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carbon paper electrocatalysts with enhanced mass transport as oxygen electrodes in unitized regenerative fuel cells
Electrochemistry Communications 64 (2016) 14–17
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Development of porous Pt/IrO2/carbon paper electrocatalysts with enhanced mass transport as oxygen electrodes in unitized regenerative fuel cells Byung-Seok Lee a,b,1,2,3, Hee-Young Park a,1,2, Min Kyung Cho a,e,2,4, Jea Woo Jung a,c,2,3, Hyoung-Juhn Kim a,2, Dirk Henkensmeier a,2, Sung Jong Yoo a,2, Jin Young Kim a,2, Sehkyu Park d,5, Kwan-Young Lee b,c,⁎,3, Jong Hyun Jang a,c,⁎,2,3 a
Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea c Green School, Korea University, Seoul 02841, Republic of Korea d Department of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea e School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 25 November 2015 Received in revised form 25 December 2015 Accepted 7 January 2016 Available online 12 January 2016 Keywords: Unitized regenerative fuel cell Oxygen electrode Platinum Iridium oxide
a b s t r a c t The oxygen electrodes in unitized regenerative fuel cells (URFC) must have high activities towards oxygen reduction reaction (ORR) as well as oxygen evolution reaction (OER), thus requiring high loading of noble metal electrocatalysts. In this study, porous Pt/IrO2/carbon paper (CP) electrocatalysts were developed to reduce the metal loading. The Pt/IrO2/CP electrodes were fabricated by sequential formation of IrO2 layers (loading 0.1 mg cm−2) and porous Pt layers (0–0.3 mg cm−2) on CP substrates by electrodeposition and spraying techniques, respectively. The fuel cell (FC) performances increased linearly up to 0.69 A cm−2 with increasing Pt loading (up to ~0.3 mg cm−2) at 0.6 V, whereas the water electrolysis (WE) activity was highest at Pt loading of 0.2 mg cm−2. The current densities in the FC and WE modes and round-trip efficiency of the developed Pt/ IrO2/CP electrodes with the oxygen electrocatalysts loadings of 0.3 and 0.4 mg cm−2 were higher or comparable to previously reported values with higher loading (1.5–4.0 mg cm−2). These high performances with low loading are probably due to the facile oxygen and water transport through well-developed macropores originating from the open CP structures, providing effective utilization of the IrO2 and Pt electrocatalysts towards OER and ORR, respectively. © 2016 Published by Elsevier B.V.
1. Introduction Renewable energy sources, such as solar power and wind power, have attracted increasing interest as alternatives to conventional systems based on fossil fuels. As the electricity generation by these renewable sources is highly dependent on weather conditions, energy storage systems that can store extra electricity are required for ensuring stable electricity supply to consumers [1]. Regenerative fuel cells (RFCs), which enable bidirectional conversion between electricity and hydrogen, is considered a promising candidate for electrochemical energy storage. Since the energy capacity of an RFC-based system can be ⁎ Corresponding authors at: Korea Institute of Science and Technology (KIST) Fuel Cell Research Center Seoul 02792 Republic of Korea. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (J.H. Jang). 1 These authors contributed equally to this work. 2 Tel.: +82 2 958 5287; fax: +82 2 958 5199. 3 Tel.: +82 2 3290 3299; fax: +82 2 926 6102. 4 Tel.: +82 2 880 1587; fax: +82 2 888 7295. 5 Tel.: +82 2 9408 676; fax: +82 2 9185 685.
http://dx.doi.org/10.1016/j.elecom.2016.01.002 1388-2481/© 2016 Published by Elsevier B.V.
