Efficient Photoelectrochemical Water Splitting Research Articles

Tuning surface hydrophilicity of a BiVO4 photoanode through interface engineering for efficient PEC water splitting.

This study presents a novel approach to enhance photoelectrochemical (PEC) water oxidation by integrating cobalt phthalocyanine (CoPc) with bismuth vanadate (BVO) via a direct solvothermal method. The as-prepared BVO@CoPc photoanode demonstrated a photocurrent density of 4.0 mA cm-2 at 1.23 V vs. RHE, which is approximately 3.1 times greater than that of the unmodified BVO, and has superior stability. The incident photon-to-current conversion efficiency (IPCE) of the BVO@CoPc photoanode reaches an impressive value of 81%, accompanied by significant enhancements in charge injection efficiency. This excellent performance can be ascribed to the enhanced hydrophilicity with the BVO/CoPc interface engineering, which facilitates interfacial interaction between the electrode and electrolyte, accelerates photogenerated charge carrier transfer and separation. Furthermore, compared to the immersion and drop-casting methods, the BVO@CoPc-S composite photoanode prepared via the solvothermal method exhibits a significant improvement in interfacial contact and surface hydrophilicity. These findings highlight the potential of the strategy based on interfacial hydrophilicity modification to overcome key limitations in PEC water splitting, providing a pathway to more efficient and durable photoanode design.

ZnO-Ta3N5 core-shell nanowire photoanodes with pyridine passivation for efficient and stable photoelectrochemical water-splitting

ZnO-Ta3N5 core-shell nanowire photoanodes with pyridine passivation for efficient and stable photoelectrochemical water-splitting

Incorporation of sulfur vacancies in the ZnIn2S4 photoanode for highly efficient photoelectrochemical water splitting and urea oxidation

We report a “one stone, two birds” strategy for highly efficient photoelectrochemical (PEC) performance by introducing sulfur vacancies (Sv) in a ZnIn2S4 (ZIS) photoanode, which exhibits a superior photocurrent density of 2.18 mA cm−2.

Interface engineering for photoelectrochemical oxygen evolution reaction.

Photoelectrochemical (PEC) water splitting provides a promising approach for solving sustainable energy challenges and achieving carbon neutrality goals. The oxygen evolution reaction (OER), a key bottleneck in the PEC water-splitting system occurring at the photoanode/electrolyte interface, plays a fundamental role in sustainable solar fuel production. Proper surface or interface engineering strategies have been proven to be necessary to achieve efficient and stable PEC water oxidation. This review summarizes the recent advances in interface engineering, including junction formation, surface doping, surface passivation or protection, surface sensitization, and OER cocatalyst modification, while highlighting the remarkable research achievements in the field of PEC water splitting. The benefits of each interface engineering strategy and how it enhances the device performance are critically analyzed and compared. Finally, the outlook for the development of interface engineering for efficient PEC water splitting is briefly discussed. This review illustrates the importance of employing rational interface engineering in realizing efficient and stable PEC water splitting devices.

Anchoring group regulation in semiconductor/molecular complex hybrid photoelectrode for photoelectrochemical water splitting

AbstractRational interface engineering via regulating the anchoring groups between molecular catalysts and light‐absorbing semiconductors is essential and emergent to stabilize the semiconductor/molecular complex interaction and facilitate the photocarriers transport, thus realizing highly active and stable photoelectrochemical (PEC) water splitting. In this mini review, following a showcasing of the fundamental details of hybrid PEC systems containing semiconductor photoelectrodes and molecular catalysts for water splitting, the state‐of‐the‐art progress of anchoring group regulation at semiconductor/molecular complex interface for efficient and stable PEC water splitting, as well as its effect on charge transfer kinetics, are comprehensively reviewed. Finally, potential research directions aimed at building high‐efficiency hybrid PEC water splitting systems are summarized.

Polymer Networks Assembled by Ruthenium Catalysts for Enhanced Water Splitting Performance in Calixarene Dye-Sensitized Photoelectrochemical Cells.

Metal-free photosensitizers for the construction of low-cost and eco-friendly dye-sensitized photoelectrochemical cells (DSPECs) have recently been greatly improved, but the optimization of water oxidation catalysts (WOCs) used in DSPECs based on metal-free dyes has received little attention. Herein, a series of polymer networks (RuTPA, RuCz, RuPr and RuTz) assembled by ruthenium WOCs (RuCHO) with various organic donors are constructed and combined with calixarene dyes to prepare DSPEC devices. The FTO|TiO2|C4BTP+RuTPA photoanode shows the best performance with 85 % Faraday efficiency for oxygen production and 477 μA cm-2 photocurrent density after 200 s chopping irradiation at 0 V vs. Ag/AgCl, one of the highest records among other reported dye-sensitized photoanodes. Compared to monomeric RuCHO, Ru-based polymers with lower Ru content have higher activity and stability due to their rapid electron transfer and anti-aggregation ability. Meanwhile, RuTPA loaded electrodes show better performance due to the lower overpotential of the water oxidation reaction caused by the higher electron donating ability of its donor. This pioneering work incorporates Ru polymer networks as WOCs into the calixarene-sensitized DSPEC system, which has significant potential as a highly efficient and stable photoelectrochemical water splitting device.

