Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0

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Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell Yuki Terayama a,*, Shoichi Furukawa b, Munemitsu Nomura b, Takamasa Haji b, Masamichi Nishihara a,c, Omar Mendoza d, Yoshitsugu Sone d,e, Hiroshige Matsumoto a,* a

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Department of Hydrogen Energy Systems, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan c Next-Generation Fuel Cell Research Center (NEXT-FC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, 252-5210, Japan e SOKENDAI, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, 252-5210, Japan

article info

abstract

Article history:

A water-absorbing porous electrolyte electrolysis cell is presented consisting of a hydrophobic

Received 16 February 2018

gas diffusion layer (GDL), a controlled-hydrophobicity electrocatalyst layer, and a hydrophilic

Received in revised form

porous electrolyte layer. The specific character of this cell is that high-pressure water is

15 April 2018

injected directly into the porous electrolyte layer and is resisted by the electrocatalyst layer

Accepted 19 April 2018

and GDL, which have strong water support force. In this study, the preparation method of the

Available online xxx

electrocatalyst layer and the porous inorganic electrolyte layer, and the evaluation of water electrolysis using the prepared layers were investigated. The optimized conditions and

Keywords:

preparation methods of each layer of the MEA (i.e. the GDL, electrocatalyst layer, electrolyte

Water electrolysis

layer) were determined. The assembly method and conditions of these three layers were also

Membrane electrode assembly

determined for fabricating MEAs for water electrolysis. The evaluation of water electrolysis

Electrocatalyst layer

tests using this MEA showed that the hydrogen evolution rate obeyed Faraday's Law in the low

Hydrophobicity

current density region (<10 mA cm
2).

Inorganic porous electrolyte

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In recent years, hydrogen production has become increasingly attractive as the application of renewable and sustainable

energy systems becomes more widespread. Effective utilization of hydrogen energy will help to resolve the issues of global warming, by replacing the combustion of fossil fuels [1,2,3]. Water electrolysis is one of the most practical and valuable ways to generate hydrogen gas using electrical power

* Corresponding authors. E-mail addresses: [email protected] (Y. Terayama), [email protected] (H. Matsumoto). https://doi.org/10.1016/j.ijhydene.2018.04.137 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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[4]. This generated hydrogen gas is expected to be reconverted to electrical power in fuel cells. Therefore, the development of water electrolysis has been studied for various applications such as decreasing environmental carbon dioxide, application in space stations, or creating hydrogen-electricity circulating systems as an alternative to using thermal or atomic power generation [5]. Polymer electrolyte membrane water electrolysis (PEMWE) is one of most practical ways to produce hydrogen gas in industry. Compared with other conventional electrolysis technologies such as steam electrolysis or alkaline water electrolysis, there are some advantageous point in PEMWE cell. Steam electrolysis is practically operated at high temperature from 800  C to 1000  C, which expected to give decreasing (theoretical) electrical voltage and availability for various kinds of metal as electrocatalyst [6]. However, there are some problems such as the requirement of heatingtreating furnace with high power and large size and high cost for operation and maintenance [7]. On the other hand, alkaline water electrolysis is operated at relatively lower temperature than 200  C compared with steam electrolysis, and have achieved operation with relatively high energy efficiency, low product cost of electrocatalyst, and application for practical and industrial use [8]. As the problems for alkaline water electrolysis, the water tank inside the cell is damaged by alkaline solution for log-time use and a purity of generated hydrogen gas is relatively low [9]. Although there are some advantageous and disadvantageous points for other conventional electrolysis (e.g. steam electrolysis and alkaline electrolysis), PEMWE have expected to behave water electrolysis system at higher temperature operation than 100  C due to less corrosion problem, higher energy efficiency, smaller size of the electrolyzer, and production of high-purified hydrogen gas [10]. Existing PEMWE cells generally utilize dense, sulfonated fluoropolymer electrolyte polymer membranes (e.g. Nafion) as the electrolyte layer, carbon-free anode and carbon-supported cathode electrocatalyst layers, and carbon fiber or titanium based GDLs. (Fig. 1 (a)) [11,12]. During operation, water is generally injected through the anode GDL to react at the electrocatalyst layer. The generated protons (Hþ) are then transported from the anode to the cathode through the Nafion membrane. However, there are some practical problems with current PEMWE technologies, such as the poor thermal stability of Nafion at >80  C [13], dehydration of the electrolyte layer at high temperature and low humidity condition to decrease proton conductivity [14], the high cost of Nafion, and the high cost of the electrocatalyst layer (e.g. platinum or iridium oxide). As described above, PEMWE is expected to be able to operation of electrolyzer with low electrical voltage and high energy efficiency at higher temperature than currently practicable temperature (80  C). However, this expect have not been achieved yet because decreasing proton conductivity and breaking film shape were frequently occurred during operation at 100e120  C, which is correspond to flection point of thermal stability of Nafion (glass transition temperature Tg) [15]. At higher temperature than Tg, the structure of microphase separation between hydrophobic fluorinated backbone and hydrophilic sulfonated side chain is easily collapsed. Therefore, various researcher have been

