The prepared photoanode further increased the photocurrent to 4.58 mA cm–2 in the glycerol oxidation reaction at 1.23 vs. RHE and selectively obtained high added value 1,3-dihydroxyacetone in a yield of 20.5 μmol cm–2h–1.

First author: Miao Yucong

Corresponding author: Professor Shao Mingfei

Communication unit: National Key Laboratory of Effective Utilization of Chemical Resources of Beijing University of Chemical Technology

Paper DOI: 10.1016/j.apcatb. 2022.122147

Full text quick review

This study reported a strategy to modify NiCo-LDH on the photoanode, and activate LDH through electrochemical to dehydrogenate hydroxyl and expose reactive oxygen species sites, achieving a significant improvement in photoelectric performance. For example, with BiVO4 as the model photoanode, the designed BiVO4/NiCo-LDH-Act photoanode achieves a photocurrent density increase of 3.13 times compared with the original BiVO4 photoanode, and this strategy is applicable to a variety of semiconductor materials (TiO2, WO3, α-Fe2O3). In addition, by replacing the anode OER process with a PEC glycerol oxidation system, the photocurrent density was further increased to 4.58 mA cm–2 at a bias voltage of 1.23 V vs. RHE, and a high added value of 1,3-dihydroxyacetone was selectively obtained in a yield of 20.5 μmol cm–2 h–1.

Background introduction

Photoelectro-catalytic technology (PEC) provides a clean and efficient way to convert solar energy into chemical energy. In a typical photoelectro-catalyzed hydrogen production system, the oxygen evolution reaction (OER) at the anode is slow, which seriously restricts the overall solar energy conversion efficiency. Therefore, exploring high-performance photoanodes has become a research hotspot in this field. Currently widely used semiconductor photoanodes (such as BiVO4, TiO2, WO3) usually have problems with slow charge transfer and slow surface catalytic kinetics, which leads to the easy recombination of photogenerated holes. Improving hole utilization efficiency often requires stacking multiple functional layers on the photoanode, which can lead to unnecessary interfacial defects and composite sites. Designing a simple dual-function interface that enables both fast interface charge transfer and efficient surface catalytic reactions is a key challenge, and currently there are few successful cases reported. In addition, the added value of oxygen produced by the anode OER in the photoelectrolytic water hydrogen production system is low, and researchers are also committed to solving this problem by finding more efficient OER alternative reactions.

Highlights of this paper

1. This work constructs a dual-function interface for charge transport and surface catalysis on the surface of various metal semiconductor photoanodes by modifying and activating cobalt-based LDH. Among them, the photocurrent density of BiVO4/NiCo-LDH-Act photoanode is 3.13 times higher than that of the original BiVO4 under the bias voltage of 1.23 V vs.RHE.

2. A series of experiments combined with theoretical calculations prove that NiCo-LDH is dehydrogenated during the activation process and is reconstituted into a Ni-CoOOH structure. The performance improvement of the photoanode is due to the induced transmission of holes by negatively charged reactive oxygen on the surface of LDH, while the surface reactive oxygen can be converted into adsorbed hydroxyl radicals and mediated surface oxidation reaction.

3. The prepared photoanode further increased the photocurrent to 4.58 mA cm-2 in the glycerol oxidation reaction at 1.23 vs. RHE, and the high added value 1,3-dihydroxyacetone can be selectively obtained in a yield of 20.5 μmol cm-2h-1.

Picture analysis

BiVO4/NiCo-LDH-Act photoanode synthesis and PEC performance

First, the author modified NiCo-LDH on the BiVO4 photoanode with a simple electrodeposition process and electrochemical activation in 0.1 M KOH to obtain BiVO4/NiCo-LDH-Act photoanode. XRD, SEM, HRTEM and HAADF-STEM-EDS mapping photos all proved that the successful synthesizing of the BiVO4/NiCo-LDH-Act photoanodes with core-shell structures was improved by 3.13 times compared with the original BiVO4 under the bias voltage of 1.23 V vs. RHE. In addition, the chopping current test can be found that the photoanode has a sensitive light response. This strategy can be applied to a variety of semiconductor materials (TiO2, WO3, α-Fe2O3).

Figure 1. XRD test of BiVO4, BiVO4/NiCo-LDH, BiVO4/NiCo-LDH-Act photoanode, SEM, HRTEM, HAADF-STEM-EDS mapping photos, as well as LSV and chopping current tests.

Effect of activation process on NiCo-LDH

In order to explore the reasons why the activation process can improve the performance of photoanode, the authors characterized the structural changes of NiCo-LDH during the activation process. From the O1s, Co 2p3/2 and Ni 2p3/2 fine spectra of XPS, it can be seen that during the activation process, the valence state of Co increases in activation, while the valence state of Ni remains unchanged. This preliminarily demonstrates that the activation process can remove hydrogen from the hydroxyl group attached to Co on LDH and expose reactive oxygen sites. Further in situ Raman spectroscopy demonstrates that NiCo-LDH is gradually reconstructed into Ni-doped CoOOH structure during activation.

Figure 2. XPS high resolution spectra of BiVO4, BiVO4/NiCo-LDH, BiVO4/NiCo-LDH-Act photoanode, and in-situ Raman spectroscopy test.

carrier transport and composite behavior analysis

A series of spectral and electrochemical tests were used to analyze the transfer and composite behavior of photogenerated carriers. The charge separation efficiency and charge injection efficiency of the photoanode are improved after modifying LDH and performing activation. EIS shows that holes on the photoanode surface have faster migration mechanics. Fluorescence spectroscopy shows that the carrier recombination of the composite photoanode is weakened, and time-resolved fluorescence spectroscopy, transient spike analysis of LSV, Bode phase diagram and Motshotky test demonstrate that the lifetime of photogenerated charges is significantly increased. This series of tests prove that NiCo-LDH-Act can serve as a photoanode interface that promotes hole transport and catalytic surface reactions, improving the performance of the photoanode.

Figure 3. Analysis of carrier transport and composite behavior of BiVO4, BiVO4/NiCo-LDH, BiVO4/NiCo-LDH-Act photoanode.

Reactive oxygen species mechanisms explored

DFT calculations found that surfactant oxygen can induce holes on BiVO4 to the photoanode surface through its negative charge properties, and then participate in the reaction. EPR monitored that a large number of hydroxyl radical signals were generated during the reaction. Further fluorescent molecular probe experiments showed that hydroxyl radicals mainly exist in the system in adsorption state. Therefore, the authors infer that surfactant oxygen induces hole migration and is oxidized to form adsorbed hydroxyl radicals, and then participates in the surface oxidation reaction.

Figure 4. DFT calculation, EPR test, fluorescent molecular probe experiment and reactive oxygen interaction mechanism diagram.

Photoelectric catalytic Glycerol oxidation performance and mechanism

The prepared photoanode was applied to the glycerol oxidation reaction, and the photocurrent density was significantly improved compared with that during the OER reaction. A photocurrent density of 4.58 mA cm-2 can be obtained under 1.23 V vs. RHE. The prepared photoanode selectively obtains high added value 1,3-dihydroxyacetone at 1.4 V vs. RHE with a yield of 20.5 μmol cm–2h–1. According to free radical quenching experiments, the glycerol oxidation reaction is also mediated by adsorbed hydroxyl radicals. On this basis, the author proposed the mechanism of BiVO4/NiCo-LDH-Act photoanode used for water decomposition to produce hydrogen coupled oxidation.

Figure 5. Glycerol oxidation performance, free radical quenching reaction and mechanism diagram of photoanode.

Summary and Prospect

This work constructs surfactant oxygen sites on the photoanode through simple modification and activation of LDH. This strategy achieves the dual purpose of promoting hole transfer and accelerating the kinetics of surface oxidation reactions. The surfactant oxygen strategy is applicable to various cobalt-based LDH and metal oxide semiconductors, where the combination of BiVO4/NiCo-LDH-Act photoanode achieves a 3.13-fold increase in photocurrent density. A series of experiments combined with DFT calculations show that during the activation process, NiCo-LDH surface dehydrogenation is exposed, negatively charged reactive oxygen sites are exposed to induce rapid migration of photogenerated holes and capture holes to catalyze the surface reaction.The prepared photoanode can also produce high added value 1,3-dihydroxyacetone with high selectivity in glycerol oxidation reaction. EPR and fluorescent probes molecule experiments show that adsorbed hydroxyl radicals produced by in-situ oxidation of surfactant oxygen mediate effective selective oxidation of glycerol. The interface regulation strategy proposed in this work can provide new photoanode design ideas for the field of photoelectrocatalytic coupled oxidation research.