NiMoO4 Nanorods Research Articles

Enhanced rate and specific capacity in nanorod-like core-shell crystalline NiMoO4@amorphous cobalt boride materials enabled by Mott-Schottky heterostructure as positive electrode for hybrid supercapacitors

The supercapacitor electrode materials suffer from structure pulverization and sluggish electrode kinetics under high current rates. Herein, a unique NiMoO4@Co-B heterostructure composed of highly conductive Co-B nanoflakes and a semiconductive NiMoO4 nanorod is designed as an electrode material to exert the energy storage effect on supercapacitors. The formed Mott-Schottky heterostructure is helpful to overcome the ion diffusion barrier and charge transfer resistance during charging and discharging. Moreover, this crystalline-amorphous heterogeneous phase could provide additional ion storage sites and better strain adaptability. Remarkably, the optimized NiMoO4@Co-B hierarchical nanorods (the mass ratio of NiMoO4/Co-B is 3:1) present greatly enhanced electrochemical characteristics compared with other components, and show superior specific capacity of 236.2 mA h g−1 at the current density of 0.5 A g−1, as well as remarked rate capability. The present work broadens the horizons of advanced electrode design with distinct heterogeneous interface in other energy storage and conversion field.

Homogeneous NiMoO4–Co(OH)2 bifunctional heterostructures for electrocatalytic oxygen evolution and urea oxidation reaction

Developing bifunctional electrocatalysts for catalyzing water oxidation and urea oxidation reactions is of vital importance for alleviating the energy crisis and environmental damage, but it is still formidably challenging. Here, the potential of a heterostructure of NiMoO4 nanorods conformally covered by metal-organic framework (MOF) derived Co(OH)2 nanosheets is demonstrated. Under the assistance of a series of advanced characterizations and electrochemical measurements, the rich heterointerfaces in the NiMoO4–Co(OH)2 are found to be beneficial for promoting the electrocatalytic performance. As a bifunctional catalyst for the electrooxidation of water and urea, the optimized NiMoO4–Co(OH)2-100 demonstrates higher catalytic activity, kinetics and durability than individual NiMoO4 and Co(OH)2. Specifically, an overpotential as low as 285 mV can drive 10 mA cm−2 for oxygen evolution reaction (OER), and a voltage of 1.32 V is demanded for achieving 10 mA cm−2 towards urea oxidation reaction (UOR), significantly lower than the benchmarked catalysts. Presented outcomes can greatly contribute to the industrial application of urea-containing wastewater purifying, by offering a deep understanding on the interface engineering of nanocatalysts.

NiMoO4 nanorods with oxygen vacancies self-supported on Ni foam towards high-efficiency electrocatalytic conversion of nitrite to ammonia

As an eco-friendly and sustainable strategy, the electrochemical reduction of nitrite (NO2–) can simultaneous generation of NH3 and treatment of NO2– contamination in the environment. Herein, monoclinic NiMoO4 nanorods with abundant oxygen vacancies self-supported on Ni foam (NiMoO4/NF) are considered high-performance electrocatalysts for ambient NH3 synthesis by reduction of NO2−, which can deliver an outstanding yield of 18089.39 ± 227.98 μg h−1 cm−2 and a preferable FE of 94.49 ± 0.42% at −0.8 V. Additionally, its performance remains relatively stable during long-term operation as well as cycling tests. Furthermore, density functional theory calculations unveil the vital role of oxygen vacancies in promoting nitrite adsorption and activation, ensuring efficient NO2–RR towards NH3. A Zn-NO2– battery with NiMoO4/NF as the cathode shows high battery performance as well.

