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Electrocatalytic reduction of dinitrogen (N2) to ammonium (NH4+) in acidic aqueous solutions was investigated at ambient temperature and pressure using a cobalt–molybdenum (CoMo) thin-film electrode prepared by magnetron reactive sputtering. Increased concentrations of ammonium ions (NH4+) were consistently detected in the electrolyte using ion chromatography (IC) after constant-potential electrolysis at various potentials (≤−0.29 V vs. RHE). Using a newly developed analytical method based on ammonia derivatization, performing the experiments with 15N2-labelled gas led however to the detection of increased 14NH4+ concentrations instead of 15NH4+. X-ray photoelectron spectroscopic (XPS) analysis of the electrode surface revealed the presence of Mo[triple bond, length as m-dash]N and Mo–NHx species. Several contamination sources were identified that led to substantial increases in the concentration of ammonium ions, including 15NH3 impurities in 15N2 gas. The observed ammonium concentrations can be consistently ascribed to leaching of nitrogen (14N) impurities incorporated in the CoMo film during the sputtering process. Researchers in the field are therefore urged to adopt extended protocols to identify and eliminate sources of ammonia contamination and to very carefully monitor the ammonium concentrations in each experimental step.\ud \ud

The stability of p-InP photocathodes performing the hydrogen-evolution reaction (HER) has been evaluated in contact with either 1.0 M H₂SO₄ (aq) or 1.0 M KOH(aq), with a focus on identifying corrosion mechanisms. Stability for the solar-driven HER was evaluated using p-InP electrodes that were either etched or coated with an electrodeposited Pt catalyst (p-InP/Pt). Variables such as trace O₂ were systematically controlled during the measurements. Changes in surface characteristics after exposure to electrochemical conditions as well as electrode dissolution processes were monitored using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS). In either H₂SO₄ or KOH, etched p-InP photoelectrodes corroded cathodically under illumination, forming metallic In⁰ at the electrode surface. In contrast, electrodeposition of Pt kinetically stabilized illuminated p-InP photocathodes in both H₂SO₄ and KOH by inhibiting the cathodic corrosion pathway. Notably, when held at 0 V vs. the reversible hydrogen electrode (RHE) in 1.0 M H₂SO₄ (aq), p-InP/Pt exhibited a stable current density (J) of ∼−18 mA cm⁻² for >285 h under simulated 1 Sun illumination. The long-term current density vs. potential (J–E) behavior at pH 0 and pH 14 of p-InP/Pt photocathodes correlated with changes in the surface chemistry as well as the dissolution of p-InP. In acidic media, the J–E behavior of p-InP/Pt photocathodes remained nearly constant with time, but the surface of a p-InP/Pt electrodes gradually turned P-rich via a slow and continuous leaching of In ions. In alkaline electrolyte, the surface of p-InP/Pt electrodes was passivated by formation of an InO_x layer that exhibited negligible dissolution but led to a substantial degradation in the J–E characteristics. Consequently, changes in the catalytic kinetics and surface stoichiometry are both important considerations for determining the corrosion chemistry and the long-term operational stability of InP photoelectrodes.

Vapor-fed electrolysis of water has been performed using membrane-electrode assemblies (MEAs) incorporating earth-abundant catalysts and bipolar membranes (BPMs). Catalyst films containing CoP nanoparticles, carbon black, and Nafion were synthesized, characterized, and integrated into cathodes of MEAs. The CoP-containing MEAs exhibited stable (>16 h) vapor-fed electrolysis of water at room temperature at a current density of 10 mA cm⁻² with 350 mV of additional overvoltage relative to MEA's formed from Pt/C cathodic electrocatalysts due to slower hydrogen-evolution reaction kinetics under vapor-fed conditions and fewer available triple-phase boundaries in the catalyst film. Additionally, catalyst films containing a [NiFe]-layered double hydroxide ([NiFe]-LDH) as well as a hydroxide ion conductor, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), were synthesized, characterized, and integrated into the anodes of the MEAs. The [NiFe]-LDH-containing MEAs exhibited overvoltages at 10 mA cm⁻² that were similar to those of IrO_x-containing MEAs for vapor-fed electrolysis of water at room temperature. A BPM was formed by pairing Nafion with HMT-PMBI, resulting in a locally alkaline environment of HMT-PMBI to stabilize the [NiFe]-LDH and a locally acidic environment to stabilize the CoP. BPM-based MEAs were stable (>16 h) for vapor-fed electrolysis of water at room temperature at a current density of 10 mA cm⁻², with a change in the pH gradient of 1 unit over 16 h of electrolysis for IrOx-containing MEAs. The stability of [NiFe]-LDH-based MEAs under vapor-fed conditions was dependent on the catalyst film morphology and resulting BPM interface, with stable operation at 10 mA cm⁻² achieved for 16 h. All MEAs exhibited a drift in the operating voltage over time associated with dehydration. These results demonstrate that earth-abundant catalysts and BPMs can be incorporated into stable, room-temperature, vapor-fed water-splitting cells operated at 10 mA cm⁻².

An ionic liquid (IL) solvent was used to synthesize small, phase-pure nickel phosphide (Ni2P) nanocrystals. In contrast, under analogous reaction conditions, substitution of the IL for the common high-boiling organic solvent 1-octadecene (ODE) results in phase-impure nanocrystals. The 5 nm Ni2P nanocrystals prepared in IL were electrocatalytically active toward the hydrogen evolution reaction. The synthesis in IL was also extended to alloyed Ni2–xCoxP nanocrystals, where 0.5 ≤ x ≤ 1.5.

Earth-abundant materials capable of catalyzing the electrochemical decomposition of water into molecular hydrogen and oxygen are necessary components of many affordable water-splitting technologies. However, water oxidation catalysts that facilitate sustained oxygen evolution at device-relevant current densities in strongly acidic electrolytes have been limited almost exclusively to precious metal oxides. Here, we show that nanostructured films of cobalt oxide (Co3O4) on fluorine-doped tin oxide (FTO) substrates, made by first depositing Co onto FTO and heating in air at 400 °C to produce films having a robust electrical and mechanical Co3O4/FTO interface, function as active electrocatalysts for the oxygen evolution reaction (OER) in 0.5 M H2SO4. The Co3O4/FTO electrodes evolve oxygen with near-quantitative Faradaic yields and maintain a current density of 10 mA/cm2 for over 12 h at a moderate overpotential of 570 mV. At lower current densities that require lower overpotentials, sustained oxygen productio...

Water oxidation is a required half-reaction for electrochemical water splitting. To date, the only well-established active oxygen-evolution catalysts stable under operating conditions and at rest in acidic aqueous media contain Ru or Ir, two of the scarcest non-radioactive elements on Earth. We report herein a nickel-manganese antimonate electrocatalyst with a rutile-type crystal structure that requires an initial voltammetric overpotential of 672 ± 9 mV to catalyze the oxidation of water to O2(g) at a rate corresponding to 10 mA cm−2 of current density when operated in contact with 1.0 M sulfuric acid. Under galvanostatic control, the overpotential initially rose from 670 mV but was then stable at 735 ± 10 mV for 168 h of continuous operation at 10 mA cm−2. We additionally provide an in-depth evaluation of the stability of the nickel-manganese antimonate electrocatalyst, including elemental characterization of the surface, bulk, and electrolyte before and after electrochemical operation.

Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides ...

Molybdenum ditelluride, MoTe2 , is emerging as an important transition-metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1T ' polymorph of MoTe2 is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H-MoTe2 is more stable and therefore more accessible synthetically. Metastable 1T '-MoTe2 forms directly in solution at 300 °C as uniform colloidal nanostructures that consist of few-layer nanosheets, which appear to exhibit an approx. 1 % lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1T ' phase in the MoTe2 nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1T '-MoTe2 nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions.

CuCo2S4 is an important mixed-metal spinel-type sulfide that is typically synthesized using high-temperature solid-state reactions, which produce agglomerated particles with low surface areas that are not optimal for applications such as heterogeneous catalysis. Here, we show that highly crystalline and nonagglomerated colloidal CuCo2S4 nanoparticles can be synthesized in solution at 200 °C, which is significantly lower than previously reported methods. The CuCo2S4 nanoparticles were found to be highly active electrocatalysts for the oxygen evolution reaction (OER) under strongly alkaline conditions (1.0 M KOH, pH 14), requiring an OER overpotential of 395 mV to produce a current density of 10 mA cm(-2). X-ray photoelectron spectroscopy (XPS) studies showed evidence of oxide formation, suggesting, in conjunction with the observed electrocatalytic properties, that the mixed-metal sulfides may serve as precursors to oxides and/or hydroxides, which are likely to be the catalytically active species.

Three-component colloidal hybrid nanoparticles, which are central to a diverse array of applications, are typically synthesized using successive seeded growth steps, which are additive in nature and driven by surface chemistry considerations and material-specific preferences for nucleation and growth. Here, we describe a new nanoparticle insertion reaction for transforming heterodimers into heterotrimers, which is based on a supersaturation-precipitation pathway that shifts the driving force for heterotrimer formation away from surface-driven nucleation and growth. To demonstrate the concept, a Ge segment is inserted between the Au and Fe3O4 domains of Au-Fe3O4 heterodimers to form Au-Ge-Fe3O4 heterotrimers. Microscopic investigations reveal important mechanistic insights, including identification of a proposed Au-Ge-Au-Fe3O4 intermediate. The process can be modified to access the analogous addition product Ge-Au-Fe3O4, allowing tuning between two distinct heterotrimer isomers with different configurations.

Metal phosphides have emerged as promising Earth-abundant alternatives to platinum for catalyzing the hydrogen evolution reaction (HER) in acidic aqueous solutions. Here, Co2P nanoparticles having a hollow, multifaceted, crystalline morphology have been evaluated as HER electrocatalysts at a mass loading of 1 mg cm–2 on Ti foil substrates. The Co2P/Ti electrodes required low overpotentials of −95 and −109 mV to produce operationally relevant cathodic current densities of −10 and −20 mA cm–2, respectively. These values establish Co2P nanoparticles as highly active Earth-abundant HER catalyst materials. Importantly, the Co2P nanoparticles are morphologically equivalent to previously reported CoP nanoparticle HER catalysts, allowing a direct side-by-side evaluation of their HER activities. Such comparisons of different metal phosphide HER catalysts with the same constituent elements and morphologies are important for identifying the key materials characteristics that lead to high activity.

The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n(+)p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H2SO4(aq). The CoP-coated (2.0 mg cm(-2)) n(+)p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (Voc) of 0.48 V, a light-limited photocurrent density (Jph) of 17 mA cm(-2), a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (ηIRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm(-2)) n(+)p-Si MW-array photocathodes produced Voc = 0.44 V, Jph = 14 mA cm(-2), ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt.

Three-component colloidal hybrid nanoparticles are important multifunctional nanoscale constructs that present interesting and complex features. Their synthesis typically requires a third nanoscale material to nucleate and grow on a two-component heterodimer seed that has two distinct surfaces exposed and accessible. Identifying and understanding chemoselective growth behavior is therefore important for predicting the most favorable product in systems where multiple three-component configurations could be anticipated. Here, we studied Pt–Au heterodimers as seeds for the formation of several Pt–Au–MxSy (metal sulfide) heterotrimers. Pt and Au have similar lattice constants but different chemical preferences and metal–sulfur bond strengths, and the chosen metal sulfides included both crystalline and amorphous nanoparticles that span a range of threshold synthetic conditions required for them to form. These systems therefore allowed us to study the key factors that would be anticipated to contribute to chemo...

Colloidal hybrid nanoparticles, which contain multiple inorganic domains that are joined together through solid–solid interfaces, exhibit particle multifunctionality as well as new and enhanced properties that can emerge from the particle–particle interactions. These hybrid nanoparticles are typically synthesized using heterogeneous seeded nucleation of one nanoparticle on the surface of another as well as using phase segregation, surface dewetting of core–shell nanoparticles, and the fusion of premade nanoparticles. However, to expand the materials diversity and the potential range of applications of such systems, alternative routes to heterogeneous seeded nucleation are needed. Here, we show that solution–liquid–solid and related supersaturation-precipitation strategies, traditionally used in the synthesis of 1D structures such as nanowires and nanorods, can also be applied to the synthesis of colloidal hybrid nanoparticles. Specifically, we show that colloidal Au and Ag nanoparticles can serve as seeds...

Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides ...

Transition metal phosphides recently have been identified as promising Earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Here, we present a general and scalable strategy for the synthesis of transition metal phosphide electrodes based on the reaction of commercially available metal foils (Fe, Co, Ni, Cu, and NiFe) with various organophosphine reagents. The resulting phosphide electrodes were found to exhibit excellent electrocatalytic HER and OER performance. The most active electrodes required overpotentials of only -128 mV for the HER in acid (Ni2P), -183 mV for the HER in base (Ni2P), and 277 mV for the OER in base (NiFeP) to produce operationally relevant current densities of 10 mA cm(-2). Such HER and OER performance compares favorably with samples prepared using significantly more elaborate and costly procedures. Furthermore, we demonstrate that the approach can also be utilized to obtain highly active and conformal metal phosphide coatings on photocathode materials, such as highly doped Si, that are relevant to solar fuels production.

Ion exchange reactions of colloidal nanocrystals provide access to complex products that are synthetically challenging using traditional hot-injection methods. However, such reactions typically achieve only partial material transformations by employing either cation or anion exchange processes. It is now shown that anion and cation exchange reactions can be coupled together and applied sequentially in one integrated pathway that leads to complete material transformations of nanocrystal templates. Although the product nanocrystals do not contain any of the original constituent elements, the original morphology is retained, thereby fully decoupling morphology and composition control. The sequential anion/cation exchange process was applied to pseudo-spherical CdO nanocrystals and ZnO tetrapods, producing fully transformed and shape-controlled nanocrystals of copper and silver sulfides and selenides. Furthermore, hollow core-shell tetrapod ZnS@CdS heterostructures were readily accessible.

CoP nanostructures that exposed predominantly (111) crystal facets were synthesized and evaluated for performance as electrocatalysts for the hydrogen-evolution reaction (HER). The branched CoP nanostructures were synthesized by reacting cobalt(II) acetylacetonate with trioctylphosphine in the presence of trioctylphosphine oxide. Electrodes comprised of the branched CoP nanostructures deposited at a loading density of ~1 mg cm^(−2) on Ti electrodes required an overpotential of −117 mV to produce a current density of −20 mA cm^(−2) in 0.50 M H_2SO_4. Hence the branched CoP nanostructures belong to the growing family of highly active non-noble-metal HER electrocatalysts. Comparisons with related CoP systems have provided insights into the impact that shape-controlled nanoparticles and nanoparticle–electrode interactions have on the activity and stability of nanostructured HER electrocatalysts.

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).

Nanoparticles of cobalt phosphide, CoP, have been prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER) under strongly acidic conditions (0.50 M H_2SO_4, pH 0.3). Uniform, multi-faceted CoP nanoparticles were synthesized by reacting Co nanoparticles with trioctylphosphine. Electrodes comprised of CoP nanoparticles on a Ti support (2 mg cm^(−2) mass loading) produced a cathodic current density of 20 mA cm^(−2) at an overpotential of −85 mV. The CoP/Ti electrodes were stable over 24 h of sustained hydrogen production in 0.50 M H_2SO_4. The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long-term viability under operating conditions. CoP is therefore amongst the most active, acid-stable, earth-abundant HER electrocatalysts reported to date.

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.

Beneath the seeming straight-forwardness of growing carbon nanotube(CNT) forests by the injection chemical vapor deposition(CVD) method, control of the forest morphology on various substrates is yet to be achieved. Using ferrocene dissolved in xylene as the precursor, we demonstrate that the concentration of ferrocene and the injection rate of the precursor dictate the CNT density of these forests. However, CNT density will also be affected by the substrates and the growth temperature which determine the diffusion of the catalyst adatoms. The CNT growth rate is controlled by the temperature and chemical composition of the gases in the CVD reactor. We show that the final height of the forest is diffusion limited, at least in the conditions of our experiments. Because of the proximity and entanglement of the CNTs in a forest, the growing CNTs can lift-up the inactive CNTs resulting in reduced density toward the base of the forest unless the nucleation rate of the new catalyst particles is sufficiently high to replenish the inactive catalyst particles. Significant loss of CNT attachment by the lift-up effect reduces the adhesion of the forest to the substrate. Optimizing the ferrocene concentration in the precursor, precursor injection rate, gas mixture, substrate, and temperature is necessary to achieve desired forest morphology for specific applications.