increased independent of its power, capital cost to expand its energy capacity is lower than that of a Li battery and is thereby expected to be suitable for large-scale applications [2]. The RFCs can be classified as unitized regenerative fuel cells (URFC) and discrete regenerative fuel cells (DRFC) [3]. The URFCs, having integrated fuel cell (FC) and a water electrolyzer (WE) units, are expected to reduce the capital cost; the DRFCs, having separated FC and WE units, have lower technical barriers as they can utilize commercially available FC and WE devices. However, generally high loadings of noble metal electrocatalysts are required for URFCs, which increases the system cost. Since the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the URFC are sluggish, Pt and Ir (or IrO2) electrocatalysts that provide high ORR and OER activities, respectively, have been used together in mixture [4–6] or double-layer form [7,8]. In addition, bifunctional electrocatalysts of a Pt-Ir alloy [9] and (RuO2-IrO2)/Pt [10] have also been investigated. For these studies, high electrocatalyst loading (1.5–4.0 mg cm−2) with 50%–85% Pt content, typically providing current densities above 0.7 A cm−2 @ 0.6 V (FC) and 1 A cm−2 @ 1.7 V(WE). There have been
B.-S. Lee et al. / Electrochemistry Communications 64 (2016) 14–17
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studies to lower the loading of noble metal electrocatalysts, but the corresponding WE and FC performances were not satisfactory [11,12]. In this study, we developed a novel electrocatalyst of porous Pt/IrO2/ carbon paper (CP) as an oxygen electrode of URFC. For this, the IrO2 layer was electrodeposited on the CP and then the porous Pt layers were sprayed, with the total electrocatalyst loading controlled up to 0.4 mg cm−2. As the Pt/IrO2/CP electrocatalyst has an open structure with macropores, originated from the CP substrates, the mass transport of oxygen and water should be highly enhanced compared to conventional film-structured electrodes. Using the Pt/IrO2/CP as oxygen electrodes, single-cell tests were performed in both FC and WE modes to demonstrate high URFC performance with decreased oxygen electrocatalyst loading (0.1–0.4 mg cm−2), focusing on the effect of Pt loading on FC performances, which is lower compared to the WE mode [4,5,7–11].
Membrane electrode assemblies (MEA) were fabricated by placing an oxygen electrode (IP0–IP4) and a hydrogen electrode on either side of N212 membranes (DuPont Co.). The URFC single cells (active area: 6.25 cm2) were prepared by assembling the MEA with graphite bipolar plates (CNL energy). For FC test, fully humidified H2 (100 mL min−1) and O2 (100 mL min− 1) were supplied to the hydrogen and oxygen electrodes, respectively, and polarization curve was obtained with increasing current density from 0 to 1.2 A cm−2. For WE operation, deionized water (15 mL min−1) was fed to the oxygen electrode of the single cell, and polarization curves were obtained over 1.35–1.8 V, with a stepwise increase at intervals of 0.05 V (HCP-803, Biologics Ltd.). The durability was analyzed by potential cycling between 1.35 and 1.80 V at a scan rate of 5 mV s−1 in the WE mode. By switching between FC and WE modes, URFC operation was carried out at 0.3 A cm−2 [4,6]. The cell temperature was 80 °C.
2. Experimental
3. Results and discussion
According to the procedure in our previous study [13], IrO2 electrodes were prepared by electrodeposition at a deposition voltage of 0.7 V, using TGPH-090 CP (Toray Inc.) as substrates. The fabricated IrO2/CP, on which the IrO2 loading was controlled to be 0.1 mg cm−2, was designated as IP0. Pt electrocatalyst ink was prepared by mixing Pt black powder (Johnson Matthey), 5 wt.% Nafion dispersion (Dupont), 1,2-propanol (J.T. Baker), and deionized water; the mixture was sprayed onto IP0 to form porous Pt layers. The Pt loadings were controlled to be 0.1 mg cm−2 (IP1), 0.2 mg cm−2 (IP2), and 0.3 mg cm−2 (IP3). As hydrogen electrodes, Pt/C (46.3 wt.%, TKK) layers (0.4 mg cm−2) on bare CP (10 BC, SGL Ltd.) were fabricated by spraying a dispersion of Pt/C and Nafion ionomers in 1,2-propanol/deionized water mixture. The microstructures and compositions of the thus-prepared electrodes were analyzed by scanning electron microscopy (SEM; Inspect F50, Field Emission Inc.) and electron-probe micro-analysis (EPMA; JXA-8500F, JEOL), respectively. Samples for cross-sectional images were prepared by focused ion beam (FIB; Nova 600, FEI) technique. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed using a Dmax2500/Server (Rigaku) and ESCA 2000 (Thermo), respectively.
