Surface excited MoO2 to master full water splitting

Boosting urea-assisted water splitting over P-MoO2@CoNiP through Mo leaching/reabsorption coupling CoNiP reconstruction

Combining the urea oxidation reaction (UOR) with the hydrogen evolution reaction (HER) is an effective technology for energy-saving hydrogen production. Herein, a bifunctional electrocatalyst with CoNiP nanosheet coating on P-doped MoO2 nanorods (P-MoO2@CoNiP) is obtained via a two-step hydrothermal followed a phosphorization process. The catalyst demonstrates exceptional alkaline HER performance due to the formation of MoO2 and the dissolution/absorption of Mo. Meanwhile, the inclusion of Co and P in the P-MoO2@CoNiP catalyst facilitated the formation of NiOOH, enhancing UOR performance. Density functional theory calculations reveal that the hydrogen adsorption Gibbs free energy (ΔGH*) of P-MoO2@CoNiP is closer to 0 eV than CoNiP, favoring the HER. The catalyst only needs -0.08 and 1.38 V to reach 100 mA cm-2 for catalyzing the HER and UOR, respectively. The full urea electrolysis system driven by P-MoO2@CoNiP requires 1.51 V to achieve 100 mA cm-2, 120 mV lower than the traditional water electrolysis.

Vanadium-doped Ni microspheres loaded with phosphatization of NiMoO4 contributing to enhanced electron transfer for stable overall water splitting

At present, there are few reports on the micron-sized catalysts for overall water splitting. In this study, phosphating method were used to construct the self-supporting catalyst (V doped Ni microspheres coated by NiMoO4/Ni12P5) with microspherical structure, providing a short path and a stable structure to guarantee quick electron transfer and excellent catalytic performance. Hence, oxygen evolution reaction (OER) only needs 254 mV to reach a current density of 50 mA cm-2 in 1.0 mol/L KOH, after 114 h without attenuation. The catalyst can achieve a current densitiy of 10 mA cm-2 with a voltage of only 158 mV for hydrogen evolution reaction (HER). When micron scale V-Ni@NiMoO4/Ni12P5 is used as both anode and cathode for overall water splitting, the device can operate at a current density of 10 mA cm-2 for more than 200 h of good stability. Its superior catalytic performance can be attributed to the construction of micron size and phosphating. DFT calculations indicate that the introduction of P better activates the adsorbed *OH and H2O*, reduces reaction the energy barrier, and improves the catalytic activity.

N-doped carbon sheets supported P-Fe3O4-MoO2 for freshwater and seawater electrolysis

Electric-driven freshwater/seawater splitting is an attractive and sustainable route to realize the generation of H2 and O2. Molybdenum-based oxides exhibit poor activity toward freshwater/seawater electrolysis. Herein, we adjusted the electronic structure of MoO2 by constructing N-doped carbon sheets supported P-Fe3O4-MoO2 nanosheets (P-Fe3O4-MoO2/NC). P-Fe3O4-MoO2/N-doped carbon sheets were precisely prepared by pyrolysis of Schiff base Fe complex and MoO3 nanosheets through phosphorization. Benefiting from the unique structures of the samples, it required 119/145 mV to drive freshwater/seawater reduction reaction at 10 mA/cm2. P-Fe3O4-MoO2/NC catalysts exhibited superior freshwater/seawater oxidation reactivity with 180/189 mV at 10 mA/cm2 compared with commercial RuO2. The low cell voltages for P-Fe3O4-MoO2/NC were 1.47 and 1.59 V towards freshwater and seawater electrolysis, respectively. Our work might shed light on the structural modulation of Mo-based oxides for enhancing freshwater and seawater electrolysis activity.

MoO2 nanosheets anchored with Co nanoparticles as a bifunctional electrocatalytic platform for overall water splitting

Electrochemical water splitting is one of the potential commercial techniques to produce clean hydrogen energy because of the high efficiency and environmental friendliness. However, development of low-cost bifunctional electrocatalysts that can replace Pt-based catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is challenging. Herein, Co nanoparticles (NPs) are anchored on MoO2 nanosheets (Co/MoO2) by thermal reduction of the CoMoO4 nanosheet array in Ar/H2. The uniformly distributed Co NPs improve the electron transfer capability and modulate the surface states of the MoO2 nanosheets to enhance hydrogen desorption and HER kinetics. Moreover, the Co/MoO2 composite is beneficial to the interfacial structure and the MoO2 nanosheets prevent aggregation of Co NPs to improve the intrinsic OER characteristics in the alkaline electrolyte. As a result, the Co/MoO2 electrocatalyst shows low HER and OER overpotentials of 178 and 318 mV at a current density of 10 mA cm-2 in 1 M KOH. The electrolytic cell consisting of the bifunctional Co/MoO2 electrodes shows a small voltage of 1.72 V for a current density of 10 mA cm-2 in overall water splitting.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Dual-anions engineering of bimetallic oxides as highly active electrocatalyst for boosted overall water splitting

Bimetallic oxides have unique advantages in driving both oxygen and hydrogen evolution reactions (OER/HER). Surface engineering of bimetallic oxides is a promising way to boost the catalytic activity by the regulation of electronic structure and surface property. Herein, a dual P, S-anions modification strategy is developed to optimize the catalytic performance of CoMoO₄ nanowire arrays. The formations of CoP and Co₃S₄ species on the CoMoO₄ surface bring heterojunction interfaces for more catalytic active sites and strong electronic interaction for faster interfacial charge transfer. Benefiting from these advantages, the P, S-CoMoO₄ catalyst on nickel foam (NF) delivers excellent catalytic activity and stability. The overpotentials at 10 mA cm^-2 of P, S-CoMoO₄/NF for HER are just 31 mV in acid media and 58 mV in alkaline media, respectively. In addition, by assembling the P, S-CoMoO₄/NF as bifunctional electrodes for overall water splitting, the electrolyzer exhibits a voltage of as low as 1.66 V at a current density of 50 mA cm^-2. This work put forward a new avenue for the development of advanced bifunctional electrocatalysts for water splitting.

Bifunctional catalysts of Ni nanoparticle coupled MoO2 nanorods for overall water splitting

The development of active and cost-effective bifunctional catalysts is crucial for water dissociation through electrolysis. In this study, bifunctional catalysts with Ni nanoparticles (NPs) anchored on MoO₂ nanorods have been synthesized via in situ dissolution of NiMoO₄-ZIF under an inert atmosphere without using hydrogen gas. The Ni-MoO₂ catalyst exhibits high electrocatalytic activity by modulating the calcination temperature. Benefitingfrom the MOF transformation and accompanying Ni particles' outward diffusion, a precisely designed interface heterostructure between Ni and MoO₂ was constructed. As a result, the optimized Ni-MoO₂ catalyst achieves extremely low overpotentials of only 24 mV and 275 mV at 10 mA cm^-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, the catalyst required a small cell voltage of 1.55 V to deliver a current density of 10 mA cm^-2 and remained stable over 20 h for overall water splitting. The proposed MOF-derived heterojunction protocol provides a general approach for designing and fabricating transition metal oxide catalysts for water electrolysis.

Ru Doped Molybdenum-based Nanowire Arrays for Efficient Hydrogen Evolution over a Broad pH Range

Searching and developing earth-abundant electrocatalysts with predominant activity and favorable stability are significant to resolve increasing environmental pollution and serious energy crisis. In this paper, Mo-based nanowire arrays (NWAs) was...

A strategy of asymmetric local structure based on mesoporous MoO2 toward efficient electrocatalysis

Combined with density functional theory (DFT) calculations, a substitutional heteroatom-doping approach is employed to design asymmetric local structures based on highly ordered mesoporous MoO2 nanostructures. Such synergistic strategies on increasing both the number and intrinsic activity of active sites jointly lead to a significant water oxidation performance boost.

Nanoporous Nickel-Molybdenum Oxide with an Oxygen Vacancy for Electrocatalytic Nitrogen Fixation under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) is regarded as a sustainable method for N₂ fixation. N₂ adsorption and N≡N cleavage are the main challenges for the NRR. Herein, we propose a potential approach to enhance N₂ activation via introducing oxygen vacancies (OVs) into nanoporous NiO/MoO₃. Nanoporous NiO/MoO₃ with OVs (np-OVs-NiO/MoO₃) is prepared by a two-step process of dealloying and solid-state reaction. np-OVs-NiO/MoO₃ exhibits a high NH₃ yield of 35.4 μg h^-1 mg_cat^-1 and a Faradaic efficiency (FE) of 10.3% in 0.1 M PBS solution. The introduction of OVs enhances the conductivity, N₂ adsorption, and catalytic performance of np-NiO/MoO₃. The dual-metal sites with OVs have a unique electronic structure in favor of the "π back-donation" behavior, which decreases the energy barrier of protonation steps and improves the whole NRR process. This approach provides new insight into the design of composite transition metal oxides with OVs for the NRR catalyst under ambient conditions.