Ten-percent solar-to-fuel conversion with nonprecious materials

Dual functionality of the BiN monolayer: unraveling its photocatalytic and piezocatalytic water splitting properties

To achieve scalable and economically viable green hydrogen (H2) production, the photocatalytic and piezocatalytic processes are promising methods. The key to successful overall water splitting (OWS) for H2 production in these processes is using suitable semiconductor catalysts with appropriate band edge potentials, efficient optical absorption, higher mechanical flexibility, and piezoelectric coefficients. Thus, we explore the bismuth nitride (BiN) monolayer using density functional theory simulations, revealing intriguing catalytic properties. The BiN monolayer is a semiconductor with an indirect electronic bandgap (Eg) of 2.08 eV and displays excellent visible light absorption (approximately 105 cm-1). Detailed analyses show that the band edges satisfy the redox potential for photocatalytic OWS via biaxial strain engineering and pH variation. Notably, the solar to hydrogen conversion efficiency (ηSTH) for the BiN monolayer can reach 17.18%, which exceeds the 10% efficiency limit of photocatalysts for economical green H2 production. The obtained in-plane piezoelectric coefficient of e11 = 16.18 Å C m-1 is superior to widely studied 2D materials. Moreover, the generated piezopotential under oscillatory strain stands at 28.34 V, which can initiate the water redox reaction via the piezocatalytic mechanism. This originates from the mechanical flexibility coupled with higher piezoelectric coefficients. The result highlights the BiN monolayer's potential application in photocatalytic, piezocatalytic, and photo-piezo-catalytic OWS.

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Janus Zn-IV-VI: Robust Photocatalysts with Enhanced Built-In Electric Fields and Strain-Regulation Capability for Water Splitting

The use of 2D materials to produce hydrogen (H2 ) fuel via photocatalytic water splitting has been intensively studied. However, the simultaneous fulfillment of the three essential requirements-high photon utilization, rapid carrier transfer, and low-barrier redox reactions-for wide-pH-range production of H2 still poses a significant challenge with no additional modulation. By employing the first-principles calculations, it has been observed that the Janus ZnXY2 structures (X = Si/Ge/Sn, Y = S/Se/Te) exhibit significantly enhanced built-in electric fields (0.20-0.36 eV Å-1 ), which address the limitations intrinsically. Compared to conventional Janus membranes, the ductile ZnSnSe2 and ZnSnTe2 monolayers have stronger regulation of electric fields, resulting in improved electron mobility and excitonic nature (Ebinding = 0.50/0.35 eV). Both monolayers exhibit lower energy barriers of hydrogen evolution reaction (HER, 0.98/0.86 eV, pH = 7) and resistance to photocorrosion across pH 0-7. Furthermore, the 1% tensile strain can further boost visible light utilization and intermediate absorption. The optimal AC-type bilayer stacking configuration is conducive to enhancing electric fields for photocatalysis. Overall, Janus ZnXY2 membranes overcome the major challenges faced by conventional 2D photocatalysts via intrinsic polarization and external amelioration, enabling efficient and controllable photocatalysis without the need for doping or heterojunctions.

First-principles studies on the electronic and photocatalytic water splitting properties of surface functionalized Y2C-based MXenes

Recently, MXenes, an emerging family of two-dimensional (2D) materials, have attracted increasing interest for photocatalytic water splitting due to their various excellent physical and chemical properties, such as large specific surface area, good hydrophilicity, and remarkable light absorption ability. However, the photocatalysts of MXenes with symmetric structures are limited by rapid recombination of photo-generated carriers and the prerequisite of a large band gap no less than 1.23 eV. Differently, Janus MXenes with different surface functional groups facilitate the separation of photo-generated electrons and holes with the help of the intrinsic electric field. And, at the same time, there is no prerequisite for the band gap of Janus MXene photocatalysts as long as they possess appropriate band edge positions. Here, we explored the structural, electronic and photocatalytic water splitting properties of symmetric Y2CT2 and Janus Y2CTT' MXenes (T, T' = H, F, Cl, OH) using the density functional theory (DFT) method. Our calculations show that all the investigated Y2CT2 are not suitable photocatalysts for photocatalytic water splitting at all pH values (pH = 0, 7, and 14). In contrast, all the investigated Janus Y2CTT' MXenes are good water splitting photocatalysts with high optical absorption coefficients and remarkable solar-to-hydrogen (STH) efficiencies larger than 18% at pH = 14. Moreover, the STH efficiencies are larger than 18% even at all investigated pH values for Y2CHCl (18.5-22.6%), Y2 CFCl (∼18.7%), and Y2 C(OH)Cl (∼19.4%). Based on the first-principles calculations, we here for the first time propose an easy strategy to design Janus MXene photocatalyst candidates with possible high STH efficiency according to the electronic properties of their symmetric counterparts. Our study is helpful for the future design of Janus MXenes and more generally Janus 2D photocatalysts for water splitting with high STH efficiency.

