Sulfur Dioxide Depolarized Electrolyzer and Method for Performance Recovery

This application claims the benefit of U.S. Provisional Application No. 63/575,536 filed 5 Apr. 2024, which is incorporated in its entirety by this reference.

This invention relates generally to the sulfur depolarized electrolysis field, and more specifically to a new and useful system and method in the sulfur depolarized electrolysis field.

The following description of the embodiments of the invention is not intended to limit the invention to these embodiments, but rather to enable any person skilled in the art to make and use this invention.

As shown in, an electrolyzercan include an anode, a cathode, a separatordisposed between the anode and the cathode, and/or other suitable components (e.g., all mounted within a housing, frame, etc.). The electrolyzer can additionally or alternatively include other suitable components.

As shown in, a method can include electrochemically oxidizing sulfur oxide and reducing water S, determining performance metrics of the electrochemical reaction S, and recovering performance of the electrochemical reaction S. The method can additionally or alternatively include any suitable steps.

The system and method can function to: oxidize sulfur dioxide to sulfuric acid contemporaneously with reducing water to hydrogen, refresh electrolyzer performance (e.g., to near starting performance) such as after degradation is detected, and/or can otherwise function. As a specific example, during operation of the electrolyzer (or cells thereof), sulfur dioxide may cross-over through the membrane or separator and become reduced to sulfur or hydrogen sulfide at the cathode. The sulfur or hydrogen sulfide can then block reactive sites on the cathode reducing both cathode surface area and surface activity. Variations of the electrolyzer can remove some or all of this sulfur and/or hydrogen sulfide (e.g., chemically such as by introducing an oxidizing agent into the cathode region without applying an electrical potential to convert the sulfur into sulfur dioxide, chemically such as by introducing a reducing agent into the cathode region without applying an electrical potential to convert the sulfur into hydrogen sulfide, electrochemically such as by introducing an oxidizing agent into the cathode region while applying an electrical potential to convert the sulfur to sulfur dioxide, electrochemically such as by introducing a reducing agent into the cathode region while applying an electrical potential to convert the sulfur to dihydrogen sulfide, physically such as by heating the electrolyzer or portions thereof to above a melting point of sulfur, etc.) thereby restoring electrolyzer performance (and increasing a lifetime of the electrolyzer). While discussed in terms of sulfur depolarized electrolysis, variants of this invention can be relevant for other electrolyzers (e.g., alkaline electrolyzers, anion exchange membrane electrolysis, proton exchange membrane electrolysis, solid oxide electrolysis, electrochemical reduction of carbon dioxide, electrified cementitious material production, etc.) particularly (but not exclusively) when input materials can include sulfurous contaminants that can result in degraded electrolysis performance over time.

In a first specific example, an electrolyzer can operate in an electrolysis mode and a recovery mode. In the electrolysis mode, sulfur dioxide can be provided to the anode and water can be provided to the cathode where the sulfur dioxide can be oxidized to sulfuric acid and the water can be reduced to hydrogen. During the electrolysis mode, some sulfur dioxide (and/or other sulfur species such as intermediates, sulfur trioxide, sulfuric acid, etc.) can pass through the separator and undergo reduction to sulfur and/or hydrogen sulfide. The resulting sulfur and/or hydrogen sulfide can occupy active sites on the cathode reducing an efficacy of the electrolyzer. When the efficacy (e.g., as measured based on applied electric potential required to achieve a target current density, and/or hydrogen output and/or sulfuric acid output, as measured based on voltage ratio between a nominal voltage and an actual voltage such as E/Ewhere Eis taken to be 0.16V) is reduced by a threshold amount (e.g., 1%, 5%, 10%, 15%, 20%, 30%, etc.; or alternatively phrased the Erises), the electrolyzer can switch to operation in the recovery mode. In the recovery mode, a reactive agent (e.g., oxidizing agent, reducing agent, etc.) can be provided to the cathode to remove sulfur (e.g., by oxidizing the sulfur to sulfur dioxide, by reducing the sulfur to hydrogen sulfide, etc.) from the cathode. In one variation, hydrogen from the electrolyzer (e.g., produced in the electrolysis mode) can be used to reduce sulfur to hydrogen sulfide. In this variation, an electric pulse can be provided to drive the reaction, the electrolyzer can be heated to promote the reaction, the electrolyzer can be pressurized to promote the reaction, and/or the electrolyzer can otherwise be operated to promote the reaction. In another variation, oxygen can be provided to the electrolyzer where the oxygen can react with the sulfur to form sulfur dioxide (which can be captured and introduced into the electrolyzer during subsequent electrolysis mode operation). However, the electrolyzer can additionally or alternatively operate in any suitable mode of operation.

