Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer

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Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer S. Foresti a,*, G. Manzolini a, S. Escribano b,c a

Politecnico di Milano, Dipartimento di Energia, Via Lambruschini 4, 20156, Milano, Italy CEA/LITEN/DEHT, 17 Rue des Martyrs, 38054, Grenoble Cedex 9, France c Univ. Grenoble Alpes, F-38000, Grenoble, France b

article info

abstract

Article history:

An innovative small-scale cogeneration system based on membrane reformer and PEM fuel

Received 27 April 2017

cells is under development within the FluidCELL project. An experimental campaign has

Received in revised form

been carried out to characterize the PEM fuel cell and to define the operative conditions

21 July 2017

when integrated within the system. The hydrogen feeding the PEM is produced by a

Accepted 7 August 2017

membrane reactor which in principle can separate pure hydrogen; however, in general,

Available online xxx

hydrogen purity is around 99.9%e99.99%. The focus of this work is the assessment of the PEM performance under different hydrogen purities featuring actual membrane selectivity

Keywords:

and gases build-up by anode off-gas recirculation. Their effects on the cells voltage and

Micro-cogeneration

local current distribution are measured at different conditions (pressure, humidity, stoi-

Polymeric electrolyte membrane

chiometry, with and without air bleeding, in flow-through and dead-end operation). In

fuel cell

flow-through mode, the cell voltage is relatively insensitive to the presence of inert gases

Hydrogen dilution

(e.g.
20 mV with inerts/H2 from 0 to 20$10
2 at 0.3 A/cm2), and resistant also to CO (e.g.

CO poisoning


35 mV with inerts/H2 ¼ 20$10
2 and CO/H2 from 0 to 20$10
6 at 0.3 A/cm2), thanks to the

Current density distribution

Ru presence in the anode catalyst. Looking at the current density distribution on the cell surface, the most critical areas are the cathode inlet, likely due to insufficient air humidification, and the anode outlet, because of low hydrogen concentration and CO poisoning of the catalyst. Dead-end operation is also investigated using humid or impure hydrogen. In this case relatively small amount of impurities in the hydrogen feed rapidly reduces the cell voltage, requiring frequent purges (e.g. every 30 s with inerts/H2 ¼ 0.5$10
2 at 0.3 A/ cm2). These experiments set the basis for the management of the PEMFC stack integrated into the m-CHP system based on the FluidCELL concept. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Micro Combined Heat and Power systems (m-CHP, or cogeneration systems) are one of the easiest options to increase energy efficiency in residential sector, which accounts for

about 25% of the primary energy consumption in EU-28 countries in 2015 [1]. Micro-CHP systems usually suffer of lower electric efficiencies and higher specific costs than large scale plants because of the size, but fuel cells can overcome this deficiency thanks to their high efficiency and modularity. Among the various fuel cell technologies, polymer

* Corresponding author. E-mail address: [email protected] (S. Foresti). http://dx.doi.org/10.1016/j.ijhydene.2017.08.046 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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electrolyte membrane (PEM) type have been deeply investigated because of low operating temperature, high power density, fast and numerous start-up capability and commercial availability. Systems for stationary applications usually adopt a fuel processor to produce hydrogen from fossil (natural gas, LPG) or renewable (bio-ethanol, bio-gas) fuels. The fuel processor consists in a steam reforming reactor followed by two watergas shift reactors (high and low temperature) and a CO abatement system (methanator or preferential oxidizer). The hydrogen-rich stream (H2 > 70%) is finally fed to the PEM stack, in flow-through mode, and the anode off-gas is usually burned in order to supply the heat to the reformer [2e4]. The quality of hydrogen for PEM fuel cells for stationary appliances is specified in ISO 14687-3 [5]. In particular, three categories are defined: two with H2  50% and CO  10 ppm, and one with H2  99.9% and CO  0.2 ppm. Hydrogen produced by conventional fuel processors can fulfill the requirements of the first two categories. In FluidCELL project, an innovative micro-cogeneration system fueled with bio-ethanol, based on an innovative fuel processor for hydrogen production and PEM fuel cells has been developed [3,6]. Bio-ethanol has been selected as renewable, non-toxic and easy-to-store feedstock, suitable also for remote installations. The fuel processor consists of an auto-thermal fluidized-bed membrane reformer where the ethanol conversion to hydrogen and the hydrogen separation are carried out in a single step. The system is rated at 5 kWel, with 40% net electric efficiency and 90% total efficiency on ethanol lower heating value (26.952 MJ/kg). Low-temperature heat (30e45  C) is recovered by cooling the fuel cell stack, the exhausts and other process streams. Design performances of the innovative system are coherent with the actual values set by DoE for PEM-based m-CHP systems, (i.e. 40% electric and 90% total efficiency for natural gas fueled devices in 2015) but still below the target of 45% electric efficiency set by 2020 [7]. The hydrogen separation is carried out by asymmetric membranes, with a PdeAg layer of 4e5 mm deposited on a ceramic support. The permeation process is driven by the pressure difference of hydrogen between the two sides of the membrane, according to the Sievert's law. In principle, this type of membranes has an infinite selectivity toward hydrogen, so the separated hydrogen is pure, but for the water in the case steam is used as sweep gas, meeting the aforementioned third category specifications for the hydrogen quality. However, due to possible imperfections in the palladium layer deposition (the membrane manufacturing is still under development) and sealing leakages (connection between the ceramic tubes at 500  C is challenging), some impurities may be present in the permeate stream. For simplicity, here we refer to the membrane reactor selectivity as the ratio of the flowrates of hydrogen over non-hydrogen species in the permeate stream (Eq. (1)), which partial pressure in the high-pressure side depends on the global operation of the membrane reactor itself. Standard procedures for membrane testing and target performances were defined by DoE [8]. An estimation of the permeate compositions as function of the membrane reactor selectivity are listed in Table 1. It must be outlined that these values may be very different depending

Table 1 e Permeate composition as function of reactor selectivity. Reactor selectivity H2 CO CO2 CH4 N2 Total inerts (CH4 CO2, N2)

104

103

102

99.99% 1 ppm 38 ppm 20 ppm 42 ppm 0.01%

99.90% 13 ppm 380 ppm 200 ppm 420 ppm 0.1%

99% 130 ppm 0.38% 0.20% 0.42% 1%

on the phenomena that causes the lack of selectivity (sparse surface defects, mechanical break of the support, degradation of the sealing material) and their location, therefore the permeate composition is described by the ratios CO/H2 and inerts/H2. In this paper the mixture of CH4 CO2, N2 is referred to as inerts. Membrane reactor selectivity ¼