The SEM image of electrode IP2 shows open structure originated from the CP substrate, randomly crossed carbon fibers in the CP (Fig. 1a). The inter-fiber spaces, which should facilitate mass transport during FC and WE operation, were as large as ~100 μm and were hardly blocked by the porous Pt layers. The Pt layers sprayed on the IrO2-coated CP shows a highly porous structure having sub-micrometer-sized pores (Fig. 1b). Cross-sectional image of a fiber indicated that porous Pt layers covered IrO2/CP with a thickness of a few micrometers (Fig. 1c), while EPMA element mapping shows a thin IrO2 film on the carbon fiber (Fig. 1d). The XRD pattern of IP0 shows diffraction peaks at 26.5°, 42.4°, and 54.7°, which correspond to the (002), (100), and (004) planes of graphite (PDF#: 65-6212, hexagonal structure), respectively [14,15] (Fig. 1e). The absence of the diffraction peak of IrO2 could be attributed to its amorphous nature, based on the TEM analysis (not shown). The XRD patterns of IP1, IP2, and IP3 show additional peaks at 39.8°, 46.2°, and 67.7°, corresponding to the (111), (200), and (220) planes of Pt (JCPDS#: 65-2868, FCC structure), respectively [16]. The XPS analysis shows that, with increased Pt loading, Ir 4f peaks significantly decreased in intensity while the Pt 4f peaks increased
Fig. 1. (a) and (b) SEM images of porous Pt/IrO2/CP electrode (IP2) surfaces. (c) Cross-sectional image and (d) Ir-mapping image of a porous Pt/IrO2 electrocatalysts coated carbon fiber. (e) XRD pattern and (f) Pt 4f and Ir 4f X-ray photoelectron spectra of the porous Pt/IrO2/CP electrodes.
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B.-S. Lee et al. / Electrochemistry Communications 64 (2016) 14–17
(Fig. 1f). Considering that photoelectron penetration depth is several nanometers [17], the increased Pt 4f and decreased Ir 4f peak intensities indicates that the porous Pt layers uniformly covered the IrO2/CP surfaces. The Pt 4f7/2 binding energy of 71.4 eV indicated metallic Pt surfaces [18]. The oxidation state of IrO2 was hardly influenced by the Pt layers as evidenced by negligible changes in the Ir 4f7/2 binding energy (62.5 eV) [13]. Fig. 2a shows the polarization curves in FC and WE operation modes. The IP0, containing 0.1 mg cm−2 of IrO2, showed high WE performance as previously reported [13], but its FC performance was very poor due to the absence of the ORR-active Pt layer in the oxygen electrode. As the Pt loading was gradually increased up to 0.3 mg cm−2, the FC performance increased significantly from IP1 to IP3, as the ORR activity of Pt is higher than that of IrO2 by 2 orders of magnitude [19]. The high WE activities were not significantly influenced by increased Pt loading. This indicates that, in the Pt/IrO2/CP electrocatalysts (IP1, IP2, and IP3), the porous Pt layers actively participate in ORR under the FC mode, and the IrO2
sublayers function as OER electrocatalysts in WE mode without any significant hindrance by the Pt layers. When the current densities at typical operating voltages (FC: 0.6 V, WE: 1.7 V) [20,21] were plotted as a function of Pt loading, the current density at 0.6 V increased linearly with Pt loading, suggesting that the sprayed Pt electrocatalysts were effectively utilized up to 0.3 mg cm−2. In the case of WE, the current density increased slightly as Pt loading increased up to 0.2 mg cm−2, and decreased with further Pt loading. The Pt electrocatalyst is expected to provide additional OER-active sites, even though its activity is much lower than that of IrO2 (~5%) [9]. However, the Pt layer also can hinder the OER on highly active IrO2 sublayers, especially with thick Pt layers. In potential cycling of IP2, activity decay was not observed, which is inconsistent with SEM analysis (Fig. 2c). In the FC/WE switching operation, both FC and WE cell voltages were constantly maintained (Fig. 2d), indicating stability during the URFC operation. Fig. 2e summarizes the FC performance of URFC single cells obtained in this study and from previous reports, as a function of the electrocatalyst
Fig. 2. (a) Polarization curves of porous Pt/IrO2/CP electrodes in FC and WE modes. (b) Current densities at 0.6 V (FC) and at 1.7 V (WE) as a function of Pt loading. (c) Cyclic voltammograms of IP2 in the WE mode and SEM images before and after potential cycling (inset). (d) Voltage–time profile of IP2 in FC/WE switching mode operation at 0.3 A cm−2. (e) Current densities at 0.6 V (FC) and (f) at 1.7 V (WE) in this study and those reported in the literature.
