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1. Electrochemical behavior of a Ni ₃ N OER precatalyst in Fe-purified alkaline media: the impact of self-oxidation and Fe incorporation

   https://doi.org/10.1039/D1MA00130B  

   Kawashima, Kenta; Márquez-Montes, Raúl A.; Li, Hao; Shin, Kihyun; Cao, Chi L.; Vo, Kobe M.; Son, Yoon Jun; Wygant, Bryan R.; Chunangad, Adithya; Youn, Duck Hyun; et al (April 2021, Materials Advances)

   null (Ed.)

   Nickel nitride (Ni 3 N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni 3 N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni 3 N self-oxidation and Fe impurity incorporation into Ni 3 N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni 3 N precatalyst layers on Ni foams (Ni 3 N/Ni foam) during extended periods of OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni 3 N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni 3 N precatalyst. Additionally, our Ni 3 N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cm geo −2 in a flow cell with unpurified electrolyte. 

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2. Electrochemical Oxide Growth on Nickel and Commercial Nickel Alloys: Influence of Chromium and Molybdenum

   https://doi.org/10.1149/1945-7111/acd7a6  

   Kretzmer, Isaac; Stuve, Eric M. (June 2023, Journal of The Electrochemical Society)

   Nickel-chromium-molybdenum (NiCrMo) alloys are well-known for having exceptional corrosion resistance, but their electrocatalytic properties have not been extensively studied. In this paper, the development of electro-active nickel-oxyhydroxide (NiOOH) phases and kinetics of the oxygen evolution reaction (OER) have been examined on alloys G35, B3, and C276 in alkaline electrolyte at 25 °C. Reproducible oxide layers were grown by potential cycling between 0.85 and 1.52 V vs RHE up to 600 cycles, and the transition between Ni(OH) 2 and NiOOH was monitored by cyclic voltammetry throughout. Onset potentials, Tafel slopes, and turnover frequencies (TOF) were measured at OER overpotentials between 270 and 390 mV. Alloys with dissimilar Cr:Mo ratios had significantly higher electrochemical surface area and increased γ -NiOOH formation, suggesting higher metal dissolution rates. The equal Cr:Mo concentration alloy and pure Ni developed a primarily β -NiOOH surface, and had 1.8–2.0 times larger TOF values than those containing significant γ -NiOOH. The NiCrMo alloys required smaller overpotentials (54–80 mV) to produce 10 mA cm −2 of OER current, and had comparable Tafel slopes to pure Ni. The findings here indicate a β -NiOOH-developed surface to be more OER-active than a γ -NiOOH-developed surface, and suggest certain NiCrMo alloys have promise as OER electrocatalysts. 

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3. Fe‐Ion Bolted VOPO ₄ ∙2H ₂ O as an Aqueous Fe‐Ion Battery Electrode

   https://doi.org/10.1002/adma.202105234  

   Xu, Yunkai; Wu, Xianyong; Sandstrom, Sean K.; Hong, Jessica J.; Jiang, Heng; Chen, Xin; Ji, Xiulei (October 2021, Advanced Materials)

   Abstract Iron ion batteries using Fe²⁺as a charge carrier have yet to be widely explored, and they lack high‐performing Fe²⁺hosting cathode materials to couple with the iron metal anode. Here, it is demonstrated that VOPO₄∙2H₂O can reversibly host Fe²⁺with a high specific capacity of 100 mAh g⁻¹and stable cycling performance, where 68% of the initial capacity is retained over 800 cycles. In sharp contrast, VOPO₄∙2H₂O's capacity of hosting Zn²⁺fades precipitously over tens of cycles. VOPO₄∙2H₂O stores Fe²⁺with a unique mechanism, where upon contacting the electrolyte by the VOPO₄∙2H₂O electrode, Fe²⁺ions from the electrolyte get oxidized to Fe³⁺ions that are inserted and trapped in the VOPO₄∙2H₂O structure in an electroless redox reaction. The trapped Fe³⁺ions, thus, bolt the layered structure of VOPO₄∙2H₂O, which prevents it from dissolution into the electrolyte during (de)insertion of Fe²⁺. The findings offer a new strategy to use a redox‐active ion charge carrier to stabilize the layered electrode materials. 

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4. Surface Interrogation of Electrodeposited MnO _x and CaMnO ₃ Perovskites by Scanning Electrochemical Microscopy: Probing Active Sites and Kinetics for the Oxygen Evolution Reaction

   https://doi.org/10.1002/anie.202008052  

   Jin, Zhaoyu; Bard, Allen J. (November 2020, Angewandte Chemie International Edition)

   Abstract Surface interrogation scanning electrochemical microscopy (SI‐SECM) of two electrodeposited manganese‐based electrocatalysts, amorphous MnO_xand perovskite CaMnO₃, was used to investigate the manganese oxidation state relating to the oxygen evolution reaction (OER) under neutral conditions. The results indicate the amounts of Mn^IIIand Mn^IVspecies in MnO_xand CaMnO₃depend on potential. A Mn^Vspecies was identified in both structures during the OER. Time‐delay titration of Mn^Vfurther revealed that MnO_xproduced two types of active sites with different OER reaction rates:k′_fast(MnO_x)=1.21 s⁻¹andk’_slow(MnO_x)=0.24 s⁻¹. In contrast, CaMnO₃perovskites in which the Mn^Vspecies formed at a less positive potential than that in MnO_x, displayed only one kinetic behavior with a faster reaction rate of 1.72 s⁻¹. 

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5. Tuning the intrinsic catalytic activities of oxygen-evolution catalysts by doping: a comprehensive review

   https://doi.org/10.1039/D1TA04032D  

   Ede, Sivasankara Rao; Luo, Zhiping (September 2021, Journal of Materials Chemistry A)

   null (Ed.)

   Electrochemical water splitting produces clean hydrogen fuel as one of the pivotal alternative energies to fossil fuels in the near future. However, the anodic oxygen evolution reaction (OER) is a significant bottleneck that curtails large-scale applications of electrochemical water splitting technology, owing to its sluggish reaction kinetics. In the past few decades, various methods have been proposed to improve the OER kinetics. Among them, doping is a simple and efficient method to mold the OER kinetics of a catalyst by incorporating different or hetero atoms into the host lattice. These efforts are vital to design highly efficient OER catalysts for real-world applications. However, the OER mechanism of a doped catalyst varies, depending on the host lattice and the dopant. This review highlights different doping strategies and associated OER mechanisms of state-of-the-art catalysts, including oxides (noble metal oxides, perovskite oxides, spinel oxides, hydroxides and others), non-oxides (metal sulfides, metal selenides, metal phosphides, metal nitrides and metal carbides), and carbon-based catalysts (graphene, carbon nanotubes and others). Fundamental understanding of the doping effects on the OER from combined experimental and theoretical research provides guidelines for designing efficient catalysts. 

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