Acidic Water Splitting Research Articles

High resistance towards electrochemical dissolution of cubic Rh@Ir core shell in acidic water splitting

The industrial development of acidic water splitting significantly counts on the activity associated with stability of oxygen evolution reaction (OER) electrocatalyst. In this work, we have constructed cubic Rh@Ir core–shell for acidic water splitting. The formed Rh@Ir electrocatalyst shows a high HER performance with only 29 mV overpotential for 10 mA cm−2, stemming from the upshifted and downshifted d band center of Ir and Rh, leading to a moderate hydrogen binding strength. The Ir atom donates its electrons to nearby Rh atoms triggering a high susceptibility in nucleophilic attack by H2O to generate *OOH; thereby, the overpotential required to achieve 10 mA cm−2 is 216 mV for Rh@Ir core–shell, better than commercial IrO2. Moreover, the stability is superior to commercial IrO2 and pure Ir electrocatalyst due to the strong electronic interplay in Rh@Ir contributing to a better resistance towards electrochemical dissolution proved by operando Raman and electrochemical impedance spectroscopies. Only 1.55 V is needed for Rh@Ir to drive overall water splitting at 10 mA cm−2.

Acidic water oxidation with ultralow 131 mV overpotential over surface-reconstructed RuIr nanoalloy: Chloride-driven high performance

The oxygen evolution reaction (OER) in kinetics is sluggish but a key electrode reaction for various energy storage and conversion devices, such as green hydrogen from water electrolysis, rechargeable metal-air battery, sustainable carbon dioxide electroreduction, synthetic ammonia, etc. However, it still remains a major and cutting-edge challenge to pioneering catalysts with simultaneously ultrahigh activity and stability for OER, even with diverse performance enhancement strategies, such as complex composition designs, surface chemical reconstructions, and multiphase engineering have been implemented to accelerate this key electrochemical process. Herein, we proposed a chloride-triggered activation and stabilization strategy for the branched RuIr alloy coated by thin IrO2 layer to achieve unparalleled high-performance toward OER in acidic water. The ultralow overpotential of the activated catalyst is reduced to about 131 mV at 10 mA cm−2, with 79-fold enhancement of mass-activity than commercial IrO2. Moreover, a modified proton exchange membrane water electrolysis (PEMWE) device was first constructed by introducing NaCl into recyclable electrolyte. It run a stably and low cell potential (1.514 V) for 170 h, having no obvious performance decay at 50 mA cm−2 in 0.5 M H2SO4 with 1.6 M NaCl, which far exceeding traditional IrO2||Pt/C PEMWE. The outstanding performance stems from that Cl- ions enable a faster lattice oxygen evolution mechanism. The novel catalyst activates asymmetric O-Ir-Cl structure induced by Cl- filled oxygen vacancies, which modify the electronic and geometric properties, improving OER activity. Moreover, these adsorbed ions can also efficiently patch and refill the oxygen vacancies that ensure the long-term structure stability. Therefore, tailoring the chloride-activated Ir-based catalysts with simultaneously improved activity and stability may be a valuable guide to achieve surprisingly high-performances for acidic water splitting.

Two-step annealing synthesis of MOF-derived Cr-Co-Ru metallic oxides for improving the oxygen evolution reaction in acidic water splitting

