Solid-State Water Electrolysis with an Alkaline Membrane

Transcription

1 Supporting information Solid-State Water Electrolysis with an Alkaline Membrane Yongjun Leng a, Guang Chen b, Alfonso J. Mendoza b, Timothy B. Tighe b, Michael A. Hickner b and Chao-Yang Wang a,b,c * a Electrochemical Engine Center, Department of Mechanical and Nuclear Engineering b Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA 16802, USA c Center for Energy Storage and Conversion, and Department of Energy and Resources Engineering, Peking University, Beijing , China * Corresponding author: cxw31@psu.edu Materials Iridium oxide (IrO 2 ) was used as anode catalyst for oxygen evolution reaction (OER), while platinum black (Pt) (HiSPEC 1000 TM, Alfa Aesar, USA) was used as cathode catalysts for hydrogen evolution reaction (HER). The untreated carbon paper (TGPH-120, Toray Inc., Japan) and titanium (Ti) foam were employed as cathode and anode gas diffusion layer (GDL), respectively. A201 membrane (Tokuyama Corporation, Japan) with a thickness of 28 µm was used as anion exchange membrane (AEM). In order to enhance the utilization of catalysts and improve the electrode performance, an anion exchange ionomer (AS-4 ionomer, Tokuyama Corporation, Japan; or A-Radel ionomer) was introduced into the electrode catalysts layer. Radel-based anion exchange polymer (A-Radel) was prepared by chloromethylation of Radel R NT 5500 (Mw = 63,000 g/mol) poly(sulfone) according to Hibbs, et al. 1 The degree of functionalizaiton (the number of chloromethyl groups per repeat unit) was monitored by 1 H NMR. The chloromethylated Radel was dissolved in dimethylformamide (5 % wt/vol) and quaternized by adding 3x molar excess of trimethyl amine in a sealed vessel for 48 h. The S1

2 quaternized polymer was cast onto a clean glass plate and then dried under vacuum at 50 C for 48 h to form a robust membrane. The membrane was ion exchanged by immersion in 1 M NaHCO 3 for 12 h and excess rinsing with DI water. The ion exchange capacity of the resulting polymer was 2.0 meq/g with a water uptake of 44 wt % and a bicarbonate anion conductivity of 9 ms/cm in liquid water at 30 C. Ionomer solutions of the bicarbonate form A-Radel with IEC 2.0 meq/g were formulated by dissolving the polymer membrane in 50:50 water:ethanol solvent mixtures. Fabrication of membrane electrode assembly with catalyst-coated membrane (CCM) method The catalysts (IrO 2 or Pt black) were mixed with de-ionized water, n-propanol and AS-4 ionomer suspension (5 wt % polymer in suspension) to obtain well-dispersed ink using magnetic stirring combined with ultra-sonication. To obtain the catalyst coated membrane (CCM), the asprepared ink was coated onto the both sides of the anion exchange membrane (AEM) such as A201 using hand-spray method with the aid of a spray gun (Iwata, Japan). The loading of catalysts was 2.9 mg/cm 2 IrO 2 and 3.2 mg/cm 2 Pt for the anode (oxygen evolution electrode) and cathode (hydrogen evolution electrode) catalysts layer, respectively. The dry AS-4 ionomer content was 16 wt% polymer for both anode and cathode catalysts layer. The size of electrode catalyst layers was 2.4 cm 2.4 cm (i.e cm 2 ). A Ti foam anode gas diffusion layer (GDL) and TGP-H-120 plain carbon paper cathode GDL were mechanically pressed against the CCM when assembling the cell hardware. S2

3 Fabrication of membrane electrode assembly with catalyst-coated substrate (CCS) method The catalysts (IrO 2 or Pt black) were mixed with de-ionized water, propanol and 5 wt% Nafion ionomer suspension (which was used as the binder) to obtain well-dispersed ink using magnetic stirring combined with ultra-sonic method. The as-prepared ink containing IrO 2 was coated onto the surface of Ti foam using hand-spray method with the aid of a spray gun to obtain a catalyst coated substrate (CCS) for the anode (oxygen evolution electrode).the as-prepared ink containing Pt black was coated onto the surface of Toray TGP-H-120 plain carbon paper to obtain a CCS for the cathode (hydrogen evolution electrode). The binder content (5 wt% Nafion ionomer) was ~5 wt% polymer for both anode and cathode catalysts layer. Then 5 wt% anion exchange ionomer suspension (A-Radel or AS-4) was applied onto the surface of CCS for both anode and cathode using hand-spray method with the aid of a spray gun until the desired loading of the ionomer was achieved. For the CCS with A-Radel ionomer, the loading of catalysts was 2.6 mg/cm 2 IrO 2 and 2.4 mg/cm 2 Pt for the anode and cathode catalysts layer, respectively; and the loading of A-Radel ionomer was 22 wt% and 27 wt% for the anode and cathode catalysts layer, respectively. For the CCS with AS-4 ionomer, the loading of catalysts was 3.5 mg/cm 2 IrO 2 and 3.3 mg/cm 2 Pt for the anode and cathode catalysts layer, respectively; and the loading of AS-4 ionomer was 22 wt% and 24 wt% for the anode and cathode catalysts layer, respectively. The size of all electrode catalysts layer was 2.35 cm 2.35 cm (i.e cm 2 ). Anode CCS, A201 membrane and cathode CCS were assembled together in the cell hardware to form an MEA. Cell performance evaluation S3

