Determination of Flowing Electrolyte Parameters in a Zinc-Air Fuel Cell by the Taguchi Method

Huang, K. David; Nguyen, Minh-Khoa; Yang, Cheng-Jung; Chen, Po-Tuan

doi:https://doi.org/10.1155/2023/5514803

International Journal of Energy Research

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Research Article | Open Access

Volume 2023 | Article ID 5514803 | https://doi.org/10.1155/2023/5514803

Determination of Flowing Electrolyte Parameters in a Zinc-Air Fuel Cell by the Taguchi Method

K. David Huang,¹Minh-Khoa Nguyen,¹Cheng-Jung Yang,²and Po-Tuan Chen¹

Academic Editor: Denis Osinkin

Received08 Mar 2023

Revised24 Mar 2023

Accepted25 Mar 2023

Published10 Apr 2023

Abstract

The flowing electrolyte in a zinc-air fuel cell (ZAFC) can reduce the problem of incomplete reaction caused by the concentration gradient of the electrolyte. In this study, we propose a self-developed ZAFC with a flowing electrolyte system and optimize control parameters. The anode of the ZAFC used Zn particles through which the electrolyte penetrates and combined with a negative pressure pump to allow the potassium hydroxide (KOH) electrolyte to have an effective chemical reaction with the Zn particles during the discharge process. However, the flow rate of the electrolyte is required to match other control parameters, such as operating temperature, KOH concentration, and cell size, to effectively improve the efficiency of ZAFC. Therefore, the Taguchi method is adopted to obtain the optimal reaction parameters and the key factors affecting the discharge efficiency of the cell. The best operation condition is the temperature at 50°C, the KOH concentration of 30% wt, and the electrolyte flow rate of 150ml/min; then, the maximum power obtained from the experimental results is 13.8 W, the maximum current density reaches 699.721mA/cm², and the maximum power density is 395.037 mW/cm². This paper provides valuable insights for the upgrade of ZAFC for future practical applications.

1. Introduction

As the greenhouse gases brought about by the use of fossil fuels to produce energy seriously impact the earth’s environment and threaten the sustainable existence of human beings, great efforts have been made to accelerate the replacement with renewable energy. The power generation mechanism of metal fuel cells has no harmful side reactions, and the oxidized metal of the products can be reused through electrolysis, which is recognized as a rising star of the energy storage system. Zinc (Zn), aluminum, magnesium, and iron are commonly used as anode fuels in metal fuel cells, and Zn is valued for its high stability and chemical properties [1]. However, during the discharge process, the Zn metal surface is oxidized to form zinc oxide (ZnO), which reduces the reactive Zn as well as the cell efficiency [2, 3]. Besides, morphology control and parasitic reactions are the critical issues that need to be adequately addressed in rechargeable Zn-air batteries (ZABs) [4]. Mechanically rechargeable ZAB has been explored and is proven to be highly effective in morphology control [5], but its practical application is still hindered due to the inconvenience of fuel exchange physically. Flowing electrolyte is introduced to the Zn-air fuel cell (ZAFC) to take the soluble zincate of Zn(OH)₄^- away before forming the solid passivation layer of ZnO and provides continuous fuel supply without mechanical exchange, which significantly enhances the performance of ZAFC [6, 7]. Therefore, determining the optimum working conditions of flowing electrolytes is obviously a crucial part of the whole discharge-recharge process. Within this working range, the cell would be able to balance between high power output, efficiency, and stability. In spite of that, limited research has been carried out regarding this aspect, mainly based on the experimental study and cell structure without any optimization methods [8, 9]. Experimental outcomes can be influenced by a variety of external factors such as instrument accuracy, ambient temperature, and test operation. Taguchi method is able to reduce the number of experiments needed, while still ensuring the reliability and accuracy of the optimization results. Thus, this paper introduces the Zn particle anode design in combination with the Taguchi method for optimum parameters of flowing electrolyte, which has not been widely reported.

