Optimization of the Electrolyte Parameters and Components in Zinc Particle Fuel Cells

energies

Article Optimization of the Electrolyte Parameters and Components in Zinc Particle Fuel Cells

Thangavel Sangeetha 1,2,† , Po-Tuan Chen 3,† , Wu-Fu Cheng 4, Wei-Mon Yan 1,2,* and K. David Huang 4,* 1 Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] 2 Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, National Taipei University of Technology, Taipei 10608, Taiwan 3 Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan; [email protected] 4 Department of Vehicle Engineering, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] * Correspondence: [email protected] (W.-M.Y.); [email protected] (K.D.H.); Tel.: +886-2-2771-2171 (ext. 3676) (K.D.H.) † These authors contributed equally to this work (TS and PTC).

Received: 18 February 2019; Accepted: 19 March 2019; Published: 21 March 2019

Abstract: Zinc (Zn)-air fuel cells (ZAFC) are a widely-acknowledged type of metal air fuel cells, but optimization of several operational parameters and components will facilitate enhanced power performance. This research study has been focused on the investigation of ZAFC Zn particle fuel with flowing potassium hydroxide (KOH) electrolyte. Parameters like optimum electrolyte concentration, temperature, and flow velocity were optimized. Moreover, ZAFC components like anode current collector and cathode conductor material were varied and the appropriate materials were designated. Power performance was analyzed in terms of open circuit voltage (OCV), power, and current density production and were used to justify the results of the study. The flow rate of the electrolyte was determined as 150 mL/min in the self-designed configuration. KOH electrolyte of 40 wt% concentration, at a temperature of 55 to 65 °C, and with a flow velocity of 0.12 m/s was considered to be beneficial for the ZAFCs operated in this study. Nickel mesh with a surface area of 400 cm2 was chosen as anode current collector and copper plate was considered as cathode conductor material in the fuel cells designed and operated in this study. The power production of this study was better compared to some previously published works. Thus, effective enhancement and upgrading process of the ZAFCs will definitely provide great opportunities for their applications in the future.

Keywords: Zn air fuel cells; potassium hydroxide; ﬂowing electrolyte Zn particle fuel; Zn oxide; Zincate ions

1. Introduction The demands for energy have triggered issues like the energy crisis, climate change, and environmental pollution. Fossil fuels are unreliable and unsustainable due to their depleting supplies and negative impacts on the environment [1]. So, in recent years, the development of renewable energy has propelled and accelerated the inevitable transition from fossil fuels to next-generation power sources. As a result, researchers are focusing on alternative, renewable, and carbon-neutral energy sources which are necessary for economic and environmental sustainability [2]. Among various renewable energy sources, fuel cells which are capable of converting chemical energy through electrochemical reaction of cathode and anode into electric energy are

Energies 2019, 12, 1090; doi:10.3390/en12061090 www.mdpi.com/journal/energies Energies 2019, 12, 1090 2 of 13 highly preferred. Among them, Zinc (Zn)-air fuel cells (ZAFCs) have various advantages such as (i) abundant availability of raw Zn in the earth; (ii) stable discharge performance; (iii) being environmentally friendly in nature; and (iv) effective utilization for energy storage purposes [3]. Mechanically rechargeable ZAFCSs are of two basic types: (1) reconstructable ZAFCs and (2) refuelable ZAFCs. In the reconstructable ones, the Zn plates are replaced and regenerated after the discharge process. In the refuelable type, aqueous flowing electrolytes are employed in the cells, which pass through a packed bed of Zn particles that corrode and are discharged by gravitational force [4]. There are research studies that have reported various mechanisms to improve ZAFC performance like employing electrolytes and optimizing their parameters and choosing the right components for the ZAFC like anode fuel type, current collectors, and conductor materials at the air cathode [5]. The electrolyte is mainly used for the purpose of charge transfer between the positive and negative electrodes inside a cell. Earlier ZAFCs used acidic electrolytes, but due to poor performance, alkaline aqueous electrolytes like potassium hydroxide (KOH) were adopted [6]. Mele et al [7] have employed a tapered-end flow ZAFC with flowing KOH electrolyte which is mechanically refuelable with Zn microspheres. They used this to estimate the effects of KOH aging on the Zn anode and observed improved performance and stability. The parameters of the KOH electrolyte, like concentration, temperature, and flow patterns, were optimized in a regenerative type ZAFC by Wang et al [8]. They reported that among all the cell components, flowing electrolytes played an inevitable role in controlling the Zn deposition and influencing its performance to a larger extent. This can also avoid issues like passivation formation, non-uniform Zn dissolution and deposition, and hydrogen evolution. The clogging problem of the unreacted Zn, solid products, and byproducts in the electrolyte will reduce the electrolyte ionic conductivity and increase the concentration polarization. While an alkaline electrolyte is pumped into the battery, the flow will wash away dissolved products and refresh the active surfaces [9]. Thus, optimization of the significant parameters of the flowing electrolyte was considered to be of utmost importance in the present study. Anode fuel type is a vital component in a ZAFC and Zn particles can be used as a fuel supply in a flow mode instead of Zn plates. In recent decade, porous Zn plates have been commercially used as anodes for ZAFCs. The porous structure possesses a high ratio of active surface which enhances the collision with electrolyte molecules and increases the reaction rate [10]. Nevertheless, they have the following positive impacts: (i) Zn particles possess larger surface areas for reaction compared to Zn plate, which is advantageous, and (ii) particle anode with flowing electrolyte is highly advantageous as this may suppress hydrogen evolution reaction (HER) and thus improve cell performance and 2+ electricity production. The reaction of Zn particles will produce zincate ions (Zn(OH)4 ), which can be dissolved in the electrolyte. Once the concentration of zincate in the electrolyte gets saturated, zinc oxide (ZnO) is formed. This product can be washed away by the flowing electrolyte to maintain reaction equilibrium. Zn particles possess a larger reaction area than Zn plate to enhance the reaction efficiency [11]. Fuel cells require a continuous supply of fuel to sustain the chemical reactions for unceasing electricity production. The adoption of a circulating flow type electrolyte system will result in the removal of spent Zn anodes, meanwhile, the Zn fuel can also be continually added to the reaction tank via the flowing electrolyte. Accordingly, the reactant elimination can still be re-supplied smoothly without mechanical exchange [12]. Thus, due to the above-mentioned advantages, the anode fuel type used in the present study was comprised of Zn particles and this study is focused on proposing a fuel cell system with flowing Zn particle fuel. Current collectors are inevitable and very important components in a ZAFC, and they are made of various metals like nickel (Ni), stainless steel, iron, copper, silver (Ag), and gold (Au). They play a very vital role in the successful operation of the fuel cell. Zn and atmospheric air are the oxidant sources in a ZAFC. Zn undergoes oxidation and electrons are collected by the anode current collector and they get converted to cations (ZnO). During the conversion of Zn to ZnO, electrons are produced and transported to the anodic current collector and they cross the external load and return to the cathode current collector, where the reduction reactions result in the generation of hydroxyl ions [13]. Energies 2019, 12, 1090 3 of 13