In a specific example of a method, sulfur dioxide and water can be provided to an electrolyzer, an electric potential can be applied to drive oxidation of the sulfur dioxide to sulfuric acid and reduce the water to hydrogen, the electric potential (for a fixed current density) can be monitored to detect whether the electric potential exceeds a threshold electric potential, and when the electric potential exceeds the threshold electric potential recovering the performance of the electrolyzer by removing sulfur deposits (e.g., adsorbed, absorbed, reacted with, intercalated within, etc.) on the cathode. The sulfur deposits can be removed, for example, using a reducing agent, an oxidizing agent, thermally (e.g., by melting the sulfur, by desorbing the sulfur, etc.), electrically (e.g., using an electric pulse in connection with an oxidizing or reducing agent), mechanically (e.g., using pressure, displacing the sulfur with another material by saturating the atmosphere, physically displacing the sulfur atoms, etc.), and/or in any suitable manner.

Variants of the technology can confer one or more advantages over conventional technologies.

First, variants of the technology can increase a lifetime of an electrolyzer (particularly a sulfur dioxide depolarized electrolyzer). Over the course of operation, a sulfur depolarized electrolyzer (SDE) can require increasing electric potential at a fixed current density to achieve the same sulfuric acid and hydrogen production (rate, quantity, concentration, etc.). The performance reductions are believed to result from sulfur species (e.g., sulfur dioxide, sulfur trioxide, sulfuric acid, etc.) crossing over the membrane and being reduced on the cathode. The inventors have discovered a favorable recovery mechanism that removes the reduced sulfur species from the cathode thereby improving performance (e.g., reducing energy requirements, extending lifetime, etc.) of the SDE. In some examples, a lifetime of the SDE can be extended from the order of 100 s of hours to the order of 10000 s of hours.

Second, variants of the technology can recycle sulfur that crosses over the separator improving the chemical efficiency (i.e., sulfur utilization) of the SDE. For instance, sulfur that crosses over the separator and is reduced on the cathode can be oxidized to form sulfur dioxide which is then captured and reintroduced into the SDE for oxidation.

However, further advantages can be provided by the system and method disclosed herein.

As shown in, an electrolyzer can include an anode, a cathode, and a separator. As shown for example in, the anode can include an anolyte, an electrode, an anolyte reaction region, an anolyte inlet, an anode distribution plate (e.g., defining the anolyte reaction region), an (oxidized) anolyte outlet, a diffusion layer, and/or any suitable components. As shown for example in, the cathode can include a catholyte, an electrode, a catholyte reaction region, a catholyte inlet, a cathode distribution plate (e.g., defining the catholyte reaction region), a (reduced) catholyte outlet, a diffusion layer, and/or any suitable components. However, the electrolyzer can include any suitable components.

The electrolyzer preferably functions to oxidize sulfur dioxide to sulfuric acid (and/or sulfur trioxide) with concurrent reduction of protons (e.g., H, from water, hydronium, etc.). However, the electrolyzer can additionally or alternatively function (e.g., using one or more alternative anolytes and/or catholytes).

The electrolyzer can be a unicell electrolyzer and/or a multicell electrolyzer (e.g., with a plurality of cells in parallel).

