Patent Application
20230366105
The invention disclosed herein generally concerns a novel microelectrode and methods implementing same.
One of the most promising technologies for long-term renewable energy storage is water electrolysis. Water electrolysis converts electricity into chemical energy in the form of hydrogen gas (H2). Once produced, H2 gas can be used as fuel, for example for fuel-cell vehicles (FCVs), converted back to electricity or used in industrial chemical processes.
Electrolyzers are electrochemical devices comprising of oxygen- and hydrogen-producing electrodes and a membrane that separates the two compartments. On the hydrogen electrode, the hydrogen evolution reaction (HER) takes place, according to (left-to-right):
4H2O+4e−40H−+2H2E0=0VRHE[1] (1)
where E0 is the reversible reaction potential. On the oxygen electrode, the oxygen evolution reaction (OER) takes place according to (right-to-left):
O2+2H2O+4e−4OH−E0=1.23VRHE. (2)
While the HER (1) is a two-electron reaction with a relatively low overpotential of <100 mV, the OER (2) is a four-electron reaction requiring a high overpotential of typically 300-500 mV. Therefore, the OER represents a kinetically challenging bottleneck that negatively impacts the overall fuel production process. It has been proposed to divide the water oxidation reaction into an electrochemical step that oxidizes the anode by conversion of Ni(OH)2 to NiOOH (in case of a Ni-based electrode), followed by a spontaneous chemical step wherein the anode is regenerated back to its initial reduced form while producing oxygen gas [1]. Accordingly, in the first step, hydrogen is produced at the cathode according to eq. (1) (left-to-right), while a nickel hydroxide (Ni(OH)2) anode is charged according to the following reaction (right-to-left):
NiOOH+H2O+e−Ni(OH)2+OH−E0=1.42VRHE. (3)
Although the reversible redox potential of eq. (3) is higher than the reversible OER potential, this is a one-electron reaction with a much lower overpotential, typically <100 mV. However, as the Ni(OH)2 anode is charged to nickel oxyhydroxide (NiOOH), the polarization losses increase and eventually the process stops. To continue operation, the anode must be regenerated to its initial (reduced) state. Since the reversible potential of the Ni(OH)2/NiOOH redox coupled is above the thermodynamic water oxidation potential (1.23 VRHE), NiOOH can spontaneously transform back to Ni(OH)2 via a chemical reaction that produces oxygen:
4NiOOH+2H2O→4Ni(OH)2+O2. (4)
The reaction depicted by eq. (4) has a small driving force of only 80 mV. Thus, although the anode regeneration reaction is spontaneous, it proceeds slowly at ambient temperature. To speed up the kinetics, the anode is heated to mild temperatures (typically between 90 and 130° C.), thereby thermally activating the process. This electrochemical-thermally activated (E-TAC) water splitting cycle therefore consists of two steps: (i) a high efficiency hydrogen production and Ni(OH)2 charging step, followed by (ii) a thermally-activated and spontaneous oxygen evolution and anode regeneration step. This allows hydrogen production at nearly thermoneutral conditions (1.48 VRHE) in a simple, cyclic, regenerative and robust system with high efficiency and with the prospect of safe operation and scale-up potential (
This process has been demonstrated using cobalt-doped nickel hydroxide anodes synthesized by an electrochemical impregnation method. Nickel foam was used as the current collector onto which Ni0.9Co0.1(OH)2 was deposited from a nickel and cobalt nitrate bath. Nickel foam has a relatively low surface area and high thermal mass. In the electrodeposited anodes discussed above, the [current collector]: [active material] mass ratio was 3:1; i.e., the nickel current collector accounted for most of the weight and cost of the anode. Moreover, a thick active layer and poor adhesion between the active layer and the nickel substrate contribute to very low material utilization.
The most common type of Ni(OH)2 electrodes used for battery applications are the “paste” type, wherein a paste comprising of Ni(OH)2 powder, a conductive powder (e.g., carbon) and a polymer binder is pressed into a conductive matrix, such as a nickel mesh or nickel foam. For more advanced applications, such as super (or pseudo-) capacitors, Ni(OH)2 electrodes are prepared by either chemical or electrochemical deposition from a solution of nickel salt onto a conductive substrate. In either case, the electrode consists of a current collecting conductive substrate coated with the active material. Despite their extensive uses, these electrodes are not without limitations, especially when applied as redox mediating anodes in decoupled water electrolysis. First, the electrodes suffer from low active material utilization, high thermal mass of the conductive core, and limited current densities. Second, the electrodes are fixed to a current collector and cannot be circulated within a system, therefore dictating a batch operation mode, similarly to battery operation wherein the electrode is first charged (electrochemically) and then discharged (chemically) in two consecutive steps. This design prohibits continuous operation, which is desired in water electrolysis.
