Abstract: The design of highly efficient, stable, and noble-metal-free bifunctional electrocatalysts for overall water splitting is critical but challenging. Herein, a facile and controllable synthesis strategy for nickel–cobalt bimetal phosphide nanotubes as highly efficient electrocatalysts for overall water splitting via low-temperature phosphorization from a bimetallic metal-organic framework (MOF-74) precursor is reported. By optimizing the molar ratio of Co/Ni atoms in MOF-74, a series of CoxNiyP catalysts are synthesized, and the obtained Co4Ni1P has a rare form of nanotubes that possess similar morphology to the MOF precursor and exhibit perfect dispersal of the active sites. The nanotubes show remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance in an alkaline electrolyte, affording a current density of 10 mA cm−2 at overpotentials of 129 mV for HER and 245 mV for OER, respectively. An electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalyst in alkaline solutions achieves a current density of 10 mA cm−2 at a voltage of 1.59 V, which is comparable to the integrated Pt/C and RuO2 counterparts and ranks among the best of the metal-phosphide electrocatalysts reported to date.

Abstract: A bimetallic-structured ternary phosphide (NiCo2 Px ) as a novel pH-universal electrocatalyst for hydrogen evolution reaction is presented. It exhibits both high activity and long-term stability in all the tested alkaline, neutral, and acidic media. The excellent catalytic performance endows it with a bright future in the large-scale electrochemical water splitting industry.

Abstract: Cobalt-based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co2P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co-host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu0.3Co2.7P/nitrogen-doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm–2 and satisfying stability for OER. Cu0.3Co2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu0.3Co2.7P/NC paired electrodes shows 60% larger current density than Pt/RuO2 couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis-based energy conversion.

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...

Abstract: The search for active, stable, and cost-efficient electrocatalysts for hydrogen production via water splitting could make a substantial impact on energy technologies that do not rely on fossil fuels. Here we report the synthesis of rhodium phosphide electrocatalyst with low metal loading in the form of nanocubes (NCs) dispersed in high-surface-area carbon (Rh2P/C) by a facile solvo-thermal approach. The Rh2P/C NCs exhibit remarkable performance for hydrogen evolution reaction and oxygen evolution reaction compared to Rh/C and Pt/C catalysts. The atomic structure of the Rh2P NCs was directly observed by annular dark-field scanning transmission electron microscopy, which revealed a phosphorus-rich outermost atomic layer. Combined experimental and computational studies suggest that surface phosphorus plays a crucial role in determining the robust catalyst properties.

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.

Abstract: Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu3P phosphide nanosheets on conductive nickel foam (Cu3P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu3P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm2 requires an overpotential of only ∼320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ∼105 mV to achieve a catalytic current density of 10 mA cm–2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu3P@NF electrode, which showed a catalytic current density of 10 mA/cm2 under an applied voltage of ∼1.67 V. The same current density can also be obta...

Abstract: Sulfur and nitrogen dual-doped molybdenum phosphides (MoP/SN) are synthesized via a (thio)urea-phosphate-assisted strategy in which the reductant (thio)urea acts as S and N source and phosphoric acid provides the P atom. The MoP/SN nanoparticles are generated by in situ phosphidation of indigenously synthesized ammonium phosphate-coated P-doped MoSx nanoparticles in a hydrogen atmosphere. Then, MoP/SN is anchored on graphene to obtain a hybrid electrocatalyst (MoP/SNG) that exhibits high activity and stability for electrochemical hydrogen evolution from water in both acidic and basic electrolytes, outperforming most MoP-based electrocatalysts reported in the literature. The dual doping and hybridization with graphene enhance electron conductivity of MoP and stabilize small MoP nanoparticles to increase activity and stability, especially in acid electrolytes.

Abstract: Exploring efficient and earth-abundant electrocatalysts for water splitting is crucial for various renewable energy technologies. In this work, iron (Fe)-doped nickel phosphide (Ni2P) nanosheet arrays supported on nickel foam (Ni1.85Fe0.15P NSAs/NF) are fabricated through a facile hydrothermal method, followed by phosphorization. The electrochemical analysis demonstrates that the Ni1.85Fe0.15P NSAs/NF electrode possesses high electrocatalytic activity for water splitting. In 1.0 M KOH, the Ni1.85Fe0.15P NSAs/NF electrode only needs overpotentials of 106 mV at 10 mA cm–2 and 270 mV at 20 mA cm–2 to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Furthermore, the assembled two-electrode (Ni1.85Fe0.15P NSAs/NF∥Ni1.85Fe0.15P NSAs/NF) alkaline water electrolyzer can produce a current density of 10 mA cm–2 at 1.61 V. Remarkably, it can maintain stable electrolysis over 20 h. Thus, this work undoubtedly offers a promising electrocatalyst for water splitting.

Abstract: Solar-driven overall water splitting is highly desirable for hydrogen generation with sustainable energy sources, which need efficient, earth-abundant, robust, and bifunctional electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, we propose a heterogeneous bimetallic phosphide/sulfide nanocomposite electrocatalyst of NiFeSP on nickel foam (NiFeSP/NF), which shows superior electrocatalytic activity of low overpotentials of 91 mV at -10 mA cm-2 for HER and of 240 mV at 50 mA cm-2 for OER in 1 M KOH solution. In addition, the NiFeSP/NF presents excellent overall water splitting performance with a cell voltage as low as 1.58 V at a current density of 10 mA cm-2. Combining with a photovoltaic device of a Si solar cell or integrating into photoelectrochemical (PEC) systems, the bifunctional NiFeSP/NF electrocatalyst implements unassisted solar-driven water splitting with a solar-to-hydrogen conversion efficiency of ∼9.2% and significantly enhanced PEC performance, respectively.

Abstract: Transition metal phosphides (TMPs) have been identified as promising nonprecious metal electrocatalyst for hydrogen evolution reaction (HER) and other energy conversion reactions. Herein, we reported a general strategy for synthesis of a series of TMPs (Fe2P, FeP, Co2P, CoP, Ni2P, and Ni12P5) nanoparticles (NPs) with different metal phases embedded in a N-doped carbon (NC) matrix using metal salt, ammonium dihydrogen phosphate, and melamine as precursor with varying molar ratios and thermolysis temperatures. The resultant TMPs can serve as highly active and durable bifunctional electrocatalyst toward HER and oxygen evolution reaction (OER). In particular, the Ni2P@NC phase only requires an overpotential of ∼138 mV to derive HER in 0.5 M H2SO4, and ∼320 mV for OER in 1.0 M KOH at the current density of 10 mA cm–2. Because of the encapsulation of NC that can effectively prevent corrosion of embedded TMP NPs, Ni2P@NC exhibits almost unfading catalytic performance even after 10 h under both acidic and alkalin...

Abstract: 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 (η) of merely 300 mV and a small Tafel slope of 78 mV dec−1 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 η = 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.

Abstract: Black phosphorus carbide (b-PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p-type mobility (≈105 cm2 V-1 s-1 ) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high-performance composite few-layer b-PC field-effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm2 V-1 s-1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.

Abstract: Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal–phosphide core–shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron–iron phosphide (Fe@FeP) core–shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpo...

Abstract: Transition metal phosphides (TMPs) are considered to be superb catalysts for water splitting. In this work, we introduce an efficient strategy to fabricate dicobalt phosphide (Co2P) quantum dots embedded in N, P dual-doped carbon (Co2P@NPC) on carbon cloth (Co2P@NPC/CC) by in situ carbonization of cobalt ion induced phytic acid (PA) and polyaniline (PANI) macromolecule precursors. As a highly efficient self-supported electrode, it has a low onset overpotential (74 mV at 1 mA cm−2) approaching that of the commercial Pt/C catalyst for the hydrogen evolution reaction (HER) in acidic media. Meanwhile, it also shows very low overpotentials of only 116 and 129 mV at 10 mA cm−2 with robust stability in acidic and alkaline media, respectively.

Abstract: Herein we converted Fe–Ni hydroxide nanosheets into an Fe–Ni phosphide nanoparticle-stack array on Ni foam by low-temperature phosphorization treatment for promoting the specific surface area of Fe–Ni phosphide and further improving its oxygen evolution reaction (OER) activities The alkaline electro-activated Fe–Ni–P nanoparticle-stack array exhibits exceptional OER activities in both alkaline and neutral media

Abstract: The sluggish kinetics of oxygen evolution reaction (OER) hampers the H2 production by H2O electrolysis, and it is very important for the development of highly efficient and low-priced OER catalysts. Herein, a representative metalloporphyrinic MOF, PCN-600-Ni, integrated with graphene oxide (GO), serves as an ideal precursor and template to afford bimetallic iron–nickel phosphide/reduced graphene oxide composite (denoted as Fe–Ni–P/rGO-T; T represents pyrolysis temperature) via pyrolysis and subsequent phosphidation process. Thanks to the highly porous structure, the synergetic effect of Fe and Ni elements in bimetallic phosphide, and the good conductivity endowed by rGO, the optimized Fe–Ni–P/rGO-400 exhibits remarkable OER activity in 1 M KOH solution, affording an extremely low overpotential of 240 mV at 10 mA/cm2, which is far superior to the commercial IrO2 and among the best in all non-noble metal-based electrocatalysts.

Abstract: High specific capacity and long cycling life of anode materials still remain major challenges in the development of sodium-ion batteries (SIBs). Nowadays, transition metal phosphides have been reckoned as promising anodes in view of their high theoretical specific capacities, low potential for sodium storage, and superior conductivity. Herein, molybdenum phosphide (MoP) nanorods wrapped with thin carbon layer have been prepared and applied as an anode for SIBs. By utilizing in situ X-ray diffraction technology, a conversion-type mechanism of MoP anode has been disclosed. This kind of MoP electrode exhibits extraordinary electrochemical properties, including excellent cycling performance with a discharge capacity of 398.4 mA h g–1 at a current density of 100 mA g–1 after 800 cycles, and remarkable rate capabilities, which remain 104.5 mA g–1 at 1600 mA h g–1 even after 10 000 loops. The distinguished performances stem from synergetic merits of the morphology, structure, and mechanism of MoP. It is expected...

Abstract: Optimizing catalysts for the hydrogen evolution reaction (HER) is a critical step toward the efficient production of H2(g) fuel from water. It has been demonstrated experimentally that transition-metal phosphides, specifically nickel phosphides Ni2P and Ni5P4, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through density functional theory with thermodynamics, the aqueous reconstructions of Ni2P(0001) and Ni5P4(0001)/(0001), and we find that the surface P content on Ni2P(0001) depends on the applied potential, which has not been considered previously. At −0.21 V ≥ U ≥ −0.36 V versus the standard hydrogen electrode and pH = 0, a PHx-enriched Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow s...

Abstract: In the present work, nickel phosphide (Ni12P5) modified graphitic carbon nitride (g-C3N4) nanosheets were synthesized by a simple grinding method. The structural characterization clearly proved that Ni12P5 nanoparticles were well loaded on the surface of g-C3N4 nanosheets. The photocatalytic activity of the composites was tested by catalyzing the reduction of water to hydrogen under visible light irradiation. The results demonstrate that Ni12P5 is an efficient co-catalyst for photocatalytic H2 production of g-C3N4 nanosheets. The maximum photocatalytic H2-production rate of 126.61 μmol g−1 h−1 could be obtained by loading 2.0% Ni12P5 nanoparticles on the surface of g-C3N4, which is about 269.4 times higher than that of pure g-C3N4. It is believed that Ni12P5 nanoparticles on the surface of g-C3N4 could act as significant active sites to boost separation of photoexcited electrons and holes and accelerate the H2-evolution kinetics, thus achieving greatly enhanced hydrogen generation. It is expected that this work could contribute to further experimental investigation for exploiting the low cost, high-efficiency, and environmentally friendly g-C3N4-based nanocomposites for photocatalytic H2 production.

Abstract: Electrochemical splitting of water to produce oxygen (O2) and hydrogen (H2) through a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER) is a promising green approach for sustainable energy supply. Here we demonstrated a porous nickel-copper phosphide (NiCuP) nano-foam as a bifunctional electrocatalyst for highly efficient total water splitting. Prepared from a bubble-templated electrodeposition method and subsequent low-temperature phosphidization, NiCuP has a hierarchical pore structure with a large electrochemical active surface area. To reach a high current density of 50 mA cm-2, it requires merely 146 and 300 mV with small Tafel slopes of 47 and 49 mV dec-1 for HER and OER, respectively. The total water splitting test using NiCuP as both the anode and cathode showed nearly 100% Faradic efficiency and surpassed the performances of electrode pairs using commercial Pt/C and IrO2 catalysts under our test conditions. The high activity of NiCuP can be attributed to (1) the conductive NiCu substrates, (2) a large electrochemically active surface area together with a combination of pores of different sizes, and (3) the formation of active Ni/Cu oxides/hydroxides while keeping a portion of more conductive Ni/Cu phosphides in the nano-foam. We expect the current catalyst to enable the manufacturing of affordable water splitting systems.

Abstract: The nickel phosphide (Ni2P) family of materials have become a hot subject in hydrogen evolution reaction (HER) electrocatalyst research. Various studies have reported their high activity, high stability, and high faradaic efficiency. To date, there have been no systematic studies regarding the influence of pH on the HER performance of Ni2P. Here we show that the pH of electrolytes can strongly influence the HER activity of Ni2P electrocatalysts. Tests in 19 electrolytes with pH ranging from 0.52 to 13.53 show that Ni2P is much more active in strongly acidic and basic electrolytes. With the increase of pH, the lower H+ concentration reduces the formation of adsorbed H atoms in the Volmer reaction, resulting in poorer activities. However, the high activity observed in the strongly basic electrolytes is not the intrinsic property of Ni2P. We found that Ni oxides/hydroxides are formed in strongly basic electrolytes under applied potentials, resulting in improved activities. Furthermore, the specific activity based on the electrochemically active surface area of recently reported Ni2P catalysts is not high and requires significant improvements for practical applications.

Abstract: Composed of earth-abundant elements, many metal phosphides have revealed remarkable advantages as photocatalytic hydrogen generation cocatalysts for their outstanding performance, stability and low cost. Taking the importance for solar energy utilization and current synthetic methods of metal phosphides into consideration together, we focus on the safe and energy-saving preparation of metal phosphide as cocatalyst for photocatalytic hydrogen evolution. Herein, taking Co x P/CdS as a case, a novel photochemical strategy to synthesize Co x P is proposed and realized. In this process, Co salt and NaH 2 PO 2 are used as the source of Co and P, and the typical preparation of Co x P can be completed within 1 h. Furthermore, the optimized hydrogen evolution rate of Co x P/CdS is about 500 mmol g −1 h −1 under visible light, which is one of the most robust photocatalytic HER systems currently. Besides, the photocatalytic H 2 evolution mechanism using Co x P as cocatalyst is also proposed, where Co x P can effectively prevent the recombination of photogenerated electrons and holes. The photochemical synthesis route opens a door for facile preparation and practical application of many other metal phosphides.

Abstract: Transition metal phosphides are often studied as hydrogen evolution reaction cathode catalysts, but scarcely as oxygen evolution reaction (OER) anode catalysts for water oxidation. Herein, we report a new amorphous Co0.63Fe0.21P0.16 OER catalyst, which was prepared through a one-step solvothermal process. This catalyst exhibits extraordinary OER catalytic activity in alkaline media and is superior to the state of the art IrO2 in 1.0 M KOH, capable of yielding a current density of 10 mA cm−2 at an overpotential of only 217 mV. This newly achieved high activity outperforms most reported transition metal phosphide catalysts. During the OER, the surface layers of the Co0.63Fe0.21P0.16 nanospheres (200–500 nm) are oxidized, which show the typical Tafel slope of oxides around 40 mV dec−1. The in situ formation of surface Co oxides in the catalysts during the OER along with an optimal doping of Fe are found to be crucial for their remarkable activity. In particular, the metal-rich amorphous phosphide cores, i.e., substrates, probably contribute to OER performance of the catalysts due to their high electrical conductivity. This new type of surface oxidized amorphous transition metal phosphide would provide a new pathway for the design of high-performance OER electrocatalysts.