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Article

Leaching Efficiency and Kinetics of Platinum from Spent Proton Exchange Membrane Fuel Cells by H2O2/HCl

by
Wei-Sheng Chen
,
Wei-Shr Liu
* and
Wei-Chung Chen
*
Department of Resources Engineering, National Cheng Kung University, No. 1, Daxue Rd., East Dist., Tainan City 701401, Taiwan
*
Authors to whom correspondence should be addressed.
Metals 2023, 13(6), 1006; https://0-doi-org.brum.beds.ac.uk/10.3390/met13061006
Submission received: 13 April 2023 / Revised: 14 May 2023 / Accepted: 19 May 2023 / Published: 23 May 2023
(This article belongs to the Special Issue EARTH 2022-Green Technologies for Resources Processing and Circulation)
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Abstract

:
The increasing carbon emissions from various fossil fuels have led to the search for efficient and clean energy sources to replace them. Proton exchange membrane fuel cells (PEMFCs) are a promising alternative, but the use of platinum as a catalyst material poses challenges due to its limited resources and low abundance. This study proposes an efficient method for platinum recovery while retaining spent membranes. The membrane and catalyst were separated using isopropanol, and the spent membrane was dissolved in a 50% ethanol solution to prepare the precursor for subsequent membrane regeneration. Hydrochloric acid (HCl) was used as the leaching agent, and the experimental parameters such as HCl concentration, H2O2 concentration, contact time, and operating temperature were optimized to achieve the highest platinum leaching rate. Finally, through isothermal leaching experiments, the leaching mechanism was investigated using the shrinking core model, indicating the involvement of both surface chemical and inner diffusion mechanisms in the platinum leaching process, primarily controlled by the inner diffusion mechanism. Under optimal conditions, the platinum leaching rate was about 90%, and the activation energy of the reaction was calculated to be 6.89 kJ/mol using the Arrhenius equation.

