Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis

Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis

international journal of hydrogen energy xxx (xxxx) xxx Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/l...

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international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Phosphoric acid doped composite proton exchange membrane for hydrogen production in mediumtemperature copper chloride electrolysis Mohd Fadhzir Ahmad Kamaroddin a,b,*, Nordin Sabli a,d,**, Pooria Moozarm Nia b, Tuan Amran Tuan Abdullah b,c, Luqman Chuah Abdullah a, Shamsul Izhar a, Adnan Ripin b,c, Arshad Ahmad b,c a Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia b Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia c School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia d Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia

highlights

graphical abstract

 Composite membranes studied for hydrogen production from CuCl membrane electrolytic.  Phosphoric

acid

promoted

the

proton conductivity by increasing the hydrogen bonding.  High purity of H2 produced from CuCl membrane electrolysis.  Polybenzimidazole-zirconium phosphate showed enhanced CuCl electrolytic performance.  PBI/ZrP proton conductivity was 3fold higher than pristine PBI.

article info

abstract

Article history:

A copper chloride (CuCl) electrolyzer that constitutes of composite proton exchange

Received 17 May 2019

membrane (PEM) that functions at medium-temperature (>100  C) is beneficial for rapid

Received in revised form

electrochemical kinetics, and better in handling fuel pollutants. A synthesized poly-

22 August 2019

benzimidazole (PBI) composite membrane from the addition of ZrO2 followed with phosphoric acid (PA) is suggested to overcome the main issues in CuCl electrolysis, including

* Corresponding author. Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. ** Corresponding author. E-mail addresses: [email protected] (M.F.A. Kamaroddin), [email protected] (N. Sabli). https://doi.org/10.1016/j.ijhydene.2019.10.030 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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international journal of hydrogen energy xxx (xxxx) xxx

Accepted 5 October 2019

the copper diffusion and proton conductivity. PBI/ZrP properties improved significantly

Available online xxx

with enhanced proton conductivity (3 fold of pristine PBI, 50% of Nafion 117), superior thermal stability (>600  C), good mechanical strength (85.17 MPa), reasonable Cu perme-

Keywords:

ability (7.9  107) and high ionic exchange capacity (3.2  103 mol g1). Hydrogen pro-

Composite membrane

duced at 0.5 A cm2 (115  C) for PBI/ZrP and Nafion 117 was 3.27 cm3 min1 and 1.85 cm3

Polybenzimidazole

min1, respectively. The CuCl electrolyzer efficiency was ranging from 91 to 97%, thus

Copper chloride electrolysis

proven that the hybrid PBI/ZrP membrane can be a promising and cheaper alternative to

Phosphoric acid

Nafion membrane.

Hydrogen production

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Proton exchange membrane

Introduction Conventional petroleum fuels are the main driving factor for energy and power generation, which are dominated by the transportation sector that dominated by vehicles with the internal combustion engine. Transportation industries contributed about 23% of the world’s CO2 emission that consumed about three-fifths of the world’s oil supply, which contributes to the additional emissions of Green House Gases (GHG) every year [1]. There is a critical needs for a solution without adding or a very minimal carbon footprint to our atmosphere. Alternatively, there is a conventional electrolysis process; however, it requires more energy to produce the equivalent hydrogen quantity as compared to steam methane reforming and coal gasification process that requires a high temperature of up 800  C [2]. Furthermore, according to Soltani et al., the single most significant contributor of about 50% for the world’s hydrogen production comes from the steam methane reforming process [3]. Therefore, the evolution from conventional petroleum fuels to sustainable and renewable energy is critical to reducing the dependency on carbon footprint resources. Probably, hydrogen could support workable solutions via comprehensive yet affordable energy systems. However, the water electrolysis requires the highest energy input for hydrogen synthesis, although with current technological advances in hydrogen production technologies [4e6]. The global demand for sustainable and renewable energy resources as the substitute for fossil-based fuels is also increasing in tandem with the advancement in renewable energy technologies, especially in wind and solar energy. The main obstacle for wind and solar energy is their inconsistency in producing energy due to weather dependencies. Therefore, for solar and wind energy harvesting, hydrogen comes as the potential candidate for an efficient instrument for massivescale energy storage as reported by Toghyani et al. [7], and Naterer et al., [8]. Hydrogen production from water electrolysis is a promising route to generate sustainable and clean energy. To date, researchers have shifted their focus in proton exchange membrane (PEM) electrolyzer technology instead of alkaline electrolyzer because of the ability to pair with solar and wind energy resources, non-hazardous electrolytes, lower power requirement, higher hydrogen purity and current density