B.-S. Lee et al. / Electrochemistry Communications 64 (2016) 14–17
loading in the oxygen electrodes. Literature values show that current density of ~ 0.7 A cm−2 was achieved with high electrocatalyst loading of 1.5–4.0 mg cm−2 [4–9], but the FC performance gradually decreased with decreasing loading to show current density b 0.5 A cm−2 [4–10]. In comparison, our porous Pt/IrO2/CP electrodes provided better FC characteristics. Especially, the IP2 and IP3 showed current densities of 0.63 and 0.89 A cm−2, even though the electrocatalyst loading was b 1 mg cm−2. The reported current densities with similar loadings were 0.5 A cm−2 (0.40 mg cm−2) [12] and 0.38 A cm−2 (0.48 mg cm−2) [11]. The high performances of the Pt/IrO2/CP electrodes with low electrocatalyst loadings probably originate from the efficient mass transport that leads to high electrocatalyst utilization. As electrocatalysts are mostly deposited around the top layers of the CPs, highly open structure with large inter-fiber spaces could be achieved in the Pt/IrO2/CP. As oxygen supply and water removal during ORR occur through the macropores extended to the electrocatalyst/membrane interface, a facile reaction is expected with negligible transport limitation, whereas conventional electrodes require oxygen and water transports through small and tortuous pores formed among the electrocatalyst particles in the electrode layers [9,22]. In addition, the porous Pt structures of the Pt/IrO2/CP are also advantageous for facile mass transport and resultant high activity. In Fig. 2f, WE performances are plotted as a function of oxygen electrocatalyst loading. It clearly indicates that the WE performance of URFC single cell with Pt/IrO2/CP electrodes (1.5 A cm− 2 at 1.7 V) is higher than reported values with higher electrocatalyst loadings. The high WE performance could be explained by the efficient mass transport by the porous Pt layers and the high mass activity of the IrO2 layer [13]. As the porous Pt layers are thin (a few micrometers) and are directly connected to large macropores, oxygen produced at the IrO2 layers could be easily removed through the porous Pt layers and would hardly hinder OER at the inner IrO2 layers. From the FC and WE performances, the round-trip efficiency (εRT) can be defined as the ratio of cell voltage in WE mode to that in FC mode at a given current density [22], representing the energy conversion efficiency of URFCs. The εRT was significantly enhanced with increased Pt loading, mainly due to the higher FC performance with higher Pt loading. The εRT of IP2 and IP3 were 0.42 and 0.46 at 0.5 A cm−2, respectively, which is comparable to the URFCs with large oxygen electrode loading (1.5–4.0 mg cm−2) [4,5,7–10]. The εRT has been reported to be much lower (b0.38) when the electrocatalyst loading is below 1 mg cm− 2 [11,12]. 4. Conclusion Using the porous Pt/IrO2/CP electrode, high-performance URFC was demonstrated with decreased oxygen electrocatalysts loading (0.1–0.4 mg cm − 2 ). In particular, IP3 (electrocatalysts loading: 0.4 mg cm− 2) showed one of the highest performances in both FC and WE modes among the reported URFCs. The FC performance increased with the increased Pt loading up to 0.3 mg cm− 2, with little influence on WE performances, and therefore, the enhanced FC performance led to high ε RT (IP3: 0.46). The high performance of the electrode developed in this study was attributed to efficient mass transport in the electrode because of the open structure of Pt/IrO2/CP electrodes. Conflict of interest The authors declare no competing financial interest. Acknowledgements This study was supported by the Korean Government through the New and Renewable Energy Core Technology Program of the Korea Institute of
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Energy Technology Evaluation and Planning (KETEP) funded by MOTIE (No. 20133030011320 and No. 20143010031770), the National Research Foundation of Korea (NRF) Grant funded by MSIP (2014, UniversityInstitute Cooperation Program and No. NRF-2015M1A2A2056555), and the Global Frontier R&D Program at the Center for Multiscale Energy System funded by the NRF, MSIP (No. 2012M3A6A7054283). This study was also financially supported by the KIST through the Institutional Project. S. P. acknowledges the financial support from the NRF (No. NRF-2014R1A1A2057667).
