Electrolysis is a promising option for carbon-free hydrogen production from renewable and nuclear resources. Electrolysis is the process of using electricity to split water into hydrogen and oxygen. The process of electrolysis is performed in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production, to large-scale, central production facilities that, for instance, could be directly connected to renewable or other non-greenhouse-gas-emitting forms of electricity production.
In 2021, the U.S. Department of Energy's (DOE's) has formulated an objective to reduce the costs of clean hydrogen by 80% to $1 per 1 kilogram in 1 decade. The objective of reducing the production of hydrogen to $1 per 1 kilogram in 1 decade is referred to as the hydrogen “1 1 1” initiative. Electrolysis is a leading hydrogen production pathway to achieve this goal.
Hydrogen produced via electrolysis can result in zero greenhouse gas emissions, depending on the source of the electricity used. The source of the required electricity, including its cost and efficiency, as well as emissions resulting from electricity generation, must be considered when evaluating the benefits and economic viability of hydrogen production via electrolysis. In many regions in the world, today's power grid is not ideal for providing the electricity required for electrolysis. The reason for this is the greenhouse gases released during the actual production of the electricity and the amount of fuel required to produce electricity due to the low efficiency of the electricity generation process.
Hydrogen production via electrolysis is being pursued for renewable and nuclear energy options, including wind, solar, hydro and geothermal energy production. These pathways result in virtually zero greenhouse gas and criteria pollutant emissions, provided the electricity that is used for electrolysis is obtained by means of renewable energy sources. Moreover, it is important that the overall production cost for the energy decrease significantly to be competitive with more mature carbon-based pathways such as natural gas reforming.
In view of the above, there is a growing need for improved electrolyzers which show improved energy efficiency and lifetime. In particular, there appears to be a need for providing improved coatings for electrodes used in electrolyzers, such as improved coatings directed to oxygen evolution as target reaction.
According to a first aspect, the disclosure relates to an electrolyzer electrocatalyst, comprising Cobalt (Co) oxide, Zirconium (Zr) and a noble metal.
According to a second aspect, the disclosure relates to an electrode for use in an electrolyzer, the electrode comprising a support and a coating, wherein the coating comprises Cobalt (Co) oxide, Zirconium (Zr) and a noble metal.
According to a third aspect, the disclosure relates to an electrochemical system comprising an electrolyser, the electrolyser having a cathode, an anode, and an electrolyte or electrolytes, wherein the cathode, the anode or both the cathode and the anode comprise an electrocatalyst, the electrocatalyst comprising Cobalt (Co) oxide, Zirconium (Zr) and a noble metal.
According to a fourth aspect, the disclosure relates to the use of an electrocatalyst for catalysing an electrolysis process, wherein the electrocatalyst comprises Cobalt (Co) oxide, Zirconium (Zr) and a noble metal.
According to a fifth aspect, the disclosure relates to a method for electrolysing water comprising the steps of:
According to a sixth aspect, the invention relates to the use of a cathode electrocatalyst comprising Cobalt (Co) oxide, Zirconium (Zr) and a noble metal for producing hydrogen via an electrolysis process.
According to a seventh aspect, the disclosure relates to a method for producing an electrode for use in an electrolyzer, the electrode comprising a support and a coating, the method comprising the steps of:
The phraseology and terminology used in this disclosure is for the purpose of description and should not be regarded as limiting. As used herein, the term “plurality” refers to two or more items or components. The terms “comprising,” “including,” “carrying,” “having,” “containing,” and “involving,” whether in the written description or the claims and the like, are open-ended terms, i.e., to mean “including but not limited to.” Thus, the use of such terms is meant to encompass the items listed thereafter, and equivalents thereof, as well as additional items. Only the transitional phrases “consisting of” and “consisting essentially of,” are closed or semi-closed transitional phrases, respectively, with respect to the claims. The use of ordinal terms such as “first,” “second,” “third,” and the like in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.
Hydrogen (H2) is an important feedstock for various branches of the chemical industry, such as petrochemicals and semiconductor manufacturing. Moreover, it holds high potential as an agent to make the global energy infrastructure more environmentally sustainable. Hydrogen can serve as energy carrier to replace fossil fuels in a hydrogen economy, and it is also able to reduce CO2 emissions in energy-intensive applications such as steel and aluminium refining.
The most prominent way of producing hydrogen that is truly ‘green’ is through water electrolysis powered by renewable energy sources. However, water electrolysis suffers from energy inefficiencies due to the difficulty of catalyzing the reaction.
