Technological aspects of an auxiliary power unit with internal reforming methanol fuel cell

https://doi.org/10.1016/j.ijhydene.2018.11.136Get rights and content

Highlights

  • 15 and 100 W prototype internal reforming methanol fuel cell (IRMFC) modules.

  • Crosslinked Advent TPS® MEAs employed for fuel cell operation up to 210 °C.

  • Internal methanol reforming achieved via an innovative double reformer arrangement.

  • Gold-coated Al-based bipolar plates employed targeting portable applications.

  • Limited tolerance of polymeric membrane against on-off cycling conditions.

Abstract

Increasing source runtime, speeding up the transient response, while minimizing weight, volume and cost of the power supply system are key requirements for portable, mobile and off-grid applications of fuel cells. In this respect, Internal Reforming Methanol Fuel Cell (IRMFC) modules were designed, constructed and tested based on an innovative double reformer (DRef) configuration and metallic bipolar plates (BPPs) with unique arrangement. Recently developed cross-linked Advent TPS® high-temperature membrane electrode assemblies (MEAs) were employed for fuel cell operation at 210 °C. Taking into account the requirement for a light-weight and low-volume stack, Cu-based methanol reforming catalyst were supported on carbon papers, resulting in ultra-thin reformers. The proposed configuration offered a significant decrease in the weight and volume of the whole power system, as compared with previous voluminous foam-based modules. Moreover, specifically designed bipolar plates were made of coated Al-metal alloys, which proved to be stable in the strong acidic environment at elevated temperatures. The prototype 32MEAs-32DRef IRMFC stack of 100 W including home-made insulation casing, was integrated for operation at 200–210 °C and at 0.2 A cm−2, demonstrating the functionality of the unit. A power output of 100.7 W (3.14 W per cell; 0.114 W cm−2) was achieved in the last run following several on-off cycles. The volumetric power density of the IRMFC stack including insulation and casing is around 30 W per lt, being among the highest reported either in the case of portable or stationary applications. Overall, the observed stability of reformers and bipolar plates was satisfactory within the timeframe of the work undertaken. Specific targets for improvement of the efficiency were identified, and the main drawback had to do with low thermal and mechanical stability of the membranes under start-up/shut-down transient operation.

Introduction

APUs (auxiliary power units) fuel cell systems running on liquid fuels, such as methanol hold a great deal of market potential, as they offer a ready and widely available source of power for mobile (delivery vans, recreational vehicles (RV), boats, trucks and unmanned aerial vehicles) and off-grid applications [1], [2]. Currently, the former market is served by diesel or gasoline generators which rely on mature and relatively low-cost technology with the main drawbacks being fuel odour, vibration and machine noise, while off-grid photovoltaic installations and batteries are weak during low solar radiation and cold temperatures. Recreational vehicles are an ideal market for fuel cell systems because the customer demand for comfort and consequently the power consumption is rapidly increasing. Among various types of fuel cells, Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are currently the leading technology for light duty vehicles and materials handling vehicles, offering high efficiency, durability, silent operation and emissions reduction [1], [2]. These characteristics make PEMFCs an attractive technology, as compared with batteries, for mobile, portable and off-grid applications. However, hydrogen storage is a limiting parameter, since the low energy density of hydrogen requires its storage as a compressed gas in pressurized tanks. Alternatively, various liquid fuels with much higher energy density can be used as hydrogen carriers. Among various alcohols and hydrocarbons, methanol offers a favorable source of hydrogen, with 5–7 times higher energy density than compressed H2, low sulfur content, production from fossil fuels and biomass, and simple reforming to hydrogen rich gas mixtures with low CO byproduct formation at temperatures as low as ca. 200 °C [3], [4], [5]. In such case, the fuel cell-based power system should be equipped with a methanol processor for onboard production of hydrogen, thus making the design and control more complicated with respect to the size limitations for portable applications. Moreover, the additional cost should be also taken into account.