The electrolyzer(e.g., components thereof such as distribution plates, total spatial extent of anolyte or catholyte flow paths, membrane, electrodes, anode, cathode, diffusion layer, etc.) can have a spatial extent between about 10 cmand 1 m(e.g., 25 cm, 50 cm, 100 cm, 250 cm, 500 cm, 1000 cm, 2500 cm, 5000 cm, 10 dm, 25 dm, 50 dm, 100 dm, 250 dm, 500 dm, 1000 dm, values or ranges therebetween, etc.). However, the spatial extent can be less than 10 cmor greater than 1 m.

The anodepreferably functions to oxidize an anode fluid mixture (e.g., anolyte). The anode fluid mixture is preferably sulfur dioxide. However, other suitable anode fluid mixtures may be realized (e.g., SO). The anode fluid mixture can be provided in the gas phase (e.g., gaseous SO), liquid phase (e.g., condensed SO, SOdissolved in water, SOdissolved in sulfuric acid, SOdissolved in sulfurous acid, etc.), and/or in any suitable phase (e.g., dissolved in a solvent). In variants where the anode fluid mixture is provided in the gas phase, the anode fluid mixture can optionally include one or more carrier gases (e.g., inert gases such as inert to electrolysis like nitrogen, oxygen, argon, air, carbon dioxide, neon, methane, krypton, etc.). For instance, the anode fluid mixture composition can range from pure sulfur dioxide gas (e.g., 100% SO) to about 10% SO(by mass, by volume, by stoichiometry) with the remainder carrier gas.

The distribution plate(e.g., bipolar plates, flow field plate, etc.) can be made from (e.g., include) carbon material(s) (e.g., graphite; composite such as polymer matrix including thermoset resins like epoxy resin, phenolic resin, furan resin, vinyl ester, etc.; thermoplastic resin such as polypropylene, polyethylene, poly (vinylidene fluoride), etc.; etc. with a filler such as graphite powder, graphite flake, exfoliated graphite, coke-graphite, carbon nanotubes, carbon fiber, cellulose fiber, cotton flock, etc.; etc.), metal-composite (e.g., layered graphite, polycarbonate plastic, and stainless steel), metallic plates (e.g., stainless steel, aluminium, titanium, nickel, etc. optionally including a coating such as metal carbide, metal nitride, noble metal, metal oxide, catalyst, graphite, conductive polymer, etc.), and/or using any suitable material. The distribution plate can be solid (e.g., with cutouts, trenches, etc. defining an anolyte flow path or flow field; with structures protruding from a broad face of the distribution plate defining an anolyte flow path or flow field; etc. and through-holes defining inlets and/or outlets), porous (e.g., with a region analogous to the anolyte flow path where the anolyte primarily undergoes oxidation, where the distribution plate can act as a diffusion layer, etc.), and/or can have any suitable structure.

The optional diffusion layercan function to allow fluids (e.g., gases, liquids, solutes dissolved in the fluid, etc.) to diffuse to an electrode or catalyst layer (e.g., where the anolyte or species thereof can undergo oxidation). The diffusion layer can be made from porous carbon paper, carbon cloth, graphitized carbon paper, porous titanium (e.g., impregnated with platinum or other platinum group metals), stainless steel mesh, metal foam (e.g., nickel foam, copper foam, etc.), and/or can be made from any suitable material(s). The diffusion layer can optionally be hydrophobic. The diffusion layer is typically between about 100-1000 μm thick. However, the diffusion layer can be thicker than 1000 μm or thinner than 100 μm.

The anode electrode can include (e.g., be made from): platinum, gold, graphite, palladium, ruthenium, rhenium, iridium, rhodium, nickel, iron, combinations thereof (e.g., platinum-gold alloys), and/or any suitable electrode material can be used. In some variants, the anode electrode can be coated with the electrode material (e.g., where the coating material can act as a catalyst, protectant, etc.) and/or a catalyst material. Examples of catalyst materials include metal oxides (e.g., ruthenium oxide, palladium oxide, iridium oxide, titanium oxide, nickel oxide, iron oxide, etc.), nanoparticles (e.g., of an electrode material), carbon-based materials (e.g., carbon nanotubes, graphene, graphite, etc.), metal-organic frameworks (e.g., MOFs), polymer(s), alloys (e.g., Pt/C, PtRu/c, PtCo/C, etc.), combinations thereof, and/or any suitable materials. A catalyst loading is preferably between about 0.01 mg/cmand 10 mg/cm. However, the catalyst loading can be less than 0.01 mg/cmor greater than 10 mg/cm.