Unraveling the Emerging Photocatalytic, Thermoelectric, and Topological Properties of Intercalated Architecture MZX (M = Ga and In; Z = Si, Ge and Sn; X = S, Se, and Te) Monolayers

The continuous advancements in studying two-dimensional (2D) materials pave the way for groundbreaking innovations across various industries. In this study, by employing density functional theory calculations, we comprehensively elucidate the electronic structures of MZX (M = Ga and In; Z = Si, Ge, and Sn; X = S, Se, and Te) monolayers for their applications in photocatalytic, thermoelectric, and spintronic fields. Interestingly, GaSiS, GaSiSe, InSiS, and InSiSe monolayers are identified to be efficient photocatalysts for overall water splitting with band gaps close to 2.0 eV, suitable band edge positions, and excellent optical harvest ability. In addition, the InSiTe monolayer exhibits a ZT value of 1.87 at 700 K, making it highly appealing for applications in thermoelectric devices. It is further highlighted that GaSnTe, InSnS, and InSnSe monolayers are predicted to be 2D topological insulators (TIs) with bulk band gaps of 115, 54, and 152 meV, respectively. Current research expands the family of 2D GaGeTe materials and establishes a path toward the practical utilization of MZX monolayers in energy conversion and spintronic devices.

The polarized electric field of CdTe/B4C3 heterostructure efficiently promotes its photocatalytic overall water splitting

Inspired by natural photosynthesis, two-dimensional van der Waals (vdW) heterostructures are considered as promising photocatalysts for solar-driven water splitting and they attract ever-growing interest. A type-II vdW hetero-photocatalyst (CdTe/B4C3) integrating the polarized CdTe into metal-free B4C3 was constructed, which could achieve solar-driven spontaneous overall water splitting at pH = 0-7 and exhibit a high solar-to-hydrogen (STH) efficiency of 19.64%. Our calculation results show that the interlayer interaction between the CdTe and B4C3 monolayers in the heterostructure creates an interfacial electric field enhanced by the intrinsic dipole of polarized CdTe, which accelerates the effective separation of photogenerated carriers and makes the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) take place separately on the B4C3 and CdTe layers. Furthermore, the CdTe/B4C3 heterostructure has decent band edge positions to promote the redox reaction to decompose water due to the significant electrostatic potential difference in the CdTe/B4C3 heterostructure and it could trigger spontaneous redox reaction under light at pH = 0-7. This work is helpful for us to design type-II heterojunction photocatalysts with high efficiency of photogenerated carrier separation for overall water splitting.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.

A novel concept of photosynthetic soft membranes: a numerical study

We focus on a novel concept of photosynthetic soft membranes, possibly able to allow the conversion of solar energy and carbon dioxide (CO[Formula: see text]) into green fuels. The considered membranes rely on self-assembled functional molecules in the form of soap films. We elaborate a multi-scale and multi-physics model to describe the relevant phenomena, investigating the expected performance of a single soft photosynthetic membrane. First, we present a macroscale continuum model, which accounts for the transport of gaseous and ionic species within the soap film, the chemical equilibria and the two involved photocatalytic half reactions of the CO[Formula: see text] reduction and water oxidation at the two gas-surfactant-water interfaces of the soap film. Second, we introduce a mesoscale discrete Monte Carlo model, to deepen the investigation of the structure of the functional monolayers. Finally, the morphological information obtained at the mesoscale is integrated into the continuum model in a multi-scale framework. The developed tools are then used to perform sensitivity studies in a wide range of possible experimental conditions, to provide scenarios on fuel production by such a novel approach.

Decoupled Water Electrolysis Driven by 1 cm2 Single Perovskite Solar Cell Yielding a Solar-to-Hydrogen Efficiency of 14.4

Solar water splitting by photovoltaic (PV) electrolysis is a promising route for sustainable hydrogen production. However, multiple PV cells connected in series are generally required to fulfil the practical electrolytic voltages, which inevitably increases the system complexity and resistance. Decoupled water electrolysis for separate hydrogen and oxygen evolution needs smaller voltage to drive each half-reaction, which provides a feasibility to achieve the single PV cell driven water electrolysis. Herein, by introducing sodium nickelhexacyanoferrate (NaNiHCF) as the redox mediator, decoupled acid water electrolyzer and amphoteric water electrolyzer were respectively constructed. The required voltages for the hydrogen or oxygen evolution steps matched with the output voltages of the perovskite solar cell (PSC). Impressively, by combining one 1 cm² FAPbI₃ -based PSC (efficiency: 18.77 %) with the decoupled amphoteric water electrolyzer, a solar-to-hydrogen (STH) efficiency of 14.4 % was achieved, which outperformed previously reported PSC-driven water electrolysis cells.

Exploring the Photocorrosion Mechanism of a Photocatalyst

Photoelectrochemical (PEC) water splitting using Ta₃N₅ anodes shows a high solar-to-hydrogen (STH) efficiency approaching 10%. However, the long-term stability of gas evolution should be improved for the commercial utilization of PEC water-splitting technology. Herein, we examined the photocurrent degradation of photoanodes prepared by uniformly loading a NiFeO_x cocatalyst onto a Ta₃N₅ semiconductor. Although spectroscopic analysis showed that the degradation was attributable to the formation of an oxide layer, several oxide growth kinetic laws and mechanisms are known. We theoretically derived the photocurrent kinetic laws instead of the oxide growth kinetic laws by generalizing the Cabrera-Mott oxidation theory of metal oxidation in air to apply it to photocorrosion. The measured photocurrent kinetics are fully consistent with the theoretical kinetic laws. We show that ion drift due to charging of the oxide layer limits oxide growth even though uniform cocatalyst loading is designed to prevent self-oxidation of Ta₃N₅.