H2 not H2

(1)

The effects of traces of CO and other gases in PEM fuel cells have already been investigated, in particular to evaluate the impact of contaminants in hydrogen bottles that fulfil the requirements of purity for the automotive (specified by ISO14687-2 [9]). Perez et al. [10] worked on single cell with low Pt loading at anode (0.05 mg/cm2) operating at high current density (1 A/cm2) with very low CO concentration at cell inlet (1 ppm) and measured the CO at the outlet by gas chromatography at different fuel utilization factors: the flow of CO at the cell outlet is higher at lower fuel utilization factors, however steady state conditions were not reached. Ahluwalia et al. [11] developed a steady-state model of PEM fuel cell to simulated the effect of CO (0e1 ppm), CO2 (0e1%), and anode offgas recirculation ratio on the stack efficiency: an optimum recirculation ratio was found for each concentration of CO and CO2 in the fresh fuel, which resulted from a trade-off between voltage decrease and vented hydrogen. An experimental campaign on single cell with real anode off-gas recirculation (operated with continuous bleed or periodic purge) was performed by Koski et al. [12]. The cell (25 cm2, 0.25 mg/cm2 of Pte Ru anodic catalyst load) operated at 1 A/cm2, with about 30% fuel utilization for each pass. Industrial quality hydrogen (H2 ¼ 99.95%) was used and the concentration of the other components was monitored: N2, CH4 and CO2 increased their concentration at the cell inlet over the time, while no accumulation of CO was observed: it was stable around 0.2e0.4 ppm, as in the fuel bottle (0.3 ± 0.1 ppm). Nikiforow et al. [13] optimized the purge of the recirculation loop for a Nedstack P8 stack fed with pure hydrogen by changing the admitted voltage drop in the range 3e9 mV and the purge time (200e400 ms). Fuel utilization per pass was 55e70%, while the overall utilization was 99e99.9%. The nitrogen crossover from cathode to anode side across the MEA is another important source of inert gas in addition to those present in the fuel. Nitrogen crossover is minimum at the Open Circuit Voltage (OCV) and increases linearly with the current [14,15] as it is favored by higher water content of the polymeric membrane. A numerical model to optimize the purge of the anode recirculation loop limiting the nitrogen accumulation in a stack for automotive applications (pure

Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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hydrogen feeding) has been studied in Ref. [16]: 1e2% hydrogen loss is sufficient to keep the nitrogen concentration below 3% at the stack inlet for both continuous bleed or periodic purge of the recirculation stream. The hydrogen crossover measurement is often used to check the integrity of the polymeric membrane: it can be measured by the Linear Sweep Voltammetry (LSV) [17,18], approach adopted also in this work, or by analyzing the hydrogen content in the cathode off-gas [19]. The thinner the membrane the higher the hydrogen crossover: flux spans form 10
9 mol/(s cm2) for 178 mm membrane [17] to 10
7 mol/ (s cm2) for 25 mm membrane (this work). Most of the works about anode off-gas recirculation consider different systems compared to the one investigated here: usually pure hydrogen is supplied by high-pressure bottles and anode off-gas recirculation is done with an ejector [20e22]. In particular in Ref. [22] the operation of a 5 kW-class stack is analyzed: increasing the current the nitrogen fraction increases and the performance of the ejector decreases (requiring a higher pressure of the primary fluid). Stack degradation for m-CHP systems fed by reformate gas, typical of conventional fuel processors, are studied by Chattot et al. [23] by accelerated stress tests protocols (current cycles) that increase by a factor 5 the voltage decay. Cells with 0.5 mg/ cm2 Pt loading on cathode and 0.3 mg/cm2 PteRu loading at anode are used to increase the CO tolerance. Comparing the electrochemical tests from Beginning of Life (BoL) to End of Life (EoL), a reduction of the Electrochemical Surface Area of the catalyst (ECSA) appears together with an increased hydrogen crossover and modifications of the catalysts structures: (i) Pt particles growth at cathode and (ii) massive dissolution of Ru at anode are the main phenomena responsible for the voltage decay. The other option to run the stack is the anode dead-end mode. Most of the works in literature focus on the optimization of closing-opening time of the purge valve [24,25]. The effects of temperature and relative humidity on the degradation of a dead-end fuel cell are studied in Ref. [26]: higher temperature (80  C vs. 65  C) and lower cathode relative humidity (50% vs 100%) result in faster degradation, with 50e70% ECSA reduction. This is due to carbon corrosion at cathode side because of fuel starvation, anode catalyst does not show significant variations, while delamination of the catalyst layers shows up in the gas inlet regions, where less humidity is present. This work aims at characterizing the PEM fuel cells to be adopted in the innovative m-CHP system for different hydrogen purities and stack operating conditions. The previous analysis of the m-CHP system [3] was devoted to the overall system assessment considering the PEM stack as a black box, that operates at fixed conditions (0.75 V per cell at 0.3 A/cm2) independently from inlet streams state; hydrogen vent to prevent inert gas accumulation was considered, but not optimized, and it was highlighted the deep impact of PEM operating conditions on the membrane reactor sizing (in fact the anode pressure corresponds to the permeate side pressure of the membrane reactor). In this work, several experiments have been performed on an 8-cell stack, paying attention to the interaction of the PEM fuel cell stack with the other components of the innovative m-

3

CHP system. The combined effects of CO and inert gases, as well as pressure and humidity, have been widely investigated for both flow-through and dead-end operation of the anode. In addition, local measurements of the current allow a detailed description of the dynamics phenomena, like CO poisoning and inert gases accumulation. Several phenomena of great interest for reformate-feeding fuel cells have been observed and described, with the merit that all they are referred to the same stack, with defined MEA and bipolar plates flow field, therefore constitute a consistent database for the comparison with other works that focused on the details of single phenomena on different types of fuel cells.

