A PEMFC and H2 membrane purification integrated plant

Available online at www.sciencedirect.com

Chemical Engineering and Processing 47 (2008) 1081–1089

A PEMFC and H2 membrane purification integrated plant Adele Brunetti a,b , Giuseppe Barbieri a,∗ , Enrico Drioli a,b a

Institute for Membrane Technology, ITM - CNR, Via Pietro Bucci, c/o The University of Calabria, Cubo 17/C, 87030 Rende CS, Italy b Department of Chemical Engineering and Materials, The University of Calabria, Cubo 44/A, Via Pietro Bucci, 87030 Rende CS, Italy Received 13 December 2006; received in revised form 22 March 2007; accepted 30 March 2007 Available online 19 April 2007

Abstract In this work an integrated system composed of a Pd–Ag membrane module and a polymeric electrolyte fuel cell (PEMFC) was studied. The Pd–Ag membrane module, allowing a pure hydrogen permeate stream to be fed directly to the PEMFC, was used as an H2 purification step with a mixture containing also CO and CO2 . The CO poisoning effect on the Pd–Ag membrane and its consequences on the hydrogen permeation were analysed in a temperature range 300–450 ◦ C in terms of permeance reduction conditioning also the H2 recovery index (RI). A comparison between the experimental data obtained feeding the fuel cell (FC) with pure H2 from a cylinder and from the purification step was also performed in order to evaluate the PEMFC performance stability. After the experimental study of each single step, particular attention was devoted to the analysis of the integrated system Pd–Ag membrane purification step—PEMFC, focusing on the influence of the Pd–Ag membrane module operating conditions on the electrical performance of the PEMFC. The PEMFC internal crossover was also considered and its effects on the electrical performance were taken into account by means of a PEMFC actual efficiency. Furthermore, integrated membrane plant chemical efficiency was defined considering the H2 converted into electricity with respect to the total amount of H2 contained in the mixture. The integrated Pd–Ag membrane purification with PEMFC shows very good results: the hydrogen is well purified and the PEMFC performance is very high near the nominal value. © 2007 Elsevier B.V. All rights reserved. Keywords: Membrane-PEMFC integrated plant; Pure hydrogen production; PEMFC; Efficiencies

1. Introduction A fuel cell (FC) is a device that directly converts chemical energy into low-voltage dc electricity. Most of the operational characteristics of the FC systems are superior to those of conventional power generation [1]. Among several types of FCs, the polymer electrolyte membrane (PEM) fuel cell is the most popular for transportation and portable applications. Hydrogen delivery for these applications must be high density (energy/weigh > 1) [2] and able to follow a varying load with only a few seconds lag at most. Furthermore, the hydrogen must be delivered relatively pure, containing few tens of ppm CO.

∗

Corresponding author. Tel.: +39 0984 492029; fax: +39 0984 402103. E-mail address: [email protected] (G. Barbieri).

0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.03.015

An immediate and abundant H2 supply for PEMFC could be derived from fossil fuel sources, for example via steam reforming or partial oxidation of light hydrocarbons. However, a critical stage in the realization of hydrogen economy is the production of cheap, reliable and commercially useful hydrogen. H2 may be separated from mixtures which contain other products such as CO, CO2 , N2 , H2 O, etc., coming out as products of the reforming or water gas shift reaction (upgrading step of reformate streams). One of the most promising H2 separation method applies Pd–alloy membranes, since they involve an efficient onestep separation process [3]. A Pd–Ag membrane purification step is constituted of a permeate where the H2 is recovered at the low pressure side and the retentate stream concentrated in the remaining other species. Furthermore, a Pd–alloy membrane showing infinite hydrogen selectivity gives a pure H2 permeate stream to employ directly in PEMFC. For this reason these devices can be successfully applied in the purification step of an integrated

