This review focuses on introducing and explaining electrodepostion mechanisms and electrodeposition-based synthesis strategies used for the production of catalysts and semiconductor electrodes for use in water-splitting photoelectrochemical cells (PECs). It is composed of three main sections: electrochemical synthesis of hydrogen evolution catalysts, oxygen evolution catalysts, and semiconductor electrodes. The semiconductor section is divided into two parts: photoanodes and photocathodes. Photoanodes include n-type semiconductor electrodes that can perform water oxidation to O2 using photogenerated holes, while photocathodes include p-type semiconductor electrodes that can reduce water to H2 using photoexcited electrons. For each material type, deposition mechanisms were reviewed first followed by a brief discussion on its properties relevant to electrochemical and photoelectrochemical water splitting. Electrodeposition or electrochemical synthesis is an ideal method to produce individual components and integrated systems for PECs due to its various intrinsic advantages. This review will serve as a good resource or guideline for researchers who are currently utilizing electrochemical synthesis as well as for those who are interested in beginning to employ electrochemical synthesis for the construction of more efficient PECs.

2,5-Furandicarboxylic acid (FDCA) is a near-market monomer that has been identified as a viable biomass-derived replacement for petroleum-derived terephthalic acid in the synthesis of polyethylene terephthalate (PET). FDCA can be produced from the oxidation of 5-hydroxymethylfurfural (HMF), which is a versatile biomass intermediate produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass. In this study, we comparatively investigated the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-acetamido-TEMPO (ACT) for electrochemical HMF oxidation to FDCA. The distinct advantage of TEMPO- and ACT-mediated electrochemical oxidation of HMF is that they can efficiently achieve HMF oxidation in mildly basic conditions (pH 9–10), while other heterogeneous catalysts typically require the use of more basic media. Since HMF oxidation in a strongly basic condition increases the chance to form humins, which are difficult to separate from FDCA and decrease the commercial viability of FDCA and FDCA-derived products, TEMPO- and ACT-mediated HMF oxidation may offer a critical advantage for producing commercial-grade FDCA. In this study, the stabilities, electrochemical properties, and electrocatalytic performances of TEMPO and ACT, which has been identified as a less expensive alternative to TEMPO, were comparatively examined for electrochemical HMF oxidation. Through investigating the effect of pH, applied potential, and ratio of nitroxyl radical to HMF in solution on HMF oxidation, two different regeneration pathways of TEMPO and ACT in the catalytic cycle and the factors that affect their regeneration pathways were identified. The stability and catalytic activity of TEMPO and ACT for electrochemical HMF oxidation at an elevated temperature were also studied. On the basis of this investigation, optimal electrochemical conditions to efficiently oxidize a concentrated HMF solution (100 mM), which is relevant to large-scale electrochemical FDCA production, were identified.

New electrochemical synthesis methods were developed to produce copper hydroxy double salt(Cu-HDS) films with four different intercalated anions (NO, SO, Cl, and dodecyl sulfate (DS)) as pure crystalline films as deposited (CuNO(OH), CuSO(OH), CuCl(OH), and CuDS(OH)). These methods are based on p-benzoquinone reduction, which increases the local pH at the working electrode and triggers the precipitation of Cu and appropriate anions as Cu-HDS films on the working electrode. The resulting Cu-HDS films could be converted to crystalline Cu(OH) and CuO films by immersing them in basic solutions. Because Cu-HDS films were composed of 2D crystals as a result of the atomic-level layered structure of HDS, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures, creating high surface areas that cannot be obtained by direct deposition of CuO, which has a 3D atomic-level crystal structure. The resulting nanostructures allowed the CuO films to facilitate electron-hole separation and demonstrate great promise for photocurrent generation when investigated as a photocathode for a water-splitting photoelectrochemical cell. Electrochemical synthesis of Cu-HDS films and their facile conversion to CuO films will provide new routes to tune the morphologies and properties of the CuO electrodes that may not be possible by other synthesis means.

Metal oxides play a key role in many emerging applications in renewable energy, such as dye-sensitized solar cells and photocatalysts. Because the separation of charge can often be facilitated at junctions between different materials, there is great interest in the formation of heterojunctions between metal oxides. Here, we demonstrate use of the copper-catalyzed azide-alkyne cycloaddition reaction, widely referred to as "click" chemistry, to chemically assemble photoactive heterojunctions between metal oxide nanoparticles, using WO(3) and TiO(2) as a model system. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy verify the nature and selectivity of the chemical linkages, while scanning electron microscopy reveals that the TiO(2) nanoparticles form a high-density, conformal coating on the larger WO(3) nanoparticles. Time-resolved surface photoresponse measurements show that the resulting dyadic structures support photoactivated charge transfer, while measurements of the photocatalytic degradation of methylene blue show that chemical grafting of TiO(2) nanoparticles to WO(3) increases the photocatalytic activity compared with the bare WO(3) film.

The formation of a "spin polaron" stems from strong spin-charge-lattice interactions in magnetic oxides, which leads to a localization of carriers accompanied by local magnetic polarization and lattice distortion. For example, cupric oxide (CuO), which is a promising photocathode material and shares important similarities with high T c superconductors, conducts holes through spin polaron hopping with flipped spins at Cu atoms where a spin polaron has formed. The formation of these spin polarons results in an activated hopping conduction process where the carriers must not only overcome strong electron−phonon coupling but also strong magnetic coupling. Collectively, these effects cause low carrier conduction in CuO and hinder its applications. To overcome this fundamental limitation, we demonstrate from first-principles calculations how doping can improve hopping conduction through simultaneous improvement of hole concentration and hopping mobility in magnetic oxides such as CuO. Specifically, using Li doping as an example, we show that Li has a low ionization energy that improves hole concentration, and lowers the hopping barrier through both the electron−phonon and magnetic couplings' reduction that improves hopping mobility. Finally, this improved conduction predicted by theory is validated through the synthesis of Li-doped CuO electrodes which show enhanced photocurrent compared to pristine CuO electrodes. We conclude that doping with nonmagnetic shallow impurities is an effective strategy to improve hopping conductivities in magnetic oxides.