including more accessible hydrogen storage [9]. The PEM water electrolysis is an exciting pathway of synthesizing zero carbon footprint hydrogen. However, this technique requires the used of the expensive noble metal [10]. Several readily available electrolysis technologies can serve as a sustainable and renewable route to produce green hydrogen. Table 1 compares several electrolysis technologies for hydrogen production, including its strengths and weaknesses. Hydrogen is the purest form of the atom and is well known as a clean source of energy with no carbon footprint emissions. Even though, hydrogen is the most lavish element in the universe, the hydrogen in nature is in the form of molecules like water and not in the state of hydrogen gas. The most popular and established technology to produce hydrogen gas is from steam methane reforming (95%) and others (5%) including water electrolysis, gasification of coal and thermochemical methods [15,16]. Proton exchange membrane is the most critical part in a fuel cell or an electrolyzer which controls the robustness, performance, and efficiency in the form of MEA. At high current densities of 0.6e2.0 A cm2, this electrolyzer can produce a high quality of hydrogen from the water electrolysis. As a comparison, Esposito [17] reported a typical alkaline electrolyzer produces hydrogen at lower current densities (0.1e0.4 Acm2). Nafion is a well known perfluorosulfonic acid (PFSA) membrane which widely used for PEM fuel cell and PEM electrolyzer. Several studies conducted by Balashov et al. [18], Aghahosseini et al. [19], and Naterer et al. [20], found that the usage of Nafion in the copper chloride electrolysis gave the promising result at low temperature (<80  C). To date, several studies for hydrogen production using the copper chloride electrolysis reported the usage of Nafion membrane as listed in Table 2. Balashov et al. [18], reported a study on the CuCl electrolysis with the usage of 0.2e1 M CuCl in the 2e10 M HCl as the electrolytes found that the Nafion functions as an excellent medium for ionic transfer with no issues with material compatibilities. Nonetheless, Subianto et al. [25], reported that Nafion utilization has still issues with high copper permeability and the costly membrane. On the other hand, polybenzimidazole (PBI) membrane offers a better thermochemical and mechanical stabilities for working temperature beyond 80  C apart from being an excellent proton

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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Table 1 e Electrolysis technologies comparison for hydrogen production. Type of technologies

Strengths

PEM electrolysis [11,12]

High current density High voltage efficiency Good partial load range Good and compact system design High gas purity (up to 99.995%) High dynamic operation Rapid system response Technologically advanced; well established Cheap and cost-effective High durability Very stable Up to 70% efficiency Readily commercialized Reversible operation; as fuel cell and electrolyzer High process efficiency Enables new reaction at high temperature Enable non-noble catalyst usage High-pressure operation

Alkaline electrolysis [11,12]

Solid oxide; high-temperature steam electrolysis [11,13,14]

conductivity when doped with PA. Furthermore, the PBI is suitable for applications at electrolysis temperature above 100  C with zero humidity as reported by Li et al. [26], and Araya et al., [27]. The main advantages of PBI over Nafion of working at higher operating temperature includes quicker electrochemical activities, enhanced and easier water management, excellent thermal, chemical stability, and better tolerance with impurities [28]. In PBI/Phosphoric Acid (PA) hybrid membrane, the conductivity is greatly influenced by the level of acid doping, which corresponds from the temperature and immersion time being taken for treating the membrane [29]. As reported by Gashoul and Javad [30], the PBI based membrane is a lot cheaper than Nafion with the advantage of low fuel crossover due to their lower phase separation. On the other hand, the Nafion membrane is four-fold costlier than PBI to acquire, yet PBI still very few studies on its usage for CuCl electrolysis at elevated temperatures [31]. Even though the PBI is cheaper to purchase, the

Weaknesses New and partially mature technology High cost of components Expensive catalyst Corrosive due to acidic electrolytes Relatively low durability Costly and limited membrane Low current density Low gas purity Concentrated corrosive electrolytes Low dynamic operations Low operational pressure Operates at very high temperature up to 1000  C High heating requirement High-cost operation and components Infancy stage Low durability due to intense heat Bulky system design

thermal stability and mechanical properties of PBI is proven to be excellent for elevated temperatures beyond 100  C. Furthermore, the PA doped PBI offers high ionic conductivity at a working temperature above 100  C [32]. The predominant benefits of having higher process temperature comprise faster electrochemical kinetics, improved water handling, better thermal properties, and more resistance with contaminants [28]. Besides, Nafion may not be suitable for higher temperature applications due to the previous study for Nafion in fuel cell the tensile strength and ionic conductivity depreciated at a temperature beyond 80  C [33]. PBI is amorphous and has aromatic components as its main molecules that translate into a polymer which has excellent thermal stability and chemical resistance. While Nafion is majorly dependent on the humidity of the membrane (water for ion Hþ movement) to have excellent conductivity properties, in PBI/Phosphoric Acid (PA) hybrid membrane, the conductivity is greatly influenced by the level

Table 2 e CuCl electrolysis system for hydrogen production via Nafion membrane. Authors Rich S.S et al., 2013 [21] Gong et al., 2010 [22] P. Edge 2013 [23] Balashov et al., 2011 [18] S. Aghahosseini et al., 2013 [19] G. Naterer et al., 2015 [20] Abdo & Easton 2016 [24]

Electrolyte(s) concentration (M) 1e2 M CuCl, 6 M HCl 0.2e1.0 M CuCl 2e6 M HCl 0.002e0.2 M CuCl 2 M HCl 0.2e1.0 M CuCl 2 M HCl 0.5e1.0 M CuCl 6e10 M HCl 0.5e1.0 M CuCl 6e10 M HCl 0.2 M CuCl, 2 M HCl DI water

Temperature ( C)

Membrane

80

Nafion

24e65

Nafion

25e80

Nafion

22e30

Nafion 115

25e60

Nafion 117

45e60

Nafion 117 HYDRion Nafion/Polyaniline (PANI)