investigated the method how to improve the thermal stability of Nafion such as other substitute with changing chemical structure [16], cell design [17], and composite material with inorganic materials [18]. Over the past 7 years, the authors have studied a new design of water electrolysis cell, namely, the water-absorbing porous electrolyte electrolysis cell. This consists of: (1) a porous inorganic electrolyte layer designed to absorb water; (2) controlled-hydrophobicity electrocatalyst layers; and (3), hydrophobic GDLs designed to trap water in the cell (Fig. 1 (b)) [19,20]. It is expected that this cell design will achieve improved thermal stability, avoid dehydration of the electrolyte, and decrease the cost compared with current PEMWE cells. One way that the cost will be decreased is that inorganic electrolytes can use relatively inexpensive industrially produced metal oxide materials such as titanium oxide or zeolites. Dehydration issues will be solved by injecting water directly into the porous electrolyte layer, rather than through the anode GDL. One key design aspect of this cell is that the electrocatalyst layer is modified to contain hydrophobic fluorinated materials such as poly (tetrafluoroethylene) (PTFE) and/or poly (vinylidene fluoride) (PVDF). These improve the hydrophobicity of the layer, resulting in improved generation and collection of hydrogen and oxygen gases, leading to decreased electrode overvoltage. Furthermore, this electrocatalyst layer is designed to resist pressurized water at a backpressure greater than 1.0 MPa, allowing the generation of pressurized hydrogen and oxygen gases. In the case of highpressure water electrolysis, our cell is expected to separate and transport generated hydrogen and oxygen gases to the outside of the cell, without gravity-assisted vapor-water separation (i.e. buoyancy). There for the main applications of this novel cell will be hydrogen and oxygen production in the microgravity conditions of space (e.g. on the international space station), or in deep-sea locations. As mentioned above, this novel cell design requires a porous inorganic electrolyte layer, and an electrocatalyst layer with controlled hydrophobicity. We have been investigating inorganic porous electrolytes (Fig. 2), for over 10 years. We first investigated hydrous titania (HT). This was essentially titanium oxide nanoparticles modified with sulfuric acid groups (eSO4). This worked effectively as an electrolyte layer in our designed cell. HT was prepared easily by hydrolysis reaction of titanyl sulfate (TiOSO4$nH2O) [21]. Relatively low voltage was observed for this cell (3.0 V at 20 mA cm
2), and quantitative hydrogen evolution obeyed Faraday's law in water electrolysis, test due to the high proton conductivity of HT [19]. However, the critical disadvantage of HT was sulfuric acid groups leaching, resulting in reduced proton conductivity after 24 h [22]. In order to avoid this, several alternative materials are proposed. For example, Wu et al. recently reported layered protonated titanate hierarchical spheres (LTHS) for lithium-ion batteries which showed excellent cycling stability and rate capability when evaluated as anode materials for high-power lithium-ion batteries [23]. LTHS may be suitable for our water-absorbing cell because it has H2Ti2O5$H2O structure, and large surface area (due to the spherical particle shape). These factors will contribute to decreasing detachment of acid groups and increasing proton conductivity, respectively.

Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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Fig. 1 e Schematic images of different water electrolysis cells: (a) a conventional PEWE cell consist of dense polymer electrolyte layer (e.g. Nafion), electrocatalyst layer, and carbon paper type gas diffusion layer. Water was supplied from outside of anode electrocatalyst layer. (b) water-absorbing porous electrolyte cell consist of hydrophilic porous electrolyte layer, controlled hydrophobic electrocatalyst layer, and fully hydrophobic gas diffusion layer. Water was injected for porous electrolyte layer directly [20].

Fig. 2 e Schematic image of a porous inorganic electrolyte layer for the water-absorbing porous electrolyte cell [19]. Protons move on the surface of inorganic porous electrolyte material in water.