Energies 2018, 11, x FOR PEER REVIEW 3 of 13 Efficient surface area, porosity, and conductivity are significant characteristics for both anode and cathode current collectors as these properties will enhance the HER and the oxygen reduction reaction 98 cathode current collectors as these properties will enhance the HER and the oxygen reduction (ORR) and eventually the fuel cell performance. Pieces of Ni mesh with different surface areas were 99 reaction (ORR) and eventually the fuel cell performance. Pieces of Ni mesh with different surface used as anode current collectors in the ZAFCs and their effects were determined in the present 100 areas were used as anode current collectors in the ZAFCs and their effects were determined in the study. Air-cathodes in ZAFCs are usually made up of catalyst and diffusion layers and materials like 101 present study. Air-cathodes in ZAFCs are usually made up of catalyst and diffusion layers and polytetrafluoroethylene (PTFE) are used as hydrophobic binders in those layers. According to that 102 materials like polytetrafluoroethylene (PTFE) are used as hydrophobic binders in those layers. the cathodes may have single or double layers of binders. Porosity, hydrophobicity, stability, better 103 According to that the cathodes may have single or double layers of binders. Porosity, hydrophobicity, oxygen, and electron and proton conductivity are some characters of air-cathodes [3]. The air-cathodes 104 stability, better oxygen, and electron and proton conductivity are some characters of air-cathodes [3]. used in the present study were made of carbon black, so a novel attempt of using a different metallic 105 The air-cathodes used in the present study were made of carbon black, so a novel attempt of using a layer for better electron conductivity has been investigated. Their effects on the power performance of 106 different metallic layer for better electron conductivity has been investigated. Their effects on the the ZAFCs was determined. 107 power performance of the ZAFCs was determined. 2. Methodology 108 2. Methodology 2.1. Construction of Experimental System Platform 109 2.1. Construction of Experimental System Platform The experimental test platform for the ZAFC included the fuel cell body, electronic load, 110 magnetThe mixer,experimental electrolyte, test platform silica gel heatingfor the ZAFC film, temperatureincluded the controlfuel cell measuring body, electronic device, load, temperature magnet 111 mixer,sensor, electrolyte, peristaltic silica pump, gel and heating monitoring film, temperature computer. Zncontrol particles measuring were employeddevice, temperature as anode fuel sensor, and 112 peristalticthe flowing pump electrolyte, and monitoring was KOH solution. computer. It was Zn particles stirred with were a magnetic employed stirrer as anode to ensure fuel anduniform the 113 flowingmixing andelectrolyte was heated was KOH through solut aion. series It ofwas heating stirred film with sheets. a magnetic The temperature stirrer to ensure of the uniform electrolyte mixing was 114 andcontrolled was heated by temperature through a controller series of and heating the flow film was sheets. adjusted The temperature by peristaltic of pump. the electrolyte The electronic was 115 controlledloading device by temperature and AC impedance controller analyzer and the were flow connected was adjusted to the cell.by peristaltic Finally, the pump. cell was The regulated electronic by 116 loadingthe computer-controlling device and AC impedance electronic analyzer loading were to discharging connected with to the constant cell. Finally, current, the while cell wa thes regulated computer 117 byrecorded the computer the performance-controlling data. electronic The configuration loading to planning discharging of the with overall consta experimentalnt current, platform while the has 118 computerbeen portrayed record ined Figure the performance1. data. The configuration planning of the overall experimental 119 platform has been portrayed in Figure 1.

Figure 1. Structure of the experiment platform system. 120 Figure 1. Structure of the experiment platform system. 2.2. Construction of Zn-air Fuel Cells 121 2.2. Construction of Zn-air Fuel Cells Zn-air fuel cell utilizes O2 and Zn particles as cathode and anode respectively. The Zn particle 122 fuelZn cell-air employed fuel cell in utilizes this experiment O2 and Zn is particles self-fabricated, as cathode as shown and anode in Figure respectively.2a. The actual The photographZn particle 123 fuelof the cell cell employed is shown in in this Figure experiment2b. The is anode self-fabricated, fuel consisted as shown of Zn in particles Figure which2a. The were actual commercially photograph 124 ofpurchased the cell is from shown First in ChemicalFigure 2b. Co., TheLtd, anode Taipei, fuel Taiwan.consisted The of Zn particles particles were which of 0.5 we mmre commercially radius with a 125 purchasedsurface area from of 3.14 First mm Chemical2. They wereCo., Ltd, inserted Taipei, into Taiwan. the anode The chamber particles through were of an 0.5 opening mm radius at the with top ofa 2 126 surfacethe frame. area of 3.14 mm . They were inserted into the anode chamber through an opening at the top 127 of the frame.

Energies 2019, 12, 1090 4 of 13 Energies 2018, 11, x FOR PEER REVIEW 4 of 13

(a) (b)

128 FigureFigure 2. 2. (a(a) )Schematic Schematic representation representation of of the the Zn Zn plate plate fuel fuel single single cell cell.. ( (bb)) Photograph Photograph of of the the fuel fuel cell cell..

129 ZnZn particles werewere soakedsoaked in in KOH KOH solution solution for for 6 h6before h before prior prior to usage to usage as anode as anode fuel in fuel the in ZAFC. the 130 ZAFC.The outer The shellouter of shell the of fuel the cell fuel was cell made was made of stainless of stainless steel steel as it wasas it resistantwas resistant to corrosion to corrosion and and also 131 alsofacilitated facilitated frequent frequent disassembling. disassembling. The The cathode cathode chamber chamber primarily primarily consisted consisted of current of current collector collector and 132 andcarbon carbon black black as air as electrode air electrode set up. set The up. external The external interface interfaceof the air electrode of the air was electrode made of was hydrophobic made of 133 hydrophobicand porous Teflon and porous sheets. Teflon The thickness sheets. The of thethickness Teflon of sheet, the Teflon air electrode, sheet, air and electrode, separator and was separator 0.3 mm, 134 wasand 0.3 that mm of,the and anode that of chamber the anode was chamber 8 mm. was Non-woven 8 mm. Non fabric-woven material fabric was material chosen was as chosen the separator as the 135 separatormaterial. materialBesides,. theBesides, Teflon the could Teflon allow could the external allow the O 2 externalto enter O the2 to active enter site the of active the catalyst site of andthe 136 catalystprevent and the overflowprevent the of internaloverflow electrolyte. of internal The electrolyte. air cathode The also air cathode possessed also a manganese-based possessed a manganese catalyst- 137 based(Evionyx-Taiwan catalyst (Evionyx Co., Ltd.,-Taiwan Taiwan), Co., Ltd., with T anaiwan), active with area an of active 3 × 16 area cm2 .of Oxygen 3x16 cm was2. Oxygen flux-fed was in flux a gas- 138 feddistribution in a gas distribu plate andtion was plate reduced and was to reduced hydroxyl to ions, hydroxyl completing ions, completing the discharging the discharging process. The process. carbon 139 Theblack carbon was ablack porous was material a porous with material embedded with catalystembedded and catalyst it was capableand it was of achieving capable of uniform achieving gas 140 uniformdiffusion. gas The diffusion. inner side The of inner Zn anode side of electrode Zn anode housed electrode a separator housed a to separator prevent theto prevent Zn anode the from Zn 141 anodecontacting from withcontacting the cathode. with the The cathode. KOH electrolyte The KOH waselectrolyte circulated was by circulated a peristaltic by a pump. peristaltic pump.

