Recovery of Platinum and Other PGM’s from Spent PEM Fuel Cells

The Pennsylvania State UniversityZachary Omcikus

Joseph SmithKehui Zhang

4 / 2 4 / 2 0 1 3

Matse 426 Final ProjectIn the process of recycling valuable metals and the PEM, proton exchange membrane, several processes need to take place. The proposed process to be outlined first specifies a physical tear down of the cell followed by a chemical separation of the Nafion© PEM from the platinum anode, the platinum and palladium cathode, and the carbon particle sheet holding the PGMs, platinum group metals. The Nafion© is then separated by adding DMSO and 0.5M of NaOH. The PGMs remaining are separated by creating a palladium hydroxide precipitate followed by a solid/liquid separation. The Palladium is then re-dissolved and the resulting hexachloro-complexes of palladium and platinum are electroplated to a palladium and platinum cathode respectively. Economic and practical considerations were weighed and a complete process flow sheet was created.

Introduction

Polymer Electrolyte Membrane (PEM, also known as Proton Exchange Membrane) fuel

cells provide an exciting source of energy to rival currently used fossil fuels. They were first

developed in the 1960s by General Electric to act as a fuel source for the space shuttle [1]. PEM

fuel cells can be used for stationary and portable power production. Regarding portable power,

PEM fuel cells are considered a leading energy option to replace gas and oil [2]. PEM fuel cell

technology is creating new opportunities to potentially provide clean energy to millions of

Americans every day.

There are numerous benefits for switching to PEM fuel cells for energy production. One

major advantage is that PEM fuel cells function with a wide variety of fuel sources. This allows

for diversified fuel options so one fuel source will not be drained at too great a rate. Functioning

with many different fuel sources also allows for the use of environmentally friendly energy

sources (solar, wind, hydro, etc). Dependence on foreign oil will dramatically decrease if

vehicles are no longer primarily powered by gasoline [3].

PEM fuel cells in cars and for stationary energy production reduces the amount of air

pollutants emitted. Cars and trucks

produce greater than fifty percent

of greenhouse gas emissions for a

typical household [4]. They emit

volatile organic compounds and

nitrogen oxides that mix to form

smog (see Figure 1).

Figure 1: Smog seen in Los Angeles (source: Al Pavangkanan via flickr)

PEM fuel cells are composed of layers that perform specific functions. They must

contain two electrodes (anode and cathode) that are separated by a membrane. The anode and

cathode are typically

composed on carbon

paper with platinum and

other platinum group

metal (PGM) catalyst

particles attached to the

carbon paper. The

polymer membrane is

usually made of a

perflourosulfonated acid –

DuPont’s® Nafion© is commonly

used [1] – that acts as a one way membrane.

The anodic oxidation takes place when hydrogen gas is pumped through the PEM fuel

cell. The PGM catalyst causes a reaction to take place in which hydrogen gas is oxidized to

hydrogen ions and electrons. The polymer electrolyte membrane allows hydrogen ions, but not

electrons to pass. The electrons must pass around the membrane and result in electrical energy.

At the cathode, oxygen from the air is reduced and reacts with hydrogen ions that pass through

the membrane to form water [1]. The reactions are as follows:

Anode: H2 → 2H+ + 2e-

Cathode: ½ O2 + 2H+ + 2e- → H2OOverall: H2 + ½ O2 → H2O

Figure 2: overview of PEM fuel cell (source: electrochem.cwru.edu)

A single fuel cell produces just over one volt (the lithium ion batter in your cell phone

produces 3.7 volts). In order to produce enough electricity to power a vehicle or provide power

at a stationary site, individual fuel cells need to be

stacked together [5]. Stacks contain many

individual fuel cells held together by end plates, seals, and cooling plates. There are also flow

field plates that allow the hydrogen gas and air to contact the anode and cathode respectively.

Figure 3 shows a fuel cell stack containing six individual membrane electrode assemblies.