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membrane plant for hydrogen production with a low CO content (<10 ppm). In this work, a Pd–alloy membrane purification step was integrated with a commercial PEMFC and the effect of the operating conditions on each single step and on the global system were evaluated also in terms of efficiency [4]. The CO poisoning effect on Pd–Ag membrane is an important key factor to be considered in the evaluation of the system performance. When a hydrogen-based mixture containing also CO is fed to a Pd–Ag membrane module, a decline in the hydrogen permeation can be observed and this effect is more evident increasing the CO concentration in the feed mixture. The CO adsorption on the Pd–alloy membrane surface exhibits a poisoning effect reducing the membrane area available to the permeation, and consequently the H2 permeating flux through the membrane [5] drastically drops down. In particular, CO poisoning [6–9] on the Pd–Ag membrane, affecting the hydrogen permeation, influences also the recovery index (RI), a useful index indicating the H2 extractive capacity of the Pd–alloy membrane purification step, represented by the ratio of the permeated H2 going to the PEMFC to the total hydrogen fed to the purification step. After the characterization of the Pd–Ag membrane module, the integrated membrane system was also evaluated in terms of electrical performance. The potentiometric tests were carried out and the polarization curves were obtained changing the operating conditions of the Pd–alloy membrane module [10]. When the hydrogen is fed to PEMFC another important effect to consider is the hydrogen crossover consisting in the molecular H2 permeation directly through the membrane without hydrogen dissociation as H+ ion. The crossover produces a reduction of the PEMFC electric performance and a partial loss of fed hydrogen. There is a strong relationship between the integrated membrane plant and the efficiency of the process [11–13]. In this optic,

the efficiency of the FC and the whole system were evaluated in terms of hydrogen streams involved in the process and their dependence on the operating conditions of the system was also analysed. 2. Materials and method The experimental apparatus used in the experiments is reported in Fig. 1. A stainless steel module containing the Pd–Ag commercial membrane was placed in a temperature controlled electric furnace (with PID control). Mass flow controllers (Brooks Instrument 5850S) were used for feeding all the inlet gaseous streams. The flow rates of the outlet streams from Pd–Ag membrane module and from PEMFC were measured by means of soap film bubble flowmeters. In particular, a three-way valve switches the Pd–alloy membrane module permeate stream to the bubble soap flow meter or to the PEMFC. Chemical analyses on the retentate and permeate streams were performed by means of a gas chromatograph (Agilent 6890N) with two parallel analytical lines. Each line is equipped with two columns: a HP-Plot-5A (for separating permanent gases such as H2 , N2 and CO) and a HP-Poraplot-Q (for other species) and a TCD. The GC configuration adopted in this plant allows (a) the analysis of the PEMFC outlet streams at the same time and (b) avoidance of any pressure variation on the reaction and permeation sides, with no modification of the steady-state. The PEMFC was placed in a temperature controlled electric furnace. The oxygen was fed by means of a mass flow controller (Brooks Instruments). Both the gaseous feed streams were humidified at the same temperature of the PEMFC. The FC temperature was measured using a thermocouple located near the electrodes. The current and voltage were measured by means of

Fig. 1. Scheme of experimental apparatus.

A. Brunetti et al. / Chemical Engineering and Processing 47 (2008) 1081–1089 Table 1 Pd–Ag membrane characteristics

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Table 3 Pd–Ag membrane module operating conditions

Membrane type

Pd–Ag commercial (Goodfellow)

Length (cm) Thickness (␮m) Superficial area (cm2 )

9 100 2.7

Temperature range (◦ C) Feed pressure range (kPa) Permeate pressure (kPa) Feed flow rate range (cm3 (STP)/min) Feed composition

H2 CO CO2 N2

Fig. 2. Pd–Ag membrane cross-section. SEM photo.

