A hematite photoanode showing a stable, record-breaking performance of 4.32 mA/cm2 photoelectrochemical water oxidation current at 1.23 V vs. RHE under simulated 1-sun (100 mW/cm2) irradiation is reported. This photocurrent corresponds to ca. 34% of the maximum theoretical limit expected for hematite with a band gap of 2.1 V. The photoanode produced stoichiometric hydrogen and oxygen gases in amounts close to the expected values from the photocurrent. The hematitle has a unique single-crystalline “wormlike” morphology produced by in-situ two-step annealing at 550°C and 800°C of β-FeOOH nanorods grown directly on a transparent conducting oxide glass via an all-solution method. In addition, it is modified by platinum doping to improve the charge transfer characteristics of hematite and an oxygen-evolving co-catalyst on the surface.

Although Li-ion batteries have emerged as the battery of choice for electric vehicles and large-scale smart grids, significant research efforts are devoted to identifying materials that offer higher energy density, longer cycle life, lower cost, and/or improved safety compared to those of conventional Li-ion batteries based on intercalation electrodes. By moving beyond intercalation chemistry, gravimetric capacities that are 2–5 times higher than that of conventional intercalation materials (e.g., LiCoO2 and graphite) can be achieved. The transition to higher-capacity electrode materials in commercial applications is complicated by several factors. This Review highlights the developments of electrode materials and characterization tools for rechargeable lithium-ion batteries, with a focus on the structural and electrochemical degradation mechanisms that plague these systems.

Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.

An ultrathin (ca. 2 nm) amorphous FeOOH overlayer was deposited conformally on a hematite nanostructure by a simple solution-based precipitation method, to generate an oxygen evolution cocatalyst for efficient solar water splitting. This uniform and highly conformal coating of the ultrathin metal oxyhydroxide is rare and is distinguished from the layers prepared by other conventional methods. With the FeOOH overlayer as the cocatalyst, the water oxidation photocurrent of hematite increased by a factor of approximately two and the onset potential shifted in the cathodic direction by 0.12 V under 1 sun illumination. The enhanced performance was attributed to the improved water oxidation kinetics and the passivation of the surface states of the hematite.

water splitting. So far, most studies employing hematite photoelectrodes were carried out in strongly basic conditions. [ 10,[14][15][16][17] It usually shows the poor performance (smaller water oxidation photocurrents and/or higher onset potential) and low stability in neutral electrolytes, although there is no systematic study on the effects of the electrolytes. In contrast, our modifi ed hematite photoanode showed almost the same performance in a neutral electrolyte at pH 7 as in a strongly basic electrolyte at pH 13.6. The modifi cation of the hematite photoanode was carried out by simply dipping the bare hematite thin fi lm in a solution containing phosphate ions and annealing it under mild conditions. The treatment led to the hematite thin fi lm being uniformly modifi ed with phosphate ions on its surface. The phosphate ions effectively prevent deterioration of performance in activity and stability in neutral electrolyte. There have been some reports of hematite photoanodes active for solar water splitting performed in neutral electrolytes, especially when the semiconductors are decorated by cobalt phosphate (Co-Pi) as an oxygen evolution reaction (OER) co-catalyst. [14][15][16][17][18][19] The Co-Pi works best in a phosphate buffer, yet the purpose and operating principle are distinguished from the present study as discussed later. Hence, this work represents the fi rst attempt at stability engineering by correlating surface electrostatic state of hematite and its performance and stability in electrolytes with different pH values.As described in the Experimental Section, the surface of the hematite photoanode was modifi ed by dipping the bare hematite thin fi lm in an aqueous sodium phosphate solution and annealing it under mild conditions. In the transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping images shown in Figure 1 , phosphorus is uniformly distributed over the hematite surface as the hematite surface was successfully coated with phosphate ions. It was also confi rmed by scanning electron microscopy (SEM) and SEM-energy dispersive X-ray spectroscopy (EDS; Figure S1 of the Supporting Information). The surface morphology does not change after the surface modifi cation with phosphate ions, as shown in the SEM images of Figure S1 (Supporting Information). According to the EDS results, phosphorus certainly exists on the hematite surface treated with a solution containing phosphate ions. On the other hand, the bare hematite surface showed no phosphorus on the surface.To identify the chemical state of the surface species, X-ray photoelectron spectroscopy (XPS) analyses were carried out for the bare and the phosphate ion-modifi ed hematite thin fi lm (Pi-Fe 2 O 3 ). Figure 2 A shows a Fe 2p 3/2 peak at 711.2 eV, a 2p 1/2 peak at 724.4 eV, and a 2p 3/2 satellite peak at 719 eV, corresponding to binding energies of Fe 3+ . These features show