Nitrogen-doped CdS/TiO2 nanorods heterojunction photoanode for efficient and stable photoelectrochemical water splitting

Photoelectrochemical water splitting represents a promising route for converting solar energy into hydrogen, but sluggish reaction kinetics associated with inefficient charge separation and migration, and poor stability limit solar-to-hydrogen conversion. In this work, we develop a N-doped-CdS/TiO2-nanorods heterojunction photoanode for photoelectrochemical water splitting by anchoring CdS on TiO2 nanorods followed by nitrogen doping. The light harvesting is significantly enhanced and the charge separation and migration are promoted due to the formed heterojunction and nitrogen doping, which greatly enhances the water oxidation reaction. As a result, the photoelectrochemical cell with the optimized N-doped-CdS/TiO2-nanorods heterojunction photoanode yields a hydrogen production rate of 42.6 μmol cm−2 h−1, which is 2.51 times higher than that of the TiO2-nanorods photoanode. In particular, doping nitrogen atoms into CdS greatly alleviates the photocorrosion problem. Therefore, the newly-developed photoanode exhibits excellent stability under a continuous 10-h running.

Surface and Defect Modulation via Flame Technique for Efficient Photoelectrochemical Water-Splitting

The conversion of solar energy into chemical fuel via photoelectrochemical (PEC) water splitting shows great promise for solving the energy crisis, achieving carbon neutrality, and a sustainable future. Iron (Fe)-based materials have attracted tremendous attention from various light absorber materials due to their earth abundance, high light absorption, and robust PEC stability under harsh conditions[1]. However, diverse shortcomings, such as the short hole diffusion length, severe bulk/interface recombination, and sluggish water oxidation kinetics, limit the photocurrent generation for achieving high solar-to-hydrogen efficiency[2,3]. In particular, the high-temperature annealing requirement necessitates suitable synthesis and activation strategies to awaken their PEC activity.In this talk, we present a novel high-temperature flame technique to activate two Fe-based model photoanodes, i.e., zinc ferrite (ZnFe2O4, ZFO) and hematite (Fe2O3). The flame-activated ZFO (just a few tens of seconds) exhibits an enhanced charge collection ability and enriched active sites compared to the non-activated ZFO. These improvements are attributed to the high-temperature induced grain growth, enhanced grain connection, and rich defect formation caused by the fuel-rich flame environment[4]. On the other hand, a unique textured and mesoporous hematite (tm-hematite) photoanode can be synthesized via the same flame technique combining solution chemistry. The resultant tm-hematite exhibited enlarged surface area, enhanced charge transport, and transfer properties. We show that the unique mesoporous morphology, dominated (110) plane, and rich Ti dopant concentration on grain surface are responsible for the enhanced PEC activity. Notably, the tm-hematite showed greatly enhanced photocurrent density (3.1 mA/cm2 at 1.23 V vs the reversible hydrogen electrode, RHE) compared to the conventional nanorod hematite (~1.4 mA/cm2 at 1.23 VRHE) under 1 sun illumination. The flame-activated ferrite photoanodes demonstrated excellent PEC stability for 50 hours, even without additional coating/treatments. Our flame technique shows extraordinary potential for synthesizing high-efficiency ferrite-based photoelectrodes and puts a step forward to achieving the goal of sustainable energy and environment future. Keywords: Photoelectrochemical water splitting, Ferrites, Flame treatment, defect control, mesoporous structure, charge collection, photocurrent density, stability References [1] R. Tan, A. Sivanantham, B. J. Rani, Y. J. Jeong, I. S. Cho, Coord. Chem. Rev. 2023, 494, 215362.[2] I. S. Cho, M. Logar, C. H. Lee, L. Cai, F. B. Prinz, X. Zheng, Nano Lett. 2014, 14, 24.[3] I. S. Cho, H. S. Han, M. Logar, J. Park, X. Zheng, Adv. Energy Mater. 2016, 6, 1501840.[4] R. Tan, Y. J. Jeong, Q. Li, M. Kang, I. S. Cho, J. Adv. Ceram. 2023, 12(3): 612-624. Figure 1