Experimental campaign Test station, instruments and PEMFC stack The stack testing laboratory of CEA/LITEN/DEHT is equipped with stations for the complete characterization of PEM fuel cells [23,27]. One station has three gas lines at the fuel side in order to simulate the reformate feed for PEM fuel cells. The desired relative humidity of the streams is obtained by steam injection coupled with electric heaters. The cooling system employs a closed loop of demineralized water, with ionic conductivity below 20 mS/cm. A high flowrate, around 80 l/h per cell is used to guarantee a uniform temperature distribution in the cell. Cooling water temperature difference measured across the stack is always around 1  C. The cell temperature is identified with the cooling water outlet temperature; inlet streams temperature is 5  C higher than the stack temperature. The scheme of the test station is depicted in Fig. 1. The stack is made of 8 cells, 220 cm2 each cell. The MEA is made of reinforced Nafion® membrane, HP type and the catalyst loadings are 0.4 mg/cm2 of PteRu and PteCo on anode and cathode respectively. Gas diffusion layers are SGL 24BC grade on both sides. Bipolar plates are designed by CEA with multi-serpentine flow fields and made of two welded stamped stainless steel plates. As depicted in Fig. 2, anode channels have five vertical passages from the inlet (top right corner) to the outlet (bottom left corner), while cathode channels have three vertical passages from the inlet (top left corner) to the outlet (bottom right corner), that leads to mixed co-counter flow regions. This convention is used along the paper to show the results of local current density and temperature distribution recorded by the printed circuit board. The row data of the cartographies have been post-treated, in order to adjust the measure of each segment to the effective area of the electric contact between the device and the plates ribs, as in Refs. [27e29]. The IeV plots (also referred to as polarization curves) are obtained in galvanostatic mode, by decreasing the current from the maximum value (for which the cell voltage is around 0.5 V) by steps of 0.05 A/cm2 every 3 min (desired conditions are maintained before the test for at least 30 min and up to 5 h when CO is used, at 0.5 A/cm2). The ECSA is calculated by the integration of the current over the time measured during CVs (scan rate 0.025e0.05 V/s depending on the instrument available in the laboratory), that

Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Fig. 1 e Test station scheme.

Fig. 2 e Serpentine flow field of anode (left) and cathode (center) channels and corresponding co-, counter- and cross-flow regions by overlapping the flow fields (right).

is the overall amount of electric charges (named coulometry, Eq. (2)) involved in the hydrogen desorption and then divided by the charge saturation of pure Pt (Qsat,Pt ¼ 210 mC/cm2, Eq. (3)). This procedure is valid only for the pure Pt catalyst (at cathode), while there is no charge saturation value for PteRu alloy (at anode), for which only the coulometry can be computed. Minimum voltage is 0.075 V and maximum is 0.6 V and 0.7 V for anode and cathode respectively because of the different catalyst composition. Boundaries are fixed to cover the potential range of interest including the hydrogen adsorption and desorption peaks while avoiding hydrogen evolution at low potential and catalysts oxidation at the high potentials. LSVs are executed in the same way of CVs, but the scan rate is 0.001 V/s. The permeation current is measured in the range 0.48e0.52 V because in this range there is no activity of the Pt catalyst, as can be inferred by the flat line of cathode CVs. Measurements of local current and temperature distributions are carried out by a printed circuit board (Sþþ® card) located in the middle of the stack, in contact with the anodic and cathodic monopolar plates of two adjacent cells. The card is designed according to the geometry of the cell, to cover the entire MEA surface. The resolution is 24  20 segments for current and 12  10 segments for temperature measurements.

The data of current recorded on the plates are corrected to represent the current distribution on the MEA with the methodology previously developed for the same geometry of bipolar plates [27,29]. The card introduces an additional electric resistance at cell 5, which voltage is always a bit lower than the others, therefore all the average and standard deviation values of voltage are computed excluding this cell. In this experimental campaign, three bottles were used to simulate the reformate gas with the composition listed in Table 2. ZV2

Zt2 I dt ¼

Q¼ t1

I dV sr

(2)

V1

Table 2 e Composition of gas mixtures. Components

Bottle n 1

Bottle n 2

Bottle n 3

CH4 CO CO2 H2 N2 O2

10.00 ± 0.02 e 47.50 ± 0.02 e 42.50 ± 0.02 e

1.00 ± 0.01 e 99.0 ± 0.5 e e

e 0.250 ± 0.003 e e 79.95 ± 0.02 19.8 ± 0.02

Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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ECSA ¼

Q Qsat;Pt

(3)

Experimental procedures and results discussion The experimental campaign can be divided into four main parts: 1. Initial and final characterization. It includes mainly IeV curves and electrochemical characterization, which were initially performed to check the quality of the stack (no manufacture defects) and then repeated along the experimental campaign to monitor the evolution of the cells and their performance (ECSA and voltage losses). 2. Dynamics of CO poisoning and inert gases build-up by emulation of anode off-gas recirculation. Allows identifying the time required to see the coupled effects of CO poisoning and inert gas dilution on the voltage decay. 3. Steady state maps as function of inerts/H2 and CO/H2 ratios, with and without air bleeding, at different current density, pressure and relative humidity. Steady state maps provide more realistic values than IeV curves, most of all about the effects of CO, which requires long time tests. Maps also provide a large set of data for the development and validation of numerical models of fuel cell, which are useful for the implementation of the control system of the m-CHP unit. 4. Dead-end operation with humidified and dry, pure and impure hydrogen. This option is investigated because it could simplify the balance of plant of the m-CHP system, despite of stack lifetime reduction.

Initial and final characterization Current density-voltage curves (IeV) are recorded in several conditions and repeated during the experimental campaign to monitor the aging of the stack. Curves depicted in Fig. 3 are obtained in conditions listed in Table 3; the column Time in Table 3 considers the overall lifetime of the stack since it was installed on the test bench, not only the operative hours. Line 1 is recorded in the same conditions assumed in Ref. [3] and confirms the values selected for the nominal operation: 0.75 V at 0.3 A/cm2 for a new stack. Corresponding power density and electric efficiency (on the basis of hydrogen LHV ¼ 120.07 MJ/kg) are 0.224 W/cm2 and 60% respectively. Maximum power density obtained is 0.463 W/cm2 at 0.85 A/ cm2. Sensitivity analysis on RHa (50e90%), RHc (50e80%), Sta (1.3e2.5) and Stc (2e2.5) does not show significant gaps on the IeV plot compared with Line 1 and therefore they are not reported. Line 2 (same conditions of Line 1, 1363 h later), Line 3 and Line 4 show the effect of the anode/cathode pressures: 1.2/1.2 bar, 1.1/1.2 bar, 1.1/1.1 bar respectively. A small reduction of the anode pressure has a negligible effect on the voltage while the effect of the cathode pressure reduction is more relevant. Reinforced Nafion membranes can stand pressure differences between anode and cathode around 0.1 bar, but larger pressure differences were not tested to avoid too much stresses on the membrane. From this plot it can be assumed that the PEM stack can be operated in the mCHP system at Pa/c ¼ 1.1/1.2 bar without efficiency losses. From

