A high-yield and ultra-low-temperature methanol reformer integratable with phosphoric acid fuel cell (PAFC)

Accepted Manuscript A High-Yield and Ultra-Low-Temperature Methanol Reformer Integratable with Phosphoric Acid Fuel Cell (PAFC)

Hsueh-Sheng Wang, Cheng-Ping Chang, Yuh-Jeen Huang, Yu-Chuan Su, FanGang Tseng PII:

S0360-5442(17)30898-8

DOI:

10.1016/j.energy.2017.05.140

Reference:

EGY 10948

To appear in:

Energy

Received Date:

24 February 2017

Revised Date:

04 May 2017

Accepted Date:

22 May 2017

Please cite this article as: Hsueh-Sheng Wang, Cheng-Ping Chang, Yuh-Jeen Huang, Yu-Chuan Su, Fan-Gang Tseng, A High-Yield and Ultra-Low-Temperature Methanol Reformer Integratable with Phosphoric Acid Fuel Cell (PAFC), Energy (2017), doi: 10.1016/j.energy.2017.05.140

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ACCEPTED MANUSCRIPT

Figure captions Fig. 1(a). Schematic of the Direct Reformer-PAFC hybrid system; (b) the real experimental setup of the hybrid test system without the gas supplying part.........5 Fig. 2. Schematics and images of the LTPOM-reformer:(a) the catalytical reaction chamber design. (b) the flow path arrangement and the packaging holder (right image). (c) the catalyst loaded micro reformer (top image). ..................................6 Fig. 3. Experimental setup for LTPOM-Reformer testing............................................7 Fig. 4. Schematic of glass microfiber/PTFE composite membrane and the images of their morphologies ..................................................................................................8 Fig. 5. Experimental setup for LTPOM-Reformer/PAFC hybrid system testing.........8 Fig. 6 SEM cross section images of the catalyst layer inside the finger type channel of the reformer with catalyst (a) one time coated, (b) three times coated, and (c) fully loaded. .....................................................................................................................9 Fig. 7. Left side from (a)-(c): the performance of reformers with various catalyst thicknesses tested at different mixing ratio C (1, 3.5 and 5.5) of reactant gases. Right side from (d)-(f): the performance of reformers tested by different reactants gas mixing ratio for different catalyst thicknesses................................................11 Fig. 8. The influence of the fuel flow rate on the reformer performance .....................13 Fig. 9. The influence of the depth of channel and repeating test on the reformer performance ..........................................................................................................13 Fig. 10. The influence of high fuel flow rates on the reformer performance................15 Fig. 11. The influence on CO concentration by various (a) flow rates and operation temperatures, and (b) depths of channel. ..............................................................16 Fig. 12. Performance of PAFC tested by reformed gas and pure hydrogen .................17

Table caption Table 1. The ICP-MS measurement of the catalyst amount for reformers with different coating times.......................................................................................................11 Table 2. The ICP-MS measurement of the catalyst amount for reformers with different depth of channel..................................................................................................14 Table 3. The activity and throughput of micro methanol reformer ............................16

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ACCEPTED MANUSCRIPT 1 2 3 4 5 6 7 8 9 10 11

A High-Yield and Ultra-Low-Temperature Methanol Reformer Integratable with Phosphoric Acid Fuel Cell (PAFC) Hsueh-Sheng Wang a, Cheng-Ping Changa, Yuh-Jeen Huang b, Yu-Chuan Sua, and Fan-Gang Tseng a, * a Department

of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan, Republic of China (R.O.C)

b Department

of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan, Republic of China (R.O.C) *email: [email protected]

Abstract

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To provide sufficient hydrogen at lower temperature (<180℃) to small phosphoric acid fuel cells

14 15

mole/min, or 644.8 ml/min/cm3, at 180℃) is developed and integrated with a high performance PAFC.

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more hydrogen throughput at a much lower temperature (180 ℃, decreased from 225 ℃) with compatible

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yield can also be obtained at much lower operation temperature of 130℃. In integration testing, a

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(PAFC), an ultra-low-temperature (130-180℃) methanol reformer with high hydrogen yield (5.9*10-4 Compared to the previous reformer [26], the performance of the current reformer can produce 39.4 folds methanol conversion rate (83%), owing to the synergic effects from optimizing the catalyst amount and reactive area, enlarging the depth of the channel, and increasing the concentration and flow rate of reactant fuel. Commendably, 79% methanol conversion rate and 5.2*10-4 mole/min hydrogen production 132mW/cm2 power density is generated by directly employing the reformed gas (41.6% H2, 28.1%H2O, 28.5% CO2, and 1.8% CO) as the fuel to a small PAFC, a roughly 45.8% power generation efficiency is obtained when compared to that by injecting pure Hydrogen gas into the same PAFC, demonstrating a compatible performance when considering hydrogen of only 41.6% purity is provided. Key words: low temperature reformer; Fuel cell; methanol partial oxidation; hydrogen generation, phosphoric acid fuel cell, 1.

Introduction To provide low-carbon power for slowing down greenhouse effect, fuel cells have been emerging

as alternative power sources potentially for various applications in transportation, portable electronics, and residential power generators for their high overall system efficiencies, high energy density, and ecofriendly nature. Among various applicable areas, portable devices are one of the promising areas for fuel cell applications, especially for small proton-exchange membrane fuel cells, such as PAFC, due to its high efficiency and high volumetric energy densities (up to 2500 Whl-1 for hydrogen or 5000 Whl-1 for liquid methanol, when compared to 700 Whl-1 provided by lithium-ion batteries)[1]. However, the major issue encountered in PAFC is the need of fuel supply from hydrogen-rich based sources and its middle rage operational temperature at 120-180℃. Compared to the conventional ways of directly using compressed/liquid hydrogen, hydrogen-rich gases in-situ generated from liquid fuel, such as methanol, 2