1. Introduction

Due to the increasing energy demand, the depletion of the earth’s resources has been exacerbated. To meet the energy demand and simultaneously reduce the negative impact on the environment caused by energy extraction [1,2], people are actively looking for suitable and clean alternative energy sources such as solar energy, wind energy, and so on [3,4]. However, this energy is greatly affected by weather and geographical conditions, and the stability of energy generation cannot be guaranteed. Hydrogen energy is a kind of renewable energy, and storing electrochemical energy in the form of hydrogen helps to overcome the stability problem of energy production caused by weather factors. Furthermore, the high energy density and high abundance of hydrogen in nature indicate its potential in next-generation alternative energy technology [5,6,7].
The proton exchange membrane fuel cell (PEMFC) fueled by hydrogen energy has attracted much attention as a renewable energy conversion device. The PEMFC has the characteristics of a relatively low operating temperature, high efficiency, high power density, and environmental friendliness, and it is widely used in both stationary and mobile applications [8]. However, after a certain time of PEMFC operation, the pin-holes generated on the PEM and the deactivation of the catalyst deteriorate the performance and life of the PEMFC, resulting in the scrapping of the PEMFC [9,10]. On the other hand, the catalyst-coated membrane (CCM) in the PEMFC contains a Pt/C catalyst layer and a proton exchange membrane (PEM), where platinum group metals (PGMs) are the main active components of the Pt/C catalyst. Platinum group metals are one of the rarest elements in the earth’s crust. Global platinum reserves are mainly concentrated in Russia and South Africa. According to previous studies, PGM reserves and demand distribution are highly mismatched [11]. PGMs, such as platinum, palladium, and iridium, are widely used in various applications due to their unique physical and chemical properties, such as high melting and boiling points, excellent thermal and electrical conductivity, and high catalytic activity. Among them, platinum is the most widely used metal in the field of catalysis, including in fuel cells, where it acts as a catalyst to promote the oxidation of hydrogen at the anode and reduction of oxygen at the cathode. PGMs are also used in electronic and electrical devices, jewelry, medical devices, dental equipment, and space materials. However, due to their high cost and limited availability, it is becoming increasingly important to develop efficient and environmentally friendly recycling processes for the recovery of noble catalysts and membranes. This will help to reduce the demand for new PGMs as well as the negative impact of their extraction on the environment [12]. For this reason, the development of efficient and environmentally friendly recycling processes for the recovery of noble catalysts and membranes is of paramount importance [13,14].
In a PEMFC, the membrane electrode assembly (MEA) is the central part that controls the performance and lifetime of the cell. An MEA is a stacked cell comprising an ion-conducting membrane, two electrodes (an anode and a cathode), and two GDLs. The membrane is a Nafion® membrane, namely, a perfluorinated sulfonic acid (PFSA) polymer membrane [15]. The membrane surface is coated with an electrocatalyst consisting of Pt nanoparticles supported on high-surface-area carbon (Pt/C) as the cathode electrocatalyst for the oxygen reduction reaction (ORR) and the Pt or Pt-based alloy nanoparticles supported on carbon (Pt/C or PtRu/C) as the anode electrocatalyst for the fuel oxidation reaction [16,17,18,19,20]. The outermost layer is a GDL with two surfaces treated with hydrophobic carbon cloth, which can transfer electrons, strengthen gas diffusion, and prevent water from entering [21,22].
According to Strategic Analysis Inc. [23], the electrode ink accounts for about 46% of the total cost of mass-producing PEMFC. Considering the cost of metals and their limited resources, it is necessary to develop efficient recycling processes for spent fuel cells. PEMFCs are also perfect examples of today’s complex technological objects, which include precious metals and complex polymers in the form of nanoparticles. The first problem that will be faced during the recovery of Pt from PEMFC is the delamination of the MEA. Since the membrane in the MEA is bonded to the GDL by an electrolyte binder (Nafion ionomer) [24,25], the physical separation method is ineffective. However, chemical methods can achieve the stratification of MEA. In past studies, it has been reported that soaking the MEA in an aqueous alcohol solution or using a 50% isopropanol (IPA) solution resulted in complete delamination of the MEA [26], and that using dilute sulfuric acid as a pretreatment reagent obtained good results [26,27]. In this study, the use of 50% IPA effectively separate PEM, GDL, and CL which undergo solid–liquid separation for subsequent Pt leaching.
The main processes for PGM recovery can generally be grouped into the following three categories: selective chlorination or gas-phase volatilization, pyrometallurgical, and hydrometallurgical [28]. The selective chlorination process uses different metal chlorides with different vapor pressures to selectively remove target metals through gas-phase adsorption. Previous studies have shown that more than 95% of platinum can be recovered from spent honeycomb-type cordierite skeleton car catalytic converters through a selective chlorination process, and it has the characteristics of a lower operating temperature than pyrometallurgy, which only needs to be below 550 °C [29]. Whilst these processes are highly tunable and are capable of producing high-purity metals, gas-phase volatilization does require the use of CO and Cl2, both hazardous gases. Pyrometallurgy usually operates at a temperature of 1500 °C to 1700 °C. By adding flux and metal collectors at high temperatures to melt spent catalysts, PGM-containing alloys are obtained, and valuable metals are recovered from them through separation and purification techniques. Due to their simple operation, short flow sheet, and large-scale treatment potential, pyrometallurgical processes are extensively used in the recovery of precious metals from car catalytic converters [30]. However, the pyrometallurgical process is energy-intensive, and under oxidative conditions may form hydrofluoric acid, carbonyl fluoride, and other COF compounds; therefore, this process is not suitable for samples containing large amounts of fluoride, such as PEMFCs [31,32]. Hydrometallurgy has developed rapidly in this field because of its wide range of reagent choices, small economic scale, and low recovery costs and energy consumption [33]. Hydrometallurgical processes can generally be characterized by leaching platinum in oxidizing solutions, and hydrometallurgical processes are also the most diverse. Most hydrometallurgical methods require extremely aggressive conditions of immersing the spent material in highly acidic and oxidizing chloride containing media such as aqua regia. Previous studies have shown that using aqua regia as an impregnating agent for leaching platinum from automotive catalytic converters can achieve a recovery efficiency of 95% [34,35]. Even though it is a highly efficient means of recovering metal value, aqua regia is an extremely corrosive solution with well-documented biological and environmental effects, making it unattractive for large-scale recycling processes. There are also other studies on the use of cyanide to extract PGM from spent catalysts to replace aqua regia. However, the kinetics of platinum leaching with the cyanide leaching solution are not ideal at ambient temperature and pressure. This means that a large amount of catalyst and cyanide leaching solution is consumed during the Pt leaching process due to side reactions of the cyanide leaching solution with other parts of the catalyst. Therefore, a higher pressure and temperature are required to achieve more efficient use of cyanide to extract platinum. Although it has been proposed that the process of pressure cyanidation at a temperature of 160 °C and a pressure of 1.5 MPa can achieve a Pt extraction rate of 90–94%, the highly toxic and polluting cyanide leaching method has been gradually eliminated [36]. A less mentioned but promising alternative to aqua regia and cyanide extraction is the complexation of PGM with iodine. Amir and Morteza (2009) studied the extraction of platinum in acidified 0.12–0.48 M iodide solutions at temperatures between 25 and 95 °C. The results showed that Pt could be leached from the waste with sufficient time and iodine oxidant content, and the Pt recovery reached 80% [37]. In order to form chlorine complexes of platinum in PEMFC, an oxidant with a reduction potential of >0.74 V is needed, so both H2O2 and HNO3 are good choices [38]. Most studies still choose to use HNO3 as the oxidant. However, in some studies H2O2 was used instead of HNO3 as a strong oxidant in the impregnation process [35]. In other studies, spent PEMFC was used as the platinum source for impregnation with the H2O2/HCl system. To maintain efficient leaching of Pt from PEMFC electrodes at ambient temperature, 3 vol.% H2O2 or 5 vol.% H2O2/HCl should be used to leach 90% of Pt [23]. This study first used sonication to separate the platinum particles from the catalyst and examined the optimal number of cycles. Then, Pt was leached with HCl/H2O2, and the process was optimized by adjusting various parameters in the acid leaching. Finally, the kinetics of Pt in HCl/H2O2 is discussed.

