Enhancement In Oxygen Evolution Reaction Activity Research Articles

FeCoNi Alloy as Noble Metal‐Free Electrocatalyst for Oxygen Evolution Reaction (OER)

AbstractThe development of efficient and durable noble metal‐free electrocatalyst for oxygen evolution reaction (OER) is highly desired in polymer electrolyte membrane (PEM) based water electrolysis and metal‐ air batteries but still remains a challenge. In this paper, we employ co‐precipitation method to prepare layered double hydroxides followed by calcination to synthesize nanoalloy catalyst, which was further characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM/EDS), high resolution transmission electron microscopy (HRTEM)and x‐ray photoelectron spectroscopy. As a novel OER electrocatalyst, the FeCoNi alloy catalyst shows remarkable OER activity enhancement with an overpotential of only 400 mV and a low Tafel slope 72 mV dec−1. This work offers a unique paradigm for the synthesis of competitive electrocatalyst for OER with excellent durability under alkaline condition.

OER activity manipulated by IrO6 coordination geometry: an insight from pyrochlore iridates.

The anodic reaction of oxygen evolution reaction (OER), an important point for electrolysis, however, remains the obstacle due to its complicated reaction at electrochemical interfaces. Iridium oxide (IrO2) is the only currently known 5d transition metal oxide possessing admirable OER activity. Tremendous efforts have been carried out to enhance the activity of iridium oxides. Unfortunately there lies a gap in understanding what factors responsible for the activity in doped IrO2 or the novel crystal structure. Based on two metallic pyrochlores (Bi2Ir2O7 and Pb2Ir2O6.5) and IrO2. It has been found that there exists a strong correlation between the specific OER activity and IrO6 coordination geometry. The more distortion in IrO6 geometry ascends the activity of Ir sites, and generates activity order of Pb-Ir > IrO2 > Bi-Ir. Our characterizations reveal that distorted IrO6 in Pb-Ir induces a disappearance of J = 1/2 subbands in valence band, while Bi-Ir and IrO2 resist this nature probe. The performed DFT calculations indicated the distortion in IrO6 geometry can optimize binding strength between Ir-5d and O-2p due to broader d band width. Based on this insight, enhancement in OER activity is obtained by effects that change IrO6 octahedral geometry through doping or utilizing structural manipulation with nature of distorted octahedral coordination.

Lateral Size Effect on Oxygen Evolution Reaction Activity of IrO2 Nanosheet

Iridium dioxide (IrO2) is well known as an electrocatalyst with good activity for the oxygen evolution reaction (OER). We demonstrated earlier that IrO2 nanosheets, which are prepared via exfoliation of layered IrO2, exhibited similar performance to IrO2 nanoparticles (2-3 nm in diameter).1) In this study, we show that the OER activity of IrO2 nanosheet can be improved by down-sizing the lateral size of IrO2nanosheets. IrO2 nanosheet was derived via exfoliation of layered H0.3Ir0.93O2×nH2O through a process reported previously. The lateral size of IrO2 nanosheet was reduced by ultrasonic treatment for 30 min with a homogenizer. The working electrode was prepared by dropping the IrO2 nanosheet on a glassy carbon electrode. A thin layer of Nafion ionomer was cast on the electrode. All electrochemical measurements were conducted in 0.1 M HClO4 (25oC). The lateral size of as-prepared IrO2 nanosheet decreased from 258 nm to 111 nm by ultrasonic treatment. The electrochemically active surface area for both nanosheets was similar. The OER activity of IrO2 nanosheet with small lateral size was 881 A (g-IrO2)‒1, 1.9 times higher than that of as-prepared IrO2 nanosheet (451 A (g-IrO2)‒1). The edge sites of small lateral nanosheet size were 1.7 times higher than that of as-prepared nanosheets, corresponding well the enhancement in OER activity. The result suggests that the OER activity of IrO2nanosheet was enhanced by down-sizing the lateral size of nanosheet, which increased the amount of edge sites. 1) W. Shimizu, T. Ishida, S. Miyasaka, Y. Ayato, and W. Sugimoto, 227th ECS meeting, Chicago, IL (2015).

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting.

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

Sol-flame synthesis of cobalt-doped TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction.

Doping nanowires (NWs) is of crucial importance for a range of applications due to the unique properties arising from both impurities' incorporation and nanoscale dimensions. However, existing doping methods face the challenge of simultaneous control over the morphology, crystallinity, dopant distribution and concentration at the nanometer scale. Here, we present a controllable and reliable method, which combines versatile solution phase chemistry and rapid flame annealing process (sol-flame), to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. Characterizations of the TiO2:Co NWs show that Co dopants exhibit 2+ oxidation state and substitutionally occupy Ti sites in the TiO2 lattice. The Co dopant concentration significantly affects the oxygen evolution reaction (OER) activity of TiO2:Co NWs, and the TiO2:Co NWs with 12 at% of Co on the surface show the highest OER activity with a 0.76 V reduction of the overpotential with respect to undoped TiO2 NWs. This enhancement of OER activity for TiO2:Co NWs is attributed to both improved surface charge transfer kinetics and increased bulk conductivity.

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.