Microplasma and quenching-induced Co doped NiMoO4 nanorods with oxygen vacancies for electrochemical determination of glucose in food and serum

In this paper, Co-doped NiMoO4 nanorods with oxygen vacancies are synthesized on carbon cloth (Co-NiMoO4 NRs/CC) via microplasma and quenching-induced method.Owing to the surface defects and metal ion doping, the electronic structure and surface properties of the catalyst are tuned.Cyclic voltammetry (CV) and amperometry are used to investigate the electrocatalytic behavior of the glucose sensor in an alkaline medium with sensitivities of 7411 and 3125 μA L mmol−1 cm−2 in the linear range 1.0 μmol L–1 to 1.0 mmol L–1 and 1.0 mmol L–1 to 7.0 mmol L–1, respectively. The detection limit is 0.079 μmol L–1 at S/N = 3. Moreover, the as-prepared catalyst electrode is also successfully used in real food and serum samples, with a recovery rate of 97.1%-107.4%.The DFT calculations show that the Co site of the catalyst significantly influenced glucose sensing performance. This idea expands the application of quenching chemistry in electrochemical sensing.

Three-dimensional NiMoO4@CoWO4 core–shell nanorod arrays for electrochemical energy storage applications

We synthesized NiMoO4@CoWO4 with CoWO4 wrapped up NiMoO4 nanorods to form NiMoO4@CoWO4 core–shell nanorod arrays on Ni foam, which exhibited high specific capacitance and excellent electrochemical stability.

Rough Ni@MoN corals for the hydrogen evolution reaction in acidic and alkaline media

Ni@MoN catalysts were prepared by reducing NiMoO4 nanorods with urea at high temperatures. The resultant Ni@MoN-700 exhibits great HER activity, with a low overpotential of 30 mV and 76 mV at 10 mA cm−2 in alkaline and acidic media.

A free-standing NiMoO4@Mg-Co(OH)F core-shell nanocomposites supported on Ni foam for asymmetric supercapacitor applications

In this paper, a facile hydrothermal treatment method was employed to successfully synthesize the NiMoO4@Mg-Co(OH)F/NF (NM-MCF) nanocomposites by taking NiMoO4 nanorods as cores, and assembling them with the shell of Mg-Co(OH)F nanosheets. The high conductivity of Mg-Co-F was combined with the nanosheet-assembled core-shell structure, and the multivalent states of Ni, Mg, and Co increased the number of active sites and improved the synergy, contributing greatly to excellent performance of NM-MCF. As expected, the optimized NM-MCF nanocomposite electrode exhibits superb capacitive performance, including excellent specific capacitance of 1630 F g −1 at 1 A g −1 and remarkable cycle stability with 86 % after 6000 cycles. Notably, the NM-MCF exhibits a regular rate capability of 56.5 % from 1 to 15 A g−1, superior to that of NiMoO4 (56.4 %) and Mg-Co(OH)F (28.8 %). Employing the as-prepared NM-MCF composites as positive electrode and activated carbon as negative electrode, the hybrid supercapacitor device with spectacular capacity of maximum energy density and power density up to 28.2 Wh kg−1 and 0.7 kW kg−1 and impressive long-term stability of 73.3 % retention after 6000 cycles is assembled.

Carbon-encapsulated Co2P/P-modified NiMoO4 hierarchical heterojunction as superior pH-universal electrocatalyst for hydrogen production

The development of hydrogen evolution reaction (HER) technology that operates stably in a wide potential of hydrogen (pH) range of electrolytes is particular important for large-scale hydrogen production. However, the rational design of low-cost and pH-universal electrocatalyst with high catalytic performance remains a huge challenge. Herein, Co2P nanoparticles strongly coupled with P-modified NiMoO4 nanorods are directly grown on nickel foam (NF) substrates through carbon layer encapsulation (denoted as C-Co2P@P-NiMoO4/NF) by hydrothermal, deposition, and phosphating processes. This novel kind of hierarchical heterojunction has abundant heterogeneous interfaces, strong electronic interactions, and optimized reaction kinetics, representing the highly-active pH-universal electrodes for HER. Remarkably, the C-Co2P@P-NiMoO4/NF catalyst shows excellent HER properties in acidic and basic electrolytes, where the overpotentials of 105 mV and 107 mV are applied to drive the current density of 100 mA cm−2. In addition, a low overpotential of 177 mV at 100 mA cm−2 along with high stability is realized in 1 M phosphate buffer solution (PBS), which is close to the state-of-the-art non-precious metal electrocatalysts. Our work not only provides a class of robust pH-universal electrocatalyst but also offers a novel way for the rational design of other heterogeneous materials bythe interface regulation strategy.