In the previous literature, numerous researchers have made substantial improvements to the flowing-electrolyte ZAFC. By designing different anode flow channels, the stacking method of Zn particles was changed to improve the discharge efficiency of the cell [10], therefore, reaching the record of the best efficiency value in the scientific research literature. The performance of ZAFC based on the anode fuel of Zn plate and particle was compared [11], indicating that Zn particle coupling with flowing electrolyte has significantly enhanced the performance in terms of open-circuit voltage, power, and current density; Nyquist and Bode’s plots also showed better resistance reduction of particle type compared to that of plate type. Flowing electrolyte is a decisive factor that contributes to the continuous fuel supply and brings considerable benefits to this kind of cell. A fuel tank mechanism including motor, inlet, outlet, and valves have been introduced [12]; thus, the innovative structure is designed to achieve the goal of automation in circulating fluid, which helps to stabilize the discharge process under certain conditions. The investigation of the electrochemical impedance and performance of ZAFC has been carried out. Various kinds of internal resistance, namely, polarization, ohmic, and mass-transport resistance have been analyzed for the determination of flowing-type ZAFC system parameters [13]. In addition, computational fluid dynamics (CFD) simulation has become a powerful tool in numerous prominent engineering fields and disciplines, which helps to predict complex phenomena based on the mathematical model. Thus, the CFD model consists of several governing equations such as the continuity equation, momentum equation, and Fick’s diffusion law have been introduced to evaluate the performance in flowing-type ZAFC [14]. The oxygen distribution at a different inlet flow rate of electrolyte to find the optimum value was, therefore, predicted. Similarly, CFD simulations have been performed to find the optimal working parameters for effective air-flow management [15]. Experimental results then can be compared and improved based on the theoretical understanding. The charging process which recovers Zn for reuse from ZnO is also a primary focus. A set of controlling parameters including charging current, stirring speed of the electrolyte, and electrolyte temperature have been optimized and used to achieve high efficiency of Zn metal recovery. A scanning electron microscope (SEM) study has been conducted to evaluate the quality of the recovered Zn. The Taguchi method has been used to optimize the parameters and ensure the maximum Zn reduction [16, 17], as well as to design parameters for an open-cathode fuel cells [18]. Likewise, an orthogonal array (OA) sampling has been applied in previous work [19] to find optimal working conditions for different input parameters in a rechargeable ZAFC. Each previous investigation has been made to improve the flowing-type ZAFC, which also makes the follow-up research and development clearer about the meaning of each parameter.

In this study, we design a novel flowing electrolyte system. Zn particles were used as anode fuel, while using a pump with negative pressure for electrolyte flowing. The product of Zn(OH)₄^- dissolved in the electrolyte and was taken away through the flowing electrolyte to reduce the resistance on the surface of the Zn metal. However, the parameter controls of operating conditions may correlate to each other, thus, it is difficult to optimize them when considering all the parameters. The Taguchi method is a valuable tool to find the parameter optimization when there is a correlation between parameters. To effectively control the parameters of the electrolyte affecting the discharge performance of the ZAFC, the KOH concentration, flow rate, temperature, and electrode spacing were selected, and the Taguchi method was used to optimize the parameter correlation for the best performance. This paper is organized as follows. The next section, “Material and Methodology,” illustrates the cell anode module, experimental platform, and the Taguchi method. The third section explains the experimental analysis results. It shows how the Taguchi method finds optimal parameters for the ZAFC. The fourth section discusses the experimental results to explore the Zn particle anode fuel cell. In the last section, based on the research results, conclusions are drawn regarding the effectiveness of the ZAFC system.

2. Material and Methodology

2.1. ZAFC Anode Module and Experimental Platform

The design section of the cell anode module is shown in Figure 1(a). To avoid leakage of electrolyte, the materials of the ZAFC body, cathode, and anode platen are all made of polyvinyl chloride (PVC). This material is quite stable and elastic at temperatures below 148°C and not easily corroded by potassium hydroxide (KOH) and toxic gases. It can ensure that the contact surfaces are completely sealed, and the O-ring is used to prevent the leakage of electrolyte, as shown in Figure 1(b).

(a)

(b)

(c)

The anode of the ZAFC is composed of a copper-tin-plated current collector material, Zn particles, and a collector network of mesh relaxation nickel mesh. Although the tin-plated copper current collector plate has well anticorrosion ability in alkali, it will generate hydrogen gas with Zn particles, which will lead to the decline of the discharge performance of the ZAFC. Since this study focuses on the effect of the discharge efficiency between the Zn particles and the four parameters of the electrolyte, the lower-cost tin-plated copper is selected as the current collector material. Zn powder with a Zn particle size of 45 μm was mixed with KOH electrolyte, and the mixing ratio was 50 g of Zn powder with 40 wt% KOH electrolyte. The cathode consists of an air electrode sheet, a conductive sheet, and a separator. The air electrode sheet is made of a Teflon diffusion layer, foamed nickel collector net, carbon black, and catalyst from the outer layer to the inner layer, respectively. The separator is a water-based nonwoven separator (NKK company, model: NKK-MPF 30AC-100; thickness (100 μm); surface density (30 g/m²); acid and alkali resistance). The experimental procedure flow chart is shown in Figure 1(c), including ZAFC system (including electrolyte and Zn particle storage tank), peristaltic pump, heating module (including heating unit and temperature controller), signal measurement (including temperature measuring rod, electronic load machine, and AC impedance analyzer), and data collection equipment (including data acquisition device and computer).

