Challenges for photocatalytic overall water splitting

Challenges and opportunities:•Photocatalytic overall water splitting is an intriguing process to produce hydrogen. However, the photocatalytic efficiency unsatisfactory. Thus, it necessary analyze challenges for splitting.•The unfavorable thermodynamics, sluggish kinetics, dissolved oxygen, backward reaction, side reaction make enormously difficult. These obstacles must be overcome achieve in splitting.•Coupling hydrogen production with selective organic synthesis can a more viable attractive option than splitting. The compounds converted into value-added oxidation products via single-electron processes, which are much easier four-electron oxygen evolution reaction. economy sunrise industry, considered ultimate solution power future society. Photocatalytic projected as potential technology H2 production. its performance still far from meeting criteria large-scale This paper argues that theoretically practically hard achieve. limiting factors, including slow rapid discussed. expected give readers better understanding of associated every subtle aspect. shortage fossil fuels their negative impacts on environment urges energy revolution renewable sources. Solar nonpolluting. solar has low density, high temporal/seasonal variability, poor portability. Therefore, photocatalysis applied convert sunlight storable chemical energy, such clean efficient energy. Apart nuclear fuels, combustion heat highest among all biofuels. In addition, burns without any pollution environment.1He B. Bie C. Fei X. Cheng Yu J. Ho W. Al-Ghamdi A.A. Wageh S. Enhancement activity CdS NRs by Ag2S NiS dual cocatalysts.Appl. Catal. 2021; 288: 119994Crossref Scopus (135) Google Scholar called source humanity.2Cheng He Fan Cao An inorganic/organic S-scheme heterojunction H2-production photocatalyst charge transfer mechanism.Adv. Mater. 33: e2100317Crossref PubMed (385) recent years, based been developing rapidly. Moreover, (OWS) pursued regarded Holy Grail.3Ryabchuk V.K. Kuznetsov V.N. Emeline A.V. Artem'ev Y.M. Kataeva G.V. Horikoshi Serpone N. Water will coal future-the untamed dream jules verne fuel.Molecules. 2016; 21: 1638Crossref (13) activities Most research works do not solar-to-hydrogen (STH) over 10%, competitive benchmark market.4Pinaud B.A. Benck J.D. Seitz L.C. Forman A.J. Chen Z. Deutsch T.G. James B.D. Baum K.N. G.N. Ardo et al.Technical economic feasibility centralized facilities photoelectrochemistry.Energy Environ. Sci. 2013; 6: 1983-2002Crossref (988) whether OWS best remains questionable. Here, evaluated terms development prospected. intrinsic nature determines photogenerated carriers undergo several processes. Figure 1 shows four main steps: (1) Photocatalyst absorbs photons light generate electron-hole pairs. (2) separate space. (3) annihilated within bulk photocatalysts or surface through Coulomb force captured traps defects. (4) surviving cocatalysts redox reactions. Only minority ultimately participate reactions, resulting extremely quantum yield. addition inherent properties photocatalysts, ambient also critical Specifically, three prerequisites essential production: removal, cocatalyst, sacrificial agent (SA).(1)Oxygen removal: reduced electrons superoxide radicals (O2·−) (φ(O2/O2·−) = −0.33 V versus normal electrode [NHE]) series competes (HER), requires proceed. Oxygen removal significantly suppress these reactions improve efficiency. It should noted remove thoroughly evacuation bubbling inert gas (e.g., N2) effective.(2)Cocatalyst: often exhibit very even no HER activity. effects mainly classified aspects: extracting holes reduce carrier recombination, storing facilitate multielectron providing active sites bind activate reactant molecules, reducing overpotential reactions.5Meng A. Zhang L. Dual TiO2 photocatalysis.Adv. 2019; 31: e1807660Crossref (664) Scholar,6Shinagawa T. Cavallo Takanabe K. Photophysics electrochemistry relevant involved at solid–electrolyte interfaces.J. Energy Chem. 2017; 26: 259-269Crossref (17) Scholar(3)SA: employment SA one most effective ways boost typical process, H+ H2, although consumed half-reaction [OER] OWS). particular, role provide facile route react holes, usually replace OER (Figure 2A ).7Antonietti M. Savateev Splitting artificial photosynthesis: excellent science but nightmare translation?.Chem. Rec. 2018; 18: 969-972Crossref (10) presents lower faster rate OER. Furthermore, SA-assisted avoid generation O2 OER, ensuring oxygen-free conditions throughout whole process. Finally, scavenging free effectively photocorrosion. proceeds pure water, 2B). Besides common problems production, this faces additional compared HER. thermodynamically feasible when both proton reduction potentials lie bandgap photocatalyst.8Bie Enhanced solar-to-chemical conversion graphitic carbon nitride two-dimensional cocatalysts.EnergyChem. 