Another important part of this water-absorbing cell is the electrocatalyst layer, which has slightly different functions compared to conventional water electrolysis cells. As well as performing the electrochemical reactions to generate hydrogen and oxygen gas, and to conduct protons/electrons, it also needs to block pressurized water (Fig. 3). To the best of our knowledge, there are no previous reports of electrocatalyst layers fulfilling all of these requirements for water electrolysis. In similar works in the field of hydrophobic

electrocatalyst layers, Alderucci et al. reported PTFE -modification of the electrode of a phosphoric acid fuel cell (PAFC) for enhanced durability and corrosion resistance [24]. Chan et al. also reported the influence of PTFE content and baking temperature on the durability of PAFC electrodes [25]. In addition, the cell performance and long-term tests were investigated for PTFE-bonded porous gas diffusion carbon electrodes by Ghouse et al. [26]. On the other hand, studies of hydrophobic electrodes in polymer electrolyte fuel cells (PEFCs) have been

Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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Fig. 3 e Schematic image of the designed structure and function of the hydrophobic electrocatalyst layer for the waterabsorbing porous electrolyte electrolysis cell. Hydrophobic electrocatalyst layer (or GDL) consist of electrocatalyst PtC (or carbon black), hydrophobic material PTFE, and binder PVDF. These layer work as water support, gas permeation, and electrical conductive in water absorbing porous electrolyte layer.

extensively reported over the past 20 years. Hirawa et al. reported that a polymer electrolyte membrane (PEM) on crosslinked PTFE showed improved open circuit voltage (OCV), due to the prevention of gas cross-over compared with conventional PEMs [27]. Kitahara et al., explored a novel hydrophobic/hydrophilic double microporous layer (MPL) -coated GDL prepared using PTFE, and showed enhanced PEFC performance [28]. However, the PTFE-based electrocatalyst layers reported in many studies are unsuitable for use in our waterabsorbing porous electrolyte electrolysis cell. Here, we investigate the preparation of inorganic porous electrolyte layers based on LTHS, a suitable electrocatalyst layer for water-absorbing electrolysis cells. For the electrolyte layer, slurry preparation of LTHS for spraying, and preparation conditions of electrolyte films of LTHS were investigated. The mixing ratio of platinum-supported carbon (Pt/C), PTFE, and PVDF in the electrocatalyst layer, and film formation conditions were investigated. The preparation of GDLs using similar technologies was also investigated. Finally, hydrogen production was carried out using the designed electrolyte layer, electrocatalyst layers, and GDLs developed in this work, in a water-absorbing porous electrolyte electrolysis cell.

Experimental Materials Titanium (IV) butoxide (TBT, 97.0%, Sigma-Aldrich Co. LLC.), N,N-Dimethylformamide (DMF, 99.5%, Kishida Chemical Co., Ltd.), 2-propanol (IPA, 99.7%, Kishida Chemical Co., Ltd.), acetone (99.7%, Kishida Chemical Co., Ltd.), ethanol (99.8%, Kishida Chemical Co., Ltd.), Pt/C (46.6 wt% of Pt, Tanaka Kikinzoku Kogyo, TEC10E50E), 1-Methyl-2-pyrrolidone (NMP, 99.0%, Wako Pure Chemical Industries, Ltd.), Poly (vinylidene fluoride) (PVDF, Mw & 180,000, Sigma-Aldrich Co. LLC.), and Acetylene Black (AB)/NMP (8653BLACK, Tokushiki Co., Ltd) were used as-received. PTFE dispersion in toluene (NS-06, 30 wt% of PTFE) was provided form NAGOYA GOSEI Co.,Ltd. Pt/ C was dispersed into NMP by Tokushiki Co., Ltd as a contract company. Distilled water (15.0 MU cm) was purified with Pure

Water Equipment Elix Advantage3 (Merck Millipore Co.). Micro-porous layer (MPL)-coated carbon paper (GDL29BC) and IrO2 paste were purchased from Chemix Co. Ltd. MPL-coated carbon paper was cut into circular disks with f23 mm diameter, using a hand-punch (Nogamigiken Co., Ltd.) before use.

Preparation of the hydrophobic GDL using an AB/PTFE/ PVDF composite Hydrophobic GDLs were attached to MPL-coated carbon paper by a transfer method (Fig. 4), using the following procedure. 3.42 g of 10 wt% AB/NMP mixture, 1.29 g of 30 wt% PTFE/ toluene mixture, and 13.62 g of 2.0 wt% PVDF/NMP mixture were added into a 100 mL glass beaker, and then stirred at room temperature for 15 min. Then, this AB/PTFE/PVDF mixture in NMP was coated onto the non-glazed surface of aluminum foil using a doctor blade with a 350 mm gap, and then dried on a hotplate at 140  C until the NMP and other solvents evaporated. This process was then repeated to increase the thickness and mechanical stability of this hydrophobic GDL. In order to remove the solvents completely, the GDL film still attached to the aluminum foil was then dried in a vacuum oven at 100  C for 15 min. Then, the AB/PTFE/PVDF film still attached to the aluminum foil was placed onto the surface of MPL-coated carbon paper, and hot-pressed at 220  C and 220 kg/cm2 for 3 min. After hot-pressing, the samples were cooled to room temperature, then placed into 4 wt% HCl aqueous solution to completely remove the aluminum foil. Finally, the sample was washed by purified water several times, then dried at room temperature.

