The efficiency of a nanostructure applied to photo‐electrocatalysis is fundamentally governed by the capability of the nanostructure surface to sustain a reaction without the occurrence of electron trapping or a recombination of the photogenerated holes. This Minireview summarizes the latest progress in the use of cocatalysts on hematite electrodes and their application in water splitting induced by sunlight. The major drawback of hole diffusion through a solid‐liquid interface is addressed through the selection of the best cocatalyst for increasing the efficiency of an iron‐based material. Finally, the most promising modification routes and outstanding materials for enhancing the low kinetics of the oxygen evolution reaction during sunlight‐driven water oxidation reactions in hematite are provided and those materials that may have a significant impact on the overall photoelectrochemical performance are discussed.

Solar water splitting-driven hydrogen fuel production is very attractive due to positive aspects as higher energy density and a renewable energy source arising from water and sunlight. 1 Despite these remarkable overall strengths, sustainable largescale clean hydrogen production is quite far from being available, in particular by the lack of efficient and economically viable electrodes for photoelectrochemical (PEC) cells. These devices involve two redox electrochemical reactions: hydrogen evolution on the cathode and oxygen evolution on the anode, where one or more photosensitive material can be enforced in the device design. 2 It is worth mentioning that oxygen evolution reaction (OER) is the rate-determining step of the photoelectrochemical water splitting, because of the four-electron reaction mechanism of water oxidation, which

Abstract

Nanomaterials can exhibit improved electrochemical performance in cathode applications, but their inherently high surface areas cause unconventional instability, leading to capacity fading after a limited number of battery cycles. This is because of their high surface reactivity, which makes them more susceptible to phenomena such as grain growth, sintering, solubilization, and phase transformations. Thermodynamically, these can be attributed to an increased contribution of interfacial enthalpies to the total free energy of the system. The lack of experimental data on the interfacial thermodynamics of lithium-based materials has hindered strategies to mitigate such degradation mechanisms. In this study, interfacial energies of LiMn2O4 nanoparticles were directly measured for the first time using calorimetry, and the possibility of thermodynamically manipulating both surface and grain boundary energies using a dopant (scandium) was explored. We show that undoped LiMn2O4 nanoparticles have a surface energy of 0.85 J/m2, which is significantly lower than that of LiCoO2. Moreover, introducing scandium further lowered the LiMn2O4 surface energy, leading to a demonstrated improved stability against coarsening and reactivity to water, which can potentially result in more stable cathode materials for battery applications.

We have produced Fe-doped paramagnetic MAPbI 3 microwires by using a novel strategy involving a self-assembly growth process of [PbI 6 ] 4− octahedral chains in the presence of liquid water. Structural and morphological studies confirmed that after the dissociation and recrystallization process, the doped samples preserved both the perovskite structure with a tetragonal phase and a microwire shape, while X-ray photoelectron spectroscopy revealed the presence of mixed-valence Fe 3+ /Fe 2+ ions with negligible change in the PbI 6 cage environment and the maximum valence band position. From first-principles calculations, we determined that Fe 2+ ions are localized in the interstitial site while Fe 3+ ones are substitutional on Pb sites. The very high mobility and static dielectric constant, achieved by photogenerated charge carriers in MAPbI 3 , are suppressed for Fe-doped MAPbI 3 samples. These results are discussed based on a nonradiative recombination process assisted by phonons that is activated by the inclusion of the Fe ions. Our ab initio calculations support this model that can be also used to explain the quenching of the photoluminescence emission peaks. The successful insertion of dopants that can tune the perovskite's physical properties is important to the development of functional devices and is also able to open new potential applications such as in magnetic/semiconducting devices.

This work addresses the role of different modifiers on the overall photocurrent response, which allowed a dual material insertion, increasing the charge separation without compromise the surface catalysis. Sn-addition onto nanoceramic hematite photoelectrodes clearly increased flat band potential, promoting a good charge separation, and shifting the onset to a higher potential, attributed to the surface-trapping state created by this modification. Notoriously, Sn-hematite photoelectrodes loaded with NiFeO x exhibited the highest photocurrent density, suggesting a partially recovered surface-trapping states created during the electrode designing. The well-known cocatalyst acted in the overall photoelectrocatalytic response with no significant effect on the turn-on voltage, in other words, with minor effect related to catalytic efficiency. The dual modification contributes to understand the role of different modifiers allowing to satisfactorily improve charge separation while maintaining the conductivity attributed to IV-group ions.

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