The development of acid-sensitive OER (Oxygen Evolution Reaction) electrocatalysts based on ruthenium oxides is a crucial step toward achieving large-scale hydrogen production in low-cost Proton Exchange Membrane (PEM) electrolyzers. However, a major challenge in this field is to overcome the tendency of ruthenium-based metal oxide dissolving in acidic environments, which affects long-term stability. In this study, we employed MIL-88B(Cr) as a precursor and used a two-step annealing method to prepare trimetallic metal oxides Cr0.5CoxRu0.5-xO2-δ. Detailed experimental measurements demonstrate that the introduction of Co reduced the volume of individual catalyst crystallites without disrupting the rutile crystal structure and inherent activity of CrO2@RuO2. Simultaneously, Co and Cr doping lowered the oxidation state of Ru within the catalyst system. Lower oxidation states of Ru are more stable during the OER reaction, ensuring the stability of this catalyst to be at least 100 hours. Furthermore, Co incorporation raises the energy required for oxygen vacancy formation, reduced the involvement of lattice oxygen, and enhanced the catalyst's activity. The catalyst Cr0.5Co0.17Ru0.33O2-δ with the best performance exhibited the overpotential of only 224 mV at 10 mA cm-2 in 0.5 M H2SO4. This study offers a viable strategy for developing highly active and stable catalysts in acidic media by controlling the electronic structure of RuO2.

Electrochemical Leaching of Ni Dopants in IrRu Alloy Electrocatalyst Boosts Overall Water Splitting

AbstractThe commercialization of water‐splitting technology strongly relies on the stable and high‐performance electrocatalyst for hydrogen and oxygen evolution reactions (HER‐OER). In this work, the nickel dopants in IrRu alloyed electrocatalyst for acidic water splitting activity are investigated. The incorporation of nickel dopants induces more oxygen atoms coordinating with Ir/Ru facilitating the formation of *OOH species via nucleophilic attack by H2O. Additionally, a fast surface reconstruction is achieved due to the electrochemical dissolution of Ni dopants; as a result, more oxygen species are incorporated, evidenced by in situ Raman spectroscopy and electrochemical impedance spectroscopy. Therefore, the mass activity for Ni@IrRu is enhanced by threefold than commercial IrO2 in acidic OER catalysis. Moreover, the nickel dopants balance the hydrogen binding strength; as a result, only 609 mV overpotential is demanded to reach 1 A cm−2 in acidic HER catalysis. The polymer electrolyte membrane water electrolyzer (PEMWE) test indicates only 1.87 V is required to attain 2 A cm−2, 160 mV lower than Pt/C‐IrO2 revealing its potential application in PEMWE.

Pt-Decorated bimetallic PdRu nanocubes with tailorable surface electronic structures for highly efficient acidic hydrogen evolution reaction

Efficient and stable catalysts are crucial for green hydrogen production from acidic water splitting. Herein, using interface engineering, the surface electronic structures of bimetallic PdRu nanocubes have been significantly regulated by Pt atoms (PdRu@Pt). Interestingly, Pt atoms are located on the corners and facets of PdRu nanocubes, resulting in lattice tension and fast electron transfer. As a result, the obtained trimetallic PdRu@Pt nanocubes with an average size of 13 nm have exhibited highly enhanced catalytic activity and stability for hydrogen evolution reaction (HER) in acidic media, delivering 18 mV lower overpotential to reach a current density of 10 mA cm−2, and maintaining a remarkable long-term stability for up to 24 h. In particular, the lower overpotential of 489 mV at 500 mA cm−2 for PdRu0.04@Pt0.12 nanocubes indicates its potential for future industrial application in the future. Such enhancements in HER performance can be attributed to the highly facet-edge-exposed Pt atoms and the significant strain effects resulting from the introducing Pt atomic layers to PdRu surfaces.

Supporting IrOx nanosheets on hollow TiO2 for highly efficient acidic water splitting

Supporting IrOx nanosheets on hollow TiO2 for highly efficient acidic water splitting

Ruthenium-Based Binary Alloy with Oxide Nanosheath for Highly Efficient and Stable Oxygen Evolution Reaction in Acidic Media.