Tunable Photocatalytic Water Splitting Performance of Armchair MoSSe Nanotubes Realized by Polarization Engineering

The photocatalytic properties of Janus transition metal dichalcogenide (TMD) nanotubes are closely correlated with the electrostatic potential difference between their inner and outer surfaces (ΔΦ). However, due to some distraction from the tubular structures, it remains a great challenge to calculate their ΔΦ directly. Here, we creatively work out the ΔΦ of Janus MoSSe armchair single-walled nanotubes (A-SWNTs) with their corresponding building block models by first-principles calculations. The ΔΦ increases as the diameter reduces. After considering ΔΦ, we find that all of these MoSSe A-SWNTs possess suitable band-edge positions required for water redox reactions and high solar-to-hydrogen (STH) conversion efficiencies. The built-in field induced by the ΔΦ promotes the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) to proceed separately on the inner and outer surfaces. Especially, the photoexcited carriers exhibit adequate driving forces for OER and HER. Besides, constructing a double-walled nanotube can dramatically increase ΔΦ, which also further improves the separation and redox capacity of photoexcited carriers as well as the STH conversion efficiency. Moreover, all of these MoSSe armchair nanotubes have outstanding optical absorption in the visible light range. Our studies provide an effective strategy to improve the photocatalytic water-splitting performance of Janus TMD nanotubes.

Photocatalytic properties of anisotropic β-PtX2 (X = S, Se) and Janus β-PtSSe monolayers

The highly efficient photocatalytic water splitting process to produce clean energy requires novel semiconductor materials to achieve a high solar-to-hydrogen energy conversion efficiency. Herein, the photocatalytic properties of anisotropic β-PtX₂ (X = S, Se) and Janus β-PtSSe monolayers were investigated based on the density functional theory. The small cleavage energy for β-PtS₂ (0.44 J m^-2) and β-PtSe₂ (0.40 J m^-2) endorses the possibility of mechanical exfoliation from their respective layered bulk materials. The calculated results revealed that the β-PtX₂ monolayers have an appropriate bandgap (∼1.8-2.6 eV) enclosing the water redox potential, light absorption coefficient (∼10⁴ cm^-1), and exciton binding energy (∼0.5-0.7 eV), which facilitates excellent visible-light-driven photocatalytic performance. Remarkably, the inherent structural anisotropy leads to an anisotropic high carrier mobility (up to ∼5 × 10³ cm² V^-1 S^-1), leading to a fast transport of photogenerated carriers. Notably, the required small external potential to realize hydrogen evolution reaction and oxygen evolution reaction processes with an excellent solar-to-hydrogen energy conversion efficiency for β-PtSe₂ (∼16%) and β-PtSSe (∼18%) makes them promising candidates for solar water splitting applications.

Water-Splitting Artificial Leaf Based on a Triple-Junction Silicon Solar Cell: One-Step Fabrication through Photoinduced Deposition of Catalysts and Electrochemical Operando Monitoring

Solar hydrogen generation via water splitting using a monolithic photoelectrochemical cell, also called artificial leaf, could be a powerful technology to accelerate the transition from fossil to sustainable energy sources. Identification of scalable methods for the fabrication of monolithic devices and gaining insights into their operating mode to identify solutions to improve performance and stability represent great challenges. Herein, we report on the one-step fabrication of a CoWO|ITO|3jn-a-Si|Steel|CoWS monolithic device via the simple photoinduced deposition of CoWO and CoWS as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalyst layers, respectively, onto an illuminated ITO|3jn-a-Si|Steel solar cell using a single-deposition bath containing the [Co(WS₄)₂]^2- complex. In a pH 7 phosphate buffer solution, the best device achieved a solar-to-hydrogen conversion yield of 1.9%. Evolution of the catalyst layers and that of the 3jn-a-Si light-harvesting core during the operation of the monolithic device are examined by conventional tools such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) together with a bipotentiostat measurement. We demonstrate that the device performance degrades due to the partial dissolution of the catalyst. Still, this degradation is healable by simply adding [Co(WS₄)₂]^2- to the operating solution. However, modifications on the protecting indium-doped tin oxide (ITO) layer are shown to initiate irreversible degradation of the 3jn-a-Si light-harvesting core, resulting in a 10-fold decrease of the performances of the monolithic device.

Two-dimensional ferroelectric MoS2/Ga2O3 heterogeneous bilayers with highly tunable photocatalytic and electrical properties

Two-dimensional van der Waals heterostructures with strong intrinsic ferroelectrics are highly promising for novel devices with designed electronic properties. The polarization reversal transition of the 2D ferroelectric Ga₂O₃ monolayer offers a new approach to tune the photocatalytic and electrical properties of MoS₂/Ga₂O₃ heterogeneous bilayers. In this work, we study MoS₂/Ga₂O₃ heterogeneous bilayers with different intrinsic polarization using hybrid-functional calculations. We closely investigate the structural, electronic and optical properties of two stable stacking configurations with opposite polarization. The results reveal a distinct switch from type-I to type-II heterostructures owing to polarization reversal transition of the 2D ferroelectric Ga₂O₃ monolayer. Biaxial strain engineering leads to type-I-to-II and type-II-to-III transitions in the two polarized models, respectively. Intriguingly, one of the MoS₂/Ga₂O₃ heterolayers has a larger spatial separation of the valence and conduction band edges and excellent optical absorption ranging from infrared to ultraviolet region under biaxial strain, thus ensuring promising novel applications such as flexible electrical and optical devices. Based on the highly tunable physical properties of the bilayer heterostructures, we further explore their potential applications, such as photocatalytic water splitting and field-controlled switch channel in MOSFET devices.

