Electrocatalyst For Full Water Splitting Research Articles

Catalytic Water Electrolysis by Co-Cu-W Mixed Metal Oxides: Insights from X-ray Absorption Spectroelectrochemistry.

Mixed metal oxides (MMOs) are a promising class of electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Despite their importance for sustainable energy schemes, our understanding of relevant reaction pathways, catalytically active sites, and synergistic effects is rather limited. Here, we applied synchrotron-based X-ray absorption spectroscopy (XAS) to explore the evolution of the amorphous Co-Cu-W MMO electrocatalyst, shown previously to be an efficient bifunctional OER and HER catalyst for water splitting. Ex situ XAS measurements provided structural environments and the oxidation state of the metals involved, revealing Co2+ (octahedral), Cu+/2+ (tetrahedral/square-planar), and W6+ (octahedral) centers. Operando XAS investigations, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), elucidated the dynamic structural transformations of Co, Cu, and W metal centers during the OER and HER. The experimental results indicate that Co3+ and Cu0 are the active catalytic sites involved in the OER and HER, respectively, while Cu2+ and W6+ play crucial roles as structure stabilizers, suggesting strong synergistic interactions within the Co-Cu-W MMO system. These results, combined with the Tafel slope analysis, revealed that the bottleneck intermediate during the OER is Co3+ hydroperoxide, whose formation is accompanied by changes in the Cu-O bond lengths, pointing to a possible synergistic effect between Co and Cu ions. Our study reveals important structural effects taking place during MMO-driven OER/HER electrocatalysis and provides essential experimental insights into the complex catalytic mechanism of emerging noble-metal-free MMO electrocatalysts for full water splitting.

Ru anchored on Co(OH)2 nanowire arrays as highly effective electrocatalyst for full water splitting

With the development of hydrogen energy, it is necessary to develop a cost-effective catalyst for electrochemical water splitting including hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). Co-based composites are regarded as good catalysts toward OER, but weak H adsorption abilities lead to bad HER activities. Herein, Ru anchored on Co(OH)2 nanowire arrays (Ru/Co(OH)2 NWAs) as bifunctional catalysts for alkaline water splitting, are fabricated by hydrothermal process and subsequent immersion method. Ru/Co(OH)2 NWAs catalyst exhibits excellent activity for HER and OER, owing to strong electronic interaction of Ru participant and Co(OH)2 NWAs. To catalyze 10 mA cm−2 current density for HER and OER, Ru/Co(OH)2 NWAs catalyst only needs the overpotentials of 96 and 186 mV, respectively. An alkali electrolyzer assembled by bifunctional Ru/Co(OH)2 NWAs catalyst delivers 100 mA cm−2 at a cell voltage of 1.76 V, even superior than the couples of Pt/C and RuO2 electrodes (1.88 V).

Recent advances in two-dimensional nanomaterials as bifunctional electrocatalysts for full water splitting

Recent advances in two-dimensional bifunctional electrocatalysts for full water splitting are systematically reviewed, discussing challenges and opportunities for further research.

Crystalline/amorphous composite interface of CoP@Ni/Fe-P as a boosted electrocatalyst for full water splitting.

Heterojunction materials have become good candidates for electrocatalysts thanks to their unique physicochemical merits. Herein, a crystalline-amorphous CoP@Ni/Fe-P heterojunction is constructed for whole water splitting. Originating from the strong electronic reaction at the amorphous-crystal interfaces, the electron density of Co, Ni, Fe and P is adjusted, which will optimize the adsorption and desorption energy of intermediates for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and lower the kinetic barrier. The CoP@Ni/Fe-P heterojunction displays overpotentials of 125 and 250 mV to drive a current density of 10 mA cm-2 in 1 M KOH. In addition, the whole water splitting performance requires a cell voltage of 1.56 V to deliver 10 mA cm-2 and shows good stability. This work provides a way to design and prepare transition-metal-based materials with good electrocatalytic activity by constructing a crystalline and amorphous heterojunction.