Preparation of the hydrophobic electrocatalyst layer using a PtC/PTFE/PVDF composite The hydrophobic electrocatalyst layer was attached onto MPLcoated carbon paper by a transfer method (Fig. 4), using the following procedure. In the case of the cathode 3.42 g of 2.6 wt % PtC/NMP mixture, 0.143 g of 30 wt% PTFE/toluene mixture, and 2.79 g of 2.0 wt% PVDF/NMP mixture were added to a 100 mL glass beaker, and stirred at room temperature for 15 min. After this, the PtC/PTFE/PVDF slurry was coated onto

Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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Fig. 4 e Preparation of the electrocatalyst layer or GDL on MPL-coated carbon paper by transfer method. the non-glazed surface of aluminum foil by doctor blade with a 350 mm gap, and then dried on hotplate at 140  C. This process was repeated another 2 times in order to increase the thickness and mechanical stability. The coated film still attached to the aluminum foil was then dried in a vacuum oven at 100  C for 15 min. This was then placed onto an MPLcoated carbon paper, and hot-pressed first at 220  C and 240 kg/cm2 for 3 min, then turned 90 and hot-pressed again at 220  C and 240 kg/cm2 for 3 min. After hot-pressing, the sample was cooled to room temperature and placed into a 4 wt % HCl aqueous solution to remove residual aluminum foil completely. Finally, the sample was washed by purified water several times, then dried at room temperature.

Evaluation of GDLs and electrocatalyst layer Electrical resistance (R, U cm2) was evaluated perpendicular to the plane using a galvanostat (Hokuto Denko Co., Potentiostat/ Galvanostat HA-151B) to apply a constant DC current (Fig. 5 (a)). The prepared sample (GDL or electrocatalyst layer) was set up between a SUS holder, as shown Fig. 5(a). The contact pressure between the sample and the test cell was controlled by compressing at 0.5 kN using a uniaxial press. The gas permeability was measured using a home-made test cell (Fig. 5 (b)) giving the pressure loss from 0.01 MPa to 0.25 MPa using Ar gas with a precisely controllable regulator. The Ar gas permeability was measured using a soap-film flow meter (HORIBA STEC Ltd., Film Flow Meter VP-2). The normalized gas permeability rate V’ (mL$cm
2$atm
1$min
1) was calculated from the obtained values V (mL$min
1) as follows: V0 ¼

V A$P

where A is the contact area between the GDL and Ar gas (cm2), and P is the applied pressure loss (atm). The water permeability was also measured using a home-made test cell (Fig. 5 (c)). Purified water was injected into the cell and pressure was applied using nitrogen gas, controlled by a regulator from 0 to 1.0 MPa. The applied pressure just before water leakage was defined as the limiting pressure Plim (MPa).

Preparation of porous inorganic electrolyte layer using layered protonated titanate hierarchical spheres (LTHS) LTHS were prepared using a solvothermal method [23]. In a 300 mL glass beaker, 10 mL of DMF and 30 mL of IPA were added as solvents and magnetically stirred at room temperature for 15 min. After that, 1.0 mL of TBT was added to the mixed organic solvent and stirred at room temperature for 15 min. This mixture was transferred into a 100 mL Teflonelined stainless steel autoclave, sealed, and heated at 200  C for 20 h. After heating, a precipitate with light-yellow color was obtained. This was washed using acetone and ethanol by centrifuge at 3000 rpm for 15 min 3 times, in order to remove unreacted TBT and DMF. Finally, the washed precipitate was dried in vacuum oven at 60  C overnight to obtain a light-yellow LTHS powder. 2 mm and 0.05 mm zirconia beads (YTZ ball, Nikkato Co. Ltd.) were used for milling. Purified water was used as a dispersion medium. 0.5 g of LTHS powder, 19.5 g of purified water, and 50 g of zirconia beads with 2.0 mm diameter were placed in a 45 mL zirconia pot, and milled at 500 rpm for 1 h using a planetary ball mill (P-7 classic, Fritsch Co. Ltd.). After that, the obtained white mixture was filtered through a stainless steel sieve with 250 mm mesh to remove the zirconia