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provides superior advantages of high volume capacity, simple refueling process, and continues utilization

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Currently, methanol has been widely selected a more favored source for hydrogen production

of the existed fuel distribution systems [2]; . Therefore, it is more practical for a PAFC to integrate with a reformer which can reform hydrogen-rich hydrocarbon fuels, such as methanol, ethanol, or acetic acid, into hydrogen at the similar operation temperature [4]. owing to its sulfur-free, high hydrogen-to-carbon ratio, low vaporization temperature, low reaction temperature, and high energy density, when compared to other hydrocarbon fuels. In many studies, oxidation of methanol can offer an easy way to transform methanol into the hydrogen-rich gas via either decomposition of methanol (DM), steam reforming methanol reaction (SRM), or partially oxidative methanol reforming reaction (POM). The equations of these three reactions are as followed (Eq.1-3) [6]

CH3OH  CO  2H2 ( Δ H 0298  92.0kJmol1 ) 49

CH3OH  H2O  3H2  CO2 (Δ H 0298  49.5kJmol1 ) CH3OH  0.5O2  CO2  2H2 (Δ H

50 51 52 53

(1)

0 298

(2) 1

 192.4kJmol )

(3)

So far, SRM reaction (Eq.2) has been the most widely utilized method to convert methanol into hydrogen for its high hydrogen yield (for one mole methanol generating 3mole hydrogen). However, the endothermic reaction requires external heat source to maintain the operation of the system normally at a

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higher temperature (>300℃). In addition, SRM produces a considerate amount of CO (>100ppm) for the

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CO2  H2  CO  H2O (Δ H 0298  41kJmol 1 )

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reverse water gas shift reaction (R-WGSR, Eq. 4) more favored at higher temperature [9], which may poison Pt catalyst in the fuel cell to deteriorate fuel cell performance [11]

(4)

An alternative method to generate hydrogen with relatively lower amount of CO in lower operating temperature is POM reaction. Although its hydrogen production rate is about 33 % lower than that of

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SRM method, POM reaction can be operated at lower temperature (<180℃) which is beneficial for

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To incorporate with fuel cell for portable power generation, miniaturized reformer has been

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integrating with HT-PEMFC like PAFC , support a quick start-up at a cold condition, provide self-heating by exothermic reaction, and require only air for reaction [13]. Hence, POM reaction is adopted in this study. reported to be integrated with micro evaporator and chancels [15] and employing micro electromechanical system (MEMS) technology to minimize reformer size [16]. The improvement of micro reformer performance was also realized by fine-tuning operation temperature, fuel feeding rate, micro channel designs, internal pressure drop, and catalyst coating quality [17]. A. Pohar et.al. applied MEMS technology to produce micro reformer, and experimental results showed that the gaseous flow would not undergo as many directional changes, which would cause a smaller pressure drop and would provide a uniform velocity distribution by positioning the inlet and outlet gates in the flow direction [18]. In examining reformer efficiency, space time (mcat/FMeOH, Kg S mole-1) is usually considered as an 3

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important parameter. When space time is getting shorter, higher efficient catalyst is required to maintain

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In addition to the selection of catalyst, there are three key factors regarding the interface between

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the same performance in the condition of increasing feeding rate. Space time is also one of the vital factors to examine the ability of the reformer to conduct conversion rate over 90% [19]. For methanol conversion into hydrogen, Pt- and Pd-type catalysts are too expensive though showing better methanol decomposition with the assistance of ceria [20], thus Cu-based catalysts as well as other transition metals are widely used instead to provide compatible performance in lower operating temperature. catalyst and reformer for ensuring reformer’s high performance, including reaction surface area, catalyst coating condition, and pressure drop of the fluidic channels. K. Y. Huang, et.al. introduced silicon-based micro columns combined with boehmite/bentonite/PVA binder to produce a porous catalyst layer on the channel’s wall by fill-and-dry coating process. The results revealed that the performance of micro reformer and the thickness as well as the profile of the catalyst layer in the channel were improved at lower operating temperature. It was also recommended that the usage of boehmite and bentonite could be reduced to provide more reaction catalyst surface area [22]. To introduce catalyst into micro channels, two major schemes were usually adopted, including onwall coating and pack-bed stacking. To enhance the adhesion between the catalyst layer and the Si-based channel wall for both on-wall coating and pack-bed stacking types of reformers, AlOOH-based binders were normally adopted. However, they would significantly block catalytic reaction in micro channels and deteriorate the performance of micro reformer [25]. In our previous study, a centrifugal coating process was introduced to increase the adhesion between the catalyst layer and the Si-based channel wall without the application of binders, and the finger-shaped channel design and a specific composition of catalyst slurry incorporating the advantage of both wall-coating and pack-bed stacking catalyst in the reformer with a tunable and thicker catalyst layer [26]. The experimental results showed that increasing thickness of catalyst layer, lowering binder’s ratio, and incorporating optimized structure aspect ratio are the three important factors to improve conversion rate especially in lower operating temperature. However, the hydrogen yield was low (1.5*10-5 mole/min) for direct fuel cell application and the optimized operation temperature is at 200-225℃, still too high for directly integration with phosphorous fuel cell (PAFC, operated at 120-180℃) in the future [26]. Furthermore, for HT-PEMFC, PAFC, the CO tolerance is a thorny issue, which should be reduced lower than 5 volume fraction (50000ppm) [3]. As a result, one of the objectives in this study focuses on the further lowering down the operation temperature

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of the reformer to 130℃ and increasing the hydrogen yield to 34.7 folds as well as restricting the

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On the other hand regarding the introduction of reforming reaction into fuel cell systems, recent

concentration of the CO as less as possible by the applications of advanced catalyst ingredient and incorporation of improved catalyst arrangement and channel design. researches demonstrated that methanol could be directly reformed on the anodes of Pt, Pd, or Ru for the middle range temperature fuel cell (e.g. PAFC), which eliminated the need for heat exchanger and allowed the operation at middle range temperature to improve CO tolerance[29]. The direct reformation, however, utilizing anode as catalytic sites resulted in not only power density reduction of micro fuel cell 4