an amperometer and a voltmeter, respectively, and an electronic load was used. The Pd–Ag self-supported membrane characteristics are resumed in Table 1. Fig. 2 reports the cross-section of the Pd–Ag membrane. Some cavities are shown in the cross-section, however they are not interconnected and this is also confirmed by no N2 or other gas permeation. The PEMFC used in the experiments is a commercial FC with a Nafion117® polymeric membrane; its characteristics are reported in Table 2. The composition of the electro-catalyst present in the MEA is unknown. The experiments were carried out in two phases. Initially, the Pd–Ag membrane module was characterized by means of permeation tests, in terms of flux, permeability, H2 recovery. The second part of the work regarded the integration of the Pd–alloy membrane purification step with the PEMFC and the evaluation of the behavior of the whole system in terms of electrical performance, analyzing also its dependence on the operating conditions of the Pd–alloy membrane purification step and identifying the best ones in terms of FC power measured. The Pd–Ag membrane module operating conditions are reported in Table 3. The choice of the operating temperature range of 300–450 ◦ C is due to the necessity to avoid the critical temTable 2 PEMFC characteristics Type

PEMFC (Nafion117® )-UDOMI-ECO commercial

Membrane area (cm2 ) Feed type Nominal voltage (mV) Nominal power (W)

15 H2 /O2 400–960 1

Pure gas

100%

300–450 200–650 100 10–50 Mixture 1

2

80% 10% 10% 0

44% 49% 6% 1%

perature (200–280 ◦ C), where the Pd–Ag membrane shows a transition phase (Shu et al. [14]) with consequent membrane embrittlement, and also to reduce the CO poisoning effect, stronger at a lower temperature. In order to study the feed pressure effect on the hydrogen permeation and its advantage on the recovery index ranging among 200–650 kPa was analysed, maintaining the permeate pressure at 100 kPa. No sweep gas was used in the experiments. The CO poisoning effect on the Pd–Ag membrane was studied feeding two different mixtures in addition to pure H2 stream. Mixture 1 has a typical reformate stream composition, similar to a mixture of industrial interest. The high CO content in mixture 2 is useful to evaluate the CO poisoning effect on the system in an evident way. The performance study of the integrated system was the main objective of this work. With this logic no analyses were performed on PEMFC behaviour in dependence on its operating conditions which were assumed to be the same in all the tests. In particular, the PEMFC is operated at 70 ◦ C with an oxygen addition in equimolecular feed ratio with the hydrogen coming from the purification step. Both the humidified inlet gases are fed at a feed pressure of 100 kPa and the H2 feed flow rate ranges between 3 and 40 cm3 (STP)/min. The Pd–Ag membrane purification step was characterized by means of permeation tests. The H2 permeation through a Pd–Ag membrane happens with the dissociative–adsorption–desorption mechanism and, when the hydrogen permeation is controlled by metal bulk diffusion, the flux through the membrane can be described by Sievert’s law (Eq. (1)). JHSievert = 2

Pe0 e−E/RT  Feed side  Permeation side − PH2 ) ( PH2 thickness

(1)

The H2 coming out from the permeate side of the Pd–Ag membrane module was fed to the PEMFC. Fig. 3 schematically summarized the possible routes of the hydrogen into the PEMFC. The different streams considered in the integrated plant are: F0 : hydrogen fed to the integrated plant (pure H2 or H2 in mixture 1 or H2 in mixture 2 stream); F1 : hydrogen in the retentate of the purification stage;

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Fig. 3. Hydrogen distribution in PEMFC scheme.