Fig. 5 e Setup of (a) electrical resistance, (b) gas permeability, and (c) water support force measurements. Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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beads. The LTHS powder was again charged into a zirconia pot with 50 g of 0.05 mm diameter zirconia beads. This was then milled at 800 rpm for 7 h with 20 min intervals for every 10 min of operation. After milling, the obtained mixture was filtered through a 25 mm mesh stainless steel sieve to remove the beads, resulting in a flesh-colored dispersion. The z-averaged particle size of LTHS after milling was evaluated by dynamic light scattering (DLS) using Zetasizer mV (Malvern Instruments Ltd). In DLS measurements, the concentration of the test solution was prepared at 0.05 wt % dilution using purified water. Electrolyte layers were prepared by a spray method using following procedure. Firstly, a slurry of LTHS after milling were concentrated to 1.8 wt% by heating on at hot-plate at 140  C. The concentrated slurry was diluted to 0.9 wt% with ethanol and magnetically stirred for 15 min at room temperature. The obtained slurry of LTHS in water/ethanol was transferred into a spray reservoir, and then sprayed onto the surface of the electrocatalyst layer using a hot-plate at 80  C. The loading of LTHS on the electrodes was 0.5 mg cm
2.

Water electrolysis test The MEA (Fig. 6) for water electrolysis cell testing was fabricated using the following procedure, using a GDL, electrocatalyst layer, and electrolyte layer. Two pieces of MPL-coated carbon paper for the cathode and anode electrodes were cut into circular disks with f23 mm diameter, using a handpunch. A layer of AB/PTFE/PVDF ¼ 35/35/30 (volume ratio) was coated onto both MPL-coated carbon papers as described above. On the cathode side, an electrocatalyst layer of PtC/ PTFE/PVDF ¼ 35/25/40 (volume ratio) was laminated onto the surface of the GDL as described above. The anode electrocatalyst layer was sprayed onto the GDL using a hand spray machine charged with IrO2 paste ink (Chemix Co. Ltd.). During spraying, the GDL was heated using a hot-plate at 70  C in order to evaporate the solvent. After spraying, the sample was hot-pressed at 140  C and 240 kg/cm2 for 3 min, twice. The loaded amount of Pt on the cathode was 0.1 mg cm
2, and of IrO2 on the anode was 1.5 mg cm
2. Before preparation of the

Fig. 7 e Structure of a water-absorbing porous electrolyte electrolysis single cell. In this cell, a porous electrolyte layer was sandwiched between a smaller cathode and larger anode electrode (electrocatalyst layer and electrolyte layer). The water was directly injected and pressurized by back-pressure. The generated gas was separated to hydrogen and oxygen gases in the electrocatalyst layers.

electrolyte layer, the catalyst-coated anode and cathode GDLs were bored into circular shapes with f16 mm and f10 mm diameters, respectively. In order to avoid injected water leaking in the test cell (Fig. 7) during water electrolysis, the edge of the cathode electrode was then sealed by painting with silicone putty (CEMEDINE Co. Ltd., 8051 N) diluted in hexane, and then dried for 6 h at room temperature to be hardened completely. The LTHS slurry in water/ethanol was then sprayed onto the surface of the anode and cathode electrodes, on a hot-plate at 80  C. The loading of LTHS on the electrodes was 0.5 mg cm
2. Current-voltage (i-V) characteristics and hydrogen gas evolution rate of the prepared MEAs for water electrolysis were evaluated using the cell (Fig. 1). In particular, the cathode MEA was immobilized using silicon putty to seal the water in the tank with back-pressure and generated gases. Electrical voltage was measured by Galvanostat (Hokuto Denko Co.,

Fig. 6 e Schematic images and pictures of fabricated MEA consisting of a gas diffusion layer, electrocatalyst layer, and electrolyte layer. Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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Potentiostat/Galvanostat HA-151B) to apply a constant DC current. The amount of generated hydrogen gas was evaluated by gas chromatography (Agilent, 3000A Micro GC) with column (Molsieve, 14 m  320 mm  12 mm) and 20 mL min
1 of Ar flow gas, and were determined by calculation using a calibration curve of 1% H2/Ar gas. The theoretical evolution rates were calculated by using Faraday's law.

