Precious Metal Oxides Research Articles

Influence of Annealing Temperature on the OER Activity of NiO(111) Nanosheets Prepared via Microwave and Solvothermal Synthesis Approaches.

Earth-abundant transition metal oxides are promising alternatives to precious metal oxides as electrocatalysts for the oxygen evolution reaction (OER) and are intensively investigated for alkaline water electrolysis. OER electrocatalysis, like most other catalytic reactions, is surface-initiated, and the catalyst performance is fundamentally determined by the surface properties. Most transition metal oxide catalysts show OER activities that depend on the predominantly exposed crystal facets/surface structure. Therefore, the design of synthetic strategies to obtain the most active crystal facets is of significant research interest. In this work, rock salt NiO OER catalysts with (111) predominantly exposed facets were synthesized by a solvothermal (ST) method either heated under supercritical or microwave-assisted (MW) conditions. Particular emphasis was placed on the influence of the post annealing temperature on the structural configuration and OER activity to compare their catalytic performances. The as-prepared electrocatalysts are pure α-Ni hydroxides which were converted to rock salt NiO (111) nanosheets with hexagonal pores after heat treatment at different temperatures. The OER activity of the electrodes has been evaluated in 0.1 M KOH using geometric and intrinsic current densities via normalization by the disk area and BET area, respectively. The lowest overpotential at a geometric current density of 10 mA/cm2 is found for samples pretreated by heating between 400 and 500 °C with a catalyst loading of 115 μg/cm2. Despite the very similar nature of the catalysts obtained from the two methods, the ST electrodes show a higher geometric and intrinsic current density for 500 °C pretreatment. The MW electrodes, however, achieve an optimal geometric current density for 400 °C pretreatment, while their intrinsic current density requires pretreatment over 600 °C. Interestingly, pretreated electrodes show consistently higher OER activity as compared to the poorly crystalline/less ordered hydroxide as-prepared electrocatalysts. Thus, our study highlights the importance of the synthesis method and pretreatment at an optimal temperature.

Design, Reactivity, and Active Site of Atomically Dispersed Pt Catalysts for Electrochemical Chlorine Evolution Reaction

The electrochemical chlorine evolution reaction (CER) is a key anodic reaction in the chlor−alkali process for Cl2 production, on-site generation of ClO–, and Cl2-mediated electrosynthesis.1−3 For more than a half-century precious metal-based mixed metal oxides (MMOs) have been mainstays as CER catalysts, however, they intrinsically suffer from the selectivity problem arising from a parasitic oxygen evolution reaction (OER). We have recently developed a new CER catalyst based on atomically dispersed Pt sites on a carbon nanotube (Pt1/CNT), which catalyzes the CER with excellent activity and selectivity.4 The Pt1/CNT catalyst shows superior CER activity to a commercial Ru/Ir-based MMO and a Pt nanoparticle-based catalyst. Importantly, Pt1/CNT exhibits very high CER selectivity near 100% even in low Cl− concentrations (0.1 M) and neutral media, under which the OER is favored over the CER. In situ X-ray absorption spectroscopy and density functional theory calculations reveal that the CER over Pt1/CNT augments with the direct adsorption of Cl− ion on Pt, which can break the scaling relationship between CER and OER. The Pt1/CNT catalyst also shows unusual potential-dependent switching behavior of CER kinetics and rate-determining step.5 Pt1/CNT shows a reaction order of ~1.8 in the low overpotential regime, where the Volmer step is the rate-determining step (RDS). Interestingly, in the high overpotential region, the CER over Pt1/CNT proceeds with a lower reaction order and the RDS switches to the Heyrovský step. We also show that the Pt1/CNT catalyst is comprised of multiple Pt sites with different coordination environments and identify a genuine active site in Pt1/CNT catalysts for the CER.6 K. B. Karlsson and A. Cornell, Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes. Chem. Rev. 116, 2982–3028 (2016).S. Exner, T. Lim, and S. H. Joo, Circumventing the OCl vs. OOH Scaling Relation in the Chlorine Evolution Reaction: Beyond Dimensionally Stable Anodes. Curr. Opin. Electrochem. 34, 100979 (2022).Trasatti, Electrocatalysis: Understanding the Success of DSA. Electrochim. Acta 45, 2377–2385 (2000).Lim, G. Y. Jung, J. H. Kim, S. O. Park, J. Park, Y.-T. Kim, S. J. Kang, H. Y. Jeong, S. K. Kwak, and S. H. Joo, Atomically Dispersed Pt−N4 Sites as Efficient and Selective Electrocatalysts for the Chlorine Evolution Reaction, Nat. Commun. 11, 412 (2020).Lim, J. H. Kim, J. Kim, D. S. Baek, T. J. Shin, H. Y. Jeong, K.-S. Lee, K. S. Exner, and S. H. Joo, General Efficacy of Atomically Dispersed Pt Catalysts for the Chlorine Evolution Reaction: Potential-Dependent Switching of the Kinetics and Mechanism, ACS Catal. 11, 12232−12246 (2021).Cho, T. Lim, H. Kim, L. Meng, J. Kim, J. H. Lee, G. Y. Jung, F. Viñes, F. Illas, K. S. Exner, S. H. Joo, and C. H. Choi, Importance of Broken Geometric Symmetry of Single-Atom Pt Sites for Efficient Electrocatalysis, Nat. Commun. 14, 3233 (2023).