142 2.3.2.3. Measurements Measurements and and Analyses Analyses ZAFC operation with an external load lead to various discharge modes such as constant current, 143 constantZAFC voltage, operation and with constant an external power. load The voltlead method to various facilitated discharge the modes occurrence such of as a constant specific chemicalcurrent, 144 constantreaction mechanism voltage, and by constantexternal voltage power. or The current volt and method the reactions facilitated varied the with occurrence the applied of a voltage specific or 145 chemicalcurrent. Thereaction constant mechanism current by method, external also voltage known or as current chronoelectric and the reactions potential varied or current with voltammetry, the applied was investigated in this study. To further check the reactor performance, they were subjected to 146 voltage or current. The constant current method, also known as chronoelectric potential or current polarization and power density curve analysis by Electrochemical Impedance Spectroscopy (EIS). 147 voltammetryAfter 4 hours, was in the investigated open circuit in condition, this study the. To polarization further check curves the reactor were measured performance, under they the were same 148 subjectedopen-circuit to conditions polarization at a and scan ratepower of 0.5A/30s density [ curve6]. The analysis discharge by curves Electrochemical of the cell under Impedance various 149 Spectroscopyloads were observed. (EIS). After The 4 hours influences in the ofopen the cir concentration,cuit condition, temperature the polarization and curves flow velocity were measured of KOH 150 underelectrolyte the same were open analyzed-circuit in conditions this study at along a scan with rate theof 0.5A/30s effects of [6]. different The discharge current curves collector of the surface cell areas and conductor materials for the air-cathode. The peristaltic pump was used to control the flow 151 under various loads were observed. The influences of the concentration, temperature and flow velocity of the electrolyte. The electrochemical characterization of polarization curves of the ZAFC was 152 velocperformedity of KOH at room electrolyte temperature were with analyzed an increasing in this currentstudy along of 1 A with per minute,the effects using of different a multifunctional current 153 collectorDC electronic surface load areas (PLZ-4W-664WA and conductor 0-150Vmaterials 132A for 660W,the air Kikusui-cathode. Electronics The peristaltic Corp., pump Japan). was used to 154 control the flow velocity of the electrolyte. The electrochemical characterization of polarization curves 3. Results and Discussion 155 of the ZAFC was performed at room temperature with an increasing current of 1 A per minute, using 156 a multifunctionalAn attempt has DC been electronic made load in this (PLZ study-4W to-664WA construct 0-150V and operate 132A 660W, ZAFCs Kikusui with Zn Electronics particle as Corp., anode fuel and flowing KOH electrolyte. The effects of the electrolyte concentration, temperature, and flow 157 Japan). velocities on the power performance of the ZAFC have been investigated. Moreover, the anode current collector surface area and cathode conductor material have been varied to determine their impacts on 158 3the. Resu fuellts cell and performance. Discussion 159 An attempt has been made in this study to construct and operate ZAFCs with Zn particle as 160 anode fuel and flowing KOH electrolyte. The effects of the electrolyte concentration, temperature,

Energies 2018, 11, x FOR PEER REVIEW 5 of 13

161 and flow velocities on the power performance of the ZAFC have been investigated. Moreover, the 162 anode current collector surface area and cathode conductor material have been varied to determine 163 their impacts on the fuel cell performance.

Energies 2019, 12, 1090 5 of 13 164 3.1. Effects of Flowing Electrolyte Parameters on ZAFC Power Performance

165 3.1. EffectsThe operation of Flowing of Electrolyte a fuel cell Parameters with a flowing on ZAFC electrolyte Power Performance has many benefits such as: (i) a flowing 166 electrolyte can exclude away discharge products in the fuel cells without affecting the electrochemical The operation of a fuel cell with a flowing electrolyte has many benefits such as: (i) a flowing 167 reactions, and (ii) it may also reduce the heat and wash away the gas bubbles generated by the electrolyte can exclude away discharge products in the fuel cells without affecting the electrochemical 168 reactions and will manage the battery thermal system [14]. Power performance of the ZAFCs with reactions, and (ii) it may also reduce the heat and wash away the gas bubbles generated by the reactions 169 respect to all the above-mentioned parameters were significantly analyzed by the polarization curves. and will manage the battery thermal system [14]. Power performance of the ZAFCs with respect to 170 They are used to determine the operative conditions under load (external resistance). The all the above-mentioned parameters were significantly analyzed by the polarization curves. They are 171 polarization curves are usually used to identify four important losses: (i) fuel crossover losses due to used to determine the operative conditions under load (external resistance). The polarization curves 172 the depletion of fuel passing through the electrolyte; (ii) activation losses due to slow reaction kinetics are usually used to identify four important losses: (i) fuel crossover losses due to the depletion of 173 on the surface of the electrodes; (iii) ohmic losses due to the resistance present for the flow of electrons fuel passing through the electrolyte; (ii) activation losses due to slow reaction kinetics on the surface 174 and ions through electrodes and electrolytes, respectively; and (iv) the concentration losses due to of the electrodes; (iii) ohmic losses due to the resistance present for the flow of electrons and ions 175 the changes in reactant concentrations on the electrode surfaces [15,16]. through electrodes and electrolytes, respectively; and (iv) the concentration losses due to the changes in reactant concentrations on the electrode surfaces [15,16].

176 33.1.1..1.1. Concentration Concentration of of Electrolyte Electrolyte 177 The dry dry separator separator layers layers were were soaked soaked in in the the KOH KOH electrolyte electrolyte prior prior to toexperiments experiments for forabout about 10 178 minutes10 minutes in order in order to toimprove improve the the ion ion transfer transfer impedance impedance.. Di Dischargescharge tests tests were were carried carried out out during during the the 179 process, thethe cell cell voltage voltage was was monitored, monitored and, whenand w ahen stable a valuestable of value 1.29 V of was 1.29 obtained, V was the obtained subsequent, the 180 subsequentmeasurements measurements were started. were The started. concentration The concentration of KOH electrolyte of KOH was electrolyte correlated was with correlated its electrical with 181 itsconductivity. electrical conductivit Moreover,y. the Moreover, OH− in the the electrolyte OH− in the was electrolyte a reactant was for a the reactant anode, for but the it was anode, a product but it 182 wasfrom a theproduct cathode, from and the therefore cathode, theand Zn therefore anode andthe Zn air cathodeanode and have air differentcathode have demands different for electrolyte demands 183 forOH electrolyte−. Generally, OH higher−. Generally, concentrations higher concentrations of electrolyte of resulted electrolyte in enhancedresulted in performance, enhanced performance, but over a 184 butcritical over concentration, a critical concentration, the performance the reduced. performance Therefore, reduced. different Therefore, electrolyte different concentrations electrolyte were 185 concentthen investigatedrations were using then constant investigated current using discharge. constant The current variations discharge. of IV The curve variation under lows of andIV curve high 186 undercurrents low were and measured. high current Thes were concentration measured. parameters The concentration were set fromparameters 20 wt% were−60 wt% set from as the 20 saturated wt%−60 187 wt%concentration as the saturated of KOH concentration is 65 wt% for of measurements. KOH is 65 wt% The for open measurements circuit voltage. The (OCV) open increasedcircuit voltage with 188 (OCV)the increase increased of electrolyte with the concentrationincrease of electrolyte from 20 concentration wt% (1.3 V) to from 30 wt% 20 wt% (1.33 (1.3 V) andV) to 40 30 wt% wt% (1.4 (1.33 V), 189 V)but and further 40 wt% increase (1.4 V), to but 50 wt% further (1.41 increase V) and to 60 50 wt%(1.41 wt% (1.41 V) V) did and not 60 result wt%(1.41 in a signiﬁcant V) did not increaseresult in ina 190 significantvoltage output increase (Figure in volt3a).age output (Figure 3a). 191 1.5

1.4 KOH 20wt% KOH 30wt%

KOH 40wt% KOH 50wt% 1.3 KOH 60wt%

1.2

1.1

1 Voltage (V) Voltage 0.9

0.8

0.7

0.6 0 50 100 150 200 Current Density (mA/cm2) (b) (a)

192 Figure 3. ((a)) Polarization curve at various potassium hydroxide (KOH) concentration. ( (b)) Current 193 density production with respect to KOH concentration.concentration.

