing operation temperature of a metal-hydrogen battery not only reduces the discharge potential at the anode of the metal-hydrogen battery, but also improves the reliability of the metal-hydrogen battery in the long term. 
     Consequently, by way of non-limiting examples, the following aspects are included in this disclosure. 
     Aspect 1 includes a method for operating a metal-hydrogen battery, the method comprising: monitoring an indicator of degradation of the metal-hydrogen battery during normal cycles of discharge and charge; determining whether the metal-hydrogen battery during normal cycles of discharge and charge is decayed based on the indicator; and in response to determining that the energy efficiency of the metal-hydrogen battery during the discharge and charge is decayed due to anode oxidation, regenerating the metal-hydrogen battery. 
     Aspect 2 includes the method of aspect 1, wherein the metal-hydrogen battery decays due to anode oxidation during normal cycles of discharge and charge. 
     Aspect 3 includes the method of aspect 1, wherein regenerating the metal-hydrogen battery includes applying reconditioning cycles of discharge and charge to the metal-hydrogen battery, wherein the reconditioning cycles of discharge and charge utilize a discharge or charge condition different from normal cycles of discharge and charge. 
     Aspect 4 includes the method of aspect 3, wherein applying the reconditioning cycles of discharge and charge to the metal-hydrogen battery comprises: applying a same current for a same duration during each of the reconditioning cycles of discharge and charge. 
     Aspect 5 includes the method of aspect 3, wherein applying the reconditioning cycles of discharge and charge to the metal-hydrogen battery comprises: applying a first current during charge operations of the reconditioning cycles of discharge and charge; and applying a second current during discharge operations of the reconditioning cycles of discharge and charge, wherein the first current is greater than the second current. 
     Aspect 6 includes the method of aspect 1, wherein determining whether the performance of the metal-hydrogen battery during normal cycles of discharge and charge is decayed based on the indicator comprises: calculating a first decaying rate of a discharge voltage during discharge operations of the normal cycles of discharge and charge; calculating a second decaying rate of a charge voltage during charge operations of the first normal cycles of discharge and charge; and determining that the metal-hydrogen battery during the first normal cycles of discharge and charge is decayed when the first decaying rate is greater than the second decaying rate. 
     Aspect 7 includes the method of aspect 1, wherein the metal-hydrogen battery during normal cycles of discharge and charge is determined to be decayed when a potential on an anode of the metal-hydrogen battery is greater than a threshold voltage. 
     Aspect 8 includes the method of aspect 7, wherein the threshold voltage is about 0.1 V vs. H 2 /H 2 O. 
     Aspect 9 includes the method of aspect 1, wherein determining whether the metal-hydrogen battery during normal cycles of discharge and charge is decayed based on the indicator comprises: obtaining sample cycles of discharge and charge of a plurality of first metal-hydrogen batteries; extracting first parameters of the sample cycles of discharge and charge indicating that the plurality of first metal-hydrogen batteries are decayed; training a machine-learning model with the first parameters to generate a trained machine-learning model; extracting second parameters of the cycles of discharge and charge of the metal-hydrogen battery; and feeding the second parameters to the trained machine-learning model to determining whether the metal-hydrogen battery during normal cycles of discharge and charge is decayed. 
     Aspect 10 includes the method of aspect 1, further comprising: conducting normal cycles of discharge and charge at a temperature above a room temperature. 
     Aspect 11 includes the method of aspect 1, further comprising: conducting the reconditioning cycles of discharge and charge at a temperature above a room temperature. 
     Aspect 12 includes the method of aspect 1, wherein regenerating the metal-hydrogen battery includes increasing a cutoff voltage to a predetermined voltage during discharge operations subsequent to the cycles of discharge and charge. 