25

Electrolyte flowrate (ml min1) CuCl: 59 HCl: 130 CuCl: 3.4e22 HCl: 4.4e27 CuCl:40e200 HCl:40e200 CuCl: 30 & 68 HCl: 28.5 CuCl: 100e500 HCl: 100e500 CuCl: 600 HCl: 600 CuCl/HCl: 60 DI water: 60

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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of acid doping, which corresponds from the temperature and immersion time subjected for treating the membrane [34]. Moreover, the PBI has an excellent ability to uptake acid for the doping process [35,36]. Undoped PBI has a satisfactory level of the tensile strength of 60e70 MPa under anhydrous and 100e160 MPa under 100% humidified. On the other hand, the doping with phosphoric acid can weaken the main structure of the PBI membrane and decrease the mechanical strength in particular at high temperature [37]. Therefore, the PA acid doping level is essential to have a balance between higher proton conductivity and weaker mechanical strength of the doped PBI membrane [32]. Former researches have exhibited that sulfuric acid gives maximum conductivity as compared to other acids. The attainable conductivity ranking are as follows; H2SO4 > H3PO4 > HClO4 > HNO3 > HCl [38]. However, researchers have chosen PA for their work because the performance of fuel cell not only rely on the conductivity of the membrane and the PA doped PBI can tolerate under 0% humidity, unlike sulfuric acid doped PBI that requires a relative humidity of at least 50% [39]. Furthermore, the conductivity mechanism of PA doped PBI membrane that replaced water with PA gives the higher temperature range (100e220  C) for fuel cell operation [40]. Extensive consideration has been dedicated to preparing the PBI membrane by PA doping and characterization of its membrane. Alteration of the PBI membrane to improve surface morphology and structure by PA doping is hugely implemented. To the best of the author’s knowledge, the modification of composite PBI/ZrO2 based membrane by PA doping to produce PBI/ZrP membrane for hydrogen production in the copper chloride-hydrochloric acid (CuCleHCl) electrolysis has not been reported. The copper chloride electrolysis process is conducted in a closed-loop system in which the primary electrolytes are cuprous chloride in hydrochloric acid in the anode while hydrochloric acid in the cathode. The heart of the process relies on the membrane electrode assembly where the proton and current exchange take place. The electrolyzer efficiency also referred to hydrogen efficiency, is by dividing the H2 production (experimental) with the H2 production (theoretical). Several studies of the CuCl electrolysis performed by Edge et al. [23], Balashov et al. [18], Naterer et al. [41], Abdo and Easton [24] and Schatz [42] used the Nafion as the based membrane for their MEA configuration. All of the researches were using CuCl electrolytes ranging from 0.002 to 2 M CuCl and 2e10 M HCl. The high concentration of HCl is vital to ensure the dissolution of CuCl substrate and remain in its form for the entire experiment. The membranes used include Nafion 115, Nafion 117, Nafion 117/hydrion, and composite Nafion/polyaniline with the process temperature of 22e80  C. None of the researches have explored the copper chloride electrolysis at a higher temperature above 80  C. Canada Nuclear Laboratories studies proved that although without expensive electrode material, the CuCl/HCl electrolysis at low voltage from 0.6 to 0.7 V with a 0.1 Acm2 current density is achievable [43]. Moreover, it is supported with a study by Balashov et al. [18], that follows Faraday’s law at 0.5 V and 0.1 Acm2 with current and voltage efficiency were 98% and 80%, respectively.

The two-fold objectives of this study are to characterize the composite membranes with Nafion 117 membrane as the reference and to select the best membrane to be fitted as the membrane electrode assembly (MEA) in the CuCl electrolytic system. The preparation of the PBI composite membrane by introducing inorganic filler zirconia ZrO2 and PA doping has been given extensive consideration unlike a typical preparation of introducing ZrP directly to the based membrane. The implementation of altering the PBI membrane by PA doping has improved the membrane’s surface morphology and structure. This paper exhibited the preparation and characterization of a composite proton exchange membrane with the emphasis on PBI/ZrP composite membrane followed with the experimental investigation of the CuCl electrolysis process to demonstrate the effect of operating parameters on the hydrogen production. Therefore, this study aims to investigate the effects of temperature and current density towards hydrogen production for the CuCl electrolytic system.

Materials and methods Materials Copper sulfate pentahydrate (CuSO4$5H2O) was purchased from System ChemAR, hydrogen peroxide (H2O2) 30%, sulphuric acid (H2SO4) 95% and hydrochloric acid 37% (HCl) were purchased from QReC company. Phosphoric acid (PA) 85%, hydrochloric acid (HCl) 37% and zirconium oxide (ZrO2) were acquired from QREC Chemicals. 1 M HCl solution was acquired from the dilution of pure HCl 37%. Dimethylacetamide (DMAc) was purchased from VChem Chemicals. A commercial PBI membrane was acquired from PBI Performance Products Inc. the USA. The PEEK polymer was purchased from Victrex Inc. and Zirconium oxide from Sigma Aldrich. The Nafion 117 membrane was acquired from DuPont Fuel Cells. The Cu diffusion cell and conductivity cell were customized according to the work of Abdo and Easton [24] with some modification. The electrolyzer is comprised of customized titanium blocks, stainless steel electrodes, graphite serpentine flow channel, and membrane electrode assembly. The CuCl electrolysis was carried out in a CuCl electrolytic system equipped with heaters, temperature sensors with controllers, pressure indicators, DC power supply, and peristaltic pumps.