F2 : hydrogen permeated the Pd–alloy membrane and then fed to the PEMFC; F3 : hydrogen loss for crossover leaving the PEMFC in the cathode purge stream as molecule; F4 : hydrogen following the desired way with its dissociation in ions and electrons on anode: the ions are transported through the PEM and the electrons travel through the external electric circuit. Hydrogen producing electricity; F5 : hydrogen permeated through the membrane as molecule and then reacted on the cathode with oxygen to give water without electron circulation in the external electric circuit; F6 : hydrogen reacted on the anode after oxygen permeation through the PEM. No electron circulation in the external electric circuit takes places; F7 : hydrogen leaving the PEMFC in the anode purge stream.

to the hydrogen consumed: i = nH2 Faraday F4

In an analogous way, the hydrogen loss can be expressed in terms of an equivalent current (iFuel loss ) (Eq. (5)) that could be supplied by the FC if all the H2 loss for the crossover reacts electrochemically [15]. It is important to consider that the crossover effect is due also to the oxygen permeation through the membrane, which reaching the anode reacts with the H2 without external current production. The Eq. (6) reports the FC efficiency defined in terms of external and fuel loss currents. iFuel loss = nH2 Faraday JHLoss + nO2 Faraday JOLoss 2 2 ηFC,actual = 1 −

The FC efficiency is generally defined as the ratio between the electricity produced and the hydrogen consumed [10]. This definition assumes the form of the Eq. (2) in accord with Faraday’s law. ηFC,Theoretical =

iFuel loss iFuel loss + iExternal

(5) (6)

3. Results and discussion 3.1. Pd–Ag membrane module analysis

Voltage 1.482

(2)

However, this type of efficiency definition does not consider the hydrogen loss due to the internal crossover [10]. The FC fuel efficiency can be defined by the ratio between the H2 amount converted in external current (F4 ) and the H2 amount totally fed to the PEMFC (F2 ) (Eq. (3)). The obtained values give an indication about the PEMFC performance and also about the crossover influence during the PEMFC operation, in dependence on the operating conditions also on the Pd–Ag membrane purification step. Fuel cell fuel efficiency = ηFC,Fuel =

(4)

F4 F2

(3)

The FC efficiency in terms of electrical variables can be expressed by means of Faraday’s law. The current is proportional

The permeation tests confirm that hydrogen flux follows Sievert’s law showing a linear dependence with respect to the square root H2 partial pressure difference at all the temperatures investigated (Fig. 4). Therefore, a constant permeance value can be assumed for each temperature. A reduction in the permeance values with respect to the H2 pure gas permeance value was observed changing the feed mixture composition at all the temperatures considered. Specifically, the higher permeance reduction is obtained at a low temperature, feeding mixture 2 containing 49% CO and thus the CO poisoning effect on the Pd–Ag membrane is relevant. The poisoning effect decreases at a higher temperature, in fact, at 450 ◦ C the permeance reduction for the mixture 1 is around 10% (Fig. 5). The high CO content in mixture 2 implies a significant H2 permeance reduction (40%) also at the highest temperature.

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Fig. 6. Arrhenius plot of H2 measured permeability as a function of 1000/temperature.

Fig. 4. Pd–Ag membrane module permeation tests. Hydrogen permeating flux as  a function of  PH2 , driving force of Sievert’s law, for different feed mixture at different temperatures. Experimental error is <2% and not reported on the graph because its representation is located on the related symbol.

The relationship between the hydrogen permeation and the temperature follows the Arrhenius law, for all the cases considered (Fig. 6). An important parameter in the analysis of the membrane purification step is the recovery index, useful to give an evaluation of the extractive capacity of the system, being the ratio between the H2 amount recovered in the permeate with respect to the total H2 fed to the purification step (Eq. (7)) Recovery index = RI =

FHPermeate 2 FHPermeate + FHRetentate 2 2

(7)

Fig. 5. H2 permeability reduction with respect to pure gas permeability as a function of temperature for the two different mixtures.

