1. Introduction
Nowadays, fuel cell vehicles (FCVs) supplied by pure H
2 are considered as a viable alternative to the internal combustion engine (ICE) powered vehicles [
1]. In a H
2 powered FCV, H
2 reacts with O
2 to generate power that is further transformed into mechanical energy,
Figure 1.
Proton exchange membrane fuel cells (PEMFCs) are the most used fuel cells typology in transportation applications [
1,
2]. They are commonly supplied by pure H
2, provided from an external source and stored in a pressurized tank [
2]. With respect to the common ICE powered vehicles, a FCV presents several advantages including silent operations, lower temperature, rapid start-up, lower leakage and corrosion concerns, and lower greenhouse gases (GHGs) emission [
3]. Typically, about 4–7 kg of pure H
2 are stored in a FCV by means of pressurized tanks [
1,
2]. In particular, 4 kg of H
2 enables a FCV driving-range autonomy without refueling for around 320 km [
1].
Nevertheless, some open issues still remain regarding the adoption of FCVs implementing an on-board H
2 production or a H
2 storage system, both presenting drawbacks needing further development prior to be proposed at larger scale [
4]. In this regard, the U.S. Department of Energy (DOE) first established, and it is still under development, the on-board H
2 storage program [
5], also to introduce new methods to meet the needs of the customers. Another important aspect of research is related to low energy-density of H
2, which is responsible for its difficult storage in a car. Indeed, too large or too heavy H
2 pressurized tanks would be required to ensure an adequate driving-range as currently guaranteed by the conventional ICEs vehicles [
4]. Consequently, the on-board H
2 generation via fuel reforming processors seems to be a quite attractive option, particularly if MSR reformers are used to generate H
2, because this solution could exploit the CH
4 pipeline infrastructure still existing for CH
4 fueled ICE vehicles, even though further complexity would be added to the system and an increase of the costs as well. The former issues were analyzed in deep by Ma and Spataru [
4]. Different studies in literature address the development of on-board H
2 generation systems using different kind of fuels such as methanol, ethanol, and other hydrocarbons. Boettner and Moran [
6] and Wu et al. [
7] worked on the on-board FCVs development, implementing methanol fuel processors for H
2 generation, and they demonstrated the economic advantage of this solution in terms of capital cost with respect to FCVs adopting a direct H
2 supplying by tank. Purnima and Jayanti [
8] simulated the on-board reforming of ethanol for a FC powered vehicle, also coupling a methane reformer to provide the heat of ethanol reforming reaction. They demonstrated that an overall efficiency of a bit less than 50% could be reached by the proposed system. Zhang et al. [
9] analyzed the use of dimethyl ether (DME) as a fuel for on-board reformer in a PEM fuel cell system. They proved that the DME reformer could supply acceptable hydrogen for a FCV [
9]. An on-board n-heptane fuel processor system was simulated for driving a 2–3 kW FCV by Karakaya and Avci [
10], coupling a methane combustor also for supplying the required heat for n-heptane steam reforming. Darwish at al. [
11] investigated the feasibility of hydrogen generation in a 50 kW FCV by on-board naphtha reforming process. They founded that a 70 L fuel tank should be required to guarantee 5 h of continuous driving time due to a consume of 14 L/h of naphtha in the reformer. Myers et al. [
12] pointed out how MSR represents an efficient process for H
2 generation for FCVs supplying, highlighting the following advantages: (1) N
2 dilution effect could be ignored and, consequently, the highest H
2 generation may be attained by MSR reaction among the other H
2 generation methods (autothermal reforming and partial oxidation); (2) the high temperature of the output gas stream could provide the heat inputs required, (3) no need to compress air to the system because the reaction pressure could be achieved by pumping the reactants; (4) the CH
4 price is consistently lower than that of other fuels.
To the aforementioned examples, a consistent number of research studies was dedicated in the last decade to find alternative solutions to the conventional reformers under the purpose of pursuing the principles of Process Intensification Strategy (PIS) [
13]. In this regard, the role of membrane engineering in the application of PIS in the fuel reforming was largely studied, demonstrating several advantages over the conventional technologies [
14,
15,
16,
17,
18,
19,
20,
21]. In the field of H
2 generation, many studies were dedicated to the conversion of CH
4 into pure H
2 via reforming reactions in membrane reformers (MRs), highlighting the operational and economic benefits over the conventional reformers [
16,
17,
18,
22,
23,
24,
25]. In particular, metallic membranes were largely studied in MR applications, with palladium and its alloys resulting the dominant materials for preparing inorganic membranes due to the high solubility and permeability of H
2 through them [
19,
20,
26,
27,
28,
29,
30,
31].
The H2 produced in a fully H2 perm-selective Pd-based MR is hence directly useful for a PEMFC supply without needing any additional H2 purification stage. This constitutes the superiority of an on-board Pd-based MR adoption over an on-board conventional reformer, which would require further H2 purification stage processes to purify the reformed H2-rich stream in order to meet the strict purity requirements of a PEMFC (CO content below 10 ppm).
The H
2 transport through a dense palladium or palladium-alloy film occurs in six stages under a driving force (from a high to a low pressure gas region): (a) diffusion of molecular H
2 at the Pd membrane surface, (b) reversible dissociative adsorption on the Pd surface, (c) dissolution of atomic H into the bulk metal, (d) diffusion of atomic H through the bulk metal, (e) association of H atom on the Pd surface, (f) desorption of molecular H
2 from the surface, (g) diffusion of molecular H
2 away from the surface [
20].
Hence, the H
2 permeation through Pd-based membranes is generally described by the Sieverts–Fick law (1):
where,
JH2 represents the H
2 permeating flux,
Q the H
2 permeability,
δ the thickness of the palladium/palladium alloy film,
phps and
plps are the H
2 partial pressures on the high pressure (feed) and low pressure (permeate) sides, respectively, while “0.5” is the Sieverts pressure exponent, representing the bulk diffusion controlling step of the H
2 permeation mechanism [
21].
Several applications are noticed in literature regarding the application of Pd-based MRs to carry out MSR reaction for generating pure H
2 to be directly supplied to PEMFCs (with standard requirements of highly H
2 concentrated streams showing CO concentration below 10 ppm) [
5,
8,
16,
18,
19], reaching high CH
4 conversions generally at lower temperatures than the equivalent conventional reformers [
17,
32] due to the selective permeation of H
2 through the membrane that is responsible for the shift of MSR reaction towards the products, enhancing both the conversion and H
2 yield [
16,
22,
23,
24,
25]. Although a large body of literature on MRs for H
2 for generation may be noticed, to our best knowledge there are no applications of FCVs adopting MRs on-board. This choice would result more convenient also than a solid oxide electrolysis cell (SOEC) utilization for generating pure hydrogen. Indeed, the former shows as main issue (still unsolved) the high degradation under a longer operation, which would result as a limit for ensuring stable and long vehicle cruising. The SOEC degradation may occur in cell components such as hydrogen/air electrodes and electrolytes due to structural, electrochemical, and thermal modifications in the components, which in turn would affect the SOEC performance.
The novelty of this work consists of the modeling of an on-board Pd-Ag based multi-tubes MR used for generating 5 kg/day of pure H
2 from MSR reaction to be supplied to a FCV (in this case, a pure CH
4 feed stream was assumed without sulphur based odorants as-on the contrary-normally present in the already existing CH
4 refueling stations). As the diffusion could be neglected in angular direction at low pressures [
26,
27], a 1-D model was adopted to evaluate macroscopically the size, the efficiency, and the hydrogen generation performance of the proposed on-board membrane-based fuel processor as a viable option for the FCVs development.