Ir Loading Research Articles

Synthesis of Ni–Ir Nanocages with Improved Electrocatalytic Performance for the Oxygen Evolution Reaction

Design and fabrication of inexpensive and efficient oxygen evolution reaction (OER) catalysts are of great importance for polymer electrolyte membrane water electrolyzers (PEMWEs). Although the best electrocatalyst is IrO2 for OER in the PEMWEs, the practical application of Ir has been impeded because of its high cost and limited activity. Herein, a galvanic replacement reaction mechanism was developed for the preparation of polycrystalline Ni–Ir nanocages (NCs) by using Ni nanoparticles as templates. The formation of Ni–Ir NCs achieves the enhancement of OER catalytic performance, accompanied by the reduction of Ir loading but significantly increasing the efficiency of Ir atoms. The as-prepared Ni2.53Ir NCs exhibit improved catalytic activity toward OER in acid solution, which requires only an overpotential of 302 mV to deliver a current density of 10 mA/cm2. At an overpotential of 300 mV, the Ir-based mass activity of Ni2.53Ir catalysts reaches 114.7 mA/mgIr, which is 2.1 times higher than that of comme...

More active Ir subnanometer clusters than single‐atoms for catalytic oxidation of CO at low temperature

This work reported the adsorption and reaction performance of FeOx supported subnanometer cluster and single‐atom Ir catalysts for the oxidation of CO at low temperature. By varying the pretreatment temperature and Ir loading, the single‐atom and subnanometer cluster Ir catalysts were obtained. The Ir subnanometer clusters exhibited higher activity for the oxidation of CO with or without the presence of H2 than the single‐atom counterpart. By using adsorption microcalorimetry and in situ infrared spectroscopy measurements, it was found that the Ir subnanometer clusters not only promoted the adsorption and reaction of CO and O2 but also facilitated the formation of OH species from reaction between H2 and O2, thus opening a new reaction pathway between CO and OH species to produce CO2 compared with that between CO and O species on the single‐atom counterpart. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4003–4012, 2017

Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings

PEM water electrolyzers are promising candidates for large-scale energy storage in combination with renewable energy sources. Typically, high loadings of platinum group metal (PGM) catalysts are used, but since most currently installed systems are relatively small (kW range), catalyst costs are minor compared to other system costs. However, for large-scale applications, the contribution of balance-of-plant costs will be much lower and catalyst costs will become a major cost contributor [1]. Increasing prices due to a higher demand of PGMs might further amplify this trend in the future [2]. Therefore, a reduction of catalyst loading is essential to improve the economic competitiveness of PEM electrolyzers. There have been efforts to reduce PGM loadings, and previous studies show that high performance is possible even at PGM loadings of ~0.5 mgPGM cm-2[3,4]. However, there are only few studies which systematically analyze the effect of catalyst loading on the voltage losses of an electrolyzer [5,6]. In this study, we investigate the influence of reduced catalyst loadings on both the anode and cathode of a PEM electrolyzer and analyze the resulting changes in performance and voltage loss contributions. MEAs based on a Nafion® 212 membrane, carbon-supported platinum catalysts (Pt/C) for the hydrogen evolution reaction (HER), and an IrO2/TiO2 catalyst (Umicore) for the oxygen evolution reaction (OER) were fabricated with different catalyst loadings. A reduction of the Pt loading on the cathode from 0.32 to 0.02 mgPt cm-2 was achieved by using a catalyst with a lower Pt content of 4.8 wt% instead of 46.1 wt% Pt, while keeping the electrode thickness similar (~10 μm). Even though the loading was reduced by more than an order of magnitude, the performance remained essentially unchanged (see red and blue curve in Figure 1a), which is mainly due to the fast kinetics of the HER [7]. On the anode, the Ir loading was reduced from 1.6 to 0.4 mgIr cm-2by decreasing the electrode thickness from ~8 to 2 μm. The resulting increase in cell voltage (see green curve in Figure 1a) is in part due to the lower electrochemically active surface area, i.e., due to an OER kinetic loss of ~30 mV. Unfortunately, we observe an additional increase in contact resistance for this very thin electrode, which we ascribe to its inhomogeneity. Contributions of proton and mass transport losses for the different electrodes will be analyzed as well. Figure 1b shows the PGM-specific power density (in gPGM kW-1) versus current density for different catalyst loadings. It can be seen that the gPGM kW-1 at the same electrolyzer efficiency (see dashed lines in Figure 1b) can be reduced roughly 3-fold for the MEA with the lowest PGM loading. Figure 1b also illustrates that the gPGM kW-1decrease substantially with increasing current density, but of course at the expense of a lower efficiency. Therefore, the preferred point of electrolyzer operation will always be a trade-off between low capital costs (determined by loading and current density) and low operating costs (determined by efficiency). Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies).