5

the system point of view, reducing the anode pressure is beneficial as it reduces the Pd-membranes area, and reactor costs, compared to the case Pa/c ¼ 1.2/1.2 bar. Polarization curves with a synthetic syngas with CO/ H2 ¼ 10$10
6 and inerts/H2 ¼ 20$10
2, (composition of bottle n 2 in Table 2), are tested with and without air bleeding (Lines 7 and 6 respectively). Lines 7 and 6 have to be compared with Line 5 (same conditions of Line 3, 668 h later). It is evident the negative effect of CO and inert gases dilution on the cell and how air bleeding can mitigate the effect of CO. Line 8, compared to Line 5, shows the effect of dry conditions at the anode side. Some IeV plots are recorded after several hours of operation of the stack and therefore, for a better comparison, the same conditions of Line 1 and Line 3 are reported respectively as Line 2 and Line 5, to highlight the irreversible decay of the cell performance. Moving to the ECSA analysis, anode CV shows the “duck beak” shape, typical of PteRu catalysts (Fig. 4, left), while cathode CV shows the peaks of hydrogen oxidation on Pt110 and Pt100 crystals around 0.15 V and 0.25 V respectively (Fig. 4, right). CVs repeated at the end of experimental campaign (but before the dead-end tests) are compared with the initial curves. On the anode side, comparing CVs at BoL (black line) and MoL (blue solid line), both at scan rate ¼ 0.05 V/s, a small hill around 0.25 V appears, attributed to Ru dissolution, therefore the catalyst shows a trend toward the pure Pt curve. A much more evident evolution of the modification of PteRu CVs is depicted in Ref. [23]. Ru dissolution is likely due to the strong water condensation that occurred during the first 1000 h when RHa ¼ 90% was adopted. During the following 1000 h, when RHa ¼ 50% was adopted, the CV did not modify, in fact there is no difference between the lines at MoL (blue dotted line) and EoL (red dotted line), both at scan rate ¼ 0.025 V/s. Cathode CV shows a reduction of measured current in the low-voltage region, more pronounced peaks around 0.25 V, and again lower current in the range 0.4e0.5 V, therefore a modification of the catalyst occurred. Analytical results of the coulometry measurements, ECSA and H2 permeation are listed in Table 4. Anode and cathode coulometry are subject to about 18% and 16% reduction respectively; hydrogen permeation is stable and therefore there is no evidence of polymeric membrane degradation, like pinholes.

Dynamics of CO poisoning and inert gases build-up The test bench is not equipped for the recirculation of the anode off-gas, therefore the recirculation is emulated increasing the flows of CO and inert gases at the stack inlet, starting from pure hydrogen until the expected values for a steady state condition, depending on the hydrogen composition resulting from the membrane reactor and the vented fraction. A simplified approach is adopted to compute the amount of gas to supply, based on the following assumptions: (i) H2 flow is set by the desired current and stoichiometry; (ii) CO is consumed (adsorbed and/or electrochemically oxidized) with the same stoichiometry of H2, (iii) gas crossover through the polymeric membrane is neglected; (iv) the off-gas vent can be controlled to fix the hydrogen loss, therefore the overall vent flow should slightly increase from the begin of poisoning

Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Fig. 3 e IeV curves of the stack performed under different conditions listed in Table 3.

Table 3 e Summary of the PEM operating conditions in IeV tests. Line# 1 2 3 4 5 6 7 8

T [ C]

Pa/c [bar]

RHa/c [%]

Sta/c [
]

Anode gas composition

Time [h]

70 70 70 70 70 70 70 70

1.2/1.2 1.2/1.2 1.1/1.2 1.1/1.1 1.1/1.2 1.1/1.2 1.1/1.2 1.1/1.2

50/50 50/50 50/50 50/50 50/50 50/50 50/50 0/50

1.5/2 1.5/2 1.5/2 1.5/2 1.5/2 1.5/2 1.5/2 1.5/2

Pure H2 (reference) Pure H2 Pure H2 Pure H2 Pure H2 H2 þ CO/H2 ¼ 10 10
6 þ inerts/H2 ¼ 20 10
2 H2 þ CO/H2 ¼ 10$10
6 þ inerts/H2 ¼ 20$10
2 þ AB (O2/CO ¼ 84) Pure H2

40 1403 1406 1418 2074 2092 2020 2150

process to the final steady state. The hydrogen vent fraction is defined on the basis of the stoichiometric amount set by the current (Eq. (4)). Residence time of gas in the stack and in the recirculation loop sized for the 5 kWel system (including pipes) is estimated around 2 s for pure hydrogen, and is kept

constant independently from the increased velocities due to the total flow increased by the inert gases. Results of this kind of dynamic tests are reported for two cases, named A and B, that show different evolutions of voltage and current distribution during the poisoning process. Common stack

Fig. 4 e CVs at anode (left) and cathode (right) at begin and end of experimental campaign; T ¼ 70  C, Pa/c ¼ 1.2/1.2 bar; RHa/c ¼ 90/50%. Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Table 4 e Electrochemical measurements at begin and end of experimental campaign. Anode

Initial Final

Cathode Coulometry [C]

ECSA [cm2Pt/cm2MEA]

H2 crossover [10
10 mol/(s cm2)]

5.1 ± 0.5 3.9 ± 0.2

4.3 ± 0.2 3.6 ± 0.2

92 ± 5 79 ± 4

102 ± 3 100 ± 2

conditions are i ¼ 0.3 A/cm2, T ¼ 70  C, Pa/c ¼ 1.2/1.2 bar; RHa/ c ¼ 90/50%; Sta/c ¼ 1.5/2, while the differences in the initial and final feed gas composition and voltage are listed in Table 5. Vent fraction ¼

Polymeric membrane

Coulometry [C]

H2 vent H2 vent ¼ I Ncells H2 stoich zF

The region with the highest average current density is the middle one, where the cathode humidification is enhanced by the water produced in the previous sections and the oxygen concentration is still high. In Case B (Fig. 5, bottom, right) clear trends appear, which can be identified also in the weak oscillations of Case A (Fig. 5, bottom, left), therefore, the Case B is described in details. The injection of the inert gas mixture does not cause evident variations in the current distribution. After 40 min, when the inlet gas composition is stable, modifications attributed to CO start to appear: the hydrogen inlet section is the first to be poisoned and reduces its current, with the minimum after about 1 h. Then the “CO poisoning front” reaches the second section (minimum at 70 min). In the meanwhile, the lower current of the first and second sections is balanced by the others. At 90 min, the poisoning front invests the third section, while the first and second rise their current to values higher than the initial ones. Then, the poisoning front invests the fourth and fifth sections, which reach the minimum (and final) current at around 2 h (about 15 min delay between them). In the final steady-state, the hydrogen inlet sections (first and second) operate at higher current than the original value, no substantial modifications occur in the middle section, while the hydrogen outlet sections (fourth and fifth) suffer most the CO poisoning and reduces their current output. The current distribution in steady state condition is described for several conditions in the next paragraph. Local variations of current caused by N2, CO2 and CO poisoning have been investigated by using a small segmented cell in Ref. [30] where the behavior of Pt/C and PteRu/C electrodes are compared. When the poisoning process is complete (galvanostatic mode), for the case with Pt/C the local current reduces in the inlet region and increases in the outlet; for the case with PteRu/C only minor differences compared with the initial situation are observed and no clear trend is deducted.