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but also hot-spots occurrence from POM reaction which leaded to inhomogeneous temperature

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graphite layer to separate the reforming catalyst from fuel-cell-catalyst to prevent the corrosion of the

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However, there was still very few studies discussing the integration between micro methanol

distribution at the MEA [30]. Therefore, this issue might be overcome by adding spatial separation of the reforming reaction chamber from the micro fuel cell system [30]. The separation could also provide more flexibility for the application of different fuels and the associated catalysts in the reformer to minimize the carbon poisoning issue on anode [30]. The other issue on direct anode reforming regarded the dissolution of the reforming catalyst into phosphoric acid leaked from MEA, which would lead to the deterioration of the catalyst. To solve this issue, G. Avgouropoulos et. al proposed to employ a thin reforming catalyst, and the experimental result showed that a power density of 116mW/cm2 was obtained via H2 directly supplied by the reformer[31]. reformer and PAFC for hybrid operation. The second part of the current work is to investigate the real performance of a reformer integrated with a PAFC by directly utilizing the reformed gases without CO2 and water filtration as a fuel for operation, as shown in Fig. 1(a-b). The adopted PAFC was earlier developed in our lab with high-performance and low-phosphoric-acid-leakage by using synergic composite membrane of micro glass fiber and nano PTFE [33]. (a)

Anode

CH3OH(g)+0.5O2

Reformed gas

O2 in

Cathode (b)

Reformed gas

Anode

O2 in

Cathode 127 128 129

CH3OH(g)+0.5O2

Fig. 1(a). Schematic of the Direct Reformer-PAFC hybrid system; (b) the real experimental setup of the hybrid test system without the gas supplying part.

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(a)

(c)

20mm

Inlet hole

channel

D

Channel wall

Cross section view of channel

channel

Inlet hole

20mm

Outlet hole

(b) CH3OH(g)+0.5O2

Outlet hole

Reformed gas

Stainless-based testing holder

O-ring Pyrex glass Micro channel 130 131 132 133

Fig. 2. Schematics and images of the LTPOM-reformer:(a) the catalytical reaction chamber design. (b)

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Experimental 2.1 Design and fabrication of LTPOM-Reformer

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the flow path arrangement and the packaging holder (right image). (c) the catalyst loaded micro reformer (top image).

Fig. 2 (a)shows the detailed design of the LTPOM-Reformer. The micro channel is made by using photolithography process and deep silicon RIE dry etching on silicon substrate (100) and each device consists of a volume 20mm*20mm*500μm, while the total volume of channels in the reformer is 0.063cm3. On the top side of this plate, a finger-shaped rectangular channel is fabricated and used as micro methanol reformer. The flow path way of reactant fuel in the channel is described as the blue dash line of Fig. 2(a). According to our previous study, the R value (R=L/W), where W and L is the width and length of the channel, plays an important role on the performance of the micro reformer, and the best conversion rate was obtained at R= 3.4 [26]. Therefore, this study follows this aspect ratio but adjusts the depth (D) of the channel from 350μm to 450μm to increase reaction volume for more hydrogen generation. Then, groove-walls were treated by oxygen plasma for improving hydrophilic property at 100W for 30 seconds. Fig. 2 (b) shows a schematic of the micro POM-reformer composed of one siliconbased micro channel and one cover plate of Pyrex glass with inlet/outlet holes. A stainless-based holder is chosen as the device package, and methanol steam-oxygen-argon mixture can be injected into the micro methanol reformer from the inlet port and the produced gases are released from the outlet port. A fabricated micro POM reformer was bound by a cover plate of Pyrex glass after loaded with catalyst, as shown in Fig. 2 (c). The preparation/fabrication steps of the catalyst and the micro channel both basically followed the 6

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processes described in our previous work [26]. Cu-Mn-Zn based catalyst was prepared via deposition

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at 70℃ under stirring condition. At the same time, 2M of Na2CO3 was dropped into the DI-based solution

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out aging process to form an appropriate crystal structure under the circumstance of stirring condition.

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105℃ for overnight. The dried catalyst was calcinated in air at 400℃ for 2 hour.

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then the chip was dried at 105℃ for 30 minutes. The loading process was completed after the reformer

precipitation following co-precipitation (CP) method [34]. The mixture of Cu-5H2O-NO3, Mn-5H2ONO3, and Zn-5H2O-NO3 precursor solutions stored in burette were dropped into 500ml of deionized water to keep the pH value at 7 to start the crystallization of catalyst during titration process until the precursor solutions was completely used up. Afterward, the pH value of catalyst solution was gradually transformed into more basic because of the release of hydrogen carbonate ion (-HCO3-) while carrying After the pH value reached pH 8, the catalyst solution containing the insolubly metallic-compound Cu/Mn/Zn precipitates were filtrated and washed fully with 2 liter of deionized water, and then dried at The catalytic slurry was first prepared with a binder’s ratio (B) and solid content (S) to be 0% (pure catalyst without the addition of binder) and 15%, respectively, before being injected into the micro reformer. After micro reformer was loaded with catalytic solution through fill-and-dry process, a centrifugal process [26] was introduced to increase the thickness and surface area of catalyst layer, and fully loaded with catalyst, and the fabrication process was finished by annealing at 400℃ for 2h.

