Phase engineering and surface reconstruction of FeNiMo alloys as high efficient electrode for oxygen evolution reaction

To establish hydrogen-based sustainable society, we need to change our primary energy from fossil fuels to renewable energy by water electrolysis. Among practical water electrolysis, alkaline water electrolysis is advantageous to the cost of materials, though the degradation of electrode under intermittent operation must be solved.[1] In this presentation, we will introduce our recent advancements on the developments of evaluation techniques for durability of oxygen evolution reaction (OER) electrodes and novel materials to improve their durability.(1) Accelerated durability test protocol for OER electrodes It is known that the degradation of OER electrodes proceeds rapidly under intermittent operation. Reverse current is generated by discharge of the electrodes upon suspension of electrolysis. To mimic the charge–discharge cycles, we have proposed an accelerated durability test (ADT) protocol shown in Figure 1.[2] The ADT protocol consists of cycles of constant current electrolysis, potential sweep, and potential hold. The OER electrode is charged during the electrolysis and discharged during the potential hold. Potential hold is essential to fully discharge the electrode instead of potential sweep only. Using the ADT protocol, we analyzed the degradation mechanism of NiCoOx-coated Ni electrodes in a 7 M KOH electrolyte. The electrode exhibited stable OER overpotential up to 600 cycles and increase in the overpotential after that, showing immediate degradation. The chemical analyses showed that Co is dissolved from the catalyst quickly within the first 200 cycles without significant degradation, which rather increased the active surface area. After that thinning of the catalyst layer occurs with the dissolution of Ni. The Ni substrate is finally exposed to the electrolyte. The significant degradation proceeds together with the delamination of the catalyst layer and corrosion of the substrate.(2) Hybrid cobalt hydroxide nanosheets as self-repairing catalystsThe detachment of catalyst from the electrode substrate causes significant degradation; thus, we developed repairing technology of OER electrodes. Hybrid cobalt hydroxide nanosheets (Co-ns),[3] Co(OH)2 nanosheets modified with tris(hydroxymethyl)aminomethane (Tris-NH2), are unique materials that possesses OER activity of Co(OH)2 and functionalities due to Tris-NH2. When Co-ns are dispersed in an electrolyte (1 M KOH), Co-ns are deposited to be CoOOH nanosheets, forming a catalyst layer on Ni electrodes, during constant current electrolysis. If the catalyst layer is detached by degradation, fresh Co-ns can be deposited during electrolysis to repair the defects. Therefore, this system achieves quite a long lifetime under the ADT condition (Figure 2).[4,5] The concept of the self-repairing catalysts could be extended to other hybrid metal hydroxides, such as NiFe layered double hydroxides with higher OER activities, and the design of composite catalysts that enables to utilize highly active catalysts with poor self-repairing properties as self-repairing catalysts.[6] In conclusion, the durability of OER electrodes for alkaline water electrolysis under intermittent operation can be evaluated by the newly proposed ADT protocol. The ADT protocol is useful to reveal the degradation mechanism of catalysts and also to accelerate the development of novel catalytic materials with high durability like self-repairing catalysts. This work was supported by the JSPS KAKENHI (grant number 24K01580).

