Materials Letters 221 (2018) 128–130
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Highly KOH doped para-polybenzimidazole anion exchange membrane and its performance in Pt/TinO2n1 catalyzed water electrolysis cell Hristo Penchev a,⇑, Galin Borisov b, Elitsa Petkucheva b, Filip Ublekov a, Vesselin Sinigersky a, Ivan Radev b,c, Evelina Slavcheva b,⇑ a b c
Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Zentrum für BrennstoffzellenTechnik ZBT GmbH, Duisburg, Germany
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Article history: Received 19 February 2018 Received in revised form 12 March 2018 Accepted 17 March 2018 Available online 19 March 2018 Keywords: Water electrolysis Non-stoichiometric titanium oxide Para-polybenzimidazole membrane Alkali doped
a b s t r a c t In this study we report the preparation and properties of KOH doped para-polybenzimidazole membrane as anion conducting polymer electrolyte for application in water electrolysis cell. The membrane demonstrated an outstanding OH conductivity measured by electrochemical impedance spectroscopy, which depending on the relative humidity varies in the range 298–1140 mS.cm1. The membrane electrode assembly prepared with this membrane and a novel non-carbon supported Pt catalyst were tested in a single electrolysis cell at close to real operation conditions both at room and at 80 °C. The results obtained showed low overpotentials concerning both partial reactions and a stable electrochemical performance at cell voltage of 2.2 V. The enhanced efficiency of the electrolysis has been attributed to the combined effects of the high membrane conductivity, the homogeneous distribution and small size of the catalytic particles, and the corrosion resistance of the selected catalyst support. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Water electrolysis based on alkali-doped polymer electrolyte membrane is an efficient method for production of very pure hydrogen. It offers several advantages over the traditional technologies like higher current density, lower ohmic resistance, and possibility for operation at higher working pressure. Both partial electrode reactions (hydrogen evolution reaction, HER, and oxygen evolution reaction, OER, are of particular interest as they appear to be the main sources of energy losses and membrane electrode degradation. A possible way for enhancing the catalytic activity and reducing the overpotential of the partial reactions is to increase the operating temperature of the cell. Unfortunately, the commercially available membranes cannot be operated at temperatures above 50 °C which results in lower energy efficiency and higher cell voltage, leading to accelerated degradation of the whole membrane electrode assembly (MEA) package which includes gas diffusion and catalyst layers for the anode and the cathode and a polymer electrolyte membrane in between [1]. Another major problem in anion exchange membrane (AEM) water electrolysis ⇑ Corresponding authors. E-mail addresses:
[email protected] (H. Penchev),
[email protected] (E. Slavcheva). https://doi.org/10.1016/j.matlet.2018.03.094 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
is the degradation of the gas diffusion electrodes, in particular of the anode. Due to the high anodic potentials, required for the OER the carbon materials, traditionally used as catalyst supports, are easily oxidized which leads to gradual corrosion of the electrodes and decreases both the efficiency of the electrolysis and durability of the hydrogen generator [2]. Recently ceramic materials such as non-stoichiometric titanium oxides are used as alternative catalytic carriers, in particular for the anodes in the polymer electrolyte membrane electrolysis cells [3–5]. These materials have excellent electrical conductivity, small particle size, and stable electrochemical behavior at highly aggressive corrosive conditions. In addition, strong metal support interaction (SMSI effect) between the sub-oxide carrier and the catalytic noble metal nanoparticles has been registered, enhancing the catalytic activity and durability of the electrolyzer [6]. In this paper Pt nano-sized catalyst deposited on Magnéli phases titania (TinO2n1) was prepared by sol - gel method and integrated as thin catalytic layer both in the cathode and anode of MEA with anion exchange polymer membrane (para-PBI, doped with 50% KOH). The conductivity of the membrane was measured by electrochemical impedance spectroscopy, while the MEA performance characteristic were investigated in a single AEM electrochemical cell at varying temperature in the range 20 °C–80 °C.
