Water Electrolyzer

This patent application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent App. No. 63/641,656, filed May 2, 2024, entitled “WATER ELECTROLYZER,” incorporated herein by reference in its entirety.

Worldwide electricity generation is expected to double by 2050, in large part due to the rapidly increasing demand from economically emerging nations. It is further expected that by 2050, two-thirds of the power generation will be from variable renewable electricity (VRE) generators, e.g., solar and wind, replacing the conventional power plants based on fossil fuels, i.e., coal, and natural gas. Temporally responsive electrical energy storage, including batteries and water electrolyzers, would be a key enabler in this transition, since solar and wind power are unpredictable and, unlike fossil-fueled power plants, cannot be turned on-or-off to meet demand. Thus, there is a need for storing excess renewable power generated during periods of low demand.

Green hydrogen (H) from electrolysis of water, is expected to serve as the central link between a variable renewable electric (VRE) grid and hard to abate energy sectors, storing as hydrogen the excess energy from VRE generators, solar and wind, when the electricity demand is low, and using it to generate electricity when the demand is high. Green hydrogen may also be employed as a fuel in transportation and/or as a feedstock in the chemical industry, where when combined with captured or recycled CO, it can replace the conventional fossil feedstocks, or can provide green ammonia when combined with Nas a Hcarrier or as a fertilizer. Further, it can help to decarbonize other large-scale industries including steel and cement manufacture. The low-temperature electrolysis (LTE) candidate technologies poised to be the most widely employed in green Hgeneration include: 1) the liquid alkaline-water electrolyzer (L-AWE) involving a porous diaphragm (PD) and an alkaline water electrolyte feed; and 2) the proton-exchange membrane pure water electrolyzer (PEM-PWE) with a pure water feed. The L-AWE is a mature, century-old technology, that however, is not suitable for direct integration with VRE. For this, the PEM-PWE is the leading candidate. However, the PEM-PWE requires ultrapure deionized (DI) water as feed to avoid fouling of membranes and catalysts by the ionic impurities in water, which adds to cost and complexity of the balance-of-plant (BOP). This invention modifies the conventional PEM-PWE such that it can tolerate a direct impure water feed, thus reducing the cost, complexity, and bulk of the BOP.

A water electrolysis device includes a membrane-electrode assembly (MEA) incorporating a catalyzed proton-exchange membrane disposed between an anode consisting of a porous-transport layer (PTL) flanked by an anode flow field connected to an aqueous input and for removing oxygen produced at the anode, and a cathode comprising of another PTL flanked by a cathode flow field for removing the hydrogen gas produced at the cathode. A voltage source connects between the anode (positive electrode) flow field and the cathode (negative electrode) flow field for imparting a voltage differential across the MEA to cause water electrolysis. A selective layer disposed between the anode and/or cathode flow field and the aqueous input prevents the passage of ions and other impurities to the MEA, such that the water need not undergo rigorous deionization and/or filtration pretreatment as in conventional PEM-PWE. The selective layer is a porous hydrophobic (water-fearing) layer, such as, Teflon®, that is further aerophilic (gas-loving) and thus allows for the passage of water vapor as well as the evolved gases while preventing feed liquid-water, along with its impurities, to pass through.

Configurations herein describe a direct impure water electrolyzer (DIWE), thus adapted from a conventional proton-exchange membrane-pure water electrolyzer (PEM-PWE), that allows direct impure, brackish, or saline water electrolysis, based on the use of one or more in-situ porous hydrophobic layers (PHLs), or membranes, for retaining dissolved salts and any other impurities in impure water feeds, while allowing only molecularly pure water vapor to permeate its hydrophobic pores to reach the MEA, and removing the gases produced. In the disclosed approach, the water vaporization necessary for transport through the PHL occurs in-situ within the cell, and does not need any external energy. Thus, the required heat of vaporization is supplied largely by the heat of the oxygen evolution reaction (OER) at the anode, combined with any heat of vapor condensation within the MEA. Further, the electrolyzer is designed to retain the condensed liquid water within the MEA so that the PEM remains well-hydrated as does the ionomer in the catalyst layers, providing good performance.