Lithium-air batteries have become attractive due to their extremely high theoretical energy density. However, conventional lithium-air cells operating with nonaqueous electrolytes suffer from poor cycle life due to the clogging of the porous air cathodes by insoluble discharge products, contamination of the organic electrolyte by moist air, and decomposition of the electrolyte during cycling. Attempts to overcome these problems by developing compatible electrolyte/catalyst combinations have invariably been disappointing. These difficulties could be overcome by adopting a new cell configuration consisting of a lithium metal anode in the conventional organic electrolyte separated by a lithium-ion conducting solid electrolyte from the air electrode in an aqueous catholyte. Accordingly, this presentation will focus on the development of catholytes that can avoid corrosion of the solid electrolyte as well as efficient, inexpensive electroctalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).A buffer catholyte solution with a moderate pH, consisting of phosphoric acid and a supporting salt, has been developed to keep the solid electrolyte stable and reduce the internal resistance and overpotential. With a high operating voltage and the utilization of all three protons of phosphoric acid, the buffer catholyte enables a rechargeable lithium-air cell with high energy density. Further increase in power density has been realized by increasing the solid-electrolyte conductivity and the operating temperature to 40 oC.The biggest challenge with lithium-air cells is the large overpotentials associated with the ORR and OER. Nanocrystalline iridium oxide has been the best OER catalyst, but it is expensive. In this regard, the low-temperature form of lithium cobalt oxide (designated as LT-lithium cobalt oxide) that adopts a lithiated spinel structure has been developed as an inexpensive, efficient electrocatalyst for OER. Its OER activity is much higher than that of the high-temperature form of lithium cobalt oxide (designated as HT-lithium cobalt oxide), which has a layered structure and is used as a cathode in lithium-ion batteries, and even slightly higher than that of iridium oxide. Although the ORR activity of LT-lithium cobalt oxide is low, a combination of high ORR and OER activity could be realized by chemically extracting lithium from the LT-lithium cobalt oxide, demonstrating the lithium-deficient, spinel-type LT-lithium cobalt oxide as a bifunctional catalyst for rechargeable metal-air batteries. The high OER and ORR activities of spinel-type LT-lithium cobalt oxides are attributed to cubane-like subunits and a pinning of the cobalt-3d energy with the top of the oxygen-2p band. Furthermore, a novel 3-D O- and N-doped carbon nanoweb (ON-CNW) has been developed as an inexpensive, metal-free catalyst for ORR. The 3-D nanoweb structure provides an ideal backbone support for catalytically active sites due to the fast electron and mass transport properties compared to 1-D or 2-D structures. Also, the synergistic effect between the O and N groups creates highly active pyridone groups all over the nanoweb surface that significantly improves the ORR activity. With a hybrid lithium-air cell, the ON-CNW exhibits performance close to that of commercial Pt/C. In addition, a novel hybrid lithium-air cell configuration with decoupled ORR and OER electrodes has been developed. With the conventional bifunctional air cathode that supports ORR during discharge and OER during charge, the ORR catalyst and carbon support suffer from degradation under the highly oxidizing conditions during the charge process, resulting in rapid increase in overpotential upon cycling and limiting the cycle life. In contrast, the hybrid lithium-air cell with decoupled ORR and OER electrodes eliminates the degradation problems and leads to high efficiency with good cycle life.

A stable and highly active oxygen evolution reaction (OER) electrode is the key for fast and robust O2 production, which is one of the essential points for various kinds of energy conversion systems, such as water splitting, lithium‐O2 battery and artificial photosynthesis. Here, superaerophobic electrodes with metal@metal‐oxide powder catalysts are shown, which demonstrate high and stable OER activity. The active‐site‐density of metal@metal‐oxide catalysts is increased over one order of magnitude than those of pure metal oxides due to the large enhancement of electrical conductivity, revealing the substantial enhancement of electrochemical OER kinetics. Furthermore, the superaerophobic property of electrodes is favorable for fast O2 desorption, which improves electrochemical active surface area (EASA) during OER. The superaerophobic electrode with metal@metal‐oxide powder catalysts provides the new insight for increase of active‐site‐density and EASA simultaneously, which are the key factors to determine the activity of OER electrode.

Anion Exchange Membrane electrolyzers (AEMELs) have increased in popularity in recent years due to their potential to combine the benefits from proton exchange membrane electrolyzers and traditional alkaline electrolyzers – namely, high current density operation, pressurized H2 discharge and lowering cost by utilizing low-cost earth abundant electrocatalysts and inexpensive component materials. In the AEMEL, the oxygen evolution reaction (OER) electrode plays a critical role dictating the overall efficiency of the cell due to sluggish OER activity and species transport. In our previous study, we optimized the OER electrode structure by investigating the effects of catalyst loading, catalyst type, the porous transport substrate and additive carbon [1] – resulting in an excellent performance of 1.55 V operation at 1.0 A/cm2.Though that study led to high performance, all of those gains were made using KOH electrolyte being fed to the cell, not deionized (DI) water. The use of DI water complicates AEMEL operation as the liquid phase can no longer be relied on to carry any of the ionic charge – this possibly can reduce the electrochemically active surface area (ECSA) [2]. It has also been stated that pure water operation may have negative consequences on cell durability. Therefore, to allow for AEMELs to operate efficiently on DI water, it is important to understand how the ion transport and charge transfer resistance change as cells are transitioned from KOH to DI water operation. It is also important to understand what effects even trace amounts of KOH can have on behavior. Lastly, the effect of the ionomer properties on the OER anode performance should be well-understood.In this study, we investigate the role of alkaline feed pH on the performance and durability of AEMELs. When shifting from alkaline feed to DI water, it will be shown that there is an increase in ohmic and charge transfer resistance, resulting in significant voltage loss. Because the alkaline feed enhances electrode conductivity by connecting catalyst active sites to the conductive ionomer and AEM [2-3], new electrode structures are needed that allow for enhanced ECSA and lower resistances with DI water feeds. In this work, that was accomplished by creating a four-layer electrode structure as well as manipulating the cell operating conditions. Lastly, our team enabled the use of a high IEC ionomer in the anode catalyst layer by introducing a cross-linker and introducing smaller, cryo-milled ionomer particles. This was important to overcome adhesion issues that can come from high water uptake and swelling in high-IEC ionomers [4-5]. This paper will focus not only on performance, but also longevity, with several cells being stably operated for more than 500 hours continuously.