Thus, to allow a continuous operation, wherein the electrodes may be charged and discharged without arresting or stopping the water electrolysis process, the electrodes must be movable, namely the monolithic electrode must be presented in a form that can be moved or circulated within the electrochemical cell and its activity regenerated.
The inventors of the technology disclosed herein have developed a process to pelletize the active material, such that each pellet can flow freely within the cell medium and act as a separate microelectrode. Thus, the pellets have the ability to be circulated between two continuously operated reactors, one that produces hydrogen and one that produces oxygen. However, since pellet circulation within the reactors with constant friction with system components may lead to erosion and pealing of an outer active layer, e.g., Ni(OH)2 layer, each pellet cannot simply be in a form of a Ni(OH)2-coated metallic particle. Moreover, a pelletized design would cover the electrically conducting metallic core with a poorly conducting Ni(OH)2, leading to high resistance and Ohmic losses.
Accordingly, the inventors of the technology disclosed herein have further developed a capsule or a microelectrode that comprises a conducting network core, which the core integrity and function being maintained, and erosion or other friction related processes limited or diminished, by coating the conducting network core with a protective coating of an active material. The conducting network core comprises a plurality of fiber-like conductive structures that exhibit open hierarchical porosity. Each capsule or microelectrode comprising the conducting network is in itself porous, enabling flow of liquids and gases, i.e., a liquid electrolyte solution and hydrogen/oxygen gases, in and out of the capsule, while keeping structural integrity and substantially reducing mechanical damage to the inner conducting material.
The capsule is electrically conducting and connected to the internal conducting network. It is robust enough to prevent erosion and breakage of the inner porous structure, yet light enough not to add significant thermal mass to the pellet.
Thus, in a first of its aspects, the invention provides a capsule comprising a conducting network core comprising a plurality of fiber-like conductive structures exhibiting open hierarchical porosity, the capsule having a porosity enabling flow of liquid and gases therethrough.
In another aspect there is provided an electrically conductive capsule comprising an electrically conducting network core comprising a plurality of fibers exhibiting open hierarchical porosity, the capsule having a porosity enabling flow of liquid and gases therethrough.
In some embodiments, the capsule being an electrically conductive porous capsule encapsulating an electrically conductive porous network of conducting fibers, each having an open hierarchical porosity, wherein the conductive porous capsule conducts electricity to the fibers contained therein and permits liquid and gaseous communication therethrough.
In another aspect, the invention provides an electrically conductive porous capsule comprising or encapsulating an electrically conductive porous network of conducting fibers, each having an open hierarchical porosity, as defined, wherein the conductive porous capsule conducts electricity to the fibers contained therein and permits liquid and gaseous communication therethrough.
The capsules of the invention are said to comprise or “encapsulate” the conductive fiber material. The term means that the core is coated by or contained within a conductive porous capsule or a membrane or a perforated shell from all its sides, forming a continuous membrane around the core. The fibers contained in the core are provided as an electrically conductive porous network of fibers; namely a collection of fibers that are arranged as a porous network or matrix. Such network may be manufactured from a fiber mat or foam, as detailed herein.
The conducting fibers present in the cores of the capsules are composed of high surface-area conductive materials having open hierarchical porosity. Putting it differently, the fibers are elongated highly porous conductive structures having nanometric thicknesses or diameters and open hierarchical porosities. The fibers are connected to each other to form a network or matrix of fibers which, in addition to the porosity exhibited by the fibers themselves, is so arranged that pores or spaces may also be present between the fibers forming the network matrix. The porosity characterizing the fibers and fiber network or matrix may be of random distribution, sizes and shapes. Nevertheless, the porosity is relatively homogenous, permitting flow of liquids and gases through the pores.
The term “open hierarchical porosity” means that the fibers exhibit two or more types of pore systems that are of distinctly different sizes and ranges, which may be in the nanopore range, in the micron range and/or in the mesopore range.