A cost-effective nanoporous ultrathin film electrode based on nanoporous gold/IrO2 composite for proton exchange membrane water electrolysis

Abstract A cost-effective nanoporous ultrathin film (NPUF) electrode based on nanoporous gold (NPG)/IrO 2 composite has been constructed for proton exchange membrane (PEM) water electrolysis. The electrode was fabricated by integrating IrO 2 nanoparticles into NPG through a facile dealloying and thermal decomposition method. The NPUF electrode is featured in its 3D interconnected nanoporosity and ultrathin thickness. The nanoporous ultrathin architecture is binder-free and beneficial for improving electrochemical active surface area, enhancing mass transport and facilitating releasing of oxygen produced during water electrolysis. Serving as anode, a single cell performance of 1.728 V (@ 2 A cm −2 ) has been achieved by NPUF electrode with a loading of IrO 2 and Au at 86.43 and 100.0 μg cm −2 respectively, the electrolysis voltage is 58 mV lower than that of conventional electrode with an Ir loading an order of magnitude higher. The electrolysis voltage kept relatively constant up to 300 h (@250 mA cm −2 ) during the course of durability test, manifesting that NPUF electrode is promising for gas evolution.

Direct Synthesis of an Iridium(III) Bipyridine Metal–Organic Framework as a Heterogeneous Catalyst for Aerobic Alcohol Oxidation

AbstractThe heterogenization of an IrIII complex is accomplished by the modification of a nanoporous Ga‐based metal–organic framework (MOF), denoted as COMOC‐4. The introduction of the Ir complex was performed successfully through two different strategies (post‐ and prefunctionalization). The obtained Ir‐based MOF materials were compared in terms of the Ir loading, specific surface area, and crystallinity. In this case, the prefunctionalization approach was a powerful way to obtain a high Ir loading and surface area and preserve the formation of the desired MOF topology. The IrIIICp*Cl@COMOC‐4 (Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) material exhibits a good catalytic performance in the oxidation of benzyl alcohol using O2 as a clean oxidant. Furthermore, the stability and reusability were examined and showed that the IrIIICp*Cl@COMOC‐4 material could be reused for at least four cycles with no decrease in activity and selectivity. Additionally, no significant leaching of the Ir and Ga species or loss of the crystallinity was observed.

Iridium Nanoparticles Supported on Magneli Phase Ti4O7 for PEM Water Electrolyzers

PEM water electrolysis is one of the promising technologies for hydrogen production from renewable energy sources. Iridium and their oxides are recognized as the most suitable catalyst showing high activity and stability for oxygen evolution reaction (OER) in acidic media [1], however, overpotential is still high and its scarcity and price are also drawbacks. To overcome these problems, a number of studies concerning particle size reduction, catalyst compositions, and utilization of catalyst support have been reported [2]. Among them, use of corrosion-resistant and highly electro-conductive oxides, especially Magneli phase Ti4O7 as catalyst support would be beneficial for reducing Ir loading and increasing durability. We have developed Ti4O7-supported Ir catalyst and examined activity and durability as an anode catalyst of PEM electrolyzer. Titanium oxide support was prepared by UV laser technique [3]. Ir/Ti4O7 catalysts obtained by using H2IrCl6 precursor showed conductivity comparable with that of commercial Pt/C. MEAs with the Ir/Ti4O7 anode can be operated up to 4 A cm-2 at low cell voltage < 2.0 V (with DuPont NR212 membrane) with very low Ir loadings (0.25 mg-Ir cm-2). Considering fluctuating nature of the renewable energy inputs, catalyst stability against voltage change should be another important issue. To simulate and accelerate such degradation modes, potential sweep between 1.0 V and 2.0 V at a sweep rate of 500 mV s-1 was applied up to 10,000 cycles. The Ir/Ti4O7 MEA shows limited degradation even after 10,000 cycles. Stability of the cell performance and the catalyst structure will be discussed at the Meeting.

Performances of Polymer Electrolyte Membrane Water Electrolysis Cells with IrOx/Doped SnO2 Oxygen Evolution Catalysts

