The application of perovskite materials in solar water splitting

Solar water splitting is a promising strategy for sustainable production of renewable hydrogen,and solving the crisis of energy and environment in the world.However,large-scale application of this method is hampered by the efficiency and the expense of the solar water splitting systems.Searching for non-toxic,low-cost,efficient and stable photocatalysts is an important way for solar water splitting.Due to the simplicity of structure and the flexibility of composition,perovskite based photocatalysts have recently attracted widespread attention for application in solar water splitting.In this review,the recent developments of perovskite based photocatalysts for water splitting are summarized.An introduction including the structures and properties of perovskite materials,and the fundamentals of solar water splitting is first provided.Then,it specifically focuses on the strategies for designing and modulating perovskite materials to improve their photocatalytic performance for solar water splitting.The current challenges and perspectives of perovskite materials in solar water splitting are also reviewed.The aim of this review is to summarize recent findings and developments of perovskite based photocatalysts and provide some useful guidance for the future research on the design and development of highly efficient perovskite based photocatalysts and the relevant systems for water splitting.

FeF_(x) and Fe_(2)ZrO_(5) Co-modified hematite for highly efficient solar water splitting

Hematite is an excellent catalyst for photoelectrochemical (PEC) water splitting but its performance has been highly limited by poor conductivity and high charge recombination.Here by a Zr-based treatment to create bulk Fe_(2)ZrO_(5) in hematite and a F-based treatment to form an ultrathin surface FeF_(x) layer,the charge transfer can be highly improved and the charge recombination can be significantly suppressed.As a result,the FeF_(x) /Zr-Fe_(2)O_(3) photoanode presents an enhanced PEC performance with a photocurrent density of 2.43 m A/cm^(2)at 1.23 V vs.RHE,which is around 3 times higher than that of the pristine Fe_(2)O_(3) .The FeF_(x) /Zr-Fe_(2)O_(3) photoanode also shows a low onset potential of 0.77 V vs.RHE (100 mV lower than the pristine hematite).The performance is much higher than that of the sample treated by Zr or F alone,suggesting the synergistic effect between bulk Fe_(2)ZrO_(5) and surface FeF_(x) .By coupling with the FeNiOOH co-catalyst,the final photoanode can achieve a high photocurrent density of 2.81 mA/cm^(2) at 1.23 V vs.RHE.The novel design of Zr and F co-modified hematite can be used as a promising way to prepare efficient catalysts for solar water splitting.

Spontaneous photoelectric field-enhancement effect prompts the low cost hierarchical growth of highly ordered heteronanostructures for solar water splitting

In this study, a potentially universal new strategy is reported for the large-scale, low-cost fabrication of visible-light-active highly ordered heteronanostructures based on the spontaneous photoelectric-field-enhancement effect inherent in pyramidal morphology. The hierarchical vertically oriented arrayed structures comprise an active molecular co-catalyst at the apex of a visible-light-active large band gap semiconductor for low-cost solar water splitting in a neutral aqueous medium without the use of a sacrificial agent.

Photoelectrochemical evaluation of SILAR-deposited nanoporous BiVO4 photoanodes for solar-driven water splitting

We report a photoelectrochemical investigation of BiVO4 photoanodes prepared by successive ionic layer adsorption and reaction(SILAR),a facile method that yields uniform nanoporous films.After characterization of the phase,morphology,composition,and optical properties of the prepared films,the efficiencies of charge separation(ηsep)and water oxidation(ηox)in solar water splitting cells employing these photoanodes were estimated following a previously reported procedure.Unexpected wavelength and illumination direction dependencies were discovered in the derived efficiencies,casting doubt on the validity of the analysis.An alternative approach using a diffusion–reaction model that explicitly considers the efficiency of electron collection resolved the discrepancies and explained the illumination direction dependence of the photocurrent.Electron diffusion lengths(Ln)of 0.45μm and 0.55μm were derived for pristine and cobalt phosphate(Co-Pi)modified BiVO4,respectively,which are much shorter than the film thickness of^2.1μm.The Co-Pi treatment also increasedηoxfrom 0.86 to^1,which is the main reason for the overall performance enhancement caused by adding Co-Pi.These findings suggest that there is little scope for improving the performance of SILAR-deposited BiVO4 photoanodes by further catalyzing water oxidation,but enhanced performance is achievable if electron transport can be improved.

Plasmon-enhanced solar water splitting with metal oxide nanostructures： A brief overview of recent trends

In the last decade, the surface plasmon resonance-enhanced solar water splitting （SWS） has been actively investigated for improved hydrogen production. In this mini-review, we briefly introduce the mechanisms for plasmon-enhanced SWS and then review some representative studies related to these mechanisms. In addition, we also briefly discuss how metal oxide geometry affects the SWS activity in combined metalsemiconductor nanostructures. Finally, we summarize the recent discoveries and proposed a future vision for plasmon-enhanced SWS with metal oxide nanostructures.