Better electrocatalysts are needed to make the process more economically competitive.
The overall reaction in water electrolysis is given by
2H2O→2H2+O2
The process is carried out in either acid or alkaline electrolyzers, where acid electrolyzers use a wet acidic ion exchange membrane as electrolyte, and alkaline electrolyzers use concentrated aqueous base, typically KOH in range of 15-30% mass, as electrolyte with a Zirfon separator.
Acidic systems benefit from compactness, low electrolyte resistance and good gas separation capabilities, which allows them to run at higher current densities of typically 10-30 kA/m2, and makes them more flexible in terms of ramping activity up and down. One of the main disadvantages is the reliance of this type of electrolyzer on iridium as electrocatalyst on the anode, which is an exceedingly rare and therefore expensive element. Alkaline systems rely much less on critical materials, but are bulkier, have higher internal resistances and lower power flexibility.
The overall reaction consists of two electrochemical half reactions, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which are described respectively in acidic and alkaline electrolytes by
4H++4e−→2H2
4H2O+4e−→2H2+4OH−
2H2O→O2+4H++4e−
4OH−→O2+2H2O+4e−
The largest energy loss originates from the oxygen-evolving anodic half-reaction. A better electrocatalyst for this reaction would have a smaller overpotential and higher energy efficiency. In this disclosure an electrode comprising an improved electrocatalyst is presented.
The electrolyzer 10 further comprises an anode 21 and a cathode 22 which are placed in the electrolyte 12. The anode 21 and the cathode 22 are connected to a source of electrical energy 30. In the electrolyzer 10, a diaphragm 13 is positioned in between the anode 21 and the cathode 22.
As indicated in
Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte have been commercially available for many years. One important parameter of alkaline hydrolysis is the type of electrodes and coatings that are used. Evolution of oxygen in alkaline water electrolyzers is usually catalyzed on anodes made with massive nickel, massive steel, or nickel coated steel. While these materials offer long lifetimes, the overpotential for oxygen evolution is relatively high. One of the effects thereof is a relatively high level of corrosion, for instance for steel-based anodes. The specific circumstances of this corrosion are not well-understood at the moment. In view of corrosion prevention, the anodes 21 and cathodes 22 used in electrolyzers 10, normally comprise an adapted coating to improve the lifetime of the electrodes.
In the prior art alternative solutions for producing electrolyzers are known, which use solid alkaline exchange membranes (AEM) as the electrolyte. This anion exchange membrane can be used with pure water or a KOH solution as additional electrolyte. These alternative solutions using an anion exchange membrane to separate anode and cathode compartments are showing promise on laboratory scale.
The present disclosure relates to an electrocatalyst which is used in the form of a coating for an electrode, in particular an anode 21, which can improve the properties of the electrode and in particular the lifetime of the electrode. The coating according to the disclosure is directed to oxygen evolution as target reaction. The coating is a Cobalt (Co) oxide based coating comprising Zirconium (Zr) as dispersing agent and a noble metal to promote electrical conductivity throughout the bulk coating. According to the disclosure, the noble metal is preferably selected from Ruthenium (Ru), Gold (Au), Iridium (Ir), Platinum (Pt) and Palladium (Pd). It has been established, as described in more detail below, that the lifetime of the coating comprising Cobalt oxygen, Zirkonium and in particular Ruthenium and/or Gold is much higher than known coatings. The coating described in the disclosure provides longer lifetimes than other well-known Ni substitutes, such as Ni—Fe oxyhydroxides, due to the much higher robustness of cobalt oxide.
In one embodiment, an anode comprising a Cobalt oxide coating comprising Zr and Ru and/or Au allows for catalyzing oxygen evolution at a lower overpotential due to the relatively high electrochemical activity of Cobalt, and benefits from the incorporation of Zr as a dispersing agent and Ru and/or Au to promote electrical conductivity throughout the bulk coating.
According to the disclosure, the mentioned coating comprising the Cobalt oxygen, Zirconium and a noble metal such as Ruthenium or Gold is deposited on an adapted metal support. Preferably, the coating is deposited on a Titanium (Ti) or Nickel (Ni) support. Alternatively, the support comprises a Titanium alloy, a Nickel alloy, steel or stainless steel.
