Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species

Hydrogen produced by electrolysis is one of the promising means by which to store electricity coming from renewables and to address its intermittency. With the growing global electricity share from renewable energy sources, large-scale storage systems are required. Alkaline electrolysis can be a sustainable solution for the large-scale energy storage system. However, high overpotential and slow kinetics particularly of the oxygen evolution reaction (OER) still remains as a hurdle to limit efficiency of the system. To enhance efficiency of alkaline electrolysis cells, catalysts which have high electrocatalytic activity for OER should be developed. Platinum-like behavior of Tungsten carbide in surface catalysis was first reported by Levy and Boudart in 1973[1]. In addition, tungsten carbide was reported to have Pt-like electronic structure by Colton in 1975[2]. After that, tungsten carbide has been studied widely and expected to substitute noble metal catalysts which are used in many electrochemical reactions. Here, we studied tungsten carbide as a support for 3d-transition metals and make bi-metallic surfaces for developing a high activity OER catalyst. First, WC-Co was synthesized by heat treatment. Carbon nitride (g-C3N4) and tungsten chloride were used as a carbide precursor and cobalt acetate was also used for helping carbonization of tungsten. After heat treatment, acidic leaching process was followed to eliminate cobalt. Through additional mixing with Nickel precursor and heat treatment, Ni/WC was synthesized and to compare electrocatalytic activity Ni/C was also prepared. In addition, many different Ni/WC which have different Ni to WC ratios were synthesized and measured in electrochemical way. Finally, the optimal ratio was determined and it show better electrocatalytic activity than precious catalysts and any other non-noble catalysts. Cyclic voltammetry was measured to evaluate OER activity in 1 M KOH in potential range from 1 to 1.8 V. Figure 1 shows that pure WC and Ni/C have 394 mV and 413 mV overpotentials at 10 mA∙cm-2 respectively. In Metal/WC cases, Cobalt/WC shows only 417 mV. But Nickel/WC shows 340 mV overpotential at 10 mA∙cm-2 and higher electrocatalytic activity for OER than RuO2 in alkaline media. η @ 10 mA cm-2 (mV) Ni/C 394 WC 413 Co/WC 417 Ni/WC 340 RuO2 355 Table1. Activities for OER at 10 mA cm-2 References [1] R. B. Levy, M. Boudart, Science, New series, Vol. 181, No. 4099 (1973) 547-579 [2] Richard J. Colton, Jan-Tsyu j. Huang, J.Wayne Rabalais, Chemical Physics Letters, Vol. 34, No. 2 (1975) 337-339 Figure1. Cyclic voltammetry for the oxygen evolution reaction in 1 M KOH of Ni/C, WC, Co/WC, Ni/WC, RuO2 Figure 1