4 A custom built water electrolysis setup was used to evaluate the performance and durability of a single water electrolysis cell. The single electrolysis cell includes a MEA, a graphite end plate with single serpentine channels on the cathode side, and a titanium end plate with parallel channels on the anode side. Cell potential and current was controlled through an Arbin testing system (Arbin Instruments, United States) and provided a direct electric current across the electrolysis cell to split the water and produce hydrogen at the cathode and oxygen at the anode. The de-ionized water was supplied into anode and/or cathode chamber at a flow rate of 3 ml/min with a liquid pump (Series I digital pump, LabAlliance, United States). The cell temperature was maintained at 50 C. The polarization curves (current density vs. cell voltage) were obtained using the Arbin testing system in galvanodynamic mode with a scan rate of 10 ma/s. For the durability tests, a constant current of 200 ma/cm 2 was applied onto the electrolysis cell, and the cell voltage as a function of test time was recorded by Arbin testing system. The durability test was interrupted periodically for the measurement of polarization curve. During the electrochemical testing, the high frequency resistance (HFR) was monitored by a milliohm meter (AC mω HiTester 3560, Hioki Company, Japan) along with the cell voltage. Comparison of the performance of degraded MEA between with the supply of water and 1 M KOH solution One of MEAs with AS-4 ionomer fabricated with the CCM method was tested under an electrolysis current of 200 ma/cm 2 at 50 C. Before the durability test, the initial polarization curve was measured with the supply water into the cathode chamber at a flow rate of 3 ml/min. S4

5 After 27 h of durability testing, the MEA performance was completely degraded and the cell voltage at 200mA/cm 2 was beyond the cutoff voltage of 2.5 V. After the durability test, the polarization curve for the degraded MEA was measured when supplying water in both the cathode and anode chambers at a flow rate of 3 ml/min and 1 ml/min, respectively. After measurement of the polarization curve at 50 C, an electrolysis current of 200 ma/cm 2 was applied onto the degraded MEA using the Arbin testing system, and the cell voltage and HFR as a function of test time were recorded with milliohm meter. During the galvanostatic test of degraded MEA, the supply of water into the anode chamber was switched to the supply of 1 M KOH solution at a flow rate of 1 ml/min, while maintaining the water supply into the cathode chamber. The change of cell voltage and HFR as a function of test time were monitored with milliohm meter. After the cell voltage and HFR decreased to a stable level, the galvanostatic test was interrupted for the measurement of the polarization curve. After the measurement of the polarization curve, the galvanostatic test was resumed. During the resumed galvanostatic test, the supply of 1 M KOH into the anode chamber was switched back to the supply of water at the same flow rate (i.e. 1 ml/min) and galvanostatic test continued for several hours. The cell voltage and HFR as function of test time were monitored with milliohm meter during the resumed galvanostatic test with water feed. Accelerated thermal aging of AS-4 and A-Radel cast ionomer films Ionomers from ethanol/water solutions were drop cast to form 5-10 µm films on KBr windows. The samples were placed in a convection oven at 135 ºC to thermally age the polymers and removed periodically for transmission measurement on a Bruker (Billerica, MA) S5

6 IFS 60 spectrometer with an MCT detector. Each FTIR spectrum was taken at 2 cm -1 resolution and 32 scans. Figure S1. Schematic of an alkaline membrane electrolyzer. Figure S1 shows the schematic diagram of an alkaline membrane water electrolysis cell. In general, an alkaline membrane water electrolysis cell is comprised of an anode for oxygen evolution reaction (including anode gas diffusion layer (GDL) and anode catalysts layer (CL)), a cathode for hydrogen evolution reaction (including cathode GDL and cathode CL), an anode bipolar plate, a cathode bipolar plate, and an alkaline membrane (i.e. anion exchange membrane (AEM)). The key element for alkaline membrane water electrolysis cell is the membraneelectrode assembly (MEA), which consists of anode GDL, anode CL, AEM, cathode CL and cathode GDL. The insert figure shows the porous microstructure of electrode catalyst layers, S6

