Electrolysis electrode featuring metal-doped nanotube array and methods of manufacture and using same

An electrolysis electrode includes a metal-doped array of nanotubes formed on a substrate. The nanotube array (NTA) may be a stabilized metal-doped black TiO2 NTA formed on a titanium substrate, and the metal dopant may include any suitable metal, for example, cobalt. The metal dopant improves the reactivity of the electrode and enhances its service life. The metal-doped NTA electrode may provide improved chlorine evolution and/or oxygen evolution activity for electrochemical wastewater treatment. The electrode may also be useful for water splitting applications. Increasing the loading of the metal dopant may lead to the formation of a metal oxide layer on top of the NTA, which improves oxygen evolution reaction (OER) overpotential.

TECHNICAL FIELD

The present disclosure generally relates to electrolysis, and more particularly, to electrodes suitable for water treatment and/or water splitting electrolysis.

BACKGROUND

Systems are being proposed for the electrochemical oxidation of pollutants in an electrolyte. Examples of these systems include wastewater treatment systems that employ electrolysis to clean wastewater. Electrolysis may also be used in other applications, for example, splitting water to produce oxygen and hydrogen gas. These electrolysis systems operate by applying a voltage potential between an anode and a cathode that are each in contact with a water medium. In the case of wastewater treatment, the anode and cathode contact wastewater to achieve electrochemical oxidation of organic matter.

The electrodes (anodes and cathodes) in these systems sometimes have one or more semiconductor materials that contact the aqueous medium. The semiconductor electrodes are often composed of expensive rare earth materials. Moreover, the semiconductor materials often degrade relatively quickly during operation of the systems, reducing the service life of the electrodes.

Further, the ability of some of the electrodes to purify water depends on the ability of the anode to generate Reactive Chlorine Species (RCS) and/or hydroxyl radicals in the water. However, some known electrodes generate reactive species at current efficiencies that are too low to be desirable for some wastewater treatment applications.

Accordingly, electrolysis electrodes are desirable that have improved reactive species generation, service life and current efficiency, as well as reduced cost.

SUMMARY

An electrolysis electrode includes a metal-doped array of nanotubes on a substrate. In accordance with an example embodiment, the nanotube array (NTA) may be a stabilized metal-doped black TiO2NTA formed on a titanium substrate, and the metal dopant may include any suitable metal, for example, cobalt. The metal dopant improves the reactivity of the electrode and significantly extends its service life. The metal-doped NTA electrode may provide improved chlorine evolution and/or oxygen evolution activity for electrochemical wastewater treatment. The electrode may also be useful for water splitting applications.

The metal-doped NTA electrode may be manufactured by anodizing a substrate to form an amorphous NTA on the substrate. The amorphous NTA and substrate structure is then subjected to a second anodization. A metal-dopant solution is applied to the anodized amorphous NTA, whereby producing a metal-doped amorphous NTA. The metal-doped amorphous NTA is then annealed in a gas stream, resulting in the metal-doped NTA electrode.

The metal-doped NTA electrode can be used as an electrocatalyst in water splitting systems for energy production, or alternatively, in electrochemical oxidation (EO) systems that purify water having organic pollutants and/or ammonia by placing it in direct physical contact with the wastewater and applying a suitable voltage potential.

The disclosure also describes a water processing system including one or more electrodes where at least one of the electrodes includes a metal-doped array of nanotubes formed on a substrate.

The foregoing summary does not define the limits of the appended claims. Other aspects, embodiments, features, and advantages will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features, embodiments, aspects, and advantages be included within this description and be protected by the accompanying claims.

DETAILED DESCRIPTION

The following detailed description, which references to and incorporates the drawings, describes and illustrates one or more examples of electrolysis electrodes, water processing systems, and methods of using electrolysis electrodes and water processing systems, and of manufacturing electrolysis electrodes. These examples, offered not to limit but only to exemplify and teach embodiments of inventive electrodes, methods, and systems, are shown and described in sufficient detail to enable those skilled in the art to practice what is claimed. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. The disclosures herein are examples that should not be read to unduly limit the scope of any patent claims that may eventual be granted based on this application.