Densely populated tiny RuO2 crystallites supported by hierarchically porous carbon for full acidic water splitting.

The exploitation of highly active bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic media has been a subject receiving immense interest. However, the existing catalysts usually suffer from low catalytic efficiency and poor corrosion resistance under acidic conditions. Herein, we report a facile molten salt method to fabricate ruthenium dioxide nanoparticles supported by hierarchically porous carbon (RuO2/PC) as a bifunctional electrocatalyst for full water splitting under strong acidic conditions. The formation of a densely populated nanocrystalline RuO2/carbon heterostructure helps expose catalytic sites, accelerates the mass transfer rate, and further enhances the acid resistance of RuO2 nanoparticles. The as-synthesized RuO2/PC consequently exhibits superior catalytic performance for the OER with an overpotential of 181 mV upon 10 mA cm-2 compared to that of the commercial RuO2 (343 mV) and a comparable performance to Pt/C for the HER (47.5 mV upon 10 mA cm-2) in 0.5 M H2SO4. The RuO2/PC shows promising stability with little degradation over ∼24 h. Impressively, the water electrolyzer based on RuO2/PC shows an overpotential of 326 mV at 10 mA cm-2, much lower than that of the electrolyzer based on the combination of Pt/C and RuO2 (400 mV), indicating its great potential towards practical application.

NiS/MoS2 complex grown on carbon paper as a bifunctional electrocatalyst for full water splitting

The development of transition metal-based materials to replace precious metal electrocatalysts in a facile and efficient approach is of great importance for water splitting. The NiS/MoS2 complex grown in situ on carbon paper (NiS/MoS2/CP) by a one-step hydrothermal method was successfully constructed as an efficient electrocatalyst. The synergetic effects of the superhydrophilic/aerophobic surface, hierarchical nanostructure and strong mechanical adhesion to a highly conductive substrate give the NiS/MoS2/CP complex outstanding catalytic activity and durability. Specifically, it exhibits relatively low overpotentials of 119 mV (at a current density of 10 mA cm−2) and 314 mV (at a current density of 100 mA cm−2) for the hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, the NiS/MoS2/CP complex can also be used as an electrolyzer that reaches a current density of 10 mA cm−2 at a cell voltage of 1.48 V without decay after 30 h of durability testing, making it an ideal electrocatalyst for water splitting towards practical application.

Enhanced electrocatalytic full water-splitting reaction by interfacial electric field in 2D/2D heterojunction

To increase the productivity of hydrogen and oxygen generation, the carbon-based electrocatalyst with 2D/2D structure, graphitic carbon nitride with reduced graphene oxide (g-C3N4/rGO), was synthesized via a facile electrostatic self-assembly method and was used as electrocatalyst for full water-splitting. In the alkaline electrolyte, g-C3N4/rGO showed lower overpotential and Tafel slope of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, than the other 2D structured materials. The DFT calculation simulates the spontaneous electron transfer, low charge transfer resistance, and internal electric field of g-C3N4/rGO electrocatalysts. Especially, the internal electric field between the g-C3N4 and rGO is beneficial in solving the local pH reduction problem and electron transport. Consequentially, our study elucidates that 2D/2D structure, inducing the internal electric field, is an effective way to enhance the performance of the full water-splitting system.

Bimetallic Ni–Mo nitride@N-doped C as highly active and stable bifunctional electrocatalysts for full water splitting

NiMoN-2 nanosheets were used as bifunctional catalysts and exhibited high total water decomposition activity. Only a cell voltage of 1.58 V was required to attain a current density of 10 mA cm−2.

Facile synthesis and self-assembling of transition metal phosphide nanosheets to microspheres as a high-performance electrocatalyst for full water splitting

The development of noble-metal-free electrocatalysts with enhanced active sites is of great significance for the production of renewable energy on large scale. Ultrathin transition metal compounds (e.g., phosphides and sulfides) have unique structural features but their stability and electroactivity must still be improved for practical applications. We propose a new strategy to synthesis ultrathin sheets of Ni-Cu-Co transition metals on porous nickel and self-assemble them into yarn-shaped microspheres. Enhance catalytic activity and stability are then attained by thermochemical phosphorization without morphological modifications. We employed various analytical techniques including XRD, SEM, TEM, and in-depth electrochemical analysis for materials characterizations. It is shown that ultrathin nanosheets with a thickness of few nanometers and lateral dimensions of a few hundred nanometers are formed and assembled to yarn-shape microspheres with an average diameter of ⁓7 µm. To demonstrate the practical application of the developed nanostructured compounds, their bifunctional electroactivity toward both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline solution (2.0 M NaOH) is demonstrated. On the best practice, co-alloying of Ni with Cu and Co with a ratio of Cu/Co=2.5 enhances the catalytic activity by ⁓30%. Further improvement (by about 55%) is attained by thermochemical phosphorization. The electrocatalyst merely required an overpotential of 175 mV (for HER) and 265 mV (for OER) to deliver a current density of 10 mA cm−2. No changes in the current density were observed after a long-time at higher current densities (−88 and +39 mA cm−2) showing the high stability of the catalyst in the alkaline solution. The mechanism of improved performance is elaborated and ascribed to the engineered morphology of the nanostructures having many active sites at the surface to promote the Volmer reaction combined with accelerated electrolyte diffusion, rapid charge transport, and gas bubbles detachment at the interface with the electrolyte.

Surface phosphorization of Ni-Co-S as an efficient bifunctional electrocatalyst for full water splitting.

Exploring efficient bifunctional electrocatalysts is of great importance for water splitting to obtain scalable and sustainable hydrogen generation. The strategy of element modification is effective for tuning the electronic structures of electrocatalysts. Herein, binder-free, phosphorus-doped NiCo2S4/CFP (Ni-Co-S-P) was synthesized under hydrothermal conditions, followed by surface phosphorization via solid reaction. The incorporation of P modulates the electronic structure of the Ni-Co-S-P catalyst, increases the electrical conductivity, and promotes the electrochemical surface area and the number of active sites, thereby improving its activity. The Ni-Co-S-P catalyst delivers a current density of 10 mA cm-2 at an overpotential of 176 mV for the hydrogen evolution reaction (HER) and an overpotential of 265 mV for the oxygen evolution reaction (OER). When assembled into an electrolyzer both as a cathode and anode, it demonstrated outstanding activity for overall water splitting to afford a potential of 1.76 V at 10 mA cm-2 with no obvious decay after 25 h.

Preparation of a bifunctional ultrathin nickel phosphide nanosheet electrocatalyst for full water splitting

Developing a noble-metal-free robust and effective electrocatalyst with enhanced active sites for overall water splitting is of great significance yet challenging.

Nanostructured copper molybdates as promising bifunctional electrocatalysts for overall water splitting and CO2 reduction.

Overall water splitting and CO2 reduction are two very important reactions from the environmental viewpoint. The former produces hydrogen as a clean fuel and the latter decreases the amount of CO2 emissions and thus reduces greenhouse effects. Here, we prepare two types of copper molybdate, CuMoO4 and Cu3Mo2O9, and electrochemically investigate them for water splitting and CO2 reduction. Our findings show that Cu3Mo2O9 is a better electrocatalyst for full water splitting compared to CuMoO4. It provides overpotentials, which are smaller than the overpotentials of CuMoO4 by around 0.14 V at a current density of 1 mA cm−2 and 0.10 V at −0.4 mA cm−2, for water oxidation and hydrogen evolution reactions, respectively. However, CuMoO4 adsorbs CO2 and the reduced intermediates/products more strongly than Cu3Mo2O9. Such different behaviors of these electrocatalysts can be attributed to their different unit cells.

