Si-based Photocathodes Research Articles

Covalent Immobilization of Mediators on Photoelectrodes for NADH Regeneration.

The reduced nicotinamide adenine dinucleotide (NADH) is a vital biomolecule involved in many biocatalytic processes, and the high cost makes it significant to regenerate NADH in vitro. The photoelectrochemical approach is a promising and environmentally friendly method for sustainable NADH regeneration. However, the free Rh-based mediator ([Cp*Rh (bpy)H2O]2+) in the electrolyte suffers from low efficiency due to the sluggish charge transfer controlled by the diffusion process. Herein, we report an efficient and facile covalent bonding of the Rh-based mediator with the Si-based photocathode for NADH regeneration. The bipyridine-containing covalent organic framework (BpyCOF) layer ensures the even distribution of mediators throughout the surface of the photoelectrode. The graphene interlayer provides a pathway for charge transport and prevents silicon from corrosion. Furthermore, during the synthesis of BpyCOF, it functions as a substrate to promote the growth of the oriented BpyCOF film. The imitated contact between the components of the photocathode favors the charge transfer to the surface to participate in a chemical reaction, thus improving the catalytic performance and the NADH regeneration efficiency, which is four times higher than the reported photocathode modified by the Rh-based mediator. This study offers a new strategy for the construction of photoelectrochemical solar energy conversion devices.

Silicon photocathode functionalized with osmium complex catalyst for selective catalytic conversion of CO2 to methane

Solar-driven CO2 reduction to yield high-value chemicals presents an appealing avenue for combating climate change, yet achieving selective production of specific products remains a significant challenge. We showcase two osmium complexes, przpOs, and trzpOs, as CO2 reduction catalysts for selective CO2-to-methane conversion. Kinetically, the przpOs and trzpOs exhibit high CO2 reduction catalytic rate constants of 0.544 and 6.41 s−1, respectively. Under AM1.5 G irradiation, the optimal Si/TiO2/trzpOs have CH4 as the main product and >90% Faradaic efficiency, reaching −14.11 mA cm−2 photocurrent density at 0.0 VRHE. Density functional theory calculations reveal that the N atoms on the bipyrazole and triazole ligands effectively stabilize the CO2-adduct intermediates, which tend to be further hydrogenated to produce CH4, leading to their ultrahigh CO2-to-CH4 selectivity. These results are comparable to cutting-edge Si-based photocathodes for CO2 reduction, revealing a vast research potential in employing molecular catalysts for the photoelectrochemical conversion of CO2 to methane.

Enhanced charge-carrier dynamics and efficient solar-to-urea conversion on Si-based photocathodes

Photoelectrochemical (PEC) coupling of CO2 and nitrate can provide a useful and green source of urea, but the process is affected by the photocathodes with poor charge-carrier dynamics and low conversion efficiency. Here, a NiFe diatomic catalysts/TiO2 layer/nanostructured n+p-Si photocathode is rationally designed, achieving a good charge-separation efficiency of 78.8% and charge-injection efficiency of 56.9% in the process of PEC urea synthesis. Compared with the electrocatalytic urea synthesis by using the same catalysts, the Si-based photocathode shows a similar urea yield rate (81.1 mg·h-1·cm-2) with a higher faradic efficiency (24.2%, almost twice than the electrocatalysis) at a lower applied potential under 1 sun illumination, meaning that a lower energy-consumption method acquires more aimed productions. Integrating the PEC measurements and characterization results, the synergistic effect of hierarchical structure is the dominating factor for enhancing the charge-carrier separation, transfer, and injection by the matched band structure and favorable electron-migration channels. This work provides a direct and efficient route of solar-to-urea conversion.