Methods Membrane preparation The commercial PBI and Nafion 117 membrane samples were prepared by cutting the proper size for a specified test. The SPEEK membrane was synthesized via the reaction between sulfuric acid with PEEK polymer in the sulfonation process. The dissolution of the PEEK polymer in the sulfuric acid was stirred for one hour at room temperature then extended for another three hours at 60  C. The sulfonation process was halted by immersing the container in the freezing water. The solution from the container was filtered, rinsed with DI water until neutral and dried in a vacuum for 24 h at 80  C.

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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The screening of the membranes was carried out by doping the PBI and SPEEK membrane in the PA. The doping process was carried out at a temperature ranging from 40 to 100  C and immersion time of 80 min. There were four (4) steps to remove the membrane impurities. The membrane was immersed into a boiling solution of 3% H2O2, 0.5 M H2SO4 and DI water in series, one after another in 60 min and at 90  C for each treatment. The membrane was then immersed in DI water for 24 h before use [3]. The composite membranes of PBI/ZrP were synthesized by dissolving a weighed PBI membrane in a beaker with a DMAc solution. A 10 wt% of ZrO2 was added into the solution and stirred for 3 h at 60  C. A 2 h sonification was performed and continued with membrane casting using a glass plate and vacuum dried for 24 h at 80  C. Then, the membrane was immersed in DI water and peeled off. The vacuum dried process was repeated before the acid doping process. The same procedure was performed for SPEEK/ZrP composite membrane preparation. Various membrane evaluation tests were conducted including water uptake, tensile strength, Cu permeability, ionic exchange capacity, proton conductivity and thermogravimetric analysis (TGA). The most important parameters that were considered are proton conductivity, Cu permeability, tensile strength, and TGA.

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Cu diffusion test of the membranes was executed using a customize Cu diffusion cell at room temperature, which comprises of two 100 mL modified Schott bottles at the center by a sectional area of 4.9 cm2. Cu diffusion test was carried out by placing the membrane in between the connection of two modified Schott bottle as performed by Abdo & Easton [24]. The sampling for Cu diffusion was taken from compartment B (1 M HCl) at 10 min and an interval of 1 h, 2 h, 3 h, 4 h, and 24 h. The electrolyte in compartment A consists of CuSO4$5H2O in 1 M HCl solution while compartment B consists of 1 M HCl solution. CuSO4$5H2O was adopted as the test electrolyte because Cu (I) was easy to oxidize Cu (II) that change the solution characteristic after 24 h of testing. The Cu diffusion test started once both electrolytes were poured inside both compartments. Both electrolytes were stirred with a magnetic stirrer at 100 rpm to get a homogenous solution. Sampling was performed at compartment B before proceeding for Cu diffusion test. UVeVisible Spectrometer was calibrated using five points CuSO4 at a different concentration at 320 nM wavelength for Cu ion detection. After that, each sample was taken at different time intervals were placed in the chamber, run for the detection and the results were displayed at the UVeVis monitor. The figure was then fit into the equation initiated from the calibration prior to the test. The Cu diffusivity through the membrane can be estimated from Eq. (3);

Membrane characterization H3PO4 doping was carried out by soaking membrane in 85% H3PO4 at varies doping temperature from 30 to 100  C and immersion time of 80 mine168 h in open flask covered with glass plate to avoid acid condensation and inconsistency in PA concentration. The acid doping level was quantified by weighing the membrane before and after doped in phosphoric acid. Firstly, a specified size of membrane strips was cut and weighed using the analytical balance. Then, the membrane strip was immersed in the PA for 80 min and 40  C. Acid doping level was calculated as the number of PA molecules per repeat unit of polymers using a formula based on the absorbed PA in Eq. (1). Acid doping level (ADL %) ¼ [(W1 e W0) / 98] / (W1 / 309.32) (1) where W1 is the weight after PA doping, W0 is the weight before PA doping, 98 is the molecular weight for PA and 309.32 is the molecular weight for PBI. For water uptake, membrane strips were cut and soaked in the DI water for 24 h. These were applied for pristine PBI, synthesized SPEEK and PA doped PBI and PBI composite membranes. The water uptake was calculated by weighing the membranes that were totally immersed in water for 24 h and membranes that were vacuum dried according to Eq. (2). The 24 h immersed membrane and vacuum dried membrane was expressed as mwet and mdry, respectively. The 24 h immersed membrane was meticulously dried by using an industrial wipe tissue and labeled as mwet. The vacuum oven dried membrane was written as mdry after treatment at 80  C for 24 h. Water uptake (%) ¼ (mwet - mdry)/ mdry x 100%

(2)