The anode catalyst preferably has a high specific surface area (e.g., a specific surface area greater than about 10 m/g, 15 m/g, 20 m/g, 25 m/g, 50 m/g, 75 m/g, 100 m/g, 150 m/g, 200 m/g, 250 m/g, 500 m/g, 1000 m/g, etc.).

The catalyst can form a coating (e.g., conformal coating, bumpy coating, porous coating, etc.), can include particles (e.g., nanoparticles such as nanospheres, nanorods, nanotubes, nanostars, nanoshells, nanopolyhedra, etc.; mesoparticles; microparticles; etc. such as hollow particles, porous particles, solid particles, etc.) that can be deposited on a surface, and/or can have any suitable structure (e.g., engineered structure).

In some variants, catalyst can be disposed on (e.g., deposited on) surfaces of the anolyte reaction region (e.g., in addition to or as an alternative to coating or being the electrode). For instance, the distribution plate can be made of the catalyst, the distribution plate can include structures made from the catalyst the define the anolyte reaction region, walls or surfaces defining the anolyte reaction region can include catalyst, and/or the catalyst can otherwise be disposed on surfaces of the anolyte reaction region.

The cathodepreferably functions to reduce a catholyte species. The catholyte is typically protons (usually provided as hydronium ions, dissolved in water, etc.) resulting in a reduced catholyte species of hydrogen (H) or isotopes thereof. For instance, the catholyte can be water, acidic water (e.g., water with one or more acids dissolved therein such as nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, triflic acid, perchloric acid, etc.), and/or other suitable species. In some examples, the catholyte (and/or anolyte) can include dissolved additives (e.g., salts such as potassium sulfate, sodium sulfate, lithium sulfate, ionic liquids, etc.) to modify viscosity, electrical conductivity, and/or other properties of the catholyte. The catholytes are typically provided dissolved in a solvent (e.g., water). However, the catholytes can be provided in gas phase, liquid phase, and/or in any suitable phase (e.g., plasma). In some variants, the catholyte can crossover the membrane (e.g., separator) and into the anolyte reaction region.

The cathode distribution plate′ (e.g., bipolar plates, flow field plate, etc.) can be the same as and/or different from the anode distribution plate. For instance, the cathode distribution plate can be made in the same manner as, from the same material as, have the same dimensions as, a catholyte reaction region (or catholyte flow path) that is the same as (e.g., mirror image of, has the same structure as, etc.) the anolyte reaction region (or anolyte flow path), and/or can otherwise have any suitable distribution plate as described for an anode distribution plate.

The cathode diffusion layer′ can be any suitable anode diffusion layer (e.g., as described above). The cathode diffusion layer can be the same as and/or different from the anode diffusion layer.

The cathode electrode can include (e.g., be made from, be coated with, etc.): platinum, gold, carbon (e.g., graphite, carbon black, etc.), palladium, ruthenium, rhenium, iridium, rhodium, nickel, iron, titanium, combinations thereof (e.g., platinum-gold alloys), and/or any suitable electrode material can be used.

In some variants, the cathode electrode (e.g., a substrate, support layer, etc. preferably with high electrical conductivity) can be coated with the electrode material (e.g., where the coating material can act as a catalyst, protectant, etc.) and/or a catalyst material (e.g., electrocatalyst). Examples of catalyst materials include metal oxides (e.g., ruthenium oxide, palladium oxide, iridium oxide, titanium oxide, nickel oxide, iron oxide, etc.), nanoparticles (e.g., of an electrode material), carbon-based materials (e.g., carbon nanotubes, graphene, graphite, etc.), metal-organic frameworks (e.g., MOFs), polymer(s), alloys (e.g., Pt/C, PtRu/c, PtCo/C, etc.), combinations thereof, and/or any suitable materials. A catalyst loading is preferably between about 0.01 mg/cmand 10 mg/cm. However, the catalyst loading can be less than 0.01 mg/cmor greater than 10 mg/cm.

