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The development of efficient non-noble metal electrocatalysts for the oxygen evolution reaction (OER) under acidic conditions remains a critical challenge. Herein, we report a N-doped carbonaceous component-engineered Co3O4 (NCEC) catalyst synthesized via the sol-gel method. Dopamine hydrochloride (DA)-derived nitrogen-doped carbonaceous components were found to boost the OER performance of Co3O4. The optimized catalyst can reach an overpotential as low as 330 mV in 1 M H2SO4 at a current density of 10 mA cm-2 and maintains a good long-term stability of 60 hours. In particular, we found that the thermodynamic overpotential was inversely proportional to the content of oxidized N and pyridinic N, whereas it was directly proportional to the pyrrolic-N content. Our experiments and density functional theory (DFT) calculations confirm that the optimized catalyst exhibits enhanced charge transfer and the oxidized N species on Co3O4 is responsible for the high catalytic activity. Our study suggests that the performance of NCEC in acidic media can be further optimized by enhancing the content of oxidized N species. [ABSTRACT FROM AUTHOR]

Interfacial engineering emerges as a potent strategy for regulating the catalytic reactivity of metal phosphides. Developing a facile and cost-effective method to construct bifunctional metal phosphides for highly efficient electrochemical overall water splitting remains an essential and challenging issue. Here, a multiphase transition metal phosphide is constructed through the direct phosphorization of a Ni–Co metal–organic framework grown on nickel foam (Ni–Co-MOF/NF), which is prepared by utilizing nickel foam as conductive substrate and nickel source. The resulting transition metal phosphide manifests a pillar-layered morphology, wherein CoP, Ni2P, and Ni5P4 nanoparticles are embedded within each carbon sheet and these carbon sheets assemble into a pillar-shaped structure on the nickel foam (Ni2P–Ni5P4–CoP–C/NF). The heterogeneous Ni2P-Ni5P4–CoP–C/NF with multiple interfaces serves as a highly efficient bifunctional electrocatalyst with overpotentials of −100 mV and 293 mV in the hydrogen evolution reaction and oxygen evolution reaction, respectively, at 50 mA cm−2 in alkaline media. This superior catalytic performance should mainly be ascribed to its enriched active centers and multiphase synergy. When directly applied for alkaline overall water splitting, the Ni2P–Ni5P4–CoP–C/NF couple demonstrates satisfactory activity (1.55 V @10 mA cm−2) along with sustained durability over 18 hours. This method brings fresh enlightenment to the economical and controllable preparation of multi-metal phosphides for energy conversion. [ABSTRACT FROM AUTHOR]

The synthesis of highly efficient NiFe-layered double hydroxides (NiFe-LDHs) to catalyze the oxygen evolution reaction (OER) is urgent and challenging. Herein, NiFe–FeCl3-x and NiFe–FeCl2-x samples (where FeCl3 and FeCl2 represent the Fe sources and x represents the imposed reaction time: 6, 12, and 24 h) were prepared via one-pot hydrothermal synthesis using Fe sources characterized by Fe(III) or Fe(II) valence states. In the presence of triethanolamine, when FeCl3 was used as the Fe source, pure NiFe-LDH was obtained, whose crystallinity increased with increasing hydrothermal treatment time. In contrast, when FeCl2 was used as the Fe source, a mixture of NiFe-LDH, Fe2O3, and trace amounts of Fe3O4 was obtained. The content of NiFe-LDH in the mixture increased under longer hydrothermal treatment and NiFe–FeCl3-x catalysts exhibited better OER performance than NiFe–FeCl2-x catalysts. Specifically, NiFe–FeCl3-6 afforded the highest OER performance with an overpotential of 246.8 mV at 10 mA cm−2 and a Tafel slope of 46.1 mV dec−1. Herein, we investigated the effects of the valence state of Fe precursors on the structures and OER activities of the prepared catalysts; the mechanism of NiFe-LDH formation via hydrothermal synthesis in the presence of triethanolamine was also proposed. [ABSTRACT FROM AUTHOR]

It is highly desirable for porous coordination polymers (PCPs), including metal–organic frameworks (MOFs) and Prussian blue analogues (PBAs), to retain their intrinsic characteristics in electrocatalysis, instead of being used as precursors or templates for further total conversion to other compounds via high-temperature calcination. Here, a S-treated two-dimensional (2D) CoFe bimetallic PBA grown on carbon fiber paper (CFP) (named S-CoFe-PBA/CFP) is assembled and applied as a highly efficient oxygen evolution reaction (OER) electrocatalyst in 1 M KOH. The resultant S-CoFe-PBA/CFP demonstrates significantly improved OER catalytic activity; overpotentials of only 235, 259, and 272 mV are needed to drive current densities of 10, 50, and 100 mA cm−2, respectively, with a super low Tafel slope of 35.2 mV dec−1. Even more noteworthy, a current density of 90 mA cm−2 can be achieved when a potential of 1.5 V vs. RHE is applied, which is 6.4 times higher than that of commercial Ir/C in the same environment. The outstanding electrocatalytic performance can be ascribed to two reasons caused by the S-treatment process. On one hand, H+ from intermediates of *OH and *OOH can be captured by –SOx distributed on the surface of the catalyst, thus accelerating the breaking of O–H; on the other hand, partial phase transformation of CoFe-PBA leads to the in situ formation of amorphous CoSx nanogauze on the surface, and the resultant electronic interactions between the two phases contribute much to the improvement of charge transfer and adsorption for OER intermediates. This work provides a new avenue for the design of highly efficient PCP-based OER electrocatalysts. [ABSTRACT FROM AUTHOR]