CB (t) ¼ AD$K x CA (t e t0) / VBL

(3)

where C is the copper concentration in bottle A or B and VB is the volume in compartment B. A represent the membrane area, L is the thickness of the membrane and t is the time of the test. DK represents the permeability which can be calculated from the slope of a straight-line correlation between concentrations vs. time [24]. For ionic exchange capacity, the membrane strips were submerged in a known volume of 3 M NaCl solution for 24 h with intermittent shaking. After the membrane has been removed, titrate the remaining solution with 0.005 M NaOH to equalize the amount of ion Hþ released into the 3 M NaCl solution. The formula for ionic exchange capacity is shown in Eq. (4). Ionic Exchange Capacity (IEC) (mol/g) ¼ [Volume NaOH (L) x Concentration NaOH (mol/L)]/ membrane weight (dry, g) (4) Electrochemical impedance spectroscopy (EIS) was employed to determine the conductivities (through-plane) of prepared membranes by using a MTS-740, Scribner Associates  Inc. at 80 C under 100% relative humidity. The measurements were conducted around open circuit potential (OCP) by applying a small amplitude of 10 mV with frequency ranging from 0.01 to 105 Hz. The following Eq. (5) was employed to calculate ionic conductivity of the samples: s ¼ t=A:R

(5) 1

where s (S.cm ) is the ionic conductivity, t is thickness of the

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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membrane (cm), A is surface area (cm2) and R is the membrane resistance (Ohm). Tensile strength tests were performed using a 2.5 kN Lloyd universal tensile tester. The thickness of the membranes was recorded using Yuwese EC-770 ultrasonic thickness tester for coating thickness gauge. Each membrane samples were triplicated to get the average reading for maximum load at break. Thermogravimetric analysis (TGA) was executed on a Shimadzu TA instrument mass spectrometer complete with a computer. Nitrogen air was used (20 ml min1), and the membrane samples were heated up from room temperature up to 900  C with a heating rate of 10e30  C min1. TGA was used to get the thermogram that represents the ability of the tested membrane to withstand or respond to the temperature increment and thermal stability. This test is significant for any high-temperature applications.

Medium-temperature CuCl electrolysis The copper chloride electrolysis tests were conducted using a titanium block electrolyzer to avoid the pitting corrosion

effect from the usage of HCl electrolytes. The membrane electrode assembly consists of carbon cloth electrode with 0.5 mg cm2 PtC 60% platinum on Vulcan, two titanium blocks, two graphite plates with the serpentine channel, Teflon gaskets and the tested membranes (composite PBI and Nafion 117). On the anode side, the copper chloride was dissolved into a 1 M HCl electrolyte to produce 0.05 M CuCl in 1 M HCl. At the cathode, only 1 M HCl electrolyte was prepared. The copper chloride electrolytic systems equipped with three heaters and three thermocouples for the heating and monitoring purposes. The schematic diagram and experimental setup for CuCl electrolytic system are illustrated in Fig. 1. The process temperatures were tested, ranging from 80 to 120  C with electrolytes flowrates of 3 ml min1. The electrolytes flowrates were regulated by peristaltic pumps acquired from Shenchen Precision Pump Co. The efficiency of the hydrogen process for CuCl electrolysis was calculated from the formula in Eq. (6). Eq. (7) and Eq. (8) are related for the VH2 real and VH2 ideal calculation.

Fig. 1 e The schematic diagram for CuCleHCl electrolytic system. Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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Efficiency (%) ¼ VH2

real

/ VH2 ideal x 100

(6)

with VH2 real ¼ VH2 measured x Tstandard / Tmeasured and;

(7)

VH2 ideal ¼ I x Vm x t / (2 x F)

(8)

where I - current (A), Vm e a molar volume of an ideal gas, t e time in seconds, and F e Faraday No. 96485 A.s/mol. The hydrogen gas produced from the CuCl electrolysis reaction was analyzed by using a GC-TCD from Agilent Technologies, model 6890 N network GC system with argon as the working gas and Carboxen 1010 plot as the GC capillary column.

Fig. 3 e Cell setup for electrode/membrane interfaces and representative equivalent circuits.

the membrane which is mainly due to presence of hydrophobic functional groups.

Results and discussions

Thermogravimetric analysis

Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy was performed for investigating the effect of PA doping of the membranes. As can be seen in Fig. 2, all the samples presented a typical impedance spectra containing a semicircle at high frequencies and a straight line at low frequencies. To obtain an equivalent circuit to fit the membrane resistance, a model of the electrode setup was developed. Fig. 3 shows the sample setup in the MTS 740 electrical measurement device. As can be seen, this behaviour can be modelled by a series of R/C equivalent circuits (which R is a resistor and C stands for capacitance behaviour). From Nyquist diagram (Fig. 2), doping process caused to decreased the total resistance of the membranes. This increasing of conductivity can be explained due to introducing the polar groups into PBI structure. In order to calculate the ionic conductivity of the samples, Eq. (5) was employed. The following trend was achieved: s (PBI/ZrP) > s (doped PBI) > s (Pristine PBI)

7

(9)

As can be seen (Fig. 2), treating the pristine PBI membrane by doping process caused to increase the ionic conductivity of

Fig. 2 e Nyquist curves pristine PBI, doped PBI and PBI/ZrP for interfacial resistance measurements.