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology which enables a production of high-purity hydrogen with high energy conversion efficiency. However, conventional PEMWE cells are very expensive because noble metal catalysts (a few mg cm–2) are used. The aim of our research is the reduction of the amount of noble metal catalysts to 1/10 maintaining high-efficiency ≥ 90%. We reported that IrOx nanoparticles dispersed on M-doped SnO2 (M=Nb, Ta and Sb) supports with fused aggregated structures exhibited much higher mass activities for the oxygen evolution reaction (OER) than a conventional catalyst (mixture of commercial IrO2 and Pt black, 50:50 in weight ratio) in 0.1 M HClO4 electrolyte at 80 oC.1 In the present research, we examined performances of membrane-electrode assemblies (MEAs) with the IrOx/SnO2 based anode catalysts. Uniform size IrOx nanoparticles (ca. 2 nm) highly dispersed on M-doped SnO2 (M=Nb, Ta and Sb) supports were prepared in the same manner as reported previously.1 The amounts of Ir loaded were quantified to be 11.3, 10.4 and 11.0 wt% for IrOx/M-SnO2 (M=Nb, Ta and Sb), respectively. The IrOx/M-SnO2, Nafion® binder (the volume ratio of Nafion binder to oxide support = 0.67), ethanol and pure water were mixed by using a planetary ball mill. The anode catalyst ink thus obtained was sprayed onto the Nafion® membrane with the thickness of 50 μm, followed by hot-pressing. The Ir loading at the anode was ca. 0.1 mgIr cm−2. Commercial Pt/graphitized carbon black (GCB) catalyst was used as the cathode with the Pt loadings of ca. 0.35 mgPt cm−2. The Japan Automobile Research Institute (JARI) standard cell with a geometric area of 25 cm2 was used for PEMWE measurements. Before the measurements, pure water at 80 oC was supplied to the cell, at least, for 12 h, and the ohmic resistance of the cell was then recorded by AC milli-ohmmeter. The I-V curves of the cells were measured at 80 oC. Figure 1 shows I-V curves of the cells with the IrOx/M-SnO2 anode catalysts. The data of catalyst loading, ohmic resistance and cell potential at 1 A cm−2 are summarized in Table 1. The cell with IrOx/Ta-SnO2 catalyst showed a lower cell potential at 1 A cm-2 and ohmic resistance than that with IrOx/Nb-SnO2 catalyst, probably due to an increase in the apparent electrical conductivities σapp of the catalyst. However, the overpotential of the cell with IrOx/Ta-SnO2 catalyst was fairly larger than the conventional cell with Pt black + IrO2. We have already reported that Pt nanoparticle contributed to improving electric conductivities for SnO2 supports, which may be related to a decrease in the electronic depletion region of SnO2.4 Then, we synthesized Pt-loaded IrOx/Ta-SnO2 catalyst by the colloidal method. The use of Pt-IrOx/Ta-SnO2 catalyst significantly reduced both ohmic resistance and overpotential, achieving to the cell potential of 1.63 V at 1 A cm−2. The improved cell performance can be mainly ascribed to increases in the σapp (as shown in Fig. 2) and utilization efficiency of the catalyst. In addition, it was found that IrOx/Sb-SnO2 catalyst prepared very recently exhibited ca. 7 times higher σapp than Pt-IrOx/Ta-SnO2 catalyst. The evaluation of cell performance with IrOx/Sb-SnO2catalyst is now in progress. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References 1) H. Ohno, S. Nohara, K. Kakinuma, A. Miyake, S. Deki, M. Watanabe, and H. Uchida, 229th ECS Meeting,abstract 1422 (San Diego, May 30, 2016). 2) K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011). 3) M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 229, 395 (1987). 4) Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv., 4, 32180 (2014). Figure 1

Olefin Upgrading over Ir/ZSM-5 catalysts under methane environment

Upgrading olefin in the synthetic oil to alkane is highly desired due to its high volatility and thermal unstability as well as low energy density. Unlike conventional hydrotreating, methane (CH4) was used in this study as the novel hydrogen donor for olefin saturation. The significant increase of H/C atomic ratio of product oil from 1.69±0.02 (over pure ZSM-5) to 2.04±0.02 (over Ir/ZSM-5 (10.0)) and the alkane content up to 83.8±2.1% in the upgraded oil indicated that methane could exhibit comparable catalytic performance to what hydrogen (H2) did for olefin (1-Decene) upgrading over the developed bifunctional catalysts with low Ir loadings. The HRTEM and XPS data revealed that the highly dispersed metallic Ir particles with average size of 1.32nm was coexisting with IrO2 species. The synergic effects of Ir/IrO2 obviously promoted the activation of methane, which supplied sufficient hydrogen for the saturation and stabilization of olefin. The results from BET indicated that the pore size and volume of the ZSM-5 support were increased upon Ir introduction, which provided more active sites for cracking olefin (1-decene). NH3-TPD results suggested that the presence of highly dispersed Ir increased the amount of surface acidity, which enhanced the formation and stabilization of carbenium ion intermediates. As a result, the produced alkanes were mainly composed of cyclopentane-derived compounds, like propylcyclopentane, 3-methylbutyl-cyclopentane and 1,2,4-trimethyl-cyclopentane, which has great application potential as immersion fluid and additive in the field of optics and petroleum.

