Water Splitting: From Electrode to Green Energy System

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm^-2 needs a cell voltage range of 1.8-2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Constructing RuCoOx /NC Nanosheets with Low Crystallinity within ZIF-9 as Bifunctional Catalysts for Highly Efficient Overall Water Splitting

Electrocatalysts play a pivotal role in accelerating the sluggish electrochemical water splitting reaction. Herein, a Ru-Co oxides and carbon nitrides hybrid (RuCoO_x /NC) electrocatalyst was constructed by employing ZIF-9 to disperse Ru precursor and deliberately regulating the calcination temperature. The moderate calcination temperature results in the RuCoO_x nanocomposites with small particle size and low crystallinity as well as the co-existence of multi-valence metal compounds, thus boosting the amount and species of active sites. Moreover, the strong interactions between Co and Ru species induce the electron transfer from Co to Ru, thus enhancing the adsorption of anion intermediates on the electron-deficient Co species and the proton capturing capacity of electron-sufficient Ru species. As a result, the optimized RuCoO_x /NC-350 catalyst behaved good electrocatalytic activities with 73 and 210 mV overpotential to achieve 10 mA cm^-2 for HER and OER, respectively. Remarkably, it showed good durability by holding at 100 mA cm^-2 for 100 h in HER and 50 mA cm^-2 for 24 h in OER with small activity decline. This study may shed new light on the rational construction of highly efficient Ru-based catalysts for electrochemical water splitting.

A Review on MoS2 Energy Applications: Recent Developments and Challenges

Molybdenum disulfide (MoS2) is a promising transition metal dichalcogenide (TMD) that has exceptional electronic, magnetic, optical, and mechanical properties. It can be semiconducting, superconducting, or an insulator according to its polymorph. Its bandgap structure changes from indirect to direct when moving towards its nanostructures, which opens a door to bandgap engineering for MoS2. Its supercapacitive and catalytic activity was recently noticed and studied, in order to include this material in a wide range of energy applications. In this work, we present MoS2 as a future material for energy storage and generation applications, especially solar cells, which are a cornerstone for a clean and abundant source of energy. Its role in water splitting reactions can be utilized for energy generation (hydrogen evolution) and water treatment at the same time. Although MoS2 seems to be a breakthrough in the energy field, it still faces some challenges regarding its structure stability, production scalability, and manufacturing costs.

The facile synthesis of core-shell PtCu nanoparticles with superior electrocatalytic activity and stability in the hydrogen evolution reaction

Pt is the most efficient electrocatalyst for the hydrogen evolution reaction (HER); however, it is a high cost material with scarce resources. In order to balance performance and cost in a Pt-based electrocatalyst, we prepared a series of PtCu bimetallic nanoparticles (NPs) with different Pt/Cu ratios through a facile synthetic strategy to optimize the utilization of Pt atoms. PtCu NPs demonstrate a uniform particle size distribution with exposed (111) facets that are highly active for the HER. A synergetic effect between Pt and Cu leads to electron transfer from Pt to Cu, which is favorable for the desorption of H intermediates. Therefore, the as-synthesized carbon black (CB) supported PtCu catalysts showed enhanced catalytic performance in the HER compared with a commercial Pt/C electrocatalyst. Typically, Pt₁Cu₃/CB showed excellent HER performance, with only 10 mV (acid) and 17 mV (alkaline) overpotentials required to achieve a current density of 10 mA cm^-2. This is because the Pt₁Cu₃ NPs, with a small average particle size (7.70 ± 0.04 nm) and Pt-Cu core and Pt-rich shell structure, display the highest electrochemically active surface area (24.7 m² g_Pt ^-1) out of the as-synthesized PtCu/CB samples. Furthermore, Pt₁Cu₃/CB showed good electrocatalytic stability, with current density drops of only 9.3% and 12.8% in acidic solution after 24 h and in alkaline solution after 9 h, respectively. This study may shed new light on the rational design of active and durable hydrogen evolution catalysts with low amounts of Pt.

Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges

The oxygen evolution reaction (OER) is the efficiency-determining half-reaction process of high-demand, electricity-driven water splitting due to its sluggish four-electron transfer reaction. Tremendous effects on developing OER catalysts with high activity and strong acid-tolerance at high oxidation potentials have been made for proton-conducting polymer electrolyte membrane water electrolysis (PEMWE), which is one of the most promising future hydrogen-fuel-generating technologies. This review presents recent progress in understanding OER mechanisms in PEMWE, including the adsorbate evolution mechanism (AEM) and the lattice-oxygen-mediated mechanism (LOM). We further summarize the latest strategies to improve catalytic performance, such as surface/interface modification, catalytic site coordination construction, and electronic structure regulation of catalytic centers. Finally, challenges and prospective solutions for improving OER performance are proposed.

Origin of the electrocatalytic oxygen evolution activity of nickel phosphides: in-situ electrochemical oxidation and Cr doping to achieve high performance

Zinc-air batteries (ZnABs) with high theoretical capacity and environmental benignity are the most promising candidates for next-generation electronics. However, their large-scale applications are greatly hindered due to the lack of high-efficient and cost-effective electrocatalysts. Transition metal phosphides (TMPs) have been reported as promising electrocatalysts. Notably, (Ni_1-xCr_x)₂P (0 ≤ x ≤ 0.15) is an unstable electrocatalyst, which undergoes in-situ electrochemical oxidation during the initial oxygen evolution reaction (OER) and even in the activation cycles, and is eventually converted to Cr-NiOOH serving as the actual OER active sites with high efficiency. Density functional theory (DFT) simulations and experimental results elucidate that the OER performance could be significantly promoted by the synergistic effect of surface engineering and electronic modulations by Cr doping and in-situ phase transformation. The constructed rechargeable ZnABs could stably cycle for more than 208 h at 5 mA cm^-2, while the voltage degradation is negligible. Furthermore, the developed catalytic materials could be assembled into flexible and all-solid-state ZnABs to power wearable electronics with high performance.

Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions

Of all the available resources given to mankind, the sunlight is perhaps the most abundant renewable energy resource, providing more than enough energy on earth to satisfy all the needs of humanity for several hundred years. Therefore, it is transient and sporadic that poses issues with how the energy can be harvested and processed when the sun does not shine. Scientists assume that electro/photoelectrochemical devices used for water splitting into hydrogen and oxygen may have one solution to solve this hindrance. Water electrolysis-generated hydrogen is an optimal energy carrier to store these forms of energy on scalable levels because the energy density is high, and no air pollution or toxic gas is released into the environment after combustion. However, in order to adopt these devices for readily use, they have to be low-cost for manufacturing and operation. It is thus crucial to develop electrocatalysts for water splitting based on low-cost and land-rich elements. In this review, I will summarize current advances in the synthesis of low-cost earth-abundant electrocatalysts for overall water splitting, with a particular focus on how to be linked with photoelectrocatalytic water splitting devices. The major obstacles that persist in designing these devices. The potential future developments in the production of efficient electrocatalysts for water electrolysis are also described.

Direct Synthesis of Molybdenum Phosphide Nanorods on Silicon Using Graphene at the Heterointerface for Efficient Photoelectrochemical Water Reduction