We used sodium hydroxide-mediated approach and tannic acid etching to prepare hollow structured trimetallic MOF-derived CoFeNi/Z-P NC nanocomposites. Remarkably, the resulting CoFeNi/Z-P NC nanocomposites have large specific surface area and mesoporous structure, making their active sites more accessible and mass transfer more effective. More complex trimetallic components provide greater possibilities for further improving electrocatalytic performance. The CoFeNi/Z-P NC nanocomposites demonstrate notable enhancements for the OER, and 10 mA cm−2 current density is achieved at a low overpotential of 244 mV, with a low Tafel slope of 66.2 mV dec−1 and have good stability in alkaline solutions. In addition, as a cathode material for overall alkaline water splitting, CoFeNi/Z-P NC is better than RuO2 with longer cycling stability. [ABSTRACT FROM AUTHOR]

The exploitation of highly active and stable catalysts for reduction of CO2 and water oxidation is one of the approaches to facilitate scalable and sustainable CO2 reduction potentially at the industrial scale. Herein, a feasible strategy to rationally build an overall CO2 + H2O electrocatalytic reaction device is the preparation and matchup of a high-performance CO2 reduction catalyst and low-cost and highly active oxygen anode catalyst. A heterostructured nanosheet, γ-NiOOH/NiCO3/Ni(HCOO)2, exhibited superior catalytic activity in the oxygen evolution reaction, and was integrated with CoPc/Fe–N–C to build an overall CO2 + H2O cell with a current density of 10 mA cm−2 at a very low cell voltage of 1.97 V, and the faradaic deficiency of CO2 to CO was maintained at greater than 90% at 1.9 V. [ABSTRACT FROM AUTHOR]

It is widely acknowledged that interface engineering strategies can significantly enhance the activity of catalysts. In this study, we developed a CoMoP nanoarray directly grown in situ on a nickel foam (NF) substrate, with the interface structure formed through the electrodeposition of MnOxHy. The resulting heterostructure MnOxHy/CoMoP/NF exhibited remarkable hydrogen evolution reaction (HER) activity, achieving overpotentials as low as 61 and 138 mV at 10 and 100 mA cm−2, respectively. Moreover, MnOxHy/CoMoP/NF demonstrated efficient oxygen evolution reaction (OER) activity with an overpotential of 330 mV at 100 mA cm−2. Remarkably, MnOxHy/CoMoP/NF maintained its catalytic properties and structural integrity even after working continuously for 20 h facilitating the HER at 10 mA cm−2 and the OER at 100 mA cm−2. The Tafel slopes of the HER and OER were determined to be as small as 14 and 55 mV dec−1, respectively, confirming that the coupled interface conferred fast reaction kinetics on the catalyst. When applied in overall water splitting, MnOxHy/CoMoP/NF delivered a voltage of 1.91 V at 100 mA cm−2 with excellent stability. This study demonstrated the feasibility of utilizing a simple electrodeposition technique to fabricate a heterogeneous structure with bifunctional catalytic activity, establishing a solid foundation for diverse industrial applications. [ABSTRACT FROM AUTHOR]

Powder electrocatalysts for oxygen evolution reactions usually need adhesives for electrocatalytic performance tests, leading to the increase of resistance, reduction of catalyst loading, and easy stripping of the catalyst under long-time or high current operation. In this study, Ce-doped CoFe layered double hydroxides were uniformly grown on nickel foam by a one-step hydrothermal route. A nanostructured self-supported electrode Ce-CoFe-LDH/NF without adhesive was obtained directly, which has a regular nanoneedle morphology with a length of ∼1.2 μm and tip width of ∼20 nm. Adopting Ce3+ ions with a large radius to partially displace Fe3+ ions with a small radius produced lattice distortion and more defects in the host layer of CoFe-LDH, whereby possessing the great potential to enhance catalytic behaviors. Once used as an electrocatalyst for the oxygen evolution reaction, Ce-CoFe-LDH/NF shows an outstanding electrocatalytic performance, including an optimized overpotential of 225 mV at 10 mA cm−2, a decreased Tafel slope of 34.34 mV dec−1, and a low charge transfer impedance of 2.4 Ω in 1 M KOH electrolyte. Moreover, the overpotential of the working electrode increased by only 0.04 V after 24 hours and was maintained at a current density of 50 mA cm−2. These results demonstrate a low-cost strategy compared to using noble metal OER electrocatalysts. Thus, this study highlights a ready universal approach to fabricate high-performance supported catalysts for energy-related applications. [ABSTRACT FROM AUTHOR]

Designing an excellent OER catalyst in an alkaline environment is severe yet essential for industrial H2 application under the electrochemical technique. This study has achieved multiple modifications on CoN nanowires, the classic OER catalyst, via a facile room-temperature NaBH4 spontaneous hydrolysis. This facile process simultaneously generates oxygen vacancies and robust BN species. It wraps hydrophilic BOx motifs on the OER response CoN nanowires, producing OER active Co–N–B species, increasing active numbers and guaranteeing structural stability. It suggests that a low NaBH4 concentration (0.1 mol L−1) treatment endows CoNNWAs/CC with excellent OER performance and robust structure, which can drive a current density of 50 mA cm−2 with only 325 mV overpotentials with more than 24 hours' durability. Even, the catalyst can drive 1000 mA cm−2 around 480 mV overpotential. This study allows a novel strategy for designing high-performance OER catalysts. [ABSTRACT FROM AUTHOR]

Developing efficient bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts is beneficial for simplifying the design of electrolytic cells and reducing the cost of device manufacturing. Herein, a metal phosphide nanoarray (NiMo–Fe–P) electrocatalyst was designed by in situ ion exchange and low-temperature phosphating to promote overall water splitting in 1 M KOH. NiMo–Fe–P demonstrates superb HER and OER activities as reflected by the low overpotentials of 73.1 mV and 215.2 mV, respectively, at a current density of 10 mA cm−2. The addition of Fe changes the electronic structure of Ni, which is conducive to the chemisorption of oxygen-containing intermediates and reduces the energy barrier for water decomposition. Besides, the metal phosphide not only acts as the active site of the HER, but also improves the conductivity of the catalyst. Furthermore, nanowire arrays and the small particles generated on their surfaces provide a high electrochemical active surface area (ECSA), which was beneficial for the exposure of active sites. Attributed to these advantages, the cell voltage of the water electrolyzer constructed with NiMo–Fe–P as both the cathode and anode is only 1.526 V at 10 mA cm−2, and it maintains excellent stability for 100 h with near negligible changes in potential. [ABSTRACT FROM AUTHOR]

The rational design of a highly efficient oxygen evolution reaction (OER) is crucial for the practical applications of water electrolysis. Herein, a hybrid Co3O4@FeOOH/NF electrode was fabricated by loading FeOOH sheets on the surface of Co3O4 nanosheet arrays via a newly developed chemical deposition protocol. The decoration of FeOOH on Co3O4 nanosheet arrays not only endows a strong electronic interaction between the two components but also offers sufficient active sites for the OER process. Benefitting from these advantages, Co3O4@FeOOH/NF exhibited outstanding OER activity in terms of a low overpotential of 209 mV at 10 mA cm−2 and a low Tafel slope of 48.9 mV dec−1. Moreover, nearly steady state operation current and negligible change in the phase and morphology of the catalyst also indicate remarkable stability. This work may provide an important guide for the design of high-performance electrocatalysts for energy conversion applications. [ABSTRACT FROM AUTHOR]

The development of hydrogen–oxygen fuel cells with an alkaline electrolyte was highly limited by the sluggish kinetics of the hydrogen oxidation reaction (HOR). Here, with a pyrolysis-reduction route, a new RuNi-based electrocatalyst was prepared, which presents an ultrathin nanowire-like structure. In alkaline media, this catalyst shows an excellent catalytic performance with an exchange current density of 1.10 mA cm−2disk for hydrogen oxidation. The exchange current density and mass activity of this catalyst are much higher than those of its single-metal counterparts and even the commercial Pt/C catalyst containing 20% Pt. Such a remarkable catalytic activity can be explained by the interaction between Ru and Ni; the incorporation of Ni may induce an electronic effect on the optimization of the Ru–Had strength and provide a functional surface that can absorb OH species, thus boosting the catalytic activity. These findings may cast a new light on the exploration of low-cost but high-efficiency catalysts for fuel cells. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) has become the main barrier to electrochemical water splitting, owing to sluggish kinetics. To accelerate the OER process, a nature-abundant non-noble metal catalyst with outstanding catalytic activity is required. Here, a hierarchical CoMoO4@CoFe layered double hydroxide (LDH, denoted by CoMoO4@CoFe/NF) heterostructure supported on three-dimensional (3D) nickel foam (NF) was synthesised by electrodepositing CoFe-LDH nanosheets on the surface of CoMoO4 nanoplates to boost the electrocatalytic performance for the OER. The coupling between CoMoO4 and CoFe-LDH established abundant boundaries, which exposed abundant active sites. Furthermore, the electron transfer in the hierarchical heterostructure effectively promoted the adsorption and dissociation of H2O molecules. Harnessing the aforementioned synergistic effect significantly improved the intrinsic reaction kinetics of the OER. At 10 mA cm−2, CoMoO4@CoFe/NF reached a small overpotential of 245 mV at room temperature and exhibited outstanding stability for at least 47 h. Furthermore, the Tafel slope for CoMoO4@CoFe/NF was only 46 mV dec−1. This study provides new ideas for the rational design of hierarchical structures and the use of interface interactions. [ABSTRACT FROM AUTHOR]

The advancement of a bifunctional electrocatalyst consisting of Earth's rich elements and exhibiting high efficiency is the key to obtain hydrogen fuel by overall water splitting (OWS). Here, a facile and extensible hydrothermal synthesis of an electrocatalyst on iron foam (MoOx/Fe1−xS/IF) as a robust bifunctional catalyst with excellent catalytic activity is designed for the hydrogen evolution reaction (HER) with an overpotential of 142 mV at 100 mA cm−2, and for the OER with lower overpotentials of 300 and 500 mV at 100 and 1000 mA cm−2. The good activity is ascribed to the controllable morphology, stronger bonding of the catalyst to a substrate and optimized electronic configuration. When used as bifunctional electrocatalysts toward alkaline overall water splitting, MoOx/Fe1−xS/IF delivers a current density of 10 mA cm−2 at a low cell voltage of 1.56 V for 110 h. Such high performance coupled with low-cost iron-based materials suggests that the present strategy may open new avenues for the rational design of electrocatalysts and for use in practical water splitting. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER), as the rate-determining step of electrochemical water splitting, is extremely crucial, and thus it is a requisite to engineer feasible and effective electrocatalysts to shrink the reaction energy barrier and accelerate the reaction. Herein, monodisperse Mn3O4 nanoparticles on a PANI substrate were synthesized by polymerization and in situ oxidation. Combining Mn3O4 nanoparticles and PANI fibers can not only maximize the strong coupling effect and synergistic effect but also construct a well-defined three-dimensional structure with extensive exposed active sites, where the permeation and adherence of the electrolyte are made exceedingly feasible, thus displaying excellent OER activity. Benefiting from the outstanding structural stability, the resulting Mn3O4/PANI/NF is able to deliver a low overpotential of 262 mV at a current density of 10 mA cm−2, which outperforms the commercial RuO2 catalyst (275 mV) as well as presently reported representative Mn-based and PANI-based electrocatalysts and state-of-the-art OER electrocatalysts. The synthetic method for Mn3O4/PANI not only provides a brand-new avenue for the rational design of inorganic material/conductive polymer composites but also broadens the understanding of the mechanism of Mn-based catalysts for highly enhanced OER. [ABSTRACT FROM AUTHOR]

In order to lower energy consumption it is critical to develop highly efficient and stable non-precious metal bifunctional catalysts. In this study, we found that rational design of novel nanostructures is able to increase the number of active sites, conductivity and accelerate electron transfer, thus promoting enhanced performance of the catalyst. We successfully synthesized carbon nanotubes (CNTs) containing a hollow polyhedral (CNTHPs) structure through annealing, etching and phosphating. The unique hollow shape not only provides a stable structure but also facilitates mass and charge transfer. Thus, CoP/CNTHPs can catalyze the hydrogen and oxygen evolution reactions effectively with overpotentials of 147 and 238 mV at 10 mA cm−2. Simultaneously, CoP/CNTHPs only needs a voltage of 1.54 V to attain a current density of 10 mA cm−2 in the electrocatalytic water splitting process, demonstrating its bifunctional activity. Furthermore, the electrolytic catalytic performance does not weaken significantly after 12 hours of electrolysis, demonstrating excellent stability. Overall, this research offers useful insights into rational design of high-performance non-noble metal catalysts. [ABSTRACT FROM AUTHOR]

Developing high-efficiency and low-cost catalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is of great significance for the commercialization of rechargeable metal–air batteries. Herein, we demonstrated the construction of graphited carbon-coated FeTiO3 (FeTiO3@C) via in situ annealing Ti3C2Tx nanosheets in a rusted-reactor and its efficient bifunctional activity for rechargeable Zn–air batteries (RZABs). The electron-transport dynamics of FeTiO3@C can be improved by using highly conductive graphited carbon derived from Ti3C2Tx. The FeTiO3@C catalyst annealed at 500 °C exhibits excellent OER and ORR activities. Specifically, FeTiO3@C shows a low overpotential of 323 mV at 10 mA cm−2 and a small Tafel slope of 53 mV dec−1 towards the alkaline OER. During the OER process, FeTiO3@C can be partially converted into highly active iron oxyhydroxide via in situ electrochemical reconstruction, which serves as the active species. After being assembled to RZABs, it shows an open-circuit potential of 1.33 V, a high trip efficiency of 63.4% and long-time cycling stability. This work can provide a new avenue for developing bifunctional electrocatalysts for RZABs used in portable devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the context of the ever-increasing energy crisis, electrocatalytic water splitting has attracted widespread attention as an effective means to provide clean energy. However, the oxygen evolution reaction (OER), which is an important anodic half reaction, shows very slow kinetics due to the multi-step electron transfer process, which severely restricts the efficiency of energy conversion. Herein, we used a simple solvothermal method to dope iron into the cobalt-containing hydroxide precursor, and successfully prepared the Fe-doped Co-oxide electrocatalyst Co3−xFexO4-0.01. It only needs an overpotential of 294 mV to perform the OER at a current density of 10 mA cm−2, and has a low Tafel slope of 47.3 mV dec−1. Moreover, Co3−xFexO4-0.01 has excellent stability. There is no significant increase in the overpotential for oxygen evolution at a current density of 10 mA cm−2 after nearly 20 h. BET surface area test and XPS spectroscopy results show that Fe doping provides more mesopores and oxygen bridges, which is conducive to the construction of active sites and electronic regulation during the OER. This work can help design more bimetallic based highly active OER materials. [ABSTRACT FROM AUTHOR]

Electrocatalytic water splitting is one of the key technologies for future energy systems envisioned for the storage of energy obtained from variable renewables and green fuels. The development of efficient, durable, Earth-abundant and cheap electrocatalysts for the oxygen evolution reaction is a scorching area of research. The oxygen evolution reaction has huge potential for fuel cell and metal–air battery applications. Herein, we reported interfacially interacted and uniformly decorated Co3O4–NiO hybrid nanostructures formed by a metal–organic framework (Co2–BDC(OH)2) using BDC as a linker to the metal center. The fine nanosheets of Co2–BDC(OH)2 were first uniformly grown over the honeycomb-like structure of nickel foam (NF). After controlled calcination of these nanosheets/NF composites, a uniformly decorated, binder-free Co3O4–NiO/NF electrocatalyst was synthesized. The transformation of Co2-BDC(OH)2/NF into Co3O4–NiO/NF was characterized by several techniques such as powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy, transmission electron microscopy, etc. The catalyst exhibits a low overpotential of 311 mV vs. RHE at 10 mA cm−2 current density. The catalyst also shows long-term stability (24 h) with a Tafel slope value of 90 mV dec−1. The obtained experimental results are also in-line with the theoretical data acquired from model systems. [ABSTRACT FROM AUTHOR]

Large-scale application of rechargeable Zn–air batteries requires low-cost electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as alternatives to noble metals. Herein, FeCo nanoparticles embedded in N-doped carbon (FeCo/N–C) were prepared by a two-step pyrolysis route. FeCo/N–C exhibits excellent activities toward both the ORR (half-wave potential of 0.84 V) and OER (overpotential of 345 mV at 10 mA cm−2), which are comparable to those of commercial Pt/C and RuO2, and by far exceeding their counterparts Fe/N–C and Co/N–C. Furthermore, the FeCo/N–C catalyst was evaluated in a rechargeable Zn–air battery for the full-cell test. The FeCo/N–C based battery is more durable with a smaller round-trip overpotential after 800 cycles than the battery using an expensive Pt/C + RuO2 mixture catalyst. [ABSTRACT FROM AUTHOR]

