Recent Development of Ni/Fe‐Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation

The oxygen evolution reaction (OER), as an important process involved in water splitting and rechargeable metal–air batteries, has drawn increasing attention in the context of clean energy generation and efficient energy storage. This review concerns the progress and new discoveries in the field of Ni/Fe‐based micro/nanostructures toward electrochemical and photo‐electrochemical (PEC) water oxidation during last few years. First, toward the design and construction of new electrocatalysis, different types of current Ni/Fe‐based compounds for OER are summarized. The mechanism studies of the active phases and positions of Ni/Fe‐based micro/nanostructures are further introduced to understand the properties of catalytic active sites, which could facilitate further improving the performance of Ni/Fe‐based OER electrocatalysts. Second, splitting water using sunlight with low overpotential is another important target in making solar‐to‐hydrogen micro/nanodevices, and thus attention is then focused on the development of several important Ni/Fe‐based PEC catalysts. Third, the recent theoretical calculations on the OER mechanism during water splitting and insights into electronic structures are analyzed; finally, the future trends and perspectives are also discussed briefly.     

Eutectic‐Derived Mesoporous Ni‐Fe‐O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting

The development of efficient and abundant water oxidation catalysts is essential for the large‐scale storage of renewable energy in the form of hydrogen fuel via electrolytic water splitting, but still remains challenging. Based upon eutectic reaction and dealloying inheritance effect, herein, novel Ni‐Fe‐O‐based composite with a unique mesoporous nanowire network structure is designed and synthesized. The composite exhibits exceptionally low overpotential (10 mA cm^?2 at an overpotential of 244 mV), low Tafel slope (39 mV dec^?1), and superior long‐term stability (remains 10 mA cm^?2 for over 60 h without degradation) toward oxygen evolution reaction (OER) in 1 m KOH. Moreover, an alkaline water electrolyzer is constructed with the Ni‐Fe‐O composite as catalyst for both anode and cathode. This electrolyzer displays superior electrolysis performance (affording 10 mA cm^?2 at 1.64 V) and long‐term durability. The remarkable features of the catalyst lie in its unique mesoporous nanowire network architecture and the synergistic effect of the metal core and the active metal oxide, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER.     

Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution,Oxygen Evolution Reactions,and Full Water Splitting

The sustainable and scalable production of hydrogen through hydrogen evolution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state‐of‐the‐art electrocatalysts of Pt and IrO₂/RuO₂ exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. Therefore, the development of alternative effective electrocatalysts for water splitting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster‐based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed.     

Highly Efficient Oxygen Evolution by a Thermocatalytic Process Cascaded Electrocatalysis Over Sulfur‐Treated Fe‐Based Metal–Organic‐Frameworks

The oxygen evolution reaction (OER) is a bottleneck process for water splitting and finding highly efficient, durable, low‐cost, and earth‐abundant electrocatalysts is still a major challenge. Here a sulfur‐treated Fe‐based metal–organic‐framework is reported as a promising electrocatalyst for the OER, which shows a low overpotential of 218 mV at the current density of 10 mA cm^?2 and exhibits a very low Tafel slope of 36.2 mV dec^?1 at room temperature. It can work on high current densities of 500 and 1000 mA cm^?2 at low overpotentials of 298 and 330 mV, respectively, by keeping 97% of its initial activity after 100 h. Notably, it can achieve 1000 mA cm^?2 at 296 mV with a good stability at 50 °C, fully fitting the requirements for large‐scale industrial water electrolysis. The high catalytic performance can be attributed to the thermocatalytic processes of H⁺ capture by –SO₃ groups from *OH or *OOH species, which cascades to the electrocatalytic pathway and then significantly reduces the OER overpotentials.     

Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution

The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high‐surface area nickel boride (Ni_x B) nanosheets as a low‐cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm^?2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm^?2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X‐ray absorption fine‐structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni‐B under OER conditions, owing to a Ni‐B core@nickel oxyhydroxide shell (Ni‐B@NiO_x H) structure, and increase in disorder of the NiO_x H layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution.     