Controlled synthesis of tubular ferrite MFe2O4 (M = Fe, Co, Ni) microstructures with efficiently electrocatalytic activity for water splitting

Ferrite MFe2O4 (M = Fe, Co, Ni) tubular microstructures were successfully synthesized through a simple two-step strategy. The tubular FeOOH microstructures were first obtained by a heating reflux route. Then, tubular ferrite MFe2O4 (M = Fe, Co, Ni) microstructures were produced by a solvothermal method. The as-prepared products were characterized by the powder X-ray diffraction (XRD), the field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and EDS mapping. Experiments showed that as-obtained ferrite catalysts demanded the overpotentials of −201∼-359 mV or ∼340–432 mV to afford the current density of −/+10 mA cm−2 for hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) with long-term durability for 50 h in 1.0 M KOH solution. Among them, NiFe2O4 tubular microstructures presented the best electrocatalytic activities for HER and OER due to the largest BET specific surface area, the highest electrochemical active surface area and the smallest charge-transfer resistance. Obviously, the present ferrite tubular microstructures have potential application as a bifunctional electrocatalyst for full water splitting.

Cups Ultrafine Nanoparticles Anchored into N-Doped Carbon Framework for Bifunctional Water Splitting

Hydrogen has been considered as the green energy carrier to substitute gradually depleted fossil fuels due to its virtues holding high energy density and zero environmental discharge. Electrocatalytic water splitting is one of the most effective approach to generate hydrogen fuel, which involves two half reactions, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The design of efficient and low-cost electrocatalysts to reduce the large energy barriers of water splitting is challenging. Nanosizing of the electrocatalysts could provide new insight into understanding the catalytic reaction at nanoscale and even molecular level. Here, a novel ultrafine CuPS nanoparticles anchored into N-doped carbon framework (CuPS/NC) was proposed as advanced electrocatalyst for full water splitting. Density functional theory calculations revealed the synergistic electronic interaction between S and P, leading to optimal electronic structure and hydrogen adsorption free energy, thus facilitating HER kinetics. The highly open yet interconnected 3D framework creates a large surface area, exposing abundant active sites and providing convenient charge and ionic diffusion pathway. The ultrafine CuPS nanoparticles enable to fully utilize accessible edge sites. As a result, the obtained CuPS/NC exhibits relatively low overpotentials and small Tafel slopes for both HER and OER, as well as superior performance toward full water splitting.

Facile electrodeposition of ternary Ni-Fe-Co alloy nanostructure as a binder free, cost-effective and durable electrocatalyst for high-performance overall water splitting

Development of highly effective and stable electrocatalyst as full water electrolyzers is essential for the energy production process. In this study, binder-free and self-made Ni-Fe-Co nanostructure electrode was developed using electrodeposition method, and its electrocatalytic properties were investigated for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media. The fabricated electrocatalyst exhibited excellent properties for the evolution of H2 and O2. Ni-Fe-Co nanostructure film required overpotentials of 91 mV for HER and 316 mV for OER in order to create a current density of 10 mA cm−2. Furthermore, the Tafel slope for HER and OER was measured as 86 and 43 mV/dec, respectively. In addition, the resulting electrode showed outstanding electrocatalytic stability, in which following a long period of electrolysis, the necessary overpotential to maintain a current density of 100 mA cm−2 remained constant. This bifunctional electrode enables alkaline water electrolyzers, which can provide a current density of 10 mA cm−2 under a cell voltage of 1.6 V. Such desirable performance of fabricated electrode as an electrocatalyst for full water splitting can be attributed to high active surface area factor, the synergistic effect of the elements, and rapid separation of bubbles from the electrode surface. This study provides a new method for the rapid construction of efficient electrocatalyst for renewable energy sources.

Atomic Heterointerface-Induced Local Charge Distribution and Enhanced Water Adsorption Behavior in a Cobalt Phosphide Electrocatalyst for Self-Powered Highly Efficient Overall Water Splitting.