2. Materials and Methods

2.1. Reagents and Material

The spent proton exchange membrane fuel cell stack (PEMFC) in this study was purchased from Fucell CO., Ltd. (Taoyuan, Taiwan). The PEMFC included bipolar plates and membrane electrode assemblies (MEAs) which consisted of the cathode and anode gas diffusion electrodes (GDEs) separated by the PEM (Nafion®). The mass ratio of each part of waste PEMFC is shown in Table 1. We used 100% v/v aqua regia to dissolve elements in the PEMFC, and the ion concentration in the solution was measured by inductively coupled plasma optical emission spectrometry (ICP-OES; VISTA-MPX, Varian, Palo Alto, CA, USA). The concentration of the main element compositions is shown in Table 2. The nitric acid (≥65.0%), hydrochloric acid (≥37%), isopropyl alcohol (IPA) (≥99%), and hydrogen peroxide (≥30%) were purchased from Uni-Onward Corp (New Taipei City, Taiwan). The chemicals used in this study were all of analytical quality.

2.2. Pretreatment

The spent proton exchange membrane fuel cell stack was disassembled into multiple single cells and manually disassembled into bipolar plates and membrane electrode assemblies. To delaminate the GDL of the MEA with the PEM, the MEA was cut into appropriate sizes, and 0.2 g of MEA was placed in 50 mL of a mixed solution of IPA and deionized (DI) water (1:1, v/v) for sonication at 70 °C for 30 min. Next, we repeated the above steps 1 to 3 times. We used a 100-mesh sieve to filter out the MEA fragments. Finally, the suspension was evaporated to dryness to obtain the waste Pt/C catalyst powder.
We put the MEA fragments into an autoclave filled with 50 mL of alcohol aqueous solution (50%, v/v) and dissolved them at 240 °C for 6 h. Finally, the solution obtained after dissolution was a membrane-recasting solution.
To optimize the efficiency of pretreatment, we used 60 mL of aqua regia (HCl:HNO3 = 3:1 v/v) stirred at 70 °C for 6 h to dissolve the platinum content of the obtained samples from 0 to 3 cycles of IPA treatment. The Pt(IV) concentration in the leaching solution was measured by ICP-OES.