In a particular configuration, the porous hydrophobic layer (PHL) for in situ water purification is interposed, along with necessary modifications to the flow-fields and gaskets, between the impure liquid-water feed flow-field channels at the anode and a conventional PEM-electrolyzer membrane-electrode assemble (MEA). The latter, for instance, typically includes a titanium (Ti) anode flow-field, a porous Au- or Pt-plated Ti felt porous transport layer (PTL) for the oxygen electrode, an IrOcatalyst for the oxygen evolution reaction (OER), a proton-exchange membrane (PEM) such as Nafion®, or a nanocomposite-Nafion®, the latter being more suited to a low humidity, higher-temperature cell environment, a Pt/C catalyst for a Hydrogen Evolution Reaction (HER), a carbon gas-diffusion layer (C-GDL) with a microporous layer (MPL) for the negative electrode (cathode), and a graphite cathode flow field.

A PEM water electrolyzer can be modified as disclosed herein for use with a direct impure or saline water feed with little modification to the cell architecture, facilitating adaptation of existing PEM-PWE manufacturing/assembly processes with small changes, along with a smaller balance of plant (BOP). Furthermore, the direct removal of evolved gases via the PHL without significant bubble formation at the electrode which otherwise cover the catalyst limiting its effectiveness and resulting in an overpotential, results in efficient operation of the overall electrolyzer system. Such an electrolyzer would be especially useful in terrestrial or extra-terrestrial applications with a paucity of fresh water sources, and/or an absence of gravity or presence of micro-gravity, where the absence of buoyancy forces makes the removal of evolved gas bubbles from the catalyst as well as gas-liquid separation substantially more challenging.

The description and drawings below depict example configurations of electrolysis using the disclosed water electrolyzer suitable for use with impure (non-deionized, non-desalinized) water sources in several configuration incorporating a selective membrane for controlling and shifting the electrolysis reaction for accommodating various input streams and output products.

is a context diagram of an electrolyzerfor producing hydrogen as an energy vector and as a raw material for various industries. Referring to, green hydrogen (H) from electrolysis of water is expected to serve as the central link between a variable renewable electric (VRE) grid and hard to abate energy sectors, storing excess energy from VRE generators, solar and wind, when the electricity demand is low, and employed as a fuel in transportation and/or as a feedstock in the chemical industry, where when combined with captured or recycled CO, it can replace the conventional fossil feedstocks, petroleum and natural gas, or can provide green ammonia when combined with Nas a hydrogen carrier or as a fertilizer. It can also potentially help to decarbonize other large-scale industries including steel and cement manufacture. It should be noted that the notion of “green” hydrogen production refers to a reduction or absence of combustion, high temperatures and carbon emissions associated with conventional hydrogen production, often relying on fossil fuels, as from methane steam reforming.

One technology likely to be most widely employed in such green Hgeneration is low-temperature electrolysis (LTE) of water, because of its technological maturity and high efficiency, and its ability to potentially directly use DC from solar and wind generators. The two commercially mature LTE technologies are: 1) the liquid alkaline-water electrolyzer (L-AWE) involving a porous diaphragm (PD) and an alkaline water electrolyte feed; and 2) the proton-exchange membrane pure water electrolyzer (PEM-PWE) with a pure deionized (DI)-water feed. The L-AWE is an inexpensive, scalable, and commercially available century-old technology, but suffers from low current density, inefficiency, and slow-start and limited operational flexibility needed for coupling to the solar or wind generators, which can instantaneously go from zero to full power or vice versa, or for using grid electricity during off-peak hours. The replacement of the porous diaphragm by an anion-exchange membrane (AEM) in an alkaline water electrolyzer (AEM-AWE) could potentially provide lower Ohmic losses and better gas separation and flexibility. Unfortunately, the durability of AEMs in alkaline electrolytes is low, although AEMs with better stability are being actively researched, so that it is not yet commercially deployed. However, the AEM-AWE technology can benefit from additional development and refinement to promote it as viable for commercial deployment.

