Evaluation of cerium oxide coated cathodes in the production of sodium chlorate via electrolysis

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Evaluation of cerium oxide coated cathodes in the production of sodium chlorate via electrolysis

Patrick Saade

KTH Royal Institute of Technology KE200X

Degree Project in Chemical Engineering June 12,2018

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Abstract

Sodium dichromate is a vital component during the electrolytic production of sodium chlorate.

This additive has many roles but the major one is the limitation of unwanted electrochemical reactions on the cathode, while allowing the hydrogen evolution to proceed. Unfortunately, sodium dichromate is an extremely toxic substance and new legislations require its elimination from the sodium chlorate production.

Cerium oxide is a usual electrode component in DSA-type electrodes which is added to improve long term stability of the electrode. In this study Cerium oxide coated electrodes are studied in order to evaluate how the cerium oxide influence the selectivity of the electrode, i.e. the activity towards the unwanted reduction of hypochlorite compared to the activity of the wanted hydrogen evolution. The electrode coating is prepared from the precursor cerium nitrate which is decomposed into a cerium oxide layer ex-situ via thermal decomposition. The resulting composition of the electrode coating after the thermal decomposition is identified as cerium oxide by Raman spectroscopic studies.

Different annealing temperatures and catalyst loadings were used to produce electrodes. The cerium oxide coating improved the selectivity of the electrodes inhibiting the side reaction, however the overpotential for the hydrogen evolution was increased. The results obtained were promising but further studies need to be conducted.

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I - Introduction:

Sodium chlorate (NaClO3) is produced by electrolysis of sodium chloride solution ¹. It is a strong oxidizing agent that is used to produce chlorine dioxide which is used in the pulp and paper industry ¹. The sodium chlorate has recently seen an increase of 30% in demand with the majority of the produced chlorate being utilized in the bleaching process ¹.

Figure 1 Industrial processes scheme of sodium chlorate production at Akzonobel²

To produce one ton of sodium chlorate an energy input of ca. 4700kWh is required ³. The process has an ideal pH in the range 6-7 and a temperature of 80 °C ³. A highly concentrated brine solution with sodium chlorate is subject to electrolysis and a series of electrochemical and chemical reactions lead to the formation of NaClO3. At the cathode, hydrogen is released while at the anode chlorine gas is produced according to equation (1) and (2).

The produced chlorine hydrolyzes in the brine solution to produce hypochlorous acid and hydrochloric acid (equation 3). The hypochlorous acid, depending on the solution pH form hypochlorite ions (equation 4). These two intermediates, the hypochlorous acid and hypochlorite ion react with each other to form chlorate (equation 5).

2𝐻₂𝑂 + 2𝑒⁻ → 2𝑂𝐻⁻+ 𝐻₂ 𝐸⁰ = −1.0253 𝑉 (1) 2𝐶𝑙⁻ → 𝐶𝑙₂+ 2𝑒⁻ 𝐸⁰ = 1.360 𝑉 (2)

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𝐶𝑙₂+ 𝐻₂𝑂 ⇌ 𝐻𝑂𝐶𝑙 + 𝐻𝐶𝑙 (3)

𝐻𝑂𝐶𝑙 ⇌ 𝐶𝑙𝑂⁻+ 𝐻⁺ (4)

2𝐻𝑂𝐶𝑙 + 𝐶𝑙𝑂⁻ → 𝐶𝑙𝑂₃⁻+ 2𝐶𝑙⁻+ 2𝐻⁺ (5)

Other unwanted reactions can occur which lower the cell efficiency and thus higher amount of energy will be required coupled with an increase loss in product yield. On the anode oxygen is formed from the oxidation of water or hypochlorite. Fortunately this is minimized by using dimensionally stable anodes ⁴. However, the unwanted electrochemical reactions happening on the cathode are of major concern. The most important of these are the reduction of chlorate and hypochlorite ions (or hypochlorous acid). Equation 6 and 7 represent the two unwanted reductions of chlorate and hypochlorite ions respectively:

𝐶𝑙𝑂₃⁻+ 3𝐻₂𝑂 + 6𝑒⁻ → 𝐶𝑙⁻+ 6𝑂𝐻⁻ (6)

𝑂𝐶𝑙⁻+ 𝐻₂𝑂 + 2𝑒⁻ → 𝐶𝑙⁻+ 2𝑂𝐻⁻ (7)

The unwanted reactions 6 and 7 are minimized by adding sodium dichromate to the electrolyte ^1,5. The sodium dichromate is reduced on the cathode to form a thin layer of chromium (III) oxide/hydroxide, which results in the previously stated benefits. Another benefit is that hydrogen evolution on the cathode is not hindered by the formed layer ^1,5. Also the addition of sodium dichromate buffers the electrolyte pH in the range of 5-7 and reduces oxygen evolution at the anode ¹.

Sodium dichromate is a dangerous chemical substance to the environment and it is also carcinogenic and mutagenic if direct exposure occurs ⁶. The European Commission has therefore issued a new legislation that urges the stoppage of use of sodium dichromate (and all chromium(VI) species), forcing the chlorate industry to search for alternatives. Unfortunately, due to the many functions that the sodium dichromate offers in the chlorate process finding one chemical that can fully substitute it is very difficult.

Cerium oxide has been recently used in several applications such as in solid oxide fuels⁷, gas sensors⁸, photocatalytic oxidation of water⁹, UV absorbents¹⁰ and surface treatment to prevent corrosion¹¹. This chemical compound is less toxic than sodium dichromate, cheap and abundant in

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nature¹². In this work, cerium oxide is studied in relevance to its ability to block the unwanted reactions.

There are several processes to synthesis cerium oxide. The most typically used methods in scientific works are hydrothermal¹³, sol-gel¹⁴, microwave¹⁵ ,homogenous precipitation¹⁶ and thermal decomposition¹⁷.

In this work the performance of cerium oxide coated cathodes is studied in relevance to their ability to allow hydrogen evolution and limit the unwanted reduction of hypochlorite. Cerium oxide layers were deposited on titanium plates ex-situ via thermal decomposition. Also different concentrations of the precursor, cerium nitrate hexahydrate, and different thermal decomposition temperatures were used to study their effect on the layer formation and cathode performance.

II - Experimental methods

Chemicals used

Cerium nitrate hexahydrate was purchased from Sigma-Aldrich. Pure ethanol (99.9%) was used to dissolve the precursor. Hydrochloric acid (37%) was used for the plate’s pretreatment step.

Sodium hydroxide (solid) and 1 M sodium hypochlorite solution were used to produce solution used in the electrochemistry experiments.

Cerium nitrate solution preparation

Cerium nitrate hexahydrate was chosen to be the precursor to form cerium oxide by thermal treatment. Depending on the desired concentration needed, the proper amount of cerium nitrate was weighed and dissolved in pure ethanol. Ethanol was used rather than water to make the drying process easier and to enhance the spreading of the solution on the pre-etched titanium plates.

Plate pretreatment

To enhance the adherence of the formed coatings, the titanium substrates (d = 6 cm) were pre- treated before the synthesis. First the plates were washed and scrubbed with a detergent to clean the surface from any contaminants. Then the plates are dropped in a boiling solution of 50 V/V%

HCl and 50 V/V% H2O, for 20 minutes. This acidic solution has a purpose of further cleaning the plate and to etch, and hence roughen the surface ¹⁸. After the 20 minutes, the plates were rinsed with water then with ethanol and dried on air.

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Coating method

There are several methods to coat the cerium nitrate solution on the cleaned titanium plates like brush coating, spray coating and dip coating ¹⁸. The chosen method is brush-coating. Using a micropipette, V = 230 µl solution is transferred on the plate (d = 6 cm) then the solution is evenly spread over the plate using a brush. To make sure to have the same spreading over the different plates, one spreading movement was adopted for all the plates that were produced. This coating method is easy to reproduce, cost effective and simple.

Thermal decomposition

Thermal decomposition was chosen due to the promising results found in articles, simplicity of the method and equipment availability. After coating, the plates are dried at 60 °C for 10 minutes, and then transferred to an annealing oven for another 10 minutes. These two steps along with the coating step were repeated until the required loading of cerium oxide on the titanium plate was reached. The calcination process is responsible for the decomposition of the precursor to form cerium oxide, shown in equation 8 and 9 ¹⁷. When the plates are taken from the calcination oven they are placed at room temperature for 5 min to cool down then weighed in order to track the film growth. The objective is to get a coherent film with as minimum cracks as possible. A lot of factors influence this outcome. In this study, we focused on the effect of the concentration of the precursor solution and the calcination temperature.

Ce(NO₃)₃ ∙ 6H₂O → Ce(NO₃)₃+ 6H₂𝑂 (8) 𝐶𝑒(𝑁𝑂₃)₃ → 𝐶𝑒𝑂₂+ 𝑁₂𝑂₅+ 𝑁𝑂₂ (9)

Photo 1 Drying oven Photo 2 Annealing oven

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Raman spectroscopy

Raman spectroscopy was used to identify the composition of the formed layers. The principle is to illuminate the sample with monochromatic light, and detect the energy of the back-scattered photons. When light is scattered from a molecule or crystal, most photons are elastically scattered meaning the scattered photons have the same energy (wavelength) as the incident photons.

However, a small fraction is scattered at optical frequencies different from the frequency of the incident photons. The process leading to this inelastic scattering is termed the Raman effect. The inelastic scattering can be described as an excitation to a virtual state, with a lower energy than a real electronic transition, coupled with a de-excitation and a change in vibrational energy. When the scattered photon’s energy is lower than the incident photon’s the event is termed as Raman Stokes scattering and when the scattered photon’s energy is higher it is termed as Raman Anti- Stokes scattering (figure 2). This difference in optical frequencies between the incident photons and scattered photons is also referred to as Raman shift. ^19,²

Figure 2 Difference between Stockes and Anti-Stokes scattering mechanism[²]

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Photo 3 BTR-111 Miniature Raman Spectrometer (wavelenght of 780nm) used to detect and identify the formed layer on the plates

Electrode preparation

In order to prepare electrodes for the electrochemical measurements the cerium oxide coated titanium plates were cut into small round plates using a puncher and a hammer. Then the small plates were welded to the titanium rod using a spot-welder (photo 4). After that step the rod was covered with a heat-shrinking tube which prevented its exposure to the solution while conducting the electrochemical testing. Epoxy was used to cover the back side of the plate, and the welding point that is present on the active side of the plate. Finally, the electrodes were kept intact for 24h so that the epoxy has enough time to dry.

Photo 4 Portaspot 230, spot welder ²

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Cyclic voltammetry

Cyclic voltammetry (CV) measurements were conducted to study the formed electrodes. Two main reactions are of interest and give important information regarding the electrode performance.

These two reactions are the hydrogen evolution and the reduction of hypochlorite. In order to conduct the CV test a standard 3 electrode cell was used and a thermal bath was also required in order to maintain a constant solution temperature. The counter electrode used was a platinum cage and the reference electrode used was an Ag/AgCl electrode, filled with concentrated KCl solution.

The working electrode was the one that is being tested. During a CV measurement, the potential of the working electrode is swept, using a potentiostat (photo5), with a constant rate (sweep rate) between a maximum and minimum value (this range covers both reactions so that an inclusive study can be made) for defined number of cycles chosen by the user. This results in several plots that display the current versus potential from which the effect of the working electrode on the different chemical reactions that are happening can be studied.

Figure 3 Schematics of the electrochemical setup used in CV test

Photo 5 EG&G Princeton Applied Research Potentiostat, Model 273A

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III - Results and discussion

Thermal decomposition of the precursor on titanium plates

In order to determine which concentration is the best suitable to use for coating the precursor, cerium nitrate, a set of concentrations was chosen (0.05 M, 0.1 M, 0.2 M, 0.3 M, 0.5 M) and the thermal decomposition temperature was set to 500 °C. To achieve a 1 mg/cm² loading of cerium oxide on the plates, the coating step along with the drying and annealing step were repeated several times and the results are presented in table 1. The idea behind this is to determine which concentration gives the most homogenous electrode with as minimal steps as possible. For each concentration, two plates were used to ensure that the experiments are reproducible and consistent but for simplicity only one plate from each concentration is displayed in figure 4.

Concentration Plate number mg/cm² layers

0.05M 1 0.68 33

0.05M 2 0.79 33

0.1M 3 1.07 16

0.1M 4 1.26 16

0.5M 5 1.23 4

0.5M 6 1.34 4

0.2M 7 1.02 8

0.2M 8 1.26 8

0.3M 9 1.21 7

0.3M 10 1.26 7

Table 1 Showing different concentration of the precursor versus how many layer are required to reach 1mg/cm² loading

As can be seen from figure 4, all the concentrations result in a linear layer growth. Also, the higher the concentration the less number of layers is required to reach 1mg/cm² loading. This would put the 0.5 M concentration as the best option but as mentioned earlier the desired plate would have a homogenous surface which is not the case. Photo 6b and 6c show that the plates with concentration of 0.3 M and 0.5M are not homogenous and that the 0.1M (photo 6a) shows better results.

Furthermore, the 0.1 M concentration required about 16 layers to reach the desired loading which is an acceptable number. This led to the selection of the 0.1 M concentration for the rest of the experimental work.

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Figure 4 Layer growth of different precursor concentrations in function of number of layers to reach the 1mg/cm² loading under a thermal decomposition temperature of 500^°C

Photo 6 Difference between plates at different concentrations : a (0.1M cerium nitrate) , b (0.3M cerium nitrate), c (0.5M cerium nitrate) at 500°C thermal deccomposition temperature.

In order to study the effect of the annealing temperature, coatings were formed at 250 °C, 300 °C, 400 °C and 500 °C. The selection of the annealing temperatures was based on previous studies, showing that cerium nitrate starts to decompose at around 185 °C ¹⁷. As for the highest temperature, 500 °C, it was chosen because the plates used are titanium based and higher temperature might affect the titanium substrate. The same experimental steps as the ones used in the concentration study are used here as well. Since the concentration is now fixed at 0.1M it is worth noting that all temperatures required roughly the same number of layers in order to achieve the 1mg/cm² loading. Table 2 shows the different temperature and different layers required to get to the desired loading.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

0 5 10 15 20

Layer growth (mg/cm2)

Number of layers

0,05M 0,1M 0,2M 0,3M

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Temperature / °C Layers required to get to 1 mg/cm²

250 13

300 12

400 12

500 16

Table 2 Thermal decomposition temperature vs layers required to reach a loading of 1 mg/cm²

Raman spectroscopic characterization of the formed coatings

Raman spectra were collected for these plates after achieving the 1mg/cm² loading. All the plates showed, except for the ones prepared at 250 °C, the same results which is a peak characteristic of a cerium oxide (at 469.5cm^-1) as can be seen in figure 5, indicating that all the cerium nitrate to cerium oxide.

Figure 5 Raman spectra on the cerium oxide powder formed at 250°C ,500°C and on cerium nitrate powder

In addition to the peak 469.5 cm^-1, the spectrum of the electrode formed at 250 °C also shows peaks related to the cerium nitrate which means that the decomposition is not complete in this case.

In order to determine wheter this phenomenon is temperature or time dependent, these plates were put back into the thermal decomposition oven at same temperature. That was done for 1h and 6h and then Raman measurement was performed and the results are shown in figure 6, indicating that if the plate is given enough time in the oven the cerium nitrate will decompose completely to cerium oxide. Thus this phenomenon is related to time and not temperature.

0 0.2 0.4 0.6 0.8 1 1.2

200 400 600 800 1000 1200 1400 1600

Normalized intensity

Raman shift (cm^-1)

Cerium oxide at 250°C Cerium oxide at 500°C Cerium nitrate

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Figure 6 Raman spectra performed on cerium oxide powder prepared at 250°C in function of time

Cyclic voltammetric characterization of the formed electrodes

The different plates that have been prepared at different temperature using precursor solution with a 0.1M concentration were subject to cyclic voltammetry test to study their performance. Two solutions were used; the first was 1M NaOH to study hydrogen evolution, and the second solution is 80mM sodium hypochlorite containing 1M NaOH in order to study the hydrogen evolution and the hypochlorite reduction. A magnetic agitator was used in order to increase mass transfer to the electrode. The stirring rate and the positioning of the electrodes were the same for each measurement. The potential range used was 0V to -2.5V and that is due to the fact that the onset potential of hydrogen evolution is E ≈ -1.5V (vs. Ag/AgCl/sat. KCl) on titanium electrode in such alkaline solutions. The following illustrates the behavior of all electrodes tested under different conditions.

Hydrogen evolution in 1M NaOH at room temperature

The various electrodes prepared at different temperatures were tested in 1M NaOH solution, along with a bare titanium plate used as a reference. Figure 7 clearly shows that around -1.5V the hydrogen evolution does occur on all electrodes and that the onset of hydrogen evolution is roughly the same for all electrodes including the titanium. As for the value of the current density achieved

0 0.2 0.4 0.6 0.8 1 1.2

200 400 600 800 1000 1200 1400 1600

Normalized intensity

Raman shift (cm^-1)

Cerium oxide at 250°C after 1 hour

Cerium oxide at 250°C after 6 hour

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for each electrode we can see some difference between electrodes. Note, that as these curves are not corrected for the IR-drop, this difference may originate from the slightly different distance between the working and the reference electrodes. This issue will be further discussed later on.

Figure 7 Cyclic voltammograms (sweep rate of 10 mV/sec) of electrodes with cerium oxide layers (loading of 1 mg/cm²) produced at different temperatures in a 1 M NaOH solution at room temperature

Hydrogen evolution and hypochlorite reduction

Figure 8 shows that the hypochlorite reduction starts around -0.6V on the titanium electrode. At more negative potentials the hydrogen evolution takes place for all electrodes (around -1.6 V). The titanium plate exhibits a clear hypochlorite reduction which is not desired as it lowers the cell efficiency and the product yield. As for the rest of the electrodes significantly lower current related to the reduction of hypochlorite is seen, while the hydrogen evolution takes place starting from the same potentials as for the Ti electrode. As very similar results were found for all the different electrodes, 250°C and 500°C temperatures were selected to continue the experimental work, and to study the effect of the coating thickness.

-2.50E-01 -2.00E-01 -1.50E-01 -1.00E-01 -5.00E-02 0.00E+00

-2.50E+00 -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00

j (A/cm2)

E (V) vs. Ag/AgCl/sat. KCl

Titanium plate

Electrode prepared at 250°C Electrode prepared at 300°C Electrode prepared at 400°C Electrode prepared at 500°C

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Figure 8 Cyclic voltammograms (sweep rate of 10 mV/sec) of electrodes with cerium oxide layer (loading of 1 mg/cm²) produced at different temperatures in a 1 M NaOH + 80 mM NaClO solution to study hypochlorite reduction and hydrogen evolution at room temperature

The effect of cerium oxide loading on hydrogen evolution and hypochlorite reduction To investigate the effect of the cerium oxide thickness, the 250 °C and 500 °C annealing temperature were chosen. For each temperature, the following coating layer numbers were produced: 1, 5, 10, 15, 20, 25 layers. All these electrodes were characterized by CV measurements in hypochlorite solution (80mM sodium hypochlorite and 1M NaOH). From figure 9 and 10 (representing the electrodes formed at 250 °C and 500 °C, respectively) it can be concluded that as the number of layers increases the limitation of hypochlorite reduction is increased. As a result, the highest coating cycle, 25 layers (1.99 mg/cm² for the electrode prepared at 250°C electrode and 1.87 mg/cm² for the electrode prepared at 500°C), is the best performing electrodes at both temperatures.

-2.60E-01 -2.10E-01 -1.60E-01 -1.10E-01 -6.00E-02 -1.00E-02 4.00E-02

-2.50E+00 -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00

j (A/cm2)

E(V) vs. Ag/AgCl/sat. KCl

Titanium plate

Electrode prepared at 250°C Electrode prepared at 300°C Electrode prepared at 400°C Electrode prepared at 500°C

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Figure 9 Cyclic voltammograms recorded for electrodes coated with cerium oxide layers with different catalyst loading prepared at 250°C in a 1 M NaOH + 80 mM NaClO solution at room temperature with a sweep rate of 10 mV/sec.

Figure 10 Cyclic voltammograms recorded for electrodes coated with cerium oxide layers with different catalyst loading prepared at 500°C in a 1 M NaOH + 80 mM NaClO solution at room temperature with a sweep rate of 10 mV/sec.

-2.50E-01 -2.00E-01 -1.50E-01 -1.00E-01 -5.00E-02 0.00E+00

-2.20E+00 -1.70E+00 -1.20E+00 -7.00E-01 -2.00E-01

j (A/cm2)

Titanium plate 1 layer 5 layers 15 layers 25 layers

-1.60E-01 -1.40E-01 -1.20E-01 -1.00E-01 -8.00E-02 -6.00E-02 -4.00E-02 -2.00E-02 0.00E+00

-2.30E+00 -1.80E+00 -1.30E+00 -8.00E-01 -3.00E-01

j (A/cm2)

Titanium plate 1layer 5layers 15layers 25layers

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The effect of the coating thickness on the hydrogen evolution and hypochlorite reduction at 80 °C solution temperature

The same procedure was adopted here as shown in the previous chapter, but the solution was heated to 80 °C using a thermal bath. By increasing the solution temperature to 80°C, which is similar to the temperature used in the industry, the rate of all electrochemical reactions occurring will be increased. Thus if the reduction of hypochlorite is happening at room temperature but at a negligible rate by increasing the temperature this reduction will be amplified. From figure 11 and 12 (representing the measurements with the electrodes prepared at 250°C and 500°C, respectively) it is clear that the bare titanium electrodes (with no cerium oxide) show that the hypochlorite reduction does occur which is explained by the absence of cerium oxide layer and this reduction happens at a high rate which is due to the increase in temperature. On the other hand, the electrodes with cerium oxide coating show that as the number of coating layers increases the reduction of hypochlorite decreases despite of the increase in solution temperature.

Figure 11 Cyclic voltammograms recorded for electrodes coated with cerium oxide layers with different catalyst loading prepared at 250°C in a 1 M NaOH + 80 mM NaClO solution at 80°C solution temperature with a sweep rate of 10 mV/sec.

-4.50E-01 -4.00E-01 -3.50E-01 -3.00E-01 -2.50E-01 -2.00E-01 -1.50E-01 -1.00E-01 -5.00E-02 0.00E+00

-2.50E+00 -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00

j (A/cm2)

E (V ) vs. Ag/AgCl/sat. KCl

Titanium plate 1 layer 5 layers 15 layers 30 layers

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Figure 12 Cyclic voltammograms recorded for electrodes coated with cerium oxide layers with different catalyst loading prepared at 500°C in a 1 M NaOH + 80 mM NaClO solution at 80°C solution temperature with a sweep rate of 10 mV/sec.

Effect of thickness in reducing the reduction current related to the hypochlorite reduction In order to quantify the effect of the cerium oxide layer coated on the plates and its thickness on the suppression of hypochlorite reduction, the hypochlorite reduction current was read from the CV curves at E = -1.3V. Figure 13 displays the hypochlorite reduction current versus the number of layers for electrodes prepared at 250°C and 500°C in room temperature and at 80°C hypochlorite solution temperature. From figure 13 it can be concluded that as the number of cerium oxide layers increases the hypochlorite reduction decreases, regardless under which temperature the experiments are conducted. When the experiment is under room temperature, the hypochlorite reduction is minimal as the rate of all chemical reaction happening is slow compared to when the solution temperature is at 80 °C. Moreover, the electrodes prepared at 500°C have a slightly better performance at room temperature than the 250°C electrodes. But when the solution temperature is raised to 80°C it is very clear that the electrodes prepared at 250°C are more successful in blocking the unwanted reaction. In fact, the performance of the 250°C at 25 layers is similar in both conditions (at room temperature and 80°C solution temperature). Ideally the desired hypochlorite reduction current would be zero. Nevertheless, the results obtained are very promising and since the industry uses a temperature of 80°C this indicates that the electrodes prepared at 250°C with a 30-layer coating are the best electrodes.

-3.80E-01 -3.30E-01 -2.80E-01 -2.30E-01 -1.80E-01 -1.30E-01 -8.00E-02 -3.00E-02 2.00E-02

-2.00E+00 -1.80E+00 -1.60E+00 -1.40E+00 -1.20E+00 -1.00E+00 -8.00E-01 -6.00E-01 -4.00E-01 -2.00E-01 0.00E+00 j (A/cm2)

1layer 5layers 15layers 30layers

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Figure 13 Cerium oxide loading effect on hypochlorite reduction current (corresponding to a fixed potential value of -1.3 (V) ) for electrodes with cerium oxide layers prepared at 500°C and 250°C in a 1 M NaOH + 80 mM NaClO solution at room temperature and 80°C solution temperature

250°C 500°C

Number of layers Loading (mg/cm²) Number of layers Loading (mg/cm²)

1 0.024 1 0.05

5 0.47 5 0.39

10 0.82 10 0.74

15 1.23 15 1.13

20 1.52 20 1.49

25 1.99 25 1.87

30 2.32 30 2.1

Table 3 Number of layers vs loading (mg/cm²) for electrodes prepared at 250°C and 500°C

IR-corrected polarization curve measurements

During the study of hydrogen evolution (figure 7), the tested electrodes displayed different slopes in the hydrogen evolution region (below -1.6V). Also while conducting some experiments the positioning of the reference electrode and the working electrode might have affected the

-0.1 -0.09 -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0

0 5 10 15 20 25 30 35

j (A/cm2)

Number of layers

250C electrode_room T 250C electrode_80C 500C electrode_room T 500C electrode_80C

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measurements. In order to investigate this issue current interruption technique was used and IR- corrected polarization curves were recorded for the cerium oxide coated electrodes. This technique switches off the current then immediately measures the potential drop²⁰.