Selective chlorine evolution reaction using Pt-RuO2 electrocatalysts in brackish water and seawater at circum-neutral pH and its application to seawater desalination

Precious metal-based mixed metal oxides have been widely used as catalysts for the chlorine evolution reaction (CER). However, a concurrent oxygen evolution reaction (OER) leads to insufficient selectivity for CER at circum-neutral pH. Herein, nanoparticulate Pt-RuO2 (PRO) is synthesized with various Pt/Ru ratios at sub-boiling temperatures, followed by post-annealing. The synthetic conditions significantly influence the physicochemical properties (crystalline structures, morphologies, surface areas, double-layer capacitance, and charge transfer resistance) and the electrocatalytic activities of CER in NaCl solutions (0.137 and 0.5 M) and seawater (with a salinity of 3.6 g L–1). Among PRO-x@y samples (where x is the mole fraction of Pt in the mixed Pt and Ru precursor solutions, and y is the annealing temperature [°C]), PRO-0.2@800 and PRO-0.8@500 show remarkable faradaic efficiencies (FEs) for CER (CER-FE). Despite their similar FEs (CER-FEs of approximately 95 % and 99 %, and OER-FEs of 4–6 % and <1 %, for PRO-0.2 and PRO-0.8, respectively), the Pt-rich PRO requires less potential for the same current density (J) and exhibits excellent stability at J = 800 mA cm–2 over 250 h in seawater at circum-neutral pH. The optimized PRO electrode is further used for seawater desalination and simultaneous production of HClO, achieving FEs of ∼100 %. Finally, various electrochemical analyses are conducted to elucidate the CER mechanism.

Low platinum loading Co3O4 electrode for highly efficient oxidation of ammonia in aqueous solution

Chlorine-mediated electrochemical advanced oxidation (Cl-EAO) is an environmentally-friendly and promising ammonia removal technology to convert ammonia to N2 and effectively reduce water pollution. The efficiency of ammonia removal in Cl-EAO hinges critically on the performance of the electrode materials. Currently, precious metal oxides are commonly used as electrode materials in the industry. However, their high cost limits their widespread application in ammonia treatment. Herein, Pt-Co3O4 catalyst with lutra-low content of Pt (1.67 wt%) is successfully synthesized, which performs superior CER activity and higher removal efficiency of ammonia. The favorable catalytic performance could be attributed to the increase in oxygen vacancy content and the strong metal-supported strong interaction (SMSI) between Pt and Co3O4, which has been confirmed by XPS. Especially, the chlorine evolution efficiency of the Ti/Pt3 %-Co3O4 electrode was as high as 96 % in dilute chloride (<50 mM) solution at a current density of 10 mA/cm2. Therefore, the Ti/Pt3 %-Co3O4 electrode has a broad application prospect as a new and efficient material for ammonia wastewater treatment.

Atomically dispersed dinuclear iridium active sites for efficient and stable electrocatalytic chlorine evolution reaction.

The electrochemical chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have long been used as CER catalysts, they suffer from high cost and poor selectivity due to the competing oxygen evolution reaction (OER). Single-atom catalysts (SACs), featuring high atom utilization efficiency, have captured widespread interest in diverse applications. However, the single-atom sites in SACs are generally recognized as independent motifs and the interplay of adjacent sites is largely overlooked. Herein, we report a "precursor-preselected" cage-encapsulated strategy to synthesize atomically dispersed dinuclear iridium active sites bridged by oxygen that are supported on nitrogen-doped carbon (Ir2-ONC). The dinuclear Ir2-ONC catalyst exhibits a CER onset potential of 1.375 V vs. normal hydrogen electrode, a high faradaic efficiency of >95%, and a high mass activity of 14321.6 A gIr -1, much better than the Ir SACs, which demonstrates the significance of coordination and electronic structure regulation for atomically dispersed catalysts. Density functional theory calculations and ab initio molecular dynamics simulations confirm that the unique dinuclear structure facilitates Cl- adsorption, resulting in improved catalytic CER performance.