Iridium-Tin oxide solid-solution nanocatalysts with enhanced activity and stability for oxygen evolution

Addressing major challenges from the material cost, efficiency and stability, it is highly desirable to develop high-performance catalysts for oxygen evolution reaction (OER). Herein we explore a facile surfactant-assisted approach for fabricating IrSn (Ir/Sn = 0.6/0.4, by mol.) nano-oxide catalysts with good morphology control. Direct proofs from XRD and X-ray photoelectron spectra indicate hydrophilic triblock polymer (TBP, like Pluronic® F108) surfactant can boost the formation of stable solid-solution structure. With the TBP hydrophilic and block-length increase, the fabricated IrSn oxides undergoing the rod-to-sphere transition obtain the relatively lower crystallization, decreased crystallite size, Ir-enriched surface and incremental available active sites, all of which can bolster the OER activity and stability. Meanwhile, it is observed that the coupled Ir oxidative etching takes a crucial role in determining the material structure and performance. Compared with commercial Ir black, half-cell tests confirm F108-assistant catalysts with over 40 wt% Ir loading reduction show 2-fold activity enhancement as well as significant stability improvement. The lowest cell voltage using 0.88 mg cm−2 Ir loading is only 1.621 V at 1000 mA cm−2 and 80 °C with a concomitant energy efficiency of 75.8% which is beyond the DOE 2017 efficiency target of 74%.

Characterization and Activity of Pd–Ir Catalysts in CO and C3H6 Oxidation Under Stoichiometric Conditions

Bimetallic Pd–Ir catalysts based on halogen free precursors supported on alumina and La-stabilized alumina were prepared and characterized. The influence of iridium co-metal loading on activity of supported palladium catalysts in CO and C3H6 oxidation was studied. In addition, the effect of thermal aging on the catalytic activity was discussed. The powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy combined with energy dispersive X-ray spectroscopy, thermogravimetric analysis were used for characterization of the catalysts. The catalytic activity of prepared catalysts was determined based on conversion of CO and C3H6 under stoichiometric condition. The activity tests showed that bimetallic catalyst with Pd:Ir molar ratio of 1.00:0.18 on both support materials has the lowest light-off temperature difference between the fresh and aged catalysts in CO and C3H6 oxidation. However, high Ir loading improved the activity of the catalysts only, if La-stabilized aluminum oxide was used as a support material. Lanthanum oxide promoted the activity of the fresh catalysts in CO and C3H6 oxidation. Overall, Ir is a promising co-metal candidate for Pd-based catalyst for improving CO and C3H6 oxidation activity of the catalyst under stoichiometric conditions.

Activity and Microstructure of Ultra-Low Iridium Loading Catalyst for PEM Electrolyzer MEA