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Development of porous Pt/IrO2/carbon paper electrocatalysts with enhanced mass transport as oxygen electrodes in unitized regenerative fuel cells Byung-Seok Lee a,b,1,2,3, Hee-Young Park a,1,2, Min Kyung Cho a,e,2,4, Jea Woo Jung a,c,2,3, Hyoung-Juhn Kim a,2, Dirk Henkensmeier a,2, Sung Jong Yoo a,2, Jin Young Kim a,2, Sehkyu Park d,5, Kwan-Young Lee b,c,⁎,3, Jong Hyun Jang a,c,⁎,2,3 a
Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea c Green School, Korea University, Seoul 02841, Republic of Korea d Department of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea e School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 25 November 2015 Received in revised form 25 December 2015 Accepted 7 January 2016 Available online 12 January 2016 Keywords: Unitized regenerative fuel cell Oxygen electrode Platinum Iridium oxide
a b s t r a c t The oxygen electrodes in unitized regenerative fuel cells (URFC) must have high activities towards oxygen reduction reaction (ORR) as well as oxygen evolution reaction (OER), thus requiring high loading of noble metal electrocatalysts. In this study, porous Pt/IrO2/carbon paper (CP) electrocatalysts were developed to reduce the metal loading. The Pt/IrO2/CP electrodes were fabricated by sequential formation of IrO2 layers (loading 0.1 mg cm−2) and porous Pt layers (0–0.3 mg cm−2) on CP substrates by electrodeposition and spraying techniques, respectively. The fuel cell (FC) performances increased linearly up to 0.69 A cm−2 with increasing Pt loading (up to ~0.3 mg cm−2) at 0.6 V, whereas the water electrolysis (WE) activity was highest at Pt loading of 0.2 mg cm−2. The current densities in the FC and WE modes and round-trip efficiency of the developed Pt/ IrO2/CP electrodes with the oxygen electrocatalysts loadings of 0.3 and 0.4 mg cm−2 were higher or comparable to previously reported values with higher loading (1.5–4.0 mg cm−2). These high performances with low loading are probably due to the facile oxygen and water transport through well-developed macropores originating from the open CP structures, providing effective utilization of the IrO2 and Pt electrocatalysts towards OER and ORR, respectively. © 2016 Published by Elsevier B.V.
1. Introduction Renewable energy sources, such as solar power and wind power, have attracted increasing interest as alternatives to conventional systems based on fossil fuels. As the electricity generation by these renewable sources is highly dependent on weather conditions, energy storage systems that can store extra electricity are required for ensuring stable electricity supply to consumers [1]. Regenerative fuel cells (RFCs), which enable bidirectional conversion between electricity and hydrogen, is considered a promising candidate for electrochemical energy storage. Since the energy capacity of an RFC-based system can be ⁎ Corresponding authors at: Korea Institute of Science and Technology (KIST) Fuel Cell Research Center Seoul 02792 Republic of Korea. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (J.H. Jang). 1 These authors contributed equally to this work. 2 Tel.: +82 2 958 5287; fax: +82 2 958 5199. 3 Tel.: +82 2 3290 3299; fax: +82 2 926 6102. 4 Tel.: +82 2 880 1587; fax: +82 2 888 7295. 5 Tel.: +82 2 9408 676; fax: +82 2 9185 685.
http://dx.doi.org/10.1016/j.elecom.2016.01.002 1388-2481/© 2016 Published by Elsevier B.V.