The word “exemplary” is used throughout this application to mean “serving as an example, instance, or illustration.” Any system, method, device, technique, feature or the like described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other features.

Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention(s), specific examples of appropriate materials and methods are described herein.

Disclosed herein are one or more embodiments of an electrolysis electrode that may be useful for treating polluted water and/or water splitting applications that produce hydrogen and oxygen gas (e.g., solar hydrogen energy production).

Concerning water treatment applications, water scarcity has been recognized as an emerging global crisis. In order to facilitate water recycling and reuse, decentralized wastewater treatment has been proposed as a supplement to the conventional urban wastewater system. In decentralized systems, electrochemical oxidation (EO) can be more efficient than biological treatment and less expensive than homogeneous advanced oxidation processes. In addition, the compact design, ease of automation for remote controlled operation, and small carbon footprint make EO an ideal candidate for small scale, decentralized wastewater treatment and reuse.

The performance of EO in wastewater applications is often determined by the electrochemical generation of reactive species, which largely depends on the nature of anode materials. A number of anode materials have been previously considered. For example, non-active anodes with high overpotentials for oxygen evolution reaction (OER), such as those based on SnO2, PbO2, and boron-doped diamond (BDD), have been investigated in the previous decades. In spite of their superior current efficiency for hydroxyl radical (.OH) generation, SnO2and PbO2anodes have poor conductivity and stability. The application of BDD anodes is hindered by their high cost and complicated fabrication. Conversely, Pt-group metal oxides (e.g., RuO2and IrO2) are efficient and stable catalysts for OER, exhibiting high chlorine evolution reaction (CER) activity in the presence of chloride, although they are typically less efficient for hydroxyl radical generation. Hence, the development of durable anodes with high activity for CER, OER, and radical generation is an ongoing challenge.

Electrolyte composition is another factor in EO performance. Previously, .OH was considered as the main contributor to organic matter removal during EO. Recent studies have pointed out that carbonate, sulfate and phosphate radicals are also potent oxidants. Compared with these anions, chloride (Cl—) in wastewater can be more readily oxidized to reactive chlorine species. Enhanced electrochemical oxidation of organic compounds observed in the presence of Cl— has been attributed to reaction with free chlorine (Cl2, HOCl and OCl−. More recent studies have suggested that Cl. and Cl2.−might be primarily responsible for organic compound degradation. Thus, an anode that promotes efficient generation of chlorine radicals may be desirable.

Applications of electrochemical wastewater treatment can be hindered by several challenges, which may include: 1) relatively high energy consumption costs per kilogram of chemical oxygen demand (COD) treated in units of kWh/kg of COD, depending on the composition of the electrodes; and 2) the relatively high cost of semiconductor electrodes due to the use of platinum group metals as the primary ohmic contact materials for transfer of electrons to the base metal.

Considering each of these challenges more specifically, the energy consumption of EO wastewater treatment processes (50-1000 kWh/kg COD) may be higher than aerobic biological treatment (3 kWh/kg COD; assuming 320 g/m3of inlet COD, 50% of removal efficiency, and 0.45 kWh/m3of energy consumption per volume). Electrolysis of chloride-containing wastewater produces chlorination byproducts such as chlorate (ClO3−) and perchlorate (ClO4−). Anodes operating at higher oxidative levels are often able to eliminate organic compound byproducts at longer reaction times, however with the tradeoff of higher yields of ClO3−and ClO4−. Currently available electrodes are relatively expensive due to the need to provide a low Schottky-barrier semiconductor in direct contact with the base-metal support of the electrode. For active electrodes, IrO2or RuO2are employed as ohmic contacts, and for nominally inactive electrodes, boron-doped diamond electrodes (BDD) are employed.