Traditional noble metal oxide, such as RuO2, is considered a benchmark catalyst for acidic oxygen evolution reaction (OER). However, its practical application is limited due to sluggish activity and severe electrochemical corrosion. In this study, Ru-Fe nanoparticles loading on carbon felt (RuFe@CF) is synthesized via an ultrafast Joule heating method as an active and durable OER catalyst in acidic conditions. Remarkably low overpotentials of 188 and 269mV are achieved at 10 and 100mA cm-2, respectively, with a robust stability up to 620 h at 10mA cm-2. When used as an anode in a proton exchange membrane water electrolyzer, the catalyst shows more than 250 h of stability at a water-splitting current of 200mA cm-2. Experimental characterizations reveal the presence of a Ru-based oxide nanosheath on the surface of the catalyst during OER tests, suggesting a surface reconstruction process that enhances the intrinsic activity and inhibits continuous metal dissolution. Moreover, density functional theory calculations demonstrate that the introduction of Fe into the RuFe@CF catalyst reduces the energy barrier and boosts its activities. This work offers an effective and universal strategy for the development of highly efficient and stable catalysts for acidic water splitting.

Layered PbO2 electrode decorated with Ni-doped Mn5O8 peach-like microspheres for reinforced acidic oxygen evolution reaction in zinc electrowinning

Developing anodic electrocatalysts with superior activity and durability suitable for acidic oxygen evolution reaction (OER) remains a challenge to overcome. Herein, a layered Pb-(0.76 wt%)Ag/α-PbO2/β-PbO2 electrode modified with Ni-doped Mn5O8 particles (PA/PbO2-NMO) is successfully synthesized by a coprecipitation coupling with anodic codeposition process. The physical characterization demonstrated that the surface of electrode is composed by β-PbO2 and portion NMO that embedded in the deposit. In addition, it presents a layer structure, in which PA serving as support, α-PbO2 as the mid layer enforcing the adherence, and β-PbO2-NMO function as active layer. Benefiting from the above unique layered structure, the as-fabricated PA/PbO2-NMO electrode with a layered structure exhibits the outstanding acidic OER performance (583 mV, 139.04 mV dec–1) at 50 mA cm−2 in 1.53 M H2SO4. Meanwhile, the responding accelerated corrosion life at 1.5 A cm−2 is up to 64 h, indicating its robust anticorrosion ability. As for the applied performance in zinc electrowinning simulation electrolyte (1.53 M H2SO4 + 0.31 M ZnSO4), the current efficiency of the PA/PbO2 electrode is promoted to 93.8 % after the modification with NMO. Moreover, the cell voltage and one-ton zinc energy consumption of the modified PA/PbO2 electrode are diminished by 14.32 %, 17.75 % compared with the industrial Pb-(0.76 wt%) Ag (PA) electrode. Therefore, this study offers a new horizon on the development of the anodic electrocatalyst for both strongly acidic watersplitting and non-ferrous metals electrowinning.

Highly Crystalline Iridium-Nickel Nanocages with Subnanopores for Acidic Bifunctional Water Splitting Electrolysis.

Developing efficient bifunctional materials is highly desirable for overall proton membrane water splitting. However, the design of iridium materials with high overall acidic water splitting activity and durability, as well as an in-depth understanding of the catalytic mechanism, is challenging. Herein, we successfully developed subnanoporous Ir3Ni ultrathin nanocages with high crystallinity as bifunctional materials for acidic water splitting. The subnanoporous shell enables Ir3Ni NCs optimized exposure of active sites. Importantly, the nickel incorporation contributes to the favorable thermodynamics of the electrocatalysis of the OER after surface reconstruction and optimized hydrogen adsorption free energy in HER electrocatalysis, which induce enhanced intrinsic activity of the acidic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Together, the Ir3Ni nanocages achieve 3.72 A/mgIr(η=350mV) and 4.47 A/mgIr(η=40mV) OER and HER mass activity, which are 18.8 times and 3.3 times higher than that of commercial IrO2 and Pt, respectively. In addition, their highly crystalline identity ensures a robust nanostructure, enabling good catalytic durability during the oxygen evolution reaction after surface oxidation. This work provides a new revenue toward the structural design and insightful understanding of metal alloy catalytic mechanisms for the bifunctional acidic water splitting electrocatalysis.