Platinum in PGM fuel cells serves as the catalyst to the reaction, meaning it facilitates the

reaction that takes place in the fuel cell. When the hydrogen fuel enters the fuel cell, it comes

into contact with the anode material, the platinum catalyst. This catalyst can be seen by the

number two in the visual below. The hydrogen, in the form of the diatomic H2 gas, reacts with

the platinum and splits into two Hydrogen ions. In the splitting of the diatomic hydrogen

molecule, two electrons are released per molecule split which are allowed to flow through a

circuit connected at the other end to the other side of the cell, the cathode side. The protons, or

hydrogen ions, created from the splitting of diatomic hydrogen are then allowed through the

Figure 3: PEM fuel cell stack (source: Mehta and Cooper)

PEM, Polymer Electrode Membrane, to be combined at the cathode with the oxygen flowing

through the other side of the membrane producing the waste material, water. [8]

Without the platinum catalyst, the hydrogen would not have the required energy to split

into ions at the cathode, and

therefore would produce no

electricity. This is forced to

happen because the platinum

cathode is able to lower the

activation energy needed to split

the hydrogen molecule into two

ions. The mechanism that creates

this reaction is that hydrogen has

a lower potential in the ionic

state, surface bound to the platinum catalyst. After the electron is drawn away by the electrical

wires connected, the hydrogen is driven through the membrane to the oxygen on the cathode side

to reduce the oxygen and create the byproduct, water.

The fuel cell is made up of only a few basic parts; the proton exchange membrane

(PEM), the anode, the cathode, and the shell. First, and most importantly, there must be proton

exchange membrane, as mentioned previously, allows for the passage of hydrogen ions from the

cathode to the anode, and the reaction would not be possible if not for this membrane. This is

generally made of Nafion©, which is proprietary DuPont® polymer. This polymer is unique in

that it is permeable to hydrogen ions, protons, but not permeable to electrons. This allows for the

catalyst that splits the hydrogen, and the catalyst that combines the protons and oxygen gas into

Figure 4: PEM Fuel Cell Schematic [9]

water to be separate.

The second part of a fuel cell is the anode. This part is necessary because the platinum

catalyst which is at the anode is what allows for the splitting of diatomic hydrogen into hydrogen

ions. The mechanisms of this reaction were previously described. The hydrogen is allowed to

pass by the anode by inlet holes in the casing above and below the anode. The platinum anode

can be held together in various ways, but in this paper it will be considered as platinum particles

housed on a sheet of carbon paper. This carbon paper is also particulate in nature, and can be

particularly problematic when the fuel cell end of life comes to be, and the platinum is desired to

be recovered. The carbon paper, which is typically burned off, will combust to carbon dioxide

gas polluting the atmosphere, so a more environmentally friendly separation method will be

proposed later.

The next part of the fuel cell is the cathode. This is situated on the other side of the

Nafion© from the anode as shown in the above figure of the entire cell. The cathode itself is

typically composed of platinum, and in higher concentration another PGM, such as ruthenium,

palladium, or rhodium. The ratios of these two metals can vary based on manufacturer and

economic efficiency considerations, but they are typically of similar amounts, or of less

platinum. Similar to the anode, the cathode is also held together by a particulate carbon paper,

and is plagued by the same end-of-life-separation issues. Along the cathode, there are inlets in

the casing that allows oxygen to flow past and react with the hydrogen ions coming through the

PEM.

The last part of a PEM fuel cell is the shell to house the cell. This shell can be made of

various materials, but with heavy water and hydrogen exposure, corrosion resistance is an

important factor in material selection. This has given rise to aluminum casing due to its passive

layer corrosion resistance. The casing has several important features, but the most important is

the inlets for hydrogen and oxygen, and the outlets for unused hydrogen, unused oxygen, and cell

waste, water. The whole cell casing generally houses many layers of fuel cell, and is bolted and

sealed tightly with gaskets to ensure no leak of fuel or waste to ensure expected efficiency.

The figure to the left shows a

commercially available PEM fuel cell

system. The main features are easily

visible. At the top, tubes allow for the

entry of hydrogen gas on one side, and

oxygen gas at the other. Waste is managed

at the bottom through another set of tubes.