TGA analyses were performed to investigate the effect of the PA doping and the inorganic fillers towards the thermal characteristics of the samples. Fig. 4 illustrates the TGA curve for the PBI/ZrP, PA doped PBI, undoped PBI, SPEEK/ZrP, undoped SPEEK, and Nafion 117 membranes. The inclusion of inorganic filler zirconia then followed with the acid doping process has significantly lifted the membrane thermal stability due to the polymer backbone and zirconia molecular interaction. The zirconia also acts as a crosslinker between the membrane and phosphoric acid. At room temperature until 150  C, the weight loss is due to vaporization of water and remaining solvent from the membrane synthesis. The steady weight reduction was detected at a temperature higher than 600  C for PBI/ZrP while PBI/ZrO2 at a lower temperature of 350  C. No rapid weight changes at a temperature of 150e500  C proved that the membrane has excellent thermomechanical stability. The vaporization of casting solvent donates the weight loss at above 200  C [44]. The membrane’s high intermolecular forces are the reason for high thermal stability from the presence of aromatic connection/ring [45] while the degradation of the membrane’s polymer backbone started at a temperature beyond 550  C [44]. The increase in thermal stability for composite membrane critical in the CuCl electrolysis that involves high temperature and acidic electrolytes. The weight reduction at 250e400  C for SPEEK based membranes was due to the disintegration of the sulfonic acid groups, and further weight loss at 500  C corresponds to the membrane’s polymeric backbone decomposition. Results showed that the composite membrane is highly thermally stable and acceptable for PEM electrolyzer that involves high-temperature applications. From the thermogram, the Nafion 117 deteriorated massively at a temperature more than 350  C while PBI and SPEEK composite membranes only started to deteriorate slowly at a temperature above 550e600  C. The order of the membrane thermal stability starts with Nafion 117 < SPEEK < SPEEK/ZrP < PA doped PBI < PBI < PBI/ZrP with PBI/ZrP being the most highly thermal resistance membrane. TGA analyses for PBI/ZrP, PBI/ZrO2, and PBI were carried out to investigate the effect of the inorganic fillers on the thermal behavior of the PBI membrane. As observed in Fig. 5,

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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Fig. 4 e Thermogravimetric analyses of Nafion 117, SPEEK, and PBI based membranes.

Fig. 5 e Thermogravimetric analyses of PBI and PBI based membranes.

both membranes showed excellent thermal stability up to 600e650  C. For pristine PBI and the PBI composite membranes, significant weight loss was shown at around 600  C. The weight

loss up to 150  C attributed to the loss of physically absorbed water and remaining vapor from dissolving solvent. The hydrophilic behavior of the PBI is contributed from the N, NeH groups in PBI and water’s hydrogen intermolecular

Fig. 6 e TG and DTG curve of SPEEK and SPEEK/ZrO2. Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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attractions [46]. From 150  C to 500  C, there was a gradual reduction with no significant weight loss for PBI/ZrP and PBI/ ZrO2 except PBI, which shows enhanced thermal stability of modified PBI polymer. The presence of aromatic rings increases thermal stability as a result of high intermolecular forces [45]. TGA results showed that introducing inorganic particles to PBI is increased the thermal properties of PBI very slightly. As indicated in the literature, decomposition onset temperatures of nanocomposites are higher than those of polymer since the thermal stability of inorganic materials is higher than the organic polymers [47]. All the membranes prepared are thermally stable and can be used in HT-PEME applications. TGA curves of SPEEK and SPEEK/ZrO is illustrated in Fig. 6. The results showed that sulfonated polyether ether ketone is a highly thermostable polymer, and thermal degradation temperature for this sample occurs in one step around 500  C. The results also show with introducing of sulfonic acid groups in the polymer structure, TGA behavior of samples are changed. The weight losses for SPEEK samples occur in three steps as reported by Javad et al., [48]. At a temperature of around 100  C, the vaporization of the moisture and remaining casting solvent come into effect. At 250 to 380  C of temperature range, the decomposition of sulfonic acid groups take place, and at beyond 500  C, the polymeric backbone of the backbone degraded gradually. The SPEEK/ZrO2 hybrid membrane has a higher weight loss than SPEEK. Therefore, from the results, the introduction of inorganic filler groups into the polymeric matrix causes thermal stability to decrease.

Mechanical strength analysis A high-temperature PEM electrolyzer requires an MEA that can withstand high temperature with the durability for long hours of operation. The tensile strength of the membrane is one of the crucial parameters that can determine the endurance of the PEM electrolyzer. The results of the tensile strength test divided into two groups. The 85.17e92.33 MPa band is for PBI and PBI/ZrP composite membrane while 27.30e62.33 MPa for Nafion 117, SPEEK and SPEEK/ZrP composite membranes. The tensile strength for the commercial PBI from PBI products international rated at 76.29 MPa, which is congruence with previous researches [27]. The doping process of PBI membrane with PA has significantly boosted the tensile strength from 87.36 MPa to highest 92.23 MPa for PA doped PBI which is an increase of about 5.6% from the pristine PBI. This is contradicted with the study from Di Noto et al.

Table 3 e Mechanical properties of the pristine and composite proton exchange membranes. Type of membranes PBI [50] PBI SPEEK Nafion 117 PBI/ZrP SPEEK/ZrP

Tensile Elongation Young’s strength (MPa) (%) modulus (GPa) 80 87.36 62.33 27.30 85.17 28.26

10 9.01 18 285.96 4.71 9.19

1.1 1.35 0.78 0.43 1.15 0.66

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(2011) that the addition of inorganic filler increases the mechanical strength and phosphoric acid doping makes the membrane weaker [49]. As seen in Table 3, PBI/ZrP had higher Young’s modulus (1.15 GPa) and tensile strength (85.17 MPa) than those SPEEK/ZrP (Young’s modulus of 0.66 GPa and tensile strength of 28.26 Mpa) representing the effect of PA doping to the composite membrane strength that weaken the backbone of PBI polymer matrix. There was a noticeable reduction in the membrane’s elongation that affected the mechanical integrity of the composite membrane due to the addition of inorganic filler and PA plasticizing effect. However, PA doped composite membranes showed reasonable mechanical strength to be used in the membrane electrode assembly setup. It is important to note that the addition of inorganic filler and PA doping has interrupted and weakened the flexibility of the membrane polymer chains resulting in lower Young’s modulus and tensile strength.

