Metal organic frameworks as electrocatalysts: Hydrogen evolution reactions and overall water splitting

The development of clean energy technologies to protect the environment is an important demand of the times. Electrocatalysis is emerging as a promising method for evolution of hydrogen and overall water splitting. Nowadays, metal organic frameworks (MOFs) have emerged as electrocatalysts having uniformly distributed active sites and high electrical conductivity. This review summarizes the latest advances in heterogeneous catalysis by MOFs and their composite/derivatives for efficient hydrogen evolution reaction (HER) and water splitting. Pristine MOFs with their recent development are summarized first followed by composites of MOFs with their enhanced electrocatalytic performances. Overall water splitting by using bifunctional electrocatalysts derived from MOFs with different synthetic approaches is provided and this review gives the metal-based categorisation of precursor MOFs. Different strategies to improve chemical stability, conductivity, and overall electrocatalytic properties have been discussed. In the last, perspectives on the synthesis of efficient MOF-based electrocatalyst materials are provided.     

Impact in rational synthesis Co-ZIF templates for derived the structural Co@NC catalysis for high efficient hydrogen and oxygen evolution reactions

Developing highly active and stable non-noble metal bifunctional electrocatalysts are urgently demanded in overall water splitting. Herein, tunable precursor ratio synthesis of cobalt-based ZIFs as a template derived active cobalt embedded N-doped carbon (Co@NC) catalyst. The rational synthesis of ZIF templates significantly impacts the complex nanostructure and properties of the catalyst (Co@NC). Consequently, the different nanostructures on Co@NC exhibit significance for the electrocatalyst of hydrogen and oxygen evolution reactions. The optimized Co@NC-20 provides excellent electrocatalytic activity with the lowest overpotential of 172 and 301 mV for HER and OER, respectively, at the current density of 10 mA cm^?2. The bifunctional Co@NC-20 reveals a potential for overall water splitting as low as 1.68 V of 10 mA cm^?2. After continuously working for 24h, the exceptional stability activity maintains 75% of the catalytic performance on Co@NC-20. The beneficial character in the synergistic effects between high-active Co species with well-protection of the metal core by carbon shell promotes their excellent performance. This study provides an essential reference for the rational design of ZIF templates for electrocatalysts with more complex structures in the future.     

Ni activated Mo2C nanoparticles supported on stereotaxically-constructed graphene for efficient overall water splitting

Exploring cost-effective, high-efficiency and stable electrocatalysts for overall water splitting is greatly desirable and challenging for sustainable energy. Herein, a novel designed Ni activated molybdenum carbide nanoparticle loaded on stereotaxically-constructed graphene (SCG) using two steps facile strategy (hydrothermal and carbonization) as a bifunctional electrocatalyst for overall water splitting. The optimized Ni/Mo₂C(1:20)-SCG composites exhibit excellent performance with a low overpotential of 150 mV and 330 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively to obtain a current density of 10 mA cm^?2 in 1.0 M KOH solution. In addition, when the optimized Ni/Mo₂C(1:20)-SCG composite is used as a bifunctional electrode for overall water splitting, the electrochemical cell required a low cell voltage of 1.68 V at a current density of 10 mA cm^?2 and long-term stability of 24 h. More significantly, the synergetic effects between Ni-activated Mo₂C nanoparticles and SCG are regarded as a significant contributor to accelerate charge transfer and promote electrocatalytic performance in hybrid electrocatalysts. Our works introduce a novel approach to design advanced bifunctional electrodes for overall water splitting.     

Zn doped ZIF67-derived porous carbon framework as efficient bifunctional electrocatalyst for water splitting

The electrochemical splitting of water is considered to be an efficient and potential technique for producing clean hydrogen and oxygen. Although, there are lots of significant developments in composite of superior hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) catalyst applied in water splitting currently, designing non-precious and low-cost bifunctional electrocatalysts with high performance is still an attractive challenging issue. In this article, we report a novel bifunctional electrocatalyst with cobalt-based nanoparticles (NPs) embedded in Zn-doped hierarchical porous three-dimension N-doped carbonization structure via an annealing process of metal organic frameworks (MOFs) connected by N-doped carbon nanotube (denoted as Co–Zn/PNC). This composite structure possesses the characteristics of more active sites, numerous mesopores and high conductivity. The resulting electrocatalyst (Co–Zn/PNC) can be used as both anode and cathode to roust the overall water splitting, getting a current density of 10 mA cm⁻² at a cell voltage of 1.63 V in 1.0 M KOH electrolyte.     

