Doping β-CoMoO4 Nanoplates with Phosphorus for Efficient Hydrogen Evolution Reaction in Alkaline Media

Bifunctional vanadium doped mesoporous Co3O4 on nickel foam towards highly efficient overall urea and water splitting in the alkaline electrolyte

Developing more active and stable electrode materials for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) is necessary for electrocatalytic water and urea oxidation which can be used to generate hydrogen. Here, a low-cost vanadium-doped mesoporous cobalt oxide on Ni foam (V/meso-Co/NF) electrodes are obtained via the grouping of an in-situ citric acid (CA)-assisted evaporation-induced self-assembly (EISA) method and electrophoretic deposition process, and work as highly efficient and long-lasting electrocatalytic materials for OER/UOR. In particular, V/meso-Co/NF electrodes require 329 mV overpotential to maintain a 50 mA/cm2, with exceptional long-term durability of 30 h. Interestingly, V/meso-Co/NF also exhibits excellent electrocatalytic UOR performance, reaching 50 and 100 mA/cm2 versus RHE at low potentials of 1.34 and 1.35 V, respectively. By employing the V/meso-Co/NF materials as both the anode and cathode, this urea electrolysis assembly V/meso-Co/NF-5 (+,-) reaches current densities of 100 mA cm-2 at 1.62 V in KOH/urea, which is nearly 340 mV lesser than classical water electrolysis. The V/meso-Co/NF-5 electrocatalysts also exhibit remarkable durability for electrocatalytic OERs and UORs. The obtained findings revealed that the synthesized V/meso-Co/NF might be a promising electrode materials for overall urea-rich wastewater management and H2 generation from wastewater.

Simultaneous Oxidative Cleavage of Lignin and Reduction of Furfural via Efficient Electrocatalysis by P-Doped CoMoO4

Electrochemical oxidative lignin cleavage and coupled 2-furaldehyde reduction provide a promising approach for producing high-value added products. However, developing efficient bifunctional electrocatalysts with noble-metal-like activity still remains a challenge. Here, an efficient electrochemical strategy is reported for the selective oxidative cleavage of C_α -C_β bonds in lignin into aromatic monomers by tailoring the electronic structure through P-doped CoMoO₄ spinels (99% conversion, highest monomer selectivity of 56%). Additionally, the conversion and selectivity of 2-furaldehyde reduction to 2-methyl furan reach 87% and 73%, respectively. In situ Fourier transform infrared and density functional theory analysis reveal that an upward shift of the E_d upon P-doping leads to an increase in the antibonding level, which facilitates the C_α -C_β adsorption of the lignin model compounds, thereby enhancing the bifunctional electrocatalytic activity of the active site. This work explores the potential of a spinel as a bifunctional electrocatalyst for the oxidative cracking of lignin and the reductive conversion of small organic molecules to high-value added chemicals via P-anion modulation.

Self-supporting nanoporous CoMoP electrocatalyst for hydrogen evolution reaction in alkaline solution

Efficient catalysts with low costs are very important for hydrogen production. In this work, a nanoporous CoMoP (np-CoMoP) bimetallic phosphide catalyst with a self-supporting structure was prepared by the electrochemical dealloying method. The introduction of Mo tuned the electronic structures around Co and P, optimized the desorption of the H atom, and improved the catalytic activity of cobalt phosphide. The prepared nanoporous Co₆₅Mo₁₅P₂₀ (np-Co₆₅Mo₁₅P₂₀) structures promoted electron transfer and provided more active sites, exhibiting superior hydrogen evolution reaction (HER) performance with the overpotential of 40.8 mV at 10 mA cm^-2 and Tafel slope of 46.2 mV dec^-1 in alkaline solution. Also, the catalysts exhibited good long-term stability.

Highly Efficient Water Splitting Catalyst Composed of N,P-Doped Porous Carbon Decorated with Surface P-Enriched Ni2P Nanoparticles

A non-noble-metal hybrid catalyst (Ni₂P/NPC-P), composed of N,P-doped porous carbon decorated with surface P-enriched Ni₂P nanoparticles, is developed to address the urgent challenges associated with mass production of clean hydrogen fuel. The synthesis features one-pot pyrolysis of inexpensive fluid catalytic cracking slurry, graphitic carbon nitride, and inorganic salts, followed by a feasible surface phosphidation process. As a non-noble metal catalyst, Ni₂P/NPC-P demonstrates excellent performance in hydrogen evolution reaction in alkaline electrolytes with a low overpotential of 73 mV at a current density of 10 mA cm^-2 (η₁₀) and a small Tafel slope of 56 mV dec^-1, meanwhile exhibits durability with no significant η₁₀ change after 2000 catalytic cycles. Theoretical calculation reveals that the negatively charged P-enriched surface accelerated the rate-determining transformation and desorption of OH*. In overall water splitting, the electrocatalyst achieves a low η₁₀ of 1.633 V, promising its potential in the cost-effective mass production of hydrogen fuel.