Efficiently unbiased solar-to-ammonia conversion by photoelectrochemical Cu/C/Si-TiO2 tandems

Photoelectrochemical nitrate reduction reaction (PEC NO3RR) is of interest as a promising route to directly realizing the solar-to-ammonia (NH3) conversion but the limited efficiency and high applied bias voltage hamper its commercial prospects. Here, we report a bias-free photoelectrochemical cell for PEC NO3RR in aqueous conditions, achieving a substantial NH3 yield rate of 13.1 μmol·h−1·cm−2, high faradaic efficiency of 93.8%, and recorded solar-to-NH3 conversion of ∼1.5% under 1 sun illumination. A hierarchical-structured Si-based photocathode with Cu+/Cu2+-containing Cu nanoparticles cocatalysts achieves a highly efficient PEC NO3RR with NH3 yield rate of 115.3 μmol·h−1·cm−2 in a three-electrode system. Integrating operando characterizations and systematic PEC measurements, the formation of Lewis acid sites on Cu nanoparticles by accepting the photoinduced electrons is the dominant factor for facilitating the absorption and hydrogenation of nitrate. This work will guide the development of a robust, high-performance, and unbiased PEC device for sustainable solar-to-NH3/other fuels conversion.

A high-performance TiO2 protective layer derived from non-high vacuum technology for a Si-based photocathode to enhance photoelectrochemical water splitting

A non-high vacuum technique was used to fabricate a TiO2 protective layer of a Si-based photocathode, in which MOF derivatives enhance carrier transfer efficiency. The photocathode has high onset potential, high photocurrent and long-term stability.

Mixed-phase WO3 Cocatalysts on Hierarchical Si-based Photocathode for Efficient Photoelectrochemical Li Extraction.

Photoelectrochemical lithium (Li) extraction can be expected to provide a useful recycle of Li+ from waste Li-containing battery, but the process is limited by the photocathodes with poor Li+ absorption and low yield rate. Here, we have designed a hierarchical silicon (Si)-based photocathode with mixed-phase tungsten oxide (WO3 ) cocatalysts for photoelectrochemical Li extraction under 1 sun illumination, achieving a high Li yield rate of ≈223.0 μg cm-2 h-1 and an excellent faradaic efficiency of 91.9 % at 0.0817 V versus Li0/+ redox couple. The WO3 cocatalysts with the mixture of amorphous and crystalline phase accelerates the Li+ insertion and precipitation and enriches the concentration of Li+ at the photocathode surface. This robust photoelectrochemical Li extraction system provides a new insight on designing green and efficient route for cyclic utilization of Li resources in the sustainable energy field.

One-step construction of buried a-Si/c-Si junction photocathodes for boosting photoelectrochemical hydrogen production

The activity of Si-based photocathode for photoelectrochemical hydrogen production is mainly restricted by inadequate photovoltage and easy (photo)-corrosion, requiring complex doping or deposition techniques for improvement. Herein, a mild and feasible strategy is proposed to fabricate an amorphous Si/crystal Si (a-Si/c-Si) junction photocathode with co-deposited Pt cocatalyst. The a-Si layer acts as an electron transfer mediator to form an a-Si/c-Si buried junction with large energy band shift, thereby ensuring facile charge transfer. Meanwhile, the a-Si layer serves as a growth substrate to obtain controllable Pt nanoparticles. The Pt/a-Si/c-Si photocathode shows an onset potential of 0.42 V vs reversible hydrogen electrode (VRHE). A photocurrent up to − 35.0 mA cm−2 at 0 VRHE is obtained on Pt/a-Si/c-Si, which is much higher than that without a-Si layer. This study provides a practical manner for improving the photoresponse of planar Si-based photocathodes based on the construction of Si junction.

Surface/Interface Engineering of Si-Based Photocathodes for Efficient Hydrogen Evolution

Surface/Interface Engineering of Si-Based Photocathodes for Efficient Hydrogen Evolution

Stable solar water splitting with wettable organic-layer-protected silicon photocathodes

Protective layers are essential for Si-based photocathodes to achieve long-term stability. The conventionally used inorganic protective layers, such as TiO2, need to be free of pinholes to isolate Si from corrosive solution, which demands extremely high-quality deposition techniques. On the other hand, organic hydrophobic protective layers suffer from the trade-off between current density and stability. This paper describes the design and fabrication of a discontinuous hybrid organic protective layer with controllable surface wettability. The underlying hydrophobic layer induces the formation of thin gas layers at the discontinuous pores to isolate the electrolyte from Si substrate, while allowing Pt co-catalyst to contact the electrolyte for water splitting. Meanwhile, the surface of this organic layer is modified with hydrophilic hydroxyl groups to facilitate bubble detachment. The optimized photocathode achieves a stable photocurrent of 35 mA/cm2 for over 110 h with no trend of decay.