Therefore, there are several reasons which require the design and development of compact and user friendly, fuel processor-fuel cell energy systems for portable applications [5]. Operation of the PEMFC at higher temperatures and incorporation of the fuel processor inside the stack simplifies the heat and air management and results in significant volume reduction. Moreover, methanol, as primary hydrogen carrier, offers higher energy densities and overcomes technical obstacles related with the use of pure hydrogen, while it may be considered as renewable fuel. More specifically, operation of the PEMFC at higher temperatures (160–210 °C) offers: (i) enhanced electrochemical reactions rates, (ii) simplified water management and cooling, (iii) recovered useful waste heat and (iv) operation with reformate gas streams containing significant amount of CO.

An Internal Reforming Methanol Fuel Cell (IRMFC) takes advantage of the aforementioned features of a High Temperature Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) and represents a high potential technological option for portable and off-grid applications [5], [6], [7], [8], [9]. The core of innovation of IRMFC is the incorporation of a methanol reforming catalyst in the anode compartment of the HT-PEMFC. This configuration eliminates the need for additional heat exchangers and the HT-PEM technology applied does not require CO removal through a preferential oxidation (PrOx) reactor. Moreover, it offers room for simplification, increase of system's electrical efficiency and minimization of system's weight and volume. Ref. 5 reviews the concept, challenges and opportunities of IRMFCs. We recently designed, assembled and tested a proof-of-concept 70 W IRMFC stack including Balance-of-Plant (BoP) [9]. The stability of the system and especially of the MEAs that operated at 200 °C, was remarkable, and depending on the liquid feed flow rate, current densities up to 0.18 A cm−2 and a power output up to 70 W could be obtained. However, voluminous and heavy foam-based reformers and graphite-based bipolar plates were employed, thus making necessary a drastic optimization in the stack design in order to reduce its volume, weight and consequently start-up time so that it can get a market potential.

The operational aspects of HT-PEMFC have been recently summarized in two comprehensive reviews [11], [12], though not reporting on the starting fuel parameter. Several research groups have tried in the past to develop integrated HT-PEMFC-fuel processor systems [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. However, in most of these configurations, the reformer was placed in the outer surface of the compression plates and was thermally attached with the stack. Schuller et al. [13] thermally coupled at 180 °C a methanol reformer to a HT-PEMFC utilizing its waste heat. The liquid cooled, 165 cm2, 12-cell stack was operated up to 0.4 A cm−2 generating an electrical power output of 427 Wel. Recently, Lotric et al. [14], [15] reviewed and examined the current state of R&D and the commercial market related to the technology of methanol reformer and HT-PEMFC stack integrated systems and presented an attractive configuration with a novel stack operating up to 300 °C.

Similarly to the IRMFC concept, Samms and Savinell [16] presented a reformed hydrogen fuel cell, where a commercial CuZnAlOx catalyst was packed inside the channels of the anode flowfield. However, direct contact of the methanol reforming catalyst with the MEA, resulted in deactivation due to the presence of phosphoric acid, as also observed in previous IRMFC configurations [5], [6], [7]. Thomas et al. [17] recently investigated different break-in procedures in a single HT-PEMFC, and concluded that the cell with reformed fuel break-in degrades much faster compared to the cell with H2 break-in. Kannan et al. [18] reported on a 5 cell co-flow stack assembled with BASF P1100W HT-MEA, with an active area of 163.5 cm2. Continuous operation and more than 1500 start stop cycles resulted in a degradation rate of 26 mV h−1 at 0.25 A cm−2, due to mechanical stress/strain effects at the membrane. Justesen and Andreasen [19] studied the optimal reformer temperature in a reformed methanol fuel cell system and concluded that it can significantly affect the fuel cell efficiency due to the CO content in the anode gas. Degradation aspects were also investigated by Kerr et al. [20], employing an integrated 5 kW stack/reformer system using methanol reformate as fuel. The performance and durability of HT-MEAs showed to fulfil several of the requirements for commercialization.