Activity and Durability of Mn Binary Oxide-Based Electrocatalyst for an Alternative Anode

Compared with alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE) has several advantages such as high current density, compact system and high purity of hydrogen gas. However, a conventional anode of PEMWE utilizes precious metal oxide such as iridium oxide (IrO2) with high cost and low resources. Recently, it has been reported that the performance of low-loading IrO2 anodes deteriorates due to the potential fluctuations simulated variable renewable energies [1-2]. Thus, it is required to develop a low-cost catalyst with high durability against the potential fluctuations. We have focused on the tantalum oxide-based electrocatalyst because of its low cost and high chemical stability in acid. In this study, we have investigated on the catalytic activity and durability of Mn added Ta oxide-based thin film (Mn-TaOx) for the alternative anode of PEMWE. For the purpose of improvement of durability, Mn-TaOx coated with ZrO2 fabricated by the Atomic Layer Deposition (ALD) was also prepared to evaluate the durability of potential cycling.A Ti rod was used as the substrate, and Mn-TaOx was prepared by RF magnetron sputtering deposition. Mn disk was used as the sputtering target, and the Ta plate was placed on the Mn target to prepare Mn-TaOx. Mn addition was fixed at 50 at%, and it was controlled by the area ratio considering sputtering rate of Mn and Ta metal. Partial pressure of Ar and O2 gas were kept constant at 0.23 Pa for 20 min during the sputtering. The substrate heating temperature was constant at 673 K during the sputtering. The prepared Mn-TaOx electrode was coated with ZrO2 fabricated by ALD (ZrO2 coating Mn-TaOx). The thickness of coating ZrO2 was varied in the range of 0.5 to 2.0 nm. All electrochemical measurements such as oxygen evolution reaction (OER) was used by conventional three electrode cell in 1 M H2SO4 at 303 K. In addition, electrochemical impedance spectroscopy (EIS) was also performed to estimate several factors such as charge transfer resistance (R ct), film resistance (R film) and so on. The protocol of potential cycling as a durability test was set the potential in the range of 1.5 to 2.0 V with 50 mV s-1 for 10000 cycles, and the slow scan voltammetry was conducted every 2000 cycle in the range of 1.2 to 2.0 V with 5 mV s-1 to compare the OER activity before and after test.The OER current of ZrO2 coating Mn-TaOx was smaller than that of Mn-TaOx without coating (Mn50-TaOx (0 nm)). In particular, the OER current of Mn-TaOx coated with 2 nm of ZrO2 (“2 nm”) was greatly decreased because the film thickness of ZrO2 is too thick so that it has low electric conductivity. Furthermore, the APD-ZrO2 covered on the surface, and it is also hard to react with reactant at active site. From results of EIS, the R ct and R film of “2 nm” increased. On the other hand, the OER current of Mn-TaOx coated with 1 nm of ZrO2 (“1 nm”) did not drastically decrease compared to Mn-TaOx and it was suggested that “1 nm” relatively maintained its activity.Figure 1 shows the geometric current density (i geo) at 2.0 V of ALD-ZrO2 coated on Mn-TaOx after potential cycling. The i geo at 2.0 V of Mn-TaOx without coating also shows in Fig. 1. The i geo of Mn50-TaOx (0 nm) was decreased about half value after 2000 cycles of durability test, but that of Mn-TaOx coated with ZrO2 did not decrease significantly even after 2000 cycles of potential cycling. In particular, the i geo of “1 nm” was the highest current after the 2000 cycles of potential cycling in this study, and the degradation didn’t observe in this cycle range. According to the results from XPS spectrum, a small amount of ZrO2 was detected on the surface of “1 nm” after the potential cycling. The redox peak of Mn(Ⅲ)/(Ⅳ) [3] observed at the ALD-ZrO2 coated on Mn-TaOx was also detected after the potential cycling, so that the Mn species as an active site of Mn-TaOx for the OER was still remained. Therefore, the reason why the “1 nm” was relatively stable is that the amounts of both active and protect site have maintained during the durability test.AcknowledgementThis work is partially supported by Suzuki Foundation.References(1) Ayers, Curr. Opin. Electrochem., 18, 9 (2019).(2) M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).(3) Morita, C. Iwakura, and H. Tamura, Electrochim. Acta, 24, 357 (1979). Figure 1