The fibers are composed of a conductive material which may be a metallic material (e.g., in a metallic form or zero valent metal, or in an oxide or hydroxide forms or combinations thereof) or a carbonaceous material (e.g., carbon nanotubes, carbon fibers) or may be of a different conductive material such as a polymeric material. In some embodiments, the material is a metallic material which may be provided in an oxidizable or reducible form.
The metal may be any metal traditionally used in construction of electrode active materials, such including, for example, nickel, zinc, aluminum and magnesium. In some embodiments, the metal is or comprises nickel. In some embodiments, the metal is or comprises a Ni-based material.
In some embodiments, the fibers comprise or consist Ni(OH)2. In some embodiments, the fibers comprise or consist Ni/Ni(OH)2. In some embodiments, the fibers are core-shell structures comprising Ni cores and Ni(OH)2 shells.
In some embodiments, the fiber material comprises nickel, optionally in combination with at least one other metal, e.g., cobalt.
In some embodiments, the fiber material comprises a metal, as defined, in combination with at least one carbonaceous material, such as CNT, carbon fibers, graphene and others.
Notwithstanding a particular fiber composition, e.g., identity of the metal, the fibers may be of different morphologies. The fibers' morphologies may range from nanosized fibers to fibers having large inner pores; from low to higher ratios of surface area to volume; from low to high pore densities; etc. The fiber morphology may vary greatly and may exhibit lamellar, nanobelt, hollow or other structural morphologies, or combinations thereof. In some embodiments, the fiber is in a form of a hollow tube. In some embodiments, the fiber is a nanobelt. In other embodiments, the fiber is a lamellar fiber. In other embodiments, the fiber is in a fibrillar.
In some embodiments, the fibers have a diameter in the nanometric range, typically in the range of 100-500 nm and a length that can reach hundreds of microns to several millimeters. As such, the fibers may be obtained, as disclosed herein, as entangled assembly or mat of fibers, which is treated, as disclosed, to obtain fibers of desired sizes that are suitable for encapsulation.
The fiber material may be prepared according to any process known in the art.
In some embodiments, the fibers utilized according to the present invention are manufactured by a process comprising treating a composite of a conductive material and a polymer binder (e.g., a metal-polymer composite) obtained, e.g., by treating fragments of an electrospun conductive material (nano) fiber, e.g., metal (nano) fibers, with a polymeric binder and treating said composite under conditions sufficient to remove the polymeric binder and to thereby obtain the conductive fiber material. The conductive fiber material may be shaped as a continuous matrix or collection of fibers that are associated to each other (shaping mats or sheets or foams), exhibiting fiber and matrix porosities.
The composite of the conductive material and polymer may be obtained by any means known in the art, provided that the conductive material, e.g., metal, is present in the composite in a highly porous form, e.g., fibrillary form, whereby the polymeric binder occupies pores in the fibrillar form. In some embodiments, the fibrillar material is provided in a form of cuttings, shreds, fragments or pieces of an electrospun metallic sheet, mat or generally metal foam that is grounded or shredded or mechanically treated to transform the mat into fragments. Once these fragments are obtained, they may be treated with the polymeric binder to form the composite.
Thus, in some embodiments, the process for manufacturing the conductive fiber material comprises
As used herein, the conditions sufficient to remove the polymeric binder and convert the metallic fibers into porous structures, as disclosed, include thermal treatment, reductive treatment, dissolving solutions and others, as may be the case, and depending, inter alia, on the composition of the composite, the type of polymer binder used, and others. In some embodiments, the reductive conditions involve use of a reducing agent such as hydrogen gas.
In some embodiments, the process comprises
In some embodiments, the fibers are electrospun.
In some embodiments, the conductive material is a metal.
In some embodiments, the metallic conductive material is in a form of a mat or foam of (nano) fibers.
In some embodiments, the process comprises
The hierarchical structure of the fibers may be modified or altered by forming the initial (nano) fiber foam by co-electrospinning with a polymer fiber of a larger diameter (i.e. >1 micron). These polymeric fibers create larger channels in the metal fiber bed, once the fiber compact is burned and reduced to produce the fiber electrode. Such channels will reduce mass transport limitation to the core of the fibers within the individual pellets.
Thus, in a step of forming (e.g., by electrospinning) a conductive (nano) fiber foam, a solution comprising a precursor of the conductive material or of the material and a polymer or a solution comprising the precursor or the conductive material and a different solution comprising the polymer may be electrospun to produce the foam, as further demonstrated in
In a process of the invention, the conductive material, e.g., metal, may be transformed from one oxidation state to another and/or from one form into the other. A non-limiting depiction of a process for the production of a nickel electrode according to the invention is provided in
As depicted in the example of
Thus, for manufacturing a Ni/Ni(OH)2 fiber material, the process comprises
In a similar fashion, other metals, metal precursors and conditions may be used to manufacture electrode materials according to the invention.