Decorating a high-efficiency oxygen evolution reaction (OER) electrocatalyst as a cocatalyst on an α-Fe2O3 photoanode is known to be one of the most efficient methods to improve the photoelectrochemical (PEC) water oxidation activity. In our work, different from traditional methods of transition metal sulfide cocatalyst synthesis, an NiFeSx–decorated Ti-Fe2O3 photoanode is synthesized through a simple one-step electrodeposition method, which benefits the interface between Ti-Fe2O3 and NiFeSx. With the help of this excellent OER electrocatalyst, the photocurrent density of the NiFeSx–Ti-Fe2O3 photoanode rises to 3 mA cm−2 at 1.23 V vs. RHE, which is 2.5 times greater than the photocurrent of Ti-Fe2O3. Moreover, the onset potential of NiFeSx–Ti-Fe2O3 shifts negatively by 170 mV compared with that of pure Ti-Fe2O3. Furthermore, surface photovoltage spectroscopy (SPV) and transient photovoltage (TPV) techniques and photoelectrochemical impedance spectroscopy (PEIS) were used to analyze the true effects of NiFeSx as an efficient cocatalyst for enhancing the PEC performance of the NiFeSx–Ti-Fe2O3 photoanode. This work provides a simple method for loading a low-cost and efficient cocatalyst to modify a Ti-Fe2O3 photoanode for the PEC water oxidation reaction. [ABSTRACT FROM AUTHOR]

Hydrogen production by electrocatalytic water splitting is a pollution-free, energy-saving, and efficient method. The low efficiency of hydrogen production, high overpotentials and expensive noble-metal catalysts have limited the development of hydrogen production from electrocatalytic water splitting. Therefore, the exploration of bifunctional electrocatalysts for water overall splitting to produce hydrogen is of profound significance. Herein, Ni3S2@Ni2P/MoS2 heterostructure electrocatalysts were synthesized on Ni foam through an environmentally friendly hydrothermal method and low-temperature phosphating method. The synergistic effects between different components and the mutual substitution principle between sulfur atoms and phosphorus atoms greatly improve the OER performance of the electrocatalyst. It is also an effective strategy to optimize the adsorption energies of intermediates by the design of heterostructured catalysts composed of multiple substances. Ni3S2@Ni2P/MoS2 only requires a low overpotential (η10) of 175 mV at a current density of 10 mA cm−2 in 1.0 M KOH solution and the stable duration exceeds 40 h. In addition, this heterogeneous structure is assembled into an electrolytic cell for overall water splitting, which exhibits a low cell voltage of 1.61 volts and retains the robust stability over 30 h at 10 mA cm−2. The Ni3S2@Ni2P/MoS2 heterostructure prepared in this research provides a strategy for exploring other heterostructured electrocatalysts with different components. [ABSTRACT FROM AUTHOR]

Rational design of a catalyst using earth abundant transition metals that can facilitate the smooth O–O bond formation is crucial for developing efficient water oxidation catalysts. The coordination environment around the metal ion of the catalyst plays a pivotal role in this context. We have chosen dinuclear mixed-valence CoIIICoII complexes of the general formula of [CoIIICoII(LH2)2(X)(H2O)] (X = OAc or Cl) which bear a coordinated water molecule in the primary coordination sphere. We anticipated that the water molecule in the primary sphere can take part in the proton coupled electron transfer (PCET) mechanism which can accelerate the facile formation of the O–O bond under strong alkaline conditions (1 M NaOH). To understand the role of the coordinated water molecule we have generated an analogous complex, [CoIIICoII(LH2)2(o-vanillin)] (o-vanillin = 2-hydroxy-3-methoxybenzaldehyde), without coordinated water. Interestingly, we have found that the water coordinated complexes show better oxygen evolution reaction (OER) activity and stability. [ABSTRACT FROM AUTHOR]

Since the large reaction energy barrier caused by multi-step electron transfer processes of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gravely restricts the practical application of electrocatalytic water splitting, it is urgent to develop a dual-functional electrocatalyst which can effectively reduce the reaction energy barrier and actually speed up the reaction. Herein, the Ru species are doped into the complex of magnetite and FeNi-layered double hydroxide by a one-step oil bath method, and a self-supporting binder-free bifunctional electrocatalyst was synthesized on the surface of iron foam (named Ru–Fe3O4@FeNi–LDH/IF). The unique 3D core–shell microflower structure of Ru–Fe3O4@FeNi–LDH/IF, the combination of active ingredient and conductive substrate, together with the doping of Ru may immensely provide a large number of active sites, adjust the electronic structure, accelerate electron transfer, and thus greatly improve the electrocatalytic activity and durability. It is worth mentioning that when Ru–Fe3O4@FeNi–LDH/IF is used as the anode and cathode for overall water splitting, only 1.52 V battery voltage can generate a current density of 10 mA cm−2, and also maintain a prominent stability for at least 36 hours. This work provides a feasible strategy for heteroatom-doping LDH as a bifunctional electrocatalyst. [ABSTRACT FROM AUTHOR]

The design and synthesis of hybrid core–shell catalysts is of great significance for obtaining an excellent performance of hydrogen evolution reaction (HER). However, it remains a challenge to explore the exact active sites and research the catalytic mechanism for HER. Here, a series of Ni3S2@MOOH/NF (M = Fe, Ni, Cu, Mn and Co) hybrid structures is firstly in-site grown on Ni foam by the typical hydrothermal and electrodeposition methods. The Ni3S2@NiOOH/NF catalyst with a core–shell structure exhibits a relatively low overpotential of 79 mV for HER at a current density of 10 mA cm−2, which is one of the best catalytic activities reported so far. Moreover, it also shows good stability in the long-term durability test. Various spectral analysis and density functional theory calculations demonstrate that NiOOH is favorable for the adsorption of water molecules, and the S atom at the interface between Ni3S2 and NiOOH is favorable for the adsorption of H intermediates, which strongly accelerates the HER process in alkaline solution. This work provides a general strategy for the synthesis of electrocatalytic materials, which can be used for efficient electrocatalytic water splitting reactions. [ABSTRACT FROM AUTHOR]