Methanol reformer

Pump 170 171

Fig. 3. Experimental setup for LTPOM-Reformer testing

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2.2 Experimental setup of LTPOM-Reformer and hybrid testing

180

Fig. 3 shows the experimental setup for testing the performance of the fabricated micro reformers, consisting of a reactant fuel supply, a hotplate and K-type thermocouples put on the surface of the micro methanol reformer to accurately control the operating temperature, and a gas chromatograph equipment to analyze the reformed gas composition. The feed rate of argon and oxygen were originally set at 81.7ml/min and 6.1ml/min by the mass flow controller (the Brooks instrument, 5850E), and the flow rate of liquid methanol was 0.02 ml/min supplied by peristaltic pump to obtain the steam flow rate of 12.2 sccm after evaporating. The composition of the reactant gas followed the POM stoichiometric reaction. Then, the fuel gas was well premixed in the mixing chamber at 90℃ before delivering into the micro 7

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Glass microporous fiber  Pore size: 1-1.5μm  Porosity: 93% 

PTFE nanoporous fiber  Pore size: 50-400nm  

Thickness: 675μm

Porosity: 70% Thickness: 25μm

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Fig. 4. Schematic of glass microfiber/PTFE composite membrane and the images of their morphologies reformer at 2 sccm via MFC to carry out the reforming reaction. In this study, in order to improve the hydrogen yield further, the flow rate of reactant gas was raised, and the ratio of argon in the reactant gas was reduced, when compared to the previous study, by following the 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑖𝑜: C = 𝑋 + 𝑌 (𝑋 + 𝑌 + 𝑍), where X, Y, and Z represents the flow rate of methanol, oxygen, and argon, respectively. The reformed steam gas was analyzed by a gas chromatography for distinguishing the composition of H2, H2O, CO, CO2, and remained MeOH in the reformed gas. The performance of the conversion rate, hydrogen selectivity and hydrogen production yield were acquired based on the analysis results from a mass spectrometer. Afterwards, the micro reformer was tested at the temperature ranging from 250 ºC to 160 ºC adjusted by hotplate. Mass flow meter

Mixing tank(90℃) Reactant gas Reformed gas Cathode

Anode

O2 Methanol reformer

CH3OH

193 194 195 196 197 198

Pump

K-type thermocouples

Reformer system

H2

O2

PAFC system

Fig. 5. Experimental setup for LTPOM-Reformer/PAFC hybrid system testing Fig. 5 shows the experimental setup for hybrid system testing which consisted of a LTPOMreformer and a PAFC. The design and fabrication process of the PAFC and the innovation of the selfmade MEA are elaborately introduced in [33] and in Fig. 4, respectively. Moreover, the PAFC here was conducted to integrate with the micro reformer to analyze the real performance of the fuel cell after 8

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employing the reformed gas as the anode fuel. For hybrid testing, the testing system for the reformer was

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temperatures. Current load was finally applied when the open circuit voltage rises to a stable value after

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starting purging fuel into the cell. Afterward, the fuel cell was tested in a 10℃ step from 120℃ to 150℃

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In order to investigate the efficiency by the reformed gas directly supplied from the reformer, the

the same but without Ar supply, while the PAFC was tested by a fuel cell testing system, Scribner 850e, USA. The fuel cell was first tested with the supply of pure hydrogen and oxygen with 100 sccm H2 (standard cubic centimeter) to the anode and 100 sccm O2 to the cathode without humidification. The fuel cell was heated up gradually from room temperature to the operation temperatures. When it rose up to 90ºC, the cell temperature was maintained for 20 minutes to evaporate all water away from the phosphorous acid in ion exchange membrane thoroughly to ensure optimal performance at elevated

to obtain the performance curves. PAFC was then connected with a LTPOM-reformer without CO2 and water filtration. The inlet gas of the anode was then switched from pure hydrogen to the outlet of the micro methanol reformer after the

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cell temperature reached 150℃. The flow rate of inlet gases was varied from 40 sccm to 50 sccm in a 5-

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2. Results and discussion 3.1 Reformer design and optimization Catalyst contents

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sccm step at 180℃ to study the influence on the performance of the PAFC.

To be integratable with an intermediate temperature fuel cell (i.e. PAFC), it’s a critical to design/operate the reformer at a lower temperature (<180℃) while still provide sufficient hydrogen gas. To achieve that, optimizing the amount of the catalyst coating in the reformer is essential to allow a high methanol conversion rate at lower operating temperature. For catalyst coating, various coating layers, from one, three times, to fully loading of catalyst to the reformer were tested at different concentrations of reactant gases of various normalized mixing ratio C, which was defined as the ratio of the reactant gases percentage (MeOH and O2) to that of the original composition (MeOH: 12.2%, O2: 6.1%, Ar: 81.7%), and it was ranged from 1 (original composition), 3.5, to 5.5 (pure MeHO and O2 without carrying gas Ar).

(a)

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(c)

(b)

100 μm

Catalyst layer

100 μm

100 μm

Fig. 6 SEM cross section images of the catalyst layer inside the finger type channel of the reformer with catalyst (a) one time coated, (b) three times coated, and (c) fully loaded. By adding the coating times, the thickness of the catalyst layer in the reformer could be increased accordingly, as shown in Fig. 6. From Fig. 6(a), the thickness of the catalyst is only about 40μm for one time coating, however, a pack-bed-like channel can be achieved by 8 times coating and the catalyst 9

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thickness increased to approximately 330μm equal to the depth of the channel, as shown in Fig. 6(c).

