Electrochemical Dissolution of Platinum and Ruthenium from Membrane Electrode Assemblies of Polymer Electrolyte Fuel Cells

To establish a recovery method for noble metals from membrane electrode assemblies (MEAs) of spent polymer electrolyte fuel cells (PEFCs) without the use of strong acids, electrochemical dissolution tests for Pt and Ru from MEAs were conducted. By using square potential waves, 93.2% of the Pt and 98.4% of the Ru dissolved from a MEA in 1 mol L−1 _{HCl at room temperature when oxidation and reduction} poten-tials were at 1.5 and 0.1 V vs. SHE and the holding times for both were 15 s per cycle. The dissolution of Pt and Ru became remarkable when the oxidation potential was 1.4 V vs. SHE and gradually decreased at more positive potentials. These results indicate that competitive reactions exist in the dissolution process. In addition, the effects of H+ _{and Cl}− _{concentrations on the dissolution ratios were investigated. The dissolution}

ratios of Pt and Ru were small in solutions with low Cl− _{and high H}+ _{concentrations ([Cl}−_{] =  0.01 mol L}−1_{, [H}+_{] =  1 mol L}−1_{); however, 50.6%} of the Pt and 27.9% of the Ru dissolved in solutions with high Cl− _{and low H}+ _{concentrations ([Cl}−_{] =  1 mol L}−1_{, [H}+_{] =  0.01 mol L}−1_{). Thus,} we verified that the electrochemical dissolution method was adaptable to the recovery of noble metals from MEAs and that strong acids were not needed. [doi:10.2320/matertrans.M2016198]

(Received June 1, 2016; Accepted August 17, 2016; Published September 16, 2016)

Keywords:  platinum, ruthenium, recovery, dissolution, fuel cell

1.  Introduction

Platinum has a high catalytic activity for the oxygen reduc-tion reacreduc-tion and the hydrogen oxidareduc-tion reacreduc-tion. Therefore, it is commonly used as an electrocatalyst for polymer electro-lyte fuel cells (PEFCs), although Pt is a precious metal. PEFCs are candidates for use as power sources in a sustain-able society because they do not emit greenhouse gases. To promote the spread of PEFCs throughout society, it is neces-sary to reduce the manufacturing costs. One solution for re-ducing the cost is to recycle Pt from waste PEFCs. Pt is con-tained in membrane electrode assemblies (MEAs), which are composed of a Pt-based catalyst, a gas diffusion layer and a proton exchange membrane. Therefore, Pt recycling technol-ogy for use with MEAs is needed. In general, recycling Pt from products, such as automotive catalysts, is conducted by using a leaching method with chloride-based solutions, such as oxyacids.1–3) _{The method is technically reliable and} appli-cable to most products, including PEFCs.4) _{However, there} are some problems with the leaching method, for example, generation of large quantities of acidic waste and corrosion of the materials by strong acid media. Therefore, we focused on an electrochemical dissolution method for Pt recycling from MEAs as an alternative without the strong acidic media used in leaching methods.

It is thought that Pt metal forms a protective oxide layer on its surface, meaning that the dissolution ratio of Pt metal into acidic media by using potentiostatic electrolysis is low. Xia-oping et al. have reported that the dissolution rates of 10 mass% Pt/C and Pt wire at 0.9 V vs. SHE in 0.57 mol L−1 HClO4 are 1.7 ×  10−14 _{and 1.4 ×  10}−14 _{g cm}−2 _s−1_, respective-ly.5) _{However, it has been reported by many researchers that} potentiodynamic electrolysis accelerates the dissolution rate in comparison to potentiostatic electrolysis.6–11) _Mitsushima

et al. have reported that the Pt dissolution rate depends on the profiles of the potential, and they have obtained a rate of 9.0 × 

10−9 _{g cm}−2 _s−1 _{in 0.5 mol L}−1 _{H2SO4 when a slow cathodic} triangular wave is applied.6) _{Topalov et al. have shown that Pt} dissolution occurs in both oxidation and reduction potential sweeps by using an online Pt detection system combined with an electrochemical flow cell and inductively coupled plasma mass spectrometry (ICP-MS).9) _{Yadav et al. have reported} that the existence of chloride anions in 0.5 mol L−1 _H

2SO4 solutions accelerates the Pt dissolution rate during potential cycling due to the formation of soluble chloro complexes, such as PtCl6− _{and PtCl}4−_.12)

Previous research has focused on improving catalyst per-formance or determining the mechanism for the decrease in the catalyst performance. Shiroishi et al. have adapted an electrochemical dissolution method to recover Pt and Ru from PEFCs.13) _{They apply potentiodynamic electrolysis to} dissolve noble metals from Pt-loaded electrodes using several potential wave profiles and have shown that a saw-tooth wave is 15 times faster than constant potentials are. In order to adapt the electrochemical dissolution method to actual fuel cells, it is necessary to confirm the dissolution of Pt metal from MEAs. Furthermore, it is worthwhile to achieve Pt dis-solution using simpler potential wave profiles, such as a square wave, for electrolysis control and comprehension of the reactions. In this study, we conducted noble metal disso-lution tests from Pt/Ru-loaded MEAs by applying square po-tential waves. Moreover, the tests were conducted in several solutions, and the effects of the concentrations of H+ _{and Cl}−

were evaluated.