     Aspect 13 includes a metal-hydrogen battery, comprising: a vessel; at least one cell comprising a cathode and an anode, the anode and the cathode each having a catalyst, the at least one cell contained within the vessel, the at least one cell supporting a hydrogen evolution reaction (HER) and a hydrogen oxidation reaction (HOR) during charge and discharge cycles; an electrolyte contained in the vessel; and a controller coupled to the at least one cell, the controller executing instructions to monitor an indicator of degradation of the metal-hydrogen battery during first normal cycles of discharge and charge; determine whether the metal-hydrogen battery during the first normal cycles of discharge and charge is decayed based on the indicator; and in response to determining that the metal-hydrogen battery during normal cycles of discharge and charge is decayed, regenerate the metal-hydrogen battery. 
     Aspect 14 includes the metal-hydrogen battery of claim 13, wherein the metal-hydrogen battery decays during normal cycles of discharge and charge due to anode oxidation. 
     Aspect 15 includes the battery of aspect 13, wherein instructions to regenerate the metal-hydrogen battery includes instructions to apply reconditioning cycles of discharge and charge to the metal-hydrogen battery, wherein the reconditioning cycles of discharge and charge utilize a discharge or charge condition different from normal cycles of discharge and charge. 
     Aspect 16 includes the battery of aspect 15, wherein instructions to apply the reconditioning cycles of discharge and charge to the metal-hydrogen battery comprises: applying a same current for a same duration during each of the reconditioning cycles of discharge and charge. 
     Aspect 17 includes the battery of aspect 15, wherein instructions to apply the reconditioning cycles of discharge and charge to the metal-hydrogen battery includes instructions to apply a first current during charge operations of the reconditioning cycles of discharge and charge; and apply a second current during discharge operations of the reconditioning cycles of discharge and charge, wherein the first current is greater than the second current. 
     Aspect 18 includes the battery of aspect 13, wherein instructions to determine the metal-hydrogen battery during normal cycles of discharge and charge is decayed based on the indicator comprises instructions to: calculate a first decaying rate of a discharge voltage during discharge operations of normal cycles of discharge and charge; calculate a second decaying rate of a charge voltage during charge operations of normal cycles of discharge and charge; and determine that the energy efficiency of the metal-hydrogen battery during normal cycles of discharge and charge is decayed when the first decaying rate is greater than the second decaying rate. 
     Aspect 19 includes the battery of aspect 13, wherein the performance of the metal-hydrogen battery during normal cycles of discharge and charge is decayed when a potential on an anode of the metal-hydrogen battery is greater than a threshold voltage. 
     Aspect 20 includes the battery of aspect 19, wherein the threshold voltage is about 0.1 V vs. H 2 /H 2 O. 
     Aspect 21 includes the battery of aspect 13, wherein instructions to determine whether the metal-hydrogen battery during normal cycles of discharge and charge is decayed based on the indicator includes instructions to obtain sample cycles of discharge and charge of a plurality of metal-hydrogen batteries; extract first parameters of the sample cycles of discharge and charge indicating that the plurality of metal-hydrogen batteries are decayed due to anode oxidation; train a machine-learning model with the first parameters to generate a trained machine-learning model; extract second parameters of the cycles of discharge and charge of the metal-hydrogen battery; and feed the second parameters to the trained machine-learning model to determining whether the metal-hydrogen battery during normal cycles of discharge and charge is decayed due to anode oxidation. 
     Aspect 22 includes the battery of aspect 13, further comprising: instructions to conduct normal cycles of discharge and charge at a temperature above a room temperature. 
     Aspect 23 includes the battery of aspect 13, further comprising: instructions to conduct the reconditioning cycles of discharge and charge at a temperature above a room temperature. 
     Aspect 24 includes the battery of aspect 13, wherein instructions to regenerate the metal-hydrogen battery includes instructions to increase a cutoff voltage to a predetermined voltage during discharge operations subsequent to the cycles of discharge and charge. 
     The foregoing description of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Many modifications and variations will be apparent to the practitioner skilled in the art. The modifications and variations include any relevant combination of the disclosed features. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical application, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalence.