Experimental Study on Water Electrolysis Using Cellulose Nanofluid

fluids

Article Experimental Study on Water Electrolysis Using Cellulose Nanoﬂuid

Dongnyeok Choi and Kwon-Yeong Lee *

Handong Global University, 558 Handong-Ro, Heunghae-eup, Buk-gu, Pohang-si, Gyeongbuk-do 37554, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-54-260-1176

Received: 28 August 2020; Accepted: 26 September 2020; Published: 28 September 2020

Abstract: Hydrogen energy is considered to be a future energy source due to its higher energy density as compared to renewable energy and ease of storage and transport. Water electrolysis is one of the most basic methods for producing hydrogen. KOH and NaOH, which are currently used as electrolytes for water electrolysis, have strong alkalinity. So, it cause metal corrosion and can be serious damage when it is exposed to human body. Hence, experiments using cellulose nanofluid (CNF, C6H10O5) as an electrolyte were carried out to overcome the disadvantages of existing electrolytes and increase the efficiency of hydrogen production. The variables of the experiment were CNF concentration, anode material, voltage applied to the electrode, and initial temperature of the electrolyte. The conditions showing the optimal hydrogen production efficiency (99.4%) within the set variables range were found. CNF, which is not corrosive and has high safety, can be used for electrolysis for a long period of time because it does not coagulate and settle over a long period of time unlike other inorganic nanofluids. In addition, it shows high hydrogen production efficiency. So, it is expected to be used as a next-generation water electrolysis electrolyte.

Keywords: water electrolysis; electrolyte; cellulose nanoﬁber; nanoﬂuid

1. Introduction Due to the recent increase in economic growth, population growth, mass production, and mass consumption, fossil fuels are rapidly depleting and there is a rise in environmental pollution. Additionally, nuclear energy, which accounts for a large part of the current electricity generation, is highly dangerous considering power plant accidents, such as the Fukushima nuclear power plant accident. Therefore, it is important to develop eco-friendly renewable energy, and investment and research on energy sources such as solar power, geothermal power, and wind power is actively being carried out. However, the above-mentioned renewable energy sources are limited due to low energy density and unstable energy production. This threatens the stability of the power network and reduces the power generation efficiency. Hence, mass power production and storage technology using hydrogen is gaining attention. Hydrogen energy has three times the energy density of natural gas and gasoline [1], a very high conversion efficiency to electric power, is relatively easy to store and transport [2], has a purity of 99.9% and does not need further purification [3], and does not emit greenhouse gas when used as energy. Thus, it can replace existing fossil fuels as an energy source and an energy storage medium. There is also a need for efficient hydrogen production technology as well as the development of hydrogen storage devices. However, the corresponding economical and environment-friendly methods have not yet been developed and various studies are being conducted across the globe for the same.

Fluids 2020, 5, 166; doi:10.3390/ﬂuids5040166 www.mdpi.com/journal/ﬂuids Fluids 2020, 5, 166 2 of 12

Currently, water electrolysis is the simplest method of producing hydrogen, but accounts for only 4% of all the hydrogen production methods [4,5]. This is because the technology is limited due to high costs associated with a higher power consumption relative to production, high installation cost, equipment maintenance cost, and low equipment durability and safety [6,7]. The durability and safety issues of electrolysis are mostly caused by electrolytes such as KOH and NAOH, which are commonly used in water electrolysis. These electrolytes are corrosive towards metals due to their strong alkalinity. This damages the electrodes, and therefore, requires frequent replacement of the electrodes. Additionally, there are limitations during operations, such as sealing of the container and frequent ventilation when storing and operating the solution, as the electrolytes can cause serious damage when exposed to the skin or eyes and can affect the respiratory system when exposed to air. In case of KOH, there is a risk of explosion due to flammability and requires special care during storage and handling. Recently, various studies have been conducted to overcome the limitations in hydrogen production and to increase its efficiency through water electrolysis. De Souza et al. [8] used imidazolium ionic as an electrolyte for hydrogen production through water electrolysis, found that the electrolyte improved efficiencies of hydrogen production. Nagai et al. [9] studied the effect of the distance between electrodes on hydrogen generation efficiency. Wang et al. [10] conducted a study to improve hydrogen production through water electrolysis using an ultra-gravity field. Mandal et al. [11] experimented and analyzed the changes in the amount of hydrogen produced according to the concentration during electrolysis. In this study, Cellulose Nanofibers (CNF) were used as electrolyte in the electrolysis of water in order to raise the efficiency of hydrogen production, reduce the production cost, and overcome the problems of using alkaline electrolytes. CNF, which is called a “dream material”, is an organic compound that forms the basic structure of plant cell walls. It is composed of the hydroxyl structure with hydrogen and oxygen and is weakly alkaline when it comes into contact with water. It is stronger than iron, is lightweight, and does not expand even when exposed to heat. Also, it can be used for electrolysis for a long period of time because it does not coagulate and settle over a long period of time unlike other inorganic nanofluids. Hence, it can be applied to various fields, such as automobile parts, glass substitutes, and biomedical materials.

2. Materials and Methods

2.1. CNF Production CNF does not have electrical conductivity. However, when CNF is fabricated through the method described by Saito et al. [12], the carboxyl group is replaced with C6OH and becomes charged. Briefly, a coffee filter was used as a raw material for cellulose and oxidized using three catalysts: 2,2,6,6-Tetramethylpiperidin-1-yl-oxyl (TEMPO), NaBr, and NaClO. This is a useful surface modifying method for fabricating nanofibers without significantly degrading the crystallinity of the cellulose. The surface modified cellulose can be obtained in the form of nanofibers dispersed in water using a simple mechanical treatment. After diluting the CNF gel in water according to the test concentration, ultrasonic dispersion was performed for 1 h using a sonicator to disperse it in water. Therefore, when CNF is completely dispersed in water, it acts as an electrolyte. Each fiber has 3–4 nm of width and 2–3 µm of length [13]. CNF is negatively charged, so zeta-potential indicates the stability of CNF. In the experiment, we used CNF of which pH was above 5, and the average value of zeta-potential was 53.3 mV (usually considered stable when the absolute value is more than 30 mV). CNF was − very stable because electrostatic force took the fiber apart, and the stability of the mixed liquid is also high because of its very low reactivity with water and very low corrosivity. Also, since it is an eco-friendly raw material extracted from plants, it is harmless to the human body, easy to dispose of, and economical due to easy resource acquisition. Additionally, Hwang et al. [13] found that the coagulation and precipitation of nanofibers did not occur even when CNF was operated for a long time. Fluids 2020, 5, 166 3 of 12 Fluids 2020, 5, x FOR PEER REVIEW 3 of 12

2.2.2.2. Experimental Experimental Apparatus Apparatus and and Method Method ForFor the the electrolysis electrolysis of of the the mixture mixture of of water water an andd CNF, CNF, the the Hofmann Hofmann electr electrolysisolysis apparatus apparatus shown shown inin Figure Figure 1 was used. It It is made of acrylic for visualizationvisualization of the gas. When oxygen and hydrogen collectcollect in in the the anode anode and and the the cathode, cathode, respectively, respectively, the the gas gas pushes pushes outout the thewater. water. The Thecapacity capacity was 200 was mL,200 the mL, distance the distance between between each each electrode electrode was was 3.2 3.2cm cm,, and and the the effective eﬀective surface surface area area of of the the electrode electrode 2 waswas 5.62 5.62 cm cm2. .The The amount amount of of gas gas that that could could be be collected collected was was 18 18 mL mL for for both both oxygen oxygen and and hydrogen. hydrogen.

(a) (b)

FigureFigure 1. 1. HofmannHofmann electrolysis electrolysis apparatus apparatus (a (a) )schematic schematic (b (b) )picture. picture.

TheThe hydrogen hydrogen production production efficiency efficiency was was obtain obtaineded by by experimenting experimenting with with the the following following four four variables:variables: the the concentration concentration (wt (wt %) %) of of the the electrolyte, electrolyte, the the material material of of the the electrode, electrode, the the voltage voltage applied applied toto the the electrode, electrode, and and the the initial initial temperature temperature of of the the electrolyte. electrolyte. The The optimum optimum hydrogen hydrogen production production efficiencyefficiency condition waswas found found by by comparing comparing the hydrogenthe hydrogen generation generation efficiency efficiency between between the variables. the variables.The hydrogen The hydrogen generation generation efficiency efficiency under these under conditions these conditions was also comparedwas also compared to the hydrogen to the hydrogengeneration generation efficiency efficiency when using when NaOH using as NaOH the electrolyte. as the electrolyte. ExperimentsExperiments were were conducted conducted for for 75 75 min. min. A A powe powerr supply supply (N8952A, (N8952A, Keysight) Keysight) with with accuracy accuracy of of ± 0.0015%0.0015% was was used. used. It Itcan can supply supply voltage voltage and and current current up up to to 210 210 A, A, 200 200 V, V, respectively. respectively. It It can can also also ± measuremeasure voltage voltage with with accuracy accuracy of of ± 0.0015%.0.0015%. Both Both the the electrolyte electrolyte temperature temperature and and the the atmosphere atmosphere ± temperaturetemperature were were measured measured using using thermocouples thermocouples te temperature,mperature, and and the the voltage voltage was was measured measured using using DMM.DMM. During During the electrolysiselectrolysis experiment,experiment, data data on on the the variations variations in temperaturein temperature and and current current with with time timewere were collected collected using using a data a acquisition data acquisition system system (34,770 (34,770 A, Keysight) A, Keysight) at 10 s intervals. at 10 s intervals. The gas producedThe gas producedat the cathode at the and cathode the anode and the was anode measured was viameasured a 0.1 mL via spacing a 0.1 mL scale. spacing Chakik scale. et al. Chakik [14] summarized et al. [14] summarizedEquations (1)–(3) Equations with respect(1)–(3) with to the resp electrolysisect to the e electrolysisfficiency. efficiency. ∗ ∗ V = (1) I VM t V 2∗= ∗ ∗ (1) H2Ideal 2 F V is the ideal hydrogen generation volume, is ∗the current A, is the Molar volume in [ ] idealVH2Ideal stateis the (24.47 ideal L/mol hydrogen), is time generations, and volume, is theI Faradayis the current constantA ,(96485VM is thes A/mol Molar). volume in ideal state (24.47 L/mol), t is time [s], and F is the Faraday constant (96,485 s A/mol). V =V ∗ (2) T V = V standard (2) V is the volume of the actual hydrogenH2Real generated,H2Measured V is the hydrogen volume obtained ∗ Tmeasured from the experiment, and and are reference temperature (298.15 K) and the average temperature K, respectively, measured during electrolysis. Fluids 2020, 5, 166 4 of 12

VH2Real is the volume of the actual hydrogen generated, VH2Measured is the hydrogen volume obtained from the experiment, and Tstandard and Tmeasured are reference temperature (298.15 K) and the average temperature [K], respectively, measured during electrolysis.

Tstandard V VH ( ) = H2Real = 2Measured ∗ Tmeasured Eﬃciency % 100 I V t (3) VH ∗ M 2Ideal ∗2 F∗ ∗ Equation (3) shows the formula for obtaining the hydrogen generation eﬃciency, which can be expressed as the ratio of the hydrogen volume actually generated to the ideal volume. In order to obtain the current of the entire electrolysis system, a 1.0 Ω shunt resistor was connected between the cathode and the power supply. After the electrolysis began, the voltage applied to the shunt resistor was measured at intervals of 10 s using the data acquisition system and the current was calculated by dividing the measured voltage by the shunt resistance value. The average current value was calculated by dividing the sum of the current data obtained during the experiment by the number of data points. The pH of the electrolyte was also measured before and after the experiment using a pH meter.

3. Results and Discussion

3.1. Experiment According to Electrolyte Concentration The electrolyte concentration was varied between 0.5 wt %, 1.0 wt %, 1.5 wt %, 1.8 wt %, and 2.2 wt % under the voltage of 10 V, an initial temperature of the electrolyte of 26 ◦C, and the platinum anode SUS304 cathode. The results obtained are shown in Table1. As shown in Figure2, the higher the concentration of the electrolyte, the higher is the current value. In addition, the highest hydrogen generation eﬃciency of 89% was obtained when the concentration was 1.8 wt %, as more ions were released in the aqueous medium with an increase in the concentration of the electrolyte. When the concentration of the electrolyte was 2.2 wt %, the generated gas did not rise because CNF was coated on the anode, and the generated oxygen was trapped due to the CNF coating, as shown in Figure3. However, except in the case of the concentration of 2.2 wt %, it seems that the generated gas was able to rise at the anode as the rising force was greater than the gas holding force exerted by the CNF coated on the anode. Figure4 shows the change in pH measured with a pH meter before and after the experiment with respect to the electrolyte concentration. The concentration of electrolyte and the number of ions are proportional and the change in pH with respect to the number of ions shows a ‘-log scale’. This indicates that the smaller the number of ions, the greater is the change in pH. The change in pH decreased very slowly from 0.5 to 1.5 wt %. However, at 1.8 wt %, the pH change was greatly reduced. It is believed that the pH change was not large even after electrolysis because there were much more ions contained at a concentration of 1.8 wt % than the amount of ions required for electrolysis. Therefore, when the concentration of the electrolyte was 1.8 wt %, it resulted in not only the best hydrogen generation eﬃciency but also a near neutral pH after the experiment due to the minor change in pH. It resulted in the high stability and the small causticity of the solution. After all the experiments in this study, agglomeration and sedimentation of CNF were not discovered.

Table 1. Experimental results according to the concentration of CNF.

Concentration Hydrogen Volume Average Temperature Average Current Eﬃciency wt % mL [K] [mA] [%] 0.5 5.0 295.4 11.6 76.3 1.0 9.2 294.5 19.8 82.5 1.5 12.8 298.4 27.0 83.1 1.8 15.0 293.8 30.0 89 FluidsFluids 20202020, 5, ,5 x, 166FOR PEER REVIEW 5 of5 of 12 12 Fluids 2020, 5, x FOR PEER REVIEW 5 of 12

Figure 2. Current according to the electrolyte concentration. FigureFigure 2. 2.Current Current according according to to the the electrolyte electrolyte concentration. concentration.

FigureFigure 3. 3. CoatedCoated CNF CNF at at the the anode. anode. Figure 3. Coated CNF at the anode. Fluids 2020, 5, x FOR PEER REVIEW 6 of 12

FluidsFluids2020 ,20205, 166, 5, x FOR PEER REVIEW 6 of6 of12 12

FigureFigureFigure 4. 4. pH4. pH pH change changechange of of the thethe electrode electrodeelectrode before before before andand and afterafter after thethe the experiment. experiment.

3.2.3.2. Comparison3.2. Comparison Comparison of of the of the the CNF CNF CNF Solution Solution Solution Sample Sample Replacement ReplacementReplacement BecauseBecauseBecause of of the of the the shortage shortage shortage of ofof the thethe first first CNF CNF sample sample sample solution, solution, solution, anotheranother another sample sample CNF CNF CNF solution solution solution was was was used used used from Experiment 3.2. In order to confirm the effect of the change of sample on the electrolysis fromfrom Experiment Experiment 3.2. 3.2. In order In order to confirm to confirm the eff ectthe ofeffe thect change of the ofchange sample of on sample the electrolysis on the eelectrolysisfficiency, efficiency, the experimental results were compared by varying the samples under a voltage of 10 V, theefficiency, experimental the experimental results were results compared were by compared varying theby varying samples the under samples a voltage under of a 10 voltage V, an initialof 10 V, an initial temperature of the electrolyte of 26 °C, and the platinum anode and SUS304 cathode. temperaturean initial temperature of the electrolyte of the ofelectrolyte 26 ◦C, and of the26 °C, platinum and the anode platinum and SUS304anode and cathode. SUS304 cathode. The hydrogen produced in the previous CNF sample experiments was 2.5 times more than that TheThe hydrogen hydrogen produced produced in thein the previous previous CNF CNF sample sample experiments experiments was was 2.5 times2.5 times more more than than that inthat in the new CNF samples, as shown in Table 2. However, as the amount of hydrogen generated thein new the CNFnew samples,CNF samples, as shown as inshown Table 2in. However,Table 2. However, as the amount as the of hydrogenamount of generated hydrogen decreased, generated the currentdecreased, also the reduced, current asalso shown reduced, in Figure as shown5, and in the Figure hydrogen 5, and generationthe hydrogen e ffi generationciency error efficiency between decreased,error between the current the samples also wasreduced, not great, as shown less than in 6%.Figure 5, and the hydrogen generation efficiency theerror samples between was the not samples great, less was than not 6%. great, less than 6%. Current[A] Current[A]

Figure 5. Current according to the CNF solution sample.

FigureFigure 5. 5.Current Current according according to to the the CNF CNF solution solution sample. sample. Fluids 2020, 5, x FOR PEER REVIEW 7 of 12 Fluids 2020, 5, 166 7 of 12 Table 2. Comparison of CNF solution sample results.

Hydrogen Average Average Table 2. Comparison of CNF solution sample results.Efficiency Error Sample volume Temperature Current Hydrogen Volume Average Temperature Average Current [%]E ﬃciency Error[%] Sample [mL] [K] [mA] [mL] [K] [mA] [%] [%] Old 15.0 293.8 30.0 89 Old 15.0 293.8 30.0 89 5.7 New 5.8 298.4 12.1 83.8 5.7 New 5.8 298.4 12.1 83.8 3.3. Experiment According to Anode Material 3.3. Experiment According to Anode Material Under the voltage of 10 V, an initial temperature of the electrolyte of 26 °C, an electrolyte concentrationUnder the of voltage 1.8 wt %, of 10and V, SUS304 an initial cathode, temperature SUS304 ofand the platinum electrolyte were of alternately 26 ◦C, an electrolyteapplied as concentrationanode materials of and 1.8 wtthe %, experiment and SUS304 was cathode, conducted SUS304 as shown and platinum Table 3 and were Figure alternately 6. When applied SUS304 as anodewas used materials as an and anode, the experiment the hydrogen was conductedgeneration asefficiency shown Table was3 and88%, Figure higher6. Whenthan that SUS304 of using was usedplatinum. as an anode, the hydrogen generation eﬃciency was 88%, higher than that of using platinum.

Table 3.3. Experimental resultresult accordingaccording toto anodeanode material.material. Hydrogen Volume Average Temperature Average Current Eﬃciency Anode Material Hydrogen Volume Average Temperature Average Current Efficiency Anode Material [mL] [K] [mA] [%] [mL] [K] [mA] [%] StainlessStainless 8.0 8.0 298.7 15.9 15.9 88.088.0 Platinum 5.8 298.4 12.1 83.9 Platinum 5.8 298.4 12.1 83.9

FigureFigure6 6a,ba,b showshow thatthat whenwhen SUS SUS is is used used as as an an anode anode material, material, current current flow flow and and hydrogen hydrogen generationgeneration isis higherhigher thanthan whenwhen platinumplatinum isis used.used. As per FigureFigure6 6a,a, immediately immediately afterafter thethe voltage voltage isis applied,applied, bothboth thethe SUS304SUS304 case case and and the the platinum platinum case case show show that that the the current current value value rapidly rapidly decreased decreased at firstat first and and then then increased. increased. This This was was due due to to the the initial initial voltage voltage barrier. barrier. In In the the case case ofof SUS304,SUS304, sincesince thethe initialinitial voltagevoltage barrierbarrier waswas smallsmall asas comparedcompared toto platinum,platinum, thethe risingrising raterate ofof thethe currentcurrent valuevalue waswas faster.faster. InIn addition,addition, thethe currentcurrent graphgraph roserose withwith timetime andand thenthen droppeddropped down.down. ThisThis waswas becausebecause thethe ionicionic activityactivity causedcaused the the ions ions to to collide collide with with each each other, other, thereby thereby decreasing decreasing the the current current flow. flow. After After the experiment,the experiment, agglomeration agglomeration and and sedimentation sedimentation of CNF of CNF were were not discovered.not discovered.

(a) (b)

Figure 6. Experimental result accordingaccording to anode material (aa)) CurrentCurrent ((bb)) HydrogenHydrogen generation.generation.

3.4.3.4. Experiment According to Applied Voltage TableTable4 4 shows shows the the results results obtained obtained byby applyingapplying voltagesvoltages ofof 33 V,V, 66 V,V, and and 10 10 V V to to the the electrolysis electrolysis apparatusapparatus under an an initial initial temperature temperature of ofthe the electrol electrolyteyte of 26 of °C, 26 an◦C, electrolyte an electrolyte concentration concentration of 1.8wt of 1.8%, wtand % the, and SUS304 the SUS304 anode anode cathode. cathode. It should It should be no beted noted that the that electrolysis the electrolysis apparatus apparatus used used until until the theexperiment experiment 3.3 3.3was was defective defective and and a new a new electrolysis electrolysis apparatus apparatus was was used used from from the the experiment experiment 3.4. Fluids 2020, 5, 166 8 of 12

The results of the old device at experiment 3.3 with the voltage 10 V and the new device at experiment 3.4 with the voltage 10 V were slightly different with efficiencies of 88.01% and 92.61%, respectively, showing 4.6% of error. As shown in Table4, it was found that the current and the amount of hydrogen generated increased as the applied voltage increased, and the highest hydrogen generation efficiency was obtained when 10 V was applied. As shown in Figure7a, the current increased with the increase in Fluidsvoltage 2020 due, 5, x FOR to ion PEER activity REVIEW in the solution. On the other hand, as the voltage was lowered, the current8 of 12 change was negligible. In Figure7b, it can be seen that the higher the applied voltage, the larger was the The results of the old device at experiment 3.3 with the voltage 10 V and the new device at experiment slope of the hydrogen volume occurring over time. In other words, the current change according to the 3.4 with the voltage 10 V were slightly different with efficiencies of 88.01% and 92.61%, respectively, voltage applied to the electrolytic apparatus and the activity of the ions in the solution are proportional. showing 4.6% of error. As shown in Table 4, it was found that the current and the amount of hydrogen generated increased as Tablethe applied 4. Experimental voltage increased, results according and the to thehighest applied hydrogen voltage. generation efficiency was obtained when 10 V was applied. As shown in Figure 7a, the current increased with the increase in voltageVoltage due to ionHydrogen activity in Volume the solution.Average On Temperature the other hand,Average as the Current voltage wasEffi ciencylowered, the current change[V] was negligible.[mL] In Figure 7b, it can[K] be seen that the higher[mA] the applied[%] voltage, the larger was the3 slope of the hydrogen 1.1 volume occurring 298.1 over time. In other 2.5 words, the current 77.1 change according to6 the voltage applied 4.0 to the electrolytic 297.0 apparatus and the 7.9 activity of the 89.1 ions in the 10 10.8 300.1 20.3 92.6 solution are proportional.

(a) (b)

FigureFigure 7. 7. ExperimentalExperimental results results according according to to the the applied applied voltage voltage (a (a) )Current Current ( (bb)) Hydrogen Hydrogen generation. generation.

Hydrogen productionTable e4.ffi Experimentalciency depends results not according only on theto the number applied of voltage. ions present in the electrolyte solution but also on the effect of mobility in the solution [11]. Therefore, the number and mobility of ions must beVoltage taken intoHydrogen account in Volume the current Average change Temperature that directly a ffectsAverage the hydrogen Current production Efficiency effi ciency. First, as[V] the magnitude[mL] of the applied voltage increased,[K] the magnitude[mA] of the lost overvoltage[%] also increased3 with time. As1.1 the temperature increased 298.1 gradually due to the2.5 lost overvoltage,77.1 the ionic activity in6 the solution and4.0 the current increased. 297.0 However, since the ion 7.9 mobility was slightly89.1 reduced by the highly10 active ions,10.8 a high voltage was not 300.1 necessarily suitable for 20.3 the electrolysis.92.6 In addition, since the current density was reduced due to the reduced effective surface area as a result of the adhesionHydrogen of the production bubbles on theefficiency electrode depends surface, not the on appliedly on voltagethe number range of should ions bepresent considered in the in electrolyteterms of hydrogen solution but generation also on ethefficiency effect [of14 mobility]. In the casein the of solution voltage experiments[11]. Therefore, at 3the V, 6number V, and and 10 V, mobilityas the voltage of ions increases, must be thetaken magnitude into account of the in current the current and the change amount that of directly generated affects hydrogen the hydrogen increase, productionso it is confirmed efficiency. that theFirst, influence as the magnitude of the reduction of the of applied the current voltage density increase dued, to the interruptionmagnitude of of the the lostion mobilityovervoltage and also the reductionincreased of with the etime.ffective As surface the temperature area is insignificant. increased Therefore, gradually it due is reasonable to the lost to overvoltage,carry out the the electrolysis ionic activity experiments in the atsolution the voltage and ofthe 10 current V, which increased. showed the However, highest average since the current ion mobilityof 0.0203 was mA slightly and the reduced hydrogen by generation the highly e activefficiency ions, of 92.6%.a high voltage was not necessarily suitable for the electrolysis. In addition, since the current density was reduced due to the reduced effective surface area as a result of the adhesion of the bubbles on the electrode surface, the applied voltage range should be considered in terms of hydrogen generation efficiency [14]. In the case of voltage experiments at 3 V, 6 V, and 10 V, as the voltage increases, the magnitude of the current and the amount of generated hydrogen increase, so it is confirmed that the influence of the reduction of the current density due to the interruption of the ion mobility and the reduction of the effective surface area is insignificant. Therefore, it is reasonable to carry out the electrolysis experiments at the voltage of 10 V, which showed the highest average current of 0.0203 mA and the hydrogen generation efficiency of 92.6%.

Fluids 2020, 5, 166 9 of 12 Fluids 2020, 5, x FOR PEER REVIEW 9 of 12

3.5.3.5. Experiment According to Initial Temperature ofof ElectrolyteElectrolyte UnderUnder thethe electrolyte concentration ofof 1.8 wt %, voltage of 10 V, and SUS304 anode and cathode, whichwhich areare thethe optimal hydrogen generation eefficiencyfficiency conditions derived from previous experiments, thethe initialinitial temperaturetemperature ofof thethe electrolyteelectrolyte waswas setset atat 5252 ◦°C,C, 2626 ◦°CC and 9 ◦°C,C, and the eeffectffect ofof thethe initialinitial temperaturetemperature onon thethe electrolysiselectrolysis waswas examined.examined. SinceSince thethe HofmannHofmann electrolyticelectrolytic apparatusapparatus waswas mademade ofof acrylicacrylic andand deformeddeformed atat aa temperaturetemperature ofof 8080 ◦°CC oror higher,higher, thethe highesthighest initialinitial temperaturetemperature waswas setset atat 5252 ◦°C,C, andand thethe electrolyteelectrolyte waswas placedplaced inin aa PyrexPyrex vesselvessel andand heatedheated usingusing aa hothot plate.plate. TheThe lowestlowest temperaturetemperature ofof 9 9◦ °C,C, which which is is the the lowest lowest temperature temperatur thate that can can be be cooled cooled through through cooling cooling water, water, was was set asset the as lowestthe lowest temperature temperature of the of experiment. the experiment. During During the experiment, the experiment, since it wassince di itffi cultwas todifficult maintain to themaintain temperature the temperature of the electrolyte of the due electrolyte to the characteristics due to the ofcharacteristics acrylic, a heat of loss acrylic, was naturally a heat loss induced. was Figurenaturally8 shows induced. the changeFigure 8 in shows temperature the change of the in electrolytetemperature during of the the electrolyte experiment. during As the experimentexperiment. progressed,As the experiment the temperature progressed, of the electrolytetemperature reached of the electrolyte closer to room reached temperature closer to (22–26room temperature◦C). Table5 and(22–26 Figure °C). 9Table show 5 thatand Figure the average 9 show current that the and average the amount current of and hydrogen the amount generated of hydrogen were highergenerated as thewere initial higher temperature as the initial of the temperature electrolyte of increased. the electrolyte In the caseincreased. of highest In the initial case temperature of highest ofinitial the electrolyte,temperature the of average the electrolyte, current was the 22.1 average mA and curre thent hydrogen was 22.1 generation mA and ethefficiency hydrogen was thegeneration highest atefficiency 99.4%. was the highest at 99.4%. p 55 52°C 26°C 50 9°C

25 Temperature[°C]

5 0 1020304050607080 Time[min]

Figure 8. Electrolyte temperature change.

Table 5. ExperimentalFigure results 8. Electrolyte according temperature to initial temperature change. of the electrolyte.

Initial ElectrolyteTable Temperature 5. ExperimentalHydrogen results Volume accordingAverage to initial Temperature temperature ofAverage the electrolyte. Current Eﬃciency [◦C] [mL] [K] [mA] [%] Initial Electrolyte Hydrogen Average Average 9 9.3 297.6 16.7Efficiency 97.8 Temperature26 Volume 10.8 Temperature 300.1 Current 20.3 92.6 [%] [°C]52 [mL] 12.7 302.1[K] 22.1[mA] 99.4 9 9.3 297.6 16.7 97.8 26 10.8 300.1 20.3 92.6 52 12.7 302.1 22.1 99.4

Fluids 2020, 5, 166 10 of 12

Figure 9. Experimental results according to initial temperature of electrolyte (a) Current (b) Hydrogen generation.

It can be seen that the higher the initial temperature of the electrolyte, the higher was the average current value including the amount of hydrogen generated. Increasing the operating temperature is a common method to improve the electrolysis efficiency because as the temperature of the electrolyte increases, the transfer coefficient and the Tafel slope, which shows the rate of hydrogen generation, decreases [15]. Moreover, additional energy by heat is increased and the required Gibb’s free energy is reduced [16]. Therefore, as the temperature of the electrolytic solution increased, lower voltage was required for generating hydrogen and the efficiency increased. The higher the temperature of the electrolyte, the higher was the current flow during electrolysis. This is because at higher temperatures of the electrolyte, the ions moved more freely, and the current flow increased. Also, in the latter half of the electrolysis, the current of the electrolyte at a high initial temperature dropped sharply and was similar to that of the electrolyte at a low initial temperature. This is because, as shown in Figure8, the temperature of the electrolyte approaches the room temperature (22–26 ◦C) over time. In this experiment, it shows higher efficiency at 9 ◦C than at 26 ◦C. This can be explained using the concept of reversible potential. Reversible potential refers to the minimum voltage applied to the two electrodes of the electrolysis cell to cause the electrolysis reaction of water. Oliver et al. reported that the lower the temperature of the solution, the higher the reversible potential [17]. Similarly, in this study, since the reversible potential at the case of initial temperature 9 ◦C was higher than that of 26 ◦C, the current flowing at the case of initial temperature 9 ◦C was lower than that of 26 ◦C. Therefore, even though more hydrogen was produced at the case of 26 ◦C, the hydrogen production efficiency was low because the current flowing was large. After the experiment, agglomeration and sedimentation of CNF were not discovered.

4. Conclusions Water electrolysis is the most basic and an important technology towards developing hydrogen energy. Various studies have been carried out for this globally, including research on electrodes and electrolytes. This study was conducted to overcome the issues of using conventional electrolytes by using CNF as an electrolyte for electrolysis of water, to improve the productivity of hydrogen, to find the optimum conditions of generating hydrogen efficiently, and to demonstrate its applicability as an electrolyte. Experiments were carried out using CNF as an electrolyte for electrolysis of water with several variables: electrolyte concentrations were 0.5, 1.0, 1.5, 1.8 wt %, 2.2 wt % and SUS304 and platinum were used as anode materials. The voltages applied to the electrodes were varied between 3 V, 6 V, and 10 V and the initial temperatures of the electrolyte were set to 9 ◦C, 26 ◦C, and 52 ◦C. The highest hydrogen generation efficiency was obtained when the electrolyte concentration was Fluids 2020, 5, 166 11 of 12

1.8 wt %, the anode material was SUS304, the applied voltage was 10 V, and the electrolyte initial temperature was 52 ◦C. The obtained efficiency was 99.4%. CNF can be a cost-effective option because it is easier to supply the main raw materials compared to KOH and NaOH, which are used in the existing electrolysis. Also, compared to conventional electrolytes, CNF has low reactivity with water and excellent fluid stability due to low corrosiveness which reduces the risk of device corrosion. Also, it can be used for electrolysis for a long period of time because it does not coagulate and settle over a long period of time unlike other inorganic nanofluids. Finally, it will improve the proportion of hydrogen energy used and help in combating the environmental problems of associated with the existing energy sources.

Author Contributions: Conceptualization, D.C. and K.-Y.L.; methodology, D.C. and K.-Y.L.; validation, D.C. and K.-Y.L.; formal analysis, D.C. and K.-Y.L.; investigation, D.C. and K.-Y.L.; resources, D.C. and K.-Y.L.; data curation, D.C. and K.-Y.L.; writing—original draft preparation, D.C.; writing—review and editing, K.-Y.L.; visualization, D.C.; supervision, K.-Y.L.; project administration, D.C. and K.-Y.L.; funding acquisition, D.C. and K.-Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported the Basic Research Program of the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT; Ministry of Science and ICT) (No. NRF-2017R1C1B5017640). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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