The fibers manufactured may be used as metal foams or as pellets comprising a plurality of elongated fibrillar structures, which porosity being defined by the distance between the metal regions, as depicted in examples provided herein.
The capsule of the invention is structured of a porous or perforated shell enclosure, which may be metallic, and which contains the fibers and protects the fibers from mechanical degradation, e.g., erosion. The capsule may be made by a variety of methodologies. For example, the capsules may be made by stamping a thin, perforated metal sheet; two such half capsules (e.g., with hemispherical ends and straight cylindrical sections, similar to medical pills) may be filled with fibers and subjected to burn-off of residual organics, reduction of the metal, e.g., nickel, oxide to metal, and permanently attach the fibers to the inner capsule walls, thus securing electrical conductivity from the capsule wall to the inner fibers. The final attachment of the fibers to the capsule walls may be achieved by a variety of thermal treatments, for example by rapid temperature processing (RTP) using, e.g., IR radiation.
A scheme describing this process is shown in
Alternatively, the capsule can be made from a conductive polymer by, e.g., injection molding. The capsules may thereafter be filled with the metallic nanofibers after their reduction.
Thus, the invention further provides a process for manufacturing an electrically conductive capsule according to the invention, the process comprising filling a capsule of at least one electrically conductive material, e.g., a metal or a conducting material such as a conducting polymer, with a conductive fiber material under conditions securing electrical conductivity between the capsule and the fiber contained therein.
As used herein, the term “fill” or any lingual variation of the term, when in reference to introducing the fibers into the capsules, does not suggest complete filling or packing of the capsule. The term simply suggests the introduction of an amount of the fiber material into the capsule, wherein the amount is determined, inter alia, by the size of the capsules, the conductive materials used and other considerations.
In a process of the invention, the capsule, independent of its final shape, may be formed of two halves of the capsule final shape that are brought together and closed. Alternatively, the capsule may be formed of a single shaped structure that can be filled and thereafter closed.
In some embodiments, the capsule is formed of two halves, each with hemispherical ends and straight cylindrical sections, providing secure attachment and closure.
Irrespective of the capsule form and mode of construction, once filled with the fiber material, the capsule is treated under conditions achieving electrical conductivity between the capsule and its content. Electrical conductivity may be achievable by attachment of the fibers within the capsule to the capsule inner walls. While not all fibers within the capsule need to be attached, attachment of at least the peripheral fibers is needed. The attachment may be achievable by thermally treating the capsules. In some embodiments, the attachment is achievable by rapid temperature processing (RTP), e.g., using IR radiation, as disclosed herein.
A conductive capsule of the invention may be regarded a “microelectrode”. As used herein, the term refers to an electrode that is of appropriate size and which is electrically conductive, perforated to allow passage of liquids and gasses and mechanically robust to prevent erosion. The small diameter of the microelectrodes makes their active surface area per electrode volume or weight particularly large. In some embodiments, the surface area per weight of microelectrodes of the invention is between 5 m2/gr 100 m2/gr or several hundreds.
Typically, the microelectrodes have a size or a cross-section of several microns (between 1 and 10 microns) to several millimeters. In some embodiments, the cross-section size is between 1 microns and 0.1 mm or between 10 microns and 2 millimeters. The capsule may have any shape and thus the indicated sizes refer to the capsule longest axis. In some embodiments, the capsule is in a shape of a sphere, a cube, an ellipsoid, a polygonal and others. In some embodiments, the capsule is spherical.
The capsule and/or the fiber material contained therein may be composed of a redox-active material or an active material that has the ability to undergo reversible oxidation-reduction, under different conditions, for example different temperatures and/or different electrical bias. The active material may be the material from which the capsule shell and/or the fibers are made from or a material which coats a region of the capsule and/or fiber surface. The redox active material may be in a reduced form (prior to oxidation) or in an oxidized form or may be in any intermediate state (e.g., partially oxidized, partially reduced). When in a reduced form or a partially reduced form, the redox-active material is capable of undergoing oxidation to revert back to (or generate) the oxidized or partially oxidized form and vice versa.