Three-dimensional nano-framework CoP/Co2P/Co3O4 heterojunction as a trifunctional electrocatalyst for metal–air battery and water splitting

Three-dimensional nano-framework CoP/Co2P/Co3O4 heterojunctions exhibit superior trifunctional electrocatalyst performances toward metal–air batteries and water splitting.

Heterostructured Ni3S2-Ni3P/NF as a Bifunctional Catalyst for Overall Urea-Water Electrolysis for Hydrogen Generation

Urea oxidation reaction (UOR) has been proposed to replace the formidable oxygen evolution reaction (OER) to reduce the energy consumption for producing hydrogen from electrolysis of water owing to its much lower thermodynamic oxidation potential compared to that of the OER. Therefore, exploring a highly efficient and stable hydrogen evolution and urea electrooxidation bifunctional catalyst is the key to achieve economical and efficient hydrogen production. In this paper, we report a heterostructured sulfide/phosphide catalyst (Ni₃S₂-Ni₃P/NF) synthesized via one-step thermal treatment of Ni(OH)₂/NF, which allows the simultaneous occurrence of phosphorization and sulfuration. The obtained Ni₃S₂-Ni₃P/NF catalyst shows a sheet structure with an average sheet thickness of ∼100 nm, and this sheet is composed of interconnected Ni₃S₂ and Ni₃P nanoparticles (∼20 nm), between which there are a large number of accessible interfaces of Ni₃S₂-Ni₃P. Thus, the Ni₃S₂-Ni₃P/NF exhibits superior performance for both UOR and hydrogen evolution reaction (HER). For the overall urea-water electrolysis, to achieve current densities of 10 and 100 mA cm^-2, cell voltage of only 1.43 and 1.65 V is required using this catalyst as both the anode and the cathode. Moreover, this catalyst also maintains fairly excellent stability after a long-term testing, indicating its potential for efficient and energy-saving hydrogen production. The theoretical calculation results show that the Ni atoms at the interface are the most efficient catalytically active site for the HER, and the free energy of hydrogen adsorption is closest to thermal neutrality, which is only 0.16 eV. A self-driven electron transfer at the interface, making the Ni₃S₂ sides become electron donating while Ni₃P sides become electron withdrawing, may be the reason for the enhancement of the UOR activity. Therefore, this work shows an easy treatment for enhancing the catalytic activity of Ni-based materials to achieve high-efficiency urea-water electrolysis.

Insight into electrocatalytic activity and mechanism of bimetal niobium-based oxides in situ embedded into biomass-derived porous carbon skeleton nanohybrids for photovoltaics and alkaline hydrogen evolution

Developing highly-efficient multifunctional electrocatalysts for energy conversion devices is of great importance. A sequence of nano-sized bimetal (Al, Cr, Fe) niobium oxide nanoparticles anchored on aloe peel-derived porous carbon skeleton hybrids (AN/APPC, CN/APPC, and FN/APPC) are successfully prepared via co-precipitation avenue and used as electrocatalysts for photovoltaics and alkaline hydrogen evolution reaction. Benefiting from the synergies between nano-sized metal niobium oxides and highly conductive porous carbon skeleton, these robust polycomponent hybrid electrocatalysts exhibit superior catalytic performances for accelerating the triiodide reduction and hydrogen evolution reaction. The solar cell with AN/APPC electrocatalyst achieves an outstanding device efficiency of 7.31%, superior to that with Pt (6.84%), and the AN/APPC electrocatalyst exhibit an overpotential (131.6 mV) when the current density is 10 mA cm^-2 and Tafel slope (54 mV dec^-1) in 1 M KOH for hydrogen evolution reaction. The AN/APPC electrocatalysts illustrate remarkable electrochemical durability in both I₃^-/I^- electrolyte and alkaline media. Furthermore, the catalytic mechanism was clarified both from the electronic structure and work function through first-principle density functional theory (DFT) calculations. This work opens a new avenue for electrocatalysis field via using nano-sized porous bio-carbon skeleton loaded with niobium-based binary metal.

Heteroatom-Doping of Non-Noble Metal-Based Catalysts for Electrocatalytic Hydrogen Evolution: An Electronic Structure Tuning Strategy

Electrocatalytic water splitting for hydrogen production is an appealing way to reduce carbon emissions and generate renewable fuels. This promising process, however, is limited by its sluggish reaction kinetics and high-cost catalysts. Construction of low-cost and high-performance non-noble metal-based catalysts have been one of the most effective approaches to address these grand challenges. Notably, the electronic structure tuning strategy, which could subtly tailor the electronic states, band structures, and adsorption ability of the catalysts, has become a pivotal way to further enhance the electrochemical water splitting reactions based on non-noble metal-based catalysts. Particularly, heteroatom-doping plays an effective role in regulating the electronic structure and optimizing the intrinsic activity of the catalysts. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the hetero-dopants in catalysts yet remains ambiguous. Herein, the recent progress is comprehensively reviewed in heteroatom doped non-noble metal-based electrocatalysts for hydrogen evolution reaction, particularly focus on the electronic tuning effect of hetero-dopants in the catalysts and the corresponding synthetic pathway, catalytic performance, and activity origin. This review also attempts to establish an intrinsic correlation between the localized electronic structures and the catalytic properties, so as to provide a good reference for developing advanced low-cost catalysts.

Electrochemical determination of rutin by using NiFe2O4 nanoparticles-loaded reduced graphene oxide

A binary transition metal oxide containing nickel and iron (NiFe2O4) and hybridization of this nanomaterial with reduced graphene oxide (rGO) are synthesized by the hydrothermal method. X-ray diffraction (XRD) and Raman spectroscopy confirm the successful synthesis of these materials. Also, scanning electron microscope (SEM) and transmission electron microscope (TEM) images illustrated the particle morphology with the particle size of 20 nm. The synthesized material is then examined as a sensor on the surface of the glassy carbon electrode to detect a very small amount of rutin. Some electrochemical tests such as cyclic voltammetry, differential pulse voltammetry (DPV), and impedance spectroscopy indicate the remarkable accuracy of this sensor and its operation in a relatively wide range of concentrations of rutin (100 nM-100 µM). The accuracy of the proposed electrochemical sensors is approximately 100 nM in 0.1 M PBS, (pH = 3) which is relatively impressive and can be reported. Also, the stability rate after 100 DPV was about 95 %, which is a considerable and relatively excellent value. Considering the very good results, it seems that the NiFe2O4-rGO can be considered as a new proposal in the development of accurate and inexpensive electrochemical sensors.

Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO4-Based Microrod Arrays in Neutral Media through Alkaline Activation

We present that activation of CoMoO₄-based microrod arrays in KOH (1.0 M, 2 h) allows us to significantly improve their electrochemical hydrogen evolution performance in phosphate buffer solution (1.0 M, pH = 7.1). The activation mechanism originates from the conversion of the surface layer of CoMoO₄ to Co(OH)₂ nanosheets, together with the release of Mo₃O₁₀^2- ions into the activation solution. Our experimental and calculated results suggest that the Co(OH)₂ nanosheets on the surface of the CoMoO₄-based microrod arrays show the ability to improve water molecule disassociation and stabilize the catalytic activity of the two-component catalysts by decreasing their overpotentials in the hydrogen evolution reaction. When extending this strategy to activate P-doped CoMoO₄ with a low hydrogen absorption free energy, we report the synthesis of a new class of superior neutral electrochemical hydrogen evolution catalysts of P-doped CoMoO₄-Co(OH)₂ microrod arrays. We show that a low overpotential of about 30 mV (obtained from bulk electrolysis) is required to deliver a current density of 10 mA cm^-2 in the neutral media. By making use of our catalyst and NiFe double hydroxide as cathodic and anodic electrodes, respectively, we fabricated a two-electrode electrolysis device for neutral overall water splitting. Our results showed a low cell voltage of 1.78 V (obtained from bulk electrolysis) that is needed for delivering a current density of about 10 mA cm^-2 in the neutral electrolyte, even outperforming the state-of-the-art catalyst combination of Pt/C∥RuO₂ in terms of catalytic activity and stability. These findings suggest that our strategy may be utilized as a facile but useful strategy toward the activation of molybdate catalysts to improve their HER performance in both basic and neutral media.

The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance

The hydrogen evolution reaction (HER) is a critical process in the electrolysis of water. Recently, much effort has been dedicated to developing low-cost, highly efficient, and stable electrocatalysts. Transition metal phosphides are investigated intensively due to their high electronic conductivity and optimized absorption energy of intermediates in acid electrolytes. However, the low stability of metal phosphide materials in air and during electrocatalytic processes causes a decay of performance and hinders the discovery of specific active sites. The HER in alkaline media is more intricate, which requires further delicate design due to the Volmer steps. In this work, phosphorus-modified monoclinic β-CoMoO4 is developed as a low-cost, efficient, and stable HER electrocatalyst for the electrolysis of water in alkaline media. The optimized catalyst shows a small overpotential of 94 mV to reach a current density of 10 mA cm-2 for the HER with high stability in KOH electrolyte, and an overpotential of 197 mV to reach a current density of 100 mA cm-2. Combined computational and in situ spectroscopic techniques show P is present as a surface phosphate ion; that electron holes localize on the surface ions and both (P-O1-) and Co3+-OH- are prospective surface active sites for the HER.

Engineering cobalt oxide by interfaces and pore architectures for enhanced electrocatalytic performance for overall water splitting

The development of efficient electrocatalysts for overall water splitting is important for future renewable energy systems. Herein, macroporous CoO covered by Co/N-doped graphitic carbon nanosheet arrays (mac-CoO@Co/NGC NSAs) were constructed by engineering a mesoporous CoO nanowire (mes-CoO NWAs) core with highly conductive Co nanoparticles coated by a N-doped graphitic carbon (Co/NGC) shell. The in situ derived Co/NGC shell not only introduces electrocatalytic active sites for the hydrogen evolution reaction (HER) but also promotes the oxygen evolution reaction (OER) through the strong interaction between the CoO core and the Co/NGC shell. Moreover, the highly conductive Co/NGC shell crosslinks the isolated mesoporous CoO nanowires into a nanosheet rich in macropores, ensuring effective electron and mass transfer. Furthermore, the chemically stable N-doped graphitic carbon layer and physically stable hierarchical nanosheet arrays ensure the stability of the catalyst. Owing to the desirable interfaces and pore architecture, the as-prepared mac-CoO@Co/NGC NSAs can serve as highly effective, binder-free electrocatalysts for overall water splitting with a stable cell voltage of 1.62 V at 10 mA cm-2 for 35 h.

Self-templated construction of 1D NiMo nanowires via a Li electrochemical tuning method for the hydrogen evolution reaction

NiMo based materials have been widely recognized as the most promising alternatives to noble Pt electrocatalysts used in alkaline electrolytes for the hydrogen evolution reaction. However, it is difficult to construct a nanostructure, especially 1D morphology, for NiMo materials via an electrochemical method. Herein, a novel Li electrochemical tuning method, for the first time, is introduced to synthesize 1D NiMo nanowires by insertion of lithium ions into parent NiMoO₄ nanorods. The as-prepared NiMo catalyst exhibits high HER activity in 1 M KOH, in terms of low overpotential (73 mV) at a current density of 10 mA cm^-2 and a small Tafel slope (37.2 mV dec^-1) and charge transfer resistance (11.3 Ω). Furthermore, no decay in catalytic performance is observed for this material when it is operated at -0.125 V (vs. RHE) for 1250 min and a high Faraday efficiency (96%) is achieved. The high activity of NiMo is ascribed to the synergistic interplay between Ni and Mo and its unique nanostructure, which can expose more active sites and facilitate the mass transfer and hydrogen bubble release.

Phosphorus and Yttrium Codoped Co(OH)F Nanoarray as Highly Efficient and Bifunctional Electrocatalysts for Overall Water Splitting

Rational design and synthesis of bifunctional electrocatalysts with high efficiency and low-cost for overall water splitting is still a challenge. A simple approach is reported to prepare a phosphorus and yttrium codoped cobalt hydroxyfluoride (YP-Co(OH)F) nanoarray on nickel foam, which displays high-performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction in alkaline solution. The codoping of yttrium and phosphorus into Co(OH)F leads to a tuned electronic environment and favorable electron transfer, thus resulting in superior water splitting activity. The YP-Co(OH)F electrode only requires an overpotential of 238 mV to reach a current density of 10 mA cm^-2 (η₁₀ ), much smaller than RuO₂ (302 mV). Moreover, it displays an overpotential of 55 mV at η₁₀ for HER, similar to that of Pt/C. When YP-Co(OH)F is used as both anode and cathode in a two-electrode configuration, it only demands a cell potential of 1.54 V at η₁₀ , lower than the IrO₂ ||Pt/C couple (1.6 V) as well as other recently reported electrocatalysts. It even maintains stable water splitting for 300 h. Such a two-electrode device can be easily driven by a 1.5 V silicon solar cell in sunlight, proving the potential of the promising catalyst for large-scale electrolytic water splitting.