In contrast to the L-AWE, the commercially available PEM-PWE is responsive and amenable to start-stop operation and thus direct VRE integration, further allowing differential operating pressure, high current and power densities, and high efficiencies. The main issues with PEM-PWE are that: 1) it requires ultra-high purity water, 2) its cost is high because of the requirement of precious-group metal (PGM) catalysts along with an anode porous transport layer (PTL) that is based on Pt- or Au-plated titanium, and 3) its life is limited in large part because the MEA eventually gets poisoned by ionic impurities in even ultra-high purity DI water. In fact, the realization of the envisioned green hydrogen economy depends in large part on the underlying expectations that: 1) the cost of the electrolyzers can be reduced dramatically; and that 2) there is an adequate supply of potable water collocated with abundant sunshine and/or wind resources across the globe. However, many arid areas that have a high VRE potential have only a limited supply of fresh water.

shows a block diagram of the proposed water electrolyzer in the context ofsuitable for use with configurations herein. Referring to, the water electrolyzeris generally defined by a containmentfor encapsulating a layered or stacked structure for fluid and gaseous exchange as provided by the respective layers. The containmentincludes an anodein an anode flow fieldin fluid communication with an aqueous input, and a cathodein a cathode flow fieldconfigured for passing a gaseous product. Both the aqueous inputand the gaseous productare in fluid communication with a respective impure water feedand a gaseous output, and form a generally contained fluid volume.

A proton-exchange membrane (PEM)is disposed between the anode flow fieldand the cathode flow fieldand provides for proton migration for hydrogen gas (H) generation, based on appropriate catalysts as described below. A voltage sourceconnects between the anodeand the cathodefor imparting a voltage differential across the proton-exchange membrane. The layered structure of the containmentprovides appropriate electrical communication between the electrode (anodeand cathode) layers, the voltage sourceand the flow fields,. A selective membrane′ is disposed between the anodeand the aqueous inputpreventing passage of contaminants in the aqueous inputand relieves the need for ultrapure water found in conventional electrolysis operations.

The selective membrane′ is typically a porous hydrophobic layer (PHL)permeable to water vapor for allowing only passage of gaseous water and a return of oxygen gas () to be expelled. It is expected, therefore that the selective membrane is hydrophobic and while permeable to water vapor and oxygen gas is impermeable to liquid water and the contaminants therein. In general, the PHLis a hydrophobic membrane resistive to the transport of dissolved ions, while other solid contaminants and particles will likely have been filtered from the impure water feed.

shows a schematic diagram of a conventional PEM electrolyzerplant showing a systememploying conventional PEM-PWE stack and the balance-of-plant (BOP) including the various water purification steps. The attainment of low-cost targets would require steep reductions in both operating and capital expenses of the electrolyzer stack, as well as those of the balance-of-pant (BOP), as shown in. Referring to, a significant factor in operating expenses is the cost of renewable electricity, which declines substantially during off-hours. Thus, electrolyzers should be able to operate dynamically to access the low-cost VRE, with frequent starts and stops.

For reducing the capital expenses, cheaper and Earth-abundant materials that are further not supply constrained must be utilized, and the BOP, i.e., the DI water purification and the power conditioning plants, needs to be further simplified and provide a smaller footprint. Conventional low-temperature electrolyzers require ultra-pure DI water as feed so as not to contaminate the membranes and the electrodes, and to not produce any chlorine containing side products. This adds substantially to the cost and footprint of the overall electrolyzer plant, which is roughly divided as a third for the power conditioning plant, a third for water purification, gas-separation, and pure water recirculation plant, and a third for the electrolyzer stack. In short, to meet the cost targets, 1) the electrolyzers must be capable of being directly coupled to the variable DC generators to reduce power conditioning costs, and 2) should preferably be able to work directly with impure water feeds containing ions to reduce water treatment and recycle costs.

There is, however, a paucity of fresh water in many arid parts of the world that are otherwise endowed with abundant sunshine and wind resources. Saline water, ranging from brackish to sea water, is characterized by a high dissolved salt content, mainly NaCl, present as Naand Clions, ranging from >1,000 (0.1%) for brackish water to 30,000 ppm (3.0%) for sea water, but also containing hard-water components such as Ca, Mg, and COions. Other impurities can include organics, microplastics, bacteria, particulates, and gases, which can foul the membrane, electrodes, and catalysts. The process from raw water to the ultrapure water required for electrolysis can be divided into two steps (): 1) pretreatment of raw water such as desalination at, and 2) polishingto the required ultrapure standard needed for the electrolyzer stack. The goal is to produce water with the preferred quality of ASTM Type I DI water, i.e., with a resistivity of >10 MΩ cm.