increased independent of its power, capital cost to expand its energy capacity is lower than that of a Li battery and is thereby expected to be suitable for large-scale applications [2]. The RFCs can be classified as unitized regenerative fuel cells (URFC) and discrete regenerative fuel cells (DRFC) [3]. The URFCs, having integrated fuel cell (FC) and a water electrolyzer (WE) units, are expected to reduce the capital cost; the DRFCs, having separated FC and WE units, have lower technical barriers as they can utilize commercially available FC and WE devices. However, generally high loadings of noble metal electrocatalysts are required for URFCs, which increases the system cost. Since the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the URFC are sluggish, Pt and Ir (or IrO2) electrocatalysts that provide high ORR and OER activities, respectively, have been used together in mixture [4–6] or double-layer form [7,8]. In addition, bifunctional electrocatalysts of a Pt-Ir alloy [9] and (RuO2-IrO2)/Pt [10] have also been investigated. For these studies, high electrocatalyst loading (1.5–4.0 mg cm−2) with 50%–85% Pt content, typically providing current densities above 0.7 A cm−2 @ 0.6 V (FC) and 1 A cm−2 @ 1.7 V(WE). There have been
B.-S. Lee et al. / Electrochemistry Communications 64 (2016) 14–17
15
studies to lower the loading of noble metal electrocatalysts, but the corresponding WE and FC performances were not satisfactory [11,12]. In this study, we developed a novel electrocatalyst of porous Pt/IrO2/ carbon paper (CP) as an oxygen electrode of URFC. For this, the IrO2 layer was electrodeposited on the CP and then the porous Pt layers were sprayed, with the total electrocatalyst loading controlled up to 0.4 mg cm−2. As the Pt/IrO2/CP electrocatalyst has an open structure with macropores, originated from the CP substrates, the mass transport of oxygen and water should be highly enhanced compared to conventional film-structured electrodes. Using the Pt/IrO2/CP as oxygen electrodes, single-cell tests were performed in both FC and WE modes to demonstrate high URFC performance with decreased oxygen electrocatalyst loading (0.1–0.4 mg cm−2), focusing on the effect of Pt loading on FC performances, which is lower compared to the WE mode [4,5,7–11].
Membrane electrode assemblies (MEA) were fabricated by placing an oxygen electrode (IP0–IP4) and a hydrogen electrode on either side of N212 membranes (DuPont Co.). The URFC single cells (active area: 6.25 cm2) were prepared by assembling the MEA with graphite bipolar plates (CNL energy). For FC test, fully humidified H2 (100 mL min−1) and O2 (100 mL min− 1) were supplied to the hydrogen and oxygen electrodes, respectively, and polarization curve was obtained with increasing current density from 0 to 1.2 A cm−2. For WE operation, deionized water (15 mL min−1) was fed to the oxygen electrode of the single cell, and polarization curves were obtained over 1.35–1.8 V, with a stepwise increase at intervals of 0.05 V (HCP-803, Biologics Ltd.). The durability was analyzed by potential cycling between 1.35 and 1.80 V at a scan rate of 5 mV s−1 in the WE mode. By switching between FC and WE modes, URFC operation was carried out at 0.3 A cm−2 [4,6]. The cell temperature was 80 °C.
2. Experimental
3. Results and discussion
According to the procedure in our previous study [13], IrO2 electrodes were prepared by electrodeposition at a deposition voltage of 0.7 V, using TGPH-090 CP (Toray Inc.) as substrates. The fabricated IrO2/CP, on which the IrO2 loading was controlled to be 0.1 mg cm−2, was designated as IP0. Pt electrocatalyst ink was prepared by mixing Pt black powder (Johnson Matthey), 5 wt.% Nafion dispersion (Dupont), 1,2-propanol (J.T. Baker), and deionized water; the mixture was sprayed onto IP0 to form porous Pt layers. The Pt loadings were controlled to be 0.1 mg cm−2 (IP1), 0.2 mg cm−2 (IP2), and 0.3 mg cm−2 (IP3). As hydrogen electrodes, Pt/C (46.3 wt.%, TKK) layers (0.4 mg cm−2) on bare CP (10 BC, SGL Ltd.) were fabricated by spraying a dispersion of Pt/C and Nafion ionomers in 1,2-propanol/deionized water mixture. The microstructures and compositions of the thus-prepared electrodes were analyzed by scanning electron microscopy (SEM; Inspect F50, Field Emission Inc.) and electron-probe micro-analysis (EPMA; JXA-8500F, JEOL), respectively. Samples for cross-sectional images were prepared by focused ion beam (FIB; Nova 600, FEI) technique. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed using a Dmax2500/Server (Rigaku) and ESCA 2000 (Thermo), respectively.