The material design of highly active catalysts for energy storage applications (e.g., metal-air batteries and fuel cells) is crucial for solving global energy problems. The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is used for charging and discharging rechargeable metal-air batteries, respectively. Although precious metal-based catalysts facilitate these reactions, the kinetics of the OER and ORR are rather sluggish due to their multistep electron transfer. It is therefore important to explore the mechanism of the OER and ORR to discover the key factors for highly active catalysts. As demonstrated by Suntivich et al. (2011)[1],[2], when the number of electrons in the eg orbital is close to unity for transition metals, perovskite oxides exhibit maximum OER and ORR activities. In other words, Mn3+ (t2g 3 eg 1 for both surface and bulk), Co3+ (t2g 5 eg 1 for surface), and Ni3+ (t2g 6 eg 1 for both surface and bulk) become OER and ORR active sites for Mn3+, Co3+ and Ni3+ based compounds . However, LaMnO3 exhibits a significantly lower specific OER activity[3] compared with LaCoO3 and LaNiO3 (~6% of LaNiO3 at 1.8 V vs. RHE). Also, Mn3O4 exhibits a low specific OER activity[4] (40% of Mn2O3 at 1.8 V vs. RHE) similar to LaMnO3. It is therefore important to explore what causes degradation of the OER activity of Mn3+-based (t2g 3 eg 1) compounds to improve their catalytic activity. We attempt to improve the performance of Mn3+-based oxides by controlling the degradation factors of their OER activities. To directly compare the OER activities of Mn3+-based oxides containing more than two cation sites, Mn3+ concentrations at the octahedral site and their initial crystal structures were maintained. Mn3-x Co x O4 (0 ≤ x < 1), a series of tetragonally distorted spinel compounds, satisfies this condition, as their octahedral sites remain occupied by only Mn3+ ions . For Mn3+-based oxides with a single cation site, Mn3+ was directly substituted by Co3+.We systematically studied the OER and ORR performances of Mn3+-based oxides by Co3+-substitution. Nanoparticles were prepared for the evaluation of catalytic activity to minimize the influence of geometric factors. Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (ALS Co., Ltd, Japan). In addition, a Pt wire counter electrode, and an Hg/HgO reference electrode (International Chemistry Co., Ltd., Japan) filled with 0.10 M KOH (Nacalai Tesque, Inc., Japan) were used. Electrochemical measurements were conducted with O2 saturation (30 min bubbling O2 gas through the solution) at ~25 °C, where the equilibrium potential of the O2/H2O redox couple was fixed at 0.304 V vs. Hg/HgO (or 1.23 V vs. RHE). During OER current density measurements for each sample, the potential of the sample-modified GC was controlled from 0.3-0.9 V vs. Hg/HgO (1.226-1.826 V vs. RHE) at 10 mV/s. For all measurements, the OER current density was iR-corrected (R = ~43 Ω) using the measured solution resistance, and capacitance-corrected by averaging the anodic and cathodic scans to remove the influence of the current related to the formation of an electrical double layer. With an increase in Co content, signicant improvements in the OER and ORR activities were observed for Mn3+-based oxides. For example, Mn2.1Co0.9O4 exhibited a high specific OER activity (1700 % of Mn3O4 at 1.76 V vs. RHE) and a long-term stability over 100 cycles [5]. The OER activities of Mn3+-based oxides increased linearly with the suppression of the Jahn–Teller distortion. We examined whether the electronic state of Mn3+ changes in favor of enhancing the OER and ORR activities. When the Jahn-Teller distortion of Mn3+O6 octahedra is suppressed due to the increase in Co content, the splitting of the Mn3+ eg orbitals becomes smaller and the electron occupying the Mn3+ eg orbital shifts to a higher energy level. Thus, the overlap of the antibonding Mn3+ eg orbitals with the O 2p orbitals of the oxygen adsorbate becomes stronger. The OER and ORR activities of Mn3+-based oxides should therefore be enhanced due to the stronger binding of OER intermediates to the catalytic surface. We therefore conclude that the suppression of Jahn–Teller distortion enhances the OER and ORR activities of Mn3+-based oxides. Our result suggests a future application of Mn3+-based oxides as bifunctional catalysts for metal-air batteries. [1] J. Suntivich et al., Science, 2011, 334, 1383. [2] J. Suntivich et al., Nature Chemistry, 2011, 3, 546. [3] B. Han et al., Phys. Chem. Chem. Phys., 2015, 17, 22576. [4] A. Ramirez et al., J. Phys. Chem. C, 2014, 118, 14073. [5] S. Hirai et al., RSC Adv., 2016, 6, 2019.

Construction of δ-FeOOH/NiMn2S4 heterointerface for efficient alkaline oxygen evolution reaction

Porous NiCo2S4/FeOOH nanowire arrays with rich sulfide/hydroxide interfaces enable high OER activity

Heterogeneous bimetallic FeP4/NiP2 nanosheets as efficient electrocatalyst for alkaline oxygen evolution reaction

The electrocatalytic water electrolysis is the most eco-friendly technique for hydrogen generation, which is governed by the electrode reaction kinetics involving cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in common alkaline electrolytes. Cobalt oxide (Co3O4) and related compounds are the most efficient OER catalysts, replacing the noble metals. In this work, the surface oxidations of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) and Co3O4 nanowires during the OER are carefully investigated by contrasting the polarization curves, Tafel plots, and x-ray photoelectron spectroscopy (XPS) spectra, before and after the 1000th cyclic voltammetry (CV) cycling in 1 M KOH alkaline solution. The overpotentials required to reach a current density (j) of 20 mA cm−2 (η 20) are estimated to be 313 mV for the 300 °C-calcined Co3O4, 350 mV for the 400 °C-calcined Co3O4, 365 mV for the 500 °C-calcined Co3O4, and 373 mV for the cobalt carbonate (Co(CO3)0.5OH·0.11H2O). The Tafel slope of cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires is the highest at 93 mV dec−1, while it is measured to be 57 mV dec−1 for the 300 °C-calcined Co3O4, 47 mV dec−1 for the 400 °C-calcined Co3O4, and 79 mV dec−1 for the 500 °C-calcined Co3O4. As a result, the oxidation from Co2+ to Co3+ within Co3O4 during the OER is detected, which improves the OER activity. On the other hand, the formation of cobalt hydoxide is found on the surface of the Co3O4 nanowires during the OER in alkaline solution, which decreases the OER activity. For the surface oxidation of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires, the increase in the amounts of Co3+ and oxygen vacancy and the formation of O-C-O and carbonates are found, which highly enhance the OER activity. These findings indicate that the surface redox kinetics during the electrocatalytic reactions should be considered important in order to enhance the electrocatalytic activity, and furthermore can provide insight into future challenges in designing advanced electrocatalysts.