The pretreatment steps, depending on the source of raw water, include, e.g., aeration and sand filtration to remove dissolved redox-active species involving iron and manganese, and filtration and other steps including ultrafiltration and UV to remove particles, organics, and microorganisms. Beyond the pretreatment steps, production of ultrapure water suitable for electrolyzers include removal of hardness due to multivalent cations such as Caand Mg, which are exchanged with soluble monovalent cation Navia water softening, and finally removal of most of the ionic load via reverse osmosis (RO). The permeate from the first RO step is passed through a secondary RO system for further reduction of salts. The permeate from the first RO process is filtered again in a secondary RO system. Thereafter, to produce ultrapure water with very low conductivity required for electrolyzers, a final deionization (DI) or an electrodeionization (EDI) step is used, wherein any remaining ions are exchanged with for Hand OH. All these steps can add substantially to the BOP footprint and cost, as typically 10 L of ultrapure water is needed per kg of Hproduced. Clearly, if impure/saline water, after the pretreatment steps, but containing most of the ions present in raw water such as sea water could be directly used in a PEM electrolyzer, it could provide a significant reduction in size and cost of the plant. This is the objective of configurations described herein.

However, there are significant challenges for using direct impure/saline/sea water feeds to PEM or alkaline water electrolyzers, including:

causing a significant increase in membrane resistance, and a reduction in lifetime.

shows a schematic of available processes that can occur during the electrolysis of saline water. Referring to, if there is salt (NaCl) in saline/sea water, it results in chloride (Cl) anion in it via ionization, which can lead to the undesirable chlorine evolution reaction (CER) at the anode, as follows:

in addition to the oxygen evolution reaction (OER), since CER has an electrode standard Nernst potential

not much higher than that of the OER

Here, SHE is the standard hydrogen electrode potential. The electrode operating potential is typically significantly higher to overcome the large overpotential of the OER under acidic conditions, so that there can be a significant overpotential promoting CER as well, as the CER is typically a more facile reaction than the OER. The generation of chlorine in a PEM water electrolyzer is undesirable because of its toxic and corrosive nature. Further, the resulting sodium cations (Na) cross over to the cathode and result in the generation of NaOH and H:

where the electrode standard Nernst potential

at a pH=14.

The overall process of these undesirable side reactions in the cell may hence be represented by the electrolysis of saline/sea water:

which, in fact, represents the commercial chlor-alkali process for producing NaOH and Clfrom brine, the Hbeing a side product in the production of chlorine and caustic soda.

The consumption of protons and the generation of NaOH at the cathode further results in a local pH increase at the electrode-electrolyte interface. This can further cause precipitation at the electrode due to the formation of sparingly soluble species such as Ca(OH), Mg(OH), and Ca(CO), from the Ca, Mg, and

ions present in saline water besides NaCl.

Under conditions when the anode is not strongly acidic but rather neutral or alkaline, chloride ion can undergo alternate electrode reactions, e.g., with the hydroxyl ion to produce the hypochlorite ion:

with the electrode standard Nernst potential

at a pH=14. In comparison, the ORR at a pH=14 has a standard Nernst potential

and an operating voltage, including OER overpotential, of around 0.7 to 0.8 V, i.e., below the Nernst potential of 0.89 V, so that conditions are typically not conducive to the formation of hypochlorite ion (OCl). In other words, interference from chloride ion chemistry at the OER electrode is thermodynamically unlikely under neutral or alkaline conditions (pH≥7.5). Thus, the maximum thermodynamic potential difference between the chloride chemistry and the water oxidation is 480 mV for pH>7.5, for chloride to be oxidized to hypochlorous acid, HClO, or to the hypochlorite ion, OCl. Thus, liquid-alkaline water electrolyzers (L-AWE) do not suffer from chloride-based side reactions, and can hence tolerate some salinity in feed water. On the other hand, the PEM-PWE are extremely sensitive to saline-water feeds.

Thus, the conventional approach to avoid chlorine generation and other side reactions, membrane fouling, and electrode precipitation in PEM-PWEs is to rigorously desalinate a saline water feed via reverse osmosis followed by deionization (DI) or electrodeionization (EDI) to reduce the salt and other ion content down to virtually zero to provide an ultra-pure water feed, as discussed above. However, this is expensive, and the water treatment plant has a large footprint. This is especially an issue where space is at a premium. For instance, there is a particular interest in offshore direct seawater electrolysis systems for their compactness, since plant footprint dominates the installation costs. However, because of microbial content and other particulate contaminants that are not dissolved, a simple pretreatment including filtration of the saline or natural water feeds is essential even for direct water feeds.