7 where the catalyst particles are in intimate contact with the ionomer. Catalyst particles contact each other to form an electron-conducting network, while the ionomer in the catalyst layers forms a network for the hydroxide ion conducting path. Electrode reactions for hydrogen evolution and oxygen evolution take place at the electrochemical active sites, i.e. triple-phase boundaries among catalysts, ionomer and gas pore. Normally water is fed into the cathode (i.e. water cathode-feed mode) since water is consumed for the hydrogen evolution reaction to produce hydroxide ions. However, cathode-feed mode increases the system complexity for the separation of hydrogen and water. It is possible to feed water to the anode (i.e. water anode-feed mode) since water can transport from the anode through the AEM to the cathode in the case of anode-feed mode. Absorbance / a.u (a) Absorbance / a.u (b) Wavenumber / cm Wavenumber / cm -1 Figure S2. Decrease in absorbance of the C-N asymmetric stretch during thermal aging at 135 ºC for (a) AS-4 and (b) A-Radel cast ionomers. Accelerated degradation studies under high temperature conditions were performed to compare the stability of AS-4 ionomer with the aminated Radel (A-Radel) ionomer. Figure S2 showed the decrease in absorbance of the C-N asymmetric stretch during thermal aging at 135 ºC S7

8 for (a) AS-4 and (b) A-Radel cast ionomers. The C-N asymmetric stretch at 974 cm -1 was monitored by FTIR 8 during thermal aging of cast ionomer films at 135 ºC. The absorbance of the 974 cm -1 band decreased significantly faster for AS-4 than for A-Radel (Figure S2). Figure S3 shows the normalized absorbance for A-Radel and AS-4 during thermal degradation at 135 C derived from the spectra presented in Figure S2 centered on the C-N asymmetric stretch at 974 cm -1. After 10 h of thermal aging under these accelerated conditions, the absorbance of the C-N band for AS-4 declined to less than 20 % of its original value while the absorbance of the C- N band for A-Radel declined by 55 % (Figure S3). The greater stability in this thermal aging test for A-Radel was correlated to its better long-term performance during electrolyzer operation, although it was not direct evidence of polymer stability in an electrochemical environment. In the future, we will develop in-situ methods to characterize the stability of ionomer/membrane in the MEA during electrolyzer operation. 1.0 Normalized absorbance AS-4 A-Radel Time (h) Figure S3. Normalized absorbance of C-N stretch for A-Radel and AS-4 AEMs during thermal degradation at 135 C. S8

9 Case 4 Case 2 E cell / V Case j / macm -2 Figure S4. Initial polarization curve at 50 o C for three MEAs fabricated with CCS method: Case 1, MEA w/a-radel ionomer, water cathode-feed mode; Case 2, MEA w/a-radel ionomer, water anode-feed mode; Case 4, MEA w/ AS-4 ionomer, water anode-feed mode. Two types of MEAs were fabricated with catalyst coated substrate (CCS) method: one type of MEA with A-Radel ionomer, and another type of MEA of similar construction with AS-4 ionomer. The effect of water feed to the anode or water feed to the cathode was explored. The initial MEA performance (Figure S4) and durability test (Figure 4) were evaluated in a 5 cm 2 single electrolysis cell for four cases: (1) MEA with A-Radel ionomer, cathode-feed mode; (2) MEA with A-Radel ionomer, anode-feed mode; (3) MEA with A-Radel ionomer, cathode-feed mode for 2 h, then anode-feed mode for the remainder of the test; and (4) MEA with AS-4 ionomer, anode-feed mode. Figure S4 shows the initial polarization curve at 50 o C for three MEAs (Case 1, 2 and 4) fabricated with CCS method. As shown in Figure S4, for the MEA with S9