Polymeric viologen-based electron transfer mediator for improving the photoelectrochemical water splitting on Sb_(2)Se_(3) photocathode

The photogenerated charge carrier separation and transportation of inside photocathodes can greatly influence the performance of photoelectrochemical(PEC)H2 production devices.Coupling TiO_(2) with p-type semiconductors to construct heterojunction structures is one of the most widely used strategies to facilitate charge separation and transportation.However,the band position of TiO_(2) could not perfectly match with all p-type semiconductors.Here,taking antimony selenide(Sb_(2)Se_(3))as an example,a rational strategy was developed by introducing a viologen electron transfer mediator(ETM)containing polymeric film(poly-1,1′-dially-[4,4′-bipyridine]-1,1′-diium,denoted as PV^(2+))at the interface between Sb_(2)Se_(3) and TiO_(2) to regulate the energy band alignment,which could inhibit the recombination of photogenerated charge carriers of interfaces.With Pt as a catalyst,the constructed Sb_(2)Se_(3)/PV^(2+)/TiO_(2)/Pt photocathode showed a superior PEC hydrogen generation activity with a photocurrent density of−18.6 mA cm^(-2) vs.a reversible hydrogen electrode(RHE)and a half-cell solar-to-hydrogen efficiency(HC-STH)of 1.54%at 0.17 V vs.RHE,which was much better than that of the related Sb_(2)Se_(3)/TiO_(2)/Pt photocathode without PV^(2+)(−9.8 mA cm^(-2),0.51%at 0.10 V vs.RHE).

Recent Advancements in Photoelectrochemical Water Splitting for Hydrogen Production

Sunlight is the most abundant and inexhaustible energy source on earth.However,its low energy density,dispersibility and intermittent nature make its direct utilization with industrial relevance challenging,suggesting that converting sunlight into chemical energy and storing it is a valuable measure to achieve global sustainable development.Carbon–neutral,clean and secondary pollution-free solar-driven water splitting to produce hydrogen is one of the most attractive avenues among all the current options and is expected to realize the transformation from dependence on fossil fuels to zero-pollution hydrogen.Artificial photosynthetic systems(APSs)based on photoelectrochemical(PEC)devices appear to be an ideal avenue to efficiently achieve solar-to-hydrogen conversion.In this review,we comprehensively highlight the recent developments in photocathodes,including architectures,semiconductor photoabsorbers and performance optimization strategies.In particular,frontier research cases of organic semiconductors,dye sensitization and surface grafted molecular catalysts applied to APSs based on frontier(molecular)orbital theory and semiconductor energy band theory are discussed.Moreover,research advances in typical photoelectrodes with the metal–insulator–semiconductor(MIS)architecture based on quantum tunnelling are also introduced.Finally,we discuss the benchmarks and protocols for designing integrated tandem photoelectrodes and PEC systems that conform to the solar spectrum to achieve high-efficiency and cost-effective solar-to-hydrogen conversion at an industrial scale in the near future.

Two-dimensional perovskite SrNbO_(2) N with Zr doping for accelerating photoelectrochemical water splitting

The superior ability of perovskite-type SrNbO_(2) N to absorb intensive visible light makes it a potential semiconductor to produce hydrogen and oxygen by photoelectrochemical(PEC)water splitting under sunlight.The surface morphologies,such as shape and structure,of the oxynitride strongly affect its photoactivity,although the mechanism has been hardly studied.Herein,we report a two-dimensional(2D)porous SrNbO_(2) N plate with Zr doping,nitrided from layered perovskite Sr_(5)Nb_(4)O_(15) and also its largely enhanced PEC water splitting activity.Zr^(4+)was doped in Sr_(5)Nb_(4)O_(15) during flux-assisted calcination using KCl,producing 2D-type truncated-octahedral Sr_(5)Nb_(4)O_(15):Zr plates approximately 50 nm in thickness.The nitridation completely transformed Sr_(5)Nb_(4)O_(15):Zr to 2D single-crystalline SrNbO_(2) N:Zr with a large surface area,which was subsequently used to fabricate a thin and uniform photoanode by the spin coating method.As a result,the Co(OH)_(x)/SrNbO_(2) N:Zr/FTO photoanode capable of absorbing visible light of up to 680 nm exhibited an activity of 2.0 mA cm^(-2) at 1.23 V vs the reversible hydrogen electrode for water splitting under AM 1.5G simulated sunlight.This improvement in photoactivity mainly originated from the 2D surface morphology of SrNbO_(2) N:Zr,which is clearly distinguishable from 3D-type oxynitrides.According to electrochemical analyses,the 2D structure of SrNbO_(2) N:Zr boosted the separation and accelerated the transfer of charges photogenerated during the water splitting,thus driving the reaction further.Therefore,the result empirically demonstrates that controlling the surface morphology of SrNbO_(2) N is an effective strategy to suppress the recombination of charges and minimize their diffusion pathway,eventually enhancing the PEC activity.