2.3. Leaching

After the pretreatment, hydrochloric acid was chosen as the leaching agent. The dissolution reaction of Pt by chloride in HCl solution is generally represented by Equation (1) [39]:
P t + 2 C l 2 + 2 H C l → H 2 P t C l 2
However, the leaching efficiency through direct HCl leaching is not ideal. The pertinent literature indicates that H2O2 serves as a beneficial oxidant in the leaching of platinum with HCl, and it exhibits higher sustainability compared with HNO3 or other inorganic acids. The mechanisms of acidic leaching using H2O2 can be reduced to the following overall reaction Equation (2) [40,41,42]:
P t ( s ) + 6 H C l ( a q ) + 2 H 2 O 2 ( a q ) ↔ P t C l ( a q ) 2 − + 2 H ( a q ) + + 4 H 2 O ( a q )
Next, parameters such as acid concentration, hydrogen peroxide concentration, reaction time, and operating temperature were studied. The effect of acid concentration from 0.1 to 5 M, hydrogen peroxide concentration from 1% v/v to 25% v/v, reaction time from 30 min to 240 min, and operating temperature from 25 °C to 85 °C were examined to increase the efficiency of leaching in this study.
After the best parameters were tested and chosen, the obtained H2PtCl6 aqueous solution was used as the final product of this study.

2.4. Leaching Reaction Mechanism

During the leaching process, the efficiency of metal dissolution is not only affected by parameters such as chemical concentration, temperature, and time but also by the reaction between the leaching agent and the reactant. Therefore, the control factors and rate-determining steps of Pt/C catalysts in the leaching process can be understood by the shrinking core model in the leaching kinetics. According to the shrinking core model theory, the reaction mechanism of impregnation can be divided into inner layer diffusion, surface chemistry, and outer layer diffusion. According to previous studies, the external mass transfer resistance can be neglected for an agitation speed in excess of 150 rpm [43]. If the leaching reaction is dominated by inner layer diffusion, the reaction equation will conform to Equation (3) [44]:
k d t = 1 − 2 3 x − ( 1 − x ) 2 3
If it is dominated by the surface chemical reaction, the leaching reaction equation will conform to Equation (4) [44]:
k c t = 1 − ( 1 − x ) 1 3
If it is controlled by the mixture of surface chemical and inner layer diffusion at the same time, the leaching reaction equation will conform to Equation (5) [44]:
k m t = ln ⁡ ( 1 − x ) 3 − 1 + ( 1 − x ) − 1 3
We substituted the leaching percentage (x) of the experiment at different temperatures into the reaction equation and used the linear fitting parameters (K, R2) of the data to determine which leaching reaction the metal is suitable for. Finally, the constant (K) of the leaching reaction step was substituted into the Arrhenius equation—Equation (6):
k = A e − E a / R T
The activation energy Ea could then be obtained.
In addition, according to the calculated reaction activation energy, the type of impregnation reaction can also be determined. When Ea is less than 12 kJ/mol, it belongs to inner diffusion; when Ea is greater than 48 kJ/mol, it belongs to surface chemistry; if it is between the two, it is the inner layer diffusion and surface chemical reaction mixing control. To explore the leaching reaction mechanism of Pt in the waste Pt/C catalyst powder, this study used (DI) water to configure 5 M HCl 10 mL and added 10% v/v H2O2 as the conditions; the waste Pt/C catalyst powder was dissolved in 25, 55, 75, and 85 °C for isothermal leaching experiments; and we set the time parameters as 5, 10, 15, and 30 min to obtain the Pt leaching percentage.