The cathode catalyst preferably has a high specific surface area (e.g., a specific surface area greater than about 10 m/g, 15 m/g, 20 m/g, 25 m/g, 50 m/g, 75 m/g, 100 m/g, 150 m/g, 200 m/g, 250 m/g, 500 m/g, 1000 m/g, etc.). However, the cathode catalyst can have a low specific surface area (e.g., <10 m/g), different specific surface area for different surfaces it is disposed on, and/or can have any suitable specific surface area.

The catalyst can form a coating (e.g., conformal coating, bumpy coating, porous coating, etc.), can include particles (e.g., nanoparticles such as nanospheres, nanorods, nanotubes, nanostars, nanoshells, nanopolyhedra, etc.; mesoparticles; microparticles; etc. such as hollow particles, porous particles, solid particles, etc.) that can be deposited on a surface, and/or can have any suitable structure (e.g., engineered structure).

The catholyte reaction region (e.g., catholyte flow path) can be analogous to any anolyte reaction region as described above. For instance, the catholyte reaction region can be the same as and/or different from the anolyte reaction region. The catholyte reaction region is preferably a mirror image of the anolyte reaction region (e.g., mirror image across the membrane).

The separator(e.g., membrane, diaphragm) preferably functions to shuttle ions (e.g., protons) and/or molecules (e.g., solvent molecules such as water) between the anode and the cathode while hindering (e.g., preventing) the anolyte, catholyte, oxidized anolyte products, reduced catholyte products, and/or other species (e.g., electrons, electricity, etc.) from crossing the separator and/or electrically insulating the anode and cathode (from one another). The separator is preferably arranged between the anolyte reaction region and the catholyte reaction region. However, the separator can be arranged in any suitable manner (e.g., a plurality of separators can be used).

The separator thickness is typically between about 10 μm and 500 μm (e.g., to optimize for selectivity in hindering anolyte or catholyte crossover and electrical conductivity). However, the separator can be thinner than 10 μm or thicker than 500 μm.

The separator can be made from fluoropolymers (e.g., nafion, fumapem, fumasep, aquivion®, etc. such as Nafion 112, Nafion 115, Nafion 117, Nafion NR211, Nafion NR212, Nafion 1110, Nafion 324, Nafion 424, Nafion 438, Nafion 551, Nafion NE1035, Nafion HP, Nafion XL, etc.), polybenzimidazole (PBI) membranes (e.g., doped with phosphoric acid, sulfuric acid, etc. such as Celtec®-L, Celtec®-P, Celazole®, etc.), sulfonated polybenzimidazole (s-PBI such as copolymers of poly[2,2′-(m-phenylen)-5,5′-bisbenzimidazole] with 3,3′-diaminobenzidine (DABD), 4,4′-oxybis(benzoic acid) (OBBA), 5-sulfoisophthalic acid (SIPA) and 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (DSNDA), etc.), sulfonated Diels-Alder poly (phenylene) membranes (SDAPP such as polymers formed by Diels-Alder polymerization of 1,4-bis(2,4,5-triphenylcyclopentadienone)benzene and 1,4-diethynylbenzene followed by sulfonation of the resulting polymer), sulfonated poly(ether sulfone)s, silicon carbide (e.g., saturated with phosphoric acid, sulfuric acid, etc.), polytetrafluoroethylene (PTFE), glass (e.g., glass fiber membrane), aromatic polymers (e.g., PEEK), protic ionic liquids, protic ionic plastic crystals, ionomers (e.g., perfluorosulfonic acid (PFSA), PFSA-silica composites, Aciplex™, Flemion™, etc.), composite membranes (e.g., composites of glass and one or more polymer such as polymers used in the production of other membranes from the above list, composites of nafion and silica, composites of nafion and titania, composites of nafion and zirconium phosphate, etc.), and/or using any suitable separator. For example, a perfluorosulfonic acid/PTFE copolymer can be used as the separator.