Cupric porphyrin frameworks on multi-junction silicon photocathodes to expedite the kinetics of CO2 turnover.

Photoelectrochemical CO2 reduction utilizing silicon-based photocathodes offers a promising route to directly store solar energy in chemical bonds, provoking the development of heterogeneous molecular catalysts with high turnover rates. Herein, an in situ surface transformation strategy is adopted to grow metal-organic frameworks (MOFs) on Si-based photocathodes, serving as catalytic scaffolds for boosting both the kinetics and selectivity of CO2 reduction. Benefitting from the multi-junctional configuration for enhanced charge separation and the porous MOF scaffold enriching redox-active metalloporphyrin sites, the Si photocathode demonstrates a high CO faradaic efficiency of 87% at a photocurrent density of 10.2 mA cm-2, which is among the best seen for heterogeneous molecular catalysts. This study highlights the exploitation of reticular chemistry and macrocycle complexes as Earth-abundant alternatives for catalyzing artificial photosynthesis.

Functional Integration of Catalysts with Si Nanowire Photocathodes for Efficient Utilization of Photogenerated Charge Carriers.

Low-cost catalysts with high activity and durability are necessary to achieve efficient large-scale energy conversion in photoelectrochemical cell (PEC) systems. An additional factor that governs the construction of photoelectrodes for PECs is the spatial control of the catalysts for efficient utilization of photogenerated charge carriers. Here, we demonstrate spatial decoupling of the light-absorbing and catalytic components in hierarchically structured Si-based photocathodes for the hydrogen evolution reaction (HER). By simply modifying a well-known metal-assisted chemical etching procedure, we fabricated a Si nanowire (NW) array-based photocathode with Ag–Pt catalysts at the base and small amounts of the Pt catalyst at the NW tips. This approach simultaneously mitigates the parasitic light absorption by the catalytic layers and recombination of charge carriers owing to the long transport distance, resulting in improved photoelectrochemical HER performance under simulated AM 1.5G illumination.

Direct Z-scheme SiNWs@Co3O4 photocathode with a cocatalyst of sludge-derived carbon quantum dots for efficient photoelectrochemical hydrogen production

Solar driven photoelectrochemical (PEC) hydrogen production has attracted considerable attention, but the design of highly efficient, robust and low-cost photocathode still remains a significant challenge. Herein, we report a novel SiNWs@Co3O4Z-scheme heterojunction photocathode with carbon quantum dots eco-friendly derived from sludge (SCQDs) as the co-catalyst. The photocathode not only leads to effective separation of electron–hole pair, lower transmission resistance, and longer lifetime of charge carriers, but also elevates the stability by preventing direct contact between the SiNWs and the electrolyte as well as the self-oxidation. Simultaneously, the excellent electron transport properties of the SCQDs further improved the PEC performance. Correspondingly, a maximum current density of 14.88 mA·cm−2 was obtained at −0.67 V with the applied bias photon-to-current efficiency (ABPE) achieving 8.4% under visible light irradiations at pH = 7. This work provides a promising scheme for Si-based photocathodes for PEC hydrogen production with a high performance, reliable stability, and low-cost.

Coupling Ru-MoS2 heterostructure with silicon for efficient photoelectrocatalytic water splitting

Photoelectrochemical water splitting for hydrogen evolution reaction (PEC-HER) is always challenged by the low charge-separation efficiency and catalytic hydrogen evolution activity. Herein, we designed a Ru-MoS2 heterostructured catalyst on a titanium (Ti) protecting p-type silicon (n+p-Si) to address these obstacles. The n+p-Si/Ti/Ru-MoS2 delivers the largest reported photocurrent density for the Si-based photocathode (−43 mA cm−2 at 0 V versus a reversible hydrogen electrode (VRHE)). A considerably high half-cell solar-to-hydrogen conversion efficiency (HC-STH) of 7.28% is achieved at 0.2 VRHE. The excellent performance of n+p-Si/Ti/Ru-MoS2 is benefited from the electronic properties of Ru-MoS2 that dramatically decrease charge transfer resistance, and enhance the built-in electric field at the photoelectrode/electrolyte interface for the charge separation. Significantly, the density functional theory (DFT) calculation further reveals that the engineered electronic structure of the Ru-MoS2 contributes to the improvements in catalytic HER efficiency by optimizing electron transportation and Gibbs free energy of hydrogen adsorption (ΔGH*) on hetero-interface sites.