Figure 2(a) depicts the actual single ZAFC, which is a combination of the cell body, anode and cathode current collector, nickel mesh, carbon air cathode, and the cover. All the components are assembled with bolts and screws. In order to minimize the experimental errors, each component is correctly assembled on the platform, consider the position of each component and connect to the ZAFC impedance meters to complete the system, as shown in Figure 2(b).

(a)

(b)

2.2. The Taguchi Method

The Taguchi method was proposed by Japanese scholar Taguchi Genichi. The feature is to use the level arrangement of each factor in the design to reduce the complexity of the experimental design and the number of experiments required and then obtain the optimization trend by analyzing the experimental data [20]. The Taguchi method uses the signal-to-noise ratio (S/N) as a quality loss function, which is further used to measure design quality. Due to the different characteristics and objectives of quality requirements under different requirements, the S/N ratio is divided into larger the better, on-target, and smaller the better. The goal of this research is to explore the optimal current density, so the larger the better is used, and the S/N ratio formula is as follows:

In this study, the L9 orthogonal table (Table 1) was used, and four control factors (electrolyte temperature, KOH concentration, flow rate, and interelectrode distance) were investigated at different levels (Table 2), which electrochemically reacted with Zn particles and were determined by the AC impedance. Analyze the ZAFC performance to find out the corresponding discharge efficiency under each combination of parameters.

3. Result

Table 3 shows the results of three experiments according to the experimental plan of Table 1. The output response of each parameter obtained through the analysis is shown in Figure 3. The S/N of the four parameters at different levels is shown in Figure 4. Through the output response and S/N ratio, the influence of the four experimental parameters on the ZAFC performance can be further analyzed, and the proportion of the influence of each parameter on the ZAFC performance can be determined. The detailed discussion will be discussed in the next chapter. The S/N response summed up with the current density as the quality characteristic is shown in Table 4, where the delta value is the difference between the maximum and minimum values in the average value of each parameter level. The rank is arranged in order of the delta values of the four parameters. Therefore, the proportion of electrolyte that affects the performance and benefits of the ZAFC is, in descending order, temperature, electrode spacing, concentration, and flow rate.

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

The optimal parameters deduced from the Taguchi method are that the temperature of the electrolyte is third level (50°C), the concentration is first level (30 wt%), the flow rate is third level (150 ml/min), and the distance between the electrodes is first level (5 mm). Compared with number 7 which is the best performing in the Taguchi experiment, only the distance between the poles differed (5 mm vs. 8 mm). After experimenting with the best parameters, the maximum current density is 699.721 mA/cm², the maximum power is 13.8 W, and the maximum power density is 395.037 mW/cm² (Figure 5). Compared with the number 7, the current density is increased by 11.084 mA/cm², and the power is increased by 0.3 W. From the experimental results, the performance optimization is indeed achieved.

In addition to the results achieved from the Taguchi method, test data is collected under optimal working conditions from the ZAFC impedance meters and plotted, as shown in Figure 5. The polarization curves provide a complete understanding of the gradual discharge process. A fairly steep drop is firstly caused by the activation polarization to the cell voltage, which is 1.36 V down to approximately 1.17 V, before it enters a sufficiently stable stage and stops at the cutoff voltage of 0.3 V. As long as the current rises, it requires energy to overcome the internal resistance and leads to voltage drop. The maximum power density of 395.037 mW/cm² is reached at 0.766 V. Hence, maintaining the cell voltage in a stable range around this specific value is needed for the practical applications.

4. Discussion

Through the innovative design of the anode module and the improvement of the electrolyte flowing method, the reaction of the anode is more ideal. From the output response and S/N ratio, the influence and proportion of the four parameters of the electrolyte on the ZAFC performance were analyzed. The characteristics of the four parameters of the electrolyte are further discussed below. It is known from experiments that the electrolyte temperature has the maximum current density at 50°C. Higher electrolyte temperature can speed up the cathodic electrodeposition reaction rate and ion diffusion rate and increase the upper limit of current density [21]. However, the increased agglomeration and consolidation of Zn particles will lead to the loss of water in the electrolyte and exacerbate the polarization of the ZAFC. This phenomenon is also encountered in the experiment with the electrolyte at 60°C, which means that it should be considered when further optimizing the ZAFC performance.