Interfacial electric field of CdIn2S4/CuS heterostructure induced S-scheme charge transfer for efficient photoelectrochemical water splitting

Currently, the two predominant heterojunction types, namely type II and S-scheme heterojunctions, are extensively employed in photoelectrochemical (PEC) water splitting for hydrogen production. However, type II heterojunctions suffer from the drawback of diminished oxidation-reduction capability. In this work, S-scheme CdIn2S4/CuS (CIS/CuS) heterojunction nanoblocks (NBs) photoanode is constructed for the first time by a hydrothermal and the successive ionic layer adsorption and reaction (SILAR) methods. The heterojunction achieves a photocurrent density of up to 4.0 mA/cm2 at 0 V vs. Ag/AgCl, 2.37 times that of the pure CIS NBs photoanode. X-ray photoelectron spectroscopy (XPS), Tauc plots, Mott-Schottky (M − S) tests and density functional theory (DFT) calculations reveal the formation of S-scheme band structure between the interface of CIS and CuS. As a result, the significantly enhanced PEC water splitting performance of CIS/CuS NBs photoanode is attributed to the superior light absorption ability of CuS and the effective charge separation achieved by S-scheme band structure between the interface of CIS and CuS. This work also provides new ideas and strategies for designing efficient S-scheme heterostructures for efficient PEC water splitting.

AlOOH nanosheets modified Ti-Fe2O3 with oxygen vacancies for highly efficient photoelectrochemical water splitting

Effective co-catalysts are demanded to unveil the complete photoelectrochemical efficacy of the Ti-Fe2O3 photoanode for water splitting. This work introduces nanosheet-structured AlOOH (AlOOH-ns) on Ti-Fe2O3 by in situ synthesis for utilization as a co-catalyst. The abundant electrochemical active surface area of AlOOH-ns guarantees efficient hole transfer, demonstrating superior performance compared to amorphous AlOOH. Additionally, AlOOH-ns is insensitive to N2 treatment, enabling it to serve as a confining layer to bury the oxygen vacancies in Ti-Fe2O3 induced by N2 treatment within the bulk of AlOOH-ns/Ti-Fe2O3, thereby resulting in a high carrier density, facilitated bulk hole transport and separation and improved stability. Eventually, with a transition metal-centered co-catalyst, namely NiFeB hydroxide (NiFeB-hydx) to co-modify the photoanode, the NiFeB-hydx/AlOOH-ns/Ti-Fe2O3(N2) exhibits a brilliant photocurrent density of 3.41 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (VRHE) with a low onset potential of 0.65 VRHE and an applied bias photon-to-current efficiency of 0.44 % at 0.94 VRHE.

Exploring Ultrafast Carrier Dynamics in Photoelectrochemical Water Splitting Using Transient Absorption Spectroscopy

AbstractHydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency.

In-situ synthesis of Fe phosphate layer on the porous hematite nanocube for efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) is considered as an ideal photoanode material in photoelectrochemical (PEC) water splitting system due to its suitable band gap, excellent stability and abundant reserves. However, the actual PEC water oxidation performance of α-Fe2O3 is greatly limited by its severe charge recombination and sluggish water oxidation kinetics. Here, an in-situ generated Fe phosphate (Fe-Pi) coating strategy is applied on the surface of porous hematite nanocube (Fe2O3 NC) to realize PEC water oxidation effectively. The obtained Fe-Pi/Fe2O3 NC photoanode exhibits a remarkable PEC property with a photocurrent density of 2.7 mA cm−2 at 1.23 V vs. RHE, which was about 5.7-fold enlarged compared to the pristine nanorod-like hematite (Fe2O3 NR) photoanode. Detailed studies have shown that the porous Fe2O3 NC possesses an enhanced ability in light absorption and charge separation than the traditional Fe2O3 NR photoanode. In addition, the in-situ loading of Fe-Pi layer as a co-catalyst can not only effectively passivate the surface trapping states of Fe2O3 NC and suppress interfacial charge recombination, but also quickly extract the holes to participate in the water oxidation reaction. Our study suggests that the further surface modification based on morphological engineering is an effective strategy to improve the PEC performance of hematite.

