Electrodeposited Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting

One of the challenges to realize large-scale water splitting is the lack of active and low-cost electrocatalysts for its two half reactions: H2 and O2 evolution reactions (HER and OER). Herein, we report that cobalt-phosphorous-derived films (Co-P) can act as bifunctional catalysts for overall water splitting. The as-prepared Co-P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mAcm¢2 at overpotentials of ¢94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100% Faradaic efficiency, rivalling the integrated performance of Pt and IrO2. The major composition of the asprepared and post-HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER

Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles

Amorphous tungsten phosphide (WP), which has been synthesized as colloidal nanoparticles with an average diameter of 3 nm, has been identified as a new electrocatalyst for the hydrogen-evolution reaction (HER) in acidic aqueous solutions. WP/Ti electrodes produced current densities of −10 mA cm^(−2) and −20 mA cm^(−2) at overpotentials of only −120 mV and −140 mV, respectively, in 0.50 M H_2SO_4(aq)

Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction

Nanoparticles of nickel phosphide (Ni_2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni_2P nanoparticles were hollow and faceted to expose a high density of the Ni_2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H_2(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media

Nanocrystalline Mo2C as a Bifunctional Water Splitting Electrocatalyst

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Mo2C is a well-known low cost catalyst for the hydrogen evolution reaction (HER), but the other water splitting half reaction, the oxygen evolution reaction (OER), has not been previously reported. To investigate both reactions and the origin of the catalytic sites, four synthesis methods were employed to prepare hexagonal Fe2N type Mo2C. A comparison of the HER activities in acidic and alkaline electrolyte and OER activities in alkaline electrolyte revealed that changes in synthesis route leads to morphological and surface composition variations resulting in different catalytic activities. In general, the trend in HER and OER activities show remarkably similar trends across the carbides synthesized via different routes irrespective of either electrolyte employed or reaction probed for electrocatalytic activities. Mo2C templated on multiwalled carbon nanotubes demonstrated the highest bifunctional catalytic activities, as well as superior electrochemical stability for both HER and OER. The writing's on the (multi)wall: Molybdenum carbide templated on multiwalled carbon nanotube is an excellent bifunctional electrocatalyst for HER catalyst in acid and base, and OER in base

Bi-Functional Iron-Only Electrodes for Efficient Water Splitting with Enhanced Stability through in Situ Electrochemical Regeneration

Scalable and robust electrocatalysts are required for the implementation of water splitting technologies as a globally applicable means of producing affordable renewable hydrogen. We demonstrate herein that iron-only electrode materials prove to be active for catalyzing both proton reduction and water oxidation in alkaline electrolyte solution with superior activity to that of previously established bi-functional catalysts containing less abundant elements. The reported bi-functionality of the iron electrodes is reversible upon switching of the applied bias through electrochemical interconversion of catalytic species at the electrode surface. Cycling of the applied bias results in in-situ electrochemical regeneration of the catalytic surfaces and thereby extends the catalyst stability and lifetime of the water electrolyzer. Full water splitting at a current density of I = 10 mA cm⁻² is achieved at a bias of approximately 2 V which is stable over at least 3 days (72 one hour switching cycles). Thus, potential-switching is established as a possible strategy of stabilizing electrode materials against degradation in symmetrical water splitting systems.The author’s thank the Oppenheimer Fund (University of Cambridge), the EPSRC (Grant EP/H00338X/2), the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy and National Foundation for Research, Technology and Development) and OMV Group for financial support. We also thank the National EPSRC XPS User’s Service (NEXUS) at Newcastle University, UK, where XPS spectra were obtained. Dr Chia-Yu Lin is acknowledged for his invaluable help in initial experiments.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/aenm.20150209

Stoichiometric Control of Electrocatalytic Amorphous Nickel Phosphide to Increase Hydrogen Evolution Reaction Activity and Stability in Acidic Medium

This work describes the electrocatalysis of amorphous nickel phosphide (Niâ P) electrodeposited onto copper metal foil, for its use as a nonâ noble metal catalyst for the hydrogen evolution reaction (HER) in 0.5 M H2SO4. Although electrodeposition offers many advantages over conventional high temperature and high pressure fabrication techniques, there are very few reports on the preparation of Niâ P electrocatalysts via electrodeposition. This Niâ P electrocatalyst exhibits good activity in acidic medium, with a potential of â 222 mV to achieve 10 mA cmâ 2 cathodic current density. This potential is comparable to that of electrodeposited Pt black (â 104 mV), and much better than that of electrodeposited Ni (â 480 mV). An unusual longâ term stability in acidic medium was demonstrated by the â 222 mV potential remaining constant after 5000 cyclic voltammetric sweeps in 0.5 M H2SO4. Importantly, the stoichiometry of the nickel phosphide films can be easily varied from an atomic % of phosphorus from 15 % to as high as 24 % by modifications to the electrodeposition conditions. Such a high phosphorous loading is greater than is generally reported with electrodeposited Niâ P materials. In addition, we observed Niâ P films electrodeposited at lower temperatures (â ¼ 3 °C) result in higher phosphorous loading, which gives rise to enhanced stability as well as activity. Electrodeposited amorphous Niâ P can therefore be used as an active, stable and Earthâ abundant metal catalyst for the HER in acidic electrolytes.Long live Niâ P: Even after 5000 cyclic voltammetric sweeps, electrodeposited nickel phosphide shows high activity and stability towards hydrogen evolution reaction (HER) in acidic electrolyte. Modifications to the deposition conditions could easily elevate the phosphorus loading on the film up to circa 24 at%, which provided enhanced activity.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138271/1/slct201701755_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138271/2/slct201701755.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138271/3/slct201701755-sup-0001-misc_information.pd

Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes

© 2015 The Authors. The water electrolysis for hydrogen production is constrained by the thermodynamically unfavorable oxygen evolution reaction (OER), which requires input of a large amount of energy to drive the reaction. One of the key challenges to increase the efficiency of the water electrolysis system is to develop highly effective and robust electrocatalysts for the OER. In the past 20-30 years, significant progresses have been made in the development of efficient electrocatalysts, including metal oxides, metal oxide-carbon nanotubes (CNTs) hybrid and metal-free CNTs based materials for the OER. In this critical review, the overall progress of metal oxides catalysts and the role of CNTs in the development of OER catalyst are summarized, and the latest development of new metal free CNTs-based OER catalyst is discussed

Vapor–solid synthesis of monolithic single-crystalline CoP nanowire electrodes for efficient and robust water electrolysis

Electrochemical water splitting into hydrogen and oxygen is a promising technology for sustainable energy storage. The development of earth-abundant transition metal phosphides (TMPs) to catalyze the hydrogen evolution reaction (HER) and TMP-derived oxy-hydroxides to catalyze the oxygen evolution reaction (OER) has recently drawn considerable attention. However, most monolithically integrated metal phosphide electrodes are prepared by laborious multi-step methods and their operational stability at high current densities has been rarely studied. Herein, we report a novel vapor–solid synthesis of single-crystalline cobalt phosphide nanowires (CoP NWs) on a porous Co foam and demonstrate their use in overall water splitting. The CoP NWs grown on the entire surface of the porous Co foam ligaments have a large aspect ratio, and hence are able to provide a large catalytically accessible surface over a given geometrical area. Comprehensive investigation shows that under the OER conditions CoP NWs are progressively and conformally converted to CoOOH through electrochemical in situ oxidation/ dephosphorization; the latter serving as an active species to catalyze the OER. The in situ oxidized electrode shows exceptional electrocatalytic performance for the OER in 1.0 M KOH, delivering 100 mA cm-2 at an overpotential () of merely 300 mV and a small Tafel slope of 78 mV dec1 as well as excellent stability at various current densities. Meanwhile, the CoP NW electrode exhibits superior catalytic activity for the HER in the same electrolyte, affording 100 mA cm-2 at = 244 mV and showing outstanding stability. An alkaline electrolyzer composed of two symmetrical CoP NW electrodes can deliver 10 and 100 mA cm-2 at low cell voltages of 1.56 and 1.78 V, respectively. The CoP NW electrolyzer demonstrates exceptional long-term stability for overall water splitting, capable of working at 20 and 100 mA cm-2 for 1000 h without obvious degradation.L. F. Liu acknowledges the financial support of a FCT Investigator grant (No. IF/01595/2014) and exploratory grant (No. IF/ 01595/2014/CP1247/CT0001) awarded by the Portuguese Foundation of Science & Technology (FCT). W. Li and D. H. Xiong are thankful for the support of the Marie Skłodowska-Curie Action COFUND programme (NanoTrainforGrowth, grant No. 600375) for postdoctoral fellowships. This work was also partly financed by the European Commission Horizon 2020 project “CritCat” (Grant No. 686053).info:eu-repo/semantics/publishedVersio