Copper permeability analysis The copper permeability becomes the problem to the electrolyzer due to copper build-up that covers the cathode surface, which produces hydrogen gas. The Cu permeability can be minimized by acid doping and integrated the membrane with inorganic filler. The copper diffusion properties for the proton exchange membranes are shown in Table 4. The Cu permeability test performed for Nafion 117 value was close to the value determined by Abdo and Easton [24]. This is an important benchmarking to ensure the method was performed correctly. Slight difference in the value of Cu permeability might be due to the room temperature and humidity. However, the Cu permeability on Nafion 117 by Schatz et al. [21], and Naterer et al. [51], were much lower due to different electrolytes (CuCl2) used for the test. The PBI/ZrP exhibited among the lowest Cu permeability (7.87  107 cm2s1) and reported better than Nafion (1.67  106). As a comparison, the Cu permeability for PBI/ZrP and SPEEK/ZrP was very close, but the advantage goes to PBI/ZrP as its tensile strength, and thermal stability are both excellent. These findings suggest that the PBI/ZrP is a potential candidate to replace Nafion for MEA setup in the CuCl/HCl PEM electrolyzer.

Ionic exchange capacity and water uptake At present, Nafion is the only established membrane medium being used for copper chloride electrolysis. PBI membrane is extensively exploited in fuel cell applications but yet to be reported as the working membrane for copper chloride electrolysis whereas SPEEK membrane is focusing more on the dimethyl ether fuel cell (DMFC) applications. The water uptake and ionic exchange capacity for several tested membranes are shown in Table 5. Water uptake representing the ability of the membrane to grasp a certain amount of humidity or moisture that attached to the membrane’s surface. Additionally, the proton conductivity that represents the movement of the proton via the vehicular movement and Grotthus mechanism. The working temperature used for PA doping adapted from previous research [34]. Nevertheless, the

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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Table 4 e Copper permeability properties of the tested proton exchange membranes. Copper permeability (cm2 s1)

Membranes Nafion 117 PBI PBI (100  C)f SPEEK SPEEK (100  C)f PBI/ZrP SPEEK/ZrP a b c e d f

1.7 2.7 2.4 1.0 5.5 7.9 6.1

      

6a

2.1  108b 4.5  108e e e e e e

10 107a 107a 106a 107a 107a 107a

4.6  107c e e e e e e

1.0  106d e e e e e e

This study, using CuSO4 5H2O to model the Cu (I) as it can easily oxidized to Cu (II), method used by Abdo and Easton 2016 [24]. Schatz et al., 2013 [21]. Naterer et al., 2011 [51]. Kim 2013 [52], using CuCl2 in the permeability test. Abdo and Easton 2016 [24] using CuSO4 5H2O for Cu permeability test. PA doping of the membrane at 100  C, otherwise acid doping temperature at 40  C.

emphasis on fuel cell studies only covers a working condition that is really dry, partial, or fully humidified. Water retention in the membrane via water uptake is essential in proton mobilization. However, if water retention is excessive, the membrane will swell and significantly decrease the tensile strength of the membrane [53]. Mechanical strength of the membrane is still within the acceptable limit when the water uptake of the membrane is around 40 to 50 wt%. From the findings, the PBI/ZrP hybrid membrane holds the highest IEC value at 3.2  104 mol g1 which escalating the proton conductivity value because of the bigger capacity of active sites in the membrane that benefit the ion transfer [54].

Proton conductivity Proton conductivity plays an important role in the CuCl electrolysis process. The movement of the ion Hþ is primarily via vehicular movement through membrane pores and Grotthus mechanism. Fig. 7 illustrates the effect of the process temperature on the proton conductivity. At 90  C, Nafion exhibited the highest proton conductivity (through-plane) at 10.06 mScm1 while PBI/ZrP at 5.67 mScm1. Although the proton conductivity of the PBI/ZrP is only about half of the Nafion, the addition of ZrO2 followed with PA doping has increased the proton conductivity

Fig. 7 e The effects of temperature on proton conductivity (mScm-1) for Nafion 117 and PBI/ZrP composite membrane.

tremendously at 4-fold compared to the original proton conductivity of pristine PBI. Previous studies have an only limited temperature at a maximum of 80  C with Nafion as the medium membrane [18,20,22e24]. It was assumed earlier that,

Table 5 e Water uptake and ionic exchange capacity for the tested proton exchange membranes. Membrane PBI/ZrP SPEEK/ZrP Nafion 117a PBI PBI (100  C)b SPEEK SPEEK (100  C)b a b

Water uptake Ionic exchange capacity (mol (%) g1) 41.7 3.5 13.0 5.5 46.7 2.8 76.8

3.2 1.7 8.0 1.3 1.2 1.2 1.6

      

103 103 104 104 103 103 103

Abdo and Easton 2016 [24] water uptake 31.4% for Nafion 117. PA doping of the membrane at 100  C, otherwise acid doping temperature at 40  C.