Titanium is an especially attractive substrate, due to its dimensional stability and high availability. A known drawback of using Titanium as a support material for obtaining an electrode, is the possibility of forming an electrically insulating oxide interlayer during the coating preparation or actual electrolysis. However, according to present disclosure, the risk of forming such an electrically insulating oxide interlayer is negated by the presence of Ru in the coating, which has the ability to form a passivation-resistant interlayer at the interface Titanium-coating.
Nickel is particularly suitable for the preparation of electrodes since it is dimensionally stable and is capable of strongly interacting with Co by forming NiCo2O4 spinels.
Cobalt oxide (Co3O4) is a well-known oxygen evolution electrocatalyst and, along with mixtures of nickel iron oxides and cobalt iron oxides, one of the materials with the highest power efficiency. This means that the material allows in use for a low overpotential. The material has a lower overpotential than nickel oxide grown on massive Ni, which is the standard material in alkaline electrolyzers today, and which tends to deactivate over time.
To utilize Co3O4 layers in alkaline electrolyzers, significant layer thicknesses need to be deposited; although it was found that the cobalt wear rate during operation is on the same scale as iridium oxide, a state-of-the-art electrocatalyst with very high rarity and price, the extremely high lifetime requirements of alkaline electrolyzers necessitate significant loadings. Co3O4 however suffers from poor bulk electrical conductivity, which makes thick layers of the pre-formed oxides unfeasible.
In one embodiment, attempts to circumvent this issue are achieved by adding to the Co3O4 layer both a) Zr and b) Ru or Au, which serve as a) a dispersing agent to increase the volume and active surface area of the electrocatalyst, and b) a conductivity agent, to improve the electrical conductivity in the bulk coating and prevent the formation of a passivating layer at the interface of the coating and the massive metal support during repeated calcination in air and electrolytic operation of the coating.
According to the disclosure it has been established that, surprisingly, very small amounts of Zirconium in combination with a very small amount of a noble metal, such as Ruthenium or Gold importantly alters and improves the properties of a coating comprising Cobalt oxide comprising coating, in particular when considering oxygen evolution.
It is noted that the coatings according to the disclosure allow electrolyzers using electrodes and in particular anodes provided with the coating to operate at a higher power efficiency. The power efficiency is the key factor in determining the OPEX, which term refers to the operating expenses. If the gain in efficiency at high currents densities is sufficient, it may also reduce the needed stack size, which decreases the CAPEX, which term refers to the capital expenses.
As further shown in
In the example of
According to
It is noted that the results shown in
In general, the coating can be painted on the support. According to an embodiment, prior to the step of applying the coating, a viscosity modifier is added. An adapted viscosity modifier for use in the production of electrodes according to the disclosure is polyethylene glycol.
After the application of the coating on the support, the production process was followed by thermal decomposition at 400° C. for 15 minutes in air. That means that the Titanium supports were heated in an oven. The mentioned step of heating could be done be a temperature between about 300° C. and 600° C., preferably by a temperature between about 350° C. and 450° C.
The metal salt referred above could, for instance, comprise CoCl2, RuCl3, and ZrCl2. Alternatively, the salts could comprise Co(NO3)2, Zr(NO3)2 and Ru(No)(NO3).
The so obtained electrodes were then electrolyzed at 600 A/m2 in strong acid (H2SO4; 25%). Although the coatings are intended to be used under strongly alkaline conditions, electrolysis in strong acid serves as an accelerated lifetime test, since one of the primary degradation mechanisms is local acidification at the catalyst surface due to the nature of oxygen evolution reaction.
The dispersing effect of Zirconium and the conductivity-promoting effect of Ruthenium were further analyzed varying their fractions and noting the effect on the initial potential and the lifetime of the coating.
The experiments shown in
The Co—Zr/Ru 100-9/1 coating show a clear activity enhancement for the Nickel support electrode when compared to pure Nickel, the benefit however is not as high as on the Titanium support electrode.
The difference in effectivity between the coating present on the Nickel support and on the Titanium support is presumably the fact that the Ruthenium component is less efficient at promoting the conductivity when the substrate is Nickel instead of Titanium. The short-term stability is sufficient for both substrates.
It is further noted that the data shown in
According to the tests presented in
As shown in
The coatings shown in
The data provided for the Cobalt oxide coating comprising Zirconium and Ruthenium in
Filing Document | Filing Date | Country | Kind |
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PCT/EP2022/075440 | 9/13/2022 | WO |
Number | Date | Country | |
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63243353 | Sep 2021 | US |