High-Property Anode Catalyst Compositing Co-Based Perovskite and NiFe-Layered Double Hydroxide for Alkaline Seawater Splitting

The progress of high-efficiency non-precious metal anode catalysts for direct seawater splitting is of great importance. However, due to the slow oxygen evolution reaction (OER) kinetics, competition of chlorine evolution reaction (ClER), and corrosion of chloride ions on the anode, the direct seawater splitting faces many challenges. Herein, we develop a perovskite@NiFe layered double hydroxide composite for anode catalyst based on Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) and NiFe layered double hydroxide (NiFe-LDH) heterostructure. The optimized BSCF@CeO2@NiFe exhibits excellent OER activity, with the potential at 100 mA cm−2 (Ej = 100) being 1.62 V in the alkaline natural seawater. Moreover, the electrolytic cell composed of BSCF@CeO2@NiFe anode shows an excellent stability, with negligible attenuation during the long-term overall seawater splitting with the remarkable self-recovery ability in the initial operation stage, and the direct seawater splitting potential increasing by about 30 mV at 10 mA cm−2. Our work can give a guidance for the design and preparation of anode catalysts for the direct seawater splitting.

Self-healing oxygen evolution catalysts

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Large scale sustainable energy storage by water splitting benefits from performing the oxygen evolution reaction under a variety of conditions. Here, the authors discuss self-healing catalysis as a new tool in the design of stable and functionally active catalysts in acidic to basic solutions, and a variety of water sources

Two-Dimensional In2X2X' (X and X' = S, Se, and Te) Monolayers with an Intrinsic Electric Field for High-Performance Photocatalytic and Piezoelectric Applications

Two-dimensional (2D) materials with excellent photocatalytic properties and unique piezoelectric response have attracted great attention. However, these characters are rare for traditional 2D structures. With an intrinsic electric field, the Janus 2D materials show great promise in photocatalytic and out-of-plane piezoelectric applications. Herein, we show that Janus In₂X₂X' (X and X' = S, Se, and Te) monolayers are desirable in the overall water splitting and piezoelectric devices. Comprehensive investigations reveal that the band gaps of these Janus monolayers are from 0.34 to 2.27 eV. With proper band edge positions, strong solar absorption, fast transfer and efficient separation of carriers, and high solar to hydrogen (STH) efficiencies (reaching 37.70%), eight members of them stand out. Besides, the electrons and holes have sufficient driving forces in the process of redox reaction. The piezoelectric response for in- and out-of-plane is superior for all monolayers. These compelling features make them suitable for photocatalysts, sensors, actuators, and energy conversion devices.

Detection of high-valent iron species in alloyed oxidic cobaltates for catalysing the oxygen evolution reaction

Iron alloying of oxidic cobaltate catalysts results in catalytic activity for oxygen evolution on par with Ni-Fe oxides in base but at much higher alloying compositions. Zero-field ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) are able to clearly identify Fe⁴⁺ in mixed-metal Co-Fe oxides. The highest Fe⁴⁺ population is obtained in the 40–60% Fe alloying range, and XAS identifies the ion residing in an octahedral oxide ligand field. The oxygen evolution reaction (OER) activity, as reflected in Tafel analysis of CoFeO_x films in 1 M KOH, tracks the absolute concentration of Fe⁴⁺. The results reported herein suggest an important role for the formation of the Fe⁴⁺ redox state in activating cobaltate OER catalysts at high iron loadings.

The capturing of high valent iron in a catalytic reaction is important but difficult task. Here, the authors report identification of a high-valent Fe(IV)-species with different spectroscopic tools such as Mössbauer spectroscopy and X-ray absorption spectroscopy during the course of an oxygen evolving reaction.

Plasmonic Enhancement in Water Splitting Performance for NiFe Layered Double Hydroxide-N10 TC MXene Heterojunction

MXene-based material has attracted wide attention due to its tunable band gap, high conductivity and impressive optical and plasmonic properties. Herein, a hetero-nanostructured water splitting system was developed based on N-doped Ti₃ C₂ (N₁₀ TC) MXene and NiFe layered double hydroxide (LDH) nanosheets. The oxygen evolution reaction performance of the NiFe-LDH significantly enhanced to approximately 8.8-fold after incorporation of N₁₀ TC. Meanwhile, the Tafel slope was only 58.1 mV dec^-1 with light irradiation, which is lower than pure NiFe-LDH nanosheets (76.9 mV dec^-1 ). All results manifested the vital role of the N₁₀ TC MXene induced plasmonic hot carriers via electrophoto-excitation in enhancing the full water splitting performance of the as-prepared system. This work is expected to provide a platform for designing various plasmonic MXenes-based heterogeneous structures for highly efficient catalytic applications.

Maximizing Oxygen Evolution Performance on a Transparent NiFeOx/Ta3N5 Photoelectrode Fabricated on an Insulator

A transparent Ta₃N₅ photoanode is a promising candidate for the front-side photoelectrode in a photoelectrochemical (PEC) cell with tandem configuration (tandem cell), which can potentially provide high solar-to-hydrogen (STH) energy conversion efficiency. This study focuses in particular on the semiconductor properties and interfacial design of transparent Ta₃N₅ photoanodes fabricated on insulating quartz substrates (Ta₃N₅/SiO₂), typically the geometric area of 1 × 1 cm² in contact with indium on its edge. This material utilizes the self-conductivity of Ta₃N₅ to make the PEC system operational, and the electrode would strongly reflect the intrinsic nature of Ta₃N₅ without a back contact that is commonly introduced. First, PEC measurements using acetonitrile (ACN)/H₂O mixed solution were made to elucidate the intrinsic photoresponse in the presence of tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate) (Ru(bpy)₃(PF₆)₂) without water contact which avoids a multielectron-transfer oxygen evolution reaction (OER) and photoinduced self-oxidation. The potential difference between the onset potential of Ru²⁺ PEC oxidation by Ta₃N₅/SiO₂ and the redox potential of Ru^2+/3+ in the nonaqueous environment was about 0.7 V. While a stable photoanodic response was observed for Ta₃N₅/SiO₂ in the nonaqueous phase, the addition of a small quantity of water into this nonaqueous system led to the immediate deactivation of Ta₃N₅/SiO₂ photoanode under illumination by self-photooxidation to form TaO_x at the solid/water interface. In aqueous phase, flatband potentials estimated from Mott-Schottky analysis varied with solution pH (constant potential against reversible hydrogen electrode (RHE)). Photoelectrode modification by a transparent NiFeO_x layer was attempted. The complete coverage of the Ta₃N₅ surface with transparent NiFeO_x electrocatalysts, achieved by an optimized spin-coating protocol with controlled Ni-Fe precursors, allowed for the successful protection of Ta₃N₅ and demonstrated an extremely stable photocurrent for hours without any additional protective layers. The stability of the resultant NiFeO_x/Ta₃N₅/SiO₂ was limited not by Ta₃N₅ but mainly by a NiFeO_x electrocatalyst due to Fe dissolution with time.

Metabolic Engineering of Cupriavidus necator H16 for Sustainable Biofuels from CO2

Decelerating global warming is one of the predominant challenges of our time and will require conversion of CO₂ to usable products and commodity chemicals. Of particular interest is the production of fuels, because the transportation sector is a major source of CO₂ emissions. Here, we review recent technological advances in metabolic engineering of the hydrogen-oxidizing bacterium Cupriavidus necator H16, a chemolithotroph that naturally consumes CO₂ to generate biomass. We discuss recent successes in biofuel production using this organism, and the implementation of electrolysis/artificial photosynthesis approaches that enable growth of C. necator using renewable electricity and CO₂. Last, we discuss prospects of improving the nonoptimal growth of C. necator in ambient concentrations of CO₂.

A rationally designed two-dimensional MoSe2/Ti2CO2 heterojunction for photocatalytic overall water splitting: simultaneously suppressing electron-hole recombination and photocorrosion

Electron–hole recombination and photocorrosion are two challenges that seriously limit the application of two-dimensional (2D) transition metal dichalcogenides (TMDs) for photocatalytic water splitting. In this work, we propose a 2D van der Waals MoSe₂/Ti₂CO₂ heterojunction that features promising resistance to both electron–hole recombination and photocorrosion existing in TMDs. By means of first-principles calculations, the MoSe₂/Ti₂CO₂ heterojunction is demonstrated to be a direct Z-scheme photocatalyst for overall water splitting with MoSe₂ and Ti₂CO₂ serving as photocatalysts for hydrogen and oxygen evolution reactions, respectively, which is beneficial to electron–hole separation. The ultrafast migration of photo-generated holes from MoSe₂ to Ti₂CO₂ as well as the anti-photocorrosion ability of Ti₂CO₂ are responsible for photocatalytic stability. This heterojunction is experimentally reachable and exhibits a high solar-to-hydrogen efficiency of 12%. The strategy proposed here paves the way for developing 2D photocatalysts for water splitting with high performance and stability in experiments.

The two challenges of electron–hole recombination and photocorrosion for two-dimensional transition metal dichalcogenides in the application of photocatalytic water splitting are simultaneously suppressed by rational design of heterojunctions.

Efficient charge separation and visible-light response of two-dimensional Janus group-III monochalcogenide multilayers

The Janus group-III monochalcogenide multilayers with enhanced built-in electric field exhibit excellent performance in harvesting sunlight and efficient separation of electron–hole pairs.

Tunable photocatalytic water splitting and solar-to-hydrogen efficiency in β-PdSe2 monolayer

Direct production of hydrogen from photocatalytic water splitting is a potential solution to overcome global energy crisis.

High Solar-to-Hydrogen Conversion Efficiency at pH 7 Based on a PV-EC Cell with an Oligomeric Molecular Anode

In the urgent quest for green energy vectors, the generation of hydrogen by water splitting with sunlight occupies a preeminent standpoint. The highest solar-to-hydrogen (STH) efficiencies have been achieved with photovoltaic-electrochemical (PV-EC) systems. However, most PV-EC water-splitting devices are required to work at extreme conditions, such as in concentrated solutions of HClO₄ or KOH or under highly concentrated solar illumination. In this work, a molecular catalyst-based anode is incorporated for the first time in a PV-EC configuration, achieving an impressive 21.2% STH efficiency at neutral pH. Moreover, as opposed to metal oxide-based anodes, the molecular catalyst-based anode allows us to work with extremely small catalyst loadings (<16 nmol/cm²) due to a well-defined metallic center, which is responsible for the fast catalysis of the reaction in the anodic compartment. This work paves the way for integrating molecular materials in efficient PV-EC water-splitting systems.