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The left-side figures of Fig. 7 (a-c) show the GC-analysis results of the reformer products based

Thicker catalyst layer not only can increase the yield of products but also lead to a significant operating temperature reduction. These characteristics result from the accumulation of heat and increase of Thiele modulus of inlet and outlet from thicker catalyst layer, resulting in a lower diffusion rate and higher retaining time of the fuel [36]. Where Thiele modulus represents the ratio of reaction to diffusion rate within the catalyst, which can be used to distinguish catalytical activity. If the value is larger than one, it means the local catatytical efficiency is high. on various catalyst coating times at various reactant gases mixing ratio C supplied at 2sccm. Several parameters were employed to identify the performance of each micro reformers, including CONV. %, (conversion rate of methanol), S H2.% (hydrogen selectivity), and H2-yield (hydrogen yield, mole/min), and they are defined as followed, respectively: CONV.%=

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𝑚2 𝑚1

× 100%, and SH2.% =

𝑚3 𝑚3+𝑚4

× 100%,

where m1-4 are the amount of the methanol fed into micro reformer, the consumed methanol, the produced hydrogen, and water, respectively. The experimental result showed that all reformers can

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achieve 99% of methanol conversion rate when operated between 225 and 250℃ at the original

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was gradually lower down to 160℃, only the reformer fully loaded with catalyst can carry out high

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concentration of reactants (C=1), revealing that the reformers with over 95% of conversion rate only require a thin catalyst layer or small amount of catalyst, as shown in Fig. 7(a). When testing temperature performance, including 88% of conversion rate and 87% of hydrogen selectivity. From the ICP-MS measurement results, as tabulated in Table 1, the amount of the catalyst stored in the reformer is increased with the increased coating times from 5.3mg to 33.6mg, which suggests a positive relationship between the increased catalyst amount and the modified efficiency of the reformer. Table 1. The ICP-MS measurement of the catalyst amount for reformers with different coating times Coating times

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Cu

Mn

Zn

*MCu+Mn+Zn (mg)

*wt.%

Fully load

28.8

19.5

51.7

33.6

5

30.7

19.2

50.1

30.5

3

30.9

19.4

49.7

19.7

1

30.4

19.4

50.2

5.3

* MCu+Mn+Zn(mg): the sum of the mass of metals contained in the catalyst *wt.% : the mass percentage of metals contained in the catalyst

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X8(reduction) X8(unreduction) X5 X3 X1

100 80 60 40 20 0 -5

1.00x10 -6 7.50x10 -6 5.00x10 -6 2.50x10 0.00 160

180

200 220 o Temp. ( C)

240

260

105 90 75 60 45 30 15 0 105 90 75 60 45 30 15 0

o

250 C o 225 C o 200 C o 180 C o 170 C o 160 C

-5

3.5x10-5 3.0x10-5 2.5x10-5 2.0x10-5 1.5x10-5 1.0x10-6 5.0x10 0.0 1

2

CONV.%

o

90 75 60 45 30 15 0 -5 3.5x10-5 3.0x10-5 2.5x10-5 2.0x10-5 1.5x10-5 1.0x10-6 5.0x10 0.0 SH2%

180

190

200

210

220

230

240

250

H2-yield (mole/min)

SH2.%

170

260

1

2

CONV.%

CONV.%

190

200

210 220 o Temp. ( C)

230

240

250

H2-yield(mole/min)

SH2.%

SH2.%

180

4

5

6

(f) fully loaded Fully loaded X5 X3 X1

170

3

Concentraion (C)

(c) C=5.5 (pure reactant gas)

H2-yield (mole/min)

6

250 C o 225 C o 200 C o 180 C o 170 C o 160 C

o

263 264 265

5

96 84 72 60 48 36

CONV.%

Fully loaded X5 X3 X1

Temp. ( C)

259 260 261 262

4

(e) 3 times

100 80 60 40 20 0 100 80 60 40 20 0 -5 3.0x10-5 2.5x10-5 2.0x10-5 1.5x10-5 1.0x10-6 5.0x10 0.0

105 90 75 60 45 30 15 90 75 60 45 30 15 0 -5 3.6x10-5 3.0x10-5 2.4x10-5 1.8x10-5 1.2x10-6 6.0x10 0.0

3

Concentration (C)

(b) C=3.5

H2-yield (mole/min)

CONV.%

90 75 60 45 30 15

SH2.%

(d) one time

H2-yield(mole/min)

H2.yield (mole/min)

SH2.%

CONV.%

(a) C=1

260

100 95 90 85 80 75 70 96 88 80 72 64 56

o

250 C o 225 C o 200 C o 180 C o 170 C o 160 C

-5

3.5x10 -5 3.0x10 -5 2.5x10 -5 2.0x10 -5 1.5x10 -5 1.0x10 1

2

3

4

5

6

Concentration X

Fig. 7. Left side from (a)-(c): the performance of reformers with various catalyst thicknesses tested at different mixing ratio C (1, 3.5 and 5.5) of reactant gases. Right side from (d)-(f): the performance of reformers tested by different reactants gas mixing ratio for different catalyst thicknesses.

The methanol concentration of the input fuel In addition to the increment of catalyst amount, it was suggested that catalyst with zero valance state have higher catalytic reaction by lacking of oxidative-state metal [37]. As a result, a reduction procedure

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was adopted to fully refresh the catalyst in the reformer at 250℃ for 30 min with 10% H2 at 40sccm prior

270 271

low temperature at 160℃ and C=1. Therefore, in the following experiments, all the reformers will

to the test, which will be operated from 250℃ to 160℃, and the result is shown in the empty-righttriangle line in Fig. 7(a). It demonstrated a very promising result regarding further increasing the methanol conversion rate and hydrogen selectivity up to 99.9% and 95%, respectively, at an extremely undergo a pre-refreshed step by the reduction process before testing. 11

ACCEPTED MANUSCRIPT 272 273

To further improve the hydrogen yield, the concentration of methanol and oxygen in the reactant gas was

274 275 276 277 278

(d)-(f) for different catalyst loadings. When testing temperature is lower than 200℃, the reactive area

279 280 281 282 283 284 285

at a high concentration reaction gas flow C=5.5 which is superior 3.4 folds to the one of C=1 at 180℃,