(4)

In Case A, high purity permeate hydrogen is considered, with only 5 ppm of CO and 0.04% of inert gases, and 0.05% as vent fraction is assumed. Inert gases concentration grows over the time up to 17.4%, while the CO/H2 ratio is stable because of the (ii) assumption aforementioned. Voltage continuously and slowly reduces until a steady state is reached (0.707 V) after about 6 h (Fig. 5, top, left). In Case B, a lower hydrogen quality is considered, with 20 ppm of CO and 0.15% of inert gases, and the vent fraction is increased to 0.1%. In this case the voltage decay can be clearly divided into two phases: the first phase corresponds to the inert gases accumulation that causes steep voltage loss due to hydrogen dilution; the second phase is longer and corresponds to the catalyst poisoning by CO. The two phases are marked (Fig. 5, top, right) by the inversion of the slope of the voltage time derivative (DV/Dt) at about 40 min. After about 2 h from the beginning of the test the voltage is stable at 0.698 V. To study the evolution of the current density, the cell is divided into five sections, roughly corresponding to the five vertical sections of the hydrogen flow field (Fig. 2). The average current of each section is monitored over the time. The hydrogen outlet region, which roughly corresponds to the air inlet, has always the lowest average value, also at the beginning with pure hydrogen feed, because it suffers the low humidification of the air in the very first part (see, for example, Fig. 8 for the global current distribution). The hydrogen inlet segment, which roughly corresponds to the air outlet, is penalized by the low concentration of oxygen at cathode side.

Table 5 e Emulation of anode off gas recirculation: conditions and results. Membrane reactor selectivity [
]

Case A

Case B

2500

H2 CO concentration; CO/H2 Inert gases concentration; Inerts/H2 H2 vent fraction [%] Time for steady state [h] Final voltage [V]

700

Permeate composition

Final composition at stack inlet

Permeate composition

Final composition at stack inlet

99.96% 5 ppm; 4.8$10
6 0.04%; 4$10
4 0.05% z6 h 0.707

83.4% 4 ppm; 4.8$10
6 17.4%; 21$10
2

99.85% 20 ppm; 20$10
6 0.15%; 15$10
4 0.1% z2 h 0.698

78% 15 ppm; 20$10
6 22%; 28$10
2

Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Fig. 5 e Emulation of anode off gas recirculation. Top plots: evolution of gas composition, cell voltage and voltage time derivative; bottom plots: average current density distribution of the five vertical segments of the anode flow field; left: case A; right: case B.

The evolution they obtain over the time is different from the one obtained in our experiments because the channels flow field is completely different (straight parallel channels, coflow between anode and cathode). This highlights the impact of the flowfield on the performance of the cells that makes not easy a direct comparison of different MEAs. To face this issue the adoption of a standardized flow field to be used for consistent comparison of MEAs is under evaluation [31].

0.2

40 35 30 25

0.1

20 15

0.05

CO/H2 x106 [-]

Inerts/H2 [-]

0.15

10 5

0

0

Time Fig. 6 e Protocol of CO/H2-inerts/H2 maps (the duration of each step depends on P, RH, St and amount of impurities).

After few hundred hours of operation in conditions similar to Case A and Case B the shape of anode CV slightly changed: as already described (Fig. 4), the hill appeared on top and bottom lines suggest Ru dissolution in liquid water. The main outcome of the first set of experiments, at system level, is the need to reduce the water content of the permeate stream at the stack inlet. This can be done by cooling the permeate stream below the cell temperature (70  C) and removing the condensed water. The relative humidity of the stream is still 100%, but then it mixes with the warmer stream from the recycle loop and finally reaches the thermal equilibrium with the cell. Cooling the permeate to about 50  C lowers the relative humidity at stack inlet around 50% (computed at Tcell) which should be enough to limit water condensation at the outlet of the anode channels whilst keeping good humidification of the polymeric membrane in the inlet region. Therefore RHa ¼ 50% is used in the future tests. The drawbacks of this kind of tests are (i) the long time required to reach a steady state starting every time from “clean” catalyst conditions and (ii) the need of several assumptions to simulate the anode off-gas recirculation loop that cannot be verified. Therefore the method of steady state maps is adopted for obtaining more reliable data to be exploited by a numerical model of the PEMFC subsystem.

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Steady state maps The scope of steady state experiments, here named maps, is to obtain the cell voltage as function of inert gases and CO concentration for a set of conditions of interest for the mCHP system. The CO/H2 and inerts/H2 molar ratios are adopted as parameters. A wide range of conditions is tested because a large number of variables at system level can change: the palladium membranes assembly selectivity, the presence of a guard system on the permeate stream to limit the CO flow to the stack, like a methanator or a preferential oxidizer, or direct air bleeding into the anode side, the amount of hydrogen vent. The presence of a guard system slightly modifies the inerts/H2 (increase) and CO/H2 (decrease) ratios compared to those at the outlet of the membrane reactor [32], therefore a parametric analysis is the preferred way to face the problem. Thanks to these maps, it is possible to predict the PEM stack performance for many sets of the variables of the system. In case of normal operation of the membrane reactor or with a guard system upstream the stack, a limited amount of CO will enter the stack, while in the cases with direct air bleeding much more CO can enter the stack and, due to the selected O2/CO ratio (or air/CO), a lot of nitrogen is introduced too, leading to a final composition of inert gases made almost exclusively of nitrogen. Therefore, the gas from Bottle n 2 is generally used for the inerts, but in the cases with air bleeding, where pure N2 is used. A protocol is defined, in order to minimize the time required for each map and limit the number of CVs to clean the cells (CO stripping), moving step by step from pure hydrogen to heavier poisoning conditions. From pure hydrogen, the inert gas flow is increased to the maximum, then the minimum amount of CO is added and inert gas flow is reduced to the minimum. Inert gas flow is set again to the maximum in one step, the CO flow is increased and the procedure continues for all the desired combinations of CO/H2 and inerts/H2 ratios, as reported in Fig. 6, waiting for a steady state value of the average cell voltage in each condition. In this way the partial pressure of CO is increasing as well as the active sites of PteRu catalyst occupied by CO (in literature referred to as CO coverage fraction, qCO [33]). At the end of each map, CO adsorbed on the catalyst is removed by CV. The independent effects of CO and inert gases are tested only in few cases because in the final integration of the PEM stack in the m-CHP system all the species will always be present together. Table 6 reports the complete list of maps and test conditions. Two nominal current density values are tested (i ¼ 0.3 and 0.5 A/cm2) and inlet streams conditions are changed. Results of cell voltage of the maps listed in Table 6 are depicted in Fig. 7 for the case i ¼ 0.3 A/cm2 as function of the CO/H2 and inerts/H2 ratios. Differences of the average cell voltage from one point to another, tested according to the aforementioned protocol are in the same order of magnitude of the standard deviation of the seven cells of the short stack, therefore only some points are reported here in order to increase the readability of the plots. Main results about the local current distributions are reported in Fig. 8. Results of voltage