Titanium dioxide (TiO2) has been recognized as a stable and reusable photocatalyst for water splitting and water treatment applications. However, it is an inefficient anode material in the absence of photoactivation due to its low electron conductivity. To overcome this limitation, conductive TiO2nanotube array electrodes have been developed that may be used in “dark” applications that do not rely on photoactivation. Even though TiO2nanotube arrays are effective for electrochemical oxidation initially, premature failure by deactivation is often observed within a few hours.

To address the foregoing limitations, an electrolysis electrode featuring a metal-doped nanotube array (NTA) is disclosed. The NTA electrode may include a cobalt-doped black-TiO2nanotube array (Co-black NTA). The metal dopants may be immobilized on the surface of the black NTA to improve water oxidation activity and reduce or prevent surface passivation. The disclosed metal-doped NTA electrode can be applied in water splitting and/or EO wastewater treatment systems, as described herein.

FIG. 1is a schematic illustration of a first exemplary water electrolysis system8that includes a vessel10for holding an aqueous medium18such as water, a first electrode (electrode1)14and second electrode (electrode2)16for use in electrolysis, and a voltage source38for providing current to the electrodes14,16. For the purposes of simplification, only a pair of electrodes14,16are illustrated, although additional electrodes can be employed.

In the system8, the first electrode14includes a substrate30and a metal-doped nanotube array (NTA)31having a bottom surface contacting the substrate30. Examples of the detailed construction of the electrode14are described herein with reference to the other Figures. The metal dopant added to the NTA improves the performance and service life of the electrode14in electrolysis applications. The second electrode16can be a metal base, such as a stainless steel or platinum cathode.

The vessel10can be any suitable container for holding the medium18, for example, it may be a metal, plastic or glass vessel.

The system8can be used for splitting water into hydrogen and oxygen gas. This application may be useful for producing hydrogen gas for energy production.

Alternatively, the system8may be used to purify water having organic matters by making use of advanced oxidation processes (AOP) to break organic matters into small and stable molecules, such as water and CO2. Wastewater may include the organic matters that are normally associated with waste products and chloride that is naturally present in urine. Accordingly, wastewater can naturally operate as electrolytic medium, or an electrolyte, such as NaCl, can optionally be added to the wastewater.

The system8can operate in a monopolar (MP mode) or a bipolar (BP) mode. In MP mode, the voltage source38provides continuous current between the electrodes14,16in one direction and does not switch voltage polarity (reverse the direction of the current flow through the electrodes14,16). In the example shown inFIG. 1, in MP mode the first electrode14acts as an anode and the second electrode16acts as a cathode.

In BP mode, each of the electrodes14,16can act as either an anode or a cathode, alternatively, depending on the polarity of the voltage source38. Operating the system8in BP mode can increase the service life and improve the performance of the electrodes14,16.

In BP mode, the voltage source38can switch polarity at a set frequency so that the electrodes14,16are alternatively employed as both anode and cathode. Switching the polarity of the source38can be accomplished by a timed switch (not shown) in the source38that changes the output voltage polarity of the source38at set times. For example, the electrodes14,16can be employed as both anode and cathode with source polarity switching at an interval having a length between 10 and 30 minutes.

The nanotube array31can include, consist of, or consist essentially of any suitable number of nanotubes and a metal oxide that includes, consist of, or consist essentially of oxygen and one or more elements, e.g., titanium. For example, the NTA may be a black TiO2nanotube array (BNTA). In accordance with an exemplary embodiment of the electrode14, the metal-doped NTA31may include a cobalt-doped black TiO2nanotube array (Co-black NTA) and the substrate30may be a valve metal, such as titanium, for example, Ti foil or mesh.

During operation of the water purification system8, the source38applies an anodic potential38between the first electrode14and the second electrode16at a level that is sufficient to generate reactive species at the electrode14, while performing as an anode.