286 287 288 289 290 291 292

250℃, but the performance of the previous reformer with catalyst coated only 1 time, resulting in rare

gradually increased from C=1 to the pure reactant fuel at C=5.5, and the results are showing in Fig. 7 and amount of catalyst in the reformer become the two dominated factors on the performance, so the conversion rate of the reformer with catalyst coating times less than 3 times are all deteriorated with decreasing temperature, as shown in Fig. 7 (d)-(f). Apparently, only the reformer with fully loaded catalyst can perform a conversion rate higher than 97% and the hydrogen yield up to 3.7*10-5(mole/min) and the reformed product are consisted of 73.8%H2, 2.8%H2O, 1.4%CO, 20.1%CO2 and 1.9%CH3OH. The others’ performance was declined quickly with the decreased operating temperature and the increased concentration of reactant gas, which reveal that the reactive area and amount of catalyst in the reformer are the two dominated factors on the performance. On the other hand, from the point of view of gas hour space velocity (GHSV), the GHSV for the reformer coated with 1, 3 and 8 times are 25930, 8640 and 3140, respectively. Despite of the higher GHSV, all of them show excellent conversion rate at catalyst content, dramatically deteriorates below 200℃ whereas the reformer with thicker thickness faced relatively lower GHSV can sustain performance at the lower temperature. These results provide initial evidences to prove that a large amount of catalyst stored in the reformer and a low valance state of the catalyst are the two key parameters to offer sufficient reactive area, catalytic ability, the higher retaining time, which can effectively reduce the diffusing rate and GHSV to keep a high efficiency of reformer at lower operating temperature for high concentration of reactive gases.

293 294 295 296 297 298 299

Reactant gas supply I In order to provide sufficient hydrogen yield to the fuel cell to generate electricity, the flow rate of the pure reactant gas was significantly increased. The flow rate was gradually adjusted from 2 sccm to 30 sccm for testing. The results are shown in Fig. 8. The highest hydrogen yield, 4.9*10-4 mole/min, of the reformer is appeared at with a 95.4% of methanol conversion rate and 30 sccm flow rate. However, this temperature is too high for the further application to integrating PAFC. On the other hand, if we chose

300 301 302

the operational temperature at 180℃-200℃, the conversion rate will be declined from 98% to 70% at the

303 304

25 sccm. Even though the current reformer exhibits decent performance at the temperature above 225℃,

305 306 307 308 309

modulus for sustaining the performance at severe operating conditions lower than 200℃.

same flow rate of 30 sccm. If we can bear a little higher temperature operated at 225℃, a high hydrogen production rate and high methanol conversion rate of 4*10-4 mole/min and over 95% will be obtained at it cannot support adequate reactive area and catalyst amount/thickness to additionally raise the Thiele However

lower operation temperature can provide the advantage to diminish the negative effect of hot spot and non-uniform temperature distribution. On the other hand, for GHSV, it is reduced from 51430 to 7410 displaying a better performance. At lower temperature operation, it’s desired to decrease GHSV while preserving the conversion rate. 12

102 96 90 84 78 72 66

H2-yield (mole/min)

SH2.%

CONV.%

ACCEPTED MANUSCRIPT 250C 225C 200C 180C

96 88 80 72 64 56 -4

4.8x10-4 4.2x10-4 3.6x10-4 3.0x10-4 2.4x10-4 1.8x10-4 1.2x10-5 6.0x10 0.0

310 311

15

20

25

30

Channel volume To further reduce the operation temperature while keeping the hydrogen yield, it’s essential to further increase the amount of catalyst fully accommodated in the reformer by enlarging the channel volume to diminish GHSV. Consequently, two depths of channel, 400μm and 450μm, were introduced in the following study. The reformer with deeper channel was also fully loaded with catalyst. The reactive and repeating test at 25 sccm on the same reformer was adopted to investigate the performance and the durability of the reformer, and the results are shown in Fig. 9. The both GHSV are 32258 and 28571, respectively, which is less than the one of the reformer with the channel of 350μm. The dash line represents the result of the repeating tests on the same reformer at the same operation parameters.

SH2.%

324 325 326 327

10

Fig. 8. The influence of the fuel flow rate on the reformer performance

H2-yield (mole/min)

322 323

5

flow rate (SCCM)

CONV.%

312 313 314 315 316 317 318 319 320 321

0

100 95 90 85 80 75 70 65 100 95 90 85 80 75 70 65

400um 450um 400um-R1 450um-R1

-4

4.4x10 -4 4.0x10 -4 3.6x10 -4 3.2x10 -4 2.8x10 120

140

160

180 200 o Temp. ( C)

220

240

260

Fig. 9. The influence of the depth of channel and repeating test on the reformer performance Fig. 9 presents that the better performance and durability can be achieved via the reformer with deeper channel owing to high reactive site and area provided by large amount of catalyst. From Table 2, the catalyst amount is further increased from 32.4mg to 45.4mg by the channel depth increment, which matches the tendency revealing from Fig. 9. Finally, a 94% of conversion rate and 4*10-4(mole/min) of 13

ACCEPTED MANUSCRIPT 328 329

hydrogen yield were achieved at a low temperature 180℃, and the operation temperature is 45℃ lower

330 331 332

extremely low one at 130℃, a 78% conversion rate and 3*10-4(mole/min) high hydrogen yield still can

333

than that on 350μm channel. Moreover, even if the operating temperature is further lowered down to an be obtained. Therefore, to improve the efficiency of the reformer, raising the operating temperature and lessening the GHSV at the same testing condition are recommended according to the results. Table 2. The ICP-MS measurement of the catalyst amount for reformers with different depth of channel Depth of channel

Cu

(μm)

Mn

Zn

*MCu+Mn+Zn (mg)