The reduction of catalyst loading is a major barrier to the path of large-scale production of PEM water electrolysis (PEMWE).[1] The state-of-the-art anode catalysts in conventional PEMWEs are Iridium oxides (IrOx) and its mixture with Ruthenium oxide [2]. Typical electrodes for commercial applications have IrOx loadings from 1 to 3 mg cm-2 [1]. While dispersing IrOx on high-surface-area supports, such as Sb-doped SnO2 and Tin-doped Indium oxide (ITO) can improve catalyst utilization and reduce loading, the IrOx loading typically ranged from 0.5 to 1.0 mg cm-2 in the literature.[3] Further reduction is required to meet the DOE target ($4-5/kg H2 production). Our previous work [4]showed the feasibility of reducing the Ir loading to 0.1 mg cm-2 using the reactive spray deposition technology (RSDT). The RSDT process combines catalyst synthesis and processing into a single step where catalyst particles are synthesized through nucleation in the gas-phase followed by deposition on the catalyst support or binder. [5,6]. We further explored the Iridium oxide catalyst layer (CL) integrity and stability by carefully choosing the catalyst binder and controlling the CL thickness. Perfluorosulfonic acid (Nafion®, DuPont) was found to have more stable performance at 1.8 A cm-2 current hold than Polytetrafluoroethylene (PTFE, DuPont). The IrOx/PTFE cell potential increased by 50 mV within one hour while the IrOx/Nafion cell potential increased by 50 mV after 6.5 hours (Figure 1 a). This can be attributed to the enhanced adhesion between IrOx/Nafion and membrane since the binder was the same type of polymer as the membrane electrolyte (Nafion ® N117). The comparison of CL cross-section supported this hypothesis (Figure 2 a,b). The IrOx/Nafion CL showed column-like agglomerates and the interface of IrOx/Nafion was adhered to the membrane more tightly than the IrOx/PTFE CL which comprised of spherical agglomerates. Figure 2c shows the TEM bright field image of IrOx/Nafion agglomerates where the IrOx particles (dark) were connected by Nafion (light grey). The CL stability at 1.8 A cm-2 current hold can be further improved by increasing the CL thickness as shown in Figure 1a and b. The Ir loading increased accordingly from 0.1 to 0.2 mg cm-2. IrOx/Nafion with doubled thickness displayed stable performance up to 25 hours at 1.8 A cm-2 current hold with < 10 mV change of cell potential. For the polarization performance, the cell potential of IrOx/Nafion with doubled thickness was slightly higher at the same current density, probably caused by the higher ohmic resistance with thicker CL. We also investigated supported IrOx catalyst to improve the IrOx dispersion at ultra-low Ir loading. Figure 2d showed the TEM bright field image of IrOx supported on Tin oxide (SnO2, Sigma Aldrich) nanopowders with Nafion® binder. The IrOx particles appeared as dark particles with ~2 nm size dispersed evenly on the surface of SnO2 particles (20-50 nm). The performance of IrOx/SnO2 was comparable to that of IrOx/Nafion but the stability at 1.8 A cm-2 current hold was not as good. Further optimization of the CL will be conducted through optimizing the binder/support ratio, CL thickness, and investigating other corrosion-resistive supports. [1] K.E. Ayers, C. Capuano, E.B. Anderson, ECS Transactions. 41 (2012) 15-22. [2] E. Fabbri, A. Habereder, K. Waltar, R. Kotz, T.J. Schmidt, Catal.Sci.Technol. 4 (2014) 3800-3821. [3] J. Xu, D. Aili, Q. Li, E. Christensen, J.O. Jensen, W. Zhang, M.K. Hansen, G. Liu, X. Wang, N.J. Bjerrum, Energy Environ.Sci. 7 (2014) 820-830. [4] K.E. Ayers, J.N. Renner, N. Danilovic, J.X. Wang, Y. Zhang, R. Maric, H. Yu, Catalysis Today. 262 (2016) 121-132. [5] H. Yu, J.M. Roller, W.E. Mustain, R. Maric, J. Power. Sources. 283 (2015) 84-94. [6] J.M. Roller, M.A. Arellano-Jimenez, R. Jain, H. Yu, C.B. Carter, R. Maric, Journal of the Electrochemical Society. 160 (2013) F716-F730. Figure 1

Nanostructured Ir-supported on Ti4O7 as a cost-effective anode for proton exchange membrane (PEM) electrolyzers.

PEM water electrolysis has recently emerged as one of the most promising technologies for large H2 production from a temporal surplus of renewable electricity; yet it is expensive, partly due to the use of large amounts of Ir present in the anode. Here we report the development and characterization of a cost-effective catalyst, which consists of metallic Ir nanoparticles supported on commercial Ti4O7. The catalyst is synthesized by reducing IrCl3 with NaBH4 in a suspension containing Ti4O7, cetyltrimethylammonium bromide (CTAB) and anhydrous ethanol. No thermal treatment was applied afterwards in order to preserve the high conductivity of Ti4O7 and the metallic properties of Ir. Electron microscopy images show an uniform distribution of mostly single Ir particles covering the electro-ceramic support, although some agglomerates are still present. X-ray diffraction (XRD) analysis reveals a cubic face centered structure of Ir nanoparticles with a crystallite size of ca. 1.8 nm. According to X-ray photoelectron spectroscopy (XPS), the ratio of metallic Ir and Ir-oxide, identified as Ir(3+), is 3 : 1 after the removal of surface contamination. Other surface properties such as primary particle size distribution and surface potential were determined by atomic force microscopy (AFM). Cyclic and linear voltammetric measurements were conducted to study the electrochemical surface and kinetics of Ir-black and Ir/Ti4O7. The developed catalyst outperforms the commercial Ir-black in terms of mass activity for the oxygen evolution reaction (OER) in acid medium by a factor of four, measured at 0.25 V overpotential and room temperature. In general, the Ir/Ti4O7 catalyst exhibits improved kinetics and higher turnover frequency (TOF) compared to Ir-black. The developed Ir/Ti4O7 catalyst allows reducing the precious metal loading in the anode of a PEM electrolyzer by taking advantage of the use of an electro-ceramic support.

Time Average Synchronization in Thermoelastic Stress Analysis

Thermoelastic stress analysis is noncontacting technique used to obtain stress field on the material surface by measuring changes of the surface temperature by infrared camera. Roughly 1MPa change in stress state causes a temperature change of 1 mK in steel. Because of extremely small temperature measurement it is necessary to reduce noise. Thermoelastic stress analysis expects adiabatic conditions so that the specimen is usually loaded by cyclic load. Lock-in principle is most frequently method used to extract signal from statistical noise. However, it is necessary to ensure a synchronization link between IR camera and loading machine. In the case of large displacement of the test object it is necessary to be eliminated. In the contribution is presented an algorithm which requires only knowledge of the temporal signal and identification of displacement by using Lock-in technology and time average synchronization to determine infrared field.