Table 6 e List of MAP tests. Fixed conditions are: T ¼ 70  C, Pa ¼ 1.1 bar, Stc ¼ 2, RHa ¼ 50%. MAP#

1 2 3 4 5 6 7 8 9

i [A/ cm2]

Pc [bar]

Sta [
]

RHc [%]

Air bleeding

0.3 0.3 0.3 0.3 0.3 0.5 0.5 0.5 0.5

1.2 1.1 1.2 1.2 1.2 1.2 1.1 1.2 1.2

1.5 1.5 2 1.5 1.5 1.5 1.5 1.5 1.5

50 50 50 50 50 50 50 30 50

e e e yes e e e e yes

DT coolant [ C] z1 z1 z1 z1 z5 z1 z1 z1 z1

and current density distribution for the tests with i ¼ 0.5 A/ cm2 (MAP6 to MAP9) are reported in the supplementary material. At i ¼ 0.3 A/cm2, the single cell voltage with pure hydrogen is around 0.715 V (Fig. 7), 35 mV lower than the initial value measured during IeV curves when the stack was new (Fig. 3). With inerts/H2 ¼ 5 10
2 and no CO, a significant drop is recorded, about 15 mV at Sta ¼ 1.5 (MAP1), and 10 mV at Sta ¼ 2 (MAP3). Further increasing the inerts/H2 ratio, whatever the CO/H2 ratio, makes the voltage slightly decrease with a linear trend. The benefit of increasing the anode stoichiometry from 1.5 to 2 appears only at high CO/H2 ratios. Lowering the cathode pressure from 1.2 bar to 1.1 bar (MAP2) causes a small voltage drop of about 5 mV in each corresponding condition. A higher cathode pressure (e.g. 1.3 bar) could be beneficial for the cell voltage, but on the long period, membrane resistance issues could rise, especially if sudden pressure difference occurs, causing pinholes formation or propagation, therefore this condition was not tested. Voltage recorded for MAP1 and MAP5 which differed by the cooling temperature difference are the same (not reported), therefore a larger temperature variation of the cooling water across the stack does not affect the cell performance. In the case of air bleeding (MAP4) it is still present a gap from pure hydrogen to contaminated hydrogen and then a linear decay with the increase of inert gas. Despite the CO/H2 ratio spans from 10 10
6 to 100 10
6, the voltage drop is limited in less than 10 mV range, both at low and high inert gas content, that is lower than with much less CO but without air bleeding. Comparing the voltage of MAP1 and MAP4 the effectiveness of air bleeding (εAB, Eq. (5)), is greater than 50%. εAB ¼ 1


UAB
UnoCO UCO;noAB
UnoCO

(5)

The current density distribution is not uniform in the reference conditions with pure hydrogen and nominal value i ¼ 0.3 A/cm2 (MAP1, Fig. 8a and b depict the row and processed data respectively): two low-current regions appear mainly at the top corners which corresponds to the inlet of the streams; the center of the cell operates close to the nominal current density and is evident the air pattern; high current density are recorded in thin regions at the top and bottom borders because of the manufacturing process of the bipolar plates

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Fig. 7 e Average cell voltage as function of the inerts/H2 (£102) ratio for different CO/H2 (£106) ratios at fixed current density i ¼ 0.3 A/cm2 for different inlet streams conditions.

and stack clamping. Maximum deviations are in the range ±50% of the nominal value. Modifications of the current density distribution due to different operative conditions (anode gas composition, Pc, Sta, Air Bleeding) depicted in Fig. 8 are about one order of magnitude lower than initial uniformities. Compared to pure hydrogen, the addition of CO and inert gases slightly changes the current distribution: the low current region grows from the anode outlet along the anode channels and is balanced by an increased current on the rest of the surface (Fig. 8c). Negative deviations are more than twice the positive deviations (
0.05 A/cm2 against þ0.02 A/ cm2) and well localized, while positive deviations are more distributed, suggesting good cathode conditions and no limitations by the Oxygen Reduction Reaction. Modifications are clearly caused by anode conditions (CO poisoning, hydrogen dilution). Reducing the pressure on the cathode side (MAP2, Fig. 8d) has a negative impact on the voltage and on the current distribution as well, in fact the phenomena already described for the MAP1 are here a slightly more pronounced in each condition of CO/H2 and inerts/H2 ratios. Increasing the anode stoichiometry from 1.5 to 2 does not modify the general current distribution, but results in a substantial cancellation of the modifications caused by CO and inert gases (MAP3 in

Fig. 8e) compared to the initial condition with pure hydrogen. The use of more severe conditions with air bleeding (MAP4 in Fig. 8f) shows the same phenomena of current reduction at the anode outlet, but also a weak reduction of current in the region of the anode inlet (Di z
0.02 A/cm2), where probably competition between hydrogen combustion and electrochemical oxidation occurs. As already mentioned, the temperature distribution in the cell is uniform (local differences limited within ±1  C) thanks to the large water flowrate. Temperature uniformity is desired during the cell characterization in nominal conditions to avoid local collateral effects, anyway in field operation typical temperature variations of the cooling water across the stack are around 5  C [34] or up to 10  C [35,36]. When the cooling water flowrate is reduced to obtain about 5  C temperature difference (MAP5, Fig. 9), a “cold spot” extended on the bottom (that corresponds to water inlet) and the left regions and a “hot spot” in the middle and top (that corresponds to the water outlet) regions appear. This is due to the asymmetry of the cooling water flow field and therefore uneven flow distribution. The temperature distribution on the cell varies within ±5  C and slightly modifies the current distribution. The coldest regions roughly correspond to the low current regions,