NiFe-Layered Double Hydroxide Synchronously Activated by Heterojunctions and Vacancies for the Oxygen Evolution Reaction

The development of earth-abundant transition-metal-based electrocatalysts with bifunctional properties (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) is crucial to commercial hydrogen production. In this work, layered double hydroxide (LDH)-zinc oxide (ZnO) heterostructures and oxygen vacancies are constructed synchronously by plasma magnetron sputtering of NiFe-LDH. Using the optimal conditions, ZnO nanoparticles are uniformly distributed on the NiFe-LDH nanoflowers, which are prepared uniformly on the three-dimensional porous Ni foam. In the LDH-ZnO heterostructures and oxygen vacancies, electrons are depleted at the Ni cations on the NiFe-LDH surface and the active sites change from Fe cations to Ni cations during OER. Our theoretical assessment confirms the change of active sites after the deposition of ZnO and reveals the charge-transfer mechanism. Owing to the significant improvement in the OER dynamics, overall water splitting can be achieved at only 1.603 V in 1 M KOH when the Ni/LDH-ZnO and Ni/LDH are used as the anode and cathode, respectively. The work reveals a novel design of self-supported catalytic electrodes for efficient water splitting and also provides insights into the surface modification of catalytic materials.

InTeI: a novel wide-bandgap 2D material with desirable stability and highly anisotropic carrier mobility

Recently, stable 2D wide-bandgap semiconductors with excellent electronic and photoelectronic properties have attracted much scientific and technological interest. In this study, we predict a novel InTeI monolayer which has a wide bandgap of 2.735 eV and a anisotropic electron mobility as high as 12 137.80 cm² V^-1 s^-1 based on first-principles calculations. With an exfoliating energy lower than that of monolayer phosphorene, it is feasible to synthesize the 2D InTeI monolayer through mechanical exfoliation from their 3D bulk crystals. Remarkably, the monolayer InTeI achieves the indirect-to-direct bandgap transition under a small in-plane uniaxial strain, while a quasi-direct bandgap can be achieved in the InTeI nanosheets with elevated thickness. The InTeI monolayer and nanosheets have suitable band alignments in the visible-light excitation region. In addition, our theoretical simulations determine that 2D InTeI materials exhibit more excellent oxidation resistance than black phosphorene. The results not only identify a novel class of 2D wide-bandgap semiconductors but also demonstrate their potential applications in nanoelectronics and optoelectronics.

Highly-efficient overall water splitting in 2D Janus group-III chalcogenide multilayers: the roles of intrinsic electric filed and vacancy defects

Two-dimensional (2D) van der Waals materials have been widely adopted as photocatalysts for water splitting, but the energy conversion efficiency remains low. On the basis of first-principles calculations, we demonstrate that the 2D Janus group-III chalcogenide multilayers: InGaXY, M₂XY and InGaX₂ (M = In/Ga; X, Y = S/Se/Te), are promising photocatalysts for highly-efficient overall water splitting. The intrinsic electric field enhances the spatial separations of photogenerated carriers and alters the band alignment, which is more pronounced compared with the Janus monolayers. High solar-to-hydrogen (STH) efficiency with the upper limit of 38.5% was predicted in the Janus multilayers. More excitingly, the Ga vacancy of InGaSSe bilayer effectively lowers the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to the levels provided solely by the photogenerated carriers. Our theoretical results suggest that the 2D Janus group-III chalcogenide multilayers could be utilized as highly efficient photocatalysts for overall water splitting without the needs of sacrificial reagents.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

From passivation to activation – tunable nickel/nickel oxide for hydrogen evolution electrocatalysis

A novel, simple and controllable approach to designing NiO/Ni heterostructures supported on carbon for the hydrogen evolution reaction (HER) was utilized.

Recent Progress and Approaches on Carbon-Free Energy from Water Splitting

Sunlight is the most abundant renewable energy resource, providing the earth with enough power that is capable of taking care of all of humanity's desires-a hundred times over. However, as it is at times diffuse and intermittent, it raises issues concerning how best to reap this energy and store it for times when the Sun is not shining. With increasing population in the world and modern economic development, there will be an additional increase in energy demand. Devices that use daylight to separate water into individual chemical elements may well be the answer to this issue, as water splitting produces an ideal fuel. If such devices that generate fuel were to become widely adopted, they must be low in cost, both for supplying and operation. Therefore, it is essential to research for cheap technologies for water ripping. This review summarizes the progress made toward such development, the open challenges existing, and the approaches undertaken to generate carbon-free energy through water splitting.

Artificial Photosynthesis at Efficiencies Greatly Exceeding That of Natural Photosynthesis

Sunlight is an abundant energy source for a sustainable society. Indeed, photosynthetic organisms harness solar radiation to build the world around us by synthesizing energy-rich compounds from water and CO₂. However, numerous energy conversion bottlenecks in the natural system limits the overall efficiency of photosynthesis; the most efficient plants do not exceed solar storage efficiencies of 1%. Artificial photosynthetic solar-to-fuels cycles may occur at higher intrinsic efficiencies, but they typically terminate at hydrogen, with no process installed to complete the cycle for carbon fixation. This limitation may be overcome by interfacing solar-driven water splitting to H₂-oxidizing microorganisms. To this end, hybrid biological-inorganic constructs have been created to use sunlight, air, and water as the only starting materials to accomplish carbon fixation in the form of biomass and liquid fuels. This artificial photosynthetic cycle begins with the Artificial Leaf, which accomplishes the solar process of natural photosynthesis-the splitting of water to hydrogen and oxygen using sunlight-under ambient conditions. To create the Artificial Leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts permit water splitting to be accomplished using any water source, which is the critical development for (1) the Artificial Leaf, as it allows for the facile interfacing of water splitting catalysis to materials such as silicon, and (2) the hybrid biological-inorganic construct, called the Bionic Leaf, as it allows for the facile interfacing of water splitting catalysis to bioorganisms. Hydrogenases in the bioorganism allow the hydrogen to be coupled to NADPH and ATP production, thus allowing the solar energy from water splitting to be converted into cellular energy to drive cellular biosynthesis. In the design of the hybrid system, water splitting catalysts must be designed that support hydrogen generation at low applied potential to ensure a high energy efficiency while avoiding reactive oxygen species. Using the tools of synthetic biology, a bioengineered bacterium, Ralstonia eutropha, converts carbon dioxide from air, along with the hydrogen produced from such catalysts of the Artificial Leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The Bionic Leaf operates at solar-to-biomass and solar-to-liquid fuels efficiencies that greatly exceed the highest solar-to-biomass efficiencies of natural photosynthesis.

