TECHNICAL FIELD

The present disclosure relates to a coated electrode comprising a conductive substrate and a coating . The disclosure also relates to a method for preparing a coated electrode as well as use of the coated electrode .

BACKGROUND

Coated electrodes are used for example in electrowinning . The commercial use of electrowinning usually requires several electrodes , such as anodes , in a single electrolytic cell . For decades lead based anodes have been used in electrowinning due to their low-cost and availability . However, lead anodes have several drawbacks related to for example their significant energy consumption, environmental issues with generation of undesirable sediments and sludge as well as cathode contamination . Other kinds of electrodes have been developed . Typically, a titanium anode or alloy is used with an expensive platinum group metalbased coating . However, as large numbers of electrodes are required, the cost becomes a significant factor for electrode selection . Further, environmental issues and sustainability are becoming increasingly important to consider . Thus , the inventor has identified a need to develop novel kinds of coated electrodes .

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description . This Summary is not intended to identify key features or essential features of the claimed subj ect matter, nor is it intended to be used to limit the scope of the claimed sub ect-matter .

Disclosed is a coated electrode comprising a conductive substrate and a coating . The coating may comprise an outer coating composition forming at least one outer coating layer . The outer coating layer may comprise manganese and iridium, wherein the molar ratio of iridium is at least 10 mol-% and below 40 mol-% and may be measured using X-ray fluorescence (XRF) . The mol-% values are based on the metal content of the outer coating composition . In the outer coating composition, manganese and iridium are in the form of their oxides . EDX technology (Energy Dispersive X-ray Spectroscopy) is suitable for the XRF measurements . I f there are several layers or coatings , especially in case of a multi-coating layer with different coating compositions , the measurements may be performed from the cross section of a coated conductive substrate to measure the mol-% content of layers separately . Another way to measure XRF could be WDX (Wavelength Dispersive X-ray analysis ) .

Disclosed is also a method for preparing a coated electrode . The method may comprise : i . providing a conductive substrate to be coated, ii . pretreating the conductive substrate , wherein the pretreatment includes subj ecting the conductive substrate to an aqueous solution, to obtain a pretreated conductive substrate , iii . painting the pretreated conductive substrate with a precursor solution comprising a solvent and a starting material composition to obtain a painted conductive substrate , iv . drying the painted conductive substrate , and v . calcining the painted conductive substrate at a temperature of at least 400 ° C to obtain a coated electrode .

Further, the coated electrode may comprise an outer coating composition comprising manganese and iridium and forming at least one outer coating layer . The molar ratio of iridium may be at least 10 mol-% and below 40 mol-% and may be measured using X-ray fluorescence (XRF) . The mol-% are based on the metal content of the outer coating composition . In the outer coating composition, manganese and iridium are in the form of their oxides .

Further, disclosed is use of the described coated electrode in a process selected from metal electrowinning, metal recovery, electrolytic foil production, metal plating, electrosynthesis , water electrolysis , water treatment , and electrolytic manganese dioxide production .

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings , which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments or aspects related to the disclosure . In the drawings :

Fig . 1 illustrates XRD patterns related to embodiments of the outer coating composition of the disclosed coated electrode .

Fig . 2A - 2D illustrate SEM images showing the surface morphology of various coating compositions . DETAILED DESCRIPTION

A coated electrode is disclosed . The coated electrode comprises a conductive substrate and a coating . The coating comprises an outer coating composition forming at least one outer coating layer, which comprises manganese and iridium, wherein the molar ratio of iridium is at least 10 mol-% and below 40 mol-% . The molar ratio may be measured using X-ray fluorescence (XRF) . The molar ratio is based on the metal content of the outer coating composition . In the outer coating composition, manganese and iridium are in the form of their oxides .

Disclosed is also a method for preparing a coated electrode , wherein the method comprises : i . providing a conductive substrate to be coated, ii . pretreating the conductive substrate , wherein the pretreatment includes subj ecting the conductive substrate to an aqueous solution, to obtain a pretreated conductive substrate , iii . painting the pretreated conductive substrate with a precursor solution comprising a solvent and a starting material composition to obtain a painted conductive substrate , iv . drying the painted conductive substrate , and v . calcining the painted conductive substrate at a temperature of at least 400 ° C to obtain a coated electrode .