A water-soluble copper complex with a diamine-tripyridine ligand was synthesized successfully and well characterized. It was found to be catalytically active for the water oxidation reaction under basic conditions. Based on the electrochemical test result, this copper complex displayed an apparent rate constant (kcat) of 0.81 s−1 for the oxygen evolution reaction in 0.1 M phosphate buffer solution at pH 11.0. More importantly, the copper complex remained stable over 3 h of a bulk electrolysis experiment at 1.60 V with a Faradaic efficiency of 90.7% for O2 evolution, and the decrement of current density was only 1.9%. These results suggest that the pentadentate copper complex is an efficient and durable homogeneous Earth-abundant electrocatalyst for water oxidation. [ABSTRACT FROM AUTHOR]

Rational design of non-noble metal electrocatalysts with high intrinsic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely impressive for sustainable electrocatalytic water splitting systems. However, it still remains a major challenge to engineer bifunctional performance. Here, we put forward a highly efficient water electrolyzer based on Ni3S2-based materials. The hierarchical structure of Ni3S2 can be well regulated for optimizing the HER catalytic activity. The best c-Ni3S2/NF electrode exhibits a much smaller overpotential of 220 mV to reach the current density of 100 mA cm−2. Upon introducing Fe species onto the Ni3S2/NF electrode by a simple dipping/drying method, the intrinsic OER activity can be extremely improved. As a result, the Fe-c-Ni3S2/NF catalyst showed excellent catalytic activity for the OER, including an overpotential of 193 mV at 10 mA cm−2, high specific current density and excellent stability. Post-characterization studies proved that the remaining S anions have an effective influence on improving the OER intrinsic activity. The assembled water electrolyzer also presented superior performance, such as a very low cell voltage of 1.50 V at 10 mA cm−2 and excellent durability for 120 h in alkaline medium. This strategy provides a promising way to design highly active and low-cost materials for overall water electrolysis. [ABSTRACT FROM AUTHOR]

Current interest in investigating non-precious group (NPG) metals for catalyzing the oxygen evolution reaction (OER) has revealed that doping of Ni hydroxides with Fe results in the dramatic enhancement of catalytic activity. Herein, a facile pathway to construct tetrataenite, an NiFe alloy of extraterrestrial origin and to address the limited electrical conductivity of metal oxides/hydroxides by directly constructing them atop graphene sheets is described. In this approach, a one-pot, bottom-up assembly of hybrid ultramicroporous materials (HUMs) was carried out, in the presence of suspended graphene (G), to homogeneously deposit the HUMs on unmodified graphene sheets, affording HUMs@G. Single metal (SIFSIX-3-Ni@G) and mixed metal (SIFSIX-3-NiFe@G) HUMs can be readily synthesized from their respective metal salts to afford a well-designed catalyst for the OER. The pyrolysis of SIFSIX-3-NiFe@G resulted in the deposition of the nanoalloy tetrataenite on G, demonstrating an exceptionally low OER onset potential of 1.44 V vs. RHE and reduced overpotential at 10 mA cm−2 (η10 = 266 mV). The synergy between the composition of the active catalyst and the electronically conductive support was attained by designing a reaction system encoding the self-assembly of a crystalline pre-catalyst on G sheets. [ABSTRACT FROM AUTHOR]

Developing highly efficient and stable electrocatalysts toward the oxygen evolution reaction (OER) is essential for large-scale sustainable energy conversion and storage technologies. Herein, we design and synthesize a ruthenium (Ru) doped NiFe bimetallic metal–organic framework (MOF) deposited on the nickel foam (Ru-NiFe-MOF/NF) by a facile one-pot hydrothermal reaction. Ru-NiFe-MOF/NF exhibits favourable electrocatalytic OER activity in alkaline solution, and requires a low overpotential of 205 mV to achieve 10 mA cm−2, a small Tafel slope of 50 mV dec−1, and long-term electrochemical stability over 100 h. This work demonstrates the rational nano-architectural design and synthesis of predominantly efficient and robust cation-doped MOF-derived materials for energy catalysis and beyond. [ABSTRACT FROM AUTHOR]

Developing state-of-the-art non-noble metal catalysts for the oxygen evolution reaction holds a key to the production of electrolytic hydrogen. Herein, self-supported hierarchical NiFe LDH/VO(OH)2 nanoflowers/nanosheets grown on a Ni foam have been synthesized via a two-step hydrothermal method. Numerous fine VO(OH)2 nanosheets grown on NiFe LDH nanoflowers enlarge the contact area for the electrolyte penetration and facilitate ion diffusion, while the three-dimensional structure of the material also provides an extensive active surface area and plentiful accessible active sites. Moreover, the strong synergistic interaction between VO(OH)2 and NiFe LDHs subtly modulates the electronic environment, accelerating the electron/charge transfer. As a result, the catalyst exhibits excellent electrochemical performance for OER giving a voltage of 1.51 V to achieve the current density of 100 mA cm−2 and possessed a Tafel slope of 65 mV dec−1 in 1.0 M KOH. In addition, the material exhibited remarkable long-term durability and stability during the 40 h measurement. This investigation provides a promising strategy for rationally designing high-efficiency metal electrocatalysts with hierarchical multi-dimensional nanostructures for OER. [ABSTRACT FROM AUTHOR]

Cobalt-Prussian blue analogues are remarkable catalysts for the oxygen evolution reaction (water oxidation) under mild conditions such as neutral pH. Although there are extensive reports in the literature about the application of these catalysts in water oxidation (the limiting step for hydrogen evolution), some limitations must be overcome in terms of improving the turnover frequency, oxygen production, long term stability, and elucidation of the mechanism. Another important feature to consider is the industrial processability of electrolytic cells for water splitting. For these reasons, we have reported herein a comparison of the electrochemical and chemical properties of three catalysts produced from cobalt-Prussian blue. Co–Co PBA 60 refers to cobalt-Prussian blue heated up to 60 °C with a high content of water. Co–Co PBA 200 is the same starting material but heated up to 200 °C with a low water content. Finally, Co3O4 is a thermal decomposition product obtained from heating cobalt-Prussian blue up to 400 °C. Although Co–Co PBA 60 has a higher overpotential for water oxidation than Co–Co PBA 200, this catalyst is kinetically faster than Co PBA 200. It is suggested that the water coordinated to Co2+ in Co–Co PBA 60 can accelerate the reaction and that there is a balance between the thermodynamic and kinetic characteristics for determining the final properties of the catalyst at pH = 7. Another important observation is that the Co3O4 catalyst has the best performance among the considered catalysts with the highest TON and TOF. This suggests that the different mechanisms and surface effects demonstrated by the Co3O4 catalyst are more conducive to efficient water oxidation than those of Prussian blue. Further studies concerning the effect of water and surface on these catalysts under mild conditions are essential to gain a better understanding of the mechanism of water oxidation and to advance the development of new catalysts. [ABSTRACT FROM AUTHOR]