3. Results and Discussion

3.1. Sonication

To stratify the MEA, it was completely soaked in a 50% IPA solution. The recommended optimal volume of 50% IPA is 50 mL, and it was sonicated for 30 min in a water bath with a constant temperature of 70°C. Although the MEA after ultrasonic treatment was not completely delaminated, the GDL gradually separated from the PEM. At this time, the MEA can be easily separated with tweezers, and a colloidal substance appears between the GDL and PEM. Previous studies have indicated that colloidal substances are PFSA ionomers with Pt/C catalysts [24,45,46]. Firstly, the MEA was put into 50 mL of 70 °C IPA solution, the MEA was slightly separated without ultrasonic treatment, and only some microparticles were separated. During the first sonication, it could be seen that most of the CLs had been isolated. CL detachment was also seen in subsequent cycles 2 and 3, but not as much as in the first sonication. In this study, the Pt in the samples pretreated for the 0th, 1st, 2nd, and 3rd cycles was dissolved by aqua regia digestion, and the Pt content was determined by ICP-OES. Table 3 shows the leaching percentage of the three samples, and the results show that almost all the Pt was separated from the MEA after the second cycle. Similar results were achieved in previous studies [9,47,48].

3.2. Effect of Acid Leaching Parameter Variations

After pretreatment, we used an acid to dissolve the platinum in the waste Pt/C catalyst powder. We chose hydrochloric acid as the leaching agent and hydrogen peroxide as the oxidizing agent. We adjusted the acid concentration, hydrogen peroxide concentration, reaction time, and operating temperature to obtain the best parameters.

3.2.1. Effect of the HCl Concentration

To understand the effect of the leaching agent concentration on Pt dissolution and reduce the amount of acid agent, the immersion solution concentration is discussed in this section. Hydrochloric acid was adjusted by dilution with deionized water; the concentration was set at 0.1, 0.5, 1, 3, 5, and 7 M; and the volume was fixed at 10 mL. The remaining parameters were adjusted to a 10% hydrogen peroxide addition, 70 °C reaction temperature, and reaction time of 120 min. First, the experiment was conducted without adding hydrogen peroxide. The results are shown in Figure 1. We can see that, although the efficiency increases with the increase in concentration, the leaching efficiency is not ideal. Figure 2 demonstrates the results obtained after the addition of hydrogen peroxide, clearly indicating a significant improvement in leaching efficiency. Moreover, as the hydrochloric acid concentration increases from 0.1 M to 7 M, there is a corresponding increase in the leaching efficiency of platinum. The reaction tends to equilibrate at an HCl concentration of 5 M. Therefore, 5 M of hydrochloric acid was chosen as the optimal concentration in this study.

3.2.2. Effect of the Reaction Time

Under the condition of the selected acid concentration of 5 M, the effect of the reaction time on the leaching of Pt was studied from 30 min to 240 min. The results are shown in Figure 3. The leaching percentage of Pt continues to increase over time until the maximum efficiency is reached in 120 min. Therefore, 120 min was chosen as the optimal time for this study. Compared with the previous research on leaching platinum with aqua regia [34], this study used a lower temperature and at the same time achieves an impregnation efficiency of more than 90% at a reaction time of about 100 min.

3.2.3. Effect of the Leaching Temperature

The leaching temperature is an important parameter affecting the leaching percentage. The increase in the temperature can increase the kinetic energy of the particles and improve their solubility [35]. Therefore, the impregnation temperature is discussed in this section. Under the optimal acid concentration and reaction time, the temperature was set to 25, 40, 55, 70, and 85 °C to explore the effect of impregnation temperature on the leaching percentage of Pt. The result is shown in Figure 4. The leaching percentage increased with increasing temperature and reached a near equilibrium at 70°C. Therefore, 70 °C was chosen as the optimal reaction temperature for this study.

3.2.4. Effect of the Hydrogen Peroxide Concentration

Hydrogen peroxide plays an important role in the leaching process. If only hydrochloric acid is used as the leaching agent, direct leaching is not effective. Therefore, hydrogen peroxide was selected as the oxidant to assist the leaching reaction in this study. Under the selected acid concentration, reaction time, and leaching temperature, the hydrogen peroxide concentration was set to 1% v/v, 5% v/v, 10% v/v, 15% v/v, and 25% v/v to explore the effect of hydrogen peroxide concentration on the leaching percentage of Pt. The result is shown in Figure 5. The leaching percentage increased with increasing concentration and reached equilibrium at an H2O2 concentration of 10% v/v. The leaching percentage increases with the increase in the concentration. The reason is that the oxidation–reduction potential of the solution can be increased by adding hydrogen peroxide so that the metal is more likely to enter the solution in the form of ions. Therefore, a 10% v/v concentration was chosen as the optimal hydrogen peroxide concentration for this study.