It is noteworthy that conventional approaches aimed at developing direct seawater electrolysis have been based on alkaline water electrolyzers, which operate at low current densities and are less sensitive to ionic impurities in feed water and can largely avoid the unwanted chloride reactions, as discussed in the preceding. However, L-AWEs are not suitable for direct integration with VRE. On the other hand, PEM-PWE operate at high current densities providing high performance and dynamic operation suitable for VRE integration, and are especially attractive where space is at a premium, e.g., on an offshore platforms. However, there have been no reports so far of efforts to develop PEM-water electrolyzers for direct saline water electrolysis. Thus, we believe that the invention described here wherein a PEM-PWE is equipped with an in-situ porous hydrophobic layer (PHL) that allows only pure water vapor to access its MEA and operates at higher current densities than the L-AWE is without precedence. However, the conceptual basis of this invention is in part supported by a report wherein the researchers proposed to utilize an externally raised vapor feed from saline water to the anode of a PEM-PWE, thus avoiding the CER and the attendant corrosion issues, while the saline water feed is sent to the cathode chamber, where chloride electrode chemistry is not a concern, and the Nafion® PEM used is able to largely reject the chloride ions diffusion from the cathode to the anode.

Turning now to the beneficial approach disclosed herein, configurations below adapt a PEM-electrolyzer for direct impure water electrolysis by equipping it with one or two in-situ porous hydrophobic layers (PHLs), that are highly effective, by retaining all dissolved salts and any other impurities in impure liquid water feeds, while allowing only molecularly pure water vapor to permeate its hydrophobic pores and access the MEA. In our approach, the membrane-based water vaporization occurs within the cell, and does not need any input of external energy. Rather, the required heat of vaporization is supplied in-situ by the heat of the oxygen evolution reaction (OER) at the anode, combined with any heat of vapor condensation within the ionomer layer in catalyst layer. Further, the electrolyzer is designed to retain liquid water within the cell so that the PEM remains well-hydrated as does the ionomer in the catalyst layers, providing good performance. In fact, especially in the two PHL configuration, since the PHL forbids any liquid to enter or exit, the PEM can also support non-volatile liquid or gel acids (e.g., phosphoric acid), or solid acids (e.g., heteropolyacids), or functionalized acidic materials (e.g., sulfated zirconia), providing higher proton conductivities and allowing higher operating temperatures.

Thus disclosed are electrochemical cells and methods that allow the direct use of impure water for electrolysis without resulting in any fouling of the PEM electrolyzer cell membrane or catalysts by the ions present in saline water, or the corrosion caused by the generation of chlorine gas or hypochlorite ion within the cell. Further, the aerophilic nature of the PHL that is in close proximity to the catalyst layer where the gases are evolved, draws the formed gases directly into the PHL pores, without significant nucleation and bubble formation on the catalyst surface that can cover the catalyst surface. This improves the catalyst utilization as well as avoids the associated overpotential.

show a particular configuration of a direct impure water electrolyzer (DIWE) cell with a single porous hydrophobic layer (PHL). Referring to, a porous hydrophobic layer (PHL)for water purification is interposed, with appropriate modifications to the flow-fields and gaskets for maintaining the layered structure of the containment, between the impure liquid-water feed channels and a PEM electrolyzer membrane-electrode assemble (MEA). The latter typically comprises of a titanium (Ti) anode flow-field, a porous Au- or Pt-plated Ti felt porous transport layer (PTL) for the oxygen electrode, or cathode, an IrOcatalystfor the OER, the proton-exchange membrane (PEM)such as Nafion®, or a nanocomposite-Nafion®, the latter being more suited to a low humidity, higher-temperature cell, a Pt/C catalystfor the HER, a carbon gas-diffusion layer (GDL) with a microporous layer (MPL) for the negative electrode (cathode), and a graphite cathode flow field. In short, it is noteworthy that the disclosed PEM water electrolyzer can be used with a direct impure water feedwith feasible modification to the cell or containmentarchitecture, facilitating adaptation of existing PEM-PWE manufacturing/assembly processes with small changes.