Each of these tubes is regulated by sensors

and a computer system to allow for fuel

pressure regulation, on the fly efficiency

checking, and changes in power output

based on need. This particular system can supply three kilowatts of power, contains 72 fuel

cells, and is approximately 40% efficient at providing energy from its 99.999% dry hydrogen

fuel.[12]

There are several difficulties that may be encountered when recycling fuel cells from

scrapped vehicles. The main impediment to recycle fuel cells is that membrane-electrode

assembly (MEA) contains high fluoropolymer content, which may cause undesired emission of

fluorine and hydrogen fluoride (HF) gas [11]. Since HF is highly toxic and corrosive, reuse of

fuel cells from scrapped vehicles become very difficult. Secondly, as we discussed in the

Figure 5: Commercial PEM Fuel Cell System [8]

following session, although the recovery of Nafion is approachable, the processes are

complicated and costly. The whole recycle processes need high temperature, which require a lot

of energy. In addition, the recycling technology is not very practical to be used in relevant

industry.

The platinum catalyst has been identified as one of the major cost contributors to the

PEM fuel cell material. Currently, platinum is identified as the most viable catalyst for PEM fuel

cell systems. However, at today’s platinum loading and price, a 50-kW fuel cell contains

approximately 46 g of platinum costing $2200. [12] Historically, platinum prices have been

sensitive to changes in demand, and the widespread development of fuel cells might significantly

drive up platinum demand and hence platinum prices. In addition, the commercialization of PEM

fuel cell systems will result in an increasing demand of the platinum group metals. Therefore, the

high cost as well as the long-term availability of platinum makes recycling of platinum

necessary.

MEA Separation

A stack of proton exchange membranes (PEMs) is called membrane electrolyte assembly

(MEAs). An MEA consists of electrolyte film, two electrode sheets made of carbon particles

carrying PGM catalysts, and two diffusion layers such as carbon papers. [13] A schematic

diagram of MEA is shown in Figure 1.

Figure 6 Schematic diagram of a MEA (5 layers) sample. [13]

The electrolytes films need to permit hydrogen ion to transport while prevent electron

conduction. Since Nafion was proved to possess this quality, it was widely used as the electrolyte

films. The separation of platinum group metals and Nafion can be done in both chemical

methods such as dissolution and/or incineration and physical methods such as solid-solid

separation. The technique we chose in our project employs most physical methods. According to

Tatsuya Ok et al., (2009) the separation can be achieved by adding organic solvents with high

ambient temperature. [13] The organic solvents accelerate the detachments of the electrode

particles, so does the high temperature. The study has shown that ethanol with less 25%

concentration generates the best separation results with 270 °C ambient temperature while

minimize any deformation of the electrolyte film. After the MEA is soaked in organic solvent,

ultrasonic wave irradiation will be added to increase the effectiveness in peeling the carbon paper

diffusion layers from the MEAs. [13] The carbon paper can be then separated from the Nafion

membrane. The whole schematic processes are shown in Figure 2 below.

Figure 7: Separation of Carbon Paper and Nafion Membrane

The recovery of carbon paper with PGM will be discussed in later sessions in this paper.

Since Nafion membranes contain stable polytetrafluoroethylene backbone, which may cause

severe environmental problems, sustainable technologies to recycle Nafion membranes are vita

important and economically efficient. Nafion membrane can be recycled using dissolution and

recasting. [14] Nafion membrane is typically insoluble at room temperature in organic solvent

and water. H-F Xu et al., (2002) proposed in their paper that by introducing a strong polar and

high boiling point solvent will facilitate the dissolution process which makes Nafion membrane

to dissolve at atmospheric pressure. [14] The solvent proposed in the paper was dimethyl

sulfoxide (DMSO). The obtained Nafion membranes can be put into 3-5% H 2 O2 solution and

boiled for one hour. Then 0.5M NaOH will be put in to the solution to convert the membranes to

N a+¿ ¿form. This step will prevent the membrane from degradation during the high temperature

dissolution. DMSO solution will be then added to the converted membranes and heated to 190

°C until all the membranes were totally dissolved to form a ~2wt% Nafion solution. The DMSO

solution with Nafion isomer will then be put in a petri dish and heated up to 150 °C until all the

DMSO evaporate. After that, the Nafion on the petri dish will be heated 2 more hours at 170 °C.