2.  Experimental

Potentiostatic and potentiodynamic electrolyses and cyclic voltammetry (CV) were carried out using a conventional three-electrode method. An MEA, which had a thickness of 0.5 mm and was cut into 10 mm squares, was used as the working electrode. The amounts of Pt and Ru loaded in the MEA were 0.91 mg cm−2 _{and 0.41 mg cm}−2_{, respectively.} The counter electrode was a Pt plate, and the reference

elec-*

trode was an Ag/AgCl/saturated KCl electrode (RE-1C, ALS). The tip of the reference electrode was enclosed in a ceramic filter to avoid the diffusion of KCl. In this paper, all potentials were converted from V vs. Ag/AgCl to V vs. SHE. Hydrochloric acid (ASS grade, Kanto Chemical) and sulfuric acid (IC grade, Kanto Chemical) were used as electrolytes after diluting them to the proper concentrations. The elec-trodes and the electrolytes were set up in an H-shaped cell, which had two chambers separated by proton exchange mem-branes. To confirm the electrochemical behavior of the pre-pared MEA, a CV test was performed in 1 mol L−1 _{HCl in} advance. A potentiostat (1280Z, Solartron) was used to con-trol the potential of the working electrode and generate square potential waves, as shown in Fig. 1. Here the signs of the ox-idation and reduction potentials were defined in relation to the immersion potential of the MEA. During the electrolysis tests, the electrolytes were stirred with a magnetic stirrer. Af-ter the electrochemical dissolution tests, the concentrations of Pt and Ru in the electrolyte solutions were measured by using inductively coupled plasma atomic emission spectrometry (ICP-AES, SPS-3520DD, SII). All electrochemical dissolu-tion tests were performed at room temperature.

3.  Results and Discussion

3.1  Cyclic voltammetry of the prepared MEA electrode

Figure 2 shows a cyclic voltammogram of the prepared MEA working electrode in 1 mol L−1 _{HCl at a scan rate of} 10 mV s−1_{. The vertical axis is the current density calculated} from the geometric area of the MEA. An increase in the anod-ic current corresponding to Cl2 gas evolution was observed around 1.3 V. From the intersection point of the voltammo-gram in the anodic region and the horizontal line correspond-ing to 0 mA cm−2_{, the potential for Cl2 gas evolution using} the MEA working electrode was estimated to be 1.35 V. The result agrees with the reported value for the reaction below.14)

Cl2(g)+2e−=2Cl− E◦=1.3583V vs.SHE (1)

In the cathodic region, an increase in the current density was observed around 0.7 V, which corresponds to the reduction of Pt oxide to Pt metal.11) _{The increase in the current density at} the cathode limit potential of 0 V is due to H2 gas evolution or an H adsorption reaction.

3.2  Electrochemical dissolution of Pt and Ru by using square potential waves

In order to confirm the effectiveness of potentiodynamic electrolysis for noble metal dissolution, potentiodynamic and potentiostatic electrolyses were conducted in 1 mol L−1 _HCl. A square potential wave was used as the potential wave pro-file in the potentiodynamic electrolysis because of its simple profile. In the potentiodynamic electrolysis, the MEA poten-tial was kept at 0.1 V for 30 s (reduction electrolysis), then the potential was changed to 1.5 V, and this potential was maintained for 30 s (oxidation electrolysis). This series of op-erations were repeated 60 times. In other words, the total ox-idation electrolysis time was 30 min. In the case of potentio-static electrolysis, a test was conducted at 1.5 V for 30 min. Figure 3 shows the time dependence of the Pt and Ru dissolu-tion ratios in both electrolysis tests. The dissoludissolu-tion ratio of metal is defined as:

Dissolution ratio=Wt/W0 (2)

Fig. 1 Diagram of the square potential wave and definitions of potentials and times.

Fig. 2 Cyclic voltammogram using an MEA working electrode in 1 mol L−1 _{HCl at R. T. Scan rate: 10 mV s}−1_.

where Wt is the mass of the metal dissolved in the electrolyte calculated from ICP-AES results, and W0 is the mass of the metal loaded in the initial MEA.