*wt.%

450

29.5

18.8

51.7

45.4

400

30.2

19.9

49.9

42.3

350

30.1

19.7

50.2

32.4

* MCu+Mn+Zn(mg): the sum of the mass of metals contained in the catalyst

334

*wt.% : the mass percentage of metals contained in the catalyst

335 336 337 338 339 340 341

Reactant gas supply II Then, to increase the hydrogen yield further at lower temperature, the flow rates higher than 25 sccm were studied on the reformer with 450μm channel. The flow rate was adjusted from 30 sccm to 45 sccm in a 5-sccm step for the testing. Fig. 10 presents the influence of the higher flow rate on the reformer performance. From the result, the hydrogen yield is enhanced with the increasing flow rate accompanied by similar conversion rate and hydrogen selectivity until 45sccm. It demonstrates that 45sccm flow rate is too high to sustain the sufficient conversion reaction at lower temperature. Surprisingly, the conversion

342 343 344 345 346

rate, the hydrogen selectivity and the hydrogen yield at 130-180℃ all keep stable in a range from 79%

347 348 349

result at the same operating temperature, 180℃, by optimizing the channel height, catalyst contents, and

350 351

(min*cm3)) up to 802.5, 689.9, and 644.8, for 225℃, 200℃ and 180℃, respectively, which are much

352 353

can still be observed at much lower temperature of 130℃. From the experimental result of this section,

354

crucial factors for ameliorating the performance in the temperature lower than 180℃.

to 83%, 73% to 77%, and 5.2*10-4(mole/min) to 5.9*10-4(mole/min), respectively, which is much superior to the reformers with shallower channel presented in the Fig. 8 and the reformed product are consisted of 45.2%H2, 14.3%H2O, 3.3%CO, 27.5%CO2 and 9.7%CH3OH. The result of hydrogen yield has been improved 39.4 folds with only sacrificing the conversion rate of 12% compared to our previous fuel flow rate. By taking the volume of channels, conversion rate, hydrogen selectivity, and space time into account, the current reformer demonstrated a very low space time 4.3 and a high H2 throughput (ml/ superior to those in the literature tabulated in Table 3. Moreover, a relatively high H2 throughput, 568.3, deeper channels and high content of catalyst to provide larger active area and lower GHSV are some

14

100 95 90 85 80 75 70 65 90 84 78 72 66 60

H2-yield(mole/min)

SH2.%

CONV.%

ACCEPTED MANUSCRIPT

357 358

-4

7.2x10 -4 6.6x10 -4 6.0x10-4 5.4x10 -4 4.8x10 -4 4.2x10-4 3.6x10 120

355 356

30SCCM 35SCCM 40SCCM 45SCCM

140

160

180

200

220

240

260

o

Temp. ( C)

Fig. 10. The influence of high fuel flow rates on the reformer performance Table 3. The activity and throughput of micro methanol reformer

Reference

Temp. ℃

CONV. %

Throughput ,H2, ml/(min*cm3)

mcat/FMeOH (Kg s mole-1)

[38]

270

60

55.6

4.4

[39]

260

99

237.9

5.1

Wash-

[40]

300

98

293.6

4.8

Coating

[41]

190

95

38.5

473

[42]

270

99

114.6

32.7

[43]

270

70

114.8

17.8

[26]

180

98

53.3

2.27

225

99.6

802.5

200

93

689.9

180

83

644.8

130

79

568.3

This study

Catalyst bed

Catalyst Type

Reaction

CuO/ ZnO

SRM

/Al2O3 Packing

CuO/ 4.3

Slurry

MnO

POM

/ZnO

359 360 361 362 363 364 365 366 367 368 369 370

3.2 Influence on CO concentration CO poison is a severe issue especially for low-temperature (LT) fuel cells in which CO tolerance is less than 10 ppm. Thus for better efficiency, the supply of pure hydrogen is desired for most LT fuel cells. Although phosphoric acid based fuel cells operated at elevated temperatures allow considerable higher tolerance to CO, yet when adsorbed on the platinum catalyst surface, CO can still form a monolayer to block the active sites for the adsorption/desorption of H2. A tolerance of CO was observed at a concentration less than 5% for Pt/C catalyst operated at 453 K and moderate values of voltages [28]. On the other hand, the presence of CO2 in the reformers only dilutes the fuel and limits the maximum current density of the fuel cell without much poisoning effect. Therefore, the concentration of CO and CO2 in the reformate should be restricted further for improving performance. According to the WGS-reaction, the CO concentration decreases with the decrease of testing temperature. From Fig. 11(a), it reveals that 15

ACCEPTED MANUSCRIPT 371 372

the CO concentration is diminished with reduced operation temperatures, from 152.8 ppm to 17.8 ppm,

373 374 375 376 377

temperatures beyond 200 ℃. However, the CO concentration slightly grows as the flow rates or the

378 379 380 381 382

channels and the GHSV also gradually increases from 28571 to 37040. As a result, the temperature

within all the flow rates, and also reduced with the increased fuel flow rates until 15 sccm in the temperatures are over 15 sccm or less than 200 ℃, respectively, which may be attribute to the poor conversion rate lower than 85 %, leading to more side reactions. Besides, Fig. 11(b) displays the effect of the varied depths of channel on the CO emission. In that the CO concentration drops with the shallower depth of the channel which might be ascribed to higher gas velocity in the reformer with shallower gradient from the interior to the surface of the catalyst layer in the reformer is enhanced through the raising flow rate and GHSV, promoting the exothermic WGS-reaction. In summary, decreasing the operating temperature as well as the depth of channel and increasing the fuel flow rate are important considerations to limit CO generation in reformers while maintain the conversion rate over 85%. (a)

(b) 160

50

o

100 80 60

25sccm

45 40

CO(ppm)

250 C o 225 C o 200 C o 180 C

120

CO(ppm)

60 55

140

35 30 25

450um 400um 350um

20

40

15 10

20

5

0

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398

5

10

15

20

25

120

Flow rate (sccm)

140

160

180

200

220

240

260

o

Temp. ( C)

Fig. 11. The influence on CO concentration by various (a) flow rates and operation temperatures, and (b) depths of channel.