Catalysis for CO2 Hydrogenation—What We Have Learned/Should Learn from the Hydrogenation of Syngas to Methanol

This short review provides an in-depth analysis of the achievements and further developments of the catalytic hydrogenation of carbon dioxide (CO2) to methanol from those that are worth learning about based on the transformation of syngas into methanol. We begin by exploring the environmental and energy-related implications of utilizing CO2 as a feedstock for methanol production by emphasizing its potential to mitigate greenhouse gas emissions and facilitate renewable energy integration. Then, different catalytic formulations focusing on precious metals, copper-based catalysts, and metal oxides are summarized, and insights into their advantages and limitations in the aspects of catalytic activity, selectivity, and stability are discussed. Precious metal catalysts, such as platinum and iridium, exhibit high activity but are cost-prohibitive, while copper-based catalysts present a promising and cost-effective alternative. Metal oxides are considered for their unique properties in CO2 activation. Mechanistic insights into reaction pathways are explored, with a particular emphasis on copper-based catalysts. Moreover, the complex steps involved in CO2 hydrogenation to methanol are discussed to shed light on the key intermediates and active sites responsible for catalysis, which is crucial for catalyst design and optimization. Finally, we stress the importance of ongoing research and development efforts to enhance catalyst efficiency, mechanistic comprehension, and process optimization. This review serves as a valuable resource for researchers, engineers, and policymakers working toward a more sustainable and carbon-neutral energy future. By harnessing CO2 as a carbon feedstock for methanol synthesis, we have the potential to address environmental concerns and advance the utilization of renewable energy sources, further contributing to the transition to a cleaner and more sustainable energy landscape.

Electrocatalysis of Selective Chlorine Evolution Reaction: Fundamental Understanding and Catalyst Design

&lt;p&gt;The electrochemical chlorine evolution reaction (CER) is an important electrochemical reaction and has been widely used in chlor–alkali electrolysis, on-site generation of ClO&lt;sup&gt;−&lt;/sup&gt;, and Cl&lt;sub&gt;2&lt;/sub&gt;-mediated electrosynthesis. Although precious metal-based mixed metal oxides (MMOs) have been used as CER catalysts for more than half a century, they intrinsically suffer from a selectivity problem between the CER and parasitic oxygen evolution reaction (OER). Hence, the design of selective CER electrocatalysts is critically important. In this review, we provide an overview of the fundamental issues related to the electrocatalysis of the CER and design strategies for selective CER electrocatalysts. We present experimental and theoretical methods for assessing the active sites of MMO catalysts and the origin of the scaling relationship between the CER and the OER. We discuss kinetic analysis methods to understand the kinetics and mechanisms of CER. Next, we summarize the design strategies for new CER electrocatalysts that can enhance the reactivity of MMO-based catalysts and overcome their scaling relationship, which include the doping of MMO catalysts with foreign metals and the development of non-precious metal-based catalysts and atomically dispersed metal catalysts.&lt;/p&gt;

Temperature Dependence of Oxygen Evolution Reaction Activity in Alkaline Solution at Ni–Co Oxide Catalysts with Amorphous/Crystalline Surfaces

The development of high-performance oxygen evolution reaction (OER) catalysts based on earth-abundant transition metal oxides as alternatives to precious metal oxides is being pursued intensively for the large-scale application of alkaline water electrolysis for hydrogen production. Active OER electrocatalysts can be designed on the basis of fundamental insights into the catalytic phenomena occurring on surfaces. In particular, the temperature dependence of OER activity as a measure of reaction energies/barriers needs to be characterized by taking into account the operating temperature (60–80 °C) of a practical water electrolyzer. In this work, we synthesized Ni–Co oxide solid solution catalysts by the flame pyrolysis method, with particular emphasis on the temperature dependence of the OER catalysis, as a way to elucidate the catalytic reactions, from both experimental and theoretical perspectives. The as-prepared catalysts structured with conductive Ni–Co alloys were heat-treated at different temperatures in the presence of O2 to regulate the crystallinity of the oxide surfaces. The OER activity has been evaluated from 20 to 80 °C in 1 M KOH, and the OER kinetic current density was found to increase with increasing temperature for all catalysts obtained. Interestingly, the Ni–Co/oxide materials with amorphous or poorly crystalline oxide surfaces showed superior activity (3–8 times higher surface-specific activity at 1.55 V vs reversible hydrogen electrode (RHE)) than their crystalline counterparts at all temperatures. The OER activation energy also decreased with decrease of the crystalline oxide phases. It was demonstrated that the catalyst with amorphous Ni–Co oxide surfaces could be readily activated to form active oxyhydroxides under alkaline OER conditions. The amorphous or disordered structure of the surface oxide with low-coordinated sites is proposed to render the surface geometry energetically favorable for the rate-determining step O + OH → OOH, which accelerates the reaction kinetics and thus improves the OER activity, as explained by density functional theory calculations.

