Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

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A review on fundamentals for designing oxygen evolution electrocatalysts

Titanium dioxide (TiO2) has been the most intensively investigated binary transition metal oxide in the past four decades as indicated by Figure S1. Furthermore, the annual number of papers published on TiO2 has seen a continuous increase, particularly since the beginning of this century (Figure S2). This is understandable when one considers the wide range of applications of TiO2 from the conventional areas (i.e., pigment, cosmetic, toothpaste, and paint) to the later developed functional areas such as photoelectrochemical cell,(1-3) dye-sensitized solar cells (DSSCs),(4-11) photocatalysis,(12-24) catalysis,(25-31) photovoltaic cell,(32-34) lithium ion batteries,(35-41) sensors,(42-46) electron field emission,(47-51) microwave absorbing material, biomimetic growth, and biomedical treatments.(52-57) Nearly all these functional applications of TiO2 fall in the scope of energy, environment, and health, which are definitely the three most important and challenging themes facing the Human race that need to be addressed in this century. Besides the apparent merits including nontoxicity, elemental abundance, good chemical stability, and easy synthesis, TiO2 has attracted strong research interest worldwide due to its physicochemical properties for realizing various functions.(15, 58, 59) Especially, very encouraging progresses in photocatalysis and DSSCs with the involvement of TiO2 have greatly stimulated the rapid development of TiO2 crystals with controllable phase, size, shape, defect, and heteroatom.(58, 60-68)

Titanium dioxide crystals with tailored facets

Metal–organic framework-derived porous materials for catalysis

Defect engineering in photocatalytic materials

Defects are critically important for metal oxides in chemical and physical applications. Compared with the often studied oxygen vacancies, engineering metal vacancies in n-type undoped metal oxides is still a great challenge, and the effect of metal vacancies on the physiochemical properties is seldom reported. Here, using anatase TiO2, the most important and widely studied semiconductor, we demonstrate that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties. Anatase Ti0.905O2 was synthesized using solvothermal treatment of tetrabutyl titanate in an ethanol–glycerol mixture and then thermal calcination. Experimental measurements and DFT calculations on cell lattice parameters show the unstoichiometry is caused by the presence of VTi rather than oxygen interstitials. The presence of VTi changes the charge density and valence band edge of TiO2, and an unreported strong EPR signal at g = 1.998 presents under room temperatu...

Titanium-Defected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance

ZnO is an important semiconductor that has been widely applied in solar cell, photocatalysis, environmental remediation. Doping and morphology control are important approaches to improve its photocatalytic performance. Herein, a facile two-step calcination method was developed to fabricate carbon(C)-doped cubic ZnO with porous structure from zeolite imidazolate frameworks (ZIF-8). Compared with one-step pyrolysis, the approach of two-step calcination not only retains the cubic morphology with inter-connected ZnO nanoparticles and porous structure but also introduces C doping in ZnO lattice effectively. This morphology has advantage in charge transfer, optical absorption and mass transfer during the photoreaction, and C doping results in high charge-separation efficiency. The sample C350-400 (C-doped ZnO, firstly calcined at 350 °C for 2 h from ZIF-8, then 400 °C for 1 h) shows the maximum photoactivity, which is ca . 3-fold and 4-fold higher than ZnO (C450) in photodegradation and PEC water splitting (under UV–vis irradiation), respectively. It is expected that the preparation of metal oxide from MOF is a very promising way to fabricate highly efficient photocatalyst.

MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis

Constructing TiO2 p-n homojunction for photoelectrochemical and photocatalytic hydrogen generation

Photocatalysis has been regarded as a promising strategy for hydrogen production and high-value-added chemicals synthesis, in which the activity of photocatalyst depends significantly on their electronic structures, however the effect of electron spin polarization has been rarely considered. Here we report a controllable method to manipulate its electron spin polarization by tuning the concentration of Ti vacancies. The characterizations confirm the emergence of spatial spin polarization among Ti-defected TiO2, which promotes the efficiency of charge separation and surface reaction via the parallel alignment of electron spin orientation. Specifically, Ti0.936O2, possessing intensive spin polarization, performs 20-fold increased photocatalytic hydrogen evolution and 8-fold increased phenol photodegradation rates, compared with stoichiometric TiO2. Notably, we further observed the positive effect of external magnetic fields on photocatalytic activity of spin-polarized TiO2, attributed to the enhanced electron-spin parallel alignment. This work may create the opportunity for tailoring the spin-dependent electronic structures in metal oxides.

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Songbo Wang

Papers

Titanium-Defected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance

MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis

Constructing TiO2 p-n homojunction for photoelectrochemical and photocatalytic hydrogen generation

Manipulating spin polarization of titanium dioxide for efficient photocatalysis.

Undoped ZnO abundant with metal vacancies