Sub-10-nm-sized Au@AuxIr1-x metal-core/alloy-shell nanoparticles as highly durable catalysts for acidic water splitting.

The absence of efficient and durable catalysts for oxygen evolution reaction (OER) is the main obstacle to hydrogen production through water splitting in an acidic electrolyte. Here, we report a controllable synthesis method of surface IrOx with changing Au/Ir compositions by constructing a range of sub-10-nm-sized core-shell nanocatalysts composed of an Au core and AuxIr1-x alloy shell. In particular, Au@Au0.43Ir0.57 exhibits 4.5 times higher intrinsic OER activity than that of the commercial Ir/C. Synchrotron X-ray-based spectroscopies, electron microscopy and density functional theory calculations revealed a balanced binding of reaction intermediates with enhanced activity. The water-splitting cell using a load of 0.02mgIr/cm2 of Au@Au0.43Ir0.57 as both anode and cathode can reach 10mA/cm2 at 1.52V and maintain activity for at least 194h, which is better than the cell using the commercial couple Ir/C‖Pt/C (1.63V, 0.2h).

Understanding the catalytic performances of metal-doped Ta2O5 catalysts for acidic oxygen evolution reaction with computations.

The design of stable and active alternative catalysts to iridium oxide for the anodic oxygen evolution reaction (OER) has been a long pursuit in acidic water splitting. Tantalum pentoxide (Ta2O5) has the merit of great acidic stability but poor OER performance, yet strategies to improve its intrinsic OER activity are highly desirable. Herein, by using density functional theory (DFT) calculations combined with aqueous stability assessment from surface Pourbaix diagrams, we systematically evaluated the OER activity and acidic stability of 14 different metal-doped Ta2O5 catalysts. Apart from the experimentally reported Ir-doped Ta2O5, we computationally identified Ru- and Nb-doped Ta2O5 catalysts as another two candidates with reasonably high stability and activity in acidic OER. Our study also underscores the essence of considering stable surface states of catalysts under working conditions before a reasonable activity trend can be computationally achieved.

An N-defective graphitic carbon nitride-supported Ir/Fe catalyst with low Ir-content as highly efficient oxygen evolution reaction catalyst for acidic water splitting

To develop a low-cost acidic water electrolysis catalyst with high efficiency and stability and low Ir content, a catalyst with highly dispersed Ir/Fe complex sites (∼ 5 wt% Ir) was synthesized by a simple carbonization and oxidation process. N-defective g-C3N4 (NCN) was introduced to enhance the support effect between metal nanoparticles and CN bases and the intrinsic catalytic activity for the oxygen evolution reaction (OER). NCN provided more active sites and adjusted the electronic structure of CN. Compared with the original single Ir catalyst, the electrons of the Ir-Fe metal catalyst were transferred from the base to the metal, which maintained the high OER performance and stability of the catalyst. Density functional theory calculations demonstrated that the Gibbs free energy of Ir/Fe@NCN was greatly reduced during O*→OOH* as the rate-determining step. The overpotential of the Ir/Fe@NCN catalyst was 242 mV at 10 mA/cm2, and its mass activity (612.6 A/g) was much higher than that of IrO2 (88.6 A/g), showing excellent OER performance. This study provides a new strategy for the design and optimization of highly efficient and low-cost acidic Ir/transition metal OER catalysts.

Robust Ru-VO2 Bifunctional Catalysts for All-pH Overall Water Splitting.