Bimetallic NiCo–NiCoO2 nano-heterostructures embedded on copper foam as a self-supported bifunctional electrode for water oxidation and hydrogen production in alkaline media

Earth-abundant, non-precious metal-based bifunctional electrocatalysts with efficient water splitting activity are of valuable importance in the limitation of energy losses in an alkaline environment. Herein, we report NiCo–NiCoO₂ nano-heterostructures embedded on the oxidized surface of copper foam (NiCo–NiCoO₂@Cu₂O@CF) as an efficient bifunctional electrocatalyst for overall water splitting in 1 M KOH electrolyte solution. In this study, metallic Ni and Co interlinkage with NiCoO₂ nanoparticles (NPs) are suggested to form by thermal decomposition of nickel-cobalt hydroxide precursors embedding on copper foam under a nitrogen environment. Bimetallic thin layered nano-heterostructures of NiCo–NiCoO₂@Cu₂O@CF exhibits a synergic effect of doubly active metals Ni and Co to achieve remarkable small overpotentials of 133 and 327 mV to achieve a current density of 10 mA cm^?2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The influential synergetic and structural effects have been extensively discussed to understand the overall water splitting for designing an efficient electrocatalyst. Hence, this phenomenon for surface modification of conductive substrate (CF) with a suitable combination of metal/metal oxide alloying as catalytic material helps us to design and synthesize low cost, highly efficient, non-precious metal-based electrocatalysts for overall water splitting.     

Sulfuration of hierarchical Mn-doped NiCo LDH heterostructures as efficient electrocatalyst for overall water splitting

Constructing efficient bifunctional electrocatalysts for both cathode and anode is of great importance for obtaining green hydrogen by water splitting. Herein, sulfuration of hierarchical Mn-doped NiCo LDH heterostructures (Mn–NiCoS₂/NF) is constructed as a bifunctional electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) via a facile hydrothermal-annealing strategy. Mn–NiCoS₂/NF shows an overpotential of 310 mV at 50 mA cm⁻² for OER and 100 mV at 10 mA cm⁻² for HER in 1.0 M KOH. Moreover, only 1.496 V@10 mA cm⁻² is required for overall water splitting by using Mn–NiCoS₂/NF as catalyst dual electrodes in a two-electrode system. The excellent performance of Mn–NiCoS₂/NF should be attributed to the ameliorative energy barriers of adsorption/desorption for HO⁻/H₂O through the modification of electronic structure of NiCo basal plane by Mn-doping and the acceleration of water dissociation steps via rich delocalized electron inside sulfur vacancies. The construction of hierarchical Mn–NiCoS₂/NF heterostructures provides new prospects and visions into developing efficient-advanced electrocatalysts for overall water splitting.     

Manganese oxide embedded graphene nanohybrid for pH-universal electrochemical overall water splitting

Efficient non-noble metal bifunctional electrocatalysts for overall water splitting in pH universal is highly desired in application. Herein, MnO₂/graphene composition are applied as efficient electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in pH-universal electrolytes with the help of plasma dc arc method. The couple of MnO₂ and graphene highly benefits to the H₂O, H⁺ and OH⁻ absorption respectively. The defects and stable Mn³⁺ contribute to the transfer of electron and charge. The low overpotentials and small Tafel slopes reveal attractive activities of HER and OER. The good electrocatalytic performances are attributed to the synergistic effect and abundant heterogeneous interfaces in MnO₂/graphene. These can offer rich electroactive sites and accelerate electron transfer. Thus, it may provide facile route for developing nonprecious electrocatalysts of water splitting.     