High-efficiency water splitting catalyzed by NiMoO4 nanorod arrays decorated with vacancy defect-rich NiTex and MoOy layers

Nickel molybdate is a promising non-noble metal candidate for alkaline hydrogen evolution reaction (HER), but its insufficient activity due to its low conductivity and slow reaction kinetics hinders its application for large-scale commercial hydrogen production. Here, we report an efficient self-supporting electrocatalyst consisting of NiMoO4 nanorods modified by MoOy with abundant internal oxygen vacancies and epitaxially grown NiTex with high specific surface area for alkaline HER. Benefiting from the structural advantages of nanorods/nanosheets integrated electrode, a larger specific surface area and abundant active centers can be provided for the HER. Such constructed NiTex/MoOy/NiMoO4 catalyst achieves a low overpotential of 59 mV to drive a current density of 10 mA cm−2 and a low Tafel slope of 32.8 mV dec−1 in 1.0 M KOH solution with sustainable durability. Additionally, a two-electrode electrolytic cell system assembled with NiTex/MoOy/NiMoO4 and NiMoO4 employed as cathode and anode, respectively, extraordinary performance with a voltage of 1.49 V at a current density of 10 mA cm−2 was achieved in alkaline electrolyte. This work demonstrates a novel idea for metalloid Te-modified transition metal oxides, which is expected to expand the family of stable and efficient transition metal oxide (such as CoMoO4, Co3O4, NiCo2O4, etc.)-based catalysts for energy electrocatalysis.

MoS2 Nanosheets Uniformly Anchored on NiMoO4 Nanorods, a Highly Active Hierarchical Nanostructure Catalyst for Oxygen Evolution Reaction and Pseudo‐Capacitors

AbstractHierarchical nanostructures have attracted considerable research attention due to their applications in the catalysis field. Herein, we design a versatile hierarchical nanostructure composed of NiMoO4 nanorods surrounded by active MoS2 nanosheets on an interconnected nickel foam substrate. The as‐prepared nanostructure exhibits excellent oxygen evolution reaction performance, producing a current density of 10 mA cm−2 at an overpotential of 90 mV, in comparison with 220 mV necessary to reach a similar current density for NiMoO4. This behavior originates from the structural/morphological properties of the MoS2 nanosheets, which present numerous surface‐active sites and allow good contact with the electrolyte. Besides, the structures can effectively store charges, due to their unique branched network providing accessible active surface area, which facilitates intermediates adsorptions. Particularly, NiMoO4/MoS2 shows a charge capacity of 358 mAhg−1 at a current of 0.5 A g−1 (230 mAhg−1 for NiMoO4), thus suggesting promising applications for charge‐storing devices.

Dehydrogenation of N2H4·H2O over NiMoO4 nanorods-stabilized NiPt bimetal nanoparticles for on-demand H2 evolution

In order to effectively and safely produce, deposit and carry H2 gas, the exploration of the“on–off”switch for demand-based H2 generation from completely N2H4·H2O dehydrogenation is a practically important matter. Herein, we first design and synthesize a battery of nickel molybdate nanorods (NMN)-stabilized NixPty bimetallic nanoparticles (NixPty/NMN), by immobilization of ultrafine NiPt nanoparticles at the surface of nickel molybdate nanorods (NMN), for the high-efficiency and selective demand-based H2 generation from completely N2H4·H2O dehydrogenation. Among them, NMN is synthesized from nickel chloride and sodium molybdate under green hydrothermal condition. Then, the optimized nano-catalyst Ni1Pt1/NMN exhibits the highest turnover frequency (TOF) of 3750 h−1 in H2 generation from N2H4·H2O dehydrogenation at 50 °C with 100 % H2 selectivity, due to its excellent synergistic effect. The comparison of other common supports, including Co3O4, Fe3O4, ZnO, ZrO2 and MoS2, has highlighted the vital role of NiMoO4 nanorods in N2H4·H2O dehydrogenation. More interestingly, the significantly selective“on–off”switch for on-demand H2 generation from N2H4·H2O dehydrogenation has been successfully developed by pH adjusting.

Enhanced electrocatalytic performance of uniformly spherical Ni-MOF decorated with NiMoO4 nanorods for oxygen evolution reaction

The creation of considerably effective electrocatalysts to reduce overpotentials is essential for speeding up the sluggish oxygen evolution reaction (OER) processes, which is imperatively required for the practical exploitation of electrochemical water-splitting. Herein, we prepared a unique composite structure, uniformly spherical Ni-MOF decorated with NiMoO4 nanorods on Ni foam (NF), and assessed its performance toward oxygen evolution reaction. The synergistic effect between NiMoO4 and Ni-MOF remarkably promoted the OER intrinsic activity. Moreover, introducing NiMoO4 endowed the composite structure with enhanced electrical conductivity and exposed more active sites. The NiMoO4/Ni-MOF/NF electrode exhibited exceptional electrocatalytic activity in 1.0 M KOH aqueous solution, with a low overpotential of 218 mV to drive the current density of 10 mA cm−2 and a low Tafel slope (67.75 mV dec−1), surpassing the benchmark RuO2. The presented strategy opens a promising candidate for preparing novel non-precious metal OER electrocatalysts derived from MOF materials for sustainable energy applications.

Preparation of NiMoO4 nanoarrays electrodes with optimized morphology and internal crystal water for efficient supercapacitors and water splitting

NiMoO4 shows good electrochemical properties for its multiple oxidation states. However, in practical application, NiMoO4 suffers from poor ion diffusion ability and slow electron transfer rate. In this work, the NiMoO4 nanoarrays with three different morphologies of sheets, rods, and fibers on the nickel foam substrate are successfully prepared, according to the difference of nucleation rate of NiMoO4 in different polar solvents during hydrothermal synthesis. Among them, NiMoO4 nanorods (NiMoO4-NRs) own the highest electrochemical activity due to their higher active surface area utilization. Furthermore, benefiting from the adequate crystal water inside, NiMoO4·1/2H2O-NRs exhibit good ionic diffusion ability. When examined for electrochemical performance experiments, NiMoO4·1/2H2O-NRs can achieve a rapid and reversible Faraday reaction, displaying a high area specific capacity of 2450 mC cm−2 at 1 mA cm−2. The asymmetric supercapacitor (ASC) using NiMoO4·1/2H2O-NRs as a positive electrode is assembled, which achieves a high energy density of 75.1 Wh kg−1 at 849.9 W kg−1 and excellent capacitance retention of 86.4% after 10,000 cycles. Moreover, NiMoO4·1/2H2O-NRs also present outstanding water electrolysis activity. It can afford ultralow overpotential in 1 M KOH solution (166 mV at 10 mA cm−2 for HER and 360 mV at 50 mA cm−2 for OER) and the current density retention of 99.4% (HER) as well as 89.1% (OER) after 10 h continuous electrolysis. The NiMoO4·1/2H2O-NRs designed in our work with optimal surface topography and moderate crystal water have a broad application prospect in both supercapacitors and electrolysis of water.

NiMoO4 nanorods derives carbon layers wrapped NiCo alloy decorated with Mo2C platinum-based catalyst for efficient methanol electrooxidation

To the aim of accelerate the methanol oxidation reaction (MOR), rationally designing novel MOR catalyst is highly desired. Hence, NiCo alloy resides in carbon layers decorated with Mo2C platinum-based catalyst (Pt/NiCo-Mo2C@NDC) is fabricate. For the fabrication, NiMoO4 nanorods (NRs) are firstly coated with CoZn-ZIF and then subjected to anneal treatment and NaBH4 reduction procedure. In the system, CoZn-ZIF serves as the sacrificial template and reducing agent, rendering bifunctional effect: CoZn-ZIF provides doped nitrogen sources, which are beneficial to the overall conductivity and deposition of Pt nanoparticles. Besides, it could act as the carbon sources for the reaction formation of the Mo2C during high temperature calcination, which delivers the excellent co-catalytic effect. Meanwhile, NiCo alloy can be fabricated during the annealing treatment and alter the structure properties of the catalyst, showing the Ni content-depended electrochemical performance. Therefore, the catalyst exhibits remarkable electrocatalytic activity and stability. Moreover, we also focus on the structure-activity relationships to provide insight into the effect of alloy and molybdenum carbide on catalytic activity.

NiMoO4 @Co3S4 nanorods with core-shell structure for efficient hydrogen evolution reactions in electrocatalysts

High-efficiency electrocatalysts are required for the hydrogen evolution reaction processes used in clean-energy applications. Single-component materials have limited physicochemical properties and do not meet the requirements of high-performance material applications. More complex materials must therefore be developed for these purposes. This study involves growing flower cluster-like NiMoO4 nanorod arrays on reduced graphene oxide-modified nickel foam (NF/rGO) substrates by a simple two-step hydrothermal method, followed by NiMoO4 nanorod modification with Co3S4 nanoparticles, to achieve an efficient hydrogen evolution reaction (HER) strategy for NF/rGO/NiMoO4 @Co3S4 core-shell structure composite catalysts. The proposed vertically aligned NiMoO4 nanorods served as the core framework, providing abundant specific surface area and fast charge transfer channels. The Co3S4 particles grown on the NiMoO4 nanorods provided sufficient electrochemical active sites for full contact with the electrolyte solution. In the 1.0 M KOH solution, the NF/rGO/NiMoO4 @Co3S4 core-shell structure exhibited a low overpotential of 40 mV for HER at a current density of 10 mA cm−2. These properties indicate that the proposed material could be used in HER processes in energy applications.

Surface/interface engineering for fabricating hierarchical Ir doped NiMoO4 covered by CoMn layered double hydroxide toward oxygen evolution reaction

Designing and fabricating outstanding electrocatalysts for anodic oxygen evolution reaction plays a key role in driving the earlier application of water splitting for large-scale hydrogen production. Surface/interface engineering is regarded as the most effective method for activating the active sites and modulating the electronic structure, which are two crucial factors determining the electrocatalytic performance. Herein, amorphous CoMn LDH ultrathin nanosheets doped by Ir are integrated with low-crystalline NiMoO4 nanorods (Ir-NiMoO4/Ir-CoMn LDH) to fabricate advanced hierarchical OER electrocatalysts. The optimized Ir-NiMoO4/Ir-Co3Mn2 LDH exhibits excellent OER performance as attested by an ultralow overpotential of 221 mV at the current density of 10 mA cm−2, as well as superb long-term durability. The intimate interface between NiMoO4 and Co LDH and modified electronic structure induced by Ir and Mn doping can greatly optimize the binding strength of between active sites and intermediated species, which improves the electrocatalytic OER performance. This study manifests how the synergistic surface/interface engineering design of nanomaterials boosts their catalytic activity.

Intrinsic Lability of NiMoO4 to Excel the Oxygen Evolution Reaction.

Nickel-based bimetallic oxides such as NiMoO4 and NiWO4, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO4 nanorods and NiWO4 nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO4/NF and NiWO4/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH)ED (ED: electrochemically derived) from NiMoO4 precatalyst, while NiWO4 remains stable. A controlled study, stirring of NiMoO4/NF in 1 M KOH without applied potential, confirms that NiMoO4 hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH)ED. Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO4 compared to the Ni-O-W bond in NiWO4 causes the transformation of NiMoO4 into NiO(OH)ED. A comparison of the OER performance of electrochemically derived NiO(OH)ED, NiWO4, ex-situ-prepared Ni(OH)2, and NiO(OH) confirmed that in-situ-prepared NiO(OH)ED remained superior with a substantial potential of 238 (±6) mV at 20 mA cm-2. The notable electrochemical performance of NiO(OH)ED can be attributed to its low Tafel slope value (26 mV dec-1), high double-layer capacitance (Cdl, 1.21 mF cm-2), and a low charge-transfer resistance (Rct, 1.76 Ω). The NiO(OH)ED/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm-2. Post-characterization of the anode proves the structural integrity of NiO(OH)ED even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)ED/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm-2. Similar to that on NF, NiMoO4 deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO4 to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH)ED succeeding the NiO phase is intrinsic to its superior activity.

