Thermochemical Water Research Articles

(Invited) Hydrogen Consortium: Advancements in Renewable Hydrogen Production

HydroGEN Energy Materials Network (EMN) is an U.S. Department of Energy (DOE) EERE Hydrogen and Fuel Cell Technologies Office (HFTO)-funded consortium that aims to accelerate the discovery and development of advanced water splitting materials (AWSM) for clean, low-cost hydrogen production. Materials innovations are key to enhancing performance, durability, and cost of hydrogen generation technologies.HydroGEN is focused on low technology readiness level AWS technologies, including low- (alkaline exchanged membrane electrolysis) and high-temperature electrolysis (proton-conducting solid oxide electrolysis), photoelectrochecmical (PEC) and solar thermochemical (STCH) water splitting. This presentation will provide an overview of the HydroGEN EMN and technical highlights of a few lab-led and FOA-awarded R&D projects. HydroGEN continues to grow its community of industry, university, and national laboratories, forming a national innovation ecosystem focused on renewable hydrogen production.

Breakthroughs in Low-Cost Hydrogen Production Methods for Fuel Cells

Traditional hydrogen production, like steam methane reforming (SMR), is limited by high greenhouse gas emissions and fossil fuel dependency. However, integrating renewable energy with innovative production technologies offers a more sustainable and cost-effective pathway. Electrolysis, powered by renewable sources such as wind and solar, is a promising method that has seen efficiency improvements through advancements in proton exchange membrane (PEM) and anion exchange membrane (AEM) technologies. Additionally, research into thermochemical water splitting, which utilizes high-temperature heat from solar or nuclear energy, presents a scalable alternative for industrial hydrogen production. Biological hydrogen production using microorganisms, such as algae and bacteria, offers further innovation by leveraging natural processes for hydrogen generation. The integration of renewable energy into hydrogen production not only lowers costs but also provides an effective means of energy storage, contributing to grid stability and reducing reliance on fossil fuels. This paper explores recent breakthroughs in low-cost hydrogen production methods, essential for advancing the hydrogen economy and promoting fuel cells as a clean energy solution.

Enhanced conversion of a magnetic oxide to metal using induction heating: Application for H2 production

In this study, a C–CoFe2O4 composite is proposed for reduction in an induction heating reactor with a continuous argon flow. An impressive conversion efficiency of 87% is achieved at a maximum temperature of 325 °C in just 8 min without the need for vacuum. The impact of the molar ratio of metal oxide to reducing agent (acetylacetone) is also examined. Results indicate that a 1:1 ratio favours the reduction towards cobalt and iron monoxides, while a 1:3 ratio yields a 31% metal phase. The highest conversion (87%) is seen with a 1:17 ratio, indicating the need for excess reducing agent for effective conversion of CoFe2O4 oxide. The molar ratio influences the induction heating temperature, with the 1:17 ratio having a lower temperature of 325 °C compared to 450 °C for other ratios. These operating temperatures (≤450 °C) are significantly lower than those reported in the literature for carbothermal reduction in an induction reactor (>1700 °C), which often requires vacuum and very high temperatures for high conversion efficiency. The proposed process offers advantages such as high conversion efficiency, fast processing, and simpler operational conditions. Additionally, the metallic phases produced are utilized for H2 production in thermochemical water splitting, showcasing these materials potential for energy generation applications.

Thermal management of sodium-oxygen-hydrogen (Na-O-H) thermochemical water splitting for sustainable hydrogen production

Owing to environmental problems caused by the consumption of fossil fuels and the growing global demand for energy, interests in usage of hydrogen as a green energy carrier has grown. In particular, water is considered a prospect source for green hydrogen production. In this paper, a three-step sodium-oxygen-hydrogen (Na–O–H) thermochemical cycle, which is a newly developed cycle that can operate between 400 and 500 °C is implemented. Also, low number of steps, which results in less system complexity and easier system design is another advantage of the Na–O–H thermochemical cycle. In the first step of this study, a basic model of the three-step pure Na–O–H cycle is modeled in order to produce 1 kg/s of hydrogen. Moreover, output data from basic modeling is used for cycle energy management. Then, heat recovery is performed within the cycle streams to reduce the external energy demand and make the Na–O–H cycle more feasible. To achieve this objective, pinch method, which is an effective method for designing and optimizing plants is used. Several heat exchanger network designs have been developed and compared from different aspects. Finally, a design with the least heat requirement among others is selected and optimized based on total annualized cost objective function to introduce the optimum final design exchanger network. As a result of this work, it is found that using the final optimized design, external heating loads accounted for just 5.8% of the total load, while external cooling loads accounted for 19.3% of the total load. This implies that the Na–O–H thermochemical cycle recovered 74.7% of the energy required for heating and cooling of the streams. The implementation of the proposed heat exchanger network design boosts the overall energy efficiency of the Na–O–H thermochemical cycle. Using final heat recover scheme the overall efficiency of the cycle increased from 45.36% to 52.86%, representing an impressive 7.50% improvement. This significant achievement underscores the pivotal role of this study in developing an effective and optimized model for the Na–O–H thermochemical cycle, which holds great promise for future applications.

Modeling Development of a Receiver–Reactor of Type R2Mx for Thermochemical Water Splitting

Abstract This work reports on the development of a transient heat transfer model for a prototype reactor of type R2Mx for thermochemical water splitting by temperature and pressure swing of ceria. Key aspects of the R2Mx concept, which are also incorporated in the prototype design, include a movable monolithic redox structure combined with a linear transport system, a reduction reactor, as well as a dedicated oxidation reactor. With the model, the operation of the prototype is simulated for consecutive water splitting cycles, in which ceria is reduced in a continuously heated reactor, oxidized in a separate oxidation reactor, and transported in between the reaction zones. A 2D axisymmetric numerical model of the prototype reactor was developed in ansys mechanical. The model includes heat transfer calculations in combination with an approximated simulation of the transport of the redox material during cyclic operation. It incorporates the chemical reaction by means of a modified heat capacity for ceria and accounts for internal radiation heat transfer inside the porous redox material by applying effective heat transfer properties. A parametric analysis has been undertaken to evaluate different modes of operation of the oxidation reactor. Model results are used to size the power demand of the reduction reactor and vacuum pump, to define durations of the process steps, as well as to assess operational parameters with respect to achieved temperatures. Findings suggest that suitable operation of the prototype reactor involves reduction durations ranging from 8 to 10 min and oxidations of 6 to 10 min.

Development of a hydrogen permselective silica membrane and its application to the thermochemical water splitting method

Abstract The thermochemical water splitting IS process is one of the hydrogen production method to use a heat directly. The temperature of the thermal decomposition of water (4000 K) can be reduced under 700 K by introducing I2 and SO2 as recycling catalysts. One of the problems in the IS process is that the conversion of the HI decomposition reaction is low at about 20%. If a membrane reactor with the H2 permselective membrane is applied to the HI decomposition reaction, the hydrogen can be extracted to improve the HI conversion. We focus on a counter diffusion CVD method for the preparation method of the membranes. Two reactants (e.g. silica precursor and oxidant) are provided at the opposite side of the porous substrates and hybrid silica is deposited inside the pore of the substrate. The pore sizes are controlled by introducing organic functional groups to silica precursor. In this study, the silica hybrid membrane with high H2 permeation performance and high H2/HI selectivity were developed by introducing organic functional groups. Effects of the organic functional groups were summarized by using a pore model. The HI gas and other inorganic gases permeation performance were tested through silica hybrid membranes.

Thermodynamic properties of hercynite (FeAl2O4) for thermochemical water splitting applications: A first-principles approach

FeAl2O4 is considered as a potential material to produce solar thermochemical hydrogen. However, its temperature and pressure dependant thermodynamic properties have not yet been well explored. In this work, first-principles calculations have been applied to investigate the structural, mechanical, electronic and thermodynamic properties of FeAl2O4. The temperature and pressure-dependent thermal expansion coefficient and Grüneisen parameter have been calculated, leading to the calculation of molar heat capacity, cp = 5.50907 + 0.39378 T-202345.44153 T-2-94039T2 at 0 GPa. Change in Gibb's energy can be calculated at a given temperature and pressure during the reduction and oxidation cycle to computationally assess the suitability of FeAl2O4, as a catalyst for thermochemical water splitting. This approach may be further extended using suitable substitution and subsequent compositional variation of Fe and Al atoms, to explore more efficient catalyst.