Activity and Durability of Iridium Nanoparticles in the Oxygen Evolution Reaction

Unsupported iridium (Ir) nanoparticles, a benchmark catalyst in acidic electrolyzers, are investigated for oxygen evolution activity in rotating disk electrode (RDE) half-cells. Mercury underpotential deposition, used to evaluate the electrochemical surface areas (ECAs) of Ir, produces similar values prior to and following oxygen evolution testing and irreversible surface oxidation; stable ECA measurements allow for the study of site-specific activities prior to and following accelerated stress tests. Durability testing results in increasing activity losses due to particle growth at moderate potential and Ir dissolution at higher potential. Although the prolonged formation of oxygen bubbles may destabilize the catalyst layer, accelerated stress tests in hydrogen peroxide and formic acid confirm that the losses are primarily potential dependent. This study establishes RDE testing protocols for Ir in oxygen evolution. The observed durability losses have significant implications on acidic electrolyzers, particularly at low Ir loadings.

Studies of MEA Durability in Proton Exchange Membrane Water Electrolysis

Hydrogen production from proton exchange membrane (PEM) water electrolysis is attractive due to its simple and clean nature. Membrane and electrode assemblies (MEAs) of PEM electrolyzers typically use iridium (Ir) as an anode catalyst and Pt as a cathode catalyst. The durability of MEAs plays an essential role for the electrolyzers’ lifetime and cost. However, unlike the well-established MEA durability protocols of PEM fuel cells, there have been a lack of effective procedures for PEM electrolyzer MEA durability evaluation. The objective of this work is to establish a series of test protocols that can help to rapidly evaluate PEM electrolyzer MEA durability. Three testing approaches have been used in this work, which include high-voltage hold (>1.8 V), voltage cycling (1.4 to 1.8 V or 2.0 V), and constant low-current operations. The electrochemical surface area (ECSA) and polarization curves of the MEAs will be obtained after each test. Two categories of anode catalysts have been evaluated using the above durability test protocols: one is the commercial Ir black and the other is Ir supported on tungsten-doped titanium oxide (Ir/WxTi1-xO2) developed at Giner. The supported Ir/WxTi1-xO2 catalyst proves to possess 8-10 x higher mass activity than the Ir black. The MEAs made of these two anode catalysts have been evaluated at a variety of loading, low (0.1 mg/cm2), medium (0.4 mg/cm2) and high (1.0 mg/cm2). Figure 1 shows the durability of these two catalysts after voltage cycling from 1.4 V to 1.8 V: (a) 2 mg/cm2 Ir from Ir black; (b) 0.4 mg/cm2 Ir from Ir/WxTi1-xO2. It can be seen that at medium to high Ir loadings, these two catalysts demonstrates minimal performance decay after voltage cycling. The morphology and structure of MEAs after durability tests will be characterized to correlate to their performance and to elucidate their degradation mechanisms. Acknowledgement: The financial support is from the Department of Energy under the Contract Grant DE-SC0007471. Figure 1

Manufacturing of Low Catalyst Loading PEM Electrolyzer Meas Using Reactive Spray Deposition Technology

Proton exchange membrane (PEM) water electrolysis is a low temperature, low carbon footprint technology for the production of high purity hydrogen. PEM electrolyzers can operate at higher current densities and higher pressures with lower gas crossover and a more compact physical footprint than other water electrolysis processes such as alkaline electrolyzers and solid oxide electrolyzers. However, PEM electrolyzers require expensive, noble metal catalysts and a considerable effort is currently underway to minimize catalyst loading to reduce cost, which crucial for the penetration of PEM electrolysis technology into the market. [1] Ir or IrO2 is generally recognized as the state-of-the-art electrocatalyst for oxygen evolution reaction (OER) at the anode, while Pt remains the best candidate for the hydrogen evolution reaction (HER) at the cathode [1,2]. The catalyst loading on the cathode is typically at least 0.5-1.0 mg/cm2, whereas the anode catalyst loading is around 2 mg/cm2[1]. Reactive spray deposition technology (RSDT) has proven to be a cost-effective method for producing high performance PEM electrolyzer membrane electrode assemblies (MEAs) [3,4]. RSDT is a flame-based method that combines catalyst synthesis and processing in a single step and has the flexibility in optimizing the catalyst composition, making gradient and other complex structures possible. This has allowed our team to reduce the cathode Pt loading below 0.2 mg/cm2with enhanced performance compared to the standard baseline(Figure 1 a). Xu and Scott studied the effect of ionomer content in the catalyst layer on PEM electrolysis using decal transfer method and the optimum ionomer content for cathode was found to be 20% [5]. However, at such low Pt loading, we found that the catalyst layer structure and ionomer content becomes more important than catalyst loading, meaning that each catalyst formulation needs to be carefully controlled. Since RSDT is a dry deposition process, the catalyst layer formation mechanism can be different from catalyst-ink spraying and drying process. Therefore, in this talk we will focus on the role of the catalyst layer pore structure and I/C ratio in dictating the performance of RSDT-derived PEM electrolyzer MEAs. Studies on ITO as fuel cell [6-7] or electrolysis catalyst [8] support have shown good stability above 1.4V and high performance. Recent work has also shown the feasibility of using RSDT to directly deposit both ITO-supported and unsupported Ir/IrO2 with ~0.1 mg/cm2 Ir loading and initial MEA performance close to the baseline. Figure 1 c shows the TEM micrograph of Ir/IrO2particles supported on ITO. The processing variables being explored to achieve stable and high performance with low Ir loading will be discussed. Figure 1. a)PEM electrolyzer performance using RSDT fabricated Pt/C as cathode and Ir/IrO2; b) electrolyzer performance using RSDT fabricated Ir/IrO2 supported on ITO as anode and Pt/C as cathode; c) TEM micrograph of Ir/IrO2supported on ITO nanoparticles