High purity water will then be needed to detach the recovered Nafion membrane from the petri

dish [14].

Platinum and Palladium Dissolution (with Hydrogen Peroxide and Hydrochloric Acid)

Once the membrane electrode assembly (MEA) has been delaminated in a tank by

alcohol, a filter cake containing platinum, palladium, and carbon must be handled. To produce

platinum and palladium, the cake must be further processed. One way to separate platinum from

palladium and palladium from carbon is to selectively dissolve and precipitate the three metals.

To do this, platinum and palladium will be dissolved in one step that reduces carbon to methane

gas (CH4).

The highly corrosive acid mixture, aqua regia (HNO3 + 3HCl), has commonly been used

to dissolve platinum to form aqueous chloroplatinate (PtCl62-). Using aqua regia for dissolution

leads to the emission of nitrosyl chloride gas (NOCl), nitric oxide gas (NO), and nitrogen dioxide

gas (NO2). These gasses help cause acid rain and ozone depletion. For this reason, it is

necessary to find another solution.

Kizilaslan et al. showed that a mixture of

hydrogen peroxide (H2O2) and hydrochloric

acid (HCl) will react without producing

nitrogen containing gasses. Kizilaslan et

Figure 8: Pt dissolution as a function of temperature [6]

al. also showed that increasing temperature provides industrially acceptable Pt dissolution rates

that are similar with a mixture of H2O2 and HCl or aqua regia (see figure 4) [6]. The following

equation has been provided to show the reaction between the present metals and the H2O2 and

HCl mixture:

H2O2 + 12HCl + Pt + Pd + 25C + 4e- → PtCl62-

(aq) + PdCl62-

(aq) + 2.5CH4 + 2H2O

ΔG=ΔGoH2 O2+12 ΔGo

HCl+ ΔGoPt+ΔGo

Pd+25 ΔGoC−ΔGo

PtCl 6−ΔGoPdCl6−2.5 ΔGo

CH 4−2 ΔGoH 2O=−120.4+12 (−131.228 )−(−430 )−2.5 (−50.72 )−2 (−237.129 )=−181.38

kJmol

Since the calculated ΔG value is less than zero, the reaction will proceed. The methane gas

produced will be sold or reused in the process as a source of heat, while the aqueous products

will be further processed.

Separating Platinum and Palladium

Following the dissolution of platinum and palladium, the problem then arises of how to

separate the two metals because they are often desired in their pure form. Therefore, there is a

few ways common in which to separate the two. One method is adding sodium bromate

(NaBrO3) to the solution containing the two hexachloro-complexes.[4] This method involves

first diluting the solution containing the complexes with water to approximately 3:1.[4] After,

the sodium bromate is added to create palladium hydroxide(Pd(OH)4(s)). [4] This solution is

brought to a boil and allowed to boil for approximately an hour and a half.[4] This provides the

energy necessary to form the palladium hydroxide, which is a dark yellow solid precipitate.[5]

The sodium bromate facilitates the reaction in that it allows for favorable kinetics in the

precipitation of Palladium. The platinum, though, will not react allowing for the physical

separation of the two metals. This reaction will have to be closely monitored for Eh and pH to

be sure one remains within the precipitation window of palladium hydroxide without

precipitating platinum’s hydrated solid. The precipitation window shown via the overlayed Eh-

Figure 9 Overlaid Eh-pH Diagrams Pd-Cl-H2O & Pt-Cl-H2O

pH diagrams of the Pt-H2O-Cl system and the Pt-H2O-Cl, and the relevant balanced chemical

equation are as follows. The purple hatched area is the precipitation window which must be

maintained in order to be sure unwanted reactions are prohibited, and desired products are

efficiently produced. The arrow shows the direction the pH will need to go to allow the reaction

to take place. The ΔGr of the following reaction is also calculated showing its tendency to go

forward with a negative Gibb’s energy.