194 Nevertheless, electrolyte electrolyte concentration concentration increase, increase, resulted resulted in in an an increase increase in in current density 195 production as as well well and and the the trend trend followed followed by by the the current current density density output output was was different different to that to that of the of −2 the voltage production (Figure3b). A maximum cell voltage at 200 mA cm was obtained at the KOH concentration of 40 wt%, but further increase in the electrolyte concentration reduced the cell voltage. The relationship between the electrolyte concentration and power output can be better explained by the Nernst equation which indicates the afﬁliation between the reversible potential (E0) of electrochemical reaction, temperature, derive current density and standard potentials [17,18]. Energies 2019, 12, 1090 6 of 13

The reversible potential increases with reactant amount, but decreases with product concentration. In the Equation (1), the anodic reaction of Zn is:

− 2− − Zn + 4OH → Zn(OH)4 + 2e (1)

Zn is solid, so the anodic reaction potential of Zn can be expressed as follows: " # a4 o RT OH− EZn = EZn + ln (2) 4F a 2− Zn(OH)4

Cathodic reaction mode: − − O2 + 2H2O + 4e → 4OH (3) The cathode reaction potential is " # a a2 o RT O2 H2O EO = E + ln (4) 2 O2 4F a4 OH−

So according to Equation (2), the reasons for the power output changes with respect to KOH concentrations can be explained as follows: (1) Zn anode reactant is OH−, and the reaction potential increases with the increase of OH− concentration. (2) Equation (3) shows that the product of air cathode is OH−. The reaction potential decreases with the increase of the concentration of OH−, and increases with the increase of the oxygen concentration of the reactant. The anode and cathode required contrasting concentrations of OH− (3). In addition, if the concentration of KOH is low, the electrolyte possesses higher resistance which leads to serious anode polarization, and when the concentration is increased, the performance of the cathode will be reduced due to polarization caused by large current discharge. These facts have been validated by Wang et al [9], where they have explained that the flowing electrolyte concentration in a ZAFC plays a potential role in determining the ionic current output and conductivity. They also explained the Nernst equation and determined the association between OH−and the electrochemical reactions at the anode and cathode. The cell kinetics are largely dependent on the effect of OH− ions on zinc deposition in ZAFC, because the convection between zincate ions and OH− during zinc deposition plays a key mechanism. The solubility and deposition of zincate ions depend on the saturation level of the electrolyte and thus governing the power performance of the fuel cell [19,20]. Zhang [21] have indicated that the solubility of Zn depended on KOH concentration. The electrode potential decreased with increasing electrolyte concentrations, but on the contrary, the current and conductivity increased. This occurred till a concentration of 30%, but then decreased with further increase in concentration. For effective ZAFC performance, the KOH concentrations should be of 20–40% as in this range the solubility is almost 1 mol/L. The effects of electrolyte concentrations on Zn deposit morphology in a ZAFC were investigated by Gavrilović-Wohlmuther et al. [22]. They used three types of KOH concentrations like 0.2, 0.5, and 1 M in a Zn plate anode ZAFC. The 0.5 M KOH concentration electrolyte was observed to have induced the ZAFC to produce 10 times higher current density values than the other two concentrations, thus enhancing the cell performance. Thus, in acceptance with the above mentioned justifications, 40% KOH concentration was found to be suitable for the ZAFC operated in the present study.

3.1.2. Flowing Electrolyte Temperature The temperature of the KOH electrolyte would prominently affect the performance behavior of the ZAFC [23]. The fuel cells were operated under ﬁve different temperatures like room temperature (28 °C), 35 °C, 45 °C, 55 °C, and 65 °C and electrolyte solutions with concentrations of 20, 40, and 60% were subjected to these temperature variations. Experiment results revealed that the temperature of the Energies 2018, 11, x FOR PEER REVIEW 7 of 13

233 3.1.2. Flowing Electrolyte Temperature 234 The temperature of the KOH electrolyte would prominently affect the performance behavior of Energies 2019, 12, 1090 7 of 13 235 the ZAFC [23]. The fuel cells were operated under five different temperatures like room temperature 236 (28 ℃), 35 ℃, 45 ℃, 55 ℃, and 65 ℃ and electrolyte solutions with concentrations of 20, 40, and 60% 237 wereelectrolyte subjected influenced to these the temperature chemical reactions variations. and Experiment increased the results chemical revealed activity that (Figure the temperature4a). The OCV of 238 theincreased electrolyte in parallel influenced with the temperature chemical reactions and a maximum and increased of 1.42 the V waschemical produced activity at 65(Fig°Cure. However, 4a). The 239 OCVthe electrolyte increased was in parallel observed with to vaporize temperature at temperatures and a maximum higher than of 1.42 65 °C V thus was further produced reducing at 65 the℃. 240 However,voltage production. the electrolyte In contrast, was observed at low to temperatures, vaporize at temperatures the performance higher of the than cell 65 was ℃ thus lower further and a 241 reducingminimum the voltage voltage of production. 1.32 V wasproduced In contrast, at 28at low°C. Thustemperatures, the figure the shows performance that the maximum of the cell OCVwas 242 lowerwere producedand a minimum only at highestvoltage currentof 1.32 V density was produced of 200 mA/m at 282 .℃ This. Thus may the be figure due to shows the decreased that the 243 maximumion transfer OCV capacity were produced of the electrolyte only at highest at low current temperatures density and of 200 drying mA/m out2. This of the may air-electrode be due to the at 244 decreasedvery high temperatures. ion transfer capacity According of theto Equation electrolyte (4), at water low depletion temperatures in the and electrolyte drying outsolution of the might air- 245 electrodelead to increased at very producthigh temperatures. concentration According at the air to electrode, Equation and (4), decreased water depletion concentration in the ofelectrolyte reactants. 246 solutionThis might might have lea beend tothe increased reason for product the reduced concentration cathode reaction at the and air OCV. electrode, The significant and decreased results 247 concentrationof the study haveof reactants been substantiated. This might byhave results been fromthe reason other for researchers. the reduced At cathode very high reaction electrolyte and 248 OCVtemperatures,. The significant ZAFCs results might of absorb the study water have from been the substantiated atmosphere through by results the from cathode other leading researchers. to air 249 Atelectrode very high flooding, electrolyte but at temperatures, very low temperatures, ZAFCs might they absorb may release water water from vapor the atmosphere through the through air cathode. the 250 cathodeThis may leading result into air further electrode leakage flooding, and practical but at very difficulties, low tem thusperatures, finding they out may an optimumrelease water electrolyte vapor 251 throughoperating the temperature air cathode. is This much may more result than in importantfurther leakage in ZAFCs and practical [24]. difficulties, thus finding out 252 an optimum electrolyte operating temperature is much more than important in ZAFCs [24].

(a) (b)

253 FigureFigure 4. ((aa)) Polarization Polarization curve curve at at different different electrolyte temperatures. ( (bb)) Voltage Voltage production production with with 254 respectrespect to to electrolyte electrolyte concentration and temperature temperature..

