Search

Photoelectrochemical (PEC) water splitting is a means to store solar energy in the form of hydrogen. Knowledge of practical limits for this process can help researchers assess their technology and guide future directions. We develop a model to quantify loss mechanisms in PEC water splitting based on the current state of materials research and calculate maximum solar-to-hydrogen (STH) conversion efficiencies along with associated optimal absorber band gaps. Various absorber configurations are modeled considering the major loss mechanisms in PEC devices. Quantitative sensitivity analyses for each loss mechanism and each absorber configuration show a profound impact of both on the resulting STH efficiencies, which can reach upwards of 25 % for the highest performance materials in a dual stacked configuration. Higher efficiencies could be reached as improved materials are developed. The results of the modeling also identify and quantify approaches that can improve system performance when working with imperfect materials.

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

Electrochemical methods for the codeposition of Cr or Mo with α-Fe2O3 (hematite) have been developed to produce doped iron oxide films with varying compositions of Cr and Mo which are active photoanodes for photoelectrochemical (PEC) water decomposition (“water splitting”). The films were characterized by scanning electron microscopy, X-ray diffraction, UV−vis optical spectroscopy, and X-ray photoelectron spectroscopy to determine the effect of the dopants on the hematite structure and PEC performance. Upon doping, the microstructures of the films varied; however, no preferred crystallographic orientation or dopant phase segregation was observed. The best performing samples were 5% Cr and 15% Mo doped which had Incident Photon Conversion Efficiencies (IPCE’s) at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2× and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. The increase in performance is attributed to an imp...

In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.

Solar-to-hydrogen (STH) conversion efficiency is the most important figure of merit to gage the potential of a semiconductor material to photoelectrochemically split water (see Chapter “ Efficiency Definitions in the Field of PEC”). It is projected that STH conversion efficiencies in excess of 10 % will be needed for practical hydrogen production systems [1]. Taken in conjunction with gas detection measurements (see Chapter “ Stability Testing”), the photocurrent density (j SC) under short-circuited conditions (i.e., zero applied bias) in a 2-electrode measurement is critical in determining the STH conversion efficiency. Moreover, applied bias experiments using the 2-electrode configuration can shed important light on the water splitting capabilities and limits of a PEC material system.

It is important to determine the conductivity and flat-band potential (Efb) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. The three different techniques that can estimate the Efb are: Illuminated OCP, Mott–Schottky and Photocurrent Onset. The Efb should be independent of the technique used to determine it. Due to the inherent shortcomings of each technique, there is often a lack of agreement of the values determined by the various analyses. Researchers should be aware of these limitations in interpreting results.

The chemical products of PEC water splitting processes are the evolved hydrogen and oxygen gases. Standard experimental methods for detecting and validating the quantity and quality of the product gases are critical. Faradaic efficiencies for the water splitting reaction in the given system can be determined. Three examples of PEC reactors are discussed, including batch, flow, and recirculating batch reactor.

In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.

Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60–$10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O_2 and H_2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energy's targeted threshold cost of $2.00–$4.00 per kg H_2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met.