3.3. Kinetics

According to the leaching percentage at different times in the isothermal leaching experiment, the leaching percentage was substituted into Equations (3) and (4). The results are shown in Figure 6 and Figure 7, and the linear fitting results are shown in Table 4. In addition, we used Equation (6) to calculate the activation energy (Ea) of the leaching reaction. The results are shown in Table 5.
From the results of the data, it was found that the correlation coefficient (R2) of the data fitting of Pt was greater than 0.80 regardless of the inner diffusion mechanism or the surface chemical mechanism. If the correlation coefficient is greater than 0.75, it can be considered that the mechanism is involved in the leaching process, indicating the possible coexistence of two controlling mechanisms.
To confirm that the leaching process of Pt in the spent Pt/C catalyst was controlled by the mixing mechanism, the isothermal leaching experimental data were substituted into Equation (5) and plotted against time and fitted. The results are shown in Figure 8 and Table 6.
The results show that the correlation coefficients under the 4 conditions and temperatures are all greater than 85%, indicating that the leaching reaction of Pt conforms to the hybrid reaction mechanism, that is, the inner diffusion mechanism and the surface chemical reaction mechanism are jointly controlled. In addition, the type of reaction mechanism can also be determined based on the magnitude of the activation energy. The activation energy for inner diffusion reactions is generally not greater than 12 kJ/mol, while for surface chemical reactions it is typically greater than 48 kJ/mol. Mixed control mechanisms usually fall within the range of 12–48 kJ/mol [49]. Finally, by substituting the calculated rate constants into the Arrhenius equation (Equation (6)), it can be observed that all the results are less than 12 kJ/mol. Therefore, it can be determined that both the inner diffusion mechanism and surface chemical mechanism are involved in the leaching process. However, the reaction is primarily controlled by the inner diffusion mechanism.

4. Conclusions

Based on our findings, we have demonstrated that ultrasonic treatment with a 50% IPA solution for 60 min effectively retains the spent proton membrane and separates more than 95% of the Pt/C catalyst for subsequent acid leaching. The leaching kinetics analysis reveals that the platinum leaching process is controlled by a combination of the surface chemical mechanism and the inner diffusion reaction mechanism. However, the primary control lies with the inner diffusion mechanism. The activation energy of platinum leaching is found to be 6.89 kJ/mol. Through the optimization of experimental parameters, we determined that 5 M hydrochloric acid with 10% hydrogen peroxide at 70 °C for 120 min is the optimal condition for platinum leaching, resulting in a leaching percentage of approximately 90%. These results demonstrate the effectiveness of our proposed method for efficient platinum recovery while retaining the spent membrane.

Author Contributions

Conceptualization, W.-S.C., W.-S.L. and W.-C.C.; data curation, W.-S.L.; formal analysis, W.-S.L.; investigation, W.-S.L.; methodology, W.-S.C., W.-S.L. and W.-C.C.; supervision, W.-S.C. and W.-C.C.; validation, W.-S.L. and W.-C.C.; visualization, W.-S.L.; writing—original draft, W.-S.L.; and writing—review and editing, W.-S.L. and W.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data sharing not applicable.