RI depends on several variables such as temperature, feed and permeate pressure, feed mixture composition, the membrane properties such as permeance and area, etc. It is shown in Fig. 7 as a function of the feed pressure at two different temperatures for the two mixtures considered. The feed pressure promotes the permeation of hydrogen through the membrane increasing the recovery on the permeate side. RI follows an increasing trend with the feed pressure at both the temperatures considered for both the feed mixtures. The RI obtained with mixture 1 is always higher than that achieved feeding mixture 2 to the Pd–Ag membrane module in the whole feed pressure range investigated. Increasing the temperature, a higher RI can be achieved, for both the mixtures considered, since the H2 permeation is favoured (Fig. 6). RI values measured were in the range 30–70% (using mixture 2) and higher (50–95%) for mixture 1. In particular, a value higher than 70% (mixture 1) was reached also at a low feed pressure (200 kPa) and at 450 ◦ C.

Fig. 7. Recovery index as a function of feed pressure at 350 ◦ C and 450 ◦ C for the two different mixtures.

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3.2. Integrate system: Pd–Ag membrane module–PEMFC The integration with the PEMFC was realized after the characterization and testing of the Pd–alloy membrane purification step. A comparative experimental analysis using both pure H2 (from cylinder) and H2 purified (by means of Pd–Ag membrane purification step) from mixtures 1 and 2 was performed, in order to evaluate the stability of power, voltage and current supplied by the PEMFC. Some experiments were carried out in order to evaluate the optimum feed flow rate value giving the best PEMFC performance (e.g., in terms of power provided), feeding both pure H2 from the cylinder and as a gas mixture. A maximum of ca. 12 cm3 (TPS)/min was the optimal H2 feed flow rate to the PEMFC coming from the cylinder or purified with the membrane module. The PEMFC performance stability is confirmed in Fig. 8 showing the electrical performance measured in terms of voltage, current and power as function of time, for hydrogen fed both from the cylinder (open symbol) and from the membrane purification step (full symbols). Constant values for all variables (voltage, current and power) as function of time were observed for the whole testing time. A very good agreement of experimental data for the two different H2 feeds was obtained confirming that the Pd–alloy membrane purification step assures a pure H2 stream, its constant flow rate and, consequently, continuity in the performance of the PEMFC. After the integration between the Pd–Ag membrane purification step and the PEMFC, the potentiometric tests were carried out obtaining the polarization curves (Fig. 9). Furthermore, the experiments were performed changing the operating conditions of the membrane module (Table 4). The potential (Ecell ) produced by the FC follows a decreasing trend with the current drawn from the cell due to several voltage

Fig. 8. Power, voltage, current as a function of time. Experimental data: FC fed by cylinder (full symbols), FC fed by Pd–Ag membrane purification step (open symbols). FC temperature = 70 ◦ C, H2 FC feed flow rate = 12 cm3 (STP)/min. Table 4 Integrated membrane plant: Pd–Ag membrane module operating conditions Feed stream composition

Temperature (◦ C)

Feed pressure range (kPa)

Mixture 1 Mixture 2

350; 450

250–600

losses, at all the operating condition considered. These voltage losses are produced from electrode polarisation, internal cell and external resistances, mass transport limitations and limitations in cell materials [16,17]. The power produced by the PEMFC exhibits a maximum with an increase of the current density. The peak power produced is often used for characterising and comparing the performance. Fig. 9a and b report the polarization diagrams of the integrated plant fed with mixtures 1 and 2, respectively, purified at 350 ◦ C with the Pd–Ag membrane. In both diagrams, three

Fig. 9. Potentiometric measurements. Voltage (full symbols) and power (open symbols) as a function of current density at different operating conditions of the Pd–Ag membrane purification step. Literature data (full triangle) [18].