255 FigFigureure 44bb revealsreveals thethe OCVOCV productionproduction inin ZAFCZAFC withwith respectrespect toto KOHKOH electrolyteelectrolyte operatedoperated withwith 256 threethree different different concentrations (20, 40 40,, and 60 wt%), at five five different temperatures (28 °C℃,, 35 35 ℃°C,, 45 45 ℃°C,, 2 257 5555 ℃°C, ,and and 65 65 ℃°C) )and and only only at atthe the maximum maximum current current density density of 200 of 200 mA/m mA/m2. The. ZAFC The ZAFC operated operated with 258 optimumwith optimum electrolyte electrolyte concentration concentration of 40% of chosen 40% chosen in this in study this study has produced has produced increased increased voltage voltage at a 259 higherat a higher electrolyte electrolyte temperature temperature of 65 ℃ of (1.42 65 °C V).(1.42 The V).results The thus results ascertained thus ascertained that the fuel that cell the operated fuel cell 260 withoperated the with optimized the optimized electrolyte electrolyte concentration concentration (60 wt%) (60 and wt%) temperature and temperature (65 ℃ (65) produced°C) produced the 2 261 maximumthe maximum OCV OCV (1.42 (1.42 V) at V) the at highest the highest current current density density (200 mA/m (200 mA/m2). Thus). proving Thus proving the fact the that fact better that 262 performancebetter performance of the fuel of the cell fuel was cell obviously was obviously dependent dependent on electrolyte on electrolyte parameters parameters and also and that also each that 263 parametereach parameter was inter was interconnected connected with with one one another another.. Gavrilović Gavrilović-Wohlmuther-Wohlmuther et et al. al. [2 [222] ]designed designed and and 264 operatedoperated ZAFCs at three different electrolyte temperatures (25(25 °C℃,, 5050 °C℃, and 70 70 ℃°C).). They They investigated investigated 265 thethe effect effect of of electrolyte electrolyte temperature temperature on Zn on plating, Zn plating, Zn crystals Zn crystals formation formation and dissolution and dissolution They reported They 266 reportedthat higher that electrolyte higher electrolyte operating operating temperatures temperatures in ZAFCs in were ZAFCs more were beneficial more as beneficial this resulted as thi ins 267 resultedincreased in electrolyteincreased electrolyte conductivity, conductivity, salt solubility, salt solubility and air electrode, and air kinetics.electrode Thekinetics. electrolyte The electrolyte viscosity 268 viscositywas also was reduced also reduced with increasing with increasing temperatures. temperature Thus,s. theThus, ionic the mobilityionic mobility of the of zincate the zincate ions ions was 269 wasincreased increased and thisand eventuallythis eventually led to le highd to limiting high limiting current current density density production production and thus, and almost thus, 10 almost times 2 270 10higher times current higher densitycurrent valuesdensity than values that than of low that temperatures of low temperatures (10 mA/cm (10 mA/cm) were2 produced) were produced at higher at 2 271 highertemperatures temperatures (100 mA/cm (100 mA/cm). The2). zincate The zincate ion formation ion formation and deposition and deposition on the on ZAFC the ZAFC electrodes electrodes were 272 wereobserved observed to be to different be different when when the temperature the temperature was varied. was varied. Lower Lower temperatures temperatures resulted resulted in mossy in type of formations but higher and very high temperatures lead zincate ions to get deposited as spongy

dendrites which had poorer attachment on the electrodes, low solubility and were easy to be washed away after the discharge processes [25]. Thus considering the above mentioned results and signiﬁcant Energies 2018, 11, x FOR PEER REVIEW 8 of 13

273 Energiesmossy2019 type, 12 of, 1090 formations but higher and very high temperatures lead zincate ions to get deposited8 of 13 274 as spongy dendrites which had poorer attachment on the electrodes, low solubility and were easy to 275 substantiations,be washed away KOH after electrolyte the discharge with aprocesses temperature[25]. of Thus 55 to considering 65 °C was considered the above tomentioned be beneficial results for 276 theand ZAFCs significant operated substantiations, in this study. KOH electrolyte with a temperature of 55 to 65 ℃ was considered to 277 be beneficial for the ZAFCs operated in this study. 3.1.3. Influence of Electrolyte Flow Velocity 278 3.1.3. Influence of Electrolyte Flow Velocity The velocity of the flowing electrolyte can have numerous influences on the ZAFC components 279 and behavior.The velocity A right of the flow flowing velocity electrolyte can have can positive have impacts numerous in ZAFC influences and willon the evidently ZAFC components enhance the 280 powerand behavior. performance, A right durability flow veloc andity stabilitycan have [ 12positive]. Therefore, impacts the in effects ZAFC ofand different will evidently electrolyte enhance flow 281 velocitiesthe power on performance, the ZAFC were durability controlled and by stability a peristaltic [12]. pumpTherefore, and investigatedthe effects ofin different this study electrolyte such as 282 0flow m/s, velocities 0.06 m/s, on 0.12 the m/s, ZAFC and were 0.18 controlled m/s with by a KOH a peristaltic concentration pump ofand 40wt investigated %. The results in thi ares study depicted such 283 inas Figure 0 m/s,5a,b, 0.06 When m/s, the0.12 electrolyte m/s, and had 0.18 no m/s flow, with the a concentration KOH concentration of products of 40wt increased %. The continuously. results are 284 OCVdepicted output in Figincreasedure 5a from and b, no When flow conditions the electrolyte (1.35 V) had tono 0.06 flow, m/s the (1.38 concentration V), till 0.12 m/s of products (1.45 V). 285 Butincreased further continuously. rise in velocity OCV to 0.18m/soutput increased resulted in from a sudden no flow fall conditions of OCV to (1.35 1.36 V) V. to 0.06 m/s (1.38 V), 286 till 0.12 m/s (1.45 V). But further rise in velocity to 0.18m/s resulted in a sudden fall of OCV to 1.36 V.

(a) (b)

287 FigureFigure 5.5. ((aa)) PolarizationPolarization curvecurve atat differentdifferent electrolyteelectrolyte ﬂowflow velocityvelocity values.values. ((b)) VoltageVoltage andand currentcurrent 288 densitydensity productionproduction inin termsterms ofof electrolyteelectrolyte ﬂowflow velocity.velocity