Developing economical and highly efficient noble metal-free electrocatalysts for overall water splitting is an essential precondition for renewable energy conversion. Herein, we highlight atomic heterointerface engineering in constructing highly efficient cobalt phosphide (CoP)/Co9S8 electrocatalysts for full water splitting. A CoP/Co9S8 hybrid was prepared for the first time by partial homogeneous transformation of in situ-formed Co9S8, in which the atomic heterointerface was formed between CoP and Co9S8. Systematic experiments and theoretical calculations confirm that the as-formed atomic heterointerface can induce local charge distribution in CoP/Co9S8, which can not only accelerate the charge transfer but also optimize the hydrogen adsorption energy of CoP in favor of the fast transformation of Hads into H2. Meanwhile, the Co9S8 component can also increase the water adsorption capability of CoP/Co9S8. Benefiting from these outstanding advantages, an alkaline electrolyzer based on CoP/Co9S8 as both electrodes achieves a low cell voltage of 1.6 V at an operating current density of 10 mA cm-2, and at the same time, it can also be self-powered by a home-assembled Zn-air battery employing the same CoP/Co9S8 as the air electrode for prospectively achieving renewable energy conversion. This work demonstrates the importance of heterostructure engineering in developing noble metal-free catalysts for high-performance water electrolysis.

Full Water Splitting Electrocatalyzed by NiWO4 Nanowire Array

It is attractive to develop an effective bifunctional electrocatalyst for full water splitting. In this Letter, we report that a NiWO4 nanowire array on a Ti mesh (NiWO4/TM) is a high-performance a...

Multifunctional Single‐Crystallized Carbonate Hydroxides as Highly Efficient Electrocatalyst for Full Water splitting

AbstractThe controllable synthesis of single‐crystallized iron‐cobalt carbonate hydroxide nanosheets array on 3D conductive Ni foam (FCCH/NF) as a monolithic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional electrocatalyst for full water splitting is described. The results demonstrate that the incorporation of Fe can effectively tune the morphology, composition, electronic structure, and electrochemical active surface area of the electrocatalysts, thus greatly enhancing the intrinsic electrocatalytic activity. The optimal electrocatalyst (F0.25C1CH/NF) can deliver 10 and 1000 mA cm−2 at very small overpotentials of 77 and 256 mV for HER and 228 and 308 mV for OER in 1.0 m KOH without significant interference from gas evolution. The F0.25C1CH‐based two‐electrode alkaline water electrolyzer only requires cell voltages of 1.45 and 1.52 V to achieve current densities of 10 and 500 mA cm−2. The results demonstrate that such fascinating electrocatalytic activity can be ascribed to the increase in the catalytic active surface area, facilitated electron and mass transport properties, and the synergistic interactions because of the incorporation of Fe.

Self-supported NiMo-based nanowire arrays as bifunctional electrocatalysts for full water splitting

Self-supported NiMo-based nanowire arrays as binder-free electrodes for full water splitting exhibited excellent electrochemical performance, superior to the one assembled with Pt and IrO2 electrocatalysts.

The unified ordered mesoporous carbons supported Co-based electrocatalysts for full water splitting

Co-based electrocatalysts were prepared by facilely low-temperature multi-step calcination using ordered mesoporous carbons (OMCs) and cobalt salt as the precursors. According to the chemical reaction of cobalt compound, the calcined product of each step could be used as different catalysts, which could complete the full water splitting in alkaline solution together. The introduction of ordered mesoporous carbon not only make the morphology was well inherited, but also enhanced the catalytic activity. The OMCs/CoP exhibited overpotential of 134 mV in acidic media and 143 mV in basic media with a current density of 10 mA cm−2 for hydrogen evolution reaction (HER). While, in basic solution, the OMCs/Co3O4 had an overpotential value of 350 mV at 10 mA cm−2 for oxygen evolution reaction (OER). Moreover, the different Co-based electrocatalysts were assembled into one system for the full water splitting in 1 M KOH solution. This work provided a facile and efficient candidate for water splitting.