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polarization and three power curves were shown to correspond at the three different feed pressures operated in the purification step. A higher feed pressure increases the RI and thus the H2 fed to the PEMFC is higher too. This allows higher power supplied by FC due to a better exploitation of the system. The potential values obtained when the Pd–Ag membrane module is fed with mixture 1 (Fig. 9a) are higher than the other reported in Fig. 9b (mixture 2) and increase with the pressure. Mixture 2 has lower H2 concentration and higher CO molar fraction: the first reduces Sievert’s driving force whereas the latter implies greater CO poisoning. Both reduce the hydrogen permeation through the Pd–Ag membrane with a consequent decreasing of the hydrogen recovered (and then fed to the FC) and, therefore, the FC gives a reduced performance. The positive effect of the feed pressure is shown for mixture 2 also at 450 ◦ C (Fig. 9d). Fig. 9c shows the better FC performance when the purification stage operates at the lowest pressure (250 kPa). At this temperature (450 ◦ C), the hydrogen purified by the first step, exceeding the H2 necessary for feeding the PEMFC, increases also the H2 crossover. The PEMFC electrolyte consists of a polymeric membrane showing a finite permeability with respect to the reactant gases (H2 , O2 ) and also, although not electrically conductive, allows the possibility to some electrons of finding also a “shortcut” through the membrane [17]. Since each hydrogen molecule contains two electrons, this fuel crossover and the so-called internal currents are essentially equivalent. Each hydrogen molecule diffusing through the polymer electrolyte membrane (stream F5 ) and reacting with oxygen on the cathode side results in two fewer electrons in the generated current of electrons that goes through the external circuit. When the FC is operated at very low current density or when the H2 feed flow rate is very high, these losses may have a dramatic effect on cell potential. An independence of the FC actual efficiency with respect to the Pd–Ag membrane purification step operating conditions was observed in Fig. 10. The FC efficiency always follows the same

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curve showing a maximum as a function of the H2 feed flow rate equal to that permeated, changing the feed mixture composition and the temperature. A low hydrogen feed flow rate causes a poor PEMFC exploitation, because the hydrogen amount is not sufficient. Lower efficiency is also obtained at a high H2 feed flow rate because the exceeding H2 is just vented. The highest efficiency is around 12.5 cm3 (STP)/min of H2 flow rate, resulted as a compromise between these two effects. The hydrogen loss in the FC operations is not the only loss by the overall plant. A mixture containing a set H2 percentage was fed to the Pd–Ag membrane purification step: a part of this permeated through the Pd–Ag membrane constitutes the PEMFC total feed (F2 ), another part (F1 ) remaining in the retentate side was lost. As previously for the single PEMFC, it could be useful to introduce another parameter: “the integrated membrane plant chemical efficiency (εIP )” that gives information concerning the quantity of the H2 fed to the integrated membrane plant with the mixture is finally converted to electrical power (Eq. (8)). Integrated plant chemical efficieincy = εIP =

F4 H2 equivalent to power produced by FC = H2 fed to Pd-based purification step FH0 2

(8)

As opposed to FC efficiency, this parameter depends on the operating conditions of the Pd–Ag purification step such as temperature, feed pressure and feed mixture composition because they influence the RI and also the operating conditions of the PEMFC (e.g., temperature, feed pressure). Fig. 11 shows the efficiencies as a function of the purification step feed pressure at two different temperature for both the mixtures investigated. Considering the case at 350 ◦ C, both the efficiencies obtained using feed mixture 1 are higher than those relative to mixture 2, owing to the lower CO content exhibiting a milder poisoning effect. In both cases, the εIP and the ηFC increase with increasing the feed pressure of the membrane purification step. In principle, a higher feed pressure increases the RI, favouring a better PEMFC performance because it increases the H2 feed (Table 5). Furthermore, a higher RI implies a lower hydrogen amount lost in the retentate stream and thus a higher integrated plant overall efficiency. A different trend can be shown at 450 ◦ C for the two different mixtures. The efficiencies of both Pd–Ag membrane purification stage and PEMFC increase with feed pressure of the purification step for mixture 2. On the contrary, they decrease for the other mixture 1 due to the high amount of H2 loss. Table 5 reports the H2 feed flow rate produced by membrane at 350 and 450 ◦ C. It is in the range desired from FC for mixture 2 at both temperatures but, at 450 ◦ C, it is much higher for mixture 1. Table 5 PEMFC H2 feed flow rate range (cm3 (STP)/min) at different Pd–Ag membrane module operating conditions

Fig. 10. PEMFC actual efficiency as a function of feed flow rate of the Pd–Ag membrane purification step at different operating conditions of the latter.