Coating of Phosphide Catalysts on p-Silicon by a Necking Strategy for Improved Photoelectrochemical Characteristics in Alkaline Media.

The methodology of coating electrocatalysts on semiconductor substrates is critical for the catalytic performance of photoelectrochemical electrodes. A weakly bound coating leads to orders of magnitude lower efficiency and reliability compared to those required to meet the commercial demand. Herein, a facile strategy based on the hydrolysis of TiCl4 is developed to solve the coating issue. Mesoporous tungsten phosphide (WP) particles were spin-coated and affixed onto TiO2-protected planar p-Si by the formation of a TiO2 necking layer between the catalyst particles and the substrates. Under 1 sun illumination, the as-prepared WP/TiO2/Si photocathode yields a saturated current density of -35 mA cm-2 and a durability of over 110 h with a current density over -15 mA cm-2 at 0 V versus a reversible hydrogen electrode in a 1.0 M KOH solution, which is among the state-of-the-art performances of commercial planar Si-based photocathodes. The Kelvin probe force microscopy results suggest the successive transfer of photoelectrons from Si to TiO2 and WP. The as-formed TiO2 necking layer plays the key role in ensuring the surface catalytic activity and durability. This necking strategy is also applicable for coating other transition-metal phosphides, for example, MoP and FeP, thus offering a practical approach to meet the commercial requirement of low-cost, highly efficient, and durable photoelectrodes.

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

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

Chlorophyll(a)/Carbon Quantum Dot Bio-Nanocomposite Activated Nano-Structured Silicon as an Efficient Photocathode for Photoelectrochemical Water Splitting.

Solar-driven water splitting is considered as a futuristic sustainable way to generate hydrogen and chemical storage of solar energy. Further, considering the technological competence, silicon is one of the potential materials for developing large-scale and cost-effective photocathodes (PCs), but it lacks efficacy and stability. Here, we show that chlorophyll(a)/carbon quantum dots (Chl/CQDs) bio-nanocomposite (b-NC)-decorated Si-nanowires (SiNWs) as PC can surpass the reported efficiency for photoelectrochemical (PEC) hydrogen generation along with stability. The optimized heterojunction (Chl/CQDs_SiNW) significantly enhances broad-band solar absorption and protects Si surface from corrosion. Further, the appropriate band alignment enforces efficient photogenerated charge separation and possesses directional exciton transport property via the Förster resonance energy transfer (FRET) mechanism. This synergic effect demonstrates an ∼18 times increase in photocurrent density (26.36 mA/cm2) compared to pristine SiNW PC at 1.07 V vs reversible hydrogen electrode (RHE). The efficiency reaches ∼7.86%, which is comparably the highest reported for hybrid Si-based photocathodes. Hydrogen evaluation rate was measured to be ∼113 μmol/h at 0.8 V vs RHE under 1 sun illumination. With Si-process line compatibility, this new finding opens a new direction toward the development of Si-based efficient and stable PCs at a large scale for commercial applications.

Improved performance and stability of photoelectrochemical water-splitting Si system using a bifacial design to decouple light harvesting and electrocatalysis

Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen is a promising way for the production of clean, and storable form of fuel but the PEC efficiency has remained low. Herein, we demonstrate enhanced light harvesting, charge carrier separation/transfer, and catalyst management with bifacial design for the Si-based photocathodes to achieve best-in-class hydrogen generation with excellent electrochemical stability. Decoupling the light harvesting side from the electrocatalytic surface nullifies parasitic light absorption and enables Si photocathodes that exhibit a photocurrent density of 39.01 mA/cm2 and stability over 370 h in 1 M H2SO4(aq) electrolyte due to fully covered a 15 nm Pt without any intentional protective layer. Furthermore, the bifacial Si photocathode system with semi-transparent Pt layer of 5 nm developed herein are capable of collecting sunlight not only on the light harvesting side but also on the back side of the device, resulting in a photocurrent density of 61.20 mA/cm2 under bifacial two-sun illumination, which yields 56.88% of excess hydrogen when compared to the monofacial PEC system. Combining the bifacial design with surface texturing and antireflection coating enables excellent omnidirectional light harvesting capability with a record hydrogen (photocurrent) generation, which provides a promising way to realize practical PEC water splitting applications.