The SDE is preferably operable in a plurality of modes. Examples of operation modes include a shutdown mode (e.g., when no energy is supplied), electrolysis mode (e.g., when energy and electrolytes are supplied), a recovery mode (e.g., when energy and/or recovery materials are supplied), and/or other suitable modes. The modes are typically distinct modes (e.g., only a single mode is performed at a time). However, in some variants, a plurality of modes can operate simultaneously (e.g., a shutdown mode and recovery mode can be considered contemporaneous when electrical energy is not provided during the recovery mode, an electrolysis mode and a recovery mode can be performed contemporaneously when a reactive or recovery agent is added to the catholyte or anolyte during operation of the electrolyzer, etc.).

During electrolysis (e.g., the electrolysis mode), the electrolyzer operating temperature is preferably between about 60° C. and 150° C. (e.g., 75° C., 80° C., 85° C., 90° C., 100° C., 105° C., 110° C., 120° C., 140° C., etc.). However, the electrolyzer can be operated at any suitable temperature. The electrolysis temperature is typically achieved primarily based on the electrolysis reaction being performed (and the amount of reaction thus proportional to the current density). However, additionally or alternatively, the temperature can be achieved using heaters, using sensible heat from other reactions (e.g., sulfur combustion), and/or using other suitable heat source(s). During electrolysis (e.g., the electrolysis mode), the electrolyzer preferably operates at a current density of between about 0.4 and 1 A/cm(e.g., 0.4 A/cm, 0.45 A/cm, 0.5 A/cm, 0.55 A/cm, 0.6 A/cm, 0.75 A/cm, 0.8 A/cm, 0.9 A/cm, etc.). However, the electrolyzer can operate at any suitable current density. During electrolysis (e.g., the electrolysis mode), the electrolyzer preferably operates at an initial electric potential that is approximately 0.75 V (e.g., 0.73-0.78V). During continued operation, the initial electric potential (i.e., an amount of overpotential) typically increases resulting from sulfur formation (e.g., sulfur dioxide crossing over through the separator and being reduced on the cathode, within a diffusion layer, within a flow field, etc.), reduction of electrode surface area (e.g., due to annealing, sintering, metal hydrogenation, etc.), membrane or separator drying out over time, and/or other mechanisms. However, the electrolyzer can operate under a higher initial electric potential (e.g., with increased energy cost) and/or lower electric potential (e.g., with a reduced sulfuric acid output concentration).

In one specific example, the electrolyzer can be operated in a manner as disclosed in U.S. patent application Ser. No. 18/598,324 titled ‘SULFUR DIOXIDE DEPOLARIZED ELECTROLYSIS AND ELECTROLYZER THEREFORE’ which was filed on 7 Mar. 2024 and/or U.S. patent application Ser. No. 19/087,106 titled ‘SULFUR DIOXIDE ELECTROLYZER WITH IMPROVED SULFURIC ACID CONCENTRATION FORMATION AND METHOD OF OPERATION’ which was filed on 21 Mar. 2025 and is incorporated in its entirety by this reference.

During recovery (e.g., the recovery mode), the electrolyzer can be operated in the same and/or different conditions (e.g., temperature, pressure, etc.) from those performed in the electrolysis mode. In some variants, the recovery mode can include heating the electrolyzer (e.g., without providing water, without providing sulfur dioxide, etc.) to a temperature greater than about 115° C. to melt sulfur, where the molten sulfur can then be collected as a liquid. In another variant, the recovery mode can include providing an electrical potential pulse (e.g., reversing polarity of the anode and cathode, increasing an applied electric potential, etc. for a threshold amount of time, in cycles, etc. while providing the sulfur dioxide and water), which can result in reduction of sulfur on the cathode to hydrogen sulfide (e.g., by reacting the sulfur with hydrogen from the electrolysis mode). In another variant, the recovery mode can include providing oxidizing agents (optionally with heating to promote the reaction) to the cathode, where the oxidizing agents (e.g., oxygen) can react with sulfur to form sulfur dioxide (which can be recovered and subsequently oxidized in an electrolysis mode). However, the recovery mode can include any suitable processes (e.g., a combination of the above variants can be applied in tandem such as sequentially during the recovery mode to improve sulfur removal in a single instantiation of the recovery mode).