On the Mechanism of Heterogeneous Water Oxidation Catalysis: A Theoretical Perspective

Earth abundant transition metal oxides are low-cost promising catalysts for the oxygen evolution reaction (OER). Many transition metal oxides have shown higher OER activity than the noble metal oxides (RuO2 and IrO2). Many experimental and theoretical studies have been performed to understand the mechanism of OER. In this review article we have considered four earth abundant transition metal oxides, namely, titanium oxide (TiO2), manganese oxide/hydroxide (MnOx/MnOOH), cobalt oxide/hydroxide (CoOx/CoOOH), and nickel oxide/hydroxide (NiOx/NiOOH). The OER mechanism on three polymorphs of TiO2: TiO2 rutile (110), anatase (101), and brookite (210) are summarized. It is discussed that the surface peroxo O* intermediates formation required a smaller activation barrier compared to the dangling O* intermediates. Manganese-based oxide material CaMn4O5 is the active site of photosystem II where OER takes place in nature. The commonly known polymorphs of MnO2; α-(tetragonal), β-(tetragonal), and δ-(triclinic) are discussed for their OER activity. The electrochemical activity of electrochemically synthesized induced layer δ-MnO2 (EI-δ-MnO2) materials is discussed in comparison to precious metal oxides (Ir/RuOx). Hydrothermally synthesized α-MnO2 shows higher activity than δ-MnO2. The OER activity of different bulk oxide phases: (a) Mn3O4(001), (b) Mn2O3(110), and (c) MnO2(110) are comparatively discussed. Different crystalline phases of CoOOH and NiOOH are discussed considering different surfaces for the catalytic activity. In some cases, the effects of doping with other metals (e.g., doping of Fe to NiOOH) are discussed.

(Digital Presentation) Oxygen Evolution Reaction on Non-Precious Metal Oxide-Based Electrocatalysts With and Without Low Potential Scan in Acidic Solution

Hydrogen energy ministerial meeting (H2 EM 2021) was held on Oct. 4, 2021 by the ministry of economy, trade and industry (METI) of Japanese government as online special event with cabinet members and officials from 29 countries, regions, and organizations. In the industrial session, 4 sessions were held, and the water electrolysis was one of the themes of industrial session. Panelists from several countries introduced state-of-art technologies of water electrolyzer [1]. In our previous study, the hydrogen producing from renewable energies is called as “Green Hydrogen” [2].Proton exchange membrane water electrolysis (PEMWE) has partly commercialized and recently applied its system for the Power-to-Gas (PtG). However, conventional anode is precious metal oxide such as IrO2 and the cost of raw material is expensive. From this point of view, the non-precious metal oxide anode should be required for green hydrogen production. We focused on tantalum and molybdenum oxide-based electrocatalyst (TaOx and MoOx) and have studied its catalytic activity for oxygen evolution reaction (OER) [3]. Moreover, TaOx with Mn addition showed much higher the OER activity than TaOx [4]. In this study, we have applied for MoOx with Mn addition for investigating the catalytic activity for the OER in acidic solution.Ti rod was used as substarate. MoOx was prepared by RF magnetron sputtering method. In the case of fabrication for Mn-MoOx, Mo metal piece for 40% surface area was set on the Mn disc as additional material. Each partial pressure of Ar and O2 gas was adjusted to 0.15 Pa in fabrication of Mn-MoOx. The substrate heating temperature was constant at 673 K during sputtering. We used conventional three electrode cell with each sample as working electrode while the reversible hydrogen electrode (RHE) and carbon plate were used as reference and counter electrode to demonstrate the electrochemical measurement in 1 M H2SO4 solution with saturated nitrogen atmosphere at 303 K. Several samples were demonstrated pretreatment from 0.05 to 1.2 V vs. RHE before the measurement of OER. The slow scan voltammetry was performed from 1.2 to 2.0 V to evaluate the OER activity. We also carried out electrochemical impedance spectroscopy (EIS) to obtain resistance components and semiconducting properties. In addition, samples before and after electrochemical measurement were analyzed by SEM-EDX and XPS.Figure 1 shows the Tafel plots of OER on Mo oxide-based electrocatalysts with and without Mn addition. The current density was based on geometric surface area. In the case of sample performing pretreatment, the Mn-MoOx with pretreatment (Mn-MoOx(Pre)) has obviously higher OER current than MoOx with pretreatment (MoOx(Pre)). Moreover, Mn-MoOx has higher OER current than Mn-MoOx(Pre), and the Tafel slope of OER on Mn-TaOx was 79 mV dec-1 the while the that on the Mn-MoOx(Pre) was 147 mV dec-1. From the results of EIS, the charge transfer resistance of Mn-MoOx was much smaller than that of Mn-MoOx(Pre) while the film resistance of was similar to that of Mn-MoOx(Pre). The results from SEM-EDX revealed that Mn amount in Mn-MoOx after the electrochemical measurement was constant compared to that before electrochemical measurement while Mn amount in Mn-MoOx(Pre) after electrochemical measurement was decreased compared to that before electrochemical measurement. From results from XPS analysis, Mn ratio in the Mn-MoOx after electrochemical measurement was constant compared to that before electrochemical measurement while Mn amount in Mn-MoOx(Pre) after electrochemical measurement was decreased compared to that before electrochemical measurement. Therefore, Mn species still remained on both surface and bulk in the case of Mn-MoOx while it was decreased in the case of Mn-MoOx(Pre). In Fig. 1, The peak over 1.5 V was detected from Tafel plots of both Mn-MoOx and Mn-MoOx(Pre). This peak corresponded to redox peak of Mn(III)/Mn(IV) [5], and it implied that Mn species affected on the OER activity. However, the electric charge of Mn(III)/Mn(IV) of Mn-MoOx was smaller than that of Mn-MnOx(Pre), and it is suggested that not only Mn oxide but also other species such as Mn-Mo compound oxide reacts as an active site of OER on Mn-MoOx.Acknowledgement: This work is partially supported by the Suzuki Foundation.Reference https://www.meti.go.jp/english/press/2021/1008_001.html Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010). Matsuzawa, S. Hirayama, K. Sumi, A. Ishihara, Abst. PRiME2020, I01F-2492, Online (2020). Matsuzawa, S. Hirayama, Y. Kohara, and A. Ishihara, ECS Trans, 104(8), 431 (2021).M. Morita, C. Iwakura, and H. Tamura, Electrochim. Acta, 24, 357 (1979). Figure 1

