How efficient could photocatalytic CO2 reduction with H2O into …
Shahzad Ali
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Article Writer
Highlights
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An efficiency evaluation model of water-based photocatalytic CO2 reduction system was established.
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Regulation of recombination coefficient and gas coverage ratio can enhance efficiency.
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Efficiency limit of photocatalytic reduction of CO2 with H2O into CH3OH is 46.7%.
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Efficiency limit for photocatalysis system combined with up-conversion materials is 59%.
Abstract
Photocatalytic carbon dioxide (CO2) reduction in aqueous media provides a potential and convenient way for fulfilling increasing fossil energy demand and relieving global warming problems. However, the efficiency limit for the photocatalytic CO2 reduction system still remains unclear. Here a comprehensive model of the photocatalytic CO2 reduction system is established to theoretically evaluate the efficiency limit, in which the light absorption, charge carrier recombination behaviors, and surface reaction processes are considered. Effects of Auger coefficient and the gas coverage ratio on the energy conversion efficiency are discussed to provide feasible enhancement approaches. Experimental efficiencies are compared with theoretical results to analyze the origin of low efficiencies. The photocatalytic reduction of CO2 into methanol is taken as an example, the energy conversion efficiency limit amounts to be 46.7%. To further improve the utilization of solar spectrum, up-conversion materials are incorporated into the photocatalytic system. The maximum efficiency of photocatalytic reduction of CO2 to methanol is predicted to be 59%. This paper unveils the upper efficiency limit of photocatalytic reduction of CO2 and provides the guidance for design of efficient photocatalysts and systems.
Introduction
Solar energy, given its abundance and cleanness, is no doubt one of the most promising resources to tackle current environmental issues and meet the increasing energy demand of human beings. One approach to harvest solar energy into the chemical energy is to use photocatalytic systems. Since its discovery in 1972, photocatalysis has drawn much attention all over the world [1]. Especially, the process of photocatalytic CO2 reduction into fuels can not only convert solar energy into stable chemical energy directly but also capture and utilize CO2 in the atmosphere. It helps to alleviate global warming effects considering that CO2 contributes mostly to the worldwide climate change. As a result, photocatalytic CO2 reduction has been considered as one of the most promising ways to obtain clean energy supply due to the following advantages: solar energy utilization, chemical energy storage, and CO2 capture [2], [3].
Among different photocatalytic CO2 reduction systems, photocatalytic CO2 reduction in aqueous media has sparked a huge interest because of reactants’ availability, low cost, system simplicity, etc. However, the performance of this photocatalytic system is too sluggish to meet the efficiency requirement of industrial commercialization. For instance, Arai et al. demonstrated that the conversion efficiency from solar to chemical energy was only 0.14% in a photo-electrochemical device composed of an InP/[RuCP] photocathode and a reduced SrTiO3 (r-STO) photoanode [4]. In the last few decades, intensive investigations have been conducted to improve the efficiency of photocatalytic CO2 reduction using water (H2O) as the hole scavenger, ranging from the exploration of novel catalysts [5], [6], bandgap engineering [7], recombination inhibition [8], to system optimization [9], [10], and etc. Nevertheless, currently reported energy conversion efficiencies are still far from what expected, which make us doubt whether photocatalytic CO2 reduction systems are theoretically inefficient.
There have been some theoretical efficiency evaluations of systems storing solar energy as chemical fuels in the literature [11], [12], [13], [14], [15]. For example, Bolton et al. [11] provided a theoretical estimation of the solar fuel conversion efficiency. The incident photon absorption, the chemical energy yield, and the product conversion were considered. The solar-to-fuel (STF) efficiency amounted to 10% to 13%. For the sake of simplicity, the photon-generated charge carrier behaviors of photocatalysts were neglected. Fountaine et al. [13] built a theoretical model for analyzing efficiencies of the photo-electrochemical (PEC) water splitting system and predicted the limiting efficiency 30.6% of the PEC system, but the important surface reaction process was not discussed. Takanabe et al. [14] recently reported numerical simulations of the particulate photocatalytic water splitting system based on the Poisson’s equation. Ardo et al. [15] calculated theoretical efficiencies of tandem Z-scheme solar water splitting devices. The photocatalyst bandgap was considered in the study, and the optimal parameter was given to get the limiting STF efficiency. However, these reported models neglected the surface chemical process while focus on photo-physical or/and -electric phenomena inside photocatalysts. Photocatalytic reduction includes photo-generated charge carrier behaviors inside photocatalysts and surface reactions, so both of them should be considered in evaluating the STF efficiency. Especially for the photocatalytic CO2 reduction in water, the gas adsorption is of the first concern during the complicated gas-liquid-solid reaction because the reaction kinetics is highly correlated with the surface gas concentration [16]. Furthermore, CO2 reduction reactions are multi-electron transfer processes driven without sacrificial agents, leading to complicated pathways, and different product types. Nevertheless, an analysis of the whole photocatalytic reduction processes is still missing. Therefore, it is necessary to establish a comprehensive model for conducting the efficiency assessment of the water-based photocatalytic CO2 reduction system.
In this paper, we propose a theoretical efficiency model of water-based photocatalytic CO2 reduction system, through which the STF efficiency variation mechanism can be understood for guiding to improve energy conversion efficiency. Firstly, photocatalyst bandgaps, charge carrier recombination behaviors, and the surface reaction process are considered to build an efficiency model for water-based photocatalytic CO2 reduction. Secondly, influences of the recombination coefficient and the gas adsorption ratio on the STF efficiency are discussed to show the efficiency variation mechanism. Experimental efficiencies are compared with theoretical results to analyze improvement methods. Then, the efficiency limit of the photocatalytic reduction of CO2 into methanol in the ideal condition is calculated. Finally, to increase the utilization of solar spectrum, we incorporate up-conversion processes into the traditional photocatalytic system and obtain a higher efficiency limit of the new photocatalytic system.
Section snippets
Theoretical energy conversion efficiency model
The process of photocatalytic reduction of CO2 using a semiconductor photocatalyst in aqueous media is illustrated in Fig. 1. It includes the following steps: (1) light absorption, (2) charge separation, (3) charge carrier migration and recombination, (4) surface redox reaction [17]. For the first step, the photocatalyst absorbs photons with energies greater than its bandgap. In the second step, electrons are excited from the valence band to the conduction band by absorbed photons, generating
Efficiency limit of the single water-based photocatalytic CO2 reduction system
Previous section has shown the dependence of the efficiency on the Auger coefficient and the gas coverage ratio when the photocatalyst bandgap is determined. However, the efficiency limit of water-based photocatalytic reduction of CO2 to CH3OH still remains unclear. In this section, the efficiency limit of photocatalysis in ideal condition is calculated.
To determine the limiting efficiency, some assumptions are made — (1) the gas coverage ratio is in the optimized range. (2) Non-radiative
Conclusions
In summary, we build the theoretical efficiency model of photocatalytic CO2 reduction in aqueous media considering complex charge carrier behaviors. Effects of recombination coefficient and gas coverage ratio on STF efficiency are discussed. In order to get the maximum efficiency, ones should design photocatalysts reasonably to get the optimized CO2 adsorption interval. A small recombination coefficient can not only reduce the energy loss of recombination, but also supply a wide range of
CRediT authorship contribution statement
Zhonghui Zhu: Conceptualization, Methodology, Writing - original draft, Investigation. Xianglei Liu: Writing - review & editing. Chuang Bao: Data curation. Kai Zhang: Writing - review & editing. Chao Song: Visualization. Yimin Xuan: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work is supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No.51888103).
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