Designing robust bifunctional catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction in all-pH conditions for overall water splitting (OWS) is an effective way to achieve sustainable development. Herein, a composite Ru-VO2 containing Ru-doped VO2 and Ru nanoparticles (NPs) is synthesized, and it shows a high OWS performance in full-pH range due to their synergist effect. In particular, the OER mass activities of Ru-VO2 at 1.53 V (vs RHE) in acidic, alkaline, and PBS solutions are ≈65, 36, and 235 times of commercial RuO2 in the same conditions. The "Ru-VO2 || Ru-VO2 " two-electrode electrolyzer only needs a voltage of 1.515 V (at 10 mA cm-2 ) in acidic water splitting, which can operate stably for 125 h at 10 mA cm-2 without significant voltage decay. In situ Raman spectra and in situ differential electrochemical mass spectrometry prove that the OER of Ru-VO2 in acid follows the adsorption evolution mechanism. Density functional theory calculations further reveal the synergistic effect between Ru NP and Ru-doped VO2 , which breaks the hydrogen bond network formed by *OH adsorbed on the Ru single-atom site, and thereby significantly enhances the OER activity. This work provides new insights into the design of novel bifunctional pH-universal catalysts for OWS.

Stabilizing ruthenium dioxide with cation-anchored sulfate for durable oxygen evolution in proton-exchange membrane water electrolyzers

Ruthenium dioxide is the most promising alternative to the prevailing but expensive iridium-based catalysts for the oxygen evolution reaction in proton-exchange membrane water electrolyzers. However, the under-coordinated lattice oxygen of ruthenium dioxide is prone to over-oxidation, and oxygen vacancies are formed at high oxidation potentials under acidic corrosive conditions. Consequently, ruthenium atoms adjacent to oxygen vacancies are oxidized into soluble high-valence derivatives, causing the collapse of the ruthenium dioxide crystal structure and leading to its poor stability. Here, we report an oxyanion protection strategy to prevent the formation of oxygen vacancies on the ruthenium dioxide surface by forming coordination-saturated lattice oxygen. Combining density functional theory calculations, electrochemical measurements, and a suite of operando spectroscopies, we showcase that barium-anchored sulfate can greatly impede ruthenium loss and extend the lifetime of ruthenium-based catalysts during acidic oxygen evolution, while maintaining the activity. This work paves a new way for designing stable and active anode catalysts toward acidic water splitting.

Stabilizing non-iridium active sites by non-stoichiometric oxide for acidic water oxidation at high current density

Stabilizing active sites of non-iridium-based oxygen evolution reaction (OER) electrocatalysts is crucial, but remains a big challenge for hydrogen production by acidic water splitting. Here, we report that non-stoichiometric Ti oxides (TiOx) can safeguard the Ru sites through structural-confinement and charge-redistribution, thereby extending the catalyst lifetime in acid by 10 orders of magnitude longer compared to that of the stoichiometric one (Ru/TiO2). By exploiting the redox interaction-engaged strategy, the in situ growth of TiOx on Ti foam and the loading of Ru nanoparticles are realized in one step. The as-synthesized binder-free Ru/TiOx catalyst exhibits low OER overpotentials of 174 and 265 mV at 10 and 500 mA cm−2, respectively. Experimental characterizations and theoretical calculations confirm that TiOx stabilizes the Ru active center, enabling operation at 10 mA cm−2 for over 37 days. This work opens an avenue of using non-stoichiometric compounds as stable and active materials for energy technologies.

Electrochemical Evolution of Ru-Based Polyoxometalates into Si,W-Codoped RuOx for Acidic Overall Water Splitting.

Despite intensive studies over decades, the development of electrocatalysts for acidic water splitting still relies on platinum group metals, especially Pt and Ir, which are scarce, expensive, and poorly sustainable. Because such problems can be alleviated, Ru-based bifunctional catalysts such as rutile RuO2 have recently emerged. However, RuO2 has a relatively low activity for hydrogen evolution reactions (HER) and low stability for oxygen evolution reactions (OER) under acidic conditions. In this study, the synthesis of a RuOx -based bifunctional catalyst (RuSiW) for acidic water splitting via the electrochemical evolution from Ru-based polyoxometalates at cathodic potentials is reported. RuSiW consists of the nanocrystalline RuO2 core and Si,W-codoped RuOx shell. RuSiW exhibits outstanding HER and OER activity comparable to Pt/C and RuO2 , respectively, with high stability. Computational analysis suggests that the codoping of RuOx with W and Si synergistically improves the HER activity of otherwise poor RuO2 by shifting the d-band center and optimizing atomic configurations beneficial for proper hydrogen adsorption. This study provides insights into the design and synthesis of unprecedented bifunctional electrocatalysts using catalytically inactive and less explored elements, such as Si and W.