Acknowledgments

We wish to acknowledge the support of the Laboratory of Resources Circulation (LRC) at National Cheng Kung University.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 13 01006 g001 550
Figure 1. Effect of HCl concentration on leaching efficiency without the addition of H2O2 (reaction time: 120 min; leaching temperature: 70 °C).
Figure 1. Effect of HCl concentration on leaching efficiency without the addition of H2O2 (reaction time: 120 min; leaching temperature: 70 °C).
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Figure 2. Effect of HCl concentration on the leaching percentage (H2O2 concentration: 10% v/v; reaction time: 120 min; leaching temperature: 70 °C).
Figure 2. Effect of HCl concentration on the leaching percentage (H2O2 concentration: 10% v/v; reaction time: 120 min; leaching temperature: 70 °C).
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Figure 3. Effect of reaction time on the leaching percentage (HCl concentration: 5 M; H2O2 concentration: 10% v/v; leaching temperature: 70 °C).
Figure 3. Effect of reaction time on the leaching percentage (HCl concentration: 5 M; H2O2 concentration: 10% v/v; leaching temperature: 70 °C).
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Figure 4. Effect of leaching temperature on the leaching percentage (HCl concentration: 5 M; H2O2 concentration: 10% v/v; reaction time: 120 min).
Figure 4. Effect of leaching temperature on the leaching percentage (HCl concentration: 5 M; H2O2 concentration: 10% v/v; reaction time: 120 min).
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Figure 5. Effect of H2O2 concentration on the leaching percentage (HCl concentration: 5 M; reaction time: 120 min; leaching temperature: 70 °C).
Figure 5. Effect of H2O2 concentration on the leaching percentage (HCl concentration: 5 M; reaction time: 120 min; leaching temperature: 70 °C).
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Figure 6. Inner Diffusion Reaction Kinetic Function.
Figure 6. Inner Diffusion Reaction Kinetic Function.
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Figure 7. Surface Chemical Reaction Kinetic Function.
Figure 7. Surface Chemical Reaction Kinetic Function.
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Figure 8. Hybrid Reaction Kinetic Function.
Figure 8. Hybrid Reaction Kinetic Function.
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Table
Table 1. The mass ratio of each physical component in the PEMFCs (wt.%).
Table 1. The mass ratio of each physical component in the PEMFCs (wt.%).
Bipolar PlatesMEA
96.7%3.3%
Table
Table 2. The concentration of the main element compositions in the MEAs (wt.%).
Table 2. The concentration of the main element compositions in the MEAs (wt.%).
ElementCPt
wt.%97.25%2.75%
Table
Table 3. The leaching percentage of Pt from 1 to 3 cycles of aqua regia-dissolved IPA treatment.
Table 3. The leaching percentage of Pt from 1 to 3 cycles of aqua regia-dissolved IPA treatment.
0th Cycle1st Cycle2nd Cycle3rd Cycle
17.66%63.14%96.11%98.73%
Table
Table 4. Linear fitting data of inner layer diffusion and surface chemical reaction.
Table 4. Linear fitting data of inner layer diffusion and surface chemical reaction.
Temperature
(°C)		Inner DiffusionSurface Chemical
kdR2kcR2
250.00070.87670.00380.8180
550.00130.92760.00510.8944
750.00150.96760.00570.9109
850.00090.99390.00790.9036
Table
Table 5. The leaching activation energy of inner diffusion and surface chemical reaction.
Table 5. The leaching activation energy of inner diffusion and surface chemical reaction.
ElementActivation Energy (Ea)
Inner Diffusion (kd)Chemical Reaction (kc)
Pt6.57 kJ/mol9.54 kJ/mol
Table
Table 6. Linear fitting data of hybrid reaction.
Table 6. Linear fitting data of hybrid reaction.
Temperature
(°C)		Hybrid Reaction
kdR2
250.00050.8957
550.00110.9458
750.00130.9876
850.00060.995
Activation Energy (Ea)6.89 kJ/mol
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Chen, W.-S.; Liu, W.-S.; Chen, W.-C. Leaching Efficiency and Kinetics of Platinum from Spent Proton Exchange Membrane Fuel Cells by H2O2/HCl. Metals 2023, 13, 1006. https://0-doi-org.brum.beds.ac.uk/10.3390/met13061006

AMA Style

Chen W-S, Liu W-S, Chen W-C. Leaching Efficiency and Kinetics of Platinum from Spent Proton Exchange Membrane Fuel Cells by H2O2/HCl. Metals. 2023; 13(6):1006. https://0-doi-org.brum.beds.ac.uk/10.3390/met13061006

Chicago/Turabian Style

Chen, Wei-Sheng, Wei-Shr Liu, and Wei-Chung Chen. 2023. "Leaching Efficiency and Kinetics of Platinum from Spent Proton Exchange Membrane Fuel Cells by H2O2/HCl" Metals 13, no. 6: 1006. https://0-doi-org.brum.beds.ac.uk/10.3390/met13061006

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