It is of extreme importance to design efficient electrocatalysts for hydrogen evolution reaction (HER), which is considered as a promising approach to provide efficient and renewable clean fuel (hydrogen). Tuning the electronic structure through heteroatom doping demonstrates one of the most effective strategies to promote the electrocatalytic performance of HER. Herein, phosphorus-doping modulation is utilized to fabricate monoclinic P-CoMoO4 with optimized electron structure supported on nickel foam (P-CoMoO4/NF) for alkaline HER via a facile hydrothermal method, followed by low-temperature phosphidation. Notably, P-CoMoO4/NF shows outstanding electrocatalytic performance for HER in 1 M KOH with a low overpotential of 89 mV at 10 mA cm−2, a remarkable Tafel slope value of 59 mV dec−1, and excellent 24 h-long stability. The excellent catalyst activity and stability merits of P-CoMoO4/NF are comparable to the reported highly efficient non-precious metal HER electrocatalysts and could be applied as a powerful electrocatalyst in water electrolysis. This work provides a superior synthesis strategy for the effective design and rational fabrication of low-cost, highly active, and highly stable non-precious metal HER electrocatalysts for electricity-to-hydrogen applications. [ABSTRACT FROM AUTHOR]

Developing earth-abundant highly efficient catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is indispensable for the widespread implementation of electrochemical water splitting to store renewable energy. Herein, amorphous bimetallic selenide (Ni–Fe–Se) hollow nanospheres electrodeposited on nickel foam (Ni–Fe–Se/NF) are developed as a bifunctional catalyst for the HER and OER. The HER and OER bifunctional activity of Ni–Fe–Se/NF outperforms those of monometallic Ni–Se/NF and Fe–Se/NF owing to the synergy of Ni and Fe in Ni–Fe–Se/NF. Moreover, the amorphous hollow spherical morphology of Ni–Fe–Se/NF increases the active site density and facilitates the mass transfer of electrolytes and H2/O2 products. Ni–Fe–Se/NF drives a current density of 10 mA cm−2 with an overpotential of ∼85 mV for the HER and 100 mA cm−2 with an overpotential of ∼222 mV for the OER. As the HER and OER bifunctional catalyst, Ni–Fe–Se/NF can split alkaline water with total voltages of ∼1.52 V and ∼1.66 V at 10 mA cm−2 and 100 mA cm−2, respectively, and remain stable over 50 hours of operation in 1 M KOH. [ABSTRACT FROM AUTHOR]

In this work, a dynamic self-optimizing material consisting of nickel–sulfide nanosheets anchored onto Ni foam (DSO-Ni3S2-NF) as the model material was constructed using a hydrothermal method, and its electrocatalytic performance for oxygen evolution was evaluated. It was found that the electrocatalytic activity of the dynamic self-optimizing (DSO) 25 h-Ni3S2-NF for oxygen evolution is significantly enhanced compared with that of pristine 0 h-Ni3S2-NF since the formed oxide layer evolves into new active sites and the specific process of activity optimization was explored dynamically. The best oxygen evolution reaction (OER) performance was achieved by 25 h-Ni3S2-NF catalyst, which required merely 241 mV overpotential to deliver a current density of 20 mA cm−2, and its Tafel slope was as low as ∼40 mV dec−1, which was superior to most nickel-based catalysts, in 1 M KOH electrolyte. The current density was found to be increased gradually at the same potential and the stability test curves were steady with ignorable decline, showing that the promising strategy of the preparation of a dynamic self-optimizing pre-catalyst may open a new pathway to prepare low-cost, high-performance and stable water splitting catalysts. [ABSTRACT FROM AUTHOR]

In this work, we present an easy and scalable electrodeposition protocol that operates in a deep eutectic solvent, used to prepare self-supported Ni–Fe alloy films directly grown on copper foils. Unlike electrodeposition in aqueous baths, alloy compositions deposited in deep eutectic solvent are found to be the same as in plating solution owing to the enlargement of the deposition window and secondary reaction suppression. By rationally tuning the Ni/Fe ratio in deep eutectic solvent plating solution, the best oxygen evolution reaction performance was achieved by a Ni75Fe25 catalyst, which requires only a 316 mV overpotential to reach a current density of 10 mA cm−2, while its Tafel slope is as low as 62 mV dec−1. This catalyst can operate at 10 mA cm−2 with negligible activity degradation for over 10 h, promising its potential use as a low-cost, high-performance and stable electrocatalyst in water splitting devices. [ABSTRACT FROM AUTHOR]