Temperature (◦ C)

Mixture 1

Mixture 2

350 450

5–10 12–36

3–9 4–14

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were evaluated in terms of H2 streams processed by the system. In this way, the defined efficiencies were evaluated taking into account also the H2 crossover. An independence of the FC actual efficiency by the Pd–Ag membrane purification step operating conditions was observed. On the contrary, a dependence was shown on the integrated membrane plant efficiency. The integrated system Pd–Ag membrane module purification step-PEMFC showed these best results: • recovery index: 50–95% (mixture 1); • stable power and current density. Best result: ∼990 mW; 130 mA/cm2 , at 250 kPa and 450 ◦ C of the purification step; • FC fuel efficiency at 350 ◦ C: 55–75% (mixture 1); • integrated membrane plant efficiency: 50–85% feeding the mixture 1 to the Pd–Ag membrane purification step at both the temperatures investigated. Acknowledgements The “Ministero degli Affari Esteri, Direzione Generale per la Promozione e la Cooperazione Culturale” and “Ministero dell’ Istruzione, Universit`a e Ricerca” Progetto “FIRB-CAMERE RBNE03JCR5 – Nuove membrane catalitiche e reattori catalitici a membrana per reazioni selettive come sistemi avanzati per uno sviluppo sostenibile” are gratefully acknowledged for the financial support. Fig. 11. Integrated plant overall efficiency and FC fuel efficiency as a function of the feed pressure of the Pd–Ag membrane module purification step for the two feed mixtures.

4. Conclusion In this work an integrated membrane plant consisting of a Pd–Ag membrane purification step and a commercial PEMFC was studied and the influence of the operating conditions on each single step and on the global system was evaluated. The Pd–Ag membrane module was characterized by means of permeation tests with pure and gas mixture in the range of 300–450 ◦ C and 200–650 kPa and the CO poisoning effect on the hydrogen permeation was analysed. In order to analyse the extractive capacity of the Pd–Ag membrane module, the H2 recovery index was evaluated. Good and stable performances were realized in terms of power, voltage and current. A good agreement of experimental data obtained feeding H2 by cylinder and by Pd–Ag membrane purification step was obtained confirming no performance loss when the PEMFC was fed by the membrane purification step. The electrical performance of the PEMFC integrated with the Pd–Ag membrane module was evaluated. The best output in terms of electric performance (power ∼990 mW and current density 130 mA/cm2 ) was achieved working with the Pd–Ag membrane module at 250 kPa and 450 ◦ C with mixture 1. An FC actual efficiency and an integrated membrane plant efficiency

Appendix A. Nomenclature

Ecell F i J L n P Pe0 Q RI T

cell voltage (V) molar flow rate (mol/s) current density (A) permeating flux (mol/m2 s) length (m) electron number pressure (Pa) permeability pre-exponential factor (mol m/m2 s Pa0.5 ) volumetric flow rate (m3 (STP)/s) recovery index temperature (◦ C or K)

Greek letters εip integrated membrane plant overall efficiency ηFC fuel cell efficiency Superscripts and subscript Catalyst referred to catalyst External external current produced by fuel cell Feed membrane module inlet stream Fuel loss current loss for the crossover effect Membrane membrane phase Permeation membrane module outlet stream on the permeation side Pd-module referred to Pd–Ag membrane module purification step

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Retentate membrane module outlet stream on the reaction side Theoretical referred to the theoretical FC efficiency [8]

Acronyms FC fuel cell PEMFC polymeric electrolyte membrane fuel cell PS purification step STP standard temperature (25 ◦ C) and pressure (100 kPa)

[9]

[10] [11]

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