(Invited) Alkaline Water Electrolysis Vs. PEM Water Electrolysis - Exploring Their Full Performance

The future of energy conversion and storage is expected to rely on the production and storage of hydrogen via water electrolysis1. In this scenario, water electrolyzers will play a key role to the establishment of an energy matrix based on renewable but intermittent power sources (e.g. wind turbines and photovoltaics). Other technologies could also be used to store the surplus of energy, such as pumped hydro, geo thermal, batteries, air compression, and the like2. However, none of those have the same value proposition as hydrogen. Hydrogen has the advantage of being able to drive multiple revenue streams like transportation, chemicals, green production of fertilizers, regeneration of electricity through fuel cells, and also initially supplement the energy gap through methanation, when coupling with CO2 sequestration1. Moreover, the production, storage, or distribution can be chosen to be centralized or decentralized and it is the only option with a multi-GWh storage capacity1. To date, only alkaline and polymer electrolyte membrane (PEM) water electrolyzers are commercially available1,3. In order to meet the future demand for water electrolyzers, investment and operational costs still have to be reduced. Moreover, it is fundamental to develop electrolyzers that are able to operate at high current densities, variable partial load, overload, and on/off conditions. These requirements usually place PEM water electrolysis as the best alternative to couple with intermittent power sources. In any case the high costs of PEM water electrolysis components (based on Pt, Ir, and Ti materials) still hamper its large-scale commercial application3. Though consistent R&D one can pursue to considerably reduce the costs of PEM water electrolyzers, so that the loading of the expensive based materials can be reduced or even to the point of being completely substituted3. On the other side, alkaline water electrolysis stays as a long-established, well matured, and comparatively low cost available technology and another approach could be pursued by improving the performance and operational characteristics of this KOH based system. The hydroxide transport across the diaphragm inside alkaline electrolyzers promoted by the KOH responds very slowly to the power input, limiting the efficiency of the electrochemical reaction, and consequently resulting in low current densities1,3. Moreover, the porous structure of the diaphragm allows the diffusion and extensive mixture of the produced hydrogen and oxygen gases when operating at low current densities, limiting the safety range for its operation. Conventional electrodes used in alkaline electrolyzer also tend to possess low active surface area, poor catalyst utilization, and many associated voltage losses1,3. When conventional diaphragms or separators are replaced by thin polymer based separators or anion exchange membranes (AEM), the performance of the alkaline electrolyzers can be substantially enhanced. These are the so-called advanced alkaline electrolyzer units and are already expected to reach performance levels close to that of PEM water electrolyzers. The direct comparison between those three presented alternatives is absolutely not trivial and yet cannot be avoided. It is therefore essential to establish a fair performance range comparison when using standard and available materials for classic alkaline, alkaline PEM, and PEM water electrolysis. As an example for the performance behavior in PEM water electrolysis, by using thin PFSA based membranes (< 50 µm), performances reaching up to 10 Acm-2 are obtained. Due to the low ohmic losses when using thin membranes, lower cell voltages are also found, mitigating the voltage inducing corrosion, allowing the use of less expensive material. Nonetheless, the thinner membrane shall increase the hydrogen permeation to the oxygen side limiting its partial load and differential pressure operation4. Another important point is the loading of noble metals used in the catalyst layer. To date, Ir loadings range between 2 and 3 mgIrcm-2, Pt loadings range between 0.8 and 1.5 mgPtcm-2.1,3By using advanced methods for the membrane electrode assembly (MEA) fabrication, loadings were dramatically reduced without statistically affecting the performance. In conclusion, a new, robust and efficient benchmark study is presented showing the up-to-date performance behavior of classic alkaline, alkaline PEM, and PEM water electrolysis. This study shall be able to contribute to validate the R&D potential for each technology and its future incorporation into our energy matrix for energy storage and conversion. [1] J. Mergel, M. Carmo and D. Fritz. in Transition to Renewable Energy, D. Stolten, V. Scherer, Editors, p. 423-450, Wiley-VCH (2013) [2] W.F. Pickard et al Energy Reviews; 13(8), 1934 (2009). [3] M. Carmo, D. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 38, 4901 (2013) [4] M. Schalenbach, M. Carmo, D. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 38, 14921 (2013) Figure 1