H. Penchev et al. / Materials Letters 221 (2018) 128–130
2. Experimental Para-polybenzimidazole (p-PBI) was synthesized in polyphosphoric acid (PPA), according to [7]. PBI/PPA solution was cast on a glass substrate using doctor blade (gap 0.6 mm). The obtained non supporting film was kept in air for 3 days to undergo the sol-gel process (conversion of PPA into phosphoric acid, PA). In this way highly PA doped p-PBI membranes (up to 42 mol PA per PBI unit) were obtained. The membranes were washed in water bath for 24 h, treated with 10% ammonia and finally conditioned in deionized water for another 24 h. The porous water filled p-PBI membrane, prepared in this way, was subsequently doped with potassium hydroxide (KOH) by simple soaking in 50 wt% KOH solution for 6 h at room temperature (exchange of water with dopant solution). The as prepared membrane contained 17 mol KOH per PBI repeat unit. If the pristine p-PBI membrane is doped at elevated temperature (90 °C) then even higher degree of doping (19.6 mol KOH) could be achieved but after cooling the dopant solution some loss of electrolyte from the membrane was observed and the final doping level was lower (15.6 mol KOH). The anion conductivity measurements were performed on potentiostat Zennium (Zahner-elektrik GmbH & Co.KG, Kronach, Germany) at 110 °C, varying the relative humidity (RH) from 10 to 100%. The membrane was adjusted in an in-plane four electrode cell, placed in the conditioning chamber of the EasyTest cell [8]. The humidity in the cell is controlled by the lowest temperature in the cell, in our case by the temperature of a Peltier element which cold side is in a thermal contact with a paper tissue soaked with liquid water and attached to the cooler side of the Peltier element with a teflon sink. The hot side of the Peltier element is in a thermal contact with the top of the cell. External water cooling is attached to the top of the cell. An Instron 1185 apparatus was used for recording the mechanical parameters of the membrane at speed of 100 mm.min1. The Pt catalyst was synthesized by sol gel methods from acetylacetonate precursor (Pt[(C5H7O2)n]m), according to [9]. Non-stoichiometric titanium oxide (TinO2n1) with particle size 60–100 nm, denoted as N82 Ti Dynamiks, Ltd. was used as catalytic carrier. The phase composition, morphology and surface structure of the catalyst were studied by XRD and SEM. The X-Ray diffraction spectra were recorded on X-ray diffractometer Philips APD15 with Cu Ka radiation at constant rate of 0.020.s-1 over an angle range 2h = 10–90°. The test electrodes were prepared using commercially available gas diffusion layer H2315 with micro porous layer (Freudenberg, Germany) that was coated with the catalytic layer containing the
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synthesized TinO2n1-supported Pt nanoparticles. The catalyst loading was 0.2 mgPt.cm2 for both the cathode and the anode. The test electrodes with geometric area 1 cm2 were pressed to the KOH doped p-PBI membrane directly in the test cell. The performance characteristics of the prepared MEA were investigated in a self-made electrolysis cell, consisting of two gas compartments where the hydrogen and oxygen evolution reactions take place, separated by the membrane electrode assembly. A reference electrode was situated in additional hydrogen compartment. The cell was fed with distilled water vapor introduced in a flow of inert gas argon. The MEA electrochemical performance was investigated by techniques of steady state polarization at temperatures of 20 °C and 80 °C. All electrochemical measurements were carried out using a commercial Galvanostat/Potentiostat POS 2 Bank Electronik, Germany. 3. Results and discussion 3.1. PBI membrane, doped with KOH One of the best materials for preparation of ion conductive membranes is poly[2,20 -(p-phenylene)-5,50 -bisbenzimidazole], denoted p-PBI. When doped with phosphoric acid (PA), it becomes a solid state proton (H+) conducting electrolyte. When alkali bases like potassium hydroxide (KOH) are used as dopant, an anion (OH) conductive membrane is obtained. For attaining high ion conductivity, high doping levels are required. The p-PBI membrane, doped with 50% KOH (17 mol KOH per PBI unit) used in this study exhibited very good mechanical properties (elastic modulus E = 3. 6 MPa, elongation at break 180%) as well as excellent OH conductivity. The EIS spectra, presented as Nyquist plots (Fig. 1) were taken until a constant anion resistance was attained (up to 3 h). The steady state polarization impedance curves were recorded at 110 °C and different relative humidity (RH). The OH conductivity attained (298–1140 mS.cm1, depending on RH) is extremely high. This has to be attributed to the very high contents of KOH, retained by the PBI matrix. For similar membrane (m-PBI, doped with 6 mol KOH/PBI unit) Bjerrum et al. report 120–130 mS.cm1 at 100% RH and 80 °C [10]. 3.2. Pt catalyst on sub oxide support Fig. 2 presents the XRD spectra and SEM image of the Pt catalyst, dispersed over the Magnéli phases titanium oxide. The characteristic peaks of the ceramic carrier are clearly visible. The average
Fig. 1. Electrochemical impedance spectra (Nyquist plots) presented as specific resistances taken at 110 °C and different relative humidity (RH), the illustration of KOH doped p-PBI and the ion conductivities calculated therefrom.