Co–Ni‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts

One promising approach to hydrogen energy utilization from full water splitting relies on the successful development of earth‐abundant, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, homologous Co–Ni‐based nanotube/nanosheet structures with tunable Co/Ni ratios, including hydroxides and nitrides, are grown on conductive substrates by a cation‐exchanging method to grow hydroxides, followed by anion exchanging to obtain corresponding nitrides. These hydroxide OER catalysts and nitride HER catalysts exhibit low overpotentials, small Tafel slopes, and high current densities, which are attributed to their large electrochemically reactive surface, 1D morphologies for charge conduction, and octahedral coordination states of metal ions for efficient catalytic activities. The homologous Co–Ni‐based nanotube hydroxides and nitrides suggest promising electrocatalysts for full water splitting with high efficiency, good stability, convenient fabrication, and low cost.     

Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction

Hydrogen is a promising alternative fuel for efficient energy production and storage, with water splitting considered one of the most clean, environmentally friendly, and sustainable approaches to generate hydrogen. However, to meet industrial demands with electrolysis‐generated hydrogen, the development of a low‐cost and efficient catalyst for the oxygen evolution reaction (OER) is critical, while conventional catalysts are mostly based on precious metals. Many studies have thus focused on exploring new efficient nonprecious‐metal catalytic systems and improving the understandings on the OER mechanism, resulting in the design of catalysts with superior activity compared with that of conventional catalysts. In particular, the use of multimetal rather than single‐metal catalysts is demonstrated to yield remarkable performance improvement, as the metal composition in these catalysts can be tailored to modify the intrinsic properties affecting the OER. Herein, recent progress and accomplishments of multimetal catalytic systems, including several important groups of catalysts: layered hydroxide, spinel, and amorphous metal oxides along with the theoretical principles of activity enhancement in multimetal systems are reviewed. Finally, this is concluded by discussing remaining challenges to achieve further improvements of OER catalyst activities.     

Ultralow Ru Loading Transition Metal Phosphides as High‐Efficient Bifunctional Electrocatalyst for a Solar‐to‐Hydrogen Generation System

Water splitting is a promising technology for sustainable conversion of hydrogen energy. The rational design of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional electrocatalysts with superior activity and stability in the same electrolyte is the key to promoting their large‐scale applications. Herein, an ultralow Ru (1.08 wt%) transition metal phosphide on nickel foam (Ru–MnFeP/NF) derived from Prussian blue analogue, that effectively drivies both the OER and the HER in 1 m KOH, is reported. To reach 20 mA cm^?2 for OER and 10 mA cm^?2 for HER, the Ru–MnFeP/NF electrode only requires overpotentials of 191 and 35 mV, respectively. Such high electrocatalytic activity exceeds most transition metal phosphides for the OER and the HER, and even reaches Pt‐like HER electrocatalytic levels. Accordingly, it significantly accelerates full water splitting at 10 mA cm^?2 with 1.470 V, which outperforms that of the integrated RuO₂ and Pt/C couple electrode (1.560 V). In addition, the extremely long operational stability (50 h) and the successful demonstration of a solar‐to‐hydrogen generation system through full water splitting provide more flexibility for large‐scale applications of Ru–MnFeP/NF catalysts.     

Highly Active Trimetallic NiFeCr Layered Double Hydroxide Electrocatalysts for Oxygen Evolution Reaction

The development of efficient and robust earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is an ongoing challenge. Here, a novel and stable trimetallic NiFeCr layered double hydroxide (LDH) electrocatalyst for improving OER kinetics is rationally designed and synthesized. Electrochemical testing of a series of trimetallic NiFeCr LDH materials at similar catalyst loading and electrochemical surface area shows that the molar ratio Ni:Fe:Cr = 6:2:1 exhibits the best intrinsic OER catalytic activity compared to other NiFeCr LDH compositions. Furthermore, these nanostructures are directly grown on conductive carbon paper for a high surface area 3D electrode that can achieve a catalytic current density of 25 mA cm^?2 at an overpotential as low as 225 mV and a small Tafel slope of 69 mV dec^?1 in alkaline electrolyte. The optimized NiFeCr catalyst is stable under OER conditions and X‐ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, and elemental analysis confirm the stability of trimetallic NiFeCr LDH after electrochemical testing. Due to the synergistic interactions among the metal centers, trimetallic NiFeCr LDH is significantly more active than NiFe LDH and among the most active OER catalysts to date. This work also presents general strategies to design more efficient metal oxide/hydroxide OER electrocatalysts.     