Application and Development of Precious Metal Oxide Oxygen Catalyst for Aqueous Electrolysis Technology

Application and Development of Precious Metal Oxide Oxygen Catalyst for Aqueous Electrolysis Technology

Metal Selenide Anchored Carbon Nanotube for Boosted Oxygen Evolution Reaction

The oxygen evolution represents an important reaction in electrochemical water splitting and metal-air batteries. To deal with the increasing energy crisis and demand for sustainable clean energy, production of low-cost, high-efficiency, and robust oxygen evolution reaction (OER) electrocatalysts is urgently needed. Recently, transition metal chalcogenides (TMCs) have been used as active electrocatalysts for OER because of their high electrical conductivity and enhanced electrochemical activity. These chalcogenides based electrocatalysts show unprecedented high efficiency for OER exhibiting very low overpotentials, thereby surpassing state-of-the-art precious metal oxides or hydroxide-based catalysts. Carbon nanostructures have been shown to significantly improve their electrocatalytic performance even further. In the present work, nickel selenide nanorods (NRs) were grown inside carbon nanotubes (NiSe@CNT) through chemical vapor deposition (CVD) wherein, the carbon nanotube formed in-situ wrapping around the growing nickel selenide nanorods. Such intimate intermixing is expected to aid in rapid electron transfer from the catalyst composite and yield significantly higher current density. The encapsulation with a CNT shell can also expectedly increase stability of the selenide phase with respect to corrosion and anion leaching. Electrocatalytic behavior was explored by various electrochemical studies, including linear sweep voltammetry (LSV), chronoamperometric experiments, electrochemical surface area determination, and Tafel slope determination, under highly alkaline condition. It was observed that this NiSe@CNT composites showed enhanced electrocatalytic activity for OER. Our results indicate that the self-grown CNT around nickel selenide increases catalytic activity of this hybrid nanostructure due to an increased number of catalytic sites and electronic conductivity of the nanocomposite. The overpotential at 10 mA cm-2 for the as-synthesized NiSe@CNT catalyst is 270 mV which is much better than precious metal based electrocatalysts for OER, such as RuO2 and IrO2. In addition, the current-voltage plots were superimposable before and after 40 h of chronoamperometry test, confirming that the as-synthesized catalyst gives stable electrocatalytic OER activity in 1 M KOH medium for an extended time period. The as-synthesized catalyst was characterized by XRD, Raman spectroscopy, XPS and TEM for morphology, elemental and chemical compositions.Keywords: oxygen evolution reaction, carbon nanotubes, In-situ wrapping Figure 1