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Fig. 8 e Current density distributions (A/cm2). Nominal current density i ¼ 0.3 A/cm2. Differences for some cases compared with the reference. Anode and cathode channels are depicted where a modification of the corresponding parameter from the reference map is applied.

in particular the last anode vertical passage where water condensation and possible flooding may be favored by lower temperature. This condition is depicted in Fig. 9 for the case with pure hydrogen. When CO and inert gases are added, the differences are even less evident. An explanation is the formation or not of liquid water in the last vertical part of the anode channel (left) caused by the lower temperature: condensation occurs with pure hydrogen, but does not occur

in presence of more inert gases that lower the water vapor partial pressure below the saturation value. The topic of liquid water formation in the same geometry of bipolar plates has been discussed by Nandjou et al. [29]. CVs performed after feeding the stack with hydrogen and the so called “inert gas mixture”, without CO, reveal a current peak starting from 0.25 V with the maximum at 0.42 V, that is the typical voltage range of CO electrochemical oxidation on

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the PteRu catalyst. Possible electrochemical reactions at anode side are listed in Table 7. Fig. 10 illustrates how the direct presence of CO (red line) makes almost disappear the current of the Hydrogen Oxidation Reaction (HOR), which peak is around 0.1 V (black line); anyway, the cell is still able to work because of the large catalyst loading and the HOR can occur on the sites still available, enough to produce the low current required (0.3 A/cm2). The blue line still shows a small peak of hydrogen oxidation, even if the hydrogen coulometry is reduced by a factor 10 compared with the value of pure H2 (Table 8), and a less pronounced peak for CO oxidation. The CO formation when feeding hydrogen with gas mixtures containing CO2 can be explained by the reverse water-gas shift reaction (R. 6) or by the CO2 reduction (R. 7). Despite Pt is not an active catalyst for RWGS at low temperature, it is widely used to model the CO2 poisoning effect [11,37,38]. Finally the CO poisoning has been described in literature also when feeding the anode with pure hydrogen: it has been explained by the presence of CO2 coming from the air at cathode by crossover of the MEA or as product of the corrosion of the carbon support [39].

Dead-end operation Dead end operation may be of interest for the m-CHP system because does not require the hydrogen recirculation blower, with associated costs and power consumption. Furthermore, when the m-CHP system adopts a vacuum pump, dry hydrogen is obtained and it can be used in the fuel cells in

Table 7 e Electrochemical reactions at anode side. H2 adsorption CO adsorption H2O adsorption H electrochemical oxidation CO electrochemical oxidation reverse WGS CO2 electrochemical reduction Carbon corrosion

H2 þ 2 M / 2 H-M CO þ M / CO-M H2O þ M / OH-M þ Hþ þ e
H-M / Hþ þ e
þ M

R. R. R. R.

CO-M þ OHM / CO2 þ Hþ þ e
þ2 M CO2 þ H2 / CO þ H2O CO2 þ 2Hþ þ 2e
þ M / COM þ H2O C þ H2O / CO þ Hþ þ e


R. 5

1 2 3 4

Measured current [A]

Fig. 9 e Effect of cooling water flowrate on temperature and current distributions (pure H2, i ¼ 0.3 A/cm2).

7 6 5 4 3 2 1 0 -1 0 -2 -3

pure H2 (reference) H2 + inerts H2+CO+inerts

0.2

0.4

0.6

Applied potenƟal [V] Fig. 10 e Comparison of CVs (sr ¼ 0.025 V/s) with pure H2 (black line) and after exposure to inert gases only (inerts/ H2 ¼ 20·10¡2, blue line) and CO þ inert gases (CO/H2 ¼ 10·10¡6; inerts/H2 ¼ 20·10¡2, red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

dead-end mode without humidification. Dead-end operation is tested for conditions which, in the knowledge of the authors, have not been yet investigated: (i) pure humid hydrogen; (ii) dry hydrogen with CO and inert gases; (iii) dry hydrogen with CO and air bleeding. Different closing-opening times (in the range 30e75 s and 0.5e1 s respectively) of the purge valve are tested to see if it is possible to have a stable operation also in such demanding conditions. Each condition is maintained up to 1 h, but only the last 30 min are considered in the data analysis. The cycles of the cell voltage and

Table 8 e Coulometry of H2 and CO oxidation.

R. 6 R. 7 R. 8

After exposure to

Coulometry H2 [C]

Coulometry CO [C]

Pure H2 H2 þ CO þ inerts H2 þ inerts

5.1 ± 0.5 z0 0.58 ± 0.04

z0 24.2 ± 0.8 16.4 ± 0.3

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analytical results are presented in the following figures. Nominal conditions common to all the tests are: i ¼ 0.3 A/cm2; T ¼ 70  C; Pa/c ¼ 1.1/1.2 bar; Stc ¼ 2; RHc ¼ 50%. The stability of the operation in each condition is checked by monitoring the peak voltage after every purge. Due to instantaneous fluctuations, the peak of each cycle is calculated considering the average of the values above the 90th percentile of the samples. Similarly, the valley voltage is computed using the values below the 10th percentile. The maximum decay assessed by this procedure is in the order of magnitude of few mV/h. This is satisfactory for the objective of this analysis, but it should be improved for a real long-term operation by the optimization of closing-opening time to limit the (reversible) decay of peaks around 101 mV/h, which is comparable with the irreversible decay of the cells. The shorttime stability of the peaks ensures that the purge time (0.5 s in the test described here) is enough to remove most of the inerts/CO and/or liquid water in the case of humid hydrogen. The time of closed-valve is significantly shorter than typical values adopted in dead-end mode with pure dry hydrogen (few minutes). The operation with humid hydrogen (Fig. 11) is stable, the peak voltage is close to the voltage of flow-through mode, the voltage drop can be limited to 15 mV per cycles of 60 s; the standard deviation of the cells increases from the beginning of the cycle (immediately after the purge) to the end, likely due to the random formation and motion of water droplets in the anode gas channels and in the stack manifold. Current density maps reveal that (i) the most critical region is the air inlet, (ii) current production shifts toward the hydrogen inlet during the period while negative and positive differences are not symmetric (negative more pronounced and localized), (iii) major penalty between the beginning and the end of the cycles, against the expectations, is not located in the 5th section, that is the very last part of the anode channel, but in the 4th section. Liquid water, observed at the anode outlet seems not to be the primary responsible for voltage drop and local current density distribution. Similar results are described in the