Zn0.35Co0.65O – A Stable and Highly Active Oxygen Evolution Catalyst Formed by Zinc Leaching and Tetrahedral Coordinated Cobalt in Wurtzite Structure

To arrive to sustainable hydrogen‐based energy solutions, the understanding of water‐splitting catalysts plays the most crucial role. Herein, state‐of‐the‐art hypotheses are combined on electrocatalytic active metal sites toward the oxygen evolution reaction (OER) to develop a highly efficient catalyst based on Earth‐abundant cobalt and zinc oxides. The precursor catalyst Zn_0.35Co_0.65O is synthesized via a fast microwave‐assisted approach at low temperatures. Subsequently, it transforms in situ from the wurtzite structure to the layered γ‐Co(O)OH, while most of its zinc leaches out. This material shows outstanding catalytic performance and stability toward the OER in 1 m KOH (overpotential at 10 mA cm^?2 η_initial = 306 mV, η_{98 h} = 318 mV). By comparing the electrochemical results and ex situ analyses to today's literature, clear structure‐activity correlations are able to be identified. The findings suggest that coordinately unsaturated cobalt octahedra on the surface are indeed the active centers for the OER.     

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

The 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 (F_0.25C₁CH/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 F_0.25C₁CH‐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.     

Integration of FeOOH and Zeolitic Imidazolate Framework‐Derived Nanoporous Carbon as an Efficient Electrocatalyst for Water Oxidation

As a cost‐effective catalyst for the oxygen evolution reaction (OER), the potential use of FeOOH is hindered by its intrinsic poor electron conductivity. Here, the significant enhancement of OER activity and long‐term stability of electrodeposited FeOOH on zeolitic imidazolate framework‐derived N‐doped porous carbons (NPCs) are reported. In alkaline media, FeOOH/NPC supported on nickel foam as a 3D electrode delivers a current density of 100 mA cm^?2 at a small overpotential of 230 mV and exhibits a low Tafel slope of 33.8 mV dec^?1 as well as excellent durability, making it one of the most active OER catalysts. Such high performance is attributed to a combined effect of the excellent electron conductivity of NPC and the synergy between FeOOH and NiO derived from Ni substrate.     

Controlling the Active Sites of Sulfur‐Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal‐free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal‐free catalysts, but the lack of understanding of active‐sites at the atomic‐level has slowed the design of highly active and stable metal‐free catalysts. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. This bidoping strategy introduces stable sulfur–carbon active‐sites. Fluorescence emission of the sulfur K‐edge by X‐ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM‐EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm^?2, but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube–graphene nanolobes behave like the state‐of‐the‐art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm^?2 at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light‐weight bidoped sulfur carbon nanotubes are potential candidates for next‐generation metal‐free regenerative fuel cells.     

NiFe‐Based Metal–Organic Framework Nanosheets Directly Supported on Nickel Foam Acting as Robust Electrodes for Electrochemical Oxygen Evolution Reaction

It is of great significance to develop highly efficient and superior stable oxygen evolution reaction (OER) electrocatalysts for upcoming electrochemical conversion technologies and clean energy systems. Here, an assembled 3D electrode is synthesized by a one‐step solvothermal process using such an original OER electrocatalyst. During the solvothermal process, Ni ions released from Ni foam in acidic solution and Fe ions added exogenously act as metal centers and coordinate with terephthalic acid (TPA) organic molecules by robust coordinate bonds, and finally, NiFe‐based metal–organic framework (MOF) nanosheets in situ grown on Ni foam, i.e., MIL‐53(FeNi)/NF, are prepared. This binder‐free 3D electrode shows superior OER activity with high current density (50 mA cm^?2) at an overpotential of 233 mV, a Tafel slope of 31.3 mV dec^?1, and excellent stability in alkaline aqueous solution (1 m KOH). It is discovered that introduction of Fe into MIL‐53 structure increases electrochemically‐active areas as well as reaction sites, accelerated electron transport capability, and modulated electronic structure to enhance catalytic performance. Besides, first principles calculations show that MIL‐53(FeNi) is more favorable for foreign atoms' adsorption and has increased 3d orbital electron density boosting intrinsic activity. This work elucidates a promising electrode for electrocatalysts and enriches direct application of MOF materials.     