Uniform cobalt nanoparticles embedded in nitrogen-doped graphene with abundant defects as high-performance bifunctional electrocatalyst in overall water splitting

Co nanoparticles with uniform size (about 5 nm) embedded in N-doped graphene (Co-NG) were explored in this work. The introduction of a second carbon source of citric acid during synthesis prevented the Co atoms from growing up, thus regulating the size of the cobalt nanoparticles. N atoms in N-doped graphene had more lone-pair electrons, making it easier to capture electrons from hydrated ions, and facilitating the dynamics procedure of HER. Furthermore, N dopant rendered larger positive charge density on the adjacent carbon atoms, which was conducive to OER and HER. At 10 mA cm−2 of the current density, the Co-NG/CC catalyst's overvoltage of HER was 78 mV, approaching that of 20% Pt/C (59 mV), an efficient precious metal electrocatalyst for HER, while its overvoltage of OER was about 225 mV, 12.5% lower than that of RuO2 (257 mV, a common precious metal oxide OER electrocatalyst). In addition, this Co-NG/CC composite bifunctional catalyst displayed good electrochemical stability in alkali solution and might be designed as a dual-function catalyst in the application of overall water splitting. The cell voltage of Co-NG/CC//Co-NG/CC was only 1.66 V, approaching to that of full precious metal cell of Pt/C//RuO2 (1.52 V), and revealing the good commercial application prospects of this composite bifunctional catalyst.

Bifunctional WC‐Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media

AbstractWe report the strong catalyst–support interaction in WC‐supported RuO2 nanoparticles (RuO2‐WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η10 of 347 mV and a mass activity of 1430 A gRu−1, eight‐fold higher than that of commercial RuO2 (176 A gRu−1). Theoretical calculations demonstrate that the strong catalyst–support interaction between RuO2 and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO2‐WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm−2. The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

Bifunctional WC‐Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media

AbstractWe report the strong catalyst–support interaction in WC‐supported RuO2 nanoparticles (RuO2‐WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η10 of 347 mV and a mass activity of 1430 A gRu−1, eight‐fold higher than that of commercial RuO2 (176 A gRu−1). Theoretical calculations demonstrate that the strong catalyst–support interaction between RuO2 and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO2‐WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm−2. The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

Front Cover Image

Since most electrocatalysts for oxygen evolution reaction (OER), except for precious metal oxides RuO2 and IrO2, are unstable in harsh acidic solutions, it is highly desirable to develop high‐performance OER electrocatalysts for acidic media. Herein, authors (DOI: 10.1002/inf2.12273)report a simple strategy to produce carboxyl‐enriched multiwalled carbon nanotubes that exhibit stable and high electrocatalytic activities for OER in acidic solutions. As far as we are aware, these results represent the best OER performance for metal‐free electrocatalysts, even comparable to those of RuO2 and IrO2. image

Carboxylated carbon nanotubes with high electrocatalytic activity for oxygen evolution in acidic conditions

AbstractSince most electrocatalysts for oxygen evolution reaction (OER), except for precious metal oxides RuO2 and IrO2, are unstable in harsh acidic solutions, it is highly desirable to develop high‐performance OER electrocatalysts for acidic media, though it is still a big challenge. Herein, we report a simple strategy to produce carboxyl‐enriched multiwalled carbon nanotubes (COOH‐MWNTs) that exhibit stable and high electrocatalytic activities for OER in acidic solutions, showing an overpotential at a current density of 10 mA cm–2 and a Tafel slope as low as of 265 mV and 82 mV dec–1, respectively. As far as we are aware, these results represent the best OER performance for metal‐free electrocatalysts, even comparable to those of RuO2 and IrO2. We have further revealed the catalytic mechanism, which involves one electron lose from the COOH‐MWNTs catalyst at the beginning of the OER process to trigger H2O molecule oxidation by forming peralcohol, followed by the recapture of one electron from water molecule to oxidize water and to recover the initial state for the COOH‐MWNTs catalyst. The unravel of this new OER mechanism is important as it provides new insights into the crucial role of organic functional groups in electrocatalytic processes. Also, the mechanistic understanding can be used to guide the design and development of novel metal‐free catalysts for acidic OER electrocatalysis and beyond. image

Facile synthesis of tubular CoP as a high efficient electrocatalyst for pH-universal hydrogen evolution