289 AnotherAnother reasonmay be be that that the the high high flow flow velocity velocity might might have have caused caused a high a high pressure pressure inside inside the 290 thecell cellthus thus leading leading to air to cathode air cathode flooding. flooding. When When the flow the flowvelocity velocity of the of KOH the KOHelectrolyte electrolyte was 0.06 was– 291 0.06–0.120.12 m/s, m/s,the voltage the voltage was higher,was higher, but when but when the flow the flow velocity velocity was was further further accelerated accelerated to 0.18 to 0.18 m/s, m/s, the 292 theflow flow regime regime of ofthe the electrolyte electrolyte inside inside the the fuel fuel cell cell was was too too fast fast,, which which might might have resulted in in 293 incompleteincomplete chemicalchemical reactionreaction andand irregularirregular voltagevoltage productionproduction andand downfall.downfall. ZincateZincate ionsions willwill getget 294 dissolveddissolved inin electrolyteelectrolyte and and will will be be brought brought by by circulating circulating flow flow to to electrolysis electrolysis tank. tank. Zn Zn fuel fuel can can also also be 295 continuallybe continually added added to the to the reaction reaction tank tank through through the flowingthe flowing electrolyte. electrolyte. The The reactant reactant supplement supplement can 296 becan effectively be effectively performed, performed, instead instead of mechanically of mechanically exchanging exchanging Zn plates. Zn plates. However, However, zincate zincate ions mayions 297 getmay precipitated get precipitated as solid as ZnOsolid ifZnO the if solution the solution is oversaturated is oversaturated with zincatewith zincate ions. Theions. reaction The reaction has been has 298 representedbeen represented below. below. 2− − Zn(OH)4 → ZnO + 2(OH) + H2O (5) Zn(OH)42− → ZnO + 2(OH)− + H2O (5) Solid ZnO are electrical insulators and possess higher molar volumes than metallic Zn; 299 Solid ZnO are electrical insulators and possess higher molar volumes than metallic Zn; consequently. Their precipitation in a ZAFC will lead to pore blockage gradually decreasing the specific 300 consequently. Their precipitation in a ZAFC will lead to pore blockage gradually decreasing the capacity of the cell. The relevant precipitation rates depended on alkali concentration, initial electrode 301 specific capacity of the cell. The relevant precipitation rates depended on alkali concentration, initial porosity, and applied current density. Therefore, the electrolyte flow velocity has been considered 302 electrode porosity, and applied current density. Therefore, the electrolyte flow velocity has been crucial in terms of adjusting the effect of ZnO exclusion. It has been vindicated with the help of 303 considered crucial in terms of adjusting the effect of ZnO exclusion. It has been vindicated with the forthcoming research works. Very high flow rates of the electrolyte may lead to non-uniform current 304 help of forthcoming research works. Very high flow rates of the electrolyte may lead to non-uniform density distribution and concentration gradients which may result in uneven redistribution of zinc 305 current density distribution and concentration gradients which may result in uneven redistribution active materials on the electrode surfaces. Other impacts are decrease of porosity, passivation and 306 of zinc active materials on the electrode surfaces. Other impacts are decrease of porosity, passivation active surface areas of the electrode, and thereby reduction of electrode kinetics [26]. Pei et al. [27] 307 and active surface areas of the electrode, and thereby reduction of electrode kinetics [26]. Pei et al. operated ZAFCs and reported that very less electrolyte flow rates may lead to insufficient KOH and 308 [27] operated ZAFCs and reported that very less electrolyte flow rates may lead to insufficient KOH oxygen delivery and too high flow rates may alter the renewal rate of electrolyte OH− concentration Energies 2018, 11, x FOR PEER REVIEW 9 of 13

309 andEnergies oxygen2019, 12 , delivery 1090 and too high flow rates may alter the renewal rate of electrolyte 9 OH− of 13 310 concentration and dissolution of the zincate ions in the electrolyte. Thus, flow rates should be able to 311 provide OH− according to reaction stoichiometry and remove the reaction product, or else ZnO and dissolution of the zincate ions in the electrolyte. Thus, flow rates should be able to provide OH− 312 formation can cause concentration polarization and reduced performance. The fundamental reasons according to reaction stoichiometry and remove the reaction product, or else ZnO formation can cause 313 for the performance of ZAFC with respect to flow may be due to mass transfer differences and concentration polarization and reduced performance. The fundamental reasons for the performance of 314 crossover effects of zincate ions due to flow [27]. When the concentration of zincate ions approached ZAFC with respect to flow may be due to mass transfer differences and crossover effects of zincate 315 saturation, the surface of Zn anode would react directly to ZnO, thus reducing the area for anode ions due to flow [27]. When the concentration of zincate ions approached saturation, the surface of Zn 316 reaction and decreasing the fuel cell performance. ZAFCs were operated under three electrolyte flow anode would react directly to ZnO, thus reducing the area for anode reaction and decreasing the fuel 317 velocities of 0.5, 4, and 15 cm/s by [10]. They observed that as flow velocity increased, the cell cell performance. ZAFCs were operated under three electrolyte flow velocities of 0.5, 4, and 15 cm/s 318 potentials also increased from 0.5 V to 2.8 V. They concluded that under linear velocities, the by [10]. They observed that as flow velocity increased, the cell potentials also increased from 0.5 V to 319 electrolyte inside the fuel cell can be continuously renewed, thus enhancing the long term 2.8 V. They concluded that under linear velocities, the electrolyte inside the fuel cell can be continuously 320 performance. Gavrilović-Wohlmuther et al. [22] designed and operated ZAFCs at three different renewed, thus enhancing the long term performance. Gavrilović-Wohlmuther et al. [22] designed and 321 electrolyte flow velocities like 3, 6, and 16 cm/s. In the ZAFCs with linear velocity of 6 cm/s produced operated ZAFCs at three different electrolyte flow velocities like 3, 6, and 16 cm/s. In the ZAFCs with 322 almost 5 times higher current density than the other velocity values. The surface analysis of the ZAFC linear velocity of 6 cm/s produced almost 5 times higher current density than the other velocity values. 323 anode revealed that medium velocities lead to spongy dendrites with poor attachment, low solubility The surface analysis of the ZAFC anode revealed that medium velocities lead to spongy dendrites 324 and easy exclusion by the electrolyte. Thus considering the above mentioned results and significant with poor attachment, low solubility and easy exclusion by the electrolyte. Thus considering the above 325 substantiations, KOH electrolyte with a flow velocity of 0.12 m/s was considered to be beneficial for mentioned results and significant substantiations, KOH electrolyte with a flow velocity of 0.12 m/s 326 the ZAFCs operated in the present study. was considered to be beneficial for the ZAFCs operated in the present study.

3.2. Effect of Current Collector Surface area 327 3.2. Effect of Current Collector Surface area Anode current collectors will facilitate the collection of electrons produced from oxidation of Zn 328 anodeAnode fuel. current As the Zncollectors particles will mightfacilitate possess the collection unsteady of and electrons irregular produced contact surface,from oxidation they may of Zn be 329 anodeinefﬁcient fuel. in As transporting the Zn particles electrons might efﬁciently possess to unsteady the external and circuitirregular [28]. contact Thus thesurface, electron they collection may be 330 inefficientshould be commencedin transporting by electrons metal current efficiently collectors to the as external they may circuit have [ better28]. Thus surface the electron area, conductivity, collection 331 shouldand steady be commenced and regular by contact metal surfacecurrent and collectors porosity. as they The may larger have the bettercontact surface area between area, conductivity, the current 332 andcollector steady and and the regular Zn particles, contact the surface more and effective porosity. the electron The larger transportation the contact area will be.between Thus, the Ni current meshes 333 withcollector different and the grid Zn shapes particles, and the surfacemore areaeffective valuesthe like electron 100 cm transportation2, 300 cm2, andwill 400 be. cm 2Thus,were Ni employed meshes 334 withas anode different current grid collectors shapes and and surface the effects area were values investigated like 100 cm on2, 300 the ZAFCscm2, and performance 400 cm2 were in employed this study. 335 asFigure anode6a current shows the collectors representation and the effectsof the currentwere investigated collectors and on the their ZAFCs respective performance surface areas.in this study. 336 Figure 6a shows the representation of the current collectors and their respective surface areas.