The SEM image of electrode IP2 shows open structure originated from the CP substrate, randomly crossed carbon fibers in the CP (Fig. 1a). The inter-fiber spaces, which should facilitate mass transport during FC and WE operation, were as large as ~100 μm and were hardly blocked by the porous Pt layers. The Pt layers sprayed on the IrO2-coated CP shows a highly porous structure having sub-micrometer-sized pores (Fig. 1b). Cross-sectional image of a fiber indicated that porous Pt layers covered IrO2/CP with a thickness of a few micrometers (Fig. 1c), while EPMA element mapping shows a thin IrO2 film on the carbon fiber (Fig. 1d). The XRD pattern of IP0 shows diffraction peaks at 26.5°, 42.4°, and 54.7°, which correspond to the (002), (100), and (004) planes of graphite (PDF#: 65-6212, hexagonal structure), respectively [14,15] (Fig. 1e). The absence of the diffraction peak of IrO2 could be attributed to its amorphous nature, based on the TEM analysis (not shown). The XRD patterns of IP1, IP2, and IP3 show additional peaks at 39.8°, 46.2°, and 67.7°, corresponding to the (111), (200), and (220) planes of Pt (JCPDS#: 65-2868, FCC structure), respectively [16]. The XPS analysis shows that, with increased Pt loading, Ir 4f peaks significantly decreased in intensity while the Pt 4f peaks increased
Fig. 1. (a) and (b) SEM images of porous Pt/IrO2/CP electrode (IP2) surfaces. (c) Cross-sectional image and (d) Ir-mapping image of a porous Pt/IrO2 electrocatalysts coated carbon fiber. (e) XRD pattern and (f) Pt 4f and Ir 4f X-ray photoelectron spectra of the porous Pt/IrO2/CP electrodes.
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(Fig. 1f). Considering that photoelectron penetration depth is several nanometers [17], the increased Pt 4f and decreased Ir 4f peak intensities indicates that the porous Pt layers uniformly covered the IrO2/CP surfaces. The Pt 4f7/2 binding energy of 71.4 eV indicated metallic Pt surfaces [18]. The oxidation state of IrO2 was hardly influenced by the Pt layers as evidenced by negligible changes in the Ir 4f7/2 binding energy (62.5 eV) [13]. Fig. 2a shows the polarization curves in FC and WE operation modes. The IP0, containing 0.1 mg cm−2 of IrO2, showed high WE performance as previously reported [13], but its FC performance was very poor due to the absence of the ORR-active Pt layer in the oxygen electrode. As the Pt loading was gradually increased up to 0.3 mg cm−2, the FC performance increased significantly from IP1 to IP3, as the ORR activity of Pt is higher than that of IrO2 by 2 orders of magnitude [19]. The high WE activities were not significantly influenced by increased Pt loading. This indicates that, in the Pt/IrO2/CP electrocatalysts (IP1, IP2, and IP3), the porous Pt layers actively participate in ORR under the FC mode, and the IrO2
sublayers function as OER electrocatalysts in WE mode without any significant hindrance by the Pt layers. When the current densities at typical operating voltages (FC: 0.6 V, WE: 1.7 V) [20,21] were plotted as a function of Pt loading, the current density at 0.6 V increased linearly with Pt loading, suggesting that the sprayed Pt electrocatalysts were effectively utilized up to 0.3 mg cm−2. In the case of WE, the current density increased slightly as Pt loading increased up to 0.2 mg cm−2, and decreased with further Pt loading. The Pt electrocatalyst is expected to provide additional OER-active sites, even though its activity is much lower than that of IrO2 (~5%) [9]. However, the Pt layer also can hinder the OER on highly active IrO2 sublayers, especially with thick Pt layers. In potential cycling of IP2, activity decay was not observed, which is inconsistent with SEM analysis (Fig. 2c). In the FC/WE switching operation, both FC and WE cell voltages were constantly maintained (Fig. 2d), indicating stability during the URFC operation. Fig. 2e summarizes the FC performance of URFC single cells obtained in this study and from previous reports, as a function of the electrocatalyst
Fig. 2. (a) Polarization curves of porous Pt/IrO2/CP electrodes in FC and WE modes. (b) Current densities at 0.6 V (FC) and at 1.7 V (WE) as a function of Pt loading. (c) Cyclic voltammograms of IP2 in the WE mode and SEM images before and after potential cycling (inset). (d) Voltage–time profile of IP2 in FC/WE switching mode operation at 0.3 A cm−2. (e) Current densities at 0.6 V (FC) and (f) at 1.7 V (WE) in this study and those reported in the literature.