In some embodiments, the electrically conductive capsule is made of a conductive material that is same or different from the material of the conducting fibers contained therein. In some embodiments, the capsule conductive material is of comprises nickel. In some embodiments, the capsule conductive material is or comprises Ni/Ni(OH)2. In some embodiments, each of the conductive fibers and the capsule conductive material is or comprises Ni, Ni(OH)2 or Ni/Ni(OH)2. In some embodiments, the capsule material is different or does not comprise nickel. In some embodiments, both the fibers and the capsule are or are comprised nickel, e.g., Ni/Ni(OH)2.
In a system of the invention utilizing capsules of the invention, wherein the system is, for example, for generation of hydrogen and oxygen gases, the electrode is provided in a form of a plurality of microelectrodes of the invention dispersed in a medium.
Thus, the invention also provides a system for generation of hydrogen and/or oxygen gas, the system comprising a plurality of microelectrodes according to the invention, dispersed in a medium. The system may be one which comprises an electrochemical device that is configured for generating gases.
Also provided is a system for generation of gases, the system comprising an electrochemical device comprising a plurality of microelectrodes of the invention, the microelectrodes being of at least one redox-active material.
In some embodiments, the electrochemical device is a reactor or a plurality of reactors arranged and configured for generating a gas. In some embodiments, the device is an electrochemical thermally activated chemical cell (E-TAC). As known in the art, an E-TAC system [2,3] enables alternate production of hydrogen gas and oxygen gas. In accordance with the technology, hydrogen gas is generated in an electrochemical step on a cathode electrode while charging the anode electrode, by water reduction, whereas oxygen gas is generated in a spontaneous chemical step during regeneration of the anode. In a modified ETAC system according to the present invention, microelectrodes of the invention may be utilized to replace the active anode electrode.
The invention further provides an electrochemical device for generation of a gas by utilizing microelectrodes of the invention (e.g., having an oxidized form and a reduced form), the device is adapted and operable to convert the microelectrodes from an oxidized form to a reduced form.
In another aspect there is provided a medium of an electrochemical cell comprising a plurality of capsules, e.g., microelectrodes according to the invention. In some embodiments, the medium is water-based or an aqueous medium or an electrolyte solution, which comprises metal electrolytes such as Li, Na, K, Rb, Cs, Ca, Sr and Ba. In some embodiments, the metal is an alkali metal. In some embodiments, the electrolyte comprises a metal hydroxide. In some embodiments, the metal hydroxide is NaOH or KOH. In some embodiments, the aqueous solution is carbonate-bicarbonate buffer electrolyte.
Similarly, the invention provides a substrate having a surface region associated with a microelectrode or an assembly of microelectrodes according to the invention.
The invention also provides an electrochemical cell comprising an electrode assembly of a cathode and an anode, wherein the anode is provided as a microelectrode according to the present invention. In some other implementation, the cathode is provided as a microelectrode of the invention.
In an electrochemical cell of the invention, the anode (or cathode) being the microelectrodes is immersed in the electrolyte or ionically conductive solution or medium. The cathode and the microelectrodes constituting the anode are electronically separated by ionically conductive solution or medium and the electrochemical reaction takes place only when they are in contact with an electron conducting electrode.
Also provided is an anode or a cathode electrode in a form of a microelectrode of the invention.
Further provided is a device which comprises a microelectrode according to the invention, the device being optionally selected from electric cells, electric furnaces, thermionic tubes, gas-discharge devices, and semiconductor devices, the device utilizing a microelectrode according to the invention.
In some embodiments, the device is an electrochemical cell (such as a battery, a flow battery, a fuel cell, an electrolyzer and a supercap).
The invention further provides a conductive material in a form of a fiber or an elongated structure having open hierarchical porosity, as disclosed herein, for use in constructing a capsule or a microelectrode according to the invention. The fibers are as disclosed herein.
In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
Electrospun Ni and NiCo Substrate Synthesis
Nickel acetate tetrahydrate with and without cobalt acetate (Ni:Co=0.9:0.1) was dissolved (10.6 wt. %) in acetic acid and ethanol (6:4 by wt.). The solution was stirred and filtered (0.22 μm), resulting in a light-green clear solution.
Next, polyvinyl pyrrolidone was added (8.8 wt %) and stirred at ambient temperature to achieve a final viscosity of 500-650 cPs.
