Oxygen Evolution Reaction In Water Electrolysis Research Articles

Use of Suspended Particles as a New Approach to Increase the Active Electrode Area in Water Electrolysis Experiments

AbstractThe development of base metal electrodes that can act as active and stable oxygen generating electrodes in water electrolysis systems, especially at low pH levels, remains a challenge. The use of suspensions as electrolytes for water splitting has until recently been limited to photoelectrocatalytic approaches. A high current density (j=30 mA/cm2) for water electrolysis has been achieved at a very low oxygen evolution reaction (OER) potential (E=1.36 V vs. RHE) using a SnO2/H2SO4 suspension–based electrolyte in combination with a steel anode. More importantly, the high charge–to–oxygen conversion rate (Faraday efficiency of 88 % for OER at j=10 mA/cm2 current density). Since cyclic voltammetry (CV) experiments show that oxygen evolution starts at a low, but not exceptionally low, potential, the reason for the low potential in chronoamperometry (CP) tests is an increase in the active electrode area, which has been confirmed by various experiments. For the first time, the addition of a relatively small amount of solids to a clear electrolyte has been shown to significantly reduce the overpotential of the OER in water electrolysis down to the 100 mV region, resulting in a remarkable reduction in anode wear while maintaining a high current density.

Ligand-mediated electrocatalytic activity of Cu-imidazolate coordination polymers for OER in water electrolysis

Copper is an Earth-abundant element promising for the synthesis of robust and efficient oxygen evolution reaction (OER) electrocatalysts. In this study, three Cu-imidazolate coordination polymers (Cu-Im, Cu-mIm, and Cu-bIm) are electrochemically synthesized and examined as OER catalysts whose activity can be modulated by the coordinating ligands of imidazole (Im), 2-methylimidazole (mIm), and benzimidazole (bIm). During the water electrolysis, Cu-Im and Cu-mIm are partially oxidized to form CuO particles, a well-known OER catalyst, while Cu-bIm does not. Due to the in situ formation of CuO during the electrolysis and weaker hydrophobicity of Im ligand, Cu-Im shows the highest electrocatalytic activity with a modest overpotential of 500 mV delivering a current density of 10 mA cm−2 and a Tafel slope of 215 mV dec−1 in 1 M KOH. On the contrary, the hydrophobic and anti-corrosion property of the bIm ligand stabilizes the Cu-bIm during the electrolysis without CuO formation. The hydrophobicity of the coordinating ligand influenced the electrocatalytic activity, resulting in the order of Cu-Im > Cu-mIm > Cu-bIm. Post-reaction analyses of the Cu-imidazolate coordination polymers reveal variations in the oxidation state of Cu, which has not been explored in previous studies. This work underlines that the ligand properties can regulate the catalytic activity of metal-ligand coordination polymer.

Robust and highly efficient electrocatalyst based on ZIF-67 and Ni2+ dimers for oxygen evolution reaction: In situ mechanistic insight

Electrochemical water splitting is a straightforward process that involves two distinct reactions: the oxygen evolution reaction (OER) which produces oxygen (O2) and the hydrogen evolution reaction (HER) which generates hydrogen (H2). However, in the whole process, the OER is a bottleneck as it requires more energy than a four-electron reaction involving critical raw materials (such as RuO2 or IrO2) as electrocatalysts. Therefore, here, we address the challenge of erratic kinetics/limited durability of OER in water electrolysis. In this paper, we demonstrate that the deposition of ultrasmall amounts of nickel(II) nitrate in zeolitic imidazolate framework-67 (ZIF-67) can be used as a general approach to enhance the electrocatalytic performance of the framework. We investigated the influence of Ni(NO3)2·x6H2O loading on ZIF-67 (from 0.1 to 0.0001 M) and found that ZIF-67 enriched with only 0.001 M of Ni(NO3)2·x6H2O (ZIF-67 0.001Ni) exhibited massive promotion in OER. The ZIF-67 0.001Ni showed a large specific surface area of 2577 m2 g−1, a low overpotential of 299 mV, a lower Tafel slope of 94.1 mA dec−1, and an outstanding overpotential retention of 99.8% (at 50 mA cm−2). By conducting electron paramagnetic resonance (EPR) measurements, we also discovered that the 0.001 M of Ni(NO3)2·x6H2O loading in ZIF-67 introduces Ni2+ dimers, which contribute to the enhanced electroactivity of the modified ZIF-67. This phenomenon was further revealed during density-functional theory (DFT) calculations, which allowed us to identify different possible forms of Ni2+ dimers and modeling of active centers. Along with in situ experiments, we provide mechanistic insight into the OER mechanism under alkaline conditions and found that it follows the lattice oxygen mechanism (LOM). Our study proposes a facile and efficient room-temperature route to boost the electrochemical performance of ZIF-67 in OER. For the first time, we demonstrate that modifying ZIF-67 with an ultrasmall amount of different nickel(II) salts opens a general route to enhance its electroactivity during water-splitting reactions.

Polymeric cobalt phthalocyanine on nickel foam as an efficient electrocatalyst for oxygen evolution reaction

Renewable energy is of prime importance for sustainable development. Hydrogen is considered as clean as well as an efficient energy source. In addition to H2 as fuel, polymer electrolyte membrane fuel cell requires oxygen as an oxidant. Electrocatalytic or photocatalytic water splitting processes yield H2 and O2. Compared to hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) in water electrolysis is a very sluggish and slow process. Even precious and costly catalysts have demonstrated higher overpotential for OER. To overcome the issues related to overpotential as well as cost and durability, bio-inspired macrocycles and polymers based on phthalocyanine are being explored extensively because of their unique and versatile properties. In this effort, a novel cobalt phthalocyanine polymer (poly-CoTPzPyCPc) was synthesised and employed as a low-cost and efficient organic-based catalyst for OER. The characterized polymeric catalyst was coated on Ni foam and the fabricated electrodes were evaluated for OER in 0.1 M KOH. The electrodes Ni-foam, Ni/poly-CoTPzPyCPc, Ni/IrO2, and Ni/poly-CoTPzPyCPc + IrO2 exhibited an onset potential for OER at 1.57, 1.53, 1.47 and 1.43 V Vs. RHE, respectively. The Ni/poly-CoTPzPyCPc exhibited an overpotential of 365 mV (±2 mV) at a current density of 10 mA cm−2. Furthermore, the composite of poly-CoTPzPyCPc with IrO2, displayed surprisingly superior OER activity. The composite achieved a current density of 10 mA cm−2 at lower overpotential of 274 mV (±2 mV) than the polymeric phthalocyanine and benchmark IrO2 catalyst which showed an overpotential of 324 mV at the same current density. The Tafel slope value was found to be 89, 67, 34 and 31 mV/dec for pure Ni foam, Ni/poly-CoTPzPyCPc, Ni/IrO2 and Ni/poly-CoTPzPyCPc + IrO2, respectively. The stability studies performed on poly-CoTPzPyCPc + IrO2 composite catalyst revealed that the proposed catalyst is stable and can be successfully employed for OER in water electrolysis. The developed composite catalyst has a small amount loading of precious catalyst with simple organic based catalyst to achieve efficiency better than benchmark catalyst for OER.

Metal‐organic frameworks‐derived novel nanostructured electrocatalysts for oxygen evolution reaction

AbstractEngineering cost‐effective catalysts with exceptional performance for the electrochemical oxygen evolution reaction (OER) remains crucial for the accelerated development of renewable energy techniques, and especially so, given the pivotal role of OER in water electrolysis. On the basis of the metal nodes (clusters) and organic linkers, metal‐organic frameworks (MOFs) and their derivatives are rapidly gaining ground in the fabrication of electrocatalysts, with promising catalytic activity and sound durability in OER, thanks to their controllable pore structures, abundant unsaturated active sites of metal ion, extensive specific surface area, as well as easily functionalized/modified surfaces. This review presents an in‐depth understanding of the established progress of MOFs‐derived materials for OER electrocatalysis. The material designing strategies of the pristine, monometallic, and multimetallic MOFs‐based catalysts are summarized to indicate the infinite possibilities of the morphology and the composition of MOF‐derived materials. While emphasis is laid on the essential features of MOF‐derived materials for the electrocatalysis of the corresponding reactions, insights about the applications in OER are discussed. Finally, this paper is concluded by presenting challenges and perspectives for MOF‐derived materials’ future applications in OER electrocatalysis.

Synthesis and electrocatalytic properties of LaFe1-xZnxO3 perovskites

In this work, perovskite-type oxides of general formula LaFe1-xZnxO3 (0 ≤ x ≤ 0.3) prepared by a sol–gel route were investigated as electrocatalysts for the oxygen evolution reaction (OER) in alkaline KOH solutions. X-ray diffraction analysis of samples indicates that the pure cubic structure was obtained for composition lower than x = 0.2. The OER studies indicate that substitution of iron by zinc increases the electrocatalytic activity of the resulting material significantly. The highest activity was achieved for x = 0.1, whereas the obtained current density was 9.02 mA cm−2 at 0.66 V, which is approximately three times higher than that of the base oxide. The Tafel slopes values for OER on each oxide in 1 M KOH are found to be ~89, 52, and 64 mV dec−1 for LaFeO3, LaFe0.9Zn0.1O3, and LaFe0.8Zn0.2O3, respectively. The stability of LaFe0.9Zn0.1O3 electrode is studied in the process of 1000 successive cycles at a current density of 10 mA.cm−2 of the OER. A small change in overpotential was found, ranged between 11 and 19 mV, indicating clearly its long electrochemical durability. These results suggest clearly that LaFe0.9Zn0.1O3 electrode is a promising anode material for the OER in water electrolysis.

Finely prepared and optimized Co/Fe double hydroxide nanofilms at an ionic layer level on rough Cu substrates for efficient oxygen evolution reaction

Co/Fe double hydroxides are effective non-precious metal semiconductor compound electrocatalysts for oxygen evolution reaction (OER) in water electrolysis. However, controlled loading of them on rough and conducting substrates is crucial due to their low conductivity. In this work, (CoFe)(OH)x nanofilms loaded on coral-like CuO/Cu (CuO/c-Cu) microarray substrates have been prepared in a controllable procedure at an ionic layer level for the first time by successive ionic layer adsorption and reaction (SILAR). The (CoFe)(OH)x nanofilm is finely prepared by adjusting the SILAR cycles and the Co/Fe ratio in the precursor solution. The optimized (CoFe)(OH)x electrocatalyst demonstrates excellent OER performance with a very low overpotential of 275 mV at 10 mA cm−2 (with no iR compensation), a small Tafel slope (34 mV dec−1 with iR compensation), and a long-term durability for at least 10 h in 1 M KOH. We also optimize nanofilms of (CoNi)(OH)x and (CoNi)Sx on CuO/c-Cu by SILAR and achieve satisfactory OER performances. Our results display that fine preparation and optimization of semiconductor nanofilms at an ionic layer level provide a promising strategy on fabrication of high-performance electrocatalysts for OER in water electrolysis.

Synthesis of a Highly Efficient Oxygen-Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides.

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co0.7 Fe0.3 )2 B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm-2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO2 and RuO2 . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)2 are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co1-x Fex )2 B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.

Carbon-Free Connected Ru, Ir Based Nanoparticle Catalysts for Polymer-Electrolyte Water Electrolysis

Oxygen evolution reaction (OER) catalysts for polymer-electrolyte water electrolysis (PEWE) suffered from their low surface area because carbon support cannot be used for OER operating at high potential. Connecting metal nanoparticles enable catalysts to conduct electrons through nanoparticle networks without any support materials while keeping their high surface area. Thus, connected nanoparticle catalysts (Fig. 1 (A)) are promising for OER. Previously, we have developed connected Pt-Fe nanoparticle catalysts with hollow capsule structure for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).[1] In this study, we proposed Ru and Ir nanoparticle catalysts for OER in water electrolysis because Ru and Ir have high OER activity. Ru and Ir nanoparticle catalysts were synthesized as follows. First, Ru and Ir nanoparticles were synthesized on silica template (PDDA/SiO2) via polyol method using tetraethylene glycol as reducing agent and Ru(III) acetylacetonate or Ir(III) acetylacetonate as the metallic precursors (M/PDDA/SiO2, M is Ru or Ir). Then, these nanoparticles were treated in supercritical ethanol at 330ºC for 90 min (Ru) or 60 min (Ir) (sc-M/SiO2). Dissolution of SiO2 in 3 M NaOH solution at 85ºC for 3 h made porous hollow structure (M capsule). OER performance were evaluated by electrochemical measurement in 0.1 M HClO4 solution. Fig. 1 (B) shows TEM image of Ir capsule. Capsule and networks structure of each catalysts were observed by TEM images. All Ru catalysts inactivated for OER at the first CV cycle. OER activity of Ru/PDDA/SiO2 did not improve by heat oxidation treatment in air at 400ºC. Fig. 1 (C) shows OER curves of Ir catalysts. All Ir catalysts showed stable OER. Among them, Ir/PDDA/SiO2 had the highest OER activity. Mass activity at 1.48 V of Ir/PDDA/SiO2 was higher than RuO2 [2], IrO2 [2] and Ir/C­[3] in previous reports as shown in Fig.1 (D). Heat treatment of Ir/PDDA/SiO2 at 200ºC further improved. In conclusion, we successfully synthesized connected Ru and Ir nanoparticle catalysts. Among them, Ir/PDDA/SiO2 showed the highest OER activity and the activity was further improved by heat oxidation treatment. The results showed that connected Ir nanoparticle catalysts are promising as carbon-free catalysts for water electrolysis. [1] T. Tamaki, H. Kuroki, T. Yamaguchi et al., Energy Environ. Sci., 8, 3545-3549 (2015). [2] Y. Lee et al., J. Phys. Chem. Lett., 3, 399-404 (2012). [3] HN. Nong et al., Chem. Sci., 5, 2955-2963 (2014). Figure 1

Three-dimensional ordered macroporous IrO2 as electrocatalyst for oxygen evolution reaction in acidic medium

Three-dimensional ordered macroporous (3-DOM) IrO2 was synthesized by using the silica colloidal crystal template method and explored as electrocatalyst for oxygen evolution reaction (OER) in acidic medium. X-Ray diffraction (XRD) patterns indicate that the prepared 3-DOM IrO2 has a rutile structure. Images of scanning and transmission electron microscopies suggest that the 3-DOM IrO2 possesses a hierarchical pore structure, in which the honeycomb array of 300 nm primary macropores were cross-linked by secondary mesopores on the walls. The presence of mesopores is also indicated by the N2 adsorption/desorption isotherms and the small-angle XRD patterns. As compared with IrO2 prepared by conventional colloidal method, the 3-DOM IrO2 exhibited much larger BET area and voltammetric charges. Accordingly, about two and half times enhancement in OER activity was achieved by using 3-DOM IrO2 as the electrode materials, showing prospect of 3-DOM materials in reducing the demand of the expensive IrO2 electrocatalyst for OER in water electrolysis.