Fig. 8 e Effect of process temperature on hydrogen production for Nafion 117 at 0.5 A.cm¡2 and 0.1 A cm¡2 current density, 0.05 M CuCl.

Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

international journal of hydrogen energy xxx (xxxx) xxx

Fig. 9 e Effect of electrolysis temperature on hydrogen production for PBI/ZrP composite membrane at 0.5 A.cm¡2 and 0.1 A cm¡2 current density, 0.05 M CuCl.

Nafion was not tested at higher process temperature probably due to its incompatibility at a higher temperature and experience a decreasing proton conductivity due to Nafion dependent on the humidity for its conductivity. However, from the findings, it could be confirmed that by increasing the process temperature in the fully humidified environment, the through-plane proton conductivity of the Nafion 117 and PBI/ ZrP certainly improved due to more active vehicular movement and Grotthus mechanism within the membrane network.

Hydrogen production in CuCl membrane electrolytic system The CuCl electrolysis process was performed using a lab-scale CuCl electrolytic system. For CuCl electrolytic test using Nafion 117 as the core component for a membrane electrode

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assembly, the hydrogen production kept increasing steadily from 100  C until 115  C as illustrates in Fig. 8. The tests were performed using anolyte of 0.05 M CuCl in 1 M HCl with catholyte of 1 M HCl. The higher electrolysis temperature promotes higher H2 production due to faster electrochemical kinetics and higher exchange current density [22,28]. Furthermore, at higher current density, the hydrogen production for 0.5 A cm2 was at 1.85 cm3 min1 which is almost 2-fold compared to hydrogen production for 0.1 A cm2 (1.05 cm3 min1). The efficiency of hydrogen production was determined to be ranging from 91 to 97%, which complies to the Faraday’s Law of electrolysis. For PBI/ZrP composite membrane, 0.05 M CuCl in 1 M HCl was used as the anolyte with 1 M HCl as its catholyte. The hydrogen production for PBI/ZrP composite membrane at different current densities with the influence of higher electrolysis temperature is shown in Fig. 9. The flowrate of both anolyte and catholyte fed into the CuCl electrolytic system was regulated at 3 ml min1. At higher current density (0.5 A cm2), the hydrogen production was much higher at 3.27 cm3 min1 compared to 1.97 cm3 min1 when exposed to lower current density of 0.1 A cm2. However, at a lower temperature below 100  C the hydrogen production was not significant due to slower electrochemical kinetics and lower exchange current density which is supported with the findings by Gong et al. [22], and Zhang et al., [55]. It is interesting to investigate the effect of higher electrolysis temperature (>120  C) towards the hydrogen yield. The hydrogen produced from the CuCl electrolysis was analyzed using a GC-TCD with 100 mL sample at 250  C oven temperature. The gas products testing was performed by firstly calibrated the GC with standard H2 gas supplied by Linde Gas Malaysia, followed with the hydrogen produced from the CuCl electrolysis. It is important to note that this Carboxen 1010 plot GC capillary column has a characteristic

Fig. 10 e The GC-TCD chromatogram for the standard H2 gas and produced H2 gas from CuCl electrolysis. Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030

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peak at a retention time of 16 min, most probably due to inherent characteristic of the GC capillary column. The GCTCD chromatogram for the standard hydrogen gas and produced gas from CuCl is shown in Fig. 10. The H2 detection for the standard gas was hovering at 4.951 min retention time while the H2 produced from CuCl electrolysis at 4.734 min. A slight difference in the retention time could be due the time required to inject the 100 mL sample manually. From Fig. 10, it can be calculated from the GC-TCD area percent report that the purity of the H2 from CuCl electrolysis origin was at 86.9% purity. Other impurities could be due to remaining HCl gas and nitrogen gas/air in the HCl reservoir and piping for the CuCl electrolytic system. This result is considered a very good achievement because the gas was collected directly after the HCl reservoir without bubbling the H2 produced in the water to capture any excess HCl vapor and ambient air.

Conclusion The CuCl electrolysis was performed over PBI/ZrP composite membrane and Nafion 117. The finding revealed that the addition of inorganic filler ZrO2 followed with acid doping has generally improved the synthesized composite membranes. Moreover, for PBI/ZrP composite membrane, it has improved its properties especially the proton conductivity, thermal stability, and ionic exchange capacity. Furthermore, the Cu permeability was significantly improved with PA doping method due to the dihydrogen phosphate ion (H2PO 4) attached to the polymer backbone. The higher current density with elevated electrolysis temperature has increased the hydrogen production (76% for Nafion 117 and 66% for PBI/ZrP) of the CuCl electrolysis with the highest hydrogen produced at 3.27 cm3 min1 for PBI/ZrP composite membrane. Therefore, the capability and performance of PBI/ZrP composite membrane as the MEA in medium temperature CuCl electrolytic system is established.

Acknowledgement Financial support from the Universiti Putra Malaysia project number 945200 GP-IPM and Universiti Teknologi Malaysia project number 03G10 Flagship Research University Grant under the Ministry of Education Malaysia are acknowledged.

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Please cite this article as: Kamaroddin MFA et al., Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.030