PtCl62- + PdCl6

2- + NaBrO3 + H2O + 2H+→ PtCl62- + Pd(OH)4(s) + NaBr + 3Cl2(g)

ΔGr = ΔGproducts – ΔGreactants = (-175.9 – 242.62) – (-430 -242.62 – 237.129) = -48.77 KJ/mol

One of the first things that one realizes when observing this reaction is that there is

chlorine gas created which can be very detrimental to human health, and cannot ethically be

released into the environment. Therefore, an additional step must be taken to assure the chlorine

Pd

Pd(OH)4

Pd(OH)2

Pd(Cl4)2-

Pd(Cl6)2-

gas does not make its way out of the plant. Fortunately other industries have need for chlorine

gas which they themselves do not produce. That has given rise to specialty gas producers such

as Advanced Specialty Gasses based in Reno, Nevada who sell tanks of chlorine gas for a profit.

Therefore, the gas that is produced can be fairly easily bottled through a series of tubes,

regulators, and tanks and subsequently sold to a retailer at little loss or potentially a profit.

A separate option for dealing with the escaping chlorine gas is using it to create

hydrochloric acid. When one combines hydrogen gas and chlorine gas in the right environment,

HCl gas will be created and can subsequently be condensed and fed back into the steps that

consume HCl. The hydrogen gas does not necessarily need to be bought, but additional may be

needed. This is because the subsequent dissolution of palladium hydroxide creates hydrogen gas.

This creation of hydrogen gas will not produce enough to consume all the chlorine gas, so this

will more than likely need to be supplemented with additional purchased hydrogen gas. Though

this is an additional monetary burden, the benefit of not needing to purchase HCl can outweigh

the cost. One can know this reaction will happen spontaneously due to the fact that the Gibb’s

energy of reaction is negative. The calculation is as follows:

H2(g) + Cl2(g) → 2HCl(g)

ΔGr = ΔGproducts – ΔGreactants = 2(-95.30) – (0+0) = -190.6 KJ/mol [12]

Since this reaction is done in a non-aqueous, gaseous system, aqueous stability diagrams need

not be considered, and this reaction can simply be done in a system of pipes, tanks, agitators, and

condensers to create a relatively concentrated HCl product.

Next, a solid liquid separation step, by means of vacuum assisted filtering, will separate

the solution containing hexachloroplatinate(IV) (PtCl62-) and the solid palladium hydroxide. This

leaves the hexachloroplatinate(IV) and palladium hydroxide separate and in forms that can be

purified through further processing. In the case of palladium hydroxide, a non-oxidative step

utilizing hydrochloric acid (HCl) will reform the hexachloropalladate(IV) by means of

dissolution. This product, separate from hexachloroplatinate(IV) will be purified in a later step.

The balanced chemical reaction for this step is shown below along with the relevant Eh-pH

diagram with the dissolution window and reaction path shown as before. Since there is no

platinum in the system in this step, the platinum diagram does not need to be overlayed, and the

dissolution window can still be viewed.

Pd(OH)4(s) + 6HCl → PdCl62- + 2H2O + 2H+ +2H2(g)

The Gibb’s energy of reaction of this reaction, as calculated before, comes out to be -145.8

KJ/mol, making the reaction favorable and spontaneous. As previously mentioned, the hydrogen

gas produced by this step can be re-utilized to create hydrochloric acid to be stored and used in

the future for this same reaction and subsequent ones. The aqueous platinum formed from this

reaction can be formed into a solid platinum product by several methods; one such method is

discussed below.

Figure 10 Eh-pH Diagram Pd-Cl-H2O

Pt and Pd Precipitation

Once hydrochloric acid is added to Pd(OH)4, aqueous PdCl62- is produced. In another

stream, aqueous PtCl62- is present. These two hexachloro complexes will be treated to precipitate

palladium and platinum respectively. In order for the precipitation to occur, electrolysis will be

used to provide hydrogen ions and reductive electrons. For the situation at hand, platinum will

be precipitated from the aqueous hexachloroplatinate ion (PtCl62-) and palladium will be

precipitated from the aqueous hexachloropalladate ion by electroplating. In electroplating,

current is used to reduce a metal ion so that it forms a precipitate on an electrode. In this case,

current is used to reduce hexachloroplatinate and hexachloropalladate by the following reactions:

PtCl62-

(aq) + 6H+ + 4e- → Pt(s) +6HCl PdCl62-

(aq) + 6H+ + 4e- → Pd(s) +6HCl

The reduction paths for platinum and palladium precipitation can be seen in figures 5a and 5b

respectively [7].