Zn particle anodes mainly rely on permeation and capillary phenomena to achieve good electrochemical reactions. Although the high-concentration KOH electrolyte can improve the performance, the phenomenon of agglomeration will still make the anode become unfavorable for the electrochemical reaction due to the agglomeration of Zn particles. In addition, a further increase in KOH concentration results in lower cell performance due to saturation. The saturated KOH solution (60 wt%) is unsuitable for ZAFC operation. With the concentration of 60 wt% or greater, the electrolyte becomes saturated and unable to provide high ionic conductivity which then results in poor performance. Simultaneous attention should be paid to the poor electrochemical reaction as the electrolyte evaporates from the Zn particle agglomerate structure, resulting in an increase in the attractive force between the particles [22]. Moreover, the lower KOH concentration can effectively reduce the damage of the ZAFC body, waterway, and surrounding hardware systems and improve the adaptability of various equipment and materials. The reduction part of Zn particles can also ensure that the most ideal crystal structure can be reduced, improve the reduction efficiency, and reduce the energy consumption required for reduction [23]. Nonetheless, lower than 30% wt KOH concentration, the electrolyte is not able to provide sufficient ions for electrochemical reactions, which also needs to be taken into account.

The variation of electrolyte flow rate affects the rate of electrolyte OH^- concentration renewal [6]. Such a trend is consistent with this study. When the flow rate is low, the OH^- regeneration efficiency of the electrolyte inside the ZAFC is insufficient, resulting in the local dry-up phenomenon inside the ZAFC to affect the internal ion exchange rate. Therefore, the best result in the experiment is 150 ml/min. The higher value of the flow rate can certainly be adjusted, but in such case, the overall efficiency will be directly impaired. However, a high flow rate may wash away the OH^- before it can react with Zn particles. Zn particle anode designs need to provide a space for filtration. According to the experimental results, the distance between the two is 5 mm. This is because the cell uses capillary and osmosis to make the electrolyte react with the Zn particles. Therefore, the requirement of the distance between the two poles is not like the plate-type method that uses the isolation film to reduce the ohmic polarization problem [18]. The gap between two electrodes does have an impact on the performance. The large gap means that the protons need to travel further to reach the cathode and also increases the internal resistance of ZAFC, resulting in poor performance and no current. On the other hand, when the gap is small, it may cause short-circuiting, and the cell may get heated up which is dangerous. The particle type needs to overcome the influence of ohmic polarization and concentration polarization on performance. The uniformity of Zn particles and electrolyte infiltration and the fineness of mixing will affect the efficiency of electrolyte infiltration. Therefore, finding the balance between the electrode distance and the polarization phenomenon is an important task to further optimize the ZAFC performance.

5. Conclusions

In this study, the anode module of ZAFC was designed, and the Taguchi method was used to investigate the order and the optimal parameter value of the four parameters related to the electrolyte characteristics on the ZAFC discharge efficiency. The maximum power density obtained by a single cell is 395.037 mW/cm², the maximum current density is 699.721 mA/cm², and the corresponding voltage of the maximum power point is 0.766 V. The maximum power is 13.826 W. The results of this study are summarized as follows: (1)The ZAFC anode module adopts the concept of filter structure and uses the peristaltic pump to extract the electrolyte to improve the problem of excessive concentration gradient generated by the existing vertical type. The simple assembly of anode parts can also reduce the assembly complexity of the ZAFC system(2)The best performance of the electrolyte is obtained at 50°C. Temperatures over 50°C will cause water dispersion and partial agglomeration of Zn particles, resulting in aggravated cell concentration polarization and worse cell life and performance(3)The KOH concentration of 30% wt can get the best performance, and the higher concentration of electrolyte will cause the agglomeration of Zn particles, which will affect the penetration and capillary phenomenon of the Zn particles, which is not conducive to the long-term discharge(4)The best performance is achieved when the electrolyte flow is controlled at 150 ml/min. It also solves the problems caused by electrolyte saturation, penetration, and capillary phenomenon. Finally, the design of the Zn particles anode will lead to a gap between the anode and cathode to fill the Zn particles. The optimal spacing obtained in the experiment is 5 mm, and if the spacing is too far, the phenomenon of concentration polarization and ohmic polarization will be aggravated

Data Availability

Data are available on reasonable request.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this article.

Acknowledgments

The authors thank the National Science and Technology Council, Taiwan, under grant numbers MOST 110-2221-E-027-098, MOST 110-2622-E-027-029, and MOST 111-2621-M-110-001.

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Copyright © 2023 K. David Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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