MoP nanorod-array catalysts were directly synthesized on graphene passivated silicon photocathodes without secondary phase. Mo-O-C covalent bondings and energy band bending at heterointerfaces facilitate the electron transfer to the reaction sites. Numerous catalytic sites and drastically enhanced anti-reflectance of MoP nanorods contribute to the high solar energy conversion efficiency. Transition metal phosphides (TMPs) and transition metal dichalcogenides (TMDs) have been widely investigated as photoelectrochemical (PEC) catalysts for hydrogen evolution reaction (HER). Using high-temperature processes to get crystallized compounds with large-area uniformity, it is still challenging to directly synthesize these catalysts on silicon photocathodes due to chemical incompatibility at the heterointerface. Here, a graphene interlayer is applied between p-Si and MoP nanorods to enable fully engineered interfaces without forming a metallic secondary compound that absorbs a parasitic light and provides an inefficient electron path for hydrogen evolution. Furthermore, the graphene facilitates the photogenerated electrons to rapidly transfer by creating Mo-O-C covalent bondings and energetically favorable band bending. With a bridging role of graphene, numerous active sites and anti-reflectance of MoP nanorods lead to significantly improved PEC-HER performance with a high photocurrent density of 21.8 mA cm^-2 at 0 V versus RHE and high stability. Besides, low dependence on pH and temperature is observed with MoP nanorods incorporated photocathodes, which is desirable for practical use as a part of PEC cells. These results indicate that the direct synthesis of TMPs and TMDs enabled by graphene interlayer is a new promising way to fabricate Si-based photocathodes with high-quality interfaces and superior HER performance.

Radical abstraction vs. oxidative addition mechanisms for the activation of the SH, OH, and CH bonds using early transition metal oxides

Quantum chemical calculations are performed to study the S-H, O-H, and C-H bond activation of H2S, H2O, and CH4 by bare and ligated ZrO+ and NbO+ units. These representative oxides bear low energy oxo and higher energy oxyl units. S-H and C-H bonds are readily activated by metal oxyl states (radical mechanism), but the O-H bond is harder to activate with either the oxyl or oxo states. Our results suggest that known practices for the C-H bond activation can be applied to S-H, but not to O-H bonds. The identified trends are rationalized in terms of the HS-H, HO-H, and H3C-H dissociation energies to the homolytic or heterolytic fragments. We also found that these dissociation energies drop to about half after coordination of H2S or H2O to the metal oxide unit. In addition, chlorine ligands are shown to stabilize the higher energy oxyl states of the transition metal oxygen unit enhancing the reactivity of the formed complexes.

A monodispersed CuPt alloy: synthesis and its superior catalytic performance in the hydrogen evolution reaction over a full pH range

The high cost and low stability of electrocatalysts are the major challenges for the commercialization of hydrogen generation in water. In this study, we demonstrated a one-pot synthesis of a monodispersed CuPt alloy with the diameter range of 20–30 nm by a hydrothermal method. Benefiting from the more available active sites and preferable d-band structure, the CuPt alloy exhibited a superior catalytic performance than pure Pt nanoparticles (Pt NPs) in the hydrogen evolution reaction (HER). In acidic media, the CuPt alloy achieved a low overpotential of 39 mV at a current density of 10 mA cm⁻² for HER, which was by 22 mV lower than that for pure Pt NPs. In a neutral solution, the stability of the CuPt alloy is ca. 100-fold as compared to pure Pt NPs. Accounting by the dissolution of Cu in the alloy phase, the performance of the CuPt alloy was elevated after yielding hydrogen for 1.2 × 10⁵ s in alkaline media. The superior catalytic activity can also be applied in other applications. In the reduction of 4-nitro-phenol (4-NP), the CuPt alloy showed 12.84-fold catalytic activity higher than pure Pt NPs. This study designed a low-cost electrocatalyst with an efficient and durable catalytic performance for HER over the full pH range, which provides an environmentally friendly strategy to cope with the challenges of hydrogen generation.

An effective approach to achieve the low cost and high stability of electro-catalysts for HER.

Synergistically enhanced alkaline hydrogen evolution reaction by coupling CoFe layered double hydroxide with NiMoO4 prepared by two-step electrodeposition

The optimized CoFe LDH/NiMoO4/Cu NW/Cu foam as HER electrocatalyst presents promising application prospect in water splitting with ultralow overpotential of 45 mV at -10 mA/cm2 and long-term durability.