Effects of Fe on electrocatalytic oxygen evolution reaction activity for CoFe layered double hydroxide nanosheets

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the fundamental processes occurring at the air electrode of metal-air batteries and reversible fuel cells. [1,2] However, the performance of those devices is hindered by the sluggish kinetics of both OER and ORR, and by the low stability and high cost of the catalysts. Here, we will present our recent studies on the one-dimensional (1D) La, Ni-based metal oxide [3] bifunctional catalysts which are characterized by excellent OER and ORR electrocatalytic activity and outstanding stability.The catalysts were prepared by hydrothermal method and, by controlling the KOH concentration, 1D hydroxide precursors with different aspect ratios were synthesized. By using graphene (G) as the supporting material, a variety of La, Ni-based metal oxides such as Ni+La2O3/G, LaNiO3, and La2NiO4+NiO/G were synthesized, Figure 1. Electrochemical characterization of the catalysts showed that the aspect ratio of the materials has a significant effect on their electrocatalytic activity. Namely, Ni+La2O3/G nanorods have proven to be electrochemically active for ORR with a half-wave potential (E1/2) of 0.715 V vs RHE, and for OER with 1. 574 V vs RHE at 10.0 mA cm−2 (E10) in 0.1 M KOH. The performance of Ni+La2O3/G is close to that of commercial Pt/C (E1/2=0.830V) which is the most efficient catalyst of ORR, and significantly higher than commercial RuO2 (E10=1.633 V) and IrO2 (E10=1.663 V) for OER. The unique structural, morphological and electrocatalytic properties of the Ni+La2O3/G catalysts will be presented and discussed. 1. Sui, S.; Wang, X.; Zhou, X.; Su, Y.; Riffat, S.; Liu, C.-j., A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells. Journal of Materials Chemistry A 2017, 5 (5), 1808-1825.2. Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J., Rapid room-temperature synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts. Nature chemistry 2011, 3 (1), 79.3. Singh, Sarika, Daria Zubenko, and Brian A. Rosen. "Influence of LaNiO3 shape on its solid-phase crystallization into coke-free reforming catalysts." ACS Catalysis 2016, 7 4199-4205. Figure 1

Facile synthesis of V-doped CoP nanoparticles as bifunctional electrocatalyst for efficient water splitting

Polymer-assisted approach to LaCo1-xNixO3 network nanostructures as bifunctional oxygen electrocatalysts

Rational surface engineering of metal–organic frameworks (MOFs) provide potential opportunities to address the sluggish kinetics of oxygen evolution reaction (OER). However, the development of MOF‐based materials with low overpotentials remains a great challenge. Herein, a post‐synthesis strategy to prepare highly efficient MOF‐based pre‐electrocatalysts via all‐solid‐phase mechanochemistry is demonstrated. The surface of a Fe‐based MOF (MIL‐53) can be reconstructed and anchored with atomically dispersed Ni/Co sites. As expected, the optimized M‐NiA‐CoN exhibits a very low overpotential of 180 mV at 10 mA cm−2 and a small Tafel slope of 41 mV dec−1 in 1 m KOH electrolyte. The superior electrocatalytic OER activity is mainly due to the formation of surface FeONi/Co bonding. Furthermore, density functional theory calculations reveal that the transformation from *OH to *O is the rate‐determining step and the electrocatalytic OER activity trend at different metal sites is Co &gt; Ni≈Fe.