3: 100051Crossref (72) That is, conduction band (CB) bottom valence (VB) top positioned above below potential, respectively.9Lin Hisatomi Takata Domen Visible-light-driven splitting: progress challenges.J. Trends 2020; 2: 813-824Abstract Full Text PDF (82) protons 0 NHE (Equation 1), whereas 1.23 2).10Bie H. Design, fabrication, mechanism nitrogen-doped graphene-based photocatalyst.Adv. e2003521Crossref (223) inherently challenging due stability H2O containing strong O–H bonds.11Takanabe quantitative approaches toward design.ACS 7: 8006-8022Crossref (519) energetically uphill 3A ), taking 237 kJ split mol 3).12Zhang G. Wang Oxysulfide semiconductors visible light.Angew. Int. Ed. Engl. 58: 15580-15582Crossref (62) Contrarily, direct between highly exothermic 3B), favored OWS.2H+ + 2e− → Eredox V(Equation 1) 2H2O 4h+ 4H+ O2, 2) 1/2O2, ΔG +237kJ·mol−1(Equation 3) thermodynamic challenges, kinetics. photocatalysis, generation, separation, belong photophysical few femto- nano-seconds.13Kranz Wächtler Characterizing atoms ultrafast processes.Chem. Soc. Rev. 50: 1407-1437Crossref electrocatalytic (i.e., reactions), other hand, takes longer (micro- seconds) accomplish rate-limiting step. Particularly, stagnant steps oxidation. produces continually provides consumes holes. 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As result, molecules desorb surface. important reason Since soluble actually carried out aerobic environment. Henry’s law constants 0.78 1.3 mmol L−1 atm−1 25°C, respectively,18Sander R. Compilation Henry's constants, (version 4.0) solvent.Atmos. 15: 4399-4981Crossref (1657) indicating amount significant state-of-the-art Meanwhile, O2. system removed advance, ratio H2-evolution O2-evolution (the escaping water) early stage greater 2:1. includes dissolution 4A formation, growth, bubbles 4B). Firstly, molecular produced state, initially generated dissolves driven concentration gradient bonding H2O. With continuous increases. Once reaches saturation, dynamic equilibrium established. subsequently formed adsorb Due unstable tends aggregate nucleate. nuclei gradually grow small while adhering tightly under effect Ostwald ripening, large assimilate bubbles. Spherical shapes maintained during growth minimize contact area decreases, corresponding decrease adhesion force. When enough, buoyancy outweighs force, water. conclusion, only saturated enough they finally evolve (hydrogen undergoes similar process). Dissolved proceed rich coexisting tremendous loss readily achieved electrochemical processes.19Mei Han Mul Driving heterogeneous photocatalysis: state illuminated semiconductor-supported nanoparticles water-splitting.ACS 8: 9154-9164Crossref (60) cocatalyst promote instance, metallic Pt widely used catalyze acting electrocatalyst fuel cells), OWS. double-edged sword More importantly, continuously produced, concentrations increase, intensified occur forms where coexist 5).(1)The arrive (such Pt) each other.(2)The reacts adsorbed surface.(3)The surface.(4)When close come further causes space newly Such vicious cycle leads waste carriers. On presence interferes effortlessly combines form (O2·−),20Vequizo J.J.M. 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Shi Fu Wu Li Alzuabi al.Photocatalytic value Pt/porous nanoflutes.Nano Energy. 67: 104287Crossref (97) Scholar(4)The product mixture gases, gaseous products, automatically separated. Considering extra cost purification, profitable HER.(5)The observably improved replacing SA. way, boosts holds promise itself product. Coupling accessible work was supported National Key Research Development Program China (2018YFB1502001), Natural Science Foundation (51961135303, 51932007, U1905215, 21871217, U1705251), Postdoctoral Innovative Talents (BX2021275), project funded (2021TQ0310). Conceptualization, J.Y., C.B., L.W.; investigation, writing – original draft, C.B.; review & editing, L.W. J.Y.; funding acquisition, supervision, J.Y. authors declare competing interests.