(Fe,Ni)S2@MoS2/NiS2 hollow heterostructure nanocubes for high-performance alkaline water electrolysis

Hollow hybrid heterostructures are regarded to be promising materials as bifunctional electrocatalysts for highly efficient water electrolysis due to their intriguing morphological features and remarkable electrochemical properties. Herein, with FeNi-PBA as both a precursor and morphological template, we demonstrate the rational construct of cost-effective (Fe,Ni)S₂@MoS₂/NiS₂ hollow hybrid heterostructures as bifunctional electrocatalysts for alkaline overall water splitting. Microstructural analysis shows that the hybrid is a kind of hierarchical heterostructure composed of MoS₂/NiS₂ nanosheets/nanoparticles in situ grown on hollow (Fe,Ni)S₂ nanocubes with abundant heterointerfaces, which effectively maximizes the electrochemical active sites to the accessible electrolyte ions, leading to the promoted charge transfer. As expected, the hybrid shows remarkable alkaline electrocatalytic performance, such as hydrogen evolution overpotential of 176 mV and oxygen evolution overpotential of 342 mV at 50 mA cm^?2, as well a cell voltage of 1.65 V at 20 mA cm^?2. Moreover, the stability and durability are greatly enhanced under harsh electrochemical conditions. This study opens a new venue for developing earth-abundant bifunctional electrocatalysts with hollow hybrid heterostructures for alkaline water electrolysis in the future.     

Heterogeneous layered multiple transition metal composite electrocatalysts with controlled composition for hydrogen production

The exploration of highly efficient and low-cost bifunctional electrocatalyst is essential for overall water splitting, especially for industrial application under alkaline conditions. Herein, we propose a controllable structural engineering strategy of constructing heterogeneous layered electrocatalyst with wetting surface for hydrogen evolution reaction and oxygen evolution reaction. Heterogeneous layered NiFe LDH (layered double hydroxide)/CoFeP/NF (Ni foam) with superhydrophilic surfaces is successfully fabricated by successive electrodeposition, phosphorization and solvothermal method. The NiFe LDH/CoFeP/NF for hydrogen evolution achieves a low overpotential of 198 mV at 50 mA cm^?2 in 1.0 M KOH. An overpotential of 269 mV is required at 50 mA cm^?2 for oxygen evolution. Meanwhile, the practical utilization of NiFe LDH/CoFeP/NF as bifunctional electrocatalysts for overall water splitting yields 1.73 V at 50 mA cm^?2 in the two-electrode cell. Moreover, NiFe LDH/CoFeP/NF can retain over 50 h without an obvious degradation at 10 mA cm^?2. The satisfactory operating stability and high activity of NiFe LDH/CoFeP/NF in alkaline solution can be attributed to the heterogeneous layered structure and excellent hydrophilic surface. The study provides a strategy to engineering heterogeneous layered structures with wetting surface for excellent electrocatalytic activities toward overall water splitting.     

Eco-designed electrocatalysts for water splitting: A path toward carbon neutrality

Realizing sustainable hydrogen fuel production through water electrolysis is crucial to achieving carbon neutrality. However, the development of cost-effective electrocatalysts continues to be a challenge. Eco-designed electrocatalysts derived from wastes and naturally abundant materials have recently received increasing attention. The development of eco-designed electrocatalysts is of great environmental and economic significance and makes green hydrogen more accessible to the wider community. Here, recent advances in eco-designed electrocatalysts for water splitting are summarized. Eco-design strategies such as pyrolysis, ball milling, wet-chemical methods, and electrochemical treatment are first analyzed. Recent achievements in eco-designed electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS) are then detailed, with an emphasis on analyzing the eco-design strategy-catalyst property-catalytic performance correlation. Perspectives in this blooming field for a greener hydrogen economy are finally outlined.     

Highly efficient overall water splitting performance of gadolinium-indium-zinc ternary oxide nanostructured electrocatalyst

Metal oxide nanostructures gain specific interest for bifunctional electrocatalytic functions in efficient water splitting reactions. From this perspective, Gd-In-Zn-based ternary oxides were solution processed and studied for oxygen and hydrogen evolution activities under different pH alkaline media. The nanostructure morphology was ascertained using high-power analytical tools such as scanning and transmission electron microscopes. Signals obtained from X-ray diffraction data revealed the prevalence of Gd₂O₃ phase in ternary oxide. Valence state of Gd, In, and Zn ions and their oxide traits was examined using X-ray photoelectron spectroscopy. Ternary oxides of Gd-In-Zn showcased their overall potential for water splitting applications. Overpotential (η) for oxygen and hydrogen (OER/HER) evolution reactions was recorded to be 282 and 271 mV for ±10 mA/cm². The results demonstrated the processed oxide as an effective OER/HER electrocatalyst with profound Tafel slopes (121/64 mV/dec for OER/HER) and excellent long-term stability.     

Self-standing and efficient bifunctional electrocatalyst for overall water splitting under alkaline media enabled by Mo1-xCoxS2 nanosheets anchored on carbon fiber paper

The layered MoS₂ nanostructures have been widely used in the electrochemical hydrogen evolution reaction (HER), but rarely applied in overall water splitting application for their ignorable oxygen evolution reaction (OER) activity. To address this issue, a novel self-standing and bifunctional electrocatalyst, consisting of Co-doped MoS₂ nanosheets anchored on carbon fiber paper, has been prepared via hydrothermal method. Taking advantage of conductive substrate of carbon fiber paper, sufficient-exposed active edges of MoS₂ sheets, and metallic character caused by Co-doping, our electrode exhibits high-efficient bifunctional activities for the overall water splitting in alkaline electrolyte (1 M KOH), which can produce a current density of 20 mA cm⁻² at an overpotential of 197 mV for HER and 235 mV for OER.     

Ultrathin MoS2 nanosheets in situ grown on rich defective Ni0.96S as heterojunction bifunctional electrocatalysts for alkaline water electrolysis

Developing earth-abundant and highly active bifunctional electrocatalysts are critical to advance sustainable hydrogen production via alkaline water electrolysis but still challenging. Herein, heterojunction hybrid of ultrathin molybdenum disulfide (MoS₂) nanosheets and non-stoichiometric nickel sulfide (Ni_0.96S) is in situ prepared via a facile one-step hydrothermal strategy, followed by annealing at 400 °C for 1 h. Microstructural analysis shows that the hybrid is composed of intimate heterojunction interfaces between Ni_0.96S and MoS₂ with exposed active edges provided by ultrathin MoS₂ nanosheets and rich defects provided by non-stoichiometric Ni_0.96S nanocrystals. As expected, it is evaluated as bifunctional electrocatalysts to produce both hydrogen and oxygen via water electrolysis with a hydrogen evolution reaction (HER) overpotential of 104 mV at 10 mA cm⁻² and an oxygen evolution reaction (OER) overpotential of 266 mV at 20 mA cm⁻² under alkaline conditions, outperforming most current noble-metal-free electrocatalysts. This work provides a simple strategy toward the rational design of novel heterojunction electrocatalysts which would be a promising candidate for electrochemical overall water splitting.     

Engineering MOF-based nanocatalysts for boosting electrocatalytic water splitting

Electrochemical water splitting is now appearing as a promising strategy for the renewable production of clean hydrogen. Constructing advanced electrocatalysts for boosting these processes is intensely expected to reduce their overpotentials and facilitate the practical applications. Benefitting from their tunable compositions, excellent porosities, and ultrahigh surface areas, metal-organic frameworks (MOFs) are thus emerging as the nonprecious candidates for driving overall water splitting. Recent years have witnessed the rapid development of MOFs toward electrochemical water splitting and beyond. Herein, we have presented the most pivotal progresses in recent research on engineering advanced MOFs for boosting electrochemical water splitting. This review is started by manifesting the advantages and disadvantages of MOFs. Subsequent, some advanced strategies for the effective modifications of MOFs are also highlighted. Finally, a summary about the future directions and challenges are also presented to provide guidance for the further development of more efficient MOF-based electrocatalysts.     