3.3 Testing of PAFC with pure hydrogen/oxygen An 86% phosphoric acid loaded GMF/mPTFE composite membrane was assembled with electrodes into a fuel cell for performance evaluation. The fuel cell was heated up gradually from room temperature to the operation temperatures. When it rose to 90ºC, the cell temperature was maintained for 20 minutes to evaporate all water vapor away from the phosphorous acid thoroughly to ensure optimal performance at elevated temperatures. Load was finally applied when the open circuit voltage rises to a stable value after delivering fuel into the cell. Afterward, the fuel cell was tested in a 10℃ step from 120℃ to 150℃ to obtain the performance by reacting with the fuel of pure hydrogen and oxygen. The best performance of a single cell achieved 288 mW/cm2 power density at 699 mA/cm2 current density under 100 sccm hydrogen, 100 sccm oxygen supply and ambient pressure at 150ºC, as shown in the right figure of Fig. 12. However, the performance started deteriorating while further raising the operating temperature since the hydrophilic modification on the surface of PTFE might be destroyed, resulting in the proton exchange rate of the membrane declining.

399 400 401

3.4 Testing of LTPOM-reformer/PAFC hybrid The micro reformer was heated up to 180℃ in advance and reacted for 15 min for stabilizing the 16

ACCEPTED MANUSCRIPT 402 403 404 405 406 407 408

POM reaction, and then was connected to the PAFC system without CO2 filtration, as shown in Fig. 3 in

409 410 411 412 413 414 415 416 417 418

even though the slight deterioration of the reformer’s performance. The result reveals that the power

419 420 421 422 423 424

The power generation efficiency, 45.8%, can be obtained by a direct ratio of the power density generated

supplementary materials. The fuel resource of the anode was switched from pure hydrogen to the reformed gas followed after the step of the section 3.3. According to the result from Fig. 10, the optimized performance, 40sccm, was first chosen as the fuel of anode. When the hydrogen source changed to the reformed gas, the nearly half of power density, 128mW/cm2, still can be generated, but the phenomenon of limitation of the mass transport for fuel cell has appeared, representing that the amount of the reactive fuel is not sufficient to sustain the reaction so that the inlet flow rate of the reformer needs to be increased density is increased to 132mW/cm2 at the case of the inlet flow rate of 45sccm (reformed gas composition: 41.6% H2,35sccm, 15.9% H2O, 25.5% CO2, 1.8% CO and 15.2% methanol), but decreased to only 123mW/cm2 at 50sccm, indicating that the conversion rate of the reformer has been decreased too severely to provide enough hydrogen to the PAFC. Furthermore, there’s no N-shaped negative differential resistance (N-NDR) type current-voltage behavior, reported by Hanke-Rauschenenbach et al [44], revealed in the I-V curve. The N-NDR curve indicates that less than 10 vol.% of hydrogen is generated via the reformer, condensed water appears at the outlet of the cathode, and the electrode behavior becomes unstable [31]. Yet, no condensed water was observed in the gas outlet of cathode after the hybrid testing. Hence, higher hydrogen yield is desired for better integrated performance in this study. by reformed gas to that by pure hydrogen (132 mW/cm2/288 mW/cm2) based on the similar inlet gas flow rate. However, considering the hydrogen purity of the reformed gas is 41.6%, it indicates the performance of the hybrid system is compatible with the pure hydrogen input one. To further improve the power generation of the hybrid system, it is important to filter out CO2 and H2O for obtaining highly pure hydrogen supply. 1.0

1.0

140

0.8

0.5 60

0.4

427 428 429 430 431

160

0.4

120 80

0.2

20

0.1

40

0.1 0

50

100

150

200

250

300

350

0 400

0.0

2

0

200

400

600

800

1000

0 1200

2

Current density (mA/cm )

426

0.5

0.3

2

0.2

40

200

0.6

2

Reformed gas

0.3

Voltage (V)

Voltage (V)

80

240

Pure H2

0.7

Power density (mW/cm )

0.6

Power density (mW/cm )

100

0.7

425

280

120

0.8

0.0

320

0.9

0.9

Current density (mA/cm )

Fig. 12. Performance of PAFC tested by reformed gas and pure hydrogen

4.

Conclusion

A micro-reformer with high conversion rate and hydrogen yield at extremely low operation temperatures between 130 and 200 ℃ is introduced in this study. The optimized designs and operation conditions include a 43% increment on channel depth, 8 times catalyst loading, 5.5 folds more concentrated fuel/gas ratio, and 50% increase on the fuel flow rate. Owing to the lower operation temperature and higher 17

ACCEPTED MANUSCRIPT 432 433 434

GHSV, CO emission is greatly restricted less than 20 ppm, which is enabled by the deeper channel and

435 436

mincm3 can be obtained at 130℃ and 225℃, respectively. In hybrid testing, the peak power density of

437 438

pure hydrogen injection case) can be achieved at an inlet flow rate of 45 sccm at 150℃. As a result, an

439

density and CO emission for hybrid reformer/fuel cell systems.

440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484

the higher flow rate conditions. The results show that the conversion rate of 79% and 99.6%, the hydrogen selectivity of 73% and 91.5%, and the hydrogen production rate of 568.3 and 802.5 ml/ 132mW/cm2 or the power generation efficiency of 45.8% (at 41.6% hydrogen purity, when compared to optimized reformer operated in an extremely low temperature is advantageous for improving the power

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Highlights



Energy



A High-Yield and Ultra-Low-Temperature Methanol Reformer

Integratable with Phosphoric Acid Fuel Cell (PAFC)



A 132mW/cm2 power density is generated by a small PAFC with a 45.8% efficiency

 

An ultra-low-temperature(130℃) POM reformer with crisscrossed channels is proposed. The methanol conversion rate over 80% is obtained at the temperature higher



130℃. The CO emission lower 20ppm is demonstrated.