A facile three-step approach for tubular CoP preparation and its catalytic activity for HER and OER are reported. The CoP microtubes show superior HER performance in a wide pH range with low overpotentials of 91, 101 and 113 mV at 10 mA cm−2 in 0.5 M H2SO4, 1 M KOH and 1 M PBS, respectively. Additionally, it also depicts superior OER performance with an overpotential of 300 mV at 10 mA cm−2, which is lower than reported precious metal oxides. The improved electrocatalytic performance of tubular CoP is likely attributed to the porous tube-like structural features, which not only afford rich exposed active sites, but also accelerate the charge or mass transfer efficiency, and thus efficiently promote the HER performance. The synthesis of tubular CoP confirms the importance of morphology features and provides a new insight to rationally design and synthesize highly effective non-noble metal phosphide-based pH-universal electrocatalysts for HER.

The Impact of Mn addition on Ta Oxide-Based Electrocatalysts for Oxygen Evolution Reaction in Acid

Hydrogen energy ministerial meeting (H2 EM 2020) was held on Oct. 14, 2020 by the ministry of economy, trade and industry (METI) of Japanese government as online special event with cabinet members and officials from 23 countries, regions, and organizations. In the open session, presenter from industrial company of water electrolysis introduced to improve its technologies for producing hydrogen from renewable energies [1]. In our previous study, the hydrogen producing from renewable energies is called as “Green Hydrogen” [2].Polymer electrolyte water electrolysis (PEWE) has partly commercialized and recently applied its system for the Power-to-Gas (PtG). However, conventional anode is precious metal oxide such as IrO2 and the cost of raw material is expensive. Moreover, it is reported that the IrO₂ is afraid to cause the degradation operating by variable renewable energy (VRE) because the catalytic activity of oxygen evolution reaction became lower by potential cycling of VRE simulation [3]. From this point of view, the non-precious metal oxide anode with high durability against VRE should be required for green hydrogen production. We focused on tatanium oxide-based electrocatalyst (TaOx) and have studied its catalytic activity for oxygen evolution reaction (OER) [4]. In this study, we have investigated the catalytic activity of TaOx with and Mn addition for the OER in acidic solution.Ti rod was used as substarate. TaOx was prepared by RF magnetron sputtering method. In the case of fabrication for Mn-TaOx, Ta metal piece for 60% surface area was set on the Mn disc as additional material. Each partial pressure of Ar and O2 gas was adjusted to 0.23 Pa in fabrication of Mn-TaOx. The substrate heating temperature was constant at 673 K during sputtering. We used conventional three electrode cell with each sample as working electrode while the reversible hydrogen electrode (RHE) and carbon plate were used as reference and counter electrode to demonstrate the electrochemical measurement. In order to evaluate the OER activity of samples, the slow scan voltammetry was performed from 1.2 to 2.5 V vs. RHE in 1 M H2SO4 solution with saturated nitrogen atmosphere at 303 K. We also carried out electrochemical impedance spectroscopy (EIS) to obtain resistance components and semiconducting properties.Figure 1 shows the polarization curve of OER on Ta oxide-based electrocatalysts with Mn addition. The results of Ti oxide-based electrocatalysts with Mo addition (Mo-TiOx) that is our past study [4] is also shown in this figure. The current density was based on geometric surface area. According to figure, the Mn-TaOx has obviously higher OER current than that on Mo-TiOx, and the Tafel slope of OER on Mn-TaOx was 105 mV dec-1 the while the that on the Mo-TiOx was 91 mV dec-1. The peak over 1.5 V was detected from polarization curve of Mn-TaOx in Fig. 1. This peak corresponded to Mn ion, and it suggests that Mn species affected on the OER activity while the amounts of Mn on the sample after electrochemical measurement showed below detection limit from XPS analysis. According to these results, the surface of Mn-TaOx was consisted of Ta oxide mainly with very few amount of Mn after electrochemical measurements. From the results of EIS, resistances of film and charge transfer on Mn-TaOx were lower than that of Mo-TiOx. In addition, the slope of Mott-Schottky plots on the Mn-TaOx was much gentle than that on the Mo-TiOx, and it means that the carrier concentration in Mn-TaOx was higher than that in Mo-TiOx. Therefore, the reason why the Mn-TaOx showed higher OER current than the Mo-TiOx is that both factors of electrical conductivity and catalytic activity were improved.Acknowledgement: This work is partially supported by Toyota Mobility Foundation and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation.Reference https://www.meti.go.jp/english/press/2020/1015_001.htmlK. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).S. M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).K. Matsuzawa, S. Hirayama, K. Sumi, A. Ishihara, Abst. PRiME2020, I01F-2492, Online (2020). Figure 1