For the reactions to proceed, electricity must be added to the cell. The anode will be

made of zinc, which breaks down as follows: Zn = Zn2+ + 2e-. Zinc ions will be released in

solution with hexachloroplatinate. In order to deposit platinum to a platinum cathode, the

reduction of hexachloroplatinate must occur before the reduction of zinc ions. The same goes for

Figure 11-a: Platinum reduction path (source: Harjanto et al.)

Figure 11-b: Palladium reduction path (source: Harjanto et al.)

depositing palladium – rather than zinc – to a palladium cathode. Reduction windows for

platinum-zinc and palladium-zinc are shown in figures 6a and 6b respectively [7].

Once it has been determined that platinum and palladium will deposit on their respective

cathodes, the amount of energy required to force electrolysis must be calculated. The minimum

Figure 12a: reduction window for platinum and zinc

Figure 12b: reduction window for palladium and zinc

electrolyzing voltage for platinum was calculated as follows (F is Faraday’s constant and z in the

number of electrons involved in the reaction):

Emin=ΔGzF

=451.728

kJmol

2(96.487kJ

mol−V)=2.34 V

PtCl62- + Zn + 6H+ + 2e- = Pt + 6HCl + Zn2+

where ΔG=ΔGoPdCl 6+ ΔGo

Zn+6 ΔGoH +¿−ΔGo

Pd−6 ΔGoHCl−ΔGo

Zn 2+¿=−430+0 +0−6 (−131.228 ) −(−147.06 )=451.728 kJmol

¿¿

Likewise, the minimum electrolyzing voltage for palladium was calculated to be 2.61 V.

Changes if Only One PGM

If there was only one PGM, the situation would become much simpler, but very similar

processes can be used. The first difference would be that there would no longer be a need for

sodium bromate due to not needing to precipitate out one PGM. Therefore, the metal could

simply be dissolved without concern as to which metals are involved. After that, a solid liquid

separation step would take place to separate any unreacted metal along with any impurities. The

methane will still be acquired after the dissolution step, and can still be sold for profit. After

that, the solution can be electroplated as before to make a solid PGM. The HCl recycling can

still take place and feed the dissolution of the PGM in the beginning.

Initial Economic Considerations

When considering the economics of the proposed process, the overhead including labor

and facilities will be ignored for the time being. To create an estimate of how much net profit

can be made, a comprehensive list of inputs and outputs was made. It was found that during the

total reaction, a very small, starting amount of HCl needs to be added, and the rest can be

recycled back into the system. Other things that need to be added, per mole of PGM produced,

is one mole of sodium bromate costing $0.69, one mole of hydrogen gas costing $0.008, and

some amount of electricity to be analyzed later. The products, which are proposed to be sold to

market are platinum costing $9289.61 per mole, palladium costing $2427.46/mol, and methane

gas costing $1270.4/mol.

The amount of electricity needed to perform the electroplating process is yet unknown

due to the need for further testing and analysis. By finding the minimum electrolyzing voltage, it

can be calculated that this process will consume approximately 1.24x10-3KW/mol. In order to

decide the amount of KW*hrs needed to produce one mole of the PGMs, a mock process setup

would have to be created to show concentration versus time data. From this data one could

derive a rate law equation that would govern the process. This rate law would allow us to

determine the moles per hour produced, and dividing that by the amount of electricity needed per

mole would show the kilowatt hours needed for the whole process.

Doing and overall cost analysis reaction, the amount and cost produced have the input

cost needed subtracted from it. Doing this math shows that per mole of each PGM produced,

when producing both PGM’s, the total net profit is $12,986.7/mol.

Flow Sheet

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