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H. Penchev et al. / Materials Letters 221 (2018) 128–130
Fig. 3. Cathode and anode polarization curves of the Pt/N82 catalyst recorded at scan speed 1 mV.s1 and cell voltage measurements at 2 V for one hour. Fig. 2. XRD spectra and SEM image of N82 carrier and Pt/N82 catalysts.
size of the deposited Pt particles is in the range 10–12 nm with predominant (1 1 1) crystallographic orientation. The SEM image (inset in Fig. 1) shows homogeneous distribution of the nanosized Pt particles over the catalytic carrier. 3.3. MEA performance characteristics The performance characteristics of the prepared MEA are shown in Fig. 3. The hydrogen evolution reaction, proceeding on the cathode is presented in black on the graph. The HER overpotential is in the range 400–450 mV (vs. RHE) at room temperature. The current density reaches 45–50 mA.cm2 at potential 620 mV. It should be noted that this catalyst is especially designed as an effective and electrochemically stable towards the sluggish and high anodic potentials demanding OER. The oxygen evolution reaction, proceeding on the anode is presented in gray. OER starts at anode potentials over 1450 mV (vs. RHE) at room temperature. The current density reaches 50–55 mA.cm2, with subsequent registration of the diffusion limitations at the anode which is due to the low water vapor pressure at room temperature. In comparison to commercial quaternized ammonia resin membrane, the MEA with KOH doped p-PBI show low over potential for both partial reactions. The steady state polarization curve (inset in Fig. 3) is recorded at 2 V (vs. RHE) and temperature of 80 °C. It shows stable electrochemical behavior with current density of about 100 mA.cm2. 4. Conclusion This study demonstrates the development of a core components of an AEM electrolysis and their successful integration in the MEA, namely: (i) an ultra-high OH conductive, chemically stable KOH
doped p-PBI membrane and (ii) a novel, carbon-free TinO2n1- supported Pt SMSI catalyst towards OER and HER. Together with the ex-situ characterization of the developed core components, their remarkable properties are confirmed in a real laboratory AEM water electrolysis cell. The results show an operational synergy of the AEM and the catalyst in a chemically and mechanically stable MEA able to efficiently generate a pure hydrogen at room as well as at elevated temperatures. Acknowledgement This work has been financially supported by the Bulgarian Science fund in the frame of Project DFNI E02/9. References [1] Javier Parrondo, Christopher G. Arges, Mike Niedzwiecki, Everett B. Anderson, Katherine E. Ayers, Vijay Ramani, Electronic supplementary material (ESI) for RSC, Advances 4 (2014) 9875–9879. [2] S. Grigoriev, P. Millet, V. Fateev, J. Power Sour. 177 (2008) 281–285. [3] E.Antolini, E.R.Gonzalezb, 180 (2009) 746-763. [4] A. Stoyanova, G. Borisov, E. Lefterova, E. Slavcheva, Int. J. Hydrogen Energy 37 (2012) 16515–16521. [5] E. Slavcheva, G. Borisov, E. Lefterova, E. Petkucheva, I. Boshnakova, Int. J. Hydrogen Energy 40 (2015) 11356–11361. [6] C.E. Kliewer, S. Miseo, J.E. Baumgartner, E. Stach, D. Zakharov, Microsc. Microanal. 11 (S02) (2005) 1552–1553. [7] L. Xia, H. Zhang, E. Scanlon, L.S. Ramanathan, B.C. Benicewicz, Chem. Mater. 17 (2005) 5328–5333. [8] I. Radev, G. Topalov, E. Slavcheva, E. Lefterova, G. Tsotridis, U. Schnakenberg, Int. J. Hydrogen Energy 35 (2010) 2428–2435. [9] G. Borisov, E. Lefterova, A. Stoyanova, E. Slavcheva, P. Angelov, S. Vassilev, J. Chem. Technol. Metall. 48 (2013) 162–167. [10] Jens Oluf Jensen, David Aili, Martin K. Hansen, Qingfeng Li, Niels J Bjerrum, Erik Christensen, ECS Trans. 64 (2014) 1175–1184.