(a) (b)

337 Figure 6.6. ((aa)) Anode Anode current current collectors collectors with different grid shapes shapes and and surface surface areas. areas. ( (b)) Power 338 production in zinc (Zn)-air(Zn)-air fuelfuel cellscells (ZAFCs) (ZAFCs) in in terms terms of of current current collectors. collectors

339 FigFigureure 6 6bb illustratesillustrates that that increasing increasing the the collector collector area area can can improve improve the the efficiency efficiency of of electron electron 340 collection and improve the power performance of the fuel cell.cell. Maximum OCV of 1.4 V was was produced produced 2 2 341 by the ZAFC with an anode current collector of 400 cm 2 surface area, whereas the others (100 cm 2 and 2 342 300 cm cm2 producedproduced 1.33 1.33 V Vand and 1.35 1.35 V respectively). V respectively The). reason The reason might mightbe that behigh that surface high area surface materials area 343 materialsare capable are of capable collecting of more collecting electrons more compared electrons to compared the lower to surface the lower area surfac ones,e thus area accelerating ones, thus 344 acceleratingthe reaction the kinetic reaction rate. kinetic The shape rate. ofThe the shape grid of may the effectively grid may effectively resolve the resolve efficiency the efficiency and improve and 345 improvethe contact the areacontact between area between the Zn particlesthe Zn particles and theand current the current collector. collector. The optimal The optimal grid shapegrid shape was 346 wasobserved observed to increase to increase the contact the contact area by area 30%. by These 30%. results These have results been have corroborated been corroborated by some previously by some Energies 2018, 11, x FOR PEER REVIEW 10 of 13 Energies 2019, 12, 1090 10 of 13 347 previously published studies as follows. Au and Ni mesh were employed as anode current collectors 348 in the fuel cells operated by Asghar et al. [29]. Voltage, power, and current density values of 0.7 V, 1 published studies as follows. Au and Ni mesh were employed as anode current collectors in the fuel 349 A/cm2, and 357 mW/m2, respectively, were produced by the fuel cell with Au current collector, but cells operated by Asghar et al. [29]. Voltage, power, and current density values of 0.7 V, 1 A/cm2, 350 when it was replaced by Ni mesh the values increased almost 2 fold to 1.1 V, 2.5 A/cm2, and 801 and 357 mW/m2, respectively, were produced by the fuel cell with Au current collector, but when it 351 mW/m2. They stated that the better performance of Ni mesh might be due to its better catalytic was replaced by Ni mesh the values increased almost 2 fold to 1.1 V, 2.5 A/cm2, and 801 mW/m2. 352 properties. The high surface area and material densification in Ni mesh were crucial for efficient ionic They stated that the better performance of Ni mesh might be due to its better catalytic properties. 353 transport and reaction kinetics. High surface area facilitated more ORR and HER reaction sites and The high surface area and material densification in Ni mesh were crucial for efficient ionic transport 354 this might have enabled high power performance of the fuel cell. Zhu et al. [5] operated fuel cells and reaction kinetics. High surface area facilitated more ORR and HER reaction sites and this might 355 with Ni as anode current collector with an initial surface area of 0.3 cm2. Ni foam was used on the have enabled high power performance of the fuel cell. Zhu et al. [5] operated fuel cells with Ni as anode 356 current collectors for active area enhancement and the surface area increased to 0.64 cm2. The current collector with an initial surface area of 0.3 cm2. Ni foam was used on the current collectors for 357 microstructure characteristics of the material like the active surface area, porosity played an active area enhancement and the surface area increased to 0.64 cm2. The microstructure characteristics 358 important role in ionic conduction and electrochemical kinetics thereby resulting in a heightened fuel of the material like the active surface area, porosity played an important role in ionic conduction and 359 cell performance. electrochemical kinetics thereby resulting in a heightened fuel cell performance.

3.3. Conductor Material at an Air Electrode 360 3.3. Conductor Material at an Air Electrode Cathodes are important elements in ZAFCs, and upgrading their structure will improve 361 the conductivity,Cathodes are catalyst important dispersibility, elements in structure, ZAFCs, and porosity, upgrading hydrophobicity, their structure and therebywill improve the ORR, the 362 conductivity,thus enhancing catalyst them. dispersibility, The cathode electrodestructure, of porosity, the ZAFC hydrophobicity was made of, Teflon,and thereby catalyst the and ORR, carbon thus 363 enhancingblack, and electronsthem. The were cathode transported electrode through of the ZAFC carbon was black made and of catalyst. Teflon, However,catalyst and the carbon conductivity black, 364 andof carbon electrons black were might transported be lesser than through that of carbon metals, black so replacing and catalyst. that However,layer with the a conductive conductivity metal of 365 clayerarbon was black necessary. might be Therefore, lesser than the that air cathodeof metals, should so replacing be covered that withlayer a with metal a conductive sheet to facilitate metal betterlayer 366 waselectron necessary. conduction. Therefore, Copper the andair cathode stainless should steel sheets be covered were usedwith asa metal conductive sheet to materials facilitate and better the 367 electronair-cathode conduction. electrodes Copper of the fueland cellsstainless were steel fabricated. sheets were As shown used as in conductive Figure7, the material power performances and the air- 368 cathodeof the fuel electrodes cells varied of the relatively fuel cells with were the fabric metalated. used As shown as conductive in Figure material. 7, the power Copper performance (1.4 V) was of 369 theobserved fuel cells to be varied much better relatively than stainless with the steel metal (1.35 used V) in as voltage conductive production. material. Replacing Copper the (1.4 material V) was of 370 observedthe cathode to conductivebe much better sheet than with stainless metal had steel a significant (1.35 V) in influence voltage production on the performance. Replacing of thethe fuelmaterial cells. 371 Theof the metal cathode might conductive have reduced sheet thewith ohmic metal overvoltagehad a significant during influence discharge on the and performance increased the of electronthe fuel 372 cells.conduction The metal efficiency might of cathodehave reduced electrode, the thus ohmic enhancing overvoltage the voltage during and discharge current density and increased production. the 373 electronFan and conduction Su [30] designed efficiency and operatedof cathode fuel electrode, cells with thus air enhancing cathodes that the possessedvoltage and triple current conducting density 374 (oxygen,production. proton, Fan and and Su electron) [30] designed capabilities. and operated They were fuel composed cells with ofair lithium, cathodes nickel, that possessed and cobalt triple and 375 conductingtheir performance (oxygen, was prot comparedon, and electron) with that capabilities. of non-metal They carbon were electrodescomposed fuelof lithium, cells. Thenickel carbon, and 376 cobaltelectrode and fuel their cell performance produced voltage was compared and current with density that of of non 0.5-metal V and carbon 0.7 A/m electrodes2 respectively, fuel cells. whereas The 377 carbonthe triple electrode metal electrode fuel cell fuelproduced cell was voltage much superiorand current in power density production of 0.5 V and with 0.7 a voltage A/m2 respectively, of 1.0 V and 378 whereascurrent density the triple of metal 1.5 A/m electrode2. They fuel reported cell was that much the significantsuperior in reason power behindproduction this waswith the a voltage excellent of 379 1.0and V extrinsic and current conductivity density of properties 1.5 A/m2 and. They outstanding reported that ORR the activity significant displayed reason by thebehind metal this electrodes. was the 380 excellentInnovative and stretchable extrinsic conductivity copper conductor properties air cathodes and outstanding were designed ORR activity in ZAFC displayed and a performanceby the metal 381 electrodes.comparison Innovative with carbon stretchable cloth cathode copper ZAFC conductor was done air by cathodes Qu et al were [31]. The designed metal in electrodes ZAFC and were a 382 performancebinder free and comparison cost effective with compared carbon cloth to carbon cathode electrodes ZAFC andwas the done fuel by cells Qu produced et al [31]. much The metal more 383 electrodenergy densityes were of binder 1084 Wh/Kg free and which cost was effective almost compared 2 fold higher to carbon than the electrodes values previously and the fuel reported cells 384 (573produced Wh/Kg). much Thus more the energy above density mentioned of 1084 results Wh/Kg made which it evident was thatalmost scaling 2 fold up higher of ZAFC than air-cathodes the values 385 withpreviously metallic reported conductors (573 isWh/Kg). very crucial. Thus the above mentioned results made it evident that scaling up 386 of ZAFC air-cathodes with metallic conductors is very crucial.