The coated electrode comprises an outer coating composition comprising manganese and iridium . The outer coating composition may form at least one outer coating layer, wherein the molar ratio of iridium is at least 10 mol-% and below 40 mol-% measured using X- ray fluorescence (XRF) and based on the metal content of the outer coating composition . In the outer coating composition, manganese and iridium are in the form of their oxides .

A coated electrode with a coating comprising a combination of iridium oxide , in an amount of at least 10 mol-% to below 40 mol-% , and manganese oxide has shown to provide a long lifetime to the electrode . At the same time , the coated electrode provides a sufficiently low cell voltage . A low cell voltage is preferred to keep the energy consumption low as well . An additional benefit is that the raw-material costs of such an electrode are kept at a feasible level . A higher amount of Mn is preferred, since it has better availability . The cell voltage of coatings with an iridium content of 10 to below 40 mol-% give a lower cell voltage than coatings with a higher iridium content . Since several electrolysis cells and electrodes are typically needed and they are used for a long time at a certain high current , even small differences in the cell voltage ( such as 0 . 01 to 0 . 1 V) can make a considerable difference . I f the cell voltage is for example 2 . 0 V ( such as in Cu electrowinning) , a change of 0 . 02 V corresponds to one percent . A one percent lower cell voltage could provide yearly energy savings for an electrowinning plant ( such as a typical industrial scale Cu or Zn plant ) in the range of 5 to 10 GWh .

To form at least one layer of outer coating composition, a precursor solution is typically applied . After applying the precursor solution, the coatings are typically calcined before use . A high calcining temperature usually provides a more stable coating . However, it usually also causes a relatively higher cell voltage as a side effect . Thus , calcining the coating usually requires optimizing the calcining temperature so that the cell voltage does not become too high. However, in the described coated electrode, this problem can be avoided. In the calcined coating, the metals are present as their oxides. In connection with this invention, it was discovered by XRD (Fig. 1) that typical iridium oxide, IrCy, peaks shift to higher angles when the iridium content is at least 10 mol-% to below 40 mol-% (such as 39 mol-%) . Also, the peak of IVkpOs disappears at the same time. Not bound by any theory, this suggests that iridium and manganese form a composite oxide when combined in certain ranges. It is believed that this oxide might be the reason for the low cell voltages and relatively long lifetime. Thus, the combination of iridium and manganese provides a new, surprising synergetic effect to the outer coating layer of the electrode. Further, SEM images (Fig. 2A to 2D) support the results and show a different surface morphology and a larger surface area at the defined molar percentages. This provides the benefit of more effective surface area for reaction, and in that way gives a longer lifetime to the coated electrode.

Figure 1 discloses XRD (X-ray diffraction) patterns of the outer layer coating. The electrodes with molar percentages of 100 and 95 mol-% Mn show clear diffraction peaks of M^Cg. The peaks of NkpCg disappear in certain compositions. The results indicate that an amount of iridium (at least 10 mol-%) may have this effect in combination with manganese. The diffraction peaks of IrCy are seen when the Ir content is 80 mol-% (Ir:Ta = 80:20 mol-%) and 90 mol-% (Mn:Ir = 10:90 mol-%) . IrCy peaks shift to higher angles when the Ir content is from 10 mol-% to less than 40 mol-% (such as 39 mol-%) , suggesting Ir and Mn make a composite oxide. Figure 2 disclosed a SEM (Scanning Electron Microscopy) image of the surface morphology of various outer coating compositions. Fig. 2A shows an outer coating with Mn:Ir = 95:5 mol%, Fig. 2B shows an outer coating with Mn:Ir:Ta = 70:10:20 mol%, Fig. 2C shows an outer coating with Mn:Ir = 70:30 mol%, and Fig. 2D shows an outer coating with Mn:Ir = 10:90 mol%. Fig. 2A and Fig. 2D are reference examples. It can be seen in the Fig. 2B and 2C show a finely divided, porous surface morphology, while 2A and 2D show clearly less porous surface morphology compared to 2B and 2C. This shows that the disclosed coatings give more effective surface area for reaction as described above.