The oxygen evolution reaction (OER) is still a barrier to energy generation technologies, motivating the development of new strategies to the rational use of electrodes for OER. This work reports the effect of the morphology of nickel oxide (NiO) nanostructures on their catalytic activity toward OER in alkaline medium. 1D hollow nanofibers (NiO-NFBs) synthesized by solution blow spinning are compared with 3D nanoparticles (NiO-NPTs) synthesized by a well-known citrate method. The intrinsic activity of the electrocatalysts was evaluated by linear sweep voltammetry (LSV), electrochemically active surface area (ECSA), turnover frequency (TOF), and electrochemical impedance spectroscopy (EIS). Results confirm the superiority of NiO-NFBs, with a remarkable difference of 133 mV versus RHE to generate j = 10 mA cm−2. The NiO-NFBs showed a TOF value 122 times higher than NiO-NPTs. The Tafel analysis and EIS revealed that the hollow structure favors kinetics through a more efficient process of mass and charge transfer. Overall, results corroborate the thesis that the morphology imposes the main rule on the electrocatalyst performance. The morphology of NiO-NFBs is fully preserved after the electrolysis test, while NiO-NPTs degrade through a nanoparticle coalescence mechanism. This work is a contribution to the state of the art of nanostructured NiO-based electrodes for OER.

Growing demand for sustainable, clean, efficient energy conversion system has derived tremendous interest to develop cost-effective and highly efficient electrocatalysts for oxygen evolution reaction (OER). Sulfides have recently attracted great attention due to their highly efficient electrocatalytic activity. MoS2 has been verified as an efficient catalyst toward OER because of its durability, cost-efficiency and high activity. However, binder and extra conductive materials are required to attach electrocatalyst on a conductive substrate which will result in restricted active surface areas, undesirable interfaces and lots of dead volumes. Binder-free self-supported material can eliminate these problems. Herein, we synthesize self-supported binder-free MoS2 on 3D porous nickel foam (NF) with excellent activity and stability via a facile hydrothermal method. The MoS2@NF electrode is tested in 0.1mol KOH by linear sweep voltammetry (LSV), electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS). Compared with different substrates (Cu foam, NF, Cu paper and carbon paper), MoS2@NF electrode where NF as substrate shows the lowest overpotential at 10mA cm-2 of 359mV for OER. Furthermore, the overpotential at 10mA cm-2 after 10hr is 419mV, which demonstrates high stability and activity. Such high electrochemical performance may attribute to the following reasons: 1) the large ECSA and good conductivity, that improves utilization of electroactive sites and electron transfer; 2) the unique 3D porous structure, that facilitates electrolyte penetration and reactant/product diffusion; and 3) the sulfur vacancy in MoS2, that accelerates the O-H bond breaking.