Tandem Si Micropillar Array Photocathodes with Conformal Copper Oxide and a Protection Layer by Pulsed Laser Deposition

This work demonstrates the influence of high-quality protection layers on Si-Cu₂O micropillar arrays created by pulsed laser deposition (PLD), with the goal to overcome photodegradation and achieve long-term operation during photoelectrochemical (PEC) water splitting. Sequentially, we assessed planar and micropillar device designs with various design parameters and their influence on PEC hydrogen evolution reaction. On the planar device substrates, a Cu₂O film thickness of 600 nm and a Cu₂O/CuO heterojunction layer with a 5:1 thickness ratio between Cu₂O to CuO were found to be optimal. The planar Si/Cu₂O/CuO heterostructure showed a higher PV performance (J_sc = 20 mA/cm²) as compared to the planar Si/Cu₂O device, but micropillar devices did not show this improvement. Multifunctional overlayers of ZnO (25 nm) and TiO₂ (100 nm) were employed by PLD on Si/Cu₂O planar and micropillar arrays to provide a hole-selective passivation layer that acts against photocorrosion. A micropillar Si/ITO-Au/Cu₂O/ZnO/TiO₂/Pt stack was compared to a planar device. Under optimized conditions, the Si/Cu₂O photocathode with Pt as a HER catalyst displayed a photocurrent of 7.5 mA cm^-2 at 0 V vs RHE and an onset potential of 0.85 V vs RHE, with a stable operation for 75 h.

Optimum Band Gap Energy of ((Ag),Cu)(InGa)Se2 Materials for Combination with NiMo–NiO Catalysts for Thermally Integrated Solar-Driven Water Splitting Applications

Solar-driven water splitting is considered one of the promising future routes to generate fuel in a sustainable way. A carbon-free solar fuel, molecular hydrogen, can here be produced along two different but intimately related routes, photoelectrochemical (PEC) water splitting or photovoltaic electrolysis (PV-electrolysis), where the latter builds on well-established solar cell and electrolysis materials with high efficiency. The PV-electrolysis approach is also possible to construct from an integrated PEC/PV-system avoiding dc–dc converters and enabling heat exchange between the PV and electrolyzer part, to a conventionally wired PV-electrolysis system. In either case, the operating voltage at a certain current needs to be matched with the catalyst system in the electrolysis part. Here, we investigate ((Ag),Cu)(In,Ga)Se2 ((A)CIGS)-materials with varying Ga-content modules for combination with NiMo–NiO catalysts in alkaline water splitting. The use of (A)CIGS is attractive because of the low cost-to-performance ratio and the possibility to optimize the performance of the system by tuning the band gap of (A)CIGS in contrast to Si technology. The band gap tuning is possible by changing the Ga/(Ga + In) ratio. Optoelectronic properties of the (A)CIGS materials with Ga/(Ga + In) ratios between 0.23 and 0.47 and the voltage and power output from the resulting water splitting modules are reported. Electrolysis is quantified at temperatures between 25 and 60 °C, an interval obtainable by varying the thermal heat exchange form a 1-sun illuminated PV module and an electrolyte system. The band gaps of the (A)CIGS thin films were between 1.08 to 1.25 eV and the three-cell module power conversion efficiencies (PCE) ranged from 16.44% with 1.08 eV band gap and 19.04% with 1.17 eV band gap. The highest solar-to-hydrogen (STH) efficiency was 13.33% for the (A)CIGS–NiMo–NiO system with 17.97% module efficiency and electrolysis at 60 °C compared to a STH efficiency of 12.98% at 25 °C. The increase in STH efficiency with increasing temperature was more notable for lower band gaps as these are closer to the overpotential threshold for performing efficient solar-driven catalysis, while only a modest improvement can be obtained by utilizing thermal exchange for a band gap matched PV-catalysts system. The results show that usage of cost-effective and stable thin film PV materials and earth abundant catalysts can provide STH efficiencies beyond 13% even with PV modules with modest efficiency.

Spontaneous full photocatalytic water splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 vdW heterostructures

Spontaneous full photocatalytic water splitting into hydrogen and oxygen under visible light irradiation without the need for sacrificial agents is a challenging task, because suitable band gaps, low overpotentials for both half-reactions and spatially-separated catalytic sites should be fulfilled simultaneously in a photocatalytic system. Here, we propose a promising strategy to achieve this goal by constructing van der Waals (vdW) heterostructures of two-dimensional (2D) materials. Using first-principles calculations, we predict two promising photocatalysts, MoSe2/SnSe2 and WSe2/SnSe2 heterostructures, with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) taking place separately on the MoSe2 (WSe2) and SnSe2 layers. More excitingly, the Se-vacancy of the MoSe2 (WSe2) monolayer effectively lowers the HER overpotential, making the catalytic reactions occur spontaneously under the potentials solely provided by the photo-generated electrons and holes in pure water. The unique band alignment of these hetero-structured photocatalysts leads to high solar-to-hydrogen (STH) energy conversion efficiencies up to 10.5%, which is quite promising for commercial applications. This work opens up an avenue for the design of highly-efficient photocatalysts for full water splitting.