According to one embodiment, the molar ratio of manganese in the outer coating composition is higher than the molar ratio of iridium. This has been shown to provide the desired composite oxide.

According to one embodiment, the molar ratio of manganese in the outer coating composition is at least 30 mol-%, or least 40 mol-%, or at least 50 mol- % , or at least 60 mol-%, or at least 70 mol-%, or at least 80 mol-% measured using X-ray fluorescence (XRF) and based on the metal content of the outer coating composition. The molar ratio of manganese may not be above 90 mol-%. Compositions with the defined ration of manganese have shown to provide the desired composite oxide, which is believed to provide the desired surface morphology. Thus, in preferred compositions, the ratio of manganese is at least 30 mol-%, or at least 40 mol-%, but the results depend on the composition as a whole. According to one very specific embodiment, the outer coating composition the molar ratio of manganese is above 60 mol-%, or 61-90 mol-%, or 65-90 mol-%, or 70-80 mol-%, and the molar ratio of iridium is 10-39 mol-%, or 10-35 mol-%, or 20-30 mol-%, based on the metal content of the outer coating composition. According to one embodiment , the outer coating composition comprises ruthenium . Ruthenium has similar properties as iridium, ruthenium oxide is also conductive and active in the electrode coating . Thus , the effect of the described coated electrode may be achieved also with an outer coating composition comprising ruthenium in addition to iridium and manganese .

According to one embodiment , the outer coating composition comprises at least one further metal , which is selected from the group consisting of ruthenium, tantalum, zirconium, tin, cobalt , niobium, tungsten, molybdenum, titanium, platinum, antimony and/or an alloy or any combination thereof . The metals , except platinum, are present as oxides in the calcined outer coating composition . The provided list of metals may be part of the composition and still the outer coating composition provides a long lifetime and a sufficiently low cell voltage .

According to one embodiment , the outer coating composition comprises cobalt in an amount of less than 5 mol-% . I f the composition comprises cobalt , it is preferred to keep the amount at a low level .

According to one embodiment , the outer coating composition comprises tantalum in an amount of not more than 50 mol-% . Tantalum oxide has shown to work well with iridium oxide and mananganese oxide . The tantalum oxide may be able to stabili ze iridium oxide and mangnanes oxide .

According to one embodiment , manganese and iridium together constitute at least 40 mol-% , or at least 50 mol-% , or at least 60 mol-% , or at least 70 mol-% , or at least 80 mol-% , or at least 85 mol-% , or at least 90 mol-% , or at least 95 mol-% measured using X-ray fluorescence (XRF) and based on the metal content of the outer coating composition . According to one embodiment, manganese oxide and iridium oxide together constitute at least 25 weight-%, or at least 35 weight-%, or at least 45 weight-%, or at least 50 weight-%, or at least 60 weight-%, or at least 70 weight-%, or at least 80 weight-%, or at least 85 weight-%, or at least 90 weight-%, or at least 95 weight-% of the outer coating composition. The composition may comprise a metal with a much higher molecular weight than Mn and Ir, for example tantalum (Ta) . Thus, the weight-% of the manganese and iridium oxides may be clearly lower than their molar percentages, even as low as 25 weight-%.

According to one embodiment, the conductive substrate comprises titanium (Ti) and/or a titanium alloy. This substrate has been shown to work well in the described coated electrode.

According to one embodiment, the conductive substrate comprises at least two layers, or at least three layers, or at least four layers, or at least five layers of the outer coating composition. The conductive substrate may even comprise at least six layers, at least seven layers, or at least eight layers of the outer coating composition. It has been shown that more layers provide a higher lifetime of the coated electrode. According to one embodiment, the conductive substrate comprises 1 to 100 layers, or 1 to 50 layers, or 2 to 20 layers of the outer coating composition. Preferably, the conductive substrate comprises 1 to 15, or 2 to 10 layers of the outer coating composition.