Disodium terephthalate as a multifunctional additive for simultaneous enhancing the energy release performance and weakening the water reactivity of aluminum

In this study, disodium terephthalate (Na2TP) has been successfully introduced as a novel multifunctional additive for simultaneous enhancing the energy release performance and weakening the water reactivity of micro-aluminum (μ-Al). Seven μ-Al/Na2TP composites with different contents of Na2TP (R-100 to R-700) and four comparison samples have been prepared to investigate the thermochemical behaviors, combustion performances and water reactivity. Importantly, structure characterizations as well as kinetic parameters (e.g., Ea, ΔS‡) have been performed to reveal the impacts of Na2TP on μ-Al oxidation. Moreover, incorporating Na2TP into μ-Al shows the enhancement of energy release, with R-400 exhibiting the highest heat release (4970 J/g) in the main oxidation stage, compared to μ-Al (1696 J/g). Interestingly, μ-Al shows an increased Ea while R-400 exhibits a decreased Ea as the oxidation proceeds. The decomposition of Na2TP generates both heat and CO2 gases, which are beneficial for destroying alumina shell and increasing oxidation kinetics, in consistent with higher positive ΔS‡ value, rougher surface, and higher surface O content for oxidation of R-400 compared to μ-Al. As a result, the enhanced combustion performances of higher peak pressure, pressurization rate and characteristic shock velocity as well as brighter/larger flame images have been achieved by μ-Al/Na2TP composites. On the other hand, water reactivity of μ-Al has been significantly suppressed by introducing Na2TP in terms of the 85.4 % decrease in heat release, elevated Ea, and alteration of reaction mechanism. Consequently, taking organic Na2TP as a multifunctional additive, two birds of enhancing the energy release and weakening the water reactivity is harvested, showing great potentials in improving the performances of metal-based energy systems.

Factors affecting the economy of green hydrogen production pathways for sustainable development and their challenges.

In a hydrogen economy, the primary energy source for industry, transportation, and power production is hydrogen gas. Green hydrogen can be generated and utilized in an environmentally friendly and sustainable manner; it seeks to displace fossil fuels. Finding a clean alternative energy source is becoming more crucial due to the depletion of fossil fuels and the major environmental pollution issues they bring when utilized extensively. The paper's objective is to analyze the factors affecting the economy of green hydrogen production pathways for sustainable development to decarbonize the world and the associated challenges faced in terms of technological, social, infrastructure, and people's perceptions while adopting green hydrogen. To achieve this, the research looked at a variety of areas relevant to green hydrogen, such as production techniques, industry applications, benefits for society and the environment, and challenges that need to be overcome before the technology is widely used. The most recent methods of producing hydrogen from fossil fuels, such as steam methane, partial oxidation, autothermal, and plasma reforming, as well as renewable energy sources including biomass and thermochemical reactions and water splitting. Grey hydrogen is now the least expensive type of hydrogen, but, in the future, green hydrogen's levelized cost of hydrogen (LCOH) is expected to be less than $2 per kilogram of hydrogen.

Reticulated porous structures of La0.8Al0.2NiO3-δ perovskite for enhanced green hydrogen production by thermochemical water splitting

The preparation and optimisation of La0.8Al0.2NiO3-δ (LANi82) perovskite shaped as reticulated porous ceramic (RPC) structures for H2 production by thermochemical water splitting is presented for the first time. The perovskite was first synthesised in powder form following a modified Pechini method. The redox properties of the LANi82 were first tested under N2/air flow in a thermogravimetric analyser. After that, the sponge replica method for preparing RPCs was optimised in terms of slurry composition and final thermal treatment to obtain a LANi82-RPC structure with porosity and strength appropriate to enhance heat and mass transfer in further solar reactors. The optimised LANi82-RPC material showed an outstanding hydrogen production of 8.3 cm3 STP/gmaterial·cycle at isothermal conditions (800 °C). This production was increased up to 11.5 cm3 STP/gmaterial·cycle if the thermal reduction was performed at 1000 °C. Additionally, a stable activity with almost constant H2 production in consecutive cycles was obtained for the optimised LANi82-RPC in both cases. The structure of the reticulated porous materials, with open macroporosity and wide interconnected channels, enhances heat and mass transfer, leading to higher hydrogen productions of the LANi82-RPC as compared to the materials as powder form in the same experimental set-up. These facts reinforce the favourable prospects of LANi82-RPC for large-scale hydrogen production, improving the coupling to current solar thermal concentration technologies developed, such as concentrated solar power tower.