Polydopamine-assisted NiMoO4 nanorods anchored on graphene as an electrode material for supercapacitor applications

In this study, we modified graphene sheets with polydopamine, used abundant oxygen-containing groups to adsorb metal ions in a hydrothermal process, and obtained graphene-supported NiMoO4 nanorod composite materials after pyrolysis. NiMoO4 nanorods were uniformly dispersed on the surface of a reduced graphene oxide layer under strong electrostatic adsorption. The specific capacitance of the composite material measured at a current density of 1 A g−1 was 856 F g−1 (514 C g−1), and an asymmetric supercapacitor composed of activated carbon exhibited an energy density of 48 Wh kg−1 at a power density of 800 W kg−1. After 5000 cycles of charging and discharging, a capacitance retention rate of 73% was achieved. Therefore, PDA-rGO@NiMoO4 composite materials are promising candidate materials for supercapacitor electrodes.

Artificial modulated Lewis pairs for highly efficient alkaline hydrogen production

Nickel phosphides are competitive non-noble candidates for alkaline hydrogen evolution reaction (HER), however, their activities are insufficient due to the inert alkaline water dissociation, which hampers application in hydrogen production. Herein, we design a highly efficient electrocatalyst consisting of monocrystalline NiMoO4 nanorods modified by broccoli-like hierarchical Ni(PO3)2-Ni2P coupling with MoOx self-supported catalyst. As HER proceeds, catalytic performance gets better due to the dissolution of MoOx to further expose more nickel sites as Lewis base sites and in-situ formation of hydroxyl-ligands on the surface of loading nanoparticle as Lewis acid sites, which effectively integrate artificial frustrated Lewis pairs (FLPs) to respectively promote adsorption of H* and activation of H2O molecules. Moreover, the electron-transfer leads to the downshift of the d-band center of Ni, which helps optimizing desorption ability of H* . Therefore, such reconstructed catalyst exhibits extremely low overpotentials of 11 and 79 mV to afford 10 and 100 mA·cm−2 in 1.0 M KOH with sustainable durability for over 110 h, outperforming most of reported advanced alkaline HER catalysts. Additionally, this catalyst used as cathode for overall alkaline water splitting at 10 and 100 mA·cm−2 with only 1.48 and 1.57 V, showing its broad application prospect in practical large-scale hydrogen production.

Sea urchin-like NiMoO4 nanorod arrays as highly efficient bifunctional catalysts for electrocatalytic/photovoltage-driven urea electrolysis

Developing multifunctional electrocatalysts with high catalytic activity, long-term stability, and low cost is essential for electrocatalytic energy conversion. Herein, sea urchin-like NiMoO4 nanorod arrays grown on nickel foam has been developed as a bifunctional electrocatalyst for urea oxidation and hydrogen evolution. The NiMoO4-200/NF catalyst exhibits efficient activity toward hydrogen evolution reaction with a low overpotential of only 68 mV in 1.0 mol/L KOH to gain a current density of 10 mA cm–2. The NiMoO4-300/NF catalyst exhibits a prominent oxygen evolution reaction (OER) catalytic activity with an overpotential of 288 mV at 50 mA cm–2, as well as for urea oxidation reaction with an ultra-low potential of 1.36 V at 10 mA cm–2. The observed difference in electrocatalytic activity and selectivity, derived by temperature variation, is ascribed to different lattice oxygen contents. The lattice oxygen of NiMoO4-300/NF is more than that of NiMoO4-200/NF, and the lattice oxygen is conducive to the progress of OER. A urea electrolyzer was assembled with NiMoO4-200/NF and NiMoO4-300/NF as cathode and anode respectively, delivering a current density of 10 mA cm–2 at a cell voltage of merely 1.38 V. The NiMoO4 nanorod arrays has also been successfully applied for photovoltage-driven urea electrolysis and hydrogen production, revealing its great potential for solar-driven energy conversion.