Incorporating MoO3 Patches into a Ni Oxyhydroxide Nanosheet Boosts the Electrocatalytic Oxygen Evolution Reaction

Robust Oxygen Evolution on Ni-Doped MoO3: Overcoming Activity-Stability Trade-Off in Alkaline Water Splitting

Electrochemical water splitting using earth-abundant materials is crucial for enabling green hydrogen production and energy storage. In recent years, molybdenum trioxide (MoO3), a semiconducting material, has been proposed as a candidate catalyst for the oxygen evolution reaction (OER). Here, we advance nickel (Ni) doping of MoO3 as a strategy to increase the activity and stability of the material during alkaline electrochemical water splitting, thereby overcoming the typical activity-stability trade-off encountered with OER catalysts. The instability of MoO3 in alkaline media can be mitigated by doping with Ni, whose oxide is stable under such conditions. Using density functional theory (DFT) with Hubbard corrections, we show that Ni doping reduces the thermodynamic OER overpotential on the MoO3 basal plane to 0.64 V. Experiments demonstrate that Ni-doped MoO3 requires an overpotential of 0.34 V for an OER current density of 10 mA/cm2 (and 0.56 V at 100 mA/cm2), as opposed to a value of 0.40 V for pure MoO3. Further, Ni-doped MoO3 exhibits a lower Tafel slope of 74.8 mV/dec, compared to 98.3 mV/dec for the pristine material under alkaline conditions. While Mo leaches in alkaline conditions, X-ray photoelectron spectroscopy reveals enhanced stability with Ni doping. Overall, our work advances Ni-doped MoO3 as a promising water-splitting electrocatalyst and provides new insights into its OER mechanism and stability in alkaline media. More generally, the work sheds light on choosing a dopant to increase a material's activity and stability, which will also find applications in other catalytic materials.

Two-Dimensional SnS Mediates NiFe-LDH-Layered Electrocatalyst toward Boosting OER Activity for Water Splitting

NiFe-layered double hydroxides (NiFe-LDHs), as promising electrocatalysts, have received significant research attention for hydrogen and oxygen generation through water splitting. However, the slow oxidation kinetics of NiFe-LDH, due to the limited number of active sites and the low conductivity, hinders the improvement of the water-splitting efficiency. Therefore, to overcome the obstacles, two-dimensional (2D) SnS was first explored to tailor the prepared NiFe-LDH via the hydrothermal method. A NiFe-LDH/SnS heterojunction is built, which is observed from the microstructural investigations. SnS incorporation could greatly improve the conductivity of the NiFe-LDH sheets, which was reflected by the reduced charge transfer resistance. Moreover, SnS layers modulated the electronic environment around the active sites, favoring the adsorption of intermediates during the oxygen evolution reaction (OER) process, which was verified by density functional theory calculations. A synergistic effect induced by the NiFe-LDH/SnS heterostructure promoted the OER activities in electrical, electronic, and energetic aspects. Consequently, the as-prepared NiFe-LDH/SnS electrocatalyst greatly improved the electrocatalytic performance, exhibiting 20% and 27% reductions in the overpotential and Tafel slope compared with those of pristine NiFe-LDH, respectively. The results provide a strategy for regulating NiFe-based electrocatalysts by using emerging 2D materials to enhance water-splitting efficiency.

Phosphorus-Modulated Generation of Defective Molybdenum Sites as Synergistic Active Centers for Durable Oxygen Evolution

It is well known that electrocatalytic oxygen evolution reaction (OER) activities primarily depend on the active centers of electrocatalysts. In some oxide electrocatalysts, high-valence metal sites (e.g., molybdenum oxide) are generally not the real active centers for electrocatalytic reactions, which is largely due to their undesired intermediate adsorption behaviors. As a proof-of-concept, molybdenum oxide catalysts are selected as a representative model, in which the intrinsic molybdenum sites are not the favorable active sites. Via phosphorus-modulated defective engineering, the inactive molybdenum sites can be regenerated as synergistic active centers for promoting OER. By virtue of comprehensive comparison , it is revealed that the OER performance of oxide catalysts is highly associated with the phosphorus sites and the molybdenum/oxygen defects. Specifically, the optimal catalyst delivers an overpotential of 287 mV to achieve the current density of 10 mA cm-2 , accompanied by only 2% performance decay for continuous operation up to 50 h. It is expected that this work sheds light on the enrichment of metal active sites via activating inert metal sites on oxide catalysts for boosting electrocatalytic properties.

pH-Activated Ce-Doped Molybdenum Oxide Nanoclusters for Tumor Microenvironment Responsive Photothermal and Chemodynamic Therapy

Molybdenum-based nanomaterials have shown promise for anticancer treatment due to their strong photothermal and redox-activated capabilities. Herein, we have fabricated cerium-doped MoO_x (Ce-MoO_v) with tunable Mo/Ce molar ratios by a one-pot method and investigated their effect on chemodynamic therapy (CDT) and photothermal therapy (PTT). It is found that Ce-MoO_v can self-assemble into nanoclusters in acidic conditions and the increasing Ce amount will generate oxygen vacancy defects and induce the valence change of Mo⁶⁺/Mo⁵⁺ and Ce⁴⁺/Ce³⁺, which leads to strong near-infrared absorption with high photothermal conversion efficiency of 71.31 and 49.86% for 808 and 1064 nm. Other than photothermal conversion, the materials demonstrate pH-/glutathione (GSH)-activated photoacoustic (PA) imaging capability in vitro. In addition, Ce-MoO_v acts as a CDT reagent capable of converting endogenous H₂O₂ to two types of reactive oxygen species (^•OH, ¹O₂) while depleting GSH. Ce-MoO_v demonstrates an excellent therapeutic effect against HCT116 cells and effectively reduces the intracellular GSH level and significantly increases the number of reactive radicals under 1064 nm laser irradiation as compared with the no-laser group in vitro. This work provides a new paradigm using lanthanide-doped polymetallic oxides for pH-/GSH-responsive photothermal/chemodynamic therapy with PA imaging ability.

F and rare V4+ doped cobalt hydroxide hybrid nanostructures: excellent OER activity with ultralow overpotential

Highly efficient and stable Earth abundant transition metal electrocatalysts are in great demand for the oxygen evolution reaction (OER), a bottleneck process involved in the water splitting reaction and metal-air batteries. Herein, we have demonstrated a single step direct fabrication of cobalt hydroxide (Co(OH)₂) nanowires doped with vanadium(V) in a less stable +4 oxidation state and fluoride (F) ions (V-Co(OH)₂) on a carbon cloth electrode that showed highly enhanced OER activity under alkaline conditions. V-Co(OH)₂ nanowires synthesized under the optimized conditions produced excellent OER activity with an ultralow overpotential of 136 mV at 10 mA cm^-2 (scan rate 1 mV s^-1), a small Tafel slope (51.6 mV dec^-1) and good stability over 72 h. To the best of our knowledge, this is the lowest overpotential reported for cobalt-based electrocatalysts to achieve a geometric current density of 10 mA cm^-2. The controlled synthesis and HR-TEM studies revealed the formation of hybrid nanostructures (nanowires along with spherical assembly of nanoparticles) and codoping of V and F ions played an important role in enhancing the OER activity. The detailed chemical composition and oxidation state analysis by X-ray photoelectron spectroscopy (XPS) confirmed the doping of V⁴⁺ and ionic F in V-Co(OH)₂ with mixed valence states of Co²⁺/Co³⁺ and a higher Co²⁺ ratio. The outstanding OER activity of V-Co(OH)₂ is attributed to the formation of a spherical assembly of nanoparticles with nanowires, which provided a high number of catalytically active sites with enhanced charge transport, and doping of higher valence V⁴⁺ and strongly electronegative F in V-Co(OH)₂ with a higher ratio of Co²⁺/Co³⁺ promoted OOH* intermediate generation and significantly boosted the OER activity. Overall, the present work highlights the possibility of achieving highly active Earth abundant OER electrocatalysts by controlling the mixed oxidation state of Co with a judicious choice of dopants along with maintaining optimal nanostructure morphologies.

Heterogeneous Fe-Doped NiCoP-MoO3 Efficient Electrocatalysts for Overall Water Splitting

Transition metal phosphides with excellent performance are one of the effective alternatives to noble metal catalysts in overall water splitting. In this paper, the Fe-NiCoP-MoO3 composite was prepared by a facile synthesis as the bifunctional electrocatalyst. Fe-NiCoP-MoO3 achieves an operating current density of 10 mA/cm2 at a low overpotential of 65 mV for hydrogen evolution reaction and drives an operating current density of 50 mA/cm2 at only 293 mV for oxygen evolution reaction. Significantly, Fe-NiCoP-MoO3 was employed as the anode and cathode for overall water splitting, which only requires a cell voltage of 1.586 V to reach 10 mA/cm2 as well as shows excellent stability. The electrocatalytic activity of Fe-NiCoP-MoO3 exceeds most of the recently reported typical bifunctional electrocatalysts. This may be due to the coupling effect between the polymetallic phosphides. In addition, heterogeneous catalysts generally expose more active sites than homogeneous catalysts. In addition, replacing MoO3 with WO3 and VO3 can also improve the performance of Fe-NiCoP. This work provides an idea for the modification of phosphides.

Fast and efficient electrocatalytic oxidation of glucose triggered by Cu2O-CuO nanoparticles supported on carbon nanotubes

Electrocatalytic glucose oxidation reaction (GOR) is the key to construct sophisticated devices for fast and accurately detecting trace glucose in blood and food. Herein, a noble-metal-free Cu/C-60 catalyst is fabricated by supporting Cu₂O-CuO nanoparticles on carbon nanotubes through a novel discharge process. For GOR, Cu/C-60 shows a sensitivity as high as 532 μA mM⁻¹ cm⁻², a detection limit as low as 1 μM and a steady-state response time of only 5.5 s. Moreover, Cu/C-60 has outstanding stability and anti-interference ability to impurities. The synergistic effect of Cu₂O-CuO could improve the adsorption and conversion of glucose, thus enhancing GOR performance. By using Cu/C-60, we fabricate a three-electrode chip. A portable and compact electrochemical system is constructed by connecting the three-electrode chip with Cu/C-60 to an integrated circuit board and a mobile phone for recording and displaying data. The portable and compact electrochemical system results in a GOR sensitivity of 501 μA mM⁻¹ cm⁻², which is close to the data measured on the bloated electrochemical workstation. The detection limit of the portable and compact electrochemical system in GOR is 50 μM. This is higher than those obtained on the bloated electrochemical workstation, but is much lower than the common blood glucose concentration of human body (>3 mM). This demonstrates the accuracy, reasonability and applicability of the portable and compact electrochemical system. The results of the present work are helpful for fabricating fast, efficient and portable devices for detecting trace amount of glucose in blood and food.

Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production

The development of highly active non-precious metal electrocatalysts is crucial for advancing the practical application of hydrogen evolution reaction (HER). Doping engineering is one of the important strategies to optimize the electrocatalytic activity of electrocatalysts. Herein, we put forward a simple strategy to optimize the catalytic activity of MoO3 material by incorporating the Cu atoms into the interlayer (denoted as Cu-MoO3). The prepared Cu-MoO3 nanosheet has a larger surface area, higher conductivity, and strong electron interactions, which contributes to optimal reaction kinetics of the HER process. As a result, the Cu-MoO3 nanosheet only needs a small overpotential of 106 mV to reach the geometric current density of 10 mA cm−2. In addition, it also delivers a low Tafel slope of 83 mV dec−1, as well as high stability and Faraday efficiency. Notably, when using the Cu-MoO3 as a cathode to construct the water electrolyzer, it only needs 1.55 V to reach the 10 mA cm−2, indicating its promising application in hydrogen generation. This work provides a novel type of design strategy for a highly active electrocatalyst for an energy conversion system.

NiFeMn-Layered Double Hydroxides Linked by Graphene as High-Performance Electrocatalysts for Oxygen Evolution Reaction

Currently, precious metal group materials are known as the efficient and widely used oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. The exorbitant prices and scarcity of the precious metals have stimulated scale exploration of alternative non-precious metal catalysts with low-cost and high performance. Layered double hydroxides (LDHs) are a promising precursor to prepare cost-effective and high-performance catalysts because they possess abundant micropores and nitrogen self-doping after pyrolysis, which can accelerate the electron transfer and serve as active sites for efficient OER. Herein, we developed a new highly active NiFeMn-layered double hydroxide (NFM LDH) based electrocatalyst for OER. Through building NFM hydroxide/oxyhydroxide heterojunction and incorporation of conductive graphene, the prepared NFM LDH-based electrocatalyst delivers a low overpotential of 338 mV at current density of 10 mA cm⁻² with a small Tafel slope of 67 mV dec⁻¹, which are superior to those of commercial RuO₂ catalyst for OER. The LDH/OOH heterojunction involves strong interfacial coupling, which modulates the local electronic environment and boosts the kinetics of charge transfer. In addition, the high valence Fe³⁺ and Mn³⁺ species formed after NaOH treatment provide more active sites and promote the Ni²⁺ to higher oxidation states during the O₂ evolution. Moreover, graphene contributes a lot to the reduction of charge transfer resistance. The combining effects have greatly enhanced the catalytic ability for OER, demonstrating that the synthesized NFM LDH/OOH heterojunction with graphene linkage can be practically applied as a high-performance electrocatalyst for oxygen production via water splitting.