The solution was electrospun (21-gauge needle, 17 kV, 1.25 ml/h, 15 cm WD), resulting in green NiAc-PVP fiber mats (
SEM images of the green fibers, the calcined fibers, the ground fibers and the final fibers after reduction are presented in
Ni(OH)2 Layer Growth by Oxidation/Reduction Cycles
In order to grow the active Ni(OH)2, the Ni pellet was connected as the working electrode (WE) to a potentiostat in in 5M KOH aqueous electrolyte solution (˜50° C.), and subjected to 60 chronopotentiometric oxidation-reduction cycles at ±150 mA/cm2 up to ±200 C/cm2 or up to 1.55 V (during oxidation) and 1 VRHE (during reduction).
Every few cycles, the electrode was dried and weighed to calculate the percent of nickel atoms oxidized to Ni(OH)2 (eq. 1):
where mi is the initial pellet mass, mf is the final pellet mass, and Δm is the weight gain.
To characterize the added charge capacity, the pellet was also subjected to chronoamperometric redox cycles.
In-Situ Layer Growth
The Ni(OH)2 can also be grown in-situ during E-TAC water electrolysis without preliminary layer growth. An as-prepared Ni pellet was connected as the WE to a potentiostat in a three electrode configuration (Hg/HgO reference electrode) in 5M KOH aqueous electrolyte at ambient temperature. In this setup, the pellet was subjected to repeated E-TAC cycles with charging at 1.48 VRHE for 100s and regeneration in a hot (95° C.) 5M KOH solution for 100s. X-ray diffractograms and SEM images of the resulting Ni(OH)2 layer are presented in
Electrochemical Characterization and E-TAC Water Splitting Tests
Electrochemical tests were carried out in a three-electrode system, with the either the nanofiber anode (NFA) or an electrochemically deposited anode (EDA) as the working electrode, with a platinum foil (1 cm2) counter electrode, and an Hg/HgO/1M NaOH reference electrode in 5M KOH. Electrodeposited anodes were prepared as described elsewhere.
E-TAC water splitting tests were carried out by first charging the pellet anode at 1.48/1.43 VRHE in 5M KOH solution at ambient temperature (for 100, 200, 400 or 800s), and then regenerating the pellet anode by dipping it in a hot (95° C.) 5M KOH solution for the same duration. Stability tests were carried out with 100s charge/regeneration cycles. In addition, the anode was also subjected to E-TAC cycles with short regeneration times, wherein the charging duration varied between 100 and 800s, but the regeneration duration was kept constant at 100s.
Phase and Morphology Characterization
The surface area of the pellets was measured using BET (
Electrochemical Characterization of Ni/Ni(OH)2 and Ni/Ni0.9Co0.1(OH)2 Nanofiber Anodes
Core-shell electrospun nickel hydroxide electrodes were prepared to serve as redox anodes for decoupled E-TAC water splitting (
Additionally, since parasitic oxygen evolution may take place upon charging the anode to a high state of charge (SOC), a lower redox potential shifts the anode charging reaction away from the OER and enables charging the anode to higher SOCs without parasitic oxygen evolution. To evaluate the OER rate at the NFAs, the anodes were subjected to chronoamperometric measurements between 1.43 and 1.48 VRHE By allowing the current to stabilize, a distinction could be made between the pseudo-capacitive Ni(OH)2 oxidation current and the steady-state OER Faradaic current. The corresponding steady-state Tafel plot for the OER Faradaic current at the Ni/Ni(OH)2 NFA is plotted in
The reversible redox potential, as well as the charging and discharging overpotentials, are also affected by incorporation of various additives and dopants into the Ni(OH)2 anode. Specifically, cobalt is known to shift the Ni(OH)2 redox potentials cathodically and improve the electron and proton conductivity of Ni(OH)2, allowing the anode to reach greater SOCs. Here, cobalt was embedded into the electrospun fibers, and evenly dispersed along the fibers, as shown in
Performance of Cobalt-Doped and Undoped NF Anodes in E-TAC Water Splitting
There are several key performance metrics in the selection and optimization of Ni(OH)2 anodes for E-TAC water splitting. As discussed in the previous section, the first critical metric is the Ni(OH)2 redox potential, which directly influences the cell voltage (see eq. 1) and therefore the process' voltage efficiency. Here, the anodes' charging potential was controlled by applying a constant polarization potential of either 1.48 or 1.43 VRHE during the hydrogen generation step. The highest value, 1.48 VRHE, was chosen since it is the thermoneutral water splitting voltage, i.e., this is the anode potential required in order to achieve a voltage efficiency of 100%HHV during the hydrogen generation step, neglecting all other losses (see eq. 1).