Recent progress in the development of anion exchange membranes (AEMs) with improved performance and durability has opened the way for the application of the AEM-based electrolyzers in low-temperature water electrolysis (LTWE),1 an important technology for producing “green” hydrogen.2 AEM-LTWEs can potentially operate on pure water, i.e., without highly concentrated and corrosive supporting electrolyte, and they allow for replacement of electrocatalysts based on platinum group metals (PGMs) with PGM-free ones, thus addressing the main drawbacks of the liquid-alkaline (LA) and proton exchange membrane (PEM) electrolyzers.3 Consequently, the development of PGM-free electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media is of primary importance for the deployment of AEM-LTWEs that has attracted significant attention of researchers.4–6 Besides improving the catalytic activity, the integration of PGM-free HER and OER electrocatalysts into electrodes for operation in AEM electrolyzers is crucial to achieving satisfactory electrolyzer performance and making them competitive with the LA and PEM systems.7,8 In this work, we measured electrocatalytic activity of a series of OER and HER catalysts in a three-electrode cell and then implemented these catalysts in electrodes for testing in an AEM electrolyzer. We investigated different classes of OER catalysts, including commercial IrO2 (a PGM ORR benchmark), LaxSr1-xCoO3-δ oxides, Ni-Fe nanofoam oxides, Ni-Fe aerogel-derived oxides, and MOF-derived Co oxides. In the HER-catalyst part of the study, we compared a commercial PtRu/C (a PGM HER benchmark) with an aerogel NiMo/C catalyst. Catalysts and electrodes before and after testing were characterized by XRD, SEM, EDS, and XPS.In addition to exploring different catalysts, we investigated the impact of several fabrication variables such as the ink deposition method, amount of ionomer, incorporation of a binding agent, and the type of anode porous transport layer on performance. The tests were carried in an electrolyzer operating with pure water and two electrolyte solutions, 0.1 M KOH and 1% K2CO3. The results show that, in addition to the OER and HER electrocatalytic activity, the electrode fabrication is an important factor affecting AEM electrolyzer performance, especially in the pure-water operation mode, in which case assuring an effective transport of the OH– ions within the catalyst layer is especially challenging. References Y. S. Kim, ACS Appl. Polym. Mater. (2021).C. Santoro et al., ChemSusChem, 202200027 (2022).H. A. Miller et al., Sustain. Energy Fuels, 4, 2114–2133 (2020).D. Xu et al., ACS Catal., 9, 7–15 (2019).H. Shi et al., Adv. Funct. Mater., 2102285, 1–10 (2021).H. Doan et al., J. Electrochem. Soc., 168, 084501 (2021).N. U. Hassan, M. Mandal, B. Zulevi, P. A. Kohl, and W. E. Mustain, Electrochim. Acta, 409, 140001 (2022).G. A. Lindquist et al., ACS Appl. Mater. Interfaces (2021).