Ultrasensitive Iron‐Triggered Nanosized Fe–CoOOH Integrated with Graphene for Highly Efficient Oxygen Evolution

Effectively active oxygen evolution reaction (OER) electrocatalysts are highly desired for water splitting. Herein, the design and fabrication of nanometer‐sized Fe‐modulated CoOOH nanoparticles by a novel conversion tailoring strategy is reported for the first time and these nanoparticles are assembled on graphene matrix to construct 2D nanohybrids (Fe? CoOOH/G) with ultrasmall particles and finely modulated local electronic structure of Co cations. The Fe components are capable of tailoring and converting the micrometer‐sized sheets into nanometer‐sized particles, indicative of ultrasensitive Fe‐triggered behavior. The as‐made Fe? CoOOH/G features highly exposed edge active sites, well‐defined porous structure, and finely modulated electron structure, together with effectively interconnected conducting networks endowed by graphene. Density functional theory calculations have revealed that the Fe dopants in the Fe? CoOOH nanoparticles have an enhanced adsorption capability toward the oxygenated intermediates involved in OER process, thus facilitating the whole catalytic reactions. Benefiting from these integrated characteristics, the as‐made Fe? CoOOH/G nanohybrids as an oxygen evolution electrocatalyst can deliver a low overpotential of 330 mV at 10 mA cm^?2 and excellent electrochemical durability in alkaline medium. This strategy provides an effective, durable, and nonprecious‐metal electrocatalyst for water splitting.     

Enabling Iron‐Based Highly Effective Electrochemical Water‐Splitting and Selective Oxygenation of Organic Substrates through In Situ Surface Modification of Intermetallic Iron Stannide Precatalyst

A strategy to overcome the unsatisfying catalytic performance and the durability of monometallic iron‐based materials for the electrochemical oxygen evolution reaction (OER) is provided by heterobimetallic iron–metal systems. Monometallic Fe catalysts show limited performance mostly due to poor conductivity and stability. Here, by taking advantage of the structurally ordered and highly conducting FeSn₂ nanostructure, for the first time, an intermetallic iron material is employed as an efficient anode for the alkaline OER, overall water‐splitting, and also for selective oxygenation of organic substrates. The electrophoretically deposited FeSn₂ on nickel foam (NF) and fluorine‐doped tin oxide (FTO) electrodes displays remarkable OER activity and durability with substantially low overpotentials of 197 and 273 mV at 10 mA cm^?2, respectively, which outperform most of the benchmarking NiFe‐based catalysts. The resulting superior activity is attributed to the in situ generation of α‐FeO(OH)@FeSn₂ where α‐FeO(OH) acts as the active site while FeSn₂ remains the conductive core. When the FeSn₂ anode is coupled with a Pt cathode for overall alkaline water‐splitting, a reduced cell potential (1.53 V) is attained outperforming that of noble metal‐based catalysts. FeSn₂ is further applied as an anode to produce value‐added products through selective oxygenation reactions of organic substrates.     

A Perovskite Nanorod as Bifunctional Electrocatalyst for Overall Water Splitting

The development of highly efficient and low‐cost electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting associated with the storage of clean and renewable energy. Here, this study reports its findings in the development of a nanostructured perovskite oxide as OER/HER bifunctional electrocatalyst for overall water splitting. Prepared by a facile electrospinning method, SrNb_0.1Co_0.7Fe_0.2O_3–δ perovskite nanorods (SNCF‐NRs) display excellent OER and HER activity and stability in an alkaline solution, benefiting from the catalytic nature of perovskites and unique structural features. More importantly, the SNCF‐NR delivers a current density of 10 mA cm^?2 at a cell voltage of merely ≈1.68 V while maintaining remarkable durability when used as both anodic and cathodic catalysts in an alkaline water electrolyzer. The performance of this bifunctional perovskite material is among the best ever reported for overall water splitting, offering a cost‐effective alternative to noble metal based electrocatalysts.     