Platinum metal scarcely dissolved into the electrolyte when constant oxidation electrolysis was conducted, although Pt metal can dissolve by forming complexes with chloro ions in solution.3

[PtCl6]2−+4e−=Pt+6Cl− E◦=0.744V vs.SHE (3)

In the case of potentiodynamic electrolysis, 82% of the Pt metal dissolved after 60 min. Ruthenium also dissolved into the electrolyte when a square potential wave was applied. The amount of dissolution reached 100%, whereas only 5.8% of the Ru metal dissolved when constant oxidation electrolysis was used. The difference in dissolution behavior between po-tentiostatic and potentiodynamic electrolyses is due to the redox reaction of the Pt oxide layer on the Pt metal surface, as shown by the following description of the mechanism and Fig. 4.

(Step 1) Platinum metal is oxidized by the environment, such as water and air, and a thin Pt oxide layer, which has resistance against electrochemical dissolution, is formed.

(Step 2) The platinum oxide layer is reduced to Pt metal via reduction electrolysis, and the Pt metal surface is exposed.

(Step 3) The exposed Pt metal dissolves into HCl, via the formation of chloro complexes via oxidation electrolysis.

(Step 4) The platinum oxide layer forms again during the above oxidation electrolysis, and Pt dissolution stops (go back to step 2).

In Step 2, deposition of Pt metal should occur when Pt ions exist in the solution, and a suitable potential for Pt deposition is applied. That is to say, the reduction of the Pt oxide layer and the deposition of Pt metal are competing reactions. The deposition of Pt metal is not favorable for Pt recovery from MEAs because it decreases the current efficiency. In addition, in Steps 3 and 4, the dissolution of Pt metal and the formation of Pt oxide layer are competing processes. This mechanism can also be used to explain the Ru dissolution behavior. Therefore, it is necessary to investigate the effects of the po-tentials and holding times on Pt and Ru dissolution ratios in noble metal recovery from MEAs. However, the results show that, for achievement of noble metal dissolution, complicated potential waves, such as triangle or saw-tooth waves, are not necessarily needed. In other words, the cycling between an-odic and cathan-odic potentials, not the potential wave profiles,

is the key to achievement of noble metal recovery from MEAs.

3.3  Effects of potentials and holding times on Pt and Ru dissolution ratios

To investigate the effects of reduction and oxidation poten-tials and the holding times on Pt and Ru dissolution ratios, electrolysis tests with different potentials and holding times were conducted. Figure 5 shows the dependence of the disso-lution ratio on the oxidation potential in 1 mol L−1 _{HCl when} the reduction potential was 0.1 V, at which hydrogen gas evo-lution does not occur. The holding times per cycle for both oxidation and reduction electrolyses were 15 s. At 1.3 V, at which Cl2 gas does not evolve on the basis of the CV results, the Pt and Ru dissolution ratios were 4.8% and 12.8%, where-as when the oxidation potential wwhere-as 1.4 V, they increwhere-ased to 79.7% and 80.6%. The maximum dissolution ratios reached 93.2% and 98.4% at 1.5 V, respectively. However, both disso-lution ratios decreased when the positive potential was great-er than 1.7 V. In the potential region from 1.7 to 1.9 V, the formation of the oxide layer was accelerated, and the metal dissolution reactions were blocked. Moreover, in the case of

Fig. 4 Schematic model of Pt dissolution using potentiodynaimic electrolysis.

Ru, Ru oxide, such as RuO2, can be oxidized to RuO4, which has a boiling point of 313 K.15)

RuO4+4H++4e−=RuO2+2H2O E◦=1.387V (4)

Sato et al. have reported that RuO4 volatilizes during electro-chemical oxidation in nitric acid solution.16) _{By analogy, the} decrease in the Ru dissolution ratio indicates that a volatiliza-tion reacvolatiliza-tion occurs at the same time as the formavolatiliza-tion of the oxide layer. Figure 6 shows the dependence of the dissolution ratio on reduction potential when the oxidation potential was 1.5 V in 1 mol L−1 _{HCl. The platinum dissolution ratio} in-creased when the cathodic potential was inin-creased, and a Pt dissolution ratio of 95.2% was obtained at 0.5 V. In the 0.5– 0.1 V potential region, the Pt dissolution ratio was nearly con-stant at about 90%. The results indicate that the Pt deposition reaction during reduction electrolysis is not dominant in the 0.5–0.1 V potential region. The ruthenium dissolution ratio increased linearly, and an Ru dissolution ratio of 98.4% was obtained at 0.1 V.

