Anode For Water Electrolysis Research Articles

Self-supporting trace Pt-decorated ternary metal phosphide as efficient bifunctional electrocatalyst for water splitting

Compared to monofunctional catalysts, the preparation of bifunctional materials with HER and OER activities holds greater significance for the separation of hydrogen from water electrolysis. In this study, we synthesized a self-supported bifunctional catalyst using ternary transition metal (Ni, Fe, Co) phosphide adorned with trace Pt for water splitting (P-Fe6Co3Ni/Pt/NF). As the main catalytic kernel toward HER, the Pt nanoparticles underwent uniform in situ reduction on the Ni foam, avoiding clustering due to the protective effect of iron and cobalt phosphides. This exposed more active sites, improving HER kinetics. In the OER process, the deposited Fe and Co were confirmed to play a pivotal role and will undergo oxidation to oxyhydroxides during structural reconstruction. In addition to enhancing conductivity, the Mott-Schottky test revealed that Pt can facilitate the formation of more active sites, leading to a lower potential for the OER reaction. Ultimately, P-Fe6Co3Ni/Pt/NF required only an overpotential of 32.8 mV in HER to reach the current density of 10 mA cm−2, demonstrating excellent OER activity with a low overpotential of 259.9 mV at current density of 20 mA cm−2. When employed as cathode and anode in water electrolysis, P-Fe6Co3Ni/Pt/NF necessitated only 1.6 V cell voltage to achieve the current density of 10 mA cm−2. This study provides a reference for designing other highly efficient Pt-based electrocatalyst, thereby promoting the widespread application of water electrolysis in hydrogen production.

CoFe2O4 deposited on the rice husk-derived cordyceps-like SiC as an effective electrocatalyst for overall water splitting

Electrocatalytic water splitting typically requires higher energy consumption. Coupling transition metals with other conductive materials is an effective strategy for reducing energy consumption. Herein, a chemical deposition method, which is both uncomplicated and efficient was devised for the preparation of non-precious metal/rice hull-based Si-carbon composite bifunctional electrocatalyst. The CoFe2O4 nanoparticles were uniformly deposited on SiC with a Cordyceps-like structure by coupling chemical deposition with carbonization. The specific structures and surface metal active sites enhance the electrical conductivity of the samples. The bifunctional catalyst (Fe/Co-RHC-500) exhibited impressive performance, HER and OER showed low overpotentials of 151 and 190 mV at a current density of 10 mA·cm-2 as well as Tafel slopes of 63.4 and 33.3 mV dec-1, respectively. When used as the cathode and anode for water electrolysis, it achieved a current density of 10 mA cm−2 at a voltage of 1.63 V. The results demonstrate that Fe/Co-RHC-500 sample is a promising candidate for bifunctional catalyst materials. This work broadens the potential application of biomass waste in electrocatalytic splitting of water.

Robust electrolytic oxygen evolution at nanostructured NiFe LDH@ in-situ functionalized graphite felt

NiFe layered double hydroxide (LDH) is among the most effective electrocatalysts for oxygen evolution reaction (OER). Tailor-designed NiFe LDH with high electronic conductivity assists in facile hydrogen production with marked energy-saving. Graphite felt (GF), a 3-D porous substrate with excellent conductivity is used herein. However, its low hydrophilicity calls for pretreatment. Herein, the potential strength of electrochemistry is applied to enhance the hydrophilicity of GF surface during the deposition of homogenous uniform distributed NiFe LDH electrocatalyst. GF is sequentially modified by sweeping the potential from the oxidative to the reductive region in a solution containing 100 mM NiSO4 and 30 mM FeCl3. Carbon structure flaws are introduced at the positive potential region. whereas during the negative potential scan, hydrogen evolution occurs with simultaneous uniform and homogeneous deposition of NiFe LDH. SEM, HR-TEM, mapping EDX, XPS, XRD, and contact angle measurements are utilized to reveal the features of the functionalized GF with NiFe LDH deposits. Furthermore, FeOOH/GF and Ni(OH)2/GF electrodes are prepared to highlight the advantages of the binary NiFe LDH/GF electrode toward the OER. Interestingly, the NiFe LDH/GF enhances the OER in an alkaline medium by throwing up a low overpotential of 273 mV to support a current density of 10 mA cm−2 alongside a Tafel slope of 48.5 mV dec−1. Furthermore, the NiFe LDH shows marked stability for 24 h of ongoing electrolysis. Therefore, the designed catalyst can be worked as an efficient anode for water electrolysis.

Achieving efficient urea electrolysis by spatial confinement effect and heterostructure

Urea oxidation reaction (UOR) can replace the oxygen evolution reaction (OER) at the anode in water electrolysis, which can reduce the thermodynamic potential of electrochemical water splitting. Currently, UOR performance is mainly limited by the 6 e− transfer process, requiring high-performance catalysts to reduce overpotential. Here, we innovatively designed and synthesized a binder-free self-standing composite electrocatalyst, viz. a carbon layer containing Fe2P/Co2P nanoparticles on a nickel foam substrate with Mo2S3 ([Fe2P/Co2P]@Mo2S3/NF). FeCo-prussian blue analogue (PBA)/nickel foam (NF) was obtained from cobalt-based metal–organic frameworks (Co-MOFs) by anion exchange at room temperature, and then [Fe2P/Co2P]@Mo2S3/NF was derived through high-temperature phosphating and hydrothermal treatment of FeCo-PBA/NF. After high-temperature phosphating treatment, FeCo-PBA collapses to form a carbon layer tightly wrapping on the NF skeleton. The carbon layer has a confinement effect on Fe2P/Co2P nanoparticles, preventing the nanoparticles from agglomerating and deactivating. Simultaneously, Fe2P/Co2P/NF and Mo2S3 form a heterostructure to optimize the electronic structure and accelerate electron transfer and urea activation, and make metal phosphides prone to interfacial oxidation during UOR catalysis. The internal carbon layer containing Fe2P/Co2P nanoparticles acts as a conductive support for better electron transfer from the interior to the exterior metal hydroxide/oxyhydroxide species and Mo2S3 species. The synergistic interaction amang Fe2P/Co2P, hydroxide/oxyhydroxide and Mo2S3 enhances the catalytic performance of [Fe2P/Co2P]@Mo2S3/NF. Strong hydrophilicity and superaerophobicity are conducive to electrolyte immersion and bubble detachment, which further promotes the [Fe2P/Co2P]@Mo2S3/NF catalyst to exhibit excellent UOR catalytic performance, only requiring potential of 1.36 V (vs. RHE) at 100 mA cm−2 with low Tafel slope of 41.5 dec−1 in alkaline solution. Remarkably, the electrochemical activity of [Fe2P/Co2P]@Mo2S3/NF displays only slightly decay after 72 h durability test.

Microstructures, thermal and electric properties of bismuth vanadium oxide (BiVO4) as an anode in electrolysis systems coated with a conductive glass indium thin oxide (ITO)

In this research, microstructures, thermal and electric properties of thin-film bismuth vanadium oxide as an anode for water electrolysis were presented. The anodes needed are obtained by centrifuging the indium thin oxides with BiVO4 powder. In order to determine the crystallinity and morphology of BiVO4, the following measurements were made: XRD, SEM, and FTIR. The thermal and electric properties were characterized by a CV meter. The results of XRD and SEM confirmed that the crystal structure of the investigated material is monoclinic and do not change from effect of spinning coating. The optimum production conditions were specify at 800 rpm and 7 s deposition time, with the average particle size is 0.3 μm. FTIR results show that for all samples, the peak received at 530 cm−1 is influenced by the vibrational bond of Bi–O. CV curves indicate increasing concentration of BiVO4 for all samples.

NiCo Nanoneedles on 3D Carbon Nanotubes/Carbon Foam Electrode as an Efficient Bi-Functional Catalyst for Electro-Oxidation of Water and Methanol

In this study, we report a 3D structured carbon foam electrode assembled from a bi-functional NiCo catalyst, carbon nanotubes (CNT), and a monolith 3D structured carbon foam (CF) as a highly active and stable electrode for oxygen evolution reaction (OER) and methanol oxidation reaction (MOR). When the NiCo@CNTs/CF electrode was used as an anode in OER, after the anodization step, the electrode required a small overpotential of 320 mV to reach the current density of 10 mA cm−2 and demonstrated excellent stability over a long testing time (total 30 h) in 1 M KOH. The as-prepared NiCo@CNTs/CF electrode also exhibited a good performance towards methanol oxidation reaction (MOR) with high current density, 100 mA cm−2 at 0.6 V vs. Ag/AgCl, and good stability in 1 M KOH plus 0.5 M CH3OH electrolyte. The NiCo@CNTs/CF catalyst/electrode provides a potential for application as an anode in water electrolysis and direct methanol fuel cells.

Integrating Ni nanoparticles into MoN nanosheets form Schottky heterojunctions to boost its electrochemical performance for water electrolysis

Water electrolysis using wind and solar as power offers a sustainable and promising approach to produce hydrogen. Bifunctional catalysts can significantly simplify the electrolysis system and enhance the overall electrochemical efficiency. Herein, three-dimensional nanostructured Ni and MoN compounds (Ni-Mo-N) with Schottky heterojunctions on the surface are developed as bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It is found that the solvents and pyrolysis temperature play a very important role in the formation of Ni-Mo-N with Schottky heterojunctions. The optimized Ni-Mo-N can deliver an overpotential of 125 mV (@100 mA cm−2 current density) in HER, which is approach to the Pt/C (20 wt%) (116 mV). Ni-Mo-N also exhibits high-performance in OER, comparable to RuO2. When Ni-Mo-N is assembled as cathode and anode for water electrolysis, at the cell voltage of 1.604 V the cell reaches the current density of 10 mA cm−2 with remarkable durability, making the Ni-Mo-N a promising bifunctional catalyst for water electrolysis.

Synthesis of Highly Active Catalysts for Oxygen Evolution Reaction: Iridium Ruthenium Oxide Nanoparticles Supported on Heteroatom-Doped Reduced Graphene Oxide

Environmental issues of global warming and depletion of fossil fuels have become significant topics in recent years. Effective utilization of renewable energies such as solar and wind energy is a critical issue to overcome the environmental problems. A large amount of effort has been invested in researches for conversion and storage of surplus energies. One of the prospective candidates is production of hydrogen via water electrolysis in polymer electrolyte water electrolyzer. However, catalytic activity, durability, and cost of water electrolysis systems are inadequate for practical application due to low kinetics and high over-potential of electrode reactions, especially, oxygen evolution reaction (OER) on the anode [1,2]. As OER catalyst, iridium oxide (IrO2) particles have been employed because of its high activity and durability. However, high cost and low abundance of iridium require reduction of iridium loading amount on the catalyst. Recently, we have succeeded to synthesize novel IrO2 nanoparticle with large surface area of IrO2 catalyst, which are supported on carbon nanotubes [3] and heteroatom-doped graphene [4], to improve OER activity. For further improvement of OER activity, we also develop alloy catalyst of IrO2 with other metals, such as ruthenium oxide (RuO2) to enhance the specific activity of the catalyst for OER.In this study, we have synthesized novel alloy nanoparticle catalysts, IrRuOx, supported on the heteroatom (N or B) doped reduced graphene oxide (rGO), IrRuOx/N-rGO and IrRuOx/B-rGO, and evaluated its catalytic activity as active catalysts for OER.The IrRuOx/N-rGO and IrRuOx/B-rGO catalyst were synthesized by hydrothermal method. Briefly, nitrogen- or boron-doped reduced graphene oxide (N-rGO, B-rGO) were prepared by thermal treatment of mixture of urea or boric anhydride and graphene oxide (GO), which was prepared by modified Hummers’ method, at 800 and 1000˚C, respectively, for 60 min. Then, required amounts of metal complexes, H2IrCl6 and RuCl3, and N-rGO, or B-rGO were dispersed in ethanol/water mixture and heated at 80˚C for 6 h. Finally, the mixture was heated at 150˚C in hydrothermal autoclave for 4 h to form IrRuOx nanoparticles. The synthesized catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The OER activity of the catalysts was examined in 0.5 M H2SO4 solution by linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) system.A TEM image of the IrRuOx/N- rGO catalyst is shown in Figure 1. The IrRuOx nanoparticles were dispersed uniformly on the surface of N-rGO substrate. An average size of the nanoparticles was ca. 1.7 nm. The loading amount of the IrRuOx catalyst is estimated to be approximately 20.1 wt% by EDX measurements. In the case of IrRuOx/B- rGO, average particle size was also ca. 1.7 nm, and the loading amount of the catalyst is 11.7 wt%. In addition, the EDX analysis reveals that the ratio of Ir to Ru of the IrRuOx nanoparticles is proportional to the molecular ratio of starting material compounds, H2IrCl6 to RuCl3. We confirmed the composition of the catalysts by XPS, as shown in Figure 2, and the results consisted with EDX measurement. Furthermore, the XPS results reveal that binding energy of Ir 4f peak on both catalysts was shifted to 0.1 eV lower energy state than that on the catalyst without alloying. The shift of the binding energy of Ir 4f peak reflects the modification of the electronic state of Ir, which affects adsorption energy of O and the catalytic activity for OER.Figure 3 shows mass activity of the prepared catalysts for OER estimated from LSV in 0.5 M H2SO4 solution. Electrochemical measurement reveals that the mass activity of the IrRuOx catalyst is higher than that on the catalyst without alloying. In addition, the catalyst supported on B-rGO has higher activity than the catalyst on non-doped or N-doped GO supports. High activity of the IrRuOx / B-rGO catalyst indicates that alloying IrO2 with RuO2 and heteroatom doping on the support materials improve the catalytic activity for OER. In summary, we successfully prepared the alloy catalysts of IrO2 and RuO2 supported on N-rGO and B-rGO substrate, and the catalysts can be promising candidates as anodes for water electrolysis.

Synthesis and Evaluation of Novel Iridium Ruthenium Oxide Catalysts Supported on Reduced Graphene Oxide for Oxygen Evolution Reaction

Recently, environmental issues such as global warming and depletion of fossil fuels have become important topic, and progress toward sustainable societies with renewable energies such as solar and wind energy is a critical issue to overcome the social problems. Conversion of surplus energies to hydrogen via water electrolysis has attracted attention as energy storage method to utilize renewable energies effectively. However, activity, durability, and cost of catalysts are insufficient for practical application in water electrolysis systems due to high over-potential and low kinetics of electrode reactions, especially, oxygen evolution reaction (OER) on the anode [1-2] for polymer electrolyte water electrolyzer (PEWE). Iridium oxide (IrO2) particles have been used as anode catalysts, because of its high activity and durability for OER. However, decreasing iridium loading amount is required to reduce catalyst cost. In a previous study, we have successfully synthesized novel IrO2 nanoparticle catalyst supported on carbon nanotubes with large surface area of IrO2 catalyst to improved OER activity [3]. For further improvement of OER activity, increasing the specific activity of the catalyst for OER is required. One of the approaches to improve catalytic activity is formation of alloy of iridium with other metals with high activity, such as ruthenium. In the present study, we have synthesized novel alloy nanoparticle catalysts, IrRuOx, supported on the reduced graphene oxide (rGO), as active catalysts for OER. The IrRuOx / rGO catalyst was characterized and evaluated in catalytic activity for OER in sulfuric acid solution.The IrRuOx / rGO catalyst was synthesized by hydrothermal method. Briefly, required amount of metal complexes, H2IrCl6 and RuCl3, and graphene oxide (GO) prepared by modified Hummers’ method were dispersed in ethanol/water mixture and the mixture was heated at 80˚C for 6 h. Then, the mixture was heated at 150˚C in hydrothermal autoclave for 4 h to form IrRuOx nanoparticles. The IrRuOx / rGO catalysts were characterized by transmission electron microscopy (TEM), energy dispersed X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods. The OER activity of the IrRuOx / rGO was examined in 0.5 M H2SO4 solution by linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) system. A TEM image of the Ir0.7Ru0.3Ox / rGO catalyst is shown in Figure 1. The Ir0.7Ru0.3Ox nanoparticles with wide particle size distribution were dispersed on the surface of rGO substrate. An average size of the nanoparticles was 1.9 ± 0.8 nm. The loading amount of the Ir0.7Ru0.3Ox catalyst is estimated to be approximately 23.9 wt% by EDX measurements. In addition, the EDX analysis reveals that the ratio of Ir to Ru of the IrRuOx nanoparticles is proportional to the ratio of starting materials, H2IrCl6 and RuCl3. The XRD patterns of the IrRuOx / rGO catalysts with different Ir/Ru ratio are shown in Figure 2. The XRD data shows the (200) peak of IrRuOx is shifted to small angle and became broader with increasing Ir ratio of the catalysts. Furthermore, the XPS results of the IrRuOx / rGO catalysts showed binding energy of Ir 4f peak was shifted to a lower energy state with increasing the ratio of Ru. The XRD and XPS data suggest that nanoparticles of the catalyst form alloy of RuO2 and IrO2. In addition, the shift of the binding energy of Ir 4f peak reflects the modification of the electronic state of Ir, which affects the catalytic activity for OER, by alloy formation with Ru atom. Figure 3 shows linear sweep voltammogram of the IrRuOx / rGO catalyst obtained in 0.5 M H2SO4 solution. Electrochemical measurement reveals activity of the IrRuOx / rGO catalyst for OER is higher than that of the IrO2 / rGO catalyst, especially, onset potential of the reaction is 68 mV lower than that on the IrO2 / rGO catalyst. High activity of the IrRuOx / rGO catalyst indicates that alloying IrO2 with RuO2 improves the catalytic activity for OER. In summary, we successfully prepared the alloy catalysts of IrO2 and RuO2 supported on rGO substrate, and the IrRuOx / rGO catalysts can be a promising candidate as anode for water electrolysis. Acknowledgements This work was supported by JSPS KAKENHI Grant number 17K05969.

Au/NiCo2O4 Arrays with High Activity for Water Oxidation

AbstractThe oxygen evolution reaction (OER) is a critical step in water splitting to produce hydrogen. In this report, Au/NiCo2O4 nanorod arrays with high activity for OER have been fabricated by a facile hydrothermal method followed by a reduction process. The Au/NiCo2O4 nanoarrays exhibited OER activity that was almost four times higher than that of Ir/C (at 1.75 V vs. RHE), and a small Tafel slope (63 mV decade−1). Moreover, this Au/NiCo2O4 hybrid‐array electrode showed good stability in alkaline solution, which is required of an active anode for water electrolysis.

Copper Delafossite Anode for Water Electrolysis

The design of highly active anode for water electrolysis is a critical issue. Here we report on copper delafossite oxide (ABO2; A = Cu) as a novel anode for the oxygen evolution reaction (OER). Among the copper delafossites we synthesized, copper rhodium delafossite (CuRhO2) showed the excellent cyclic voltammogram characteristic for OER in 1 M KOH (aq), comparable to that of Co3O4. We also confirmed that the eg orbital occupancy at the B site of delafossites, calculated from the bulk structure, had a good accordance with the catalytic activity of those materials. The CV characteristics of Cu delafossites were more accurately explained by considering the electronic conductivity.

Preparation of new titanium oxy nitride based electro catalysts using an anhydrous sol-gel method for water electrolysis in acid medium

Titanium oxy nitride has been fabricated for the first time from anhydrous Sol-gel Method and used as anode for water electrolysis in acid medium. It is shown that the properties of this material are highly dependent on the calcination temperature. The conductivity increases from insulator (titanium dioxide type) to metallic (titanium nitride type) when the calcination temperature rose from 500 °C to 1000 °C. When the catalyst material is calcinated at 700 °C, the rutile structure of titanium dioxide is obtained and agrees well with those obtained with the TGA-DSC results. These results also show that the structure of the osbornite of titanium nitride appears at 1000 °C. The BET surface area also increases with the calcination temperature. The morphology of the calcinated samples gradually breaks into smaller conchoidal fracture particles when the sample preparation temperature increases. Electrochemical voltage–current polarization curves of the oxygen evolution reaction in sulphuric acid on these anodes electrodes confirm that the materials calcinated at 800 °C or even at higher temperatures exhibit a low Tafel slope and a high electrochemistry surface area which are both known as good values for the oxygen evolution in acid medium. The results of this study suggest that our titanium oxy nitride prepared by an anhydrous sol-gel method followed by calcination is an interesting anode catalyst for water electrolysis. Its performance for this reaction is compared to those of some noble metal oxides.

Size-Dependent Activity of Co3O4 Nanoparticle Anodes for Alkaline Water Electrolysis

Cubic Co3O4 nanoparticles with average diameters of 5.9, 21.1, and 46.9 nm (hereafter small, medium, and large) have been synthesized and characterized by pXRD, TEM, and BET. The nanoparticles were loaded onto Ni foam supports for evaluation as anodes for water electrolysis in 1.0 M KOH. Current densities of 10 mA/cm2 were achieved at overpotentials of 328, 363, and 382 mV for anodes loaded with 1 mg/cm2 of small, medium, and large sized Co3O4 nanoparticles, respectively. The activity correlates with the BET surface area of the isolated particles. A plot of the electrochemical overpotential at 10 mA/cm2 against the log of the BET surface area gives a linear relation with a slope of −47 ± 7 mV/dec, showing unequivocally that the activity increase is a function of accessible catalyst surface area.

Determination of photo conversion efficiency of nanotubular titanium oxide photo-electrochemical cell for solar hydrogen generation

Anodized and annealed titanium oxide nanotubes show enhanced photo activity and can be used as photo anodes for water electrolysis in hydrogen generation. Application of an external potential to the photo anode is required for enhancement of the photocurrent. This additional electrical energy input complicates the photo conversion efficiency calculation. In this investigation, the photo-electrochemical behavior of anodized titanium oxide nanotubular arrays have been characterized in various electrolytes. Increase in the applied potential increased the photocurrent under illumination with visible light. A simple experimental method for calculating the photo conversion efficiency has been proposed. According to this method, the potential difference between the photo anode and cathode is measured with and without light illumination. The product of the photocurrent and the increase in potential due to light irradiation is considered as the net power output. The photocurrent and the conversion efficiency increased with increase in the pH of the electrolyte. TiO 2 nanotubular arrays annealed at 350 °C for 6 h in nitrogen atmosphere showed a maximum photo conversion efficiency of ∼4% in 1 M KOH electrolyte and ∼3% in 3.5 wt.% sodium chloride solution. The results indicate that nanotubular TiO 2 can be potentially used for the photo electrolysis of seawater to generate hydrogen.