The development of non-precious metal electrocatalysts with high activity for the oxygen evolution reaction (OER) is a crucial and challenging task. In this work, we proposed a solvent-free in situ metal–organic framework (MOF) growth strategy for the fabrication of an Fe-doped CoO/Co electrocatalyst. This approach not only partially granted the MOF's porous structure to the catalyst but also resulted in a tighter combination between the Co metal and CoO, thereby enhancing its electrical conductivity. Furthermore, this method enabled the Fe species to be more uniformly dispersed on CoO/Co, which significantly exposed more active sites for efficient electrocatalysis. The entire synthesis process was solvent-free, except for a small amount of water and ethanol used during catalyst washing. The as-synthesized Fe-CoO/Co electrocatalyst exhibited superior OER activity on a glass carbon electrode, with η = 276 mV at a current density of 10 mA cm−2, even higher than that of the commercial precious IrO2/C catalyst. Additionally, it was also extended to prepare a Ni-doped CoO/Co electrocatalyst by the same procedure with satisfactory OER performance. This work presents a new preparation approach for MOF-derived catalysts with potential applications in energy conversion and beyond. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) with a large overpotential is the key step common to artificial photosynthesis. In semiconductor photocatalysts, the light available to the reactions is usually limited to UV or visible with wavelengths shorter than the absorption edge of the semiconductors. On the other hand, gold nanoparticle (Au NP)-based plasmonic photocatalysts, particularly hot-electron transfer (HET)-type plasmonic photocatalysts, have the capability to utilize visible-to-near infrared light that makes up most sunlight as a driving force for the energetically uphill reactions. In recent years, experimental and theoretical studies on HET-type plasmonic photocatalysts consisting of Au NPs and a semiconductor have been intensively pursued. This perspective article highlights the fundamentals and recent progress of Au NP-based HET-type plasmonic photocatalysts for OER. After the introduction, the basics for the rational design of plasmonic photocatalysts are treated first. Secondly, the concrete design for the plasmonic photocatalysts is dealt with in the order of semiconductors, Au NPs, and their interface. Thirdly, recent advanced studies on plasmonic photocatalysts for OER are described. Finally, the conclusions are summarized with a direction for future research on plasmonic photocatalysts. [ABSTRACT FROM AUTHOR]

Here, a new strategy that combines accessible active sites and multiphase synergy in a simple process is developed for constructing bifunctional electrocatalysts toward overall water splitting. By using metal–organic framework (MOF) nanosheets hydrothermally grown on pre-oxidized nickel foam (denoted by Co2(OH)2(BDC)/NiO/NF) as a precursor, two novel heterogeneous nanosheet arrays including a cobalt phosphide nanoparticle embedded carbon nanotube nanosheet array supported by phosphorized nickel foam (denoted by CoP-CNT/Ni2P/NF) and a cobalt phosphide nanorod decorated carbon nanosheet array supported by oxidized nickel foam (denoted by CoP-C/NiO/NF) are prepared. Both were confirmed to be highly efficient for hydrogen and oxygen evolution reactions. In particular, CoP-C/NiO/NF exhibits higher catalytic activity toward the hydrogen evolution reaction (η100 = −131 mV), promoted by the synergy of oxidized nickel foam. CoP-CNT/Ni2P/NF performs better in the oxygen evolution reaction (η50 = 301 mV), benefiting mainly from its improved electrochemically active surface area. The two catalysts match well in overall water splitting with satisfactory activity (η10 = 1.57 V) and stability when directly applied in a two-electrode cell. This method will bring new inspiration to maximize the electrocatalytic efficiency of MOF-derived catalysts for energy conversion applications in the future. [ABSTRACT FROM AUTHOR]

We synthesized new oxygen-evolving molecular Ru(II) catalysts with one or two carbazole moieties on the axial pyridyl ligands, namely [Ru(bda)(cbz-py)(py)] and [Ru(bda)(cbz-py)2] [C1 and C2; bdaH2 = 2,2′-bipyridyl-6,6′-dicarboxylic acid, py = pyridine, and cbz-py = 9-(pyridin-4-yl)-9H-carbazole] to investigate the effect of cbz modification on the photophysical and catalytic properties of the well-known molecular catalyst [Ru(bda)(py)2] (C0). The initial oxygen-evolving catalytic activities of C1 and C2 were higher than that of C0 in both a chemical reaction driven by the strong oxidant (NH4)2[Ce(NO3)6] (CAN = ceric ammonium nitrate) and photochemical oxidation using a [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) photosensitizer with Na2S2O8 as the sacrificial oxidant. The higher activities were ascribed to the electron-withdrawing cbz groups, which promoted the radical coupling reaction to form a RuIV–O–O–RuIV species. A unique oxygen-evolution rate change behaviour was observed for both C1 and C2 in the presence of a large excess of CAN, suggesting the competitive oxidation of the cbz moiety during the chemical oxygen evolution reaction. This work suggests that the cbz modification of an oxygen evolution molecular catalyst is a promising approach for integrating the hole accumulator near the oxygen evolution catalytic centre. [ABSTRACT FROM AUTHOR]

The development of high-performance and cost-effective bifunctional water splitting catalysts has enormous significance in the hydrogen production industry from water electrolysis. Herein, an in situ Co and N co-doping method was developed to improve the electrocatalytic performance of Ni3S2 catalysts. The Co–N-Ni3S2/NF is successfully synthesized for the first time by a one-step hydrothermal method, wherein nickel foam, thioacetamide and Co(NO3)2·6H2O are used as the nickel source, sulfur source, nitrogen source and cobalt source. Co–N-Ni3S2/NF exhibits excellent oxygen evolution reaction activity (an overpotential of 285 mV@50 mA cm−2) and hydrogen evolution reaction activity (an overpotential of 215 mV@10 mA cm−2) in 1 M KOH solution. The electrolytic cell displayed a low cell voltage of 1.50 V when the Co–N-Ni3S2/NF material was used as the bifunctional water splitting electrocatalyst, which is one of the best catalysts reported so far. Density functional theory calculations show that Co–N-Ni3S2/NF exhibits stronger water adsorption energy than those of N-Ni3S2/NF, Co-Ni3S2/NF and Ni3S2/NF. It is proved that the doping of Co and N can effectively regulate the electron cloud density of Ni, thus enhancing the electrochemical activity of Co–N-Ni3S2/NF. [ABSTRACT FROM AUTHOR]

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal–organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided. [ABSTRACT FROM AUTHOR]