The electrode14has a relatively high rate of Reactive Chlorine Species (RCS) generation and/or oxygen evolution reaction (OER). This makes the electrode14highly suitable for use in wastewater electrolysis systems and/or water splitting systems.

FIG. 2is a schematic illustration of a second exemplary water electrolysis system21that includes the vessel10for holding the aqueous medium18such as water, the first electrode (electrode1)14and second electrode (electrode2)19for use in electrolysis, and the voltage source38for providing current to the electrodes14,19. For the purposes of simplification, only a pair of electrodes14,19are illustrated, although additional electrodes can be employed.

The system21may be used for either water splitting or water purification, as discussed above in connection withFIG. 1. Additionally, the system21, like the system8shown inFIG. 1, may be configured as either a BP mode system or MP mode system, depending on the application. The predominate difference between the systems8and21is that the second system21includes a metal-doped NTA electrode19as the second electrode. The second electrode19includes a substrate23and a metal-doped nanotube array (NTA)33contacting the substrate23, which may be the same as or similar to the NTA31of the first electrode14. In accordance with an exemplary embodiment of the electrode19, the metal-doped NTA33may include a cobalt-doped black TiO2nanotube array (Co-black NTA) and the substrate23may be a valve metal, such as titanium, for example a Ti foil or mesh.

In BP mode, each of the electrodes14,19can act as either an anode or a cathode, alternatively, depending on the polarity of the voltage source38. Operating the system21in BP mode can increase the service life and improve the performance of the electrodes14,19. In BP mode, the system21also has at least one metal-doped NTA electrode14,19in an anodic state at all time while the voltage potential is applied, increasing the reaction rate of the system21.

FIG. 3illustrates an example of another suitable electrolysis system15, such as a water purification system, or alternatively, a water splitting system, that includes multiple first electrodes14and second electrodes16. The system15includes a vessel10having a reservoir. First electrodes14and second electrodes16are positioned in the reservoir such that first electrodes14and second electrodes16alternate with one another. The first electrodes14and second electrodes16are parallel or substantially parallel with one another. An aqueous medium18is positioned in the reservoir such that first electrodes14and the second electrodes16are in contact with the medium18.

In some embodiments, the medium18may include one or more electrolytes and can be a liquid, a solution, a suspension, or a mixture of liquids and solids. In one example, the medium18is wastewater that includes organic matters, ammonia, and chloride (Cl−). The chloride can be present in the medium18as a result of adding a salt to the medium18or the medium18can include urine that is a natural source of the chloride. The electrolysis system15also includes a voltage source (not shown) configured to drive an electrical current through the first electrodes14and second electrodes16so as to drive a chemical reaction in the medium18. The system15can operate in MP mode or alternatively in BP mode.

The electrolysis system15illustrated inFIG. 3includes an inlet20and an outlet22. The electrolysis system15can operate as a continuous reactor in that the medium18flows into the reservoir through the inlet20and out of the reservoir through the outlet22. Alternately, the electrolysis system can also be operated as a batch reactor. When the electrolysis system15is operated as a batch reactor, the medium18can be a solid, a liquid, or a combination.

In an alternative configuration of the system15, at least some of the second electrodes16may include the metal-doped NTA electrode19, instead of the metal base electrode16.

In accordance with exemplary embodiments of the systems8,15,21, the aqueous medium18may have a pH>7.

Any of the systems8,15,21may be employed for efficient continuous removal of one or more contaminants, which contaminants may include pharmaceuticals, biological matter, biocides such as herbicides, pesticides and fungicides, human or animal waste, pathogenic contaminants such as viruses, bacteria or parasites, chemical pollutants such as PCBs, TCEs, phthalates, or the like, semiconductor manufacturing wastewater which may contain contaminants such as perfluoroalkylsulfonate surfactants (PFAS), tetramethylammonium hydroxide (TMAH), and/or residual photopolymers, any combination of the foregoing, and the like.