According to one embodiment, the coated electrode comprises at least one layer of an intermediate coating composition between the conductive substrate and the outer coating composition. The intermediate coating layer has the added utility of providing a longer lifetime. According to one embodiment , the coated electrode comprises at least two layers , or at least three layers , or at least four, or at least five layers intermediate coating composition between the conductive substrate and the outer coating composition . Several intermediate coating layers have the added utility of providing a longer lifetime by protecting the substrate . This is important especially in case cracks are formed in the outer coating .

According to one embodiment , the intermediate coating composition comprises iridium oxide . Iridium is suitable in the intermediate coating layer, since iridium oxide is conductive and active .

According to one embodiment , the intermediate coating composition comprises platinum . Platinum is very stable , but it is also active to gas evolution reactions in an aqueous solution and thus suitable to use in the intermediate coating layer . Platinum is not present as an oxide , but as a metal .

According to one embodiment , the intermediate coating composition comprises iridium oxide and tantalum oxide . Iridium oxide starts to dissolve rather easily alone . Tantalum oxide again is very stable especially in an acidic aqueous solution, such as an electrowinning solution, and, thus , the combination is more stable than iridium alone and it decreases the consumption rate of iridium oxide .

According to one embodiment , the intermediate coating composition comprises one or more metals selected from the group consisting of tantalum, niobium, tungsten, molybdenum and titanium and/or an alloy or combination of any of these metal oxides . Tantalum, niobium, tungsten, molybdenum, and titanium all have the same properties . Their oxides are stable and can stabili ze iridium oxide .

According to one embodiment , the intermediate coating composition comprises at least 40 mol-% , or at least 50 mol-% , or at least 60 mol-% , or at least 70 mol-% , or at least 80 mol-% iridium measured using X- ray fluorescence (XRF) and based on the and based on the metal content of the intermediate coating composition .

To form the at least one layer of intermediate coating composition, a precursor solution is typically applied, and a calcination step is required . Typically, the in the calcination step of the intermediate coating composition layer, a high temperature is preferred to provide a long li fetime of the coated electrode . When using iridium in the intermediate coating composition layer, while calcinating the intermediate layer, results have shown that iridium oxide seems to have a good adhesion to a titanium ( Ti ) substrate . Thus , in order to achieve good adhesion or in other words "a strong bond" between the Ti substrate and Ir comprising intermediate layer, it is preferable to have a rather high amount of Ir in the intermediate layer . However, it is also preferred to include at least a small amount of for example tantalum ( Ta) , which has a stabili zing effect on iridium oxide . Thus , according to one very specific embodiment , the intermediate coating composition layer comprises , in mol-% , Ir-Ta 80 : 20 , or Ir-Ta 90 : 10 , or Ir-Ta 70 : 30 .

In addition, a relatively high calcining temperature for the intermediate layers provide more crystalline IrCy , which is stable and gives a long lifetime to the electrode . Furthermore , a high calcining temperature might be able to reduce the cracks .

According to one embodiment , the coated electrode is an anode .

According to one embodiment , the conductive substrate is in the form of a plate or a mesh . Preferably, in the form of a rectangular plate or mesh. The form may also be round or a rod.

In the method for preparing the coated electrode, an aqueous solution is used for pretreating the conductive substrate. According to one embodiment, the aqueous solution is and acidic or alkaline solution. Preferably, an acidic solution is used.

According to one embodiment, the pretreatment includes subjecting the conductive substrate to an acidic solution for 1 to 10 h, or for 2 to 9.5 h, or for 3 to 9, or for 4 to 8.5 h, or for 5 to 8 h.

According to one embodiment, the precursor solution comprises at least one solvent selected from the group consisting of water, butanol, isopropanol, other alcohols, other organic solvents, and any combinations thereof.

According to one embodiment, the pretreated conductive substrate is painted with a precursor solution comprising a solvent and a starting material composition. The painting is performed by spraying, roller coating, spinning and/or dip coating. Other similar painting methods may also be used.

According to one embodiment, the coating is calcined by thermal decomposition at a temperature in the range of 400° to 650 °C.

According to one embodiment, the steps iii. and iv. are repeated at least two times, or at least three times, or at least four times, or at least five times. Repeating the steps several times gives a thicker coating and increases the lifetime. Several coating layers are achieved, and this may reduce crack formation in the coating.