Nanostructured Pt@RuOx catalyst for boosting overall acidic seawater splitting

In the domain of acidic water splitting, designing bifunctional catalysts that marry high activity with enduring stability is a formidable challenge. Herein, we have constructed platinum-containing ruthenium oxide nanoparticles (Pt@RuOx NPs) to achieve excellent overall water splitting performance in acidic electrolytes. Pt@RuOx NPs demonstrate exceptional catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic seawater, requiring overpotentials of only 20 and 157 mV, respectively to achieve a current density of 10 mA cm−2. Furthermore, in 0.5 M H2SO4, this catalyst exhibits high HER (19 mV) and OER (236 mV) catalytic activities. The two-electrode water splitting system composed of bifunctional Pt@RuOx NPs requires cell voltages of only 1.442 and 1.465 V to deliver a current density of 10 mA cm−2 in acidic seawater and 0.5 M H2SO4. Remarkably, this catalyst displays remarkable stability in the water splitting process. It is shown that the introduction of Pt could augment oxygen vacancies and enhance catalytic activity significantly. The synergy between Pt and Ru further contributes to the improved performance. Additionally, we have observed that acidic seawater holds distinct advantages for acidic water splitting, along with a thorough exploration of the relationship between Cl− concentration and catalytic performance. This study not only provides a strategy to improve the catalytic activity and stability of ruthenium-based catalysts for water splitting in acidic environments, but also unveils the promoting effect of Cl− on the catalytic activity in acidic water splitting.

Hydrazine-Assisted Acidic Water Splitting Driven by Iridium Single Atoms.

Water splitting, an efficient technology to produce purified hydrogen, normally requires high cell voltage (>1.5V), which restricts the application of single atoms electrocatalyst in water oxidation due to the inferior stability, especially in acidic environment. Substitution of anodic oxygen evolution reaction (OER) with hydrazine oxidation reaction (HzOR) effectually reduces the overall voltage. In this work, the utilization of iridium single atom (Ir-SA/NC) as robust hydrogen evolution reaction (HER) and HzOR electrocatalyst in 0.5m H2 SO4 electrolyte is reported. Mass activity of Ir-SA/NC is as high as 37.02 A mgIr -1 at overpotential of 50mV in HER catalysis, boosted by 127-time than Pt/C. Besides, Ir-SA/NC requires only 0.39V versus RHE to attain 10mA cm-2 in HzOR catalysis, dramatically lower than OER (1.5V versus RHE); importantly, a superior stability is achieved in HzOR. Moreover, the mass activity at 0.5V versus RHE is enhanced by 83-fold than Pt/C. The in situ Raman spectroscopy investigation suggests the HzOR pathway follows *N2 H4 →*2NH2 →*2NH→2N→*N2 →N2 for Ir-SA/NC. The hydrazine assisted water splitting demands only 0.39V to drive, 1.25V lower than acidic water splitting.

Misoriented high-entropy iridium ruthenium oxide for acidic water splitting.