Fundamentals and applications of photocatalytic CO2 methanation

The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of recent research efforts, “solar methane” can now be produced through the photocatalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane.

While natural gas and fossil fuels power human activities, increasing concerns over fuel reserves and environmental impacts require finding alternative, renewable resources. Here, authors review the fundamental science and progress on solar-powered conversion of carbon dioxide to methane.

Seebeck-voltage-triggered self-biased photoelectrochemical water splitting using HfOx/SiOx bi-layer protected Si photocathodes

The use of a photoelectrochemical device is an efficient method of converting solar energy into hydrogen fuel via water splitting reactions. One of the best photoelectrode materials is Si, which absorbs a broad wavelength range of incident light and produces a high photocurrent level (~44 mA·cm-2). However, the maximum photovoltage that can be generated in single-junction Si devices (~0.75 V) is much lower than the voltage required for a water splitting reaction (>1.6 V). In addition, the Si surface is electrochemically oxidized or reduced when it comes into direct contact with the aqueous electrolyte. Here, we propose the hybridization of the photoelectrochemical device with a thermoelectric device, where the Seebeck voltage generated by the thermal energy triggers the self-biased water splitting reaction without compromising the photocurrent level at 42 mA cm-2. In this hybrid device p-Si, where the surface is protected by HfOx/SiOx bilayers, is used as a photocathode. The HfOx exhibits high corrosion resistance and protection ability, thereby ensuring stability. On applying the Seebeck voltage, the tunneling barrier of HfOx is placed at a negligible energy level in the electron transfer from Si to the electrolyte, showing charge transfer kinetics independent of the HfOx thickness. These findings serve as a proof-of-concept of the stable and high-efficiency production of hydrogen fuel by the photoelectrochemical-thermoelectric hybrid devices.

Toward practical solar hydrogen production - an artificial photosynthetic leaf-to-farm challenge

Solar water splitting is a promising approach to transform sunlight into renewable, sustainable and green hydrogen energy. There are three representative ways of transforming solar radiation into molecular hydrogen, which are the photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-electrolysis (PV-EC) routes. Having the future perspective of green hydrogen economy in mind, this review article discusses devices and systems for solar-to-hydrogen production including comparison of the above solar water splitting systems. The focus is placed on a critical assessment of the key components needed to scale up PEC water splitting systems such as materials efficiency, cost, elemental abundancy, stability, fuel separation, device operability, cell architecture, and techno-economic aspects of the systems. The review follows a stepwise approach and provides (i) a summary of the basic principles and photocatalytic materials employed for PEC water splitting, (ii) an extensive discussion of technologies, procedures, and system designs, and (iii) an introduction to international demonstration projects, and the development of benchmarked devices and large-scale prototype systems. The task of scaling up of laboratory overall water splitting devices to practical systems may be called "an artificial photosynthetic leaf-to-farm challenge".

Carbon-Based Photocathode Materials for Solar Hydrogen Production

Hydrogen is considered a promising environmentally friendly energy carrier for replacing traditional fossil fuels. In this context, photoelectrochemical cells effectively convert solar energy directly to H₂ fuel by water photoelectrolysis, thereby monolitically combining the functions of both light harvesting and electrolysis. In such devices, photocathodes and photoanodes carry out the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Here, the focus is on photocathodes for HER, traditionally based on metal oxides, III-V group and II-VI group semiconductors, silicon, and copper-based chalcogenides as photoactive material. Recently, carbon-based materials have emerged as reliable alternatives to the aforementioned materials. A perspective on carbon-based photocathodes is provided here, critically analyzing recent research progress and outlining the major guidelines for the development of efficient and stable photocathode architectures. In particular, the functional role of charge-selective and protective layers, which enhance both the efficiency and the durability of the photocathodes, is discussed. An in-depth evaluation of the state-of-the-art fabrication of photocathodes through scalable, high-troughput, cost-effective methods is presented. The major aspects on the development of light-trapping nanostructured architectures are also addressed. Finally, the key challenges on future research directions in terms of potential performance and manufacturability of photocathodes are analyzed.

Triple hierarchy and double synergies of NiFe/Co9S8/carbon cloth: a new and efficient electrocatalyst for the oxygen evolution reaction

Electrochemical water splitting requires an efficient water oxidation catalyst to accelerate the oxygen evolution reaction (OER). A triple hierarchy catalyst structure based on abundant transition metals in soil (Fe, Co, and Ni) was prepared via chemical bath deposition, followed by a hydrothermal and electrodeposition method for the high-efficiency OER. The obtained electrode consisted of a three-layer porous structure, with a carbon cloth (CC) substrate as the bottom layer, vertically aligned Co9S8 nanotubes as the intermediate layer, and NiFe hydroxide as the top layer, resulting in two synergistic effects between Co9S8 and CC, and NiFe and Co9S8. This layered structure contained no binder, which facilitated catalytic site exposure, enhanced electron transfer, and accelerated the dissipation of gases generated during the catalytic process. This triple hierarchy multimetal/carbon electrode exhibited remarkable OER activity in an alkaline medium with a small overpotential of 219 mV (vs. RHE) at 10 mA·cm-2 and low Tafel slope of 55 mV·dec-1. Furthermore, this triple hierarchy electrode was stable for up to 20 h. The prepared triple hierarchy polymetallic material can be considered to be among the most promising oxygen evolution catalysts.