(Invited) High-Performance and Long-Lifetime Oxygen Evolution Catalysts for Proton Exchange Membrane Water Electrolysis

Hydrogen production from proton exchange Hydrogen production from proton exchange membrane (PEM) water electrolysis is attractive due to its simple and clean nature. However, the technology is still expensive due to high materials cost for the catalysts, membrane, and bipolar plate. In most commercial processes, iridium (Ir) black is used as an oxygen evolution reaction (OER) catalyst at the anode. A high precious group metal (PGM) loading (> 4mg/cm2) has been used due to significant anode activation loss caused by sluggish kinetics of the OER. Instead of using commercial Ir black, we have developed two types of high-performance OER catalysts for PEM water electrolysis. One is Ir dispersed on some supports with high oxidation resistance; the other is extended continuous nanostructures including Ir nanowires and Ir nanotubes (see Figure 1). First, Ir nanoparticles have been dispersed on tungsten-doped titanium oxide (Ir/W-TiO2). Doping of TiO2 with an element W can substantially enhance the electronic conductivity of TiO2 while maintaining its high corrosion resistance at high electrolyzer operating voltages. A series of Ir/ W-TiO2 catalysts with decreasing Ir loading from 70 wt. % to 25 wt. % has been synthesized. TEM images illustrates that Ir nanoparticles vary from 1 nm to 5 nm. The electrochemical surface area (ECSA) has been measured via mercury underpotential deposition (UPD). In addition to Ir/W-TiO2, Ir nanotubes have been synthesized via a porous alumina template. The template was coated with an iridium precursor by subliming iridium precursor at an appropriate temperature. The template support was removed via chemical etchings. The average length of the Ir nanotubes is 5mm and the diameter ranges from 250 – 400 nm with a wall thickness of ≤50 nm. The template pore size and process conditions are being varied to change the dimension of the Ir nanotubes. The activity and durability of these catalysts have been evaluated in rotating disk electrodes (RDEs) and electrolyzers. Selected catalysts have reduced PGM loading of electrodes by a factor of 10 while maintaining equivalent electrolyzer performance. These catalysts will help to lower the PEM electrolyzer capital cost making PEM water electrolysis more viable for a variety of applications. Figure 1

Electrochemical Fabrication of Well-Defined Spherical Iridium Nanoparticles and Electrocatalytic Activity towards Carbon Monoxide Adlayer Oxidation

This study outlines the methodology to electrodeposit well-defined iridium nanospheres onto glassy carbon (GC) electrodes at room temperature, using a square wave potential (SWP) technique. Similar work has recently been carried out for other noble metal nanoparticles, in particular platinum and palladium. In this study, particle size and Ir loading can be controlled by altering two key parameters in the electrodeposition process; the nucleation potential and the growth duration. Ir nanoparticles were characterized using cyclic voltammetry (CV) and scanning electron microscopy (SEM). It was found that the nucleation potential and time influence not only the nucleation density but also the particle size. In addition, the electrochemical activity of the particles towards carbon monoxide (CO) adlayer oxidation was investigated as a surface test reaction. The results showed that surface morphology of the deposited particles, which could be altered using potential cycling, plays a pivotal role in determining the activity of the particles. Particles that were treated by potential cycling showed a significantly lower overpotential, in contrast to particles of similar size that were untreated. This indicates an enhancement in electrocatalytic activity due to the enrichment of surface defects.

Oxide‐Supported IrNiOx Core–Shell Particles as Efficient, Cost‐Effective, and Stable Catalysts for Electrochemical Water Splitting

AbstractActive and highly stable oxide‐supported IrNiOx core–shell catalysts for electrochemical water splitting are presented. IrNix@IrOx nanoparticles supported on high‐surface‐area mesoporous antimony‐doped tin oxide (IrNiOx /Meso‐ATO) were synthesized from bimetallic IrNix precursor alloys (PA‐IrNix /Meso‐ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA‐IrNix /Meso‐ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core–shell particles. The core–shell IrNiOx /Meso‐ATO catalyst displayed high water‐splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.