There is a critical need to develop clean, environmental friendly and sustainable energy sources to substitute the rapidly depleting conventional fossil fuels - causing serious global environmental concerns. In the pursuit of this need, hydrogen is attracting immense attention due to its clean, non- carbonaceous nature and also, high energy density as compared to petroleum based energy sources[ 1 ]. Over the past few decades, catalytic methane conversion, steam reforming and coal gasification etc. have been widely used for hydrogen production. However, the toxic greenhouse gases (CO/CO2) produced render these techniques environmentally less attractive. Therefore, hydrogen production from water splitting (H2O® H2+ 0.5 O2) is considered a far cleaner and more efficient technique among all other conventional hydrogen production methods. However, the efficiency of water electrolysis has been severely constrained by the energy intensive anodic oxygen evolution reaction (OER) (4OH−→ O2+ 2H2O + 4e−) which is the other important half reaction in addition to hydrogen evolution reaction (HER). To date, platinum group metal (PGM) based materials such as Pt, RuO2, IrO2 are well known for their superior electrochemical performance towards OER. Commercial implementation of these electrocatalsts for water splitting has been largely limited due to their high cost and scarcity[ 2 ]. Thus, identification and development of earth abundant, cheap and PGM-free electro-catalysts, demonstrating excellent electro-catalytic activity and robust stability (for OER); comparable/superior to state-of-the art PGM/noble metals based electro-catalysts is critically desired[ 3 ]. Therefore, in the present study, exploiting theoretical first principles approaches, we have engineered anionic fluorine (F) doped transition metal non-oxide pnictide (TMN) based electro-catalyst for OER in alkaline mediated water electrolysis. We have observed remarkable enhancement in the electro-catalytic activity by incorporation of F into the as-synthesized TMN based electro-catalyst. The electro-catalyst powder with F content of 5 wt. % (Fig. 1) has been synthesized using solid state approaches. The as-synthesized electro-catalyst powder was coated on porous titanium (Ti) substrate (total catalyst loading=1 mg/cm2) and used as an anode. The electro-catalyst with F content of 5 wt. % exhibited excellent electro-catalytic performance, outperforming the benchmark IrO2 for OER in alkaline water splitting. The electrochemical response of the electro-catalysts has been evaluated in a three-electrode configuration system, using 1N KOH solution. Correspondingly, Pt wire and Hg/Hg2SO4 are used as a counter electrode and reference electrode (+0.65 V with respect to normal hydrogen electrode, NHE) respectively. Electrochemical characterization of the electrocatalyst has been performed with a scan rate of 10 mV/sec at 40oC. The generated PGM-free electro-catalyst exhibited significantly lower charge transfer resistance (Rct) than benchmark IrO2. In addition, this PGM-free electro-catalyst displayed remarkable ~7.6 fold higher electro-catalytic OER activity (i.e. current density, at 1. 55V) than that of IrO2 and reached a current density of ~ 10 mA/cm2 at an overpotential of ~ 300 mV. The chronoamperometry test conducted in 1N KOH solution at ~1.55 V (vs NHE) for 24 hours shows the minimal loss in current density, signifying a robust electrochemical stability of the as-prepared electro-catalyst. In summary, we have synthesized high performance PGM-free F doped TMN pnictide based electro-catalyst for alkaline mediated water splitting. The observed enhanced electrocatalytic activity of this electro-catalyst is mainly attributed to the modified electronic structure (as supported by the theoretical study) and lower charge transfer resistance (i.e. lower activation polarization). Based on these results we believe that this PGM-free electro-catalyst is indeed a promising and reliable system for sustainable and economic hydrogen production. Results of this work will be presented and discussed.

MXene supported nickel-cobalt layered double hydroxide as efficient bifunctional electrocatalyst for hydrogen and oxygen evolution reactions

The growing demand to shift energy production to sustainable sources poses some serious challenges. One of the biggest are the intermittent energy (wind and sun). To mitigate this issue, there is a critical need for large energy storage technologies, to store excess energy during the peak hours and carry it over for the rest of the day, weeks, and seasons. The best way to do so is through the Hydrogen Economy. At the heart of this scheme is green hydrogen production using water electrolysis.There are two distinct chemical reactions that take place in water electrolyzers: the cathodic hydrogen evolution reaction (HER), and the anodic oxygen evolution reaction (OER). Both reaction require catalysts to execute at high rates, and while the HER is considered to be relatively facile and takes place at low overpotentials, the OER requires relatively high overpotentials and high loadings of precious metal catalysts. It is considered the bottleneck reaction. The OER is a four electrons oxidation reaction per generated O2 molecule and proceeds in four distinct reaction steps. This leads to a very sluggish reaction kinetics and high overpotentials to reach viable current densities.In recent years, more and more non-precious metal OER catalyst have been developed. Most notably is the family of mixed nickel-iron oxyhydroxides (NiFeOOH), which are relatively cheap, selective and efficient catalysts in alkaline media, and their performance has been increased by optimizing the Ni:Fe ratio. One challenge that still remains is to increase the NiFeOOH surface area, and by that the electrochemically active site density (EASD).In this regard, one class of materials that has been attracting the attention of materials’ scientists in recent years are aerogels. Aerogels can be made from many different materials, such as silicates, carbons, metal organic materials, bio-inspired molecules, metals, and metal oxides. They consist of distinct units which form a porous 3D covalent framework (COF). Because of their diversity, aerogels have many different applications, e.g. as insulators, sensors, or catalysts.In this talk we will report the synthesis of NixFeyMzOOH aerogels, with a modified easy synthetic method via an epoxide route. These aerogels show much higher utilization of the material and overall increase in mass activity when catalyzing the OER when compared to other NiFeOOH derived materials. They were tested for their OER electrocatalytic activity and to the best of our knowledge these are the first aerogel materials that propagate OER themselves, rather than being used merely as support material for OER catalysts. The catalytic activity depends largely on the Ni:Fe ratio and not the surface area, which can lead to mass transport limitations when too high, showing an optimum for the ratio and the surface area.