Stainless steel mesh-based CoSe/Ni3Se4 heterostructure for efficient electrocatalytic overall water splitting

The construction of high-efficiency bifunctional electrocatalysts is still a main challenge for hydrogen production from water splitting, in which comprehensive structure regulation plays a key role for synergistically boosting the intrinsic activity and charge collection. Here, we used a two-step hydrothermal method for construction of an interjaculated CoSe/Ni₃Se₄ heterostructure from NiCo LDH nanosheets grown on stainless steel (SS) meshes as bifunctional electrocatalysts for overall water splitting. The SS meshes containing Fe and Ni act as an excellent 3D scaffold for catalyst growth and charge collection. The SS@CoSe/Ni₃Se₄ composite exhibits outstanding electrocatalytic performances with low overpotentials of 97 mV for hydrogen evolution and 230 mV for oxygen evolution to reach a current density of 10 mA cm⁻², respectively. Moreover, by using SS@CoSe/Ni₃Se₄ as both the cathode and anode, the assembled electrolyze only required 1.55 V to reach 10 mA cm⁻² for overall water splitting. The outstanding performance of SS@CoSe/Ni₃Se₄ benefits from the synergy between excellent charge collection capability of SS meshes and the abundant active sites at the CoSe/Ni₃Se₄ heterointerface formed with the in-situ conversion of NiCo LDH nanosheets. Electrochemical active surface area and impedance spectrum indicate that the CoSe/Ni₃Se₄ loaded on SS has the most abundant electrochemically active sites and the smallest electrochemical resistance, thereby exposing more active sites and enhancing the charge transfer to promote the catalytic activity. By integrating the delicate nanoscale heterostructure engineering with the microscale SS mesh scaffold, our work provides a new perspective for the preparation of high-performance and cheap electrocatalysts that are easy to be integrated with industrial applications.     

Nitrogen doped FeCoNiS nanoparticles on N,S-co-doped vertical graphene as bifunctional electrocatalyst for water splitting

It's important to develop an economical and efficient electrocatalyst for water splitting. Nitrogen-doped FeCoNiS nanoparticles are supported on N, S-co-doped vertical graphene (N–FeCoNiS/SVG) by one-step electrodeposition. The SEM and TEM results show that 30–115 nm nanoparticles grow uniformly on SVG. The XRD results show that N–FeCoNiS/SVG is a polycrystalline material. N–FeCoNiS/SVG exhibits excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) potentials of 44 mV and 138 mV (10 mA cm^?2), respectively, in 1 M KOH. The Faraday efficiency of N–FeCoNiS/SVG for overall water splitting is about 97%. The electrocatalyst also shows excellent stability over 24 h. The XPS results illustrate that N-doping promotes the electron transfer between metal and heteroatom and effectively modulates the electronic structure of FeCoNiS. N–FeCoNiS/SVG has excellent electrocatalytic performance for water splitting. This work provides theoretical and technical support for the study of water splitting bifunctional electrocatalysts.     

Combustion-derived BaNiO3 nanoparticles as a potential bifunctional electrocatalyst for overall water splitting

Electrochemical water electrolyser though an assuring solution for clean hydrogen production, the sluggish kinetics and high cost of existing precious metal electrocatalyst remains a barrier to its effective utilization. Herein, solution combustion route derived perovskite type barium nickelate (BaNiO₃) nanoparticles were developed and studied for their bifunctional electrocatalytic properties towards overall water splitting. The unannealed BaNiO₃ nanoparticles exhibited the highest OER and HER activity with overpotentials 253 mV and 427 mV respectively to attain 10 mAcm⁻² in 1.0 M KOH. Using unannealed BaNiO₃ as a bifunctional electrocatalyst in a two-electrode alkaline electrolyser, the cell was able to achieve the benchmark current density at a low cell voltage of 1.82 V. Impressively the setup's electrocatalytic performance improved 4.9% after continuous overall water splitting for 24 h at 30 mAcm⁻². Therefore, BaNiO₃ nanoparticles can be a low-cost and efficient alternative for noble metal electrocatalysts for clean H₂ production.     