Unconventional strategies to break through the efficiency of light-driven water splitting:A review

Semiconductor-based solar-driven water splitting technology is an environmentally friendly and cost-effective approach for the production of clean fuels.The overall solar-to-hydrogen efficiency of semiconductorbased photo(electro)catalysts is jointly determined by factors,such as light absorption efficiency of the photo(electro)catalysts,internal separation efficiency of charge carriers,and injection efficiency of surface charges.However,the traditional improvement strategies,such as morphology control,functional layer modification,and band alignment engineering,still have certain limitations in enhancing the conversion efficiency of the photo(electro)catalytic water splitting.Recently,unconventional enhancement strategies based on surface plasmonic effects,piezoelectric effects,thermoelectric effects,and magnetic effects have provided unique pathways for improving the solar-to-hydrogen efficiency of photo(electro)catalysts.Therefore,this review outlines the fundamental concepts of these physical effects and elucidates their intrinsic mechanisms in enhancing the efficiency of photo(electro)catalysts for water splitting process through practical application examples.Ultimately,the future development of unconventional strategies for enhancing photo(electro)catalytic water splitting is envisioned.

Heterojunction between bimetallic metal-organic framework and TiO_(2):Band-structure engineering for effective photoelectrochemical water splitting

Bimetallic Fe/Ni-based metal-organic frameworks(MOFs)with different Fe/Ni ratios were coated on TiO_(2)nanorods(NRs),and the performances of the heterojunction photoanodes in photoelectrochemical water splitting were investigated.The bandgaps and band positions of the MOFs could be modulated by changing the ratio of the Fe and Ni components.An ideal band alignment was achieved between the TiO_(2)NRs and bimetallic MOFs with an optimum ratio of[Fe]/[Ni]=0.25/0.75,which allowed efficient light absorption and charge separation.The coating of NH_(2)-MIL(Fe)-88 layer on the TiO_(2)NRs decreased the photocurrent density by 33%.In comparison,TiO_(2)/NH_(2)-MIL(Ni)-88 showed a modest improvement in photocurrent density(0.85 mA·cm^(−2)at 1.23 V vs.a reversible hydrogen electrode(RHE)).When bimetallic NH_(2)-MIL(Fe_(0.25)Ni_(0.75))-88 was coated on the TiO_(2)NRs,the photocurrent density reached 1.56 mA·cm^(−2),which was an efficiency enhancement of 3.2 times.The mechanism underlying high photoelectrochemical performance was investigated.

In situ optical spectroscopic understanding of electrochemical passivation mechanism on sol–gel processed WO_(3) photoanodes

A prevailing understanding on electrochemical activation of photoelectrodes is that electrochemical treatment leads to increased charge carrier densities thereby improved photoelectrode performances.Contrary to this understanding,in this study enhanced photoactivity of WO_(3) photoanode upon electrochemical treatment is ascribed to an extraordinary mechanism of surface trap passivation.The associated mechanism is analyzed by in situ optical spectroscopy,using which the optical property changes of WO_(3) electrode during electrochemical treatment are monitored.The results suggest surface W^(5+)species,the origin of surface traps on WO_(3) photoanodes,are converted to W^(6+) ions by electrochemical treatment.This study demonstrates the particular ability of the electrochemical strategy to passivate surface traps of photoanodes,and also shows the advantages of in situ optical spectroscopy to investigate the real-time electronic structure variations of electrodes during electrochemical treatment.

Cu_(2)O作为空穴提取层修饰BiVO_(4)光阳极以增强电荷分离和转移

光生电荷的分离和转移被认为是影响BiVO_(4)基光阳极光电性能的核心因素之一.本文设计了在BiVO_(4)光阳极与析氧助催化剂之间插入空穴提取层的方法.Cu_(2)O作为空穴提取层引入到助催化剂层(FeOOH/NiOOH)和BiVO_(4)之间,可以有效优化空穴的迁移路径,延长光生空穴的寿命,从而提高电极的光电化学性能.与BiVO_(4)相比,调整后的BiVO_(4)/Cu_(2)O/FeOOH/NiOOH光阳极的电荷分离效率从70.6%提高到了92.0%.此外,该光阳极在1.23 VRHE(AM 1.5G照明下)下,还显示出了3.85 mA cm^(-2)的高光电流密度,是BiVO_(4)的2.77倍.我们的研究结果表明,电沉积Cu_(2)O空穴提取层是一种简单且可扩展的方法,能够有效提高BiVO_(4)的光电活性,可用于太阳能驱动水分解领域.