FIG. 4is a process flow diagram40illustrating example methods of manufacturing one or more types of the NTA electrodes disclosed herein, which may be used in any of the systems shown inFIGS. 1-3.

First, a titanium substrate (e.g., Ti foil, plate, mesh or the like) is provided (box41). For example, the Ti substrate may be a Ti plate or Ti mesh. Next, as shown by box42, an amorphous TiO2NTA (Am-NTA) electrode may be prepared by a first anodization of the Ti substrate at 42 Volts in ethylene glycol (EG) electrolyte with 0.25 wt % NH4F and 2 wt % H2O for 3 to 6 hours. After the first anodization, the Am-NTA formed on the substrate is subjected to second anodization in 5 wt % H3PO4/EG electrolyte at 42 Volts for one hour to enhance its mechanical stability.

A metal loaded Am-NTA (M/Am-NTA) electrode may be prepared by dipping an Am-NTA electrode into a coating solution one or more times (box45). For example, the Am-NTA electrode may be dipped in a coating solution for about one minute, pulled up at a rate of 10 mm/minute, and then dried at room temperature for about two minutes. The foregoing dipping procedure may be repeated two or more times to vary the loading. For example, the dipping procedure may be repeated three times. Other procedures may be used to metal load the Am-NTA, such as atomic layer deposition and/or electrodeposition. Any suitable metal may be used for loading and doping the Am-NTA electrode, for example, cobalt, iron, nickel, manganese, cerium, vanadium, lead, tin, any of the noble metals, such as Pt, Ir, Au or Ag, any combination of the foregoing, or the like.

The coating solution may be prepared by dissolving a metal nitrate salt in ethanol. Example metal nitrate salts may include Co(NO3)2, Fe(NO3)3, Ni(NO3)2, Mn(NO3)2, and Ce(NO3)2. The concentration of the metal coating solution may be 250 mM.

For some embodiments of the NTA electrode, a cobalt loaded Am-NTA (Co/Am-NTA) electrode may be prepared by dipping an Am-NTA electrode into 250 mM Co(NO3)2/ethanol solution. For example, the Am-NTA may be dipped into the coating solution for one minute, pulled up at the rate of 10 mm/minute, and finally dried at room temperature for two minutes. The dip-coating process may be repeated three times. This process may result in a Co loading of the NTA of about 0.54±0.12 μmol/cm2, as determined by ICP-MS (Agilent 8800), forming a Co-black NTA electrode.

The amount of metal included in the metal-doped nanotube array may be tuned to a predetermined amount based on the application of the electrode. Any suitable metal loading amount may be used, such as <5 μmol/cm2of the effective area of the NTA. For example, lowering the Co(NO3)2concentration in the dip-coating solution to 50 or 25 mM and following the above three-dip procedure produces different embodiments of the Co-black NTA electrode, with Co loadings of 0.25 and 0.17 μmol/cm2, respectively, referred to herein as Co(0.25)-black NTA and Co(0.17)-black NTA, respectively.

A metal-doped black NTA (M-black NTA) electrode may be obtained by annealing the M/Am-NTA electrode in a stream of 5% H2/Ar at 450° C. for 30 min and then naturally cooling down to room temperature.

For example, a cobalt-doped black NTA (Co-black) electrode may be obtained by annealing a Co/Am-NTA electrode in a stream of 5% H2/Ar at 450° C. for 30 min and then naturally cooling it down to room temperature (box47).

A black NTA electrode may be obtained by annealing an Am-NTA electrode in a stream of 5% H2/Ar at 450° C. for 30 min and then naturally cooling it down to room temperature (box48).

Annealing a Co/Am-NTA electrode in air at 450° C. for one hour yields a Co-NTA electrode (box46).