According to one embodiment, the method further comprises the following steps iii. a to v.a before step iii: iii. a. painting the pretreated conductive substrate with a precursor solution comprising a solvent and a starting material composition for forming an intermediate layer to obtain an intermediate painted conductive substrate, and iv. a. drying the intermediate painted conductive substrate comprising, and v. a. calcining the intermediate painted conductive substrate comprising at a temperature of at least 400 °C to obtain a coated electrode comprising an intermediate coating composition.

According to one embodiment, the steps iii. a to v.a are repeated at least two times, or at least three times, or at least four times, or at least five times. Repeating the steps several times gives a thicker coating and increases the lifetime. Several coating layers are achieved, and this may reduce crack formation in the coating.

According to one embodiment, the outer coating composition the molar ratio of manganese is above 60 mol-%, or 61-90 mol-% or 65-90 mol-%, 70-85 mol-%, or 70-80 mol-%, and the molar ratio of iridium is 10-39 mol-%, or 10-35 mol-%, or 15-30 mol-%, or 20- 30 mol-%, based on the metal content of the outer coating composition.

According to one embodiment, the conductive substrate comprises at least two layers, or at least three layers, or at least four layers, or at least five layers of the outer coating composition. Several coating layers may reduce crack formation in the coating and increases the lifetime of the electrode.

According to one embodiment, the coated electrode may be used in a process selected from metal electrowinning, metal recovery, electrolytic foil production, metal plating, electrosynthesis , water electrolysis , water treatment , and electrolytic manganese dioxide production .

The coated electrode described herein have several advantages compared to lead alloy anodes . For example ; there is no lead comtaminat ion of the cathode metal , no additives are needed to the electrolyte , which brings cost savings , and there i s no lead sludge generation, which improves sustainability and safety overall .

The coated electrodes described herein and the method for preparing them provide energy efficient electrodes , with a low cell voltage . The described outer coating composition provides a long lifetime , but still at a feasible cost . Also , the coated electrodes require relatively little maintenance . As large numbers of electrodes are required in industrial scale use , the benef its are considerable . The low cell voltage provides energy savings . The long lifetime of the electrodes requires less frequent exchange of the electrodes , which provides cost savings as well as a more sustainable product due to efficient utili zation of the raw materials .

EXAMPLES

Reference will now be made in detail to various test results related to and embodiments of this disclosure .

The description below discloses some embodiments in such a detai l that a person skilled in the art is able to utili ze the embodiments based on the disclosure . Not all steps or features of the embodiments are discussed in detail , as many of the steps or features will be obvious for the person skilled in the art based on this specification .

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components .

Fig . 1 illustrates XRD patterns related to an embodiment the outer coating composition of the disclosed coated electrode .

Fig . 2A - 2D illustrate SEM images showing the surface morphology of various coating compositions .

Example 1 - Measurements of cell voltage

Various outer coating compositions were prepared and a conductive substrate ( Ti ) was coated forming coated electrodes with an outer coating composition . The coated electrode was used as an anode in an electrolytic cell as defined in Table 1 . Platinum plate was used as the cathode . Table 1 : Cell voltage measurements in 180 g/L H2SO4 solution at 40 °C at 250 A/m ² . Example 2 - Measurements of electrode lifetime

Various outer coating compositions were prepared and a conductive substrate ( Ti ) was coated forming coated electrodes with an outer coating composition . The coated electrode was used as an anode in an electrolytic cell as defined in Table 2 . Table 2 : Lifetime measurements in 180 g/L H2SO4 solution at room temperature at 5500 A/m ² .

* * The sample was not considered economically feasible due to high cell voltage etc . , so no li fetime measurements were made . It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways . The embodiments are thus not limited to the examples described above ; instead they may vary within the scope of the claims .

The embodiments described hereinbefore may be used in any combination with each other . Several of the embodiments may be combined to form a further embodiment .

A coated electrode , a method, or a use , disclosed herein, may comprise at least one of the embodiments described hereinbefore . It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It will further be understood that reference to ' an ' item refers to one or more of those items .

The term "comprising" is used in this specification to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .