Interfacial Se-O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation.

Surface engineering is an effective strategy to improve the photoelectrochemical (PEC) catalytic activity of hematite, and the defect states with abundant coordinative unsaturation atoms can serve as anchoring sites for constructing intimate connections between semiconductors. On this basis, we anchored an ultrathin FeSe2 layer on Nb5+-doped Fe2O3 (FeSe2/Nb:Fe2O3) via interfacial Se-O chemical bonds to tune the surface potential. Density functional theory (DFT) calculations indicate that amorphous FeSe2 decoration could generate electron delocalization over the composite photoanodes so that the electron mobility was improved to a large extent. Furthermore, electrons could be transferred via the newly formed Se-O bonds at the interface and holes were collected at the surface of electrode for PEC water oxidation. The desired charge redistribution is in favor of suppressing charge recombination and extracting effective holes. Later, work function calculations and Mott-Schottky (M-S) plots demonstrate that a type-II heterojunction was formed in FeSe2/Nb:Fe2O3, which further expedited carrier separation. Except for spatial carrier modulation, the amorphous FeSe2 layer also provided abundant active sites for intermediates adsorption according to the d band center results. In consequence, the target photoanodes attained an improved photocurrent density of 2.42 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), 2.5 times as that of the bare Fe2O3. This study proposed a defect-anchoring method to grow a close-connected layer via interfacial chemical bonds and revealed the spatial charge distribution effects of FeSe2 on Nb:Fe2O3, giving insights into rational designation in composite photoanodes.

Roles of Cobalt-Coordinated Polymeric Perylene Diimide in Hematite Photoanodes for Improved Water Oxidation.

Interfacial charge recombination is a permanent issue that impedes the photon energy utilization in photoelectrochemical (PEC) water splitting. Herein, a conjugated polymer, urea linked perylene diimide polymer (PDI), is introduced to the designation of hematite-based composite photoanodes. On account of its unique molecule structure with abundant electronegative atoms, the O and N atoms with lone electron pairs can bond with Fe atoms at the surface of Zr4+ doped α-Fe2 O3 (Zr:Fe2 O3 ) and thus establish charge transfer channels for expediting hole separation and migration. Meanwhile, PDI molecules can passivate the surface states in Zr:Fe2 O3 , which is in favor of suppressing carrier recombination. Particularly, Co2+ is used to coordinate with PDI (Co-PDI) to accelerate hole extraction as well as utilization, and the as-obtained Co-PDI form type-II heterojunction with Zr:Fe2 O3 . Such a photoanode configuration takes advantage of the unique molecule structure of PDI, and the target Co-PDI/Zr:Fe2 O3 photoanodes eventually attain a photocurrent density of 2.17 mA cm-2 , which is inspirational for unearthing the potential use of conjugative molecules in PEC fields.

Construction of nickel sulfide phase-heterostructure for alkaline hydrogen evolution reaction.

Constructing transitionmetalsulfides (TMSs) heterostructure is an effective strategy to optimize the catalytic performance for hydrogen evolution reaction (HER) in alkaline medium. Herein, the rhombohedral nickel sulfide/hexagonal nickel sulfide (r-NiS/h-NiS) catalysts with the NiS phase-heterostructure were successfully fabricated by a simple one pot method. The r-NiS/h-NiS (1.25) (1.25 means the theoretical mole ratio of S and Ni added to reaction) displayed the excellent HER performance with low overpotential (101 ± 1 mV@10 mA cm-2) and small Tafel slope (62.10 ± 0.1 mV dec-1), which were superior to the pure phase r-NiS and h-NiS. In this work, the improved HER catalytic performances were attributed to the dense coupling interfaces between the r-NiS and h-NiS. This work shows the feasibility of construction NiS phase-heterostructure and provides a novel strategy for the application of NiS for water splitting.

Expediting hole transfer via surface states in hematite-based composite photoanodes.

Regarding the indirect hole transfer route in hematite-based photoelectrodes, the widely accepted viewpoint is that the FeIVO states act as a hole transfer medium, while other types of surface states act as recombination centers. Alternatively, it has rarely been reported that the recombining surface states may contribute to the charge transport in modified photoelectrodes. In this study, we employed CoCr layered double hydroxide (LDH)/Fe2O3 and CoCr LDH/Zr:Fe2O3 as research models to investigate the distinct charge transfer pathways in composite photoanodes. Different from the adverse role of surface states at ∼0.7 V versus the reversible hydrogen electrode (r-SS) in the bare hematite photoelectrodes (Fe2O3 or Zr:Fe2O3), the r-SS in the composite photoanodes (CoCr LDH/Fe2O3 or CoCr LDH/Zr:Fe2O3) served as a hole transfer station to induce high-valent Co cations, and the position of r-SS determined the onset potential of the composite photoelectrodes. Moreover, the FeIVO states still acted as active intermediates to transport numerous holes to the cocatalyst, which enhanced the charge utilization efficiency at 1.23 V versus the reversible hydrogen electrode (RHE) to a large extent. Besides, a noteworthy fact is that Zr doping increased the number of active FeIVO states, which significantly contributed to the enhancement in current density. However, it led to a delayed onset potential because of the positively shifted surface states (r-SS and FeIVO). Evidently, the different surface state distributions between Fe2O3 and Zr:Fe2O3 gave rise to anisotropic charge transfer and recombination behavior in the composite photoanodes. This study gives extensive insight into the hole transfer route in composite photoanodes and reveals the surface state-tuning effects of dopants and cocatalysts, which are significant for a deep understanding of the surface states and optimal design of composite photoanodes via surface state modulation.

Zr(OH)4 -Catalyzed Controllable Selective Oxidation of Anilines to Azoxybenzenes, Azobenzenes and Nitrosobenzenes.

The selective oxidation of aniline to metastable and valuable azoxybenzene, azobenzene or nitrosobenzene has important practical significance in organic synthesis. However, uncontrollable selectivity and laborious synthesis of the expensive required catalysts severely hinders the uptake of these reactions in industrial settings. Herein, we have pioneered the discovery of Zr(OH)4 as an efficient heterogeneous catalyst capable of the selective oxidation of aniline, using either peroxide or O2 as oxidant, to selectively obtain various azoxybenzenes, symmetric/unsymmetric azobenzenes, as well as nitrosobenzenes, by simply regulating the reaction solvent, without the need for additives. Mechanistic experiments and DFT calculations demonstrate that the activation of H2 O2 and O2 is primarily achieved by the bridging hydroxyl and terminal hydroxyl groups of Zr(OH)4 , respectively. The present work provides an economical and environmentally friendly strategy for the selective oxidation of aniline in industrial applications.

Interface engineering triggered by carbon nanotube-supported multiple sulfides for boosting oxygen evolution.

Finding an efficient, stable and cheap oxygen evolution reaction (OER) catalyst is very important for renewable energy conversion systems. There are relatively few related research reports due to the thermodynamic instability of transition metal sulfides (TMSs) at the oxidation potential and these are usually focused on single metal sulfides or bimetal sulfides. Metal sulfide mixture systems are rarely studied. The fabrication of a TMS/TMS interface is a feasible method to improve the kinetics of the OER. Here, we constructed TMS hybrid electrocatalysts with multiple phase interfaces for the oxygen evolution reaction, named S-CoFe/CNTs. The results show that the S-CoFe/CNT catalyst exhibits a low overpotential of 258 mV to achieve a current density of 10 mA cm-2, and has high activity in the OER process. Meanwhile, the catalyst also shows a low Tafel slope (69 mV dec-1) and good stability. This can be attributed to the synergistic catalysis of the multiphase interface in the catalyst and the rapid electron transfer pathway brought by CNTs. The new strategy for the synthesis of catalysts containing the TMS/TMS interface provides a new idea and method for the development of efficient and practical water splitting catalysts.

Bifunctional citrate-Ni0.9Co0.1(OH)x layer coated fluorine-doped hematite for simultaneous hole extraction and injection towards efficient photoelectrochemical water oxidation.

Surface modification by loading a water oxidation co-catalyst (WOC) is generally considered an efficient means to optimize the sluggish surface oxygen evolution reaction (OER) of a hematite photoanode for photoelectrochemical (PEC) water oxidation. However, the surface WOC usually exerts little impact on the bulk charge separation of hematite. Herein, an ultrathin citrate-Ni0.9Co0.1(OH)x [Cit-Ni0.9Co0.1(OH)x] is conformally coated on the fluorine-doped hematite (F-Fe2O3) photoanode for PEC water oxidation to simultaneously promote the internal hole extraction and surface hole injection of the target photoanode. Besides, the conformally coated Cit-Ni0.9Co0.1(OH)x overlayer passivates the redundant surface trap states of F-Fe2O3. These factors result in a superior photocurrent density of 2.52 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (V vs. RHE) for the target photoanode. Detailed investigation manifests that the hole extraction property in Cit-Ni0.9Co0.1(OH)x is mainly derived from the Ni sites, while Co incorporation endows the overlayer with more catalytic active sites. This synergistic effect between Ni and Co contributes to a rapid and continuous hole migration pathway from the bulk to the interface of the target photoanode, and then to the electrolyte for water oxidation.

The enhanced water splitting activity of a ZnO-based photoanode by modification with self-doped lanthanum ferrite.

The difficult separation and transfer of photoexcited charge carriers in composite photoelectrodes is a decisive factor limiting the efficiencies of semiconductor-based photoelectrochemical water splitting systems. Herein, to further enhance the photoelectrochemical properties of ZnO-based photoanodes, we constructed composite ZnO nanoarray photoanodes with Fe-self-doped lanthanum ferrite (denoted as La1-xFe1+xO3/ZnO NRs), which had the effect of killing two birds with one stone. This improvement strategy differs from the previously popular multi-step modification process, and integrates the dual benefits of a heterojunction and cocatalyst using the same material, the doped LaFeO3, which bypasses the shortcomings of multi-step charge transfer. Gratifyingly, benefitting from the suitable energy bands and excellent electrocatalytic oxygen evolution activity of La0.9Fe1.1O3, the photoanode exhibits outstanding bulk charge separation and surface charge utilization efficiencies, as well as achieving a photocurrent density that is over three times higher than that of pristine ZnO NRs, with a small onset potential (0.33 V vs. RHE). This electrode modification concept provides guidance for the development of other highly active photoelectrodes.

A oxygen vacancy-modulated homojunction structural CuBi2O4 photocathodes for efficient solar water reduction.

The photoelectrochemical (PEC) water reduction performance of CuBi2O4 (CBO)-based photocathodes is still far from their theoretical values due to low bulk and surface charge separation efficiencies. Herein, we propose a regrowth strategy to prepare a photocathode with CBO coating on Zn-doped CBO (CBO/Zn-CBO). Furthermore, NaBH4 treatment of CBO/Zn-CBO introduced oxygen vacancies (Ov) on CBO/Zn-CBO. It was found that Zn-doping not only increases the charge carrier concentration of CBO, but also leads to appropriate band alignment to form homojunctions. This homojunction can effectively promote the separation of electron-hole pairs, thus obtaining excellent photocurrent density (0.5 mA cm-2 at 0.3 V vs. RHE) and charge separation efficiency (1.5 times than CBO). The following surface treatment induced Ov on CBO/Zn-CBO, which significantly increased the active area of the surface catalytic reaction and further enhanced the photocurrent density (0.6 mA cm-2). In the absence of cocatalysts, the electron injection efficiency of Ov/CBO/Zn-CBO was 1.47 times improved than that of CBO. This work demonstrates a homojunction photocathode with Ov modulation, which provides a new view for future photoelectrochemical water splitting.

Coupling interface structure in NixS/Cu5FeS4 hybrid with enhanced electrocatalytic activity for alkaline hydrogen evolution reaction.

Bornite (Cu5FeS4) exhibits great potential for the alkaline hydrogen evolution reaction (HER) and few studies have been conducted on its electrocatalytic activity. Herein, we successfully fabricate NixS/Cu5FeS4 hybrid catalyst with interface structure between NixS nanoparticles (NPs) and Cu5FeS4 NPs. The NixS/Cu5FeS4 hybrid catalyst exhibits favorable HER performances in 1.0 M KOH electrolyte and demonstrates smaller overpotential and lower Tafel slope than bare NixS NPs and Cu5FeS4 NPs. The remarkable HER performances are attributed to the strongly coupling interface structure between NixS NPs and Cu5FeS4 NPs, which leads to synergistic effect optimizing the HER activity and enhancing the charge transfer during catalytic process. This work provides a promising strategy for the construction of Cu5FeS4-based hybrid catalyst and its application in energy systems.

Surfactant-free self-assembly to the synthesis of MoO3 nanoparticles on mesoporous SiO2 to form MoO3/SiO2 nanosphere networks with excellent oxidative desulfurization catalytic performance.

Recently, oxidative desulfurization (ODS) is favoured by researchers because it is based on mild conditions and does not consume hydrogen. However, the preparation process of catalyst for ODS was not green or costly, which limits its further industrial applications. In this study, a facile route has been explored to grow the mesoporous MoO3/SiO2 nanosphere networks (MoO3/SiO2 NN) using low-cost air without surfactants. Herein, the air not only served as the template to self-assemble and form the nanosphere network structure but acted as a mesopore-directing agent to make mesopores on the MoO3/SiO2 nanosphere. Moreover, the recovered waste mother liquor was also successfully applied to prepare nanomaterials. Gratifyingly, the nanocomposites of MoO3/SiO2 NN displayed remarkable pore structure, large specific surface area (201 m2  g-1) and excellent amphipathy (CA = 24.7° and 13.6° of water and n-octane, respectively) making it a promising catalyst for two-phase ODS reaction with H2O2 as an oxidant. Meanwhile, the high TOF value (56.6 h-1) and outstanding durability were obtained under optimum conditions (Yield > 99 % at 70 °C and O/S = 8:1 for 1 h, 20 mg catalyst) and the products were detected by GC-MS and 1H NMR. Therefore, an environmentally benign self-assembly procedure can facilely prepare more types of mesoporous catalysts for large-scale industrial application.

Vertically aligned FeOOH nanosheet arrays on alkali-treated nickel foam as highly efficient electrocatalyst for oxygen evolution reaction.

The adverse effects caused by global climate warming continue to be a great impetus to develop electrocatalytic water splitting technology for hydrogen source production. However, there is an urgent necessity but it is still a significant challenge to explore electrocatalysts with excellent performance, low cost, and environmental benignity for expediting the oxygen evolution reaction (OER) owing to the sluggish reaction kinetics. Fe-based materials, especially FeOOH, have great potential as OER electrocatalysts but suffers from poor electrical conductivity. Herein, we rationally designed and successfully synthesized FeOOH nanosheet arrays supported on alkali-treated nickel foam (FeOOH NSAs/ATNF) and applied it as an electrocatalyst toward OER. The FeOOH NSAs/ATNF catalyst exhibited outstanding performance with small overpotential, fast kinetics and superior stability in alkaline medium. Our research opens up a facile and effective approach to develop cost-effective and high-performance electrocatalysts for energy conversion, especially for these Fe-based materials with poor electrical conductivity.

Activating a hematite nanorod photoanode via fluorine-doping and surface fluorination for enhanced oxygen evolution reaction.

Poor charge separation and sluggish oxygen evolution reaction (OER) kinetics are two typical factors that hinder the photoelectrochemical (PEC) applications of hematite. Dual modification via heteroatom doping and surface treatment is an attractive strategy to overcome the above problems. Herein, for the first time, a hematite nanorod photoanode was ameliorated via the fluorine treatment (F-treatment) of both bulk and surface, enabling simultaneous charge separation from the interior to the interface. Accordingly, the novel photoanode (FeFx/F-Fe2O3) exhibited an outstanding PEC water oxidation activity, with a 3-fold improved photocurrent density than that obtained using unmodified α-Fe2O3. More specifically, fluorine doping (F-doping) in the hematite bulk remarkably increased the concentration of charge carriers and endowed it with favorable electrical conductivity for rapid charge transfer. Further surface F-treatment on F-doped α-Fe2O3 (F-Fe2O3) enriched the F-Fe bonds on the surface, which significantly boosted the OER kinetics and thereby inhibited the detrimental charge recombination. As a consequence, the efficiencies of bulk electron-hole pair separation and surface hole injection increased by 2.8 and 1.7 times, respectively. This study points to fluorine modulation as an attractive avenue to advance the PEC performance of metal oxide-based photoelectrode materials.

Layered Double Hydroxide onto Perovskite Oxide-Decorated ZnO Nanorods for Modulation of Carrier Transfer Behavior in Photoelectrochemical Water Oxidation.

Despite the fact that perovskite oxides with high photoelectrochemical (PEC) stability have gained widespread concern in the field of photo(electro)catalytic water splitting, the potential as a photoelectrode has not yet fully exploited. Herein, perovskite oxide-decorated ZnO nanorod photoanode improves the vital issue that photoproduced electron-hole pairs are apt to be quenched, in which type II band alignment between perovskite oxide and ZnO plays a crucial role in extracting carriers. Further, coupling with layered double hydroxide (LDH) onto the heterostructure not only tunes surface injection behavior of charge carriers by facilitating the interface reaction dynamics but also suppresses ZnO self-corrosion for extended durability. As a result, the optimized CoAl-LDH/LaFeO3/ZnO nanorod photoanode yields a much enhancive effect for the PEC property in terms of photocurrent density (2.46 mA cm-2 at 1.23 V vs reversible hydrogen electrode under AM 1.5G), onset potential, and stability. This work signifies a feasible design to combine promising perovskite oxides with the traditional photoelectrode system for achieving efficient water splitting.

Electrohydrogenation of Carbon Dioxide using a Ternary Pd/Cu2 O-Cu Catalyst.

A simple one-pot method has been developed to synthesize a palladium/cuprous oxide-copper (Pd/Cu2 O-Cu) material with a well-defined structure, by modification of Cu2 O-Cu with Pd through a galvanic replacement reaction. Compared with the well-known copper/cuprous oxide (Cu/Cu2 O) catalysts, the Pd/Cu2 O-Cu material can catalyze the electroreduction of CO2 into C1 products with much higher faradaic efficiencies at lower overpotentials in a CO2 -saturated 0.5 m NaHCO3 solution. In particular, the highest faradaic efficiencies of 92 % for formate and 30 % for methane were achieved at -0.25 and -0.65 V (vs. the reversible hydrogen electrode), respectively. The improvement is suggested to be the result of a synergistic effect between PdH and the catalytically active copper sites during electrochemical CO2 reduction.

Strongly Coupled Interface Structure in CoFe/Co3 O4 Nanohybrids as Efficient Oxygen Evolution Reaction Catalysts.

The quest for developing electrochemical energy-storage and -conversion technologies continues to be a great impetus to develop cost-effective, highly active, and electrochemically stable electrocatalysts for overcoming the activation energy barriers of the oxygen evolution reaction (OER). Co3 O4 nanocrystals have great potential as OER catalysts, and research efforts on improving the catalytic activity of Co3 O4 are currently underway in many laboratories. Herein, CoFe layered double hydroxide (LDH) nanosheets were directly grown on the active Co3 O4 substrate to form nanohybrid electrocatalysts for OER. The CoFe LDH/Co3 O4 (6:4) nanohybrid exhibited superior catalytic performance with a low overpotential and a small Tafel slope in alkaline solution. The outstanding performance of the CoFe LDH/Co3 O4 (6:4) nanohybrid was primarily owing to the synergistic effects induced by the strongly coupled interface between CoFe LDH and Co3 O4 ; this feature enhanced the intrinsic OER catalytic activity of the nanohybrid and favored fast charge transfer. Compared with other Co3 O4 -based catalysts, the nanohybrid shows advantages and offers a feasible avenue for improving the activity of Co3 O4 -based catalysts.

Interfacial N-Cu-S coordination mode of CuSCN/C3N4 with enhanced electrocatalytic activity for hydrogen evolution.

Nitrogen/carbon layer coordinated transition metal complexes are the most important alternatives to improve the catalytic performance of catalysts for energy storage and conversion systems, which require systematic investigation and improvement. The coordination mode of transition metal ions can directly affect the catalytic performance of catalysts. Herein, this paper reports that two kinds of Cu-based composites (CuSCN and CuSCN/C3N4) are prepared by in situ controllable crystallization of copper foam (CF) through electropolymerization and calcination. As a comparison, it is clarified that the different coordination modes of Cu1+ ions determine the different catalytic properties. The samples can be switched freely by tuning the electropolymerization period, which leads to different coordination modes of Cu1+ ions dramatically, thus affecting the electrocatalytic performance of composite materials for the hydrogen evolution reaction (HER) in turn. Thorough characterization using techniques, including X-ray photoelectron spectroscopy (XPS) and synchrotron-based near edge X-ray absorption fine structure (EXAFS) spectroscopy, reveals that strong interactions between CuSCN and C3N4 of CuSCN/C3N4 facilitate the formation of subtle coordinated N-Cu-S species, of which electronic structures are changed. Density Functional Theory (DFT) calculations indicate that the electrons can penetrate from CuSCN to N atoms present in C3N4. As a result, CuSCN/C3N4 demonstrates a better catalytic performance than the conventional transition-metal-based electrocatalysts. Besides, CuSCN/C3N4 reflects almost identical hydrogen evolution reaction (HER) activity and stability in an acid electrolyte with Pt/C.

Amorphous CoFe Double Hydroxides Decorated with N-Doped CNTs for Efficient Electrochemical Oxygen Evolution.

The development and design of a highly active and affordable nanostructured material as an efficient electrocatalyst for electrochemical oxygen evolution is a pressing necessity to realize industrial production of hydrogen by water electrolysis. Amorphous nanocomposites have recently attracted interest owing to their superior electrocatalytic activity derived from their unique structure. Herein, amorphous CoFe double hydroxides (Am-CFDH) decorated with N-doped carbon nanotubes (NCNTs) is synthesized by a facile and simple one-pot approach under room temperature. Through electrochemical measurement, the bare Am-CFDH nanocomposite already exhibits a comparable oxygen evolution reaction (OER) activity to the commercial IrO2 catalyst on account of its amorphous nature and the interaction between Co and Fe. The introduced NCNTs can provide better electrical conductivity, more anchoring sites, and functional groups for enhancing the transfer of electrons and reactants, preventing the agglomeration of Am-CFDH to expose more active sites, and improving the synergistic effect between Am-CFDH and NCNTs. Thus, the Am-CFDH/NCNTs hybrid displays favorable durability beyond 20 h and advanced OER activity, owning a small overpotential of 270 mV at 10 mA cm-2 and a low Tafel slope of 56.88 mV dec-1 in alkaline medium.

Optimization of iron-doped Ni3S2 nanosheets by disorder engineering for oxygen evolution reaction.

Nowadays, disorder engineering of catalytic materials has attracted significant attention because it can increase catalytic active sites and thus enhance their catalytic activity for electrocatalytic reactions. However, it is extremely important to uncover the relationship between disorder engineering and catalytic activity. Particularly, deep exploration of the relationship is very important for fabricating excellent highly active catalysts for oxygen evolution reaction (OER), which is one of the promising technologies in energy transition. In this study, we prepared Fe-doped Ni3S2 materials and simultaneously controlled the disorder degree by regulating the ion concentration to improve the activity for OER. By investigating the as-prepared catalysts with various disorder degrees for OER, we also explored the relationship between the disordered structure and OER catalytic performance. In particular, the optimized electrocatalyst with an appropriate disorder degree showed excellent activity and stability. We hope that this study provides a feasible direction to fabricate and optimize transition metal chalcogenide (TMC) electrocatalysts as efficient and stable electrocatalysts for OER.

Nano-Cu-Mediated Multi-Site Approach to Ultrafine MoO2 Nanoparticles on Poly(diallyldimethylammonium chloride)-Decorated Reduced Graphene Oxide for Hydrogen Evolution Electrocatalysis.

Catalysts with high atom utilization efficiency accompanied by improved reactivity and durability are highly desired. Metallic MoO2 with its small size easily agglomerates, making it difficult to use in water splitting to obtain hydrogen by electrolysis. Here, the nano-Cu-mediated multi-site method is proposed to prepare ultrafine MoO2 nanoparticles (NPs) dispersed on poly(diallyldimethylammonium chloride)-decorated reduced graphene oxide (denoted as MoO2 /PDDA-rGO). The introduction of Cu NPs increases the number of growth sites for MoO2 on the PDDA-rGO and simultaneously promotes the growth rate of MoO2 on PDDA-rGO. As a consequence, the resulting size of the MoO2 NPs is only 2 nm and these are evenly dispersed on PDDA-rGO. Significantly, the optimized catalyst has a low onset potential of -42 mV versus reversible hydrogen electrode (RHE), a calculated Tafel slope of only 42 mV dec-1 , and good cycling stability of more than 40 h. This favored hydrogen evolution reaction (HER) activity is caused by the synergistic effects of MoO2 and PDDA-rGO, rapid charge transport, and sufficient exposed active sites of MoO2 /PDDA-rGO.