Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi₂O₄ Electrodes for Photoelectrochemical Water Splitting

Abstract

One of the principal challenges for solar-driven hydrogen production via water splitting is to improve the solar-to-hydrogen conversion efficiency. We have employed combinatorial chemistry using a materials inkjet printer, and selected CuBi 2 O 4 as a promising p-type material. The steady-state photocurrent corresponding to water reduction for a 280 nm film at 0.2 V (RHE) was about 0.12 mA cm −2 , significantly lower than that attainable for a 2 eV band gap semiconductor. We have applied intensity-modulated photocurrent spectroscopy (IMPS) to distinguish between the photoelectrochemical processes involved and to determine the associated time constants, in order to gain insight into the loss processes responsible for the low efficiency. The charge separation efficiency reaches up to 0.66 at sufficiently negative potential, however, the recombination rate constant is larger than that corresponding to electron transfer to the solution. This results in a relative charge transfer efficiency of 0.2−0.4, explaining the low photocurrent. At low light intensity, the relative charge transfer efficiency increases up to 0.8, indicating the promise of the material. Interestingly, at sufficiently positive applied potential, the IMPS spectrum of the CuBi 2 O 4 photoelectrode switches sign, indicating a net modulated positive photocurrent. However, the rate constant for hole transfer to the solution is small resulting in a negligible steady-state anodic photocurrent. Strategies to improve the efficiency are discussed.

Progress in Ternary Metal Oxides as Photocathodes for Water Splitting Cells: Optimization Strategies

“…Some AB 2 O 4 compounds not containing iron have also attracted considerable interest. For instance, CuBi 2 O 4 has also been widely investigated, especially in the last years, [90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108] since Arai et al discovered its potential use as a new visible-lightresponsive p-type semiconductor in 2007. [90] It crystallizes in a kusachiite-type structure consisting of CuO 4 units staggered along the c-axis and separated by Bi 3þ ions, each of which is linked to six O atoms (Figure 5a).…”

Recent advances and prospects of inkjet printing in heterogeneous catalysis

“…Parkinson et al [67][68][69] developed libraries of electrocatalysts (e.g., Cr, Fe, Cu, Pt, Ru, Ir) for oxygen evolution reaction via combinatorial studies using the inkjet printing technology. Metal salt precursors (e.g., metal nitrates and chlorides) were added to the ink solutions containing water and glycols.…”

“…94,133,134 Normally the catalyst species precursors are prepared in the form of inks and a variety of composition ratios can be produced by changing the gradient of designed patterns. 135 Parkinson et al [67][68][69][135][136][137] followed the combinatorial approach to prepare and print metal precursors for the (photo)-electrochemical water-splitting reaction. They studied a variety of precious and non-precious metal combinations from various metal precursors such as metal nitrates, chlorides and organometallic precursors.…”

Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling

“…On the other hand, ternary oxides comprise a wide number of promising materials, such as spinels, delafossites, perovskites, scheelites and other families, which could potentially fulfill the requirements for light absorption, stability and photoactivity for solar hydrogen generation. Among them, CuBi 2 O 4 has attracted considerable interest, [118][119][120] reaching relatively high photocurrents, although instability under illumination has also been reported. 121 In this context, CuFe 2 O 4 spinel stands out because of its non-toxicity, environmental benignity and its composition based on Cu and Fe, which are earth-abundant elements.…”