Results and discussion Evaluation of mixing method, hot-press temperature and materials ratio on GDL performance First, the optimized conditions of GDL preparation using the AB/PTFE/PVDF composite were investigated. This is important, since the design of electrocatalyst layers is more complicated using a composite of electrocatalyst (PtC), hydrophobic material (PTFE), and binder (PVDF), compared with the design of conventional GDLs. In previous research [20], our group reported that AB/PTFE composite films coated on the surface of MPL-coated carbon paper by transfer method (Table 1, Run 1) showed reasonable water support force, gas permeability, and electrical resistance. However, this GDL did not show sufficient gas permeability and electrical resistance to satisfy requirements for use in a water-absorbing porous electrolyte electrolysis cell. This was attributed to the fact that hot-pressing at 360  C melts the PTFE, which filled the pores in the GDL, decreasing gas permeability and increasing electrical resistance. In order to resolve this problem, we investigated the effect of adding PVDF binder into the AB/PTFE composite films. Since PVDF has a lower melting point (178  C) [29] compared with PTFE (327  C) [30], we first decreased the hotpress temperature to 180  C and 220  C (Table 1, Run 2 and 3). The GDL prepared at 180  C (Run 2) shows no water support force. On the other hand, the GDL hot-pressed at 220  C (Run 3) has a water support force of 1.0 MPa. There are no cracks in the surface morphologies of the GDLs (Run 3 and 4). After hot pressing at 180  C, the PVDF did not melt, so that the AB/PTFE/ PVDF film was not strongly bound to the MPL-coated carbon paper and broke easily during water permeability tests. A hotpressing temperature of 220  C was high enough to melt the PVDF and bind the layer to the MPL-coated carbon paper, resulting in higher water support force of 1.0 MPa. Hotpressing at higher than 220  C risks melting the PTFE

particles [31]. Therefore 220  C was deemed a suitable temperature for transferring AB/PTFE/PVDF films to MPL-coated carbon paper. The effect of AB/PTFE/PVDF ratio on the GDL performance was also investigated. In our previous research [11], the performance of fabricated GDLs with different AB/PTFE volume ratio from AB-rich conditions (AB/PTFE ¼ 92/8 (v/v)) to PTFErich conditions (AB/PTFE ¼ 16/84 (v/v)) were investigated. It was concluded that there is a trade-off in the dependence of AB/PTFE ratio on the water support force, gas permeability, and electrical resistance. Increasing the PTFE ratio decreased the gas permeability and increased the electrical resistance due to the pore-filling and insulating properties of PTFE. However, the water support force was decreased with increasing AB ratio because of the low film-toughness of ABrich layers. According to several experiments, we determined that an AB/PTFE ratio of 31/69 (v/v) was optimal. Therefore, here we investigate the effect of AB/PTFE/PVDF ratio on GDL performances close to the AB/PTFE ratio of 31/69 (v/v) (Table 1, Run 3e6). Uniformly mixing AB, PTFE, and PVDF in the dispersion medium is a serious difficulty in the preparation AB/PTFE/PVDF composite films, because of the different affinities of AB, PTFE, and PVDF for the different solvents. In our understanding, there has been no previous report on the preparation of uniform AB/ PTFE/PVDF composite films. Our strategy focused around finding suitable solvents with good dispersibility for AB, PTFE, and PVDF. Commercially-available dispersions of AB in NMP dispersion; PTFE in toluene dispersion; and PVDF soluble in NMP were obtained, and could be mixed uniformly using only magnetic stirring. This AB/PTFE/PVDF slurry was coated onto aluminum foil using doctor blade, and then successfully transferred to MPL-coated carbon paper. We prepared the GDLs with different AB/PTFE/PVDF ratio, and evaluated the performances (Run 3e6). The GDLs with AB/PTFE/PVDF ¼ 35/35/30 vol ratio showed the most suitable performances, with low electrical resistance of 0.01 U cm2, suitable gas permeability of 93 mL atm
1 cm
2 min
1, and a water support force of 1.0 MPa. The overall performance of optimized GDLs with AB/PTFE/ PVDF composite coatings is superior to that of our previously reported GDLs with only AB/PTFE. This result suggests that PVDF works as effective reinforcing material in the GDL composite. It is postulated that PVDF polymer chains are finely entangled between AB and PTFE particles to form uniformlydispersed AB/PTFE/PVDF composite films. In addition, the

Table 1 e Preparation parameters of different gas diffusion layers and electrocatalyst layers. Run 1 2 3 4 5 6 7 8 9 10

Layer

Materials Volume Ratio

Temp. of Hot-press

Water Support Force (MPa)

Gas Permeability (mL min
1 atm
1 cm
2)

GDL GDL GDL GDL GDL GDL Electrocatalyst Electrocatalyst Electrocatalyst Electrocatalyst

AB/PTFE ¼ 30/70 AB/PTFE/PVDF ¼ 35/35/30 AB/PTFE/PVDF ¼ 35/35/30 AB/PTFE/PVDF ¼ 35/45/20 AB/PTFE/PVDF ¼ 35/55/10 AB/PTFE/PVDF ¼ 55/35/10 PtC/PTFE/PVDF ¼ 35/35/30 PtC/PTFE/PVDF ¼ 35/55/10 PtC/PTFE/PVDF ¼ 35/45/20 PtC/PTFE/PVDF ¼ 35/25/40