Figure 14 IR corrected polarization curve for titanium and electrode with cerium oxide layer (25 layer coating) produced at 250°C with a 25 layer coating in a 1M NaOH solution at room temperature

From figure 14 it can be seen that the kinetics of the hydrogen evolution reaction in the presence of the cerium oxide covered electrode is slightly hindered (larger potential is required to achieve the same current density) when it is compared to the bare titanium. These two curves are not perfectly parallel and this is amplified at higher currents. It can be concluded that hydrogen evolution is affected by the presence of the cerium oxide layer but it can still proceed even at high number of layers.

Rotating disk electrode measurements

To further study the best performing electrode (0.1M cerium nitrate precursor, 250 °C annealing temperature, 30 layers with a loading of 2.3 mg/cm²) a rotating disk electrode was made and tested in 80mM hypochlorite and 1M NaOH solution. Rotating disc electrode, used in the CV setup at 80 °C, will drag the electrolyte to the electrode surface which will guarantee a good and uniform

-1.95 -1.85 -1.75 -1.65 -1.55 -1.45 -1.35 -1.25

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

E IR-corrected (V) vs Ag/AgCl/sat.KCl

Log|j| (A/m²)

Titanium plate

250°C electrode 25 layers

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flow. This is done by the uniform rotation which results in the constant contact of fresh solution with the electrode. ²⁰

This constant flow of fresh solution to the electrode will result in higher hypochlorite reduction currents due to the increased mass transport. As can be seen in figure 15, the reduction of hypochlorite on the rotating disk electrode with cerium oxide coating (30 layers) is blocked which is not the case with the titanium rotating electrode.

Figure 15 Reduction current vs potential for a rotating disk electrode (3000 rpm rotation speed) with cerium oxide layer (30layer coating) prepared at 250°C in a 1 M NaOH + 80 mM NaClO 80°C solution temperature.

-1.00E+00 -9.00E-01 -8.00E-01 -7.00E-01 -6.00E-01 -5.00E-01 -4.00E-01 -3.00E-01 -2.00E-01 -1.00E-01 0.00E+00

-3.00E+00 -2.50E+00 -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00

j (A/cm2)

E(V) vs Ag/AgCl/sat. KCl

Titanium rotating disk electrode

Rotating disk electrode with 30 layer coating

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IV - Conclusions

To produce a good performing cathode for electrolytic chlorate production, many factors need to be considered. Cerium nitrate was chosen as a precursor in order to synthesize a cerium oxide layer on titanium electrodes to limit the unwanted reduction of hypochlorite, while allowing hydrogen evolution to proceed.

During the experimental work, the concentration of the precursor solution was systematically varied to find optimal conditions to form these electrodes by thermal decomposition method. The 0.1M concentration was found as the best one since it resulted in a homogenous surface with the least cracks. The second factor was the thermal decomposition temperature. The temperatures ranged from 250 °C to 500 °C and all resulted in cerium oxide formation. The third factor was the thickness of cerium oxide deposited on the titanium plate. All experiments showed that as the number of coating layers increase, regardless of the annealing temperature, the reduction of hypochlorite is decreased and no effect on the hydrogen evolution was noticed. Experiments performed in higher temperature hypochlorite solutions (80 °C) showed that the electrodes prepared at 250°C with a 0.1M concentration and a coating of 30 layers was found to be the best in limiting hypochlorite reduction.

V - Future work

Cerium oxide layer has shown promising results in limiting hypochlorite reduction without hindering the hydrogen evolution. While the effect of temperature, layer thickness and concentration were studied in this work other factors influencing the layer formation were not. Of those factors that can have an impact on the layer formation are: other precursor that will generate cerium oxide, the use of a different solvent for the precursor solution, different coating method, different time in drying oven and thermal decomposition oven. Another study that can further reinforce the validity of the best electrode found is to quantify the hydrogen evolution selectivity by mass spectroscopy.

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