Annealing an Am-NTA electrode in air at 450° C. for one hour yields an NTA electrode (box43). A blue NTA electrode may be prepared by applying a cathodic current of 5 mA cm−2on the NTA electrode for ten minutes in a 0.1 M potassium phosphate buffer solution (KPi) (box44).

The thermally-treated NTA electrodes, including the Co-black NTA, may be in the anatase phase with a preferential exposure of (101) planes.

FIG. 5is a schematic conceptual cross-sectional view showing electron flow through an exemplary Co-black nanotube34, which may be included in any of the NTA electrodes14,19ofFIGS. 1-3. The nanotube34is formed of one or more walls36protruding from substrate35. The tube walls26form a hollow interior section37. The walls36can be Co-black TiO2formed on the substrate35, as disclosed herein. The substrate35may be a valve metal, such as Ti.

As shown inFIG. 5, during anodic operation, electron flow proceeds from the aqueous medium down the Co-black TiO2walls36of each nanotube34of an array into the substrate35. Bulk oxygen vacancies (Ov) are the primary source of the enhanced conductivity of Co-black NTA.

Conductive NTAs supported on titanium plates, consisting of a multitude of nanotubes, such as nanotube34, have an advantage over particulate electrocatalysts, since they can be utilized directly as electrodes without the need for additional adhesive substrates or organic binders. However, deactivation of both blue and black NTAs have been observed after a few hours of electrocatalysis due to the surface passivation.

To overcome this problem, doping trace amounts of a metal, such as cobalt, onto a black TiO2NTA (Co-black NTA) dramatically increases electrode stability. It also provides further advantage by lowering of the OER overpotential. Generally, in Co-black NTAs CoOxis immobilized and stable on the black NTAs even at circum-neutral pH. Cobalt doping of black TiO2NTA significantly extends the lifetime of black NTA electrodes via tuning of the concentration and stability of surficial oxygen vacancies. Cobalt doping of black TiO2NTA may both create and stabilize surficial Ov, preventing surface passivation.

Enhanced stability was observed for the disclosed Co-black NTA electrode. Under comparative testing, the Co-black NTA electrode had an operational life of over 200 hours. Previously reported operational lifetimes of some conductive NTA electrodes were less than three hours.

Testing also demonstrated that the disclosed Co-black NTA electrodes have higher OER activity due to their lower onset potentials and higher current densities than known anodes. Co-black NTA electrodes with Co loading of about 0.54 μmol/cm2exhibit 200 times higher current density compared with Co—TiO2film electrodes at 2.3 VRHE. This finding shows the improvement provided by a conductive NTA substrate over the OER activity of a Co—TiO2electrode.

The improvement in OER provided by the Co-black NTA electrode is a result of the cobalt doping. Catalytic activity, such as OER, is determined by the number of active sites. Surficial Ovis generally considered as an active site for OER. It exposes unsaturated metal ions, which in turn, lead to the adsorption and dissociation of H2O. Cobalt doping of the black NTA reduces the level of surficial Ti4+in Co-black NTA to a lower valence state, and thus, creates more surficial Ov(about 25% more).

Further testing demonstrated that certain disclosed Co-black NTA electrodes outperformed IrO2-based dimensionally stable anode (DSA) for oxidative electrochemical wastewater treatment. For example, the increase of Co loading may form a CoOxfilm on top of a Co-black NTA substrate. The resultant CoOx/Co-black composite electrode (Co*-black NTA and Co**-black NTA electrodes, described herein below) exhibit high OER activity (e.g., overpotential of 352 mV vs. 434 mV for IrO2DSA) and stability (>200 operational hours) in 1 M KOH electrolyte at 10 mA/cm2.

FIG. 6is a scanning electron microscope (SEM) image of the top surface50of an exemplary Co-amorphous (Co—Am) NTA produced from a process depicted byFIG. 4. As shown, a plurality of nanotubes52are formed adjacent to one another with cobalt rich areas54.

FIG. 7is a scanning electron microscope (SEM) image of the top surface100of an exemplary Co-black NTA electrode resulting from a process depicted byFIG. 4. As shown, a plurality of nanotubes102are formed adjacent to one another with cobalt rich areas104.

FIG. 8is a scanning electron microscope (SEM) perspective image of an exemplary Co-black NTA150resulting from a process depicted byFIG. 4and includable in the Co-black NTA electrodes disclosed herein. The Co-black NTA150includes a multitude of nanotubes152, each having a tubular wall and hollow center.

FIGS. 9A-Bare graphs comparing experimental results of wastewater treatment using a disclosed Co-black NTA electrode and prior electrodes. The graphs compare the performance of a commercially-available IrO2dimensionally stable anode (C-DSA), a laboratory-made IrO2dimensional stable anode (DSA), and a disclosed Co-black NTA anode.FIG. 9Ashows the decay of NH4+as a function of electrolysis time, andFIG. 9Bshows the decay of chemical oxygen demand (COD) as a function of electrolysis time.

To obtain these experimental results, a Co-black NTA anode was applied for the treatment of latrine wastewater that was collected on the Caltech campus in a prototype solar toilet system. Chloride (40 mM) that originated from human waste (i.e., urine) is oxidized to chlorine (e.g., HOCl, ClO−). Hypochlorus acid, HOCl, reacts with ammonia (NH3/NH4+) to form chloramines (e.g., NH2Cl, NHCl2), which in turn undergo a self-reaction leading to denitrification with the off-gassing of N2leading eventually to breakpoint chlorination. The Co-black anode outperformed the IrO2DSA due to its higher CER activity (FIG. 10A). Although the C-DSA had higher CER, it exhibited inferior NH4+removal performance than Co-black NTA anode. This may be because the Co-black NTA anode is more active for the removal of organics, which compete with NH4+to react with chlorine. Both chlorine and .OH contribute to the removal of organic pollutants (indexed in terms of chemical oxygen demand, COD). As shown, the COD removal capability of the Co-black anode is superior to that of IrO2DSA and C-DSA (FIG. 9B). The effluent after eight hours of treatment was clear in appearance and suitable for non-potable water reuse.

FIGS. 10A-Care graphs of example experimental results comparing certain radical species generation during electrolysis using disclosed Co-black NTA anodes and prior anodes.FIG. 10Ashows a comparison of the chlorine evolution rate in 30 mM NaCl of certain anodes.FIG. 10Ashows a comparison of hydroxyl radical production measured by electrochemical oxidation of benzoic acid for a Co-black NTA anode, a Co(0.17)-black NTA anode, and an IrO2anode.FIG. 10cshows a comparison of the direct oxidation efficiency measured by electrochemical oxidation of oxalate ion for a Co-black NTA anode, a Co(0.17)-black NTA anode, and an IrO2anode. A constant current of 10 mA cm−2was applied to each of the anodes in each of the above tests.

RegardingFIG. 10B, hydroxyl radical can be generated during water electrolysis. Benzoic acid (BA) was selected as a radical probe compound (r.OH=5.90×109M−1s−1). Faster degradation kinetics observed for the Co-black NTA anode compared to IrO2anode indicates the Co-black anode has a higher activity toward .OH generation than the IrO2anode. The direct electron transfer (DET) mechanism may also contribute to the oxidation of organic compounds. RegardingFIG. 10C, the oxalate ion was selected to investigate the DET activity, as it is known to be reactive via DET due to surface complex formation, but at the same time, oxalate reacts slowly with .OH (r.OH=1.4×106M−1s−1) compared to typical hydroxyl radical second-order rate constants.FIG. 10Cindicates that the Co-black anode shows higher DET activity than the corresponding IrO2electrode. As shown in the graphs ofFIGS. 10A-C, the Co(0.17)-Black NTA electrode shows comparable activity with Co-black NTA electrode with regard to chlorine evolution, radical production, and DET reaction.

Additional embodiments of the disclosed electrodes include a Co*-black NTA electrode and a Co**-black NTA electrode. These electrodes have higher OER activity compared to the Co-black NTA electrode.

The Co*-black NTA and Co**-black NTA electrodes can each be made by increasing the Co loading of a Co-black NTA electrode to form a CoOxfilm on top of the NTA. To make a Co*-black NTA electrode, a Co(NO3)2/ethanol solution may be drop-cast onto a Co-black NTA electrode. The electrode is then reduced in 5% H2/Ar at 450° C. for 30 minutes. During annealing, a discrete film layer of amorphous CoOxis formed on top of the Co-black NTA. The CoOxfilm may have a higher valence (3+/2+) than that of Co-black NTA (2+) due to the absence of Co—TiO2interaction. The Co*-Black composite NTA electrode with Co loading of 2.1 μmol cm−2may have an OER overpotential of 360 and 434 mV, respectively, at 1.0 and 10 mA cm−2constant current in 1 M KOH.

The Co**-black NTA electrode is made following the same steps as making the Co*-black NTA electrode, but increasing the Co loading to 4.2 μmol cm−2. The Co**-black NTA electrode may have greater OER activity; this electrode may have an overpotential of 289 and 352 mV at 1.0 and 10 mA cm−2constant current, respectively, in 1 M KOH. The performance of the Co**-Black NTA electrode is not only higher than IrO2DSA, C-DSA, and Co(OH)x/Ti, but also superior to the reported activities of a benchmarking Co(OH)x/GC (400 mV at 10 mA cm−2), Co3O4nanowires (320 mV at 1 mA cm−2), Co3O4nanosheets (390 mV at 10 mA cm−2), and Co@Co3O4nanoparticles (420 mV at 10 mA cm−2).

A higher OER activity may be achieved by doping Ni and Fe into the CoOxfilm of the Co*-black NTA and Co**-black NTA electrodes.

The higher OER activity of Co**-black NTA electrode may be attributed to two primary factors. First, more OER active sites are created by the CoOxfilm. A double-layer capacitance may be formed, which is proportional to the electrochemically active surface area (ECSA), increases in the order of Co-black (7.5)<Co*-black (12.4)<Co**-black (21.4 mF cm−2). A Co**-black NTA electrode with 6 cm2geometric area has a large ECSA of 3210 cm2, giving a roughness factor of 535. Co**-black NTA has higher Ovconcentration (32%) than Co-black NTA (25%). The oxygen vacancies of CoOxare surrounded by Co ions (Co—Ov—Co), which may be intrinsically more OER active than the Co—Ov—Ti and Ti—Ov—Ti sites of Co-black NTA. Secondly, the anti-passivation functionality of Co-black facilitates charge transport from the CoOxfilm to Co-black NTA, then to the underneath Ti metal support. A Co**-black NTA electrode was tested to be stable for more than 200 hours in 1.0 M KOH at 10 mA cm−2. In both the Co*-black NTA and Co**-black NTA electrodes, the presence of Co-black NTA as an interlayer may prevent the passivation of the catalyst/Ti interface, thus dramatically improving the stability of the electrodes.

The disclosed electrodes may be employed in solar powered toilets and waste treatment systems, for example, those disclosed in U.S. Published Patent Application 2014/0209479, which is incorporated by reference herein in its entirety. For example, the source38ofFIG. 1herein may be a photovoltaic source. And the electrolysis can be done on human waste, such as the electrolysis of urine depicted in FIG. 17C of U.S. Published Patent Application 2014/0209479.

The foregoing description is illustrative and not restrictive. Although certain exemplary embodiments have been described, other embodiments, combinations and modifications involving the invention will occur readily to those of ordinary skill in the art in view of the foregoing teachings. Therefore, this invention is to be limited only by the following claims, which cover at least some of the disclosed embodiments, as well as all other such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.