Figure 7 shows the dependence of the dissolution ratio on the holding time during oxidation electrolysis in 1 mol L−1 HCl when the reduction time per cycle was 15 s, and the re-duction and oxidation potentials were 0.1 and 1.5 V, respec-tively. The total oxidation times were 30 min. Dissolution ratio curves for both Pt and Ru are concave down. The local maximum values of the Pt and Ru dissolution ratios were de-termined to be 93.2% and 98.4% when the reduction time was 15 s. For oxidation times over 15 s, the dissolution ratios de-creased, indicating that competitive reactions occurred during oxidation electrolysis. The noble metal dissolution reaction was dominant for 15 s. After 15 s, the metal oxide layer for-mation reaction became dominant, and the metal dissolution reaction was suppressed by the oxide layer. Therefore, long oxidation times have a negative effect on the metal dissolu-tion reacdissolu-tion, and dissoludissolu-tion ratios decrease. Figure 8 shows the dependence of the dissolution ratio on the holding time during reduction electrolysis with a oxidation time per cycle of 30 s. The other test conditions were the same. The

dissolu-tion ratio curve for Pt is concave down with a local maximum value of 90.0% when the reduction time per cycle was 15 s. This result suggests that the reduction of the Pt oxide layer is slow. In other words, it takes 15 s to obtain a Pt metal surface that can undergo electrochemical dissolution. The Pt dissolu-tion ratio decreases when the reducdissolu-tion time is 30 s due to the competitive Pt deposition reaction. In the case of Ru, the dis-solution ratio curve is not concave down, and the rate mono-tonically increases for reduction times less than 30 s.

3.4  Effect of proton and chloride concentrations on the Pt and Ru dissolution ratios

We showed that it was effective to conduct the potentiody-namic electrolysis for Pt and Ru dissolution in 1 mol L−1 HCl. To verify the effects of the solutions for electrochemical dissolution, electrolysis tests with different proton and chlo-ride concentrations were conducted. The oxidation and

re-Fig. 6 Dependence of the Pt and Ru dissolution ratios on the reduction po-tential in 1 mol L−1 _{HCl at R. T. Oxidation potential: 1.5 V vs. SHE.} Ox-idation and reduction times were 15 s cycle−1_{, and the cycling was} repeat-ed 120 times.

Fig. 7 Dependence of the Pt and Ru dissolution ratios on oxidation time in 1 mol L−1 _{HCl at R. T. Reduction time was 15 s cycle}−1_{. Oxidation and} reduction potentials were 1.5 and 0.1 V vs. SHE, respectively.

duction potentials were 1.5 and 0.1 V, respectively. The hold-ing times were 15 s per cycle, and the cycle was repeated 120 times. Figure 9(a) shows the dependence of the Pt and Ru dissolution ratios on Cl− _{concentration. The H}+ _{concentration} was 1 mol L−1_{, and the Cl}− _{concentration was varied by}

mix-ing HCl and H2SO4. The Pt and Ru dissolution ratios were 1.1% and 11.4%, respectively, when the Cl− _{concentration}

was 0.01 mol L−1_{. However, the dissolution ratios linearly} in-creased versus the logarithm of the Cl− _{concentration. As}

mentioned in Section 3.2, the noble metals form soluble chloro complexes under the electrolysis conditions. There-fore, the observed rate behavior is reasonable. Figure 9(b) shows the dependence of Pt and Ru dissolution ratios on the H+ _{concentration with a Cl}− _{concentration of 1 mol L}−1_{. To} maintain a Cl− _{concentration of 1 mol L}−1_{, NaCl was added} to the electrolyte solution. When the H+ _{concentration was} 0.01 mol L−1 _{(0.01 mol L}−1 _{HCl +  0.99 mol L}−1 _NaCl solu-tion), 50.6% of the Pt and 27.9% of the Ru dissolved during potentiodynamic electrolysis. In a solution of 0.5 mol L−1

the use of strong acids, such as aqua regia.

4.  Conclusions

To recover noble metals from MEAs of spent PEFCs, an electrochemical dissolution method involving potentiody-namic electrolysis was developed. By using a square potential wave, 93.2% of the Pt and 98.4% of the Ru dissolved from a MEA in 1 mol L−1 _{HCl at room temperature. The dissolution} ratios were affected by the oxidation and reduction potentials and holding times at those potentials. In addition, the effects of the H+ _{and Cl}− _{concentrations on the Pt and Ru dissolution}

ratios were investigated. The Pt and Ru dissolution ratios were 50.6% and 27.9%, respectively, in a solution with an H+ concentration of 0.01 mol L−1 _{and a Cl}− _{concentration of}

1 mol L−1_{; however, 1.1% of the Pt and 11.4% of the Ru} dis-solved in a solution with an H+ _{concentration of 1 mol L}−1 and a Cl− _{concentration of 0.01 mol L}−1_{. The results indicate} that Cl− _{anions are required to dissolve Pt and Ru metals as}

chloro complexes and H+ _{cations promote the dissolution} ra-tio of Pt and Ru metals.

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Fig. 9 Dependence of the Pt and Ru dissolution ratios on the (a) Cl− _{and (b)}