360  C 180  C 220  C 220  C 220  C 220  C 220  C 220  C 220  C 220  C

1.0 0.01 1.0 0.4 0.4 0.4 0.3 0.1 0.3 1.0

24.7 ± 0.1 e 93.9 ± 0.4 47.9 ± 0.5 208.9 ± 2.4 92.5 ± 0.1 66.6 ± 0.6 145.9 ± 5.5 182.2 ± 1.0 120 ± 1.1

Electrical Resistance (U cm2) 2.6 e 1.0 2.3 2.1 7.8 2.6 5.4 1.3 3.9

 10
2        

10
2 10
2 10
2 10
3 10
2 10
2 10
2 10
2

Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

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other effect of adding PVDF is that the hot-press temperature in the transfer method can be decreased from 360  C to 220  C. The previously reported 360  C hot-pressing transfer process was not ideal, due to thermal degradation of the electrocatalyst taking place at 360  C, decreasing the catalytic performance for the generation of hydrogen or oxygen gas. In this AB/PTFE/PVDF system, a hot pressing temperature of 220  C is sufficient to form the GDL, which is an advantage in the preparation of electrocatalyst layers.

Evaluation of mixing method and materials ratio on electrocatalyst layer performance The preparation conditions of the electrocatalyst layer were also investigated. PtC was dispersed into NMP by Tokushiki Co., Ltd, and was successfully mixed with PTFE/toluene and PVDF/NMP using magnetic stirring to obtain a uniform PtC/ PTFE/PVDF slurry. This slurry was successfully transferred to MPL-coated carbon paper, and evaluated (Table 1, Run 7e10). Run 7 (PtC/PTFE/PVDF ¼ 35/35/30 vol ratio) shows poor water support force compared with Run 3 (AB/PTFE/PVDF ¼ 35/35/ 30). This may be due to the fact that the Ketjen black (KB) used as a PtC catalyst support in this case has less hydrophobic character than that of AB. To enhance the water support force whilst maintaining gas permeability and electrical resistance, we increased the PTFE or PVDF volume ratio (Run 8e10). Among these, Run 10 (PtC/PTFE/PVDF ¼ 35/25/40) shows the most reasonable performance, with low electrical resistance (0.04 U cm2), suitable gas permeability (120 mL atm
1 cm
2 min
1), and a water support force of 1.0 MPa. These results showed that addition of PVDF can be also applied for preparation of electrocatalyst layers, similar to the case of GDL

preparation. Therefore, this optimized PtC/PTFE/PVDF electrocatalyst layer was used for fabricating an MEA for cell tests of water electrolysis.

Preparation of porous inorganic electrolyte layer using LTHS LTHS light-yellow powder was obtained by procedures outlined in previous work [23]. The electrical conductivity of LTHS powder before milling was measured to be 9.0  10
3 S cm
1, evaluated by electrical conductivity tests using compacted bar-shape samples immersed in water at 95  C. After this LTHS powder was crushed by planetary-milling using water as a dispersion medium. The appearance of the resulting slurry is uniform, clear and light-yellow in color (Fig. 8 (a)). The zaveraged particle size was 133 nm, evaluated by DLS (Fig. 8 (b)). For electrical conductivity tests, this LTHS slurry was evaporated by heating at 140  C using a hot-plate to remove the dispersion medium completely. The obtained powder (Fig. 8 (a)) was formed into a bar-shape, and the conductivity was found to be 1.1  10
2 S cm
1 by electrical conductivity tests in water at 95  C. From these results, LTHS after milling showed relatively high electrical conductivity, higher than the target of 10 mS cm
1, and suitable for the electrolyte layer of the water-absorbing porous electrolyte electrolysis cell. Fig. 8 (c) and (d) shows the SEM image of the top-surface of LTHS after milling and spraying onto the electrocatalyst layer. Although there are some cracks, and the layer is relatively dense. In addition, we also confirm that this LTHS layer works as electron insulator by using impedance measurements. Therefore, we deemed this electrolyte layer prepared by spray method using LTHS after milling was suitable for use in the water-absorbing porous electrolyte cell.