Chemical synthesis of a microsphere-like copper molybdate electrode for oxygen evolution reaction

This study represents a green synthesis method for fabricating an oxygen evolution reaction (OER) electrode by depositing two-dimensional CuFeOx on nickel foam (NF). Two-dimensional CuFeOx was deposited on NF using in situ hydrothermal synthesis in the presence of Aloe vera extract. This phytochemical-assisted synthesis of CuFeOx resulted in a unique nano-rose-like morphology (petal diameter 30–70 nm), which significantly improved the electrochemical surface area of the electrode. The synthesized electrode was analyzed for its OER electrocatalytic activity and it was observed that using 75% Aloe vera extract in the phytochemical-assisted synthesis of CuFeOx resulted in improved OER electrocatalytic performance by attaining an overpotential of 310 mV for 50 mA cm−2 and 410 mV for 100 mA cm−2. The electrode also sustained robust stability throughout the 50 h of chronopotentiometry studies under alkaline electrolyte conditions, demonstrating its potential as an efficient OER electrode material. This study highlights the promising use of Aloe vera extract as a green and cost-effective way to synthesize efficient OER electrode materials.

Oxygen vacancy defects modulated electrocatalytic activity of iron-nickel layered double hydroxide on Ni foam as highly active electrodes for oxygen evolution reaction

A highly efficient and durable self-standing iron-cobalt-nickel trimetallic phosphide electrode for oxygen evolution reaction

A highly active and stable oxygen evolution reaction (OER) electrocatalyst is critical for hydrogen production from water splitting. Herein, three-dimensional Ni3S2@graphene@Co92S8 (Ni3S2@G@Co9S8), a sandwich-structured OER electrocatalyst, was grown in situ on nickel foam; it afforded an enhanced catalytic performance when highly conductive graphene is introduced as an intermediary for enhancing the electron transfer rate and stability. Serving as a free-standing electrocatalytic electrode, Ni3S2@G@Co9S8 presents excellent electrocatalytic activities for OER: A low onset overpotential (2 mA·cm−2 at 174 mV), large anode current density (10 mA·cm−2 at an overpotential of 210 mV), low Tafel slope (66 mV·dec−1), and predominant durability of over 96 h (releasing a current density of ∼14 mA·cm−2 with a low and constant overpotential of 215 mV) in a 1 M KOH solution. This work provides a promising, cost-efficient electrocatalyst and sheds new light on improving the electrochemical performance of composites through enhancing the electron transfer rate and stability by introducing graphene as an intermediary.

Highly disordered cobalt oxide nanostructure induced by sulfur incorporation for efficient overall water splitting