Co-optimization of CuBi2O4 photocathode by heterojunction and hole-selective layer for efficient photoelectrochemical water splitting

Co-optimization of CuBi2O4 photocathode by heterojunction and hole-selective layer for efficient photoelectrochemical water splitting

P–N homojunction and heteroatom active site engineering over Fe2O3 nanorods for highly efficient photoelectrochemical water splitting

Limited charge transfer and slow oxygen evolution (OER) kinetics significantly impede the practical realization of photoanodes for photoelectrochemical (PEC) water splitting. Here, pn-type-Fe2O3 homojunction photoanode catalysts with P active sites are designed by doping phosphorus (P) into outer lattice of n-type Fe2O3 nanorods (pn-P-Fe2O3). The optimized pn-P-Fe2O3 photoanode shows the maximum photocurrent density of 2.61 mA/cm2 at 1.23 VRHE, and the value is 6.4 times greater than that of pristine Fe2O3. Experimental and theoretical results clearly show that the P–N homogeneous junctions constructed in Fe2O3 through P-doping increase active sites for H2O adsorption and activation, reduce OER reaction energy barrier, and promote effective separation of photogenerated electron-hole pairs and water splitting kinetics. This not only makes the photoelectric water decomposition performance outstanding, but also produces excellent durability. This work provides a novel simple and environmentally friendly strategy for designing effective photoanodes for PEC water splitting.

Enhanced Photoelectrochemical Water Splitting Using NiMoO4/BiVO4/Sn-Doped WO3 Double Heterojunction Photoanodes.

Efficient photoelectrochemical (PEC) water splitting systems in photoelectrodes are primarily challenged by electron-hole pair recombination. Constructing a heterostructure is an effective strategy to overcome this issue and to enhance PEC efficiency. In this study, we integrated NiMoO4, known for its proper electrocatalytic conductivity, into a BiVO4/Sn-doped WO3 heterojunction using solution-based hydrothermal and spin-coating methods, forming an innovative double heterojunction concept. The resulting NiMoO4/BiVO4/Sn:WO3 triple-layer heterojunction photoanode exhibits a photocurrent density of 2.06 mA cm-2 in a potassium borate buffer (KBi) electrolyte at 1.23 V vs RHE, outperforming the bilayer BiVO4/Sn:WO3 heterojunction (1.45 mA cm-2) and Sn:WO3 photoanodes (0.55 mA cm-2) by approximately 1.4 and 3.7 times, respectively. Remarkably, the NiMoO4/BiVO4/Sn:WO3 double heterojunction photoanode exhibits notable stability, showing only an approximate 30% reduction in initial photocurrent density after 10 h of measurement in the KBi electrolyte without a hole scavenger. This stability is attributed to the excellent corrosion resistance of the thin NiMoO4 layer, effectively protecting the bilayer BiVO4/Sn:WO3 heterojunction photoanode from photocorrosion. Our findings show how this novel double heterojunction, established through simple and cost-effective solution-based methods, offers a promising approach to enhancing PEC water splitting applications.

Optimization of carrier transfer kinetics of BiOIO3 using TFA·/TFA- reversible redox pairs of TFA molecules as co-catalysts for efficient photoelectrochemical water splitting system

Co-catalyst loading on photoelectrodes is considered to be an effective way to accelerate charge transfer. However, the bonding between most of the co-catalysts and photoelectrodes tends to be weak, which severely limits the role of co-catalysts in the catalytic system. Based on the above problems, we introduced trifluoroacetic acid (TFA) as a homogeneous molecular co-catalyst in the layered BiOIO3 catalytic system for the first time. The BiOIO3-TFA photoelectrode exhibited excellent photoelectrochemical performance under light and applied bias voltage. The photocurrent density reached up to 0.228 μA cm−2, which was 2.11 times higher than that before modification, while the overpotential was negatively shifted by 183 mV. The results showed that the photogenerated electron-hole pairs in BiOIO3 are excited and transferred to the TFA causing the CO double bond to break to form a TFA radical (TFA·), which is then reduced to TFA-. This cyclic redox reaction accompanied by fast hole transfer can greatly reduce the recombination behavior during carrier transport. This work demonstrates a new form of co-catalyst for photoelectrochemical water splitting. Highly water-soluble molecules and TFA·/TFA- reversible reactions have considerable potential for the development of new co-catalysts. It also provides ideas for the establishment of new, efficient catalytic systems for inhibiting carrier recombination.

PCDA/ZnO Organic-Inorganic Hybrid Photoanode for Efficient Photoelectrochemical Solar Water Splitting.