Stability and Degradation Mechanismof Si-based Photocathodes for Water Splitting with Ultrathin TiO2 Protection Layer

Abstract Using transmission and scanning electron microscopy, we study mechanisms which determine the stability of Silicon photocathodes for solar driven water splitting. Such tandem or triple devices can show a promising stability as photocathodes if the semiconductor surface is protected by an ultrathin TiO2 protection layer. Using atomic layer deposition (ALD) with Cl-precursors, 4–7 nm thick TiO2 layers can be grown with high structural perfection. The layer can be electrochemically covered by Pt nanoparticels serving as electro-catalysts. However, Cl-remnants which are typically present in such layers due to incomplete oxidation, are the origin of an electrochemical degradation process. After 1 h AM1.5G illumination in alkaline media, circular shaped corrosion craters appear in the topmost Si layer, although the TiO2 layer is intact in most parts of the crater. The crater development is stopped at local inhomogenities with a higher Pt coverage. The observations suggests that reduced Titanium species due to Cl−/O2− substitution are nucleation sites of the initial corrosion steps due to enhanced solubility of reduced Ti in the electrolyte. This process is followed by electrochemical dissolution of Si, after direct contact between the electrolyte and the top Si layer surface. To increase the stability of TiO2 protected photocathodes, formation of reduced Ti species must be avoided.

(Invited) Toward Highly Efficient Solar Water Splitting: A Concurrent Electrical, Optical, and Catalytic Design

Energy crisis is a broad and complex global topic. Natural resources such as gas and oil are in limited in supply. Development towards renewable resources is one of the most important technologies in the world. Currently, photocatalytic and photoelectrochemical (PEC) water splitting devices under the irradiation of sunlight have received much attention for the production of renewable hydrogen from water. Solar energy conversion and storage through photoelectrolysis of water using semiconductors as both light absorber and energy converter to store solar energy in simple chemical bond, H2, become highly desirable approaches to solving the energy shortage challenge. We focus on the effort to develop an efficient Si-based PEC water splitting device. We introduce the surface textured Si heterojunction PEC cell consisting of ultrathin amorphous Si/crystalline Si as efficient and robust photoelectrodes. The solar to hydrogen conversion efficiency has been improved to 13.26%, which is the highest ever reported for Si-based photocathodes. Later on, we design the cascading energy band structure in Si via doping for facilitating carrier separation and novel electrode structures for 360° light harvesting for hydrogen generation with ultrahigh current densities of 61.2 mAcm-2. The cells have been further demonstrated with excellent hydrogen production rate. In addition, our method can significantly improve the stability of Si-based solar cells in water to sustain up to 300 hr. These multifunctional designs provide the potential for the future development in the renewable energy market.

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO2.

Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under conditions compatible with those for photoanodes are desired. Herein, Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO2 protective layer, followed by decorating the electrode surface with Ni and Ni-Mo catalysts. The carbon nanolayer, denoted as CPDA, was formed directly on the Si surface by in situ self-polymerization of dopamine, followed by carbonization of the polydopamine (PDA) coating. The performance of the as-fabricated Si photocathodes with and without the CPDA layer was comparatively studied for PEC hydrogen evolution reaction (HER) inalkaline electrolytes to evaluate the effect of the sandwich CPDA layer in between the Si substrate and the TiO2 layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the CPDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the CPDA layer. Accordingly, the short-circuit photocurrents of the Ni and Ni-Mo-decorated photocathodes with the CPDA layer were enhanced by a factor of 2.8-3.3, and their open-circuit photovoltages were enlarged by 0.14-0.22 V, compared to those of the corresponding electrodes without the CPDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the CPDA layer also exhibited an improved stability for PEC HER in alkaline solutions, with a faradaic efficiency of 90-97% in the initial hour.