The second key metric is the anode's charge capacity, as anodes with higher charge capacities can sustain longer cycles, reducing the cycling frequency. In this respect, Ni(OH)2 anodes for E-TAC water splitting are similar to battery electrodes, which also benefit from high charge capacities that extend their operation duration between recharges. Thus, similarly to battery electrodes, the Ni(OH)2 anodes are expected to exhibit high energy densities normalized by the anode's volume and mass. To achieve this, a sufficiently high mass loading is necessary, along with a high utilization efficiency of the active mass. It should be noted that, unlike battery electrodes, the Ni(OH)2 anodes in E-TAC water splitting are regenerated in a chemical process, rather than an electrochemical one (discharge), and therefore the rate of their regeneration cannot be directly controlled. As a result, the charge that can be chemically regenerated, Qregen, is often lower than the charge that can be extracted by electrochemical discharge; in electrochemical discharging, a preset electrochemical driving force or reaction rate is imposed by an external power source, whereas in chemical discharging the driving force and reaction rate decrease progressively as the NiOOH anode is discharged. In addition, the chemical discharging (regeneration) reaction takes place at the electrode-electrolyte interface and is limited by the diffusion of OH− ions into the active material. Thus, while a high mass loading may improve the electrochemical charge capacity, it may not contribute to, and perhaps even reduce, the chemically-regenerated capacity, especially if it is achieved by increasing the bulk volume at the expense of surface area.
The third key metric is the current density during the hydrogen generation step, which is proportional to the hydrogen production rate. In this respect, Ni(OH)2 anodes should support operation under high current densities normalized by the geometric area, similarly to alkaline electrolyzers, which typically operate at 200-400 mA/cm2. However, in contrast to electrolyzer anodes wherein the OER takes place at the surface of the otherwise inert electrocatalyst, the charging reaction in the E-TAC water splitting process involves an electrochemical transformation of the anode itself from Ni(OH)2 to NiOOH. Since the Ni(OH)2 mass loading must be high enough to sustain high charge capacities, as discussed above, there is a trade-off between rate capabilities and charge capacity, similar to the trade-off observed in electrodes for supercapacitors.
In each test, the average regenerated charge was calculated based on the current×time product, according to: Qregen=∫i(t)·dt=<i>·t, (eq. 2); where t is the time, i(t) is the current as a function of time, and <i> is the average current over the entire test duration. Accordingly, for the tests carried out at 100, 200, 400 and 800s, the ratio between the average current (in A) and the regenerated charge (in A·s) is 100, 200, 400 and 800, respectively.
At a charging potential of 1.48 VRHE (full line data series in
Both the cobalt-doped and undoped NFAs displayed similar regenerated charge capacities at a charging potential of 1.48 VRHE It therefore appears that under these experimental conditions, the benefits of cobalt doping were lost. Furthermore, the extent of the OER is greater at this potential for the cobalt-doped NFA compared to the undoped NFA. Indeed, 1.48 VRHE lies anodicaly of the oxidation waves of both samples, and above the OER onset potential of the cobalt-doped NFA (
As expected, the charge that could be extracted from both NFAs at 1.43 VRHE was lower than at 1.48 VRHE. Nevertheless, while the undoped NFA's retained only 34-49% (depending on the test duration) of its charge compared to that at 1.48 VRHE, the cobalt-doped NFA retained 54-57% of its charge under the same conditions. Moreover, the regenerated charge for the cobalt-doped NFA, even at the low charging potential of 1.43 VRHE, surpassed that of the EDA when charged at 1.48 VRHE (
The high surface area of the hierarchical meso/macro porous structure of the NFAs resulted in a significant increase in the obtainable current density at each potential. Compared to the EDA, the average current densities of the undoped and cobalt-doped NFAs at 1.48 VRHE were higher by up to 6.5 and 7.6 times, respectively. Furthermore, even at 1.43 VRHE, the current densities of the NFAs were higher compared to the EDA when charged at 1.48 VRHE (by up to 2.2 and 4 times for the undoped and doped NFAs, respectively). At both 1.48 and 1.43 VRHE charging potentials, the cobalt-doped NFA achieved higher current densities compared to its undoped counterpart, which is attributed to the larger driving force, i.e., a larger difference between the applied potential (1.48 VRHE) and the anode's redox potential.