With respect to the title application areas of portable and off grid power systems, UltraCell's XX55™ Fuel Cell [25] is the only available commercial reformed methanol fuel cell. This power system delivers 50 W of continuous power and up to 85 W of peak power. It is fueled with methanol/water mixture and the whole system weight without battery or cartridge is 1.6 kg (32 kg kW−1) while the corresponding specific volume is 92 lt kW−1 (without battery pack). Regarding direct methanol fuel cells (DMFCs), portable fuel cell based battery chargers have been on the market for about seven years with Smart Fuel Cell (SFC) EFOY comfort as the leading product [26]. The EFOY comfort series is based on DMFCs, and their largest unit, EFOY comfort 210, has a nominal power of 105 W. The weight is 8.2 kg corresponding to a specific size of 75 kg kW−1 and specific volume of 245 lt kW−1 (fuel excluded). Although, EFOY comfort is an excellent product, with efficiency just above 20%, low power densities and efficiency limits its market potential.

In this work, we particularly emphasize on catalytic, electrocatalytic and technological aspects of the key-components (reformer, MEA, bipolar plates, BoP) of the IRMFC power unit and report on recent progress from last generation laboratory modules up to 120 W, aiming at specific weight and size lower than 20 kg kW−1 and 35 lt kW−1, respectively. Our designs are based on an innovative double reformer (DRef) arrangement and metallic bipolar plates with unique flow fields arrangement. The anode side of each cell has a bi-functional electro/reforming anode, which consists of two layers. The function of the first layer is to reform methanol, and the function of the second layer is to catalyze H2 oxidation. More specifically, the second layer contains a Pt-based catalyst and is indirectly adjoined to a highly active Cu-based methanol reformer (innovative double layer arrangement), which provides the required flow rate of hydrogen for fuel cell operation at 210 °C, while directly absorbing the high-grade heat produced by the MEA for the methanol steam reforming endothermic reaction. Taking into account the requirement for a light-weight and low-volume stack, Cu-based methanol reforming catalyst was supported on carbon papers, resulting in ultra-thin reformers. The proposed configuration may offer a significant decrease in the weight and volume of the whole power system, as compared with previous voluminous foam-based cells [7], [9]. Moreover, special bipolar plates were made of metal alloys, being stable at elevated temperatures and having high corrosion resistance in the strong acidic environment of the high temperature membranes. Recently developed cross-linked Advent TPS® membrane electrode assembly was employed for fuel cell operation at 210 °C.

Section snippets

Fuel cell modules design

Various IRMFC modules (single cells, 15 W short stack and 100 W stack) were designed, constructed, assembled and tested following the configuration presented in Fig. 1. This design follows the internal reforming concept of our previous publications [5], [6], [7], [8], [9], however the main technological advances have to do with the reformer arrangement and the bipolar plates material and flow fields. Taking into account the requirement for a light-weight and low-volume stack, the design of the

Results and discussion

Double reformer arrangement. Long time catalytic runs spanning a period of 300 h were presented in a recent publication [8] for single layer methanol reformer in a IRMFC-type reactor. These tests were carried out at 210 °C with the CuZnAlOx-based methanol reformer in the absence of MEA. Under concentrated methanol feed streams, the reforming catalyst performance was quite stable and less than 20% decline in methanol conversion was observed during the 300 h period of the catalytic run. It has to

Conclusions

Light weight and volume, high temperature PEM fuel cell modules (specific size and volume of 20 kg kW−1 and 30 W lt−1, respectively, for the IRMFC stack) with double reformer arrangement based on a highly active CuZnAlOx methanol reforming catalyst and gold-coated aluminium alloy bipolar plates were developed for portable, mobile and off-grid energy applications. Taking advantage of the characteristics of Advent TPS® MEAs for operation at temperatures as high as 210 °C, the tests with

Acknowledgements

Financial support from The Fuel Cells and Hydrogen Joint Undertaking (FCH JU; Grant agreement no. 325358) is gratefully acknowledged; Advent Technologies S.A. is also acknowledged for successful cooperation during the IRMFC project and providing the MEAs of the tested modules.

References (32)

Cited by (0)

View full text