Introduction In dimensionally stable electrodes (DSE® type electrodes) technology, reduction of precious metal loading is significant issue to reduce cost of system. As improvements of the DSE® type electrode, many efforts of preparation conditions have been reported. Higher preparation temperature reduces electrocatalyst consumption, but corrosion of the base substrate/electrocatalyst interface makes limitation of service lifetime [1-4]. Here, intermediate layer should be important to improve durability. As the intermediate layer, 4 and 5th group transition metal oxide or composite with the transition metal oxide and precious metal was proposed, but their property has not be clarified. In addition, these material technology would be important basic for polymer electrolyte membrane water electrolysis, too. In this study, corrosion resistance of low oxygen partial pressure oxidized titanium electrodes and oxygen evolution reaction activity and durability of low IrO2 loaded electrodes of the low oxygen partial pressure oxidized titanium have been investigated as basic study of intermediate layer of DSE® type electrode in industrial electrolysis and electroconductive oxide supported precious metal oxide electrocatalyst for polymer electrolyte membrane water electrolysis. Experimental Titanium electrode was oxidized in H2 – O2 – Ar of estimated p(O2) of 1.84×10-17 atm at 1050°C for 10 or 20 h, H2 – O2 – Ar of the p(O2) of 1.43×10-17 atm at 1050°C for 10 h, air at 500°C for 1 h. For comparison, Titanium electrode was also used. Surface oxide was determined with X-ray diffraction for thin – film of 2.0° of incidence angle. 0.02 mg cm-2 of IrO2, which is ca. 1/50 level of usual industrial electrode, was loaded on the titanium electrodes by thermal decomposition of hydrogen hexachloroiridate(IV) hydrate butanol solution at 400°C for 1 h in air Electrochemical measurement was performed in 0.1 or 1.0 M (= mol dm-3) of H2SO4 at 30°C. Pretreatment was 300 cycles of cyclic voltammetry between 0.05 to 1.2 V vs. RHE with 200 mV s-1 for the all electrodes. The electrochemical stability of the titanium electrodes and the electrocatalytic activity for oxygen evolution reaction for the IrO2 coated titanium electrodes were determined with cyclic voltammetry of 5 mV s-1 of sweep rate. Resistances of the electrolyte and of surface oxide were determined with the electrochemical impedance spectroscopy (EIS). Results and discussion The surface of the titanium electrode oxidized in H2 – O2 – Ar of the p(O2) of 1.84×10-17 atm for 20 h and that of the 1.43×10-17 atm for 10 h at 1050°C was mixture of Ti4O7 and TiO2, and that of the p(O2) of 1.84×10-17 at 1050°C for 10 h and air at 500°C was TiO2. So, the notations of the titanium electrodes are Ti4O7_1050_20h/Ti, Ti4O7_1050_10h/Ti, TiO2_1050_10h/Ti, and TiO2_500/Ti, respectively. Figure 1 shows the surface oxide layer resistance of the titanium electrodes determined by the higher frequency region arc of the EIS measurement as a function of higher limit potential of the slow scan cyclic voltammetry. The resistances of the high temperature (1050°C) oxidized electrodes were constant, while that of the Ti and the 500°C oxidation Ti increased with higher limit potential of the slow scan voltammetry. In addition, the Ti4O7 containing oxide layer showed lower resistance than the TiO2 Therefore, high temperature prepared electroconductive oxidation layer has low resistance with stability as a support material. Figure 2 shows the surface oxide layer resistance of the IrO2 loaded titanium electrodes determined by higher frequency region arc of the EIS. Basically, the surface oxide layer resistance decreased with the IrO2 loading, and especially, the surface oxide layer resistance of the TiO2_1050_10h/Ti significantly decreased rather than others. Figure 3 and 4 shows the IR corrected initial performance and the lifetime test at 150 mA cm-2 for the IrO2 loaded titanium electrodes for OER. The IrO2/Ti, which has no intermediate layer, has the highest initial OER activity, but the shortest lifetime in these electrodes, and TiO2_1050_10h/Ti has relatively high OER activity and the longest lifetime in these electrodes. In the design of the intermediate layer, not only the property of the intermediate material itself but also synergy effect of 4 and 5th group metal oxide and precious metal oxide should be taken into account. This research was supported by the JSPS Grant-in-Aid for Scientific Research (C) JP 16K12345. The authors want to thank the person concern with. Reference K. Nagai, K. Nagasawa, and S. Mitsushima, Electrocatalysis, 7, 441 (2016) http://dx.doi.org/10.1007/s12678-016-0325-y.J. Hu, X. Sun, Y. Hou, J. Zhang, C. Cao, Electrochimica Acta, 53, 3127 (2008).H. Asano, T. Shimamune, USP 4,481,097 (1984).T. Shimamune, Y. Nakajima, USP 6,103,299 (2000). Figure 1

Metal organic framework nanofibers derived Co3O4-doped carbon-nitrogen nanosheet arrays for high efficiency electrocatalytic oxygen evolution

Highly efficient low-cost electrocatalysts for water splitting have attracted increasing interest in the development of energy storage and conversion. Here, we utilized copper (Cu) weaving mesh to in situ fabricate earth-abundant elements-based integrated electrodes for high performance water splitting, where NiFe layered double hydroxide (NiFe-LDH) ultrathin nanoarrys for oxygen evolution reaction (OER) and Cu3P nanowires for hydrogen evolution reaction (HER) were successfully constructed on the Cu mesh. Notably, large stable current densities are obtained for both OER (600 mA cm–2) and HER (200 mA cm–2) electrodes under low overpotential, which is superior to most of the nanoparticle-based electrodes. The large current density is mainly because of the excellent conductivity and clean surface (binder-free) of the Cu mesh-based electrode, and which is extremely important for industrial application. The prepared integrated electrodes are coupled with a macroscopic porous sieve and microscopic nanostructures. The assembled NiFe-LDH∥Cu3P electrolyzer exhibits a small cell working voltage of 1.72 V under the current density of 10 mA cm–2 at room temperature, as well as long-term stabilities (>10 h) in 1 M KOH. These excellent performances of our earth-abundant elements-based weaving mesh electrode result from their improved charge transfer, surface area, mass transport, and faster kinetics of catalytic reactions.