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loading in the oxygen electrodes. Literature values show that current density of ~ 0.7 A cm−2 was achieved with high electrocatalyst loading of 1.5–4.0 mg cm−2 [4–9], but the FC performance gradually decreased with decreasing loading to show current density b 0.5 A cm−2 [4–10]. In comparison, our porous Pt/IrO2/CP electrodes provided better FC characteristics. Especially, the IP2 and IP3 showed current densities of 0.63 and 0.89 A cm−2, even though the electrocatalyst loading was b 1 mg cm−2. The reported current densities with similar loadings were 0.5 A cm−2 (0.40 mg cm−2) [12] and 0.38 A cm−2 (0.48 mg cm−2) [11]. The high performances of the Pt/IrO2/CP electrodes with low electrocatalyst loadings probably originate from the efficient mass transport that leads to high electrocatalyst utilization. As electrocatalysts are mostly deposited around the top layers of the CPs, highly open structure with large inter-fiber spaces could be achieved in the Pt/IrO2/CP. As oxygen supply and water removal during ORR occur through the macropores extended to the electrocatalyst/membrane interface, a facile reaction is expected with negligible transport limitation, whereas conventional electrodes require oxygen and water transports through small and tortuous pores formed among the electrocatalyst particles in the electrode layers [9,22]. In addition, the porous Pt structures of the Pt/IrO2/CP are also advantageous for facile mass transport and resultant high activity. In Fig. 2f, WE performances are plotted as a function of oxygen electrocatalyst loading. It clearly indicates that the WE performance of URFC single cell with Pt/IrO2/CP electrodes (1.5 A cm− 2 at 1.7 V) is higher than reported values with higher electrocatalyst loadings. The high WE performance could be explained by the efficient mass transport by the porous Pt layers and the high mass activity of the IrO2 layer [13]. As the porous Pt layers are thin (a few micrometers) and are directly connected to large macropores, oxygen produced at the IrO2 layers could be easily removed through the porous Pt layers and would hardly hinder OER at the inner IrO2 layers. From the FC and WE performances, the round-trip efficiency (εRT) can be defined as the ratio of cell voltage in WE mode to that in FC mode at a given current density [22], representing the energy conversion efficiency of URFCs. The εRT was significantly enhanced with increased Pt loading, mainly due to the higher FC performance with higher Pt loading. The εRT of IP2 and IP3 were 0.42 and 0.46 at 0.5 A cm−2, respectively, which is comparable to the URFCs with large oxygen electrode loading (1.5–4.0 mg cm−2) [4,5,7–10]. The εRT has been reported to be much lower (b0.38) when the electrocatalyst loading is below 1 mg cm− 2 [11,12]. 4. Conclusion Using the porous Pt/IrO2/CP electrode, high-performance URFC was demonstrated with decreased oxygen electrocatalysts loading (0.1–0.4 mg cm − 2 ). In particular, IP3 (electrocatalysts loading: 0.4 mg cm− 2) showed one of the highest performances in both FC and WE modes among the reported URFCs. The FC performance increased with the increased Pt loading up to 0.3 mg cm− 2, with little influence on WE performances, and therefore, the enhanced FC performance led to high ε RT (IP3: 0.46). The high performance of the electrode developed in this study was attributed to efficient mass transport in the electrode because of the open structure of Pt/IrO2/CP electrodes. Conflict of interest The authors declare no competing financial interest. Acknowledgements This study was supported by the Korean Government through the New and Renewable Energy Core Technology Program of the Korea Institute of
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Energy Technology Evaluation and Planning (KETEP) funded by MOTIE (No. 20133030011320 and No. 20143010031770), the National Research Foundation of Korea (NRF) Grant funded by MSIP (2014, UniversityInstitute Cooperation Program and No. NRF-2015M1A2A2056555), and the Global Frontier R&D Program at the Center for Multiscale Energy System funded by the NRF, MSIP (No. 2012M3A6A7054283). This study was also financially supported by the KIST through the Institutional Project. S. P. acknowledges the financial support from the NRF (No. NRF-2014R1A1A2057667).
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