Ultrafine Metal Nanoparticles/N‐Doped Porous Carbon Hybrids Coated on Carbon Fibers as Flexible and Binder‐Free Water Splitting Catalysts

By employing in situ reduction of metal precursor and metal‐assisted carbon etching process, this study achieves a series of ultrafine transition metal‐based nanoparticles (Ni–Fe, Ni–Mo) embedded in N‐doped carbon, which are found efficient catalysts for electrolytic water splitting. The as‐prepared hybrid materials demonstrate outstanding catalytic activities as non‐noble metal electrodes rendered by the synergistic effect of bimetal elements and N‐dopants, the improved electrical conductivity, and hydrophilism. Ni/Mo₂C@N‐doped porous carbon (NiMo‐polyvinylpyrrolidone (PVP)) and NiFe@N‐doped carbon (NiFe‐PVP) produce low overpotentials of 130 and 297 mV at a current density of 10 mA cm^?2 as catalysts for hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, these binder‐free electrodes show long‐term stability. Overall water splitting is also demonstrated based on the couple of NiMo‐PVP||NiFe‐PVP catalyzer. This represents a simple and effective synthesis method toward a new type of nanometal–carbon hybrid electrodes.     

Interface Engineering of Hierarchical Branched Mo‐Doped Ni3S2/NixPy Hollow Heterostructure Nanorods for Efficient Overall Water Splitting

Rational design and construction of bifunctional electrocatalysts with excellent activity and durability is imperative for water splitting. Herein, a novel top‐down strategy to realize a hierarchical branched Mo‐doped sulfide/phosphide heterostructure (Mo‐Ni₃S₂/Ni_xP_y hollow nanorods), by partially phosphating Mo‐Ni₃S₂/NF flower clusters, is proposed. Benefitting from the optimized electronic structure configuration, hierarchical branched hollow nanorod structure, and abundant heterogeneous interfaces, the as‐obtained multisite Mo‐Ni₃S₂/Ni_xP_y/NF electrode has remarkable stability and bifunctional electrocatalytic activity in the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in 1 m KOH solutions. It possesses an extremely low overpotential of 238 mV at the current density of 50 mA cm^?2 for OER. Importantly, when assembled as anode and cathode simultaneously, it merely requires an ultralow cell voltage of 1.46 V to achieve the current density of 10 mA cm^?2, with excellent durability for over 72 h, outperforming most of the reported Ni‐based bifunctional materials. Density functional theory results further confirm that the doped heterostructure can synergistically optimize Gibbs free energies of H and O‐containing intermediates (OH*, O*, and OOH*) during HER and OER processes, thus accelerating the catalytic kinetics of electrochemical water splitting. This work demonstrates the importance of the rational combination of metal doping and interface engineering for advanced catalytic materials.     

Multifunctional Coatings from Scalable Single Source Precursor Chemistry in Tandem Photoelectrochemical Water Splitting

The straightforward and inexpensive fabrication of stabilized and activated photoelectrodes for application to tandem photoelectrochemical (PEC) water splitting is reported. Semiconductors such as Si, WO₃, and BiVO₄ can be coated with a composite layer formed upon hydrolytic decomposition of hetero­bimetallic single source precursors (SSPs) based on Ti and Ni, or Ti and Co in a simple single‐step process under ambient conditions. The resulting 3d‐transition metal oxide composite films are multifunctional, as they protect the semiconductor electrode from corrosion with an amorphous TiO₂ coating and act as bifunctional electrocatalysts for H₂ and O₂ evolution based on catalytic Ni or Co species. Thus, this approach enables the use of the same precursors for both photoelectrodes in tandem PEC water splitting, and SSP chemistry is thereby established as a highly versatile low‐cost approach to protect and activate photoelectrodes. In an optimized system, SSP coating of a Si photocathode and a BiVO₄ photoanode resulted in a benchmark noble metal‐free dual‐photoelectrode tandem PEC cell for overall solar water splitting with an applied bias solar‐to‐hydrogen conversion efficiency of 0.59% and a half‐life photostability of 5 h.