Cu infiltrated Ni-YSZ cathode in CO2 (+H2) stream: Reverse water gas shift vs. CO2 electrolysis

Ni-Yttria Stabilized Zirconia (Ni-YSZ) cermet electrode is known to perform in CO2 (+H2) stream. Introducing H2 in CO2 containing streams enables thermochemical reverse water gas shift reaction (rWGS: CO2 + H2 → CO + H2O) at open circuit. Without the application of any bias, the rWGS responds positively to an increase in temperature and concentrations of CO2 and H2. Application of bias results in enhancement in CO yield above the rWGS baseline value. With bias, both CO2 and H2O electrolysis are enabled. The infiltration of Cu on the Ni-YSZ backbone results in significant improvement of the reaction kinetics and increases H2 and CO production. Impedance analysis indicates that the kinetic limitation originates from reaction steps with slower time constants with Ni{Cu}x-YSZ outperforming Ni-YSZ in this aspect. Cu infiltration suppresses particle coarsening typically observed in Ni-YSZ.

Improving green hydrogen production: Microwave-assisted synthesis of diverse Ni-based nanocatalysts on Al2O3 for thermochemical water splitting

This study investigated the role of microwave-assisted synthesis in enhancing green hydrogen (H2) production via thermochemical water splitting (TWS) using Ni-based nanocatalysts on an Al2O3 support. H2-TPR analysis showed that the microwave-assisted synthesized (MW) catalysts exhibited stronger peaks at lower temperatures, indicating improved Ni dispersion on the Al2O3 support. Moreover, higher H2-uptake values were observed with increasing Ni concentration for both synthesis methods, indicating the presence of more accessible reducible sites crucial for catalytic activity. The MW-prepared catalysts displayed smaller particle sizes, narrower pore size distributions, and higher hydrogen adsorption capacities than those synthesized via impregnation. Additionally, they demonstrated superior catalytic activity and efficiency for hydrogen generation, with yields (∼55.42 %). Overall, our research underscores the potential of microwave-assisted synthesis as a promising method to develop efficient catalysts for hydrogen production applications.

Advances in Functional Ceramics for Water Splitting: A Comprehensive Review

The global demand for sustainable energy sources has led to extensive research regarding (green) hydrogen production technologies, with water splitting emerging as a promising avenue. In the near future the calculated hydrogen demand is expected to be 2.3 Gt per year. For green hydrogen production, 1.5 ppm of Earth’s freshwater, or 30 ppb of saltwater, is required each year, which is less than that currently consumed by fossil fuel-based energy. Functional ceramics, known for their stability and tunable properties, have garnered attention in the field of water splitting. This review provides an in-depth analysis of recent advancements in functional ceramics for water splitting, addressing key mechanisms, challenges, and prospects. Theoretical aspects, including electronic structure and crystallography, are explored to understand the catalytic behavior of these materials. Hematite photoanodes, vital for solar-driven water splitting, are discussed alongside strategies to enhance their performance, such as heterojunction structures and cocatalyst integration. Compositionally complex perovskite oxides and high-entropy alloys/ceramics are investigated for their potential for use in solar thermochemical water splitting, highlighting innovative approaches and challenges. Further exploration encompasses inorganic materials like metal oxides, molybdates, and rare earth compounds, revealing their catalytic activity and potential for water-splitting applications. Despite progress, challenges persist, indicating the need for continued research in the fields of material design and synthesis to advance sustainable hydrogen production.

Insights into the effect of operating parameters and hydrodynamics on hydrolysis reaction in Copper–Chlorine thermochemical water splitting cycle for hydrogen production: Modelling and experimental validation

Copper–Chlorine (Cu–Cl) thermochemical cycle is being explored as one of the promising technologies for large scale production of clean hydrogen from water using nuclear or renewable energy. In this method, water splitting is achieved through a series of reactions in a loop producing only hydrogen and oxygen with complete recycle of all the intermediate copper and chlorine compounds. The first step of the cycle is the hydrolysis reaction wherein steam reacts with solid CuCl2 to form solid Cu2OCl2 (copper oxychloride) and HCl gas. The products enter the subsequent steps of the cycle and are recycled back to complete the loop. Multiple side reactions in the hydrolysis step result in reduced yield and loss of chemicals from the cycle. Efficient reactor design and optimal operating conditions of hydrolysis reactor is imperative towards achieving sustained closed loop operation and reducing the energy demand of Cu–Cl cycle. This paper presents a mathematical model for the simulation of hydrolysis reaction in a bubbling fluidized bed reactor. The one-dimensional model is developed based on modified two-phase theory of fluidization and is coupled with kinetics of multiple side reactions to investigate the effect of operating parameters and hydrodynamics on the conversion and yield of Cu2OCl2. The model results are validated with experimental data in 50 mm and 100 mm semi-batch fluidized bed reactors. The developed model clearly brings out the effect of different process parameters for process optimization and scale-up of the hydrolysis reactor for efficient integration in the cycle.