Designing an efficient catalyst for acidic oxygen evolution reaction (OER) is of critical importance in manipulating proton exchange membrane water electrolyzer (PEMWE) for hydrogen production. Here, we report a fast, nonequilibrium strategy to synthesize quinary high-entropy ruthenium iridium-based oxide (M-RuIrFeCoNiO2) with abundant grain boundaries (GB), which exhibits a low overpotential of 189 millivolts at 10 milliamperes per square centimeter for OER in 0.5 M H2SO4. Microstructural analyses, density functional calculations, and isotope-labeled differential electrochemical mass spectroscopy measurements collectively reveal that the integration of foreign metal elements and GB is responsible for the enhancement of activity and stability of RuO2 toward OER. A PEMWE using M-RuIrFeCoNiO2 catalyst can steadily operate at a large current density of 1 ampere per square centimeter for over 500 hours. This work demonstrates a pathway to design high-performance OER electrocatalysts by integrating the advantages of various components and GB, which breaks the limits of thermodynamic solubility for different metal elements.

Cobalt Nanocube Electrocatalysts Comprising Sub-Nanometer Oxide-Phosphide Hetero-Interface for Highly Efficient Water Electrolysis

Hydrogen production from electrochemical water splitting is known as an eco-friendly energy conversion process, which has drawn attention for a few decades. Typically, the water electrolysis is divided into PEM and AEM electrolysis where acidic and alkaline electrolyte are used, respectively. In case of PEM water electrolysis, the acidic water splitting reveals critical drawback of poor counterpart OER efficiency. For this reason, AEM water electrolysis in alkaline condition has huge advantages in high efficiency of full cell owing to the lower overpotential for oxygen evolving anode, and accessibility of using transition metal-based materials for each anode and cathode. However, alkaline HER is 2-3 orders of magnitude slower than acidic HER due to the lack of proton nearby the surface.Thus, it is necessary to find highly efficient alkaline HER electrocatalysts for the future commercialization of AEM electrolyzer.Herein, by phosphidation of 10 nm Co3O4 nanocube (CO) loaded on nickel foam (NF) substrate, unique heterojunction interface of CoP and Co3O4 within 10 nanometers were realized, which converged both the excellent electrocatalytic performance of CoP and high stability of Co3O4. Unique sub-nanometer oxide-phosphide heterojunction was discovered via inverse fast fourier transform (IFFT), which confirmed the phase transition alongside the particle, while maintaining its nanocube morphology. X-ray photoelectron spectroscopy (XPS) was brought out to further confirm the co-exsistence of cobalt oxide and phosphide materials with respect to the chemical structure.Experimental investigations revealed the excellent water splitting activity of the hybrid-phosphidated Co3O4 (H-PCO) catalyst, only requiring an overpotential of 162, 171 and 395 mV to reach 100 mA cm-2, each for acidic HER, alkaline HER, and alkaline OER, respectively. H-PCO/PNF showed excellent stability over 20 hours under various working conditions, which was superior to that of conventionally fabricated CoP catalysts. Such outstanding alkaline water electrolysis performance and stability stems from the evolution of new active sites are expected at the interface because the interface has different chemical and electronic structures compared to bulk materials. In addition, the scalability of the electrocatalyst was demonstrated where a 34.56 cm-2 catalyst foam was uniformly fabricated As a result, all 15 points of as-prepared H-PCO electrode showed negligible distribution of the overpotentials at 100 mA cm-2, and the large unit-cell scale measurement showed remarkably improved HER performance compared to bare Ni foam. Applicability of the electrocatalyst was demonstrated by the construction of unbiased PV-EC water splitting system, where a high STH efficiency of 11.5% over 100 hours was achieved with the help of Si solar cells.In conclusion, we developed water splitting electrode containing hybrid interface of cobalt oxide and phosphide in sub-10 nm cube structure. The hybrid structure of H-PCO comprising sub-nanometer hetero-interface between cobalt oxide and phosphide evoluted synergetic effect that greatly improved either HER performance, pH indenpendency, or long-term stability, which outperformed single CO and CoP electrodes. We believe our rationally designed hetero-interfacial electrocatalyst can outperform previously established electrocatalysts via its high efficiency, versatility, and scalability.