10 A-Radel, the anode-feed mode (Case 2) showed slightly better performance than the cathodefeed mode (Case 1) at low current density; while at high current density, the anode feed mode (Case 2) showed lower performance due to the membrane limiting the flux of water from the anode through the membrane to the cathode. In the case of anode-feed mode, the MEA with AS- 4 ionomer (Case 4) showed better performance than the MEA with A-Radel ionomer (Case 2) at low current densities, which is mainly due to higher catalyst loading in the AS-4 MEA (IrO 2 /Pt = 3.5/3.3 mg/cm 2 for Case 4 vs. IrO 2 /Pt = 2.6/2.4 mg/cm 2 for Case 2). However, at high current density, the MEA with AS-4 ionomer (Case 4) showed lower performance than that of the MEA with A-Radel ionomer (Case 2) due to higher HFR (0.44 Ω cm 2 at 2.0 V for AS-4 vs Ω cm 2 at 2.0 V for A-Radel) caused by the thicker catalysts layer with high catalyst loading and possible AS-4 ionomer degradation. Figure S5 shows the polarization curve as a function of test time at 50 C during the durability test for the MEA with A-Radel ionomer (Case 3 in Figure 4) fabricated with the CCS method. As shown in Figure S5, the MEA performance degraded significantly. For example, the current density at 2.0 V decreased from 780 ma/cm 2 initially to 206 ma/cm 2 after 175 h, and then continued to decrease to 135 ma/cm 2 after 486 h. The performance degradation over this long test time may be related to, but not limited to, the degradation of ionomer in the electrode catalysts layer, the degradation of AEM, and/or the delamination between cell components, as evidenced by the gradual increase of HFR resistance during the durability testing (Figure 4). The OCV slightly decreased from V initially to V after 326 h. This result suggested that there is no obvious pinhole formation that developed in the AEM during the 326 h durability test. However, the OCV dropped quickly from V to V during the durability test from 326 h to 486 h, which indicated the possible pinhole formation of the AEM occurred at longer cell S10

11 operation times. The pinhole formation of AEM may be related to the formation of peroxide at the anode, which can damage the AEM; and/or the chemical/electrochemical degradation of AEM due to nucleophilic attack on the cationic fixed charged sites by hydroxide ions E cell / V j / ma cm -2 Initially, CF After 2h, CF After 2h, AF After 175h, AF After 326h, AF After 486h, AF Figure S5. Polarization curve as a function of test time at 50 C during the durability test for MEA with home-made A-Radel ionomer fabricated with CCS method (Case 3 in Figure 4: MEA was run in water cathode-feed (CF) mode for initial 2h, then switched to the anode-feed (AF) for the remainder of the test). S11

12 Table S1. MEA specification and performance of alkaline membrane electrolysis cell in this work and compared with other reported work in the literature Type Anode Cathode Membrane Temperature Performance Reference IrO AEM 2 Pt black E=1.8V, 2.9mg/cm 2 3.2mg/cm 2 A C 399mA/cm 2 This work Liquid alkaline PEM PEM PEM PEM PEM Raney Ni + Co 3 O 4 Ir 0.6 Ru 0.4 O 2 2mg/cm 2 Ir 0.2 Ru 0.8 O 2 1.5mg/cm 2 IrO 2 3.0mg/cm 2 Ir black 1.5mg/cm 2 Ir black 3.8mg/cm 2 Activated Mocontaining Raney Ni Zero-gap diaphragm filled with 30wt%KOH 50 C 20wt%Pt/C 0.4mg/cm 2 Nafion C 28wt%Pt/C 0.5mg/cm 2 Nafion C 28wt%Pt/C 0.5mg/cm 2 Nafion C Pt black 1.0mg/cm 2 Nafion C Pt black 2.3mg/cm 2 Nafion C E=1.8V, ~470mA/cm 2 Ref. 2 E=1.567V, 1.0A/cm 2 Ref. 3 E=1.622V, 1.0A/cm 2 Ref. 4 E=1.63V, 1.0A/cm 2 Ref. 5 E=1.8V, ~1.0A/cm 2 Ref. 6 E=1.8V, 680mA/cm 2 Ref. 7 S12

13 References (1) Hibbs, M. R.; Hickner, M. A.; Alam, T. M.; McIntyre, S. K.; Fujimoto, C. H.; Cornelius, C. J. Chem. Mater. 2008, 20(7), (2) Schiller, G.; Henne, R.; Mohr, P.; Peinecke, V. Int. J. Hydrogen Energy 1998, 23, 761. (3) Marshall, A. T.; Sunde, S.; Tsypkin, M.; Tunold, R. Int. J. Hydrogen Energy 2007, 32, (4) Cheng, J. B.; Zhang, H. M.; Chen, G. B.; Zhang, Y. N. Electrochim. Acta 2009, 54, (5) Song, S. D.; Zhang, H. M.; Ma, X. P.; Shao, Z. G.; Baker, R. T.; Yi, B. L. Int. J. Hydrogen Energy 2008, 33, (6) Ma, L. R.; Sui, S.; Zhai, Y. C. Int. J. Hydrogen Energy 2009, 34, 678. (7) Wei, G. Q.; Wang, Y. X.; Huang, C. D.; Gao, Q. J.; Wang, Z. T.; Xu, L. Int. J. Hydrogen Energy 2010, 35, (8) Vico, S.; Palys, B.; Buess-Herman, C. Langmuir, 2003, 19, (9) Arges, C. G.; Ramani, V.; Pintauro, P. N. The Electrochem. Soc. Interface 2010, 19(2), 31. S13