13

supplementary material for the case 75e0.5 s close-open time of the purge valve. The operation with impure dry hydrogen (inerts/ H2 ¼ 0.5$10
2; CO/H2 ¼ 10$10
6) becomes more critical (Fig. 12): the peak voltage is about 30 mV lower than the case with humid pure hydrogen, the voltage drop per cycle is up to 50 mV despite the cycle duration is only 30 s. The reasons of this behavior are the catalyst poisoning and the fuel starvation, while the lack of water seems not to have an immediate impact on the voltage, as also shown by the IeV plot comparing humid and dry operation (respectively Line 5 and Line 8 in Fig. 3). The voltage is affected by disturbances and the standard deviation still increases from the beginning to the end of the cycle. Similar considerations to the dead-end operation with humid hydrogen can be drawn about the current density distribution. It is worth to highlight that this small content of CO and inerts corresponds to an overall selectivity of the membrane reactor assembly lower than 103 (that would be inerts/ H2 ¼ 0.1$10
2, but such small flow rate was below the lower bound of the flow controller), therefore already represents a severe condition. The operation with air bleeding (CO/H2 ¼ 10$10
6; O2/ CO ¼ 84; air/H2 ¼ 0.4$10
2) is also tested (Fig. 13): on one side the air is able to effectively contrast the CO, in fact the peak voltage is back to about 0.7 V; on the other hand it acts as an inert that may cause fuel starvation. The voltage drop is about 40 mV with cycle duration of 45 s. The air/H2 ratio (0.4 10
2) is a bit lower than the inerts/H2 of the previous experiments (0.5 10
2), and allows a bit longer cycles, causing more relevant non-uniformity of local current density. Similar results are described in the supplementary material for the case 60e0.5 s close-open time of the purge valve. Stack efficiency, that would be a very interesting output to link these experiments with the global m-CHP system, is not computed because the cumulated amount of hydrogen measured by the mass flow controller during the tests is up to 5% lower than the theoretical amount given by the

Fig. 11 e Dead-end operation with pure humid hydrogen (RHa ¼ 50%); time close-open of the purge valve: 60-05 s. Black line: cell voltage; grey lines: average ± standard deviation. Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Fig. 12 e Dead-end operation with impure dry hydrogen; inerts/H2 ¼ 0.5·10¡2; CO/H2 ¼ 10·10¡6; time close-open of the purge valve: 30e0.5 s. Left: voltage (black line: average cell voltage; grey lines: average ± standard deviation; right: current density evolution [A/cm2]. stoichiometric consumption (neglecting the vented amount). The reason is that the time response of the instrument is about 500 ms, the same order of magnitude of the opening time of the valve, therefore too slow to obtain a reliable measure of the pulsing flow.

The experimental campaign with dead-end operation is less extensive then the one for the steady state maps, but some conclusions can be drawn: a stable dead-end operation is possible also with humid or impure hydrogen; the dead-end mode is extremely sensitive to very small amount of inert

Fig. 13 e Dead-end operation with impure dry hydrogen and air bleeding: CO/H2 ¼ 10·10¡6; O2/CO ¼ 84; air/H2 ¼ 0.4·10¡2; time close-open of the purge valve: 45e0.5 s. Left: voltage (black line: average cell voltage; grey lines: average ± standard deviation. (*) only 6 cells); right: current density evolution [A/cm2]. Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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gases; higher amounts need more frequent purges, with a negative impact on the fuel utilization and on the overall system efficiency.

Conclusions and outlook An experimental campaign is carried out to characterize the PEM fuel cells to be integrated into a m-CHP system, with hydrogen produced by a membrane reactor. The operative conditions of the stack, like anode inlet pressure and relative humidity, are defined in order to allow its best operation and integration within the system. Tests are performed for a wide set of conditions due to the number of variables which can occur in the real system, focusing on the effects of CO and inert gas dilution on cell voltage and local current distribution. Main observations are:  Light impact of anode pressure (1.1e1.2 bar), stoichiometry (1.5e2) and relative humidity (50e90%) on the cell voltage  Great impact of cathode pressure (1.1e1.2 bar) and relative humidity (30e50%) on the cell voltage  Voltage evolution until steady state when the anode is exposed to gas mixtures with CO, CO2, CH4, N2 can last several hours; the effect of few ppm of CO is dominant on the effect of other gases, tested up to about 30%  The CO poisoning front, identified by a local current reduction, moves from the anode inlet to the outlet, that, in steady state conditions, is the most penalized region  CO formation occurs at the anode side when feeding a COfree gas mixture, reducing by a factor ten the catalyst surface available for the Hydrogen Oxidation Reaction.  Possible Ru dissolution and consequent decreased CO tolerance can be caused by liquid water  Local current density is lower in the anode outlet region due to hydrogen dilution and CO poisoning and in the air inlet region due to local insufficient humidification  In dead-end mode, light concentration of inert gases in the hydrogen feed (z0.5%) causes fast voltage drop, and frequent purges (e.g. every 30e60 s) are necessary to reject them and recover the voltage The experimental results described in this work are useful to define the design conditions of the FluidCELL system prototype as well as to develop and validate numeric models of PEM fuel cells for reformate feeding that take into account local heterogeneities due to the combined effects of streams conditions and cell geometry. The model of the fuel cell in turn can help in the management of the inert gases rejection and can be integrated in the complete system to simulate transients, part-load and offdesign operation and tune the control system of the prototype.

Acknowledgements The presented work is funded within the FluidCELL project as part of the European Union's Seventh Framework Programme

15

(FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement nº 621196. Note: “The present publication reflects only the authors' views and the FCH JU and the Union are not liable for any use that may be made of the information contained therein”. The authors kindly thank the technicians of LQS laboratory of CEA/LITEN/ DEHT in Grenoble for their support.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.08.046.

Nomenclature Acronyms A Area AB Air bleeding BoL Beginning of life CHP Combined heat and power CV Cyclic voltammetry D Diffusivity ECSA Electrochemical surface area EIS Electrochemical impedance spectroscopy EoL End of life F Faraday's constant HOR Hydrogen oxidation reaction i Current density I Current LSV Linear sweep voltammetry LHV Lower heating value MEA Membrane electrode assembly MoL Middle of life N Number of cells OCV Open circuit voltage ORR Oxygen reduction reaction PEM Polymeric electrolyte membrane fuel cell Q Coulometry R Universal gas constant RH Relative humidity sr Scan rate St Stoichiometry U Voltage z Number of electrons transferred in electrochemical reactions Subscripts a/c Anode/cathode avg Average perm Permeate ret Retentate Greek letters ε Effectiveness h Efficiency q Coverage factor

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Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046

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Please cite this article in press as: Foresti S, et al., Experimental investigation of PEM fuel cells for a m-CHP system with membrane reformer, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.08.046