387 FigureFigure 7. 7.PowerPower performance performance of ofthe the ZAFCs ZAFCs in interms terms of ofair air-cathode-cathode electrode electrode conductive conductive material material.

Energies 2019, 12, 1090 11 of 13

Energies 2018, 11, x FOR PEER REVIEW 11 of 13 3.4. Power Density Curves 388 3.4. Power Density Curves The fuel cell system was tested to discharge at constant power 3 W and the termination voltage 389 was setThe at fuel 0.8 V.cell The system discharge was wastested measured to discharge at constant at constant power power using 3 25 W g and of Zn the powder. termination As illustrated voltage 390 inwas Figure set at8a, 0.8 the V. voltage The discharge of the cell wa exceededs measured 0.8 V at at constant a time of power about 230using minutes. 25 g of The Zn peak powder. power As 391 densityillustrated was in 456 Fig Wh/kg.ure 8a, the The voltage voltage of dropthe cell attributed exceeded to 0.8 the V dissolutionat a time of ofabout Zn particles230 minute afters. The reaction. peak 392 Thepower reaction density surface was 456 between Wh/kg. the The Zn voltage particles drop and theattributed collector to gridthe dissolution decreased, whichof Zn particles made the after cell 393 unsustainablereaction. The reaction for voltage surface maintenance. between the A follow-up Zn particles Zn particle-feedingand the collector design grid decreased, continuously which supplied made 394 Znthe particlescell unsustainable to the fuel for cell voltage for further maintenanc reactions.e. MaintainingA follow-up theZn contactparticle area-feeding between design Zn continuously particles and 395 currentsupplied collector Zn particles enabled to the cellfuel tocell operate for further continuously reaction withs. Maintain improveding the performance. contact area The between maximum Zn 396 powerparticles of and the fuelcurrent cell iscollector illustrated enable in Figured the 8cb,ell andto operate the peak continuously power was aboutwith improved 12 W. The performance. peak current 397 densityThe maximum was 300 power mA/cm2 of the and fuel the cell highest is illustrated power in corresponded Figure 8b, and to the the peak voltage power of 0.8 was V. about The power 12 W. 398 startedThe peak decreasing current density when the wa voltages 300 mA/cm2 was lower and thanthe highest 0.8 V. Therefore, power corresponded the operating to voltage the voltage should of 0.8 be 399 maintainedV. The power at 0.8started V or decreasing higher for enhanced when the performance. voltage was lower than 0.8 V. Therefore, the operating 400 voltage should be maintained at 0.8 V or higher for enhanced performance.

(a) (b)

401 FigureFigure 8. 8. ((aa)) Anode current collectors with different different grid grid shapes shapes and and surface surface areas. areas. (b) Power Power 402 productionproduction inin ZAFCsZAFCs inin termsterms ofof currentcurrent collectors.collectors.

403 4.4. Conclusions 404 The investigation of the flowflow electrolyteelectrolyte parametersparameters and their optimizationoptimization in ZAFCs has beenbeen undertaken in this present study. The products of Zn(OH) 2− can be dissolved in electrolyte and carried 405 undertaken in this present study. The products of Zn(OH)4 42− can be dissolved in electrolyte and 406 awaycarried by away flow. Theby flow. Zn anode The Zncan anode be maintained can be maintained in the best reaction in the best state. react By controllingion state. By parameters controlling of 407 electrolyteparameters flow, of electrolytethe active polarization, flow, the active ohmic polarization, polarization, and ohmic concentration polarization polarization, and concentration of the cell 408 reactionpolarization were of reported the cell to reaction be reduced. were Thereported following to be are reduced. the findings The andfollowing conclusions are the of findings the study and 409 conclusions of the study 1. A maximum cell voltage at 200 mA cm−2 was obtained at the KOH concentration of 40 wt%, 410 1. A maximum cell voltage at 200 mA cm-2 was obtained at the KOH concentration of 40 wt%, but a further increase in the electrolyte concentration to 50 wt% and 60 wt% reduced the cell 411 but a further increase in the electrolyte concentration to 50 wt% and 60 wt% reduced the cell voltage. voltage. However, higher voltages can be maintained while discharging at high current densities. 412 However, higher voltages can be maintained while discharging at high current densities. 2. Furthermore, the increase of electrolyte temperature, resulted in an increase in the ionic 413 2. Furthermore, the increase of electrolyte temperature, resulted in an increase in the ionic conductivity. The temperature of the electrolyte increased from room temperature to 65 °C, 414 conductivity. The temperature of the electrolyte increased from room temperature to 65 ℃, and the and the cell voltage increased obviously. The loss of H2O in electrolyte at higher temperature, 415 cell voltage increased obviously. The loss of H2O in electrolyte at higher temperature, aggravated the aggravated the polarization concentration of the fuel cell, leading to a slow rise in voltage. 416 polarization concentration of the fuel cell, leading to a slow rise in voltage. 3. The electrolyte flow velocity should be maintained at 0.12 m/s for an enhanced performance. 417 3. The electrolyte flow velocity should be maintained at 0.12 m/s for an enhanced The experimental results display that as the surface area and grid shape of the anode current 418 performance. The experimental results display that as the surface area and grid shape of the anode conductor (Ni mesh) increased, more electrons were conducted and the fuel cell power production 419 current conductor (Ni mesh) increased, more electrons were conducted and the fuel cell power was increased. The shape and area of the collector grid should be designed to facilitate the 420 production was increased. The shape and area of the collector grid should be designed to facilitate addition of Zn particles and increase the area in a limited space. 421 the addition of Zn particles and increase the area in a limited space. 422 4. 4.The air-cathodeThe air-cathode conductor conductor material material should should be made be made of copper of copper metal metal rather rather than carbon than carbon black, 423 black,which which can can increaseincrease thethe ionic conductivity conductivity and and facilitate facilitate heightened heightened power power performance performance of th ofe 424 ZAFC.the ZAFC.

425 Author Contributions: Conceptualization, P.-T.C. and K.D.H.; Methodology, W.-F.C. and K.D.H.; Validation, 426 W.-F.C.; Data Curation, W.-F.C.; Formal analysis, P.-T.C. and T.S.; Writing—Review and Editing, P.-T.C. and 427 T.S.; Supervision, W.-M.Y. and K.D.H.

Energies 2019, 12, 1090 12 of 13

Author Contributions: Conceptualization, P.-T.C. and K.D.H.; Methodology, W.-F.C. and K.D.H.; Validation, W.-F.C.; Data Curation, W.-F.C.; Formal analysis, P.-T.C. and T.S.; Writing—Review and Editing, P.-T.C. and T.S.; Supervision, W.-M.Y. and K.D.H. Funding: The authors appreciate the financial support from Ministry of Science and Technology, Taiwan, under grant number MOST 106-2218-E-027-014-MY2. The authors also acknowledge the financial support by the “Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. P.T.C. is thankful for the financial support from the Center of Atomic Initiative for New Materials, National Taiwan University, from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan (108L9008). Acknowledgments: We thank the graduate students in VULCES for their great efforts on the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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