Hematite Photoanodes For Water Splitting Research Articles

High-throughput parallel testing of ten photoelectrochemical cells for water splitting: case study on the effects of temperature in hematite photoanodes.

High-throughput testing of photoelectrochemical cells and materials under well-defined operating conditions can accelerate the discovery of new semiconducting materials, the characterization of the phenomena occurring at the semiconductor-electrolyte interface, or the understanding of the coupled multi-physics transport phenomena of a complete working cell. However, there have been few high-throughput systems capable of dealing with complete cells and applying variations in real-life operating conditions, like temperature or irradiance. Understanding the effects of the variations of these real-life operating conditions on the performance of photoelectrode materials requires reliable and reproducible measurements. In this work, we report on a setup that simultaneously tests ten individual, identical photoelectrochemical cells whilst controlling temperature. The effects of temperature from 26 to 65 °C were studied in tin-doped hematite photoanodes for water splitting - as a reference case - through cyclic voltammetry and electrochemical impedance spectroscopy. The increase of surface-state-mediated charge recombination with temperature mainly penalized the energy conversion efficiency due to the reduction of the photovoltage produced. For parallel measurements in the ten individual cells, standard deviations from 20 to 60 mV for the onset potentials and less than 0.2 mA cm-2 for saturation current densities quantified the reproducibility of the results.

From doping to composites: zirconia (ZrO2) modified hematite photoanodes for water splitting.

Herein, a ZrO2 added α-Fe2O3 photoanode that can split water at low applied potential is reported. First, the pristine hematite α-Fe2O3 photoanode was synthesized using an aerosol-assisted chemical vapour deposition (AACVD) method followed by modification with various amounts of ZrO2 (2 to 40%) in the form of thin films on conducting glass substrate. The XRD, Raman spectroscopy and scanning electron microscopy (SEM) analyses confirmed the presence of the monoclinic phase of ZrO2 in the composites with multifaceted particles of compact morphology. The optical analysis showed an increase in the absorbance and variation in band gap of the composites ascribed to the heterogeneity of the material. The photoelectrochemical studies gave a photocurrent density of 1.23 mA cm-2 at 1.23 V vs. RHE for the pristine hematite and remarkably higher value of 3.06 mA cm-2 for the optimized amount of ZrO2 in the modified α-Fe2O3 photoanode. To the best of our knowledge, this is the highest photocurrent reported for a ZrO2 containing photoanode. The optimized composite electrode produced nine times more oxygen than that produced by pristine hematite.

Recent strategies to enhance the efficiency of hematite photoanodes in photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is one of the most promising approaches toward achieving the conversion of solar energy to hydrogen. Hematite is a widely applied photoanode material in PEC water splitting because of its appropriate band structure, non-toxicity, high stability, and low cost. Nevertheless, its relatively low photochemical conversion efficiency limits its application, and enhancing its PEC water splitting efficiency remains a challenge. Consequently, increasing efforts have been rendered toward improving the performance of hematite photoanodes. The entire PEC water splitting efficiency typically includes three parts: the photon absorption efficiency, the separation efficiency of the semiconductor bulk, and the surface injection efficiency. This review briefly discusses the recent advances in studies on hematite photoanodes for water splitting, and through the enhancement of the three above-mentioned efficiencies, the corresponding strategies toward improving the PEC performance of hematite are comprehensively discussed and summarized.

Electron collection in host-guest nanostructured hematite photoanodes for water splitting: the influence of scaffold doping density.

Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host-guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host-guest counterparts, and a quantitative evaluation of the benefits of the host-guest architecture is still lacking. In this paper, we examine the impact of host-guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO2 scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host-guest electrodes based on an undoped SnO2 scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO2, showing that the scaffold conductivity is crucial for the device performance.

The colloidal nanocrystal deposition process: an advanced method to prepare high performance hematite photoanodes for water splitting

The association of colloidal deposition of magnetorheological fluid in the presence of an external magnetic field with a sintering process facilitates the attainment of hematite photoanodes with high performance for water splitting.

Design of solar cell materials via soft X-ray spectroscopy

This overview illustrates how spectroscopy with soft X-rays can assist the development of new materials and new designs for solar cells. The starting point is the general layout of a solar cell, which consists of a light absorber sandwiched between an electron donor and an electron acceptor. There are four relevant energy levels that can be measured with a combination of X-ray absorption spectroscopy and photoelectron spectroscopy, as illustrated for an organic dye as absorber attached to a p-doped diamond film as donor. Systematic measurements of organometallic dyes (phthalocyanines and porphyrins) as a function of the metal atom are presented for the metal 2p and N 1s absorption edges. In combination with density functional theory one can discern trends that are useful for tailoring absorber molecules. A customized porphyrin molecule is investigated that combines an absorber with a donor and a linker to an oxide acceptor. The bridge to device fabrication is crossed by correlating spectroscopic features with the photocurrent in hematite photoanodes for water splitting. For speeding up the development of new materials and designs of solar cells a feedback loop between spectroscopy, theory, synthesis and device fabrication is envisioned.

A facile strategy to passivate surface states on the undoped hematite photoanode for water splitting

A facile strategy was found to passivate surface states on the undoped hematite photoanode by scanning the photocurrent-time curves in saturated NaCl aqueous electrolyte. Incident photon to current efficiency (IPCE) of the hematite photoanode was increased by a factor of 2.6 (from 5% to 13%) at 365nm and 1.23 VRHE (reversible hydrogen electrode) after this pretreatment. This enhancement was ascribed to the surface states passivation, which diminishes the recombination centers of the photogenerated carriers. The surface states passivation was confirmed by current transients and photoluminescence (PL) spectra.