Fig. 8 e LTHS after planetary milling. (a) Picture of the dispersion of milled LTHS. (b) Particle size distribution measured by DLS. SEM image of the top surface of the prepared electrolyte layer by spraying of a LTHS slurry (c) 100 X and (d) 500 X. Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0

Water electrolysis test MEAs consisting of GDLs, electrocatalyst layers, and electrolyte layers were assembled (Fig. 6). The surface of the MPLcoated carbon paper was covered by three steps: (1) transfer of AB/PTFE/PVDF composite films; (2) transfer of PtC/PTFE/ PVDF; and (3) spraying of LTHS after milling in ethanol/water dispersion. In previous sections, we already showed that Step (1) was successful. In Step (2), the PtC/PTFE/PVDF electrocatalyst layer was placed on top of the GDL prepared in Step (1). In order to firmly attach this layer to the GDL, the hot-press pressure was set to 240 kg cm
2. In the case of weaker hotpressing pressure, the PtC/PTFE/PVDF layer was easily delaminated during HCl treatment. When higher hot-pressing pressure was used, the film was strongly attached to the GDL, but was too dense for effective gas permeation. The electrolyte layer must work as a proton conductor and an electron insulator between the anode and cathode electrodes. Therefore, a smooth and continuous electrolyte layer is required for low operation voltage and quantitative hydrogen generation. In our cell, the electrolyte layer consisted of only inorganic LTHS without any polymer binder, therefore there is a high possibility of cracking during spraying or drying. In pre-experiments in which LTHS dispersion was sprayed directly onto the electrocatalyst layer without a GDL, the sprayed electrolyte layer cracked whilst drying on the hot-plate. In order to avoid cracking of electrolyte layer, the GDL layer was introduced between the electrocatalyst layer and the MPL-coated carbon paper. Due to this concept change of the MEA structure, we could fabricate improved MEAs with a little cracking occurring (Fig. 8 (c) and (d)). Fig. 9 shows the obtained i-V curve and hydrogen evolution rate obtained from the water-absorbing porous electrolyte cell (Fig. 7), measured at 95  C, with an applied pressure

9

DP ¼ 0.10 MPa. Significantly, the hydrogen gas evolution rate (Fig. 9 (c)) is similar to the theoretical hydrogen evolution rate calculated from Faraday's Law. On the other hand, the hydrogen evolution rate (Fig. 9 (d)) of a reference MEA consisting of a commercially available non-hydrophobic electrocatalyst layer and a Nafion membrane electrolyte layer in the same test cell was almost 50% lower than the water-absorbing cell. This quantitative hydrogen evolution rate is due to the precisely designed electrocatalyst layer; the hydrophobic electrocatalyst layer prevents the injected water from transforming into the vapor phase, meaning that almost all of the generated hydrogen gas can be collected outside of the cell without dissolving into the liquid water phase (as is the case with conventional electrolysis cells). However, the voltage is relatively high, at 8 mA cm
2. This is due to concentration overvoltage deriving from the fact that the electrocatalyst layer does not contain a proton-conductive ionomer phase. Therefore, optimization by addition of an ionomer to the electrocatalyst layer is planned for the near-future. We will also consider the effect of dehydration of the porous inorganic electrolyte layer and the electrode overvoltage when voltage lower than 2.0 V was obtained at >50 mA cm
2.

Conclusions The preparation and optimization of a hydrophobic electrocatalyst layer (and GDL) and porous inorganic electrolyte layer for a water-absorbing porous electrolyte electrolysis cell was reported. The GDL and electrocatalyst layers were prepared by a transfer method using AB/PTFE/PVDF or PtC/PTFE/PVDF films, respectively. These GDL and electrocatalyst layers showed appropriate water support force, gas permeability, and electrical resistance. Dense, continuous electrolyte layers were successfully prepared on the surface of the electrocatalyst layer by spraying a LTHS nanoparticle slurry. An MEA was fabricated by assembling the GDL, electrocatalyst layer, and electrolyte layers. Water electrolysis tests using the fabricated MEA were carried out and showed that the generated hydrogen evolution rate was similar compared with the theoretical rate calculated by Faraday's Law. This suggests that our optimized hydrophobic electrocatalyst layer and electrolyte layer are suitable for use in this novel water-absorbing porous electrolyte electrolysis cell.

Acknowledgements

Fig. 9 e ieV characteristic and H2 gas evolution rate for water electrolysis using the prepared MEA (and a Nafion-based reference MEA) at 95  C (25  C for the reference MEA), and DP ¼ 0.10 MPa. (a) Closed squares (-) and (b) open squares (,) show current i-V curves of the water-absorbing and the reference MEA, respectively. (c) Closed triangle (:) and open triangles (D) show the hydrogen gas evolution rates. The dotted line shows theoretical hydrogen evolution rate calculated from Faraday's Law.

This work was supported by CREST (Creation of Innovative Core Technology for Manufacture and Use of Energy Carriers from Renewable Energy, No. JPMJCR1442), JST, Japan. The authors wish to acknowledge Dr. Stephen M. Lyth, Associate Professor at Kyushu University for editing the manuscript, and help in interpreting the significance the results in this study.

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Please cite this article in press as: Terayama Y, et al., Preparation of hydrophobic electrocatalyst layer and inorganic porous electrolyte layer for water absorbing porous electrolyte electrolysis cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.04.137