Towards chemical equilibrium in thermochemical water splitting. Part 2: Re-oxidation

Chemical equilibrium represents the highest efficiency achievable by a thermochemical cycle under specific operational conditions. This study delves into two-step thermochemical water splitting cycles, which dissociate water into hydrogen (H2) and oxygen (O2) via sequential thermal reduction and re-oxidation processes. Building on the thermal reduction analysis presented in Part 1, this paper zeroes in on the re-oxidation reaction within the optimal reactor framework – the counter-current reactor. It elucidates the equilibrium pathway, delineating the progression of chemical equilibrium at each incremental stage of re-oxidation. Using ceria (CeO2) as a model redox-active metal oxide, the investigation analyzes the influence of operational parameters on the re-oxidation reaction. Findings reveal the theoretical feasibility of maintaining near constant temperature (±2.5 °C) during re-oxidation, achieving a total extent of reduction of 0.038 and a hydrogen conversion yield of 32%. While near adiabatic operation is also achievable, practicality is constrained by a maximum total extent of reduction of 6.5·10−3. The study also explores the economic implications of thermochemical water splitting, particularly focusing on the steam-to-hydrogen output ratio. It highlights the severe economic hurdles associated with hydrogen yields below approximately 1%. Furthermore, the investigation reveals that the addition of inert gas to the re-oxidation step offers no significant advantages. Concluding the analysis, a comparative assessment exposes negligible differences in outcomes when substituting carbon dioxide (CO2) for water (H2O) as the oxidant in low-temperature scenarios (800 °C). This comprehensive study not only advances the understanding of re-oxidation dynamics in thermochemical water splitting but also informs practical and economic considerations crucial for the advancement of this technology.

Comprehensive assessment of green hydrogen potential in Jordan: economic, environmental and social perspectives

PurposeThis paper aims to assess the economic, environmental, policy-related and social implications of establishing green hydrogen production in Jordan.Design/methodology/approachThe comprehensive analysis has been investigated, including economic assessments, environmental impact evaluations, policy examinations and social considerations. Furthermore, the research methodology encompasses energy demand, sector, security and supply analysis, as well as an assessment of the availability of renewable energy resources.FindingsThe results indicate substantial economic benefits associated with green hydrogen production, including job creation, increased tax revenue and a reduction in energy imports. Additionally, the study identifies positive environmental impacts, such as decreased greenhouse gas emissions and air pollution. Noteworthy, two methods could be used to produce hydrogen, namely: electrolysis and thermochemical water splitting. As a recommendation, the study proposes that Jordan, particularly Aqaba, take proactive measures to foster the development of a green hydrogen industry and collaborate with international partners to exchange best practices and establish the necessary infrastructure.Originality/valueTo the best of the authors’ knowledge, this paper is among the first to provide a comprehensive perspective on the potential of green hydrogen production as a driving force for Jordan’s economy, while also benefiting the environment and society. However, the research recognizes several challenges that must be addressed to materialize green hydrogen production in Jordan, encompassing high renewable energy costs, infrastructure development requirements and community concerns. Despite these obstacles, the study asserts that the potential advantages of green hydrogen production outweigh the associated risks.

Chemical Potential Analysis as an Alternative to the van't Hoff Method: Hypothetical Limits of Solar Thermochemical Hydrogen.

The van't Hoff method is a standard approach for determining reaction enthalpies and entropies, e.g., in the thermochemical reduction of oxides, which is an important process for solar thermochemical fuels and numerous other applications. However, by analyzing the oxygen partial pressure pO2, e.g., as measured by thermogravimetric analysis (TGA), this method convolutes the properties of the probe gas with the solid-state properties of the examined oxides, which define their suitability for specific applications. The "chemical potential method" is here proposed as an alternative. Using the oxygen chemical potential ΔμO instead of pO2 for the analysis, this method does not only decouple gas-phase and solid-state contributions but also affords a simple and transparent approach to extracting the temperature dependence of the reduction enthalpy and entropy, which carries important information about the defect mechanism. For demonstration of the approach, this work considers three model systems; (1) a generic oxide with noninteracting, charge-neutral oxygen vacancy defects, (2) Sr0.86Ce0.14MnO3(1-δ) alloys with interacting vacancies, and (3) a model for charged vacancy formation in CeO2, which reproduces the extensive experimental TGA data available in the literature. The reduction behavior of these model systems obtained from the chemical potential method is correlated with simulated results for the thermochemical water splitting cycle, highlighting the exceptional behavior of CeO2, which originates from defect ionization. The theoretical performance limits for solar thermochemical hydrogen within the charged defect mechanism are assessed by considering hypothetical materials described by a variation of the CeO2 model parameters within a plausible range.