Boosting high-current water electrolysis: Superhydrophilic/superaerophobic nanosheet arrays of NiFe LDH with oxygen vacancies in situ grown on iron foam

Developing cost-effective and superior bifunctional electrocatalysts for alkaline water splitting is crucial to realizing hydrogen economy. However, in industrial applications, especially at high current densities, the sluggish kinetic process and the dissatisfactory prolonged stability of electrocatalysts astrict their applications. Herein, the superhydrophilic/superhydrophobic NiFe layered double hydroxide (LDH) with oxygen vacancies was designed in situ grown on iron foam (NiFe/IF) as a high-performance bifunctional electrocatalyst. The unique feature of the superhydrophilic/superaerophobic surface makes for electrolyte penetration and bubbles release, and the existence of oxygen vacancies grants the catalyst with enhanced inherent catalytic activity. Moreover, in situ Raman analysis reveals NiFe LDH could undergo surface reconstruction into active NiOOH species in the electrooxidation environment. Profiting from the above superiorities, the prepared NiFe/IF displays superior OER activity in 1 M KOH with low overpotentials of 245.2, and 480.2 mV to supply 100 and 1000 mA cm⁻², respectively. And the NiFe/IF exhibits prominent stability at 1000 mA cm⁻² under a simulated industrial condition (6 M KOH and 85 °C). Moreover, the water electrolysis device based on NiFe/IF as anode and cathode was assembled with a commercial solar cell to simulate a photovoltaic-driven water splitting system, which revealed a superior efficiency of 15.13% for solar hydrogen production.     

Insight into the role of sulfur in increasing intrinsic activity of Ni–Fe for efficient water splitting electrocatalysis

It is of great significance to explore and design low-cost and efficient electrocatalysts for the storage and conversion of intermittent renewable resources to clean hydrogen by water splitting. Herein, the amorphous Ni–Fe–S electrocatalysts are rapidly synthesized on Cu sheets and Ni foams using the simple electrodeposition method. After optimizing the S concentration, the Ni–Fe–S electrocatalysts exhibit the simultaneously boosted hydrogen and oxygen evolution reaction performances compared to the as-synthesized Ni–Fe and Ni. In addition, the Ni–Fe–S electrocatalysts as the bifunctional electrodes only require a cell voltage of 1.584 V (on Ni foam) and 1.705 V (on Cu sheet) to reach 10 mA/cm² with excellent stability in the electrocatalytic activity and surface properties. The results exhibit that the enhanced electrocatalytic activity can be attributed to the role of the doped S in formatting the amorphous structure, improving the hydrophilic and aerophobic properties, optimizing the electronic structure as well as enhancing the electrochemically active sites. This work might offer a new insight into the design of the cheap and highly efficient electrodes for generation of hydrogen by water splitting.     

Evidencing enhanced oxygen and hydrogen evolution reactions using In–Zn–Co ternary transition metal oxide nanostructures: A novel bifunctional electrocatalyst

Ternary transition metal oxides are gaining popularity for cost effective bifunctional electrocatalytic activities and to realization of novel water splitting devices. In this regard, In₂O₃/ZnO/Co₃O₄ based ternary oxide nanostructures were investigated in detail for their oxygen/hydrogen evolution reaction (OER/HER) in alkaline environment. The ternary oxides were at first processed through a simple chemical route involving hydrothermal treatment. The prepared nanostructures were then investigated by using high-resolution transmission electron microscopy (TEM/HRTEM) to ascertain their morphological traits. X-ray diffraction, Raman signals and photoluminescence data demonstrated the In₂O₃ phase to be prevalent in the ternary mixture on par with that of ZnO and Co₃O₄. The valence state of various metal ions and the In–O, Zn–O and Co–O bonding was verified using XPS. The ternary oxide coated electrodes exhibited excellent overall water splitting activity. Overpotential values of 398 and 510 mV were registered for OER and HER experiments under a current density of ±10 mA cm⁻², demonstrating the material to be an ideal OER/HER electrocatalyst at room temperature. The exceptional long-term stability in ternary oxides and their Tafel slope (88 mV/dec for OER and 60 mV/dec for HER) further affirmed their unique anodic/cathodic characteristics for water splitting applications.