In this study, we developed well-aligned ZnO nanoflowers coated with poly-10,12-pentacosadiyonic acid (p-PCDA@ZnO) and modified with Pt nanoparticle (Pt/p-PCDA@ZnO) hybrid photoanodes for highly efficient photoelectrochemical (PEC) water splitting. The scanning electron microscope (SEM) image shows that thin films of the p-PCDA layer were well coated on the ZnO nanoflowers and that Pt nanoparticles were on it. The photoelectrochemical characterizations were made under simulated solar irradiation AM 1.5. The current density of the p-PCDA@ZnO and the Pt/p- PCDA@ZnO was 0.227 mA/cm2 and 0.305 mA/cm2, respectively, and these values were three times and four times higher compared to the 0.071 mA/cm2 of the bare ZnO nanoflowers. The UV-visible spectrum showed that the absorbance of coated p-PCDA films was extended in visible light region, which agrees with the enhanced PEC data for p-PCDA@ZnO. Also, adding Pt nanoparticles on top of the films as co-catalysts enhanced the PEC performance of Pt/p-PCDA@ZnO further. This indicates that Pt/p- PCDA@ZnO has a great potential to be implemented in solar water splitting.

Creating electron traps in BiOBr nanosheet arrays by bulk-phase F doping to restrain carrier recombination for efficient photoelectrochemical water splitting system

Bismuth oxide semiconductors in the field of photoelectrochemical (PEC) water splitting face the problems of narrow photo response range and fast carrier compounding. On the basis of the above problems, we used a solvothermal method to prepare BiOBr nanosheet arrays (NSAs), which have a special structure that facilitates carrier transport. Subsequently, the F element was introduced into the bulk phase of BiOBr using a simple hydrothermal method. The results show that the BiOBr-F photoelectrode exhibits excellent photoelectrocatalysis performance under light and bias voltage. The photocurrent density of the BiOBr-F photoelectrode reached 28.35 μA cm−2 at 1.23 V vs. RHE, which is 2.01 times higher than that of the pure BiOBr, while a negative shift of the onset potential of 119 mV was obtained. The optical absorption tests show that the introduction of the F element forms an impurity energy level thereby extending the optical absorption range of BiOBr. Photoluminescence tests reveal that the introduction of F element effectively restricts the electron-hole pair recombination. This is due tothe fact that the F atoms replace some of the O atoms in BiOBr to create electron traps, thus forming a large number of electron-rich regions in the structure, which in turn reduces the electron-hole pair recombination. This work provides an effective solution to reduce the recombination between carriers by creating electron traps in semiconductor materials through bulk phase doping.

The CuS-coated CuO nanostructures grown by ion exchange reaction for efficient photoelectrochemical water splitting

CuO is a promising solar photocathode for hydrogen production. However, it suffers serious photocorrosion in the electrolyte, which hinders the efficiency of photoelectrochemical (PEC) water splitting. In the study, Cu(OH)2 nanowires were in situ grown on copper foils through a straightforward wet chemical oxidation process, followed by annealing in air to synthesize CuO photocathode. To enhance their performance, the surface of CuO photocathode was modified by loading CuS with the ion exchange reaction (IER). Under AM 1.5 G, the CuO photocathode modified by CuS showed a significant improvement in the photo-current density, achieving the highest photo-current density was −4.83 mA·cm−2 at 0 V vs. RHE, which was far superior to the photo-current density (-2.02 mA·cm−2) of pristine CuO photocathode at the same potential. Furthermore, the maximum applied bias photon-to-current efficiency (ABPE) of the CuO/CuS photocathode is 0.77 %, which is superior to that of pristine CuO (0.24 %). The enhanced PEC performance of CuO/CuS photocathode is attributed to the improved light absorption and the efficient separation of photogenerated electron-hole pairs.

Engineering Facet‐Dependent Interface Charge Transfer in Liquid Metal‐Embraced BiVO4 Photoelectrodes for Efficient Photoelectrochemical Water Splitting

AbstractCrystal facet‐dependent photogenerated charge transfer at the semiconductor/current collector interface of photoelectrodes is equally important compared to that at the semiconductor/liquid interface for efficient photoelectrochemical (PEC) water splitting, which, however, is difficult to explore due to the rigid solid/solid interface in conventional photoelectrodes. Here, the facet‐dependent charge transfer at both semiconductor/liquid and semiconductor/collector interfaces in liquid metal‐embraced photoelectrodes is systematically investigated using faceted BiVO4 micro‐particles with different ratios of {110} and {010} facets as model materials. The results from the photo(electro)chemical and photophysical characterizations reveal that {010} facets outperform {110} facets at the semiconductor/liquid interface in triggering water oxidation due to the lower valence band maximum (VBM) of {010} facets, while {110} facets are more efficient at the semiconductor/metal interface for the collection of photogenerated electrons due to their higher conduction band minimum (CBM). Consequently, the photoelectrodes of liquid metal‐embraced BiVO4 particles exposing 53% {110} facets yield the best PEC performance for water splitting due to the well‐balanced photogenerated charge transfer at the interfaces of semiconductor/metal and semiconductor/liquid.