In variants of the recovery mode that include electrochemical performance restoration (e.g., oxidation or reduction of sulfur, oxidation or reduction to increase electrode surface area, etc.), the recovery mode can be performed with substantially constant electrical potential, with substantially constant current density, with variable electrical potential, with variable current density, and/or with other suitable electrical properties. In these variants, the electrodes are typically switched (e.g., connected to the power source, power supply, etc. at opposite terminals or polarities compared to during operation in electrolysis mode) as such oxidation occurs at the ‘cathode’ and reduction occurs at the ‘anode’ (where the electrodes are being named based on their operation or electrical polarity during electrolysis mode). Typically, the electrical potential in the recovery mode will be limited to within a maximum threshold (typically around 1 V) and a minimum threshold (typically around −0.5 V) electrical potential (e.g., based on the standard reduction potential of the anode and/or cathode to avoid phase changes, irreversible oxidation or reduction, irreversible material changes, etc. resulting from applying too large of an electrical potential of either polarity). Typically, the current density in the recovery mode are significantly smaller than (e.g., 1/10, 1/20, 1/50, 1/100, 1/200, 1/500, 1/1000, 1/10000, 1/20000, 1/50000, 1/100000, etc. of) the electrolysis mode (which can be performed in this manner to slow or hinder degradation; because the amount of available material to react, such as sulfur, is significantly less than during electrolysis mode; etc.). As a specific example, the current density can be approximately 1 mA/cmin the recovery mode.

As a first illustrative example (as shown for instance in), in the recovery mode, a periodic electrical potential (e.g., sinusoidal waveform, triangle waveform, sawtooth waveform, cycloid waveform, etc.) can be applied to the electrolyzer, where the electric potential can rise or fall cyclically between a first and second electric potential (where the first and second electrical potential are each bounded by the maximum and minimum threshold electrical potential for the recovery mode operation). In a variation of the first illustrative example (as shown for instance in), in the recovery mode, a pulsed electrical potential (e.g., square waveform, rectangular waveform, etc.) can be applied to the electrolyzer, where the electric potential can be the same or different for each electrical potential pulse. In the first illustrative example or variations thereof, the electrical potential can have a time-varying electrical potential (e.g., amplitude modified electrical potential). In the first illustrative example or variations thereof a pulse duration (e.g., peak to peak time, peak to valley time, valley to peak time, etc.) or relatedly frequency (e.g., proportional to the inverse of the pulse duration) can be constant and/or can vary (e.g., can have a time dependent frequency, time dependent phase, etc.). For instance, a pulse duration can be on the order of about 1 s (e.g., a value within the bounded range of 0.5 s and 5 s). However, the pulse duration can be less than 0.5 s or greater than 5 s.

As a second illustrative example (as shown for instance in), in the recovery mode, a substantially constant electrical potential (e.g., varies by less than 20%) can be applied to the electrolyzer, where the electric potential can be between the maximum and minimum electrical potential for the recovery mode operation.

In some variants of the recovery mode (as shown for instance in), the recovery mode can be performed in a manner as described for the first and second illustrative examples (or variations thereof), where instead of electric potential being varying and/or constant, the current density can be constant and/or varying. Additionally, or alternatively, these variants can be combined (e.g., for some phases of recovery electric potential can be varied while in other phases current density can be varied).

