Showing papers by "Prashanth W. Menezes published in 2017"

Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting

Abstract: A systematic structural elucidation of the near-surface active species of the two remarkably active nickel phosphides Ni12P5 and Ni2P on the basis of extensive analytical, microscopic, and spectroscopic investigations is reported. The latter can serve as complementary efficient electrocatalysts in the hydrogen (HER) versus oxygen evolution reaction (OER) in alkaline media. In the OER Ni12P5 shows enhanced performance over Ni2P due to the higher concentration of nickel in this phase, which enables the formation of an amorphous NiOOH/Ni(OH)2 shell on a modified multiphase with a disordered phosphide/phosphite core. The situation is completely reversed in the HER, where Ni2P displayed a significant improvement in electrocatalytic activity over Ni12P5 owing to a larger concentration of phosphide/phosphate species in the shell. Moreover, the efficiently combined use of the two nickel phosphide phases deposited on nickel foam in overall electrocatalytic water splitting is demonstrated by a strikingly low cell v...

Boosting Visible-Light-Driven Photocatalytic Hydrogen Evolution with an Integrated Nickel Phosphide–Carbon Nitride System

Abstract: Solar light harvesting by photocatalytic H2 evolution from water could solve the problem of greenhouse gas emission from fossil fuels with alternative clean energy. However, the development of more efficient and robust catalytic systems remains a great challenge for the technological use on a large scale. Here we report the synthesis of a sol–gel prepared mesoporous graphitic carbon nitride (sg-CN) combined with nickel phosphide (Ni2P) which acts as a superior co-catalyst for efficient photocatalytic H2 evolution by visible light. This integrated system shows a much higher catalytic activity than the physical mixture of Ni2P and sg-CN or metallic nickel on sg-CN under similar conditions. Time-resolved photoluminescence and electron paramagnetic resonance (EPR) spectroscopic studies revealed that the enhanced carrier transfer at the Ni2P–sg-CN heterojunction is the prime source for improved activity.

From a Molecular 2Fe-2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting.

Abstract: A highly active FeSe2 electrocatalyst for durable overall water splitting was prepared from a molecular 2Fe-2Se precursor. The as-synthesized FeSe2 was electrophoretically deposited on nickel foam and applied to the oxygen and hydrogen evolution reactions (OER and HER, respectively) in alkaline media. When used as an oxygen-evolution electrode, a low 245 mV overpotential was achieved at a current density of 10 mA cm-2 , representing outstanding catalytic activity and stability because of Fe(OH)2 /FeOOH active sites formed at the surface of FeSe2 . Remarkably, the system is also favorable for the HER. Moreover, an overall water-splitting setup was fabricated using a two-electrode cell, which displayed a low cell voltage and high stability. In summary, the first iron selenide material is reported that can be used as a bifunctional electrocatalyst for the OER and HER, as well as overall water splitting.

A facile corrosion approach to the synthesis of highly active CoOx water oxidation catalysts

Abstract: Ultra-small rock salt cobalt monoxide (CoO) nanoparticles were synthesized and subjected to partial oxidation (‘corrosion’) with ceric ammonium nitrate (CAN) to form mixed-valence CoOx (1 < x < 2) water oxidation catalysts. Spectroscopic, microscopic and analytical methods evidenced a structural reformation of cubic CoO to active CoOx with a spinel structure. The superior water oxidation activity of CoOx has been established in electrochemical water oxidation under alkaline conditions. Electrochemical water oxidation with CoOx was recorded at a considerably low overpotential of merely 325 mV at a current density of 10 mA cm−2 in comparison to 370 mV for CoO. Transformation of both octahedral CoII and CoIII sites into amorphous Co(OH)2–CoOOH is the key to high electrochemical activity while the presence of a higher amount of octahedral CoIII sites in CoOx is imperative for an efficient oxygen evolution process.

Active and Stable Nickel‐Based Electrocatalysts Based on the ZnO:Ni System for Water Oxidation in Alkaline Media

Abstract: The synthesis of monodisperse, surfactant-free, nickel-substituted ZnO nanocrystallites (ZnO:Ni) by the mild solvolysis of heterobimetallic Ni4-xZnxO4 cubane-like precursors (x = 1-3) in benzylamine is reported which were grafted by electrophoretic deposition onto fluorine-doped tin oxide covered glass substrates and used as superior active and long-term stable working electrodes for water oxidation. Upon applying a voltage at the electrodes, the ZnO:Ni precatalyst leads to a superiorly active composite material which can durably oxidize water (>15 h) with increasing catalytic current. In contrast, the performance of homometallic NiO references decreases rapidly over time, and is clearly surpassed by the composite from the ZnO:Ni precatalyst in terms of both stability and activity. Extensive characterization of the as-prepared and activated ZnO:Ni precatalyst by hard x-ray photoelectron spectroscopy revealed that the excellent performance of the electrode material is due to the formation of a unique self-supported turbostratically disordered mixture of γ-NiOOH/α-Ni(OH)2-like phases from rapid dissolution of ZnII in the ZnO:Ni precatalyst into the electrolyte during activation.