The voltage efficiency of the E-TAC water splitting process using our cobalt-doped NFA in a two-electrode configuration was also compared to our previously reported anode. At a current density of 50 mA/cm2 (normalized by the geometric area), the low charging potential of the cobalt-doped NFA resulted in an overall lower cell voltage, with an increase in average voltage efficiency from 98.7% to 100%.
The fourth key metric in E-TAC water splitting is the Faradaic efficiency of the Ni(OH)2 charging reaction, (1−QOER/Qtotal, where QOER is the parasitic charge of the OER during the hydrogen generation step, and Qtotal is the total charge passed during the hydrogen generation step). The Faradaic efficiency is influenced by the cycle duration, which is linked to the anode's SOC, as well as by the anode's charging potential and its relation to the OER onset potential at the same anode. QOER was calculated at each E-TAC water splitting test from the measured amount of dissolved oxygen. At 1.48 VRHE charging potential, the Faradaic efficiency of the cobalt-doped NFA was high for the short tests (99%±1% for the 100s test and 98.5%±0.7% for the 200s test), but it decreased with increasing test duration, i.e., upon charging to higher SOCs, reaching 94%±3% for the 400s test, and only 85%±2% for the 800s test. However, by lowering the charging potential to 1.43 VRHE, the Faradaic efficiency for the cobalt-doped NFA increased, reaching 98%±2% at the longest (800s) test. This result was expected, as the oxygen evolution rate is negligible at this potential. In fact, the ratio between the steady-state OER rate (
In addition to the current density during the hydrogen generation step of the E-TAC water splitting cycle, it is advantageous for the oxygen generation step to be faster than the hydrogen generation step. This way the overall hydrogen production throughput is maximized. The fifth key metric is therefore the regeneration reaction rate, or the charge that can be regenerated at a given regeneration time, which is correlated to the hydrogen production throughput. To examine the influence of the anode preparation method on the regeneration rate, the E-TAC water splitting tests were repeated with shorter regeneration steps of only 100s. For the EDA, decreasing the regeneration time resulted in a capacity drop of up to 31±14%. However, for the undoped and cobalt-doped NFAs, the capacity dropped by only 12±4% and 13±4%, respectively. The enhanced surface area and availability of active sites of the NFAs thus also facilitate higher chemical reaction rates, increasing the hydrogen production throughput without a substantial loss in capacity. For example, a two-fold increase in hydrogen production duration (from 100s to 200s) with the cobalt-doped NFA corresponds to an overall hydrogen production throughput increase of 33%, with a capacity loss of less than 4%.
Finally, a highly important performance metric is the anode's cycling stability. Ni(OH)2 battery electrodes can withstand thousands of electrochemical charge-discharge cycles by avoiding overcharging, as discussed above. Moreover, we have previously shown that the electrochemical discharge (reduction) of NiOOH can be seamlessly replaced by chemical regeneration without damage or degradation of the anode's microstructure, phase composition or chemical composition. Here, the cycling stability of an undoped NFA was demonstrated by subjecting it to a series of 100 E-TAC water splitting cycles. After about 20 cycles, both anodes displayed high stability up the 100th cycle (Q=5.2±0.3C and Q=1.8±0.2 C for the EDA and NFA, respectively), with similar X-ray diffraction pattern and morphology to those observed after layer growth by galvanic cycling. This is a preliminary positive indication of the anode's cycling stability in E-TAC water splitting.
Anode Optimization: Effect of Fiber Morphology
The electrochemical properties of a core-shell Ni/Ni(OH)2 NFA depend on the microstructure and surface area of the nickel substrate. Tuning the anode's microstructure and consequent electrochemical performance is possible by controlling the various synthesis process parameters. To demonstrate this, we explored the influence of the fiber microporosity and diameter on the charge density of the resulting anodes. SEM images of three nickel nanofiber substrates prepared with maximum sintering temperatures of 500, 600, and 700° C. are shown in
Next, we looked at the fiber diameter, and its effect on the anode's performance. We postulated that reducing the fiber diameter would contribute to an enhanced performance both by increasing the active surface area, and by increasing the weight ratio of the active shell layer and the nickel core. Thinning the fiber diameter was achieved by diluting the original precursor solution, so that its viscosity decreased twofold, and by raising the voltage during the ES process, which resulted in extra stretching of the fibers during electrospinning.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IL2021/051155 | 9/23/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63082741 | Sep 2020 | US |