Investigation on the activity of Ni doped Ce0.8Zr0.2O2 for solar thermochemical water splitting combined with partial oxidation of methane

The incredible potential of metal oxide-based two-step solar-driven thermochemical water splitting technology lies in its ability to harness limitless solar energy to produce clean and renewable hydrogen fuel. However, achieving high redox kinetics and lowering the reaction temperature remain the primary challenges in hydrogen production. The CeO2–ZrO2 system is a well-known oxygen carrier for thermochemical water splitting on account of its high temperature stability, quick kinetics, and redox cyclability. While recent studies have explored the combination of methane partial oxidation with metal oxide reduction, the reduction effect of cerium-zirconium oxide is relatively low, and reactivity still needs improvement. The nickel-doped system exhibits good redox performance and is regenerative. In this study, different mass percentages nickel-loaded oxygen carriers mNi/Ce0.8Zr0.2O2 (m = 0, 1, 3, 5, 7, 9) were prepared using the impregnation method. Redox properties of oxygen carriers were investigated in a fixed bed reactor and various characterizations were performed. To assess the durability of mNi/Ce0.8Zr0.2O2, 60 long-cycle experiments were also conducted. The results indicate that the lattice oxygen in mNi/Ce0.8Zr0.2O2 exhibits remarkable selectivity for the partial oxidation of CH4 at temperatures ranging from 880 to 925 °C. The best reactivity performance was achieved with a nickel loading ratio of 5 wt%, and after 60 consecutive redox cycles, 5Ni/Ce0.8Zr0.2O2 demonstrates remarkable robustness, maintaining its redox activity.

An integrated alkali metal thermoelectric converter with sodium hydroxide thermoelectric water splitter and organic Rankine cycle for efficient power and hydrogen production

This study proposes the integration of the NaOH thermochemical water splitting cycle with both an alkali metal thermoelectric converter and an organic Rankine cycle. The objective of the proposed model is to enhance the overall efficiency by utilizing the waste heat generated by the NaOH thermochemical water splitting process and redirecting the heat from the alkali metal thermoelectric converter using bypass when hydrogen demand is low.Performance parameters such as system efficiency, system power, hydrogen production, AMTEC power production, and ORC power production were examined in the study. The results indicated an increase in power and hydrogen production with higher hot-end temperatures, along with an overall reduction in system efficiency for low values of load resistance. The system achieved a maximum efficiency of 53.37%.The system efficiency reached its peak at an AMTEC hot end temperature of 1173 °C, achieving 53.37 % efficiency at an RL value of 1.01 O. When the system operates at its maximum AMTEC power production, the AMTEC modules supply 12.81 kW, while the ORC generates 5.57 kW, and the hydrogen compressor consumes 0.13 kW. The total electrical power generation is 18.38 kW. The findings also indicate that the utilization of the bypass led to a reduction in hydrogen production while increasing the power output of the organic Rankine cycle.

Bulk‐Heterojunction Electrocatalysts in Confined Geometry Boosting Stable, Acid/Alkaline‐Universal Water Electrolysis

AbstractAlkaline water splitting electrocatalysts have been studied for decades; however, many difficulties remain for commercialization, such as sluggish hydrogen evolution reaction (HER) kinetics and poor catalytic stability. Herein, by mimicking the bulk‐heterojunction morphology of conventional organic solar cells, a uniform 10 nm scale nanocube is reported that consists of subnanometer‐scale heterointerfaces between transition metal phosphides and oxides, which serves as an alkaline water splitting electrocatalyst; showing great performance and stability toward HER and oxygen evolution reaction (OER). Interestingly, the nanocube electrocatalyst reveals acid/alkaline independency from the synergistic effect of electrochemical HER (cobalt phosphide) and thermochemical water dissociation (cobalt oxide). From the spray coating process, nanocube electrocatalyst spreads uniformly on large scale (≈6.6 × 5.6 cm2) and is applied to alkaline water electrolyzers, stably delivering 600 mA cm−2 current for >100 h. The photovoltaic‐electrochemical (PV‐EC) system, including silicon PV cells, achieves 11.5% solar‐to‐hydrogen (STH) efficiency stably for >100 h.