The mode of operation is typically switched based on one or more properties determined during the electrolysis mode (e.g., from sensor readings during operation in the electrolysis mode). However, the mode of operation can additionally or alternatively be switched based on a maintenance schedule (e.g., after a threshold amount of time operating in the electrolysis mode such as 50 hours, 100 hours, 150 hours, 200 hours, 300 hours, 500 hours, etc. the electrolyzer can be switched to a recovery mode) and/or in any suitable condition(s). Examples of properties used to determine whether to change an operation mode can include: electric potential applied, current density, hydrogen production rate, sulfuric acid concentration, sulfuric acid production rate, temperature, hydrogen selectivity (e.g., hydrogen purity, selectivity with which hydrogen is formed at the cathode, etc.), and/or other suitable properties (e.g., where a triggering condition is evaluated for at least one or more fixed state of the other properties). As an illustrative example (as shown in), the electric potential (at a fixed current density) can be evaluated to determined whether a recovery mode should be performed. When the electric potential exceeds a threshold (e.g., a fixed threshold such as 0.85 V, 0.9 V, 0.92 V, 0.93 V, 0.95 V, 0.97 V, 1V, 1.05 V, 1.1 V, 1.2 V, etc.; a threshold relative to initial operating conditions such as an electrical potential 5%, 10%, 15%, 20%, 25%, 30%, 33%, 50%, etc. greater than an initial or target electric potential; etc.), the recovery mode can be performed (e.g., to return the electric potential at the fixed current density to substantially a baseline or initial electric potential, to decrease the electric potential at the fixed current density, etc.). However, any suitable property (with an associated threshold) can be used to switch operation modes.

The recovery mode is preferably performed at most once per every 100 hours of electrolysis mode operation. However, the recovery mode can be performed with any suitable frequency (e.g., as the electrolyzer separator degrades and sulfur dioxide cross over increases the recovery mode can be performed more frequently, performed less frequently when sulfur deposition is not observed or detected, etc.). Having relatively rare instances of the recovery mode can be beneficial, for instance, as the recovery mode can also result in degradation of the electrolyzer (e.g., carbon in the electrolyzer or components thereof can be removed in addition to sulfur deposits resulting in degradation of the electrolyzer over time). In some variants, the recovery mode conditions can be tuned to optimize for removal of sulfur rather than carbon (e.g., temperature, oxidizing agent, reducing agent, flow rate, location of introduction, etc.). Relatedly, in multicell electrolyzer, typically different cells will have different amounts of deposited sulfur, but are not individually addressable (i.e., normally all cells of a multicell electrolyzer have substantially the same operating conditions). In one variant, the electrolyzer can be configured to enable individually address each cell (at least in a recovery mode of operation). In another variant, each cell can include an additional (preferably passive, but potentially active) electrical element that can shut-down operation of each cell individually based on electrical properties and/or measurement of residual sulfur within individual cells. As a third variant (as shown for instance in), the recovery mode can be operated using pulsed electric potentials (rather than continuous electric potentials) which can reduce the amount of stress on individual cells while enabling the recovery mode to improve performance of all cells.

In some variants, the recovery mode can additionally or alternatively act as a start-up mode (e.g., after a shut-down mode, before a first use of an SDE, etc.), can be performed after an incorrect shut-down, and/or can perform other suitable functions (e.g., be used in other suitable situations).

As shown in, a method can include electrochemically oxidizing sulfur oxide and reducing water S, determining performance metrics of the electrochemical reaction S, and recovering performance of the electrochemical reaction S. The method can additionally or alternatively include any suitable steps.

The method preferably functions to produce (and maintain a rate, purity, efficiency, etc. of said production) sulfuric acid and hydrogen. However, the method can additionally or alternatively function.

The method can be performed continuously and/or intermittently (e.g., only when green energy sources are available, when demand for hydrogen exceeds a threshold demand, when demand for sulfuric acid exceeds a threshold demand, etc.). All or portions of the method can be performed in real time (e.g., responsive to a request), iteratively, concurrently, asynchronously, periodically, and/or at any other suitable time. All or portions of the method can be performed automatically, manually, semi-automatically, and/or otherwise performed.

The method (and/or steps thereof) are preferably performed using an electrolyzer (e.g., an SDE as described above, an electrolyzer as described in U.S. patent application Ser. No. 18/598,324 titled ‘SULFUR DIOXIDE DEPOLARIZED ELECTROLYSIS AND ELECTROLYZER THEREFORE’ filed 7 Mar. 2024 which is incorporated in its entirety by this reference). However, the method can be performed using any suitable system(s).

Electrochemically oxidizing sulfur dioxide and reducing water Sfunctions to generate sulfuric acid and hydrogen. Sis typically performed by an electrolyzer operating in an electrolysis mode. However, Scan be performed in any system.