Optional construction of Cu2O@Fe2O3@CC architecture as a robust multifunctional photoelectronic catalyst for overall water splitting and CO2 reduction
Graphical abstract
Introduction
Nowadays, the environmental and energy crisis arising from the increasing global energy demands all over the world promot scientists to develop sustainable, renewable and carbon–neutral alternative sources for energy conversion [1], [2], [3]. Overall water splitting and CO2 reduction are two very important reactions from environmental and energy viewpoint, which have high potential to overcome these challenges in near future [4], [5]. The former produces hydrogen as clean fuel with zero carbon waste and the latter decreases the amount of CO2 emissions, thus reduces greenhouse effects. Currently, hydrogen production via electrolysis of water is regarded as a promising and effective strategy [6]. As we all know, the water splitting is driven by two half-cell reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The large thermodynamic barrier and sluggish kinetics are usually considered as the bottleneck of water splitting, which require efficient electrocatalysts to decrease the overpotential [7]. Noble metal, especially RuO2 and IrO2 materials are unwanted for the large-scale industrial applications, considering their prohibitive cost, electrochemical instability and scarcity [8], [9]. Scientists are devoted to develop an easily available, inexpensive and durable electro-catalyst to replace noble metal electrocatalysts. The hematite (α-Fe2O3) with great abundance, environmental friendliness, high stability and good electrochemical activity has got great attention in several potential applications [10]. However, the low electrical conductivity and slow oxygen evolution kinetics still limit the application of α-Fe2O3 for electrolysis of water. The heterojunction, which can induce abundant electron densities, and thus accelerating the electrical conductivity, is considered to be an expert strategy to boost the electrolytic performance [11], [12]. Meanwhile, the recent experimental results have suggested that cuprous oxide (Cu2O) could be effective catalyst because of its high abundance, high electrocatalytic activity, suitable redox potential and low overpotential [13]. In 2017, Zhang and co-workers reported that the obtained Cu2O nanoparticles coated with carbon (Cu2O@C) exhibited good OER efficiency with an onset overpotential of 250 mV and a Tafel slope of 63 mV dec−1 [14]. Still now, a few Cu2O electrocatalysts have been reported still now, although Cu2O is an attractive photocathode material [15]. More importantly, the doping of photocatalytic Cu2O may favor the photocatalytic performance, generating the multifunctional photo-electrochemical catalyst.
Additionally, the conversion of CO2 into valuable chemical fuels (e.g. CO, CH4, CH3OH, HCOOH, etc) may offer a promising solution to meet the global energy and environmental challenges [16]. Among them, the CO2 conversion by means of solar energy under mild conditions (low temperature and atmospheric pressure) is a more economical, sustainable and environmental-friendly process [17]. In 1979, Inoue et al. firstly reported photo-reduction of CO2 into renewable fuels [18]. The energy conversion efficiency and selectivity are still relatively low due its extraordinary stability [19], [20]. As reported in the literature, Cu2O is one of the excellent photocatalyst candidates for CO2 reduction because of its suitable band gap (2.0–2.2 eV) and strong reducing ability [21]. In 2015, Wang and co-workers have reported the Z-scheme α-Fe2O3/Cu2O heterojunction with good photocatalytic CO2 reduction performance [22]. In this paper, we presented the synthesis of flexible carbon clothes-based (CC) self-supported multifunctional catalysts and their applications in overall water electrolysis and photocatalytic CO2 reduction. The Cu2O-doped Fe2O3@CC hybrid electrode was thoroughly investigated via X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Interestingly, the obtained Cu2O/Fe2O3@CC self-supported electrode displayed boosted OER and HER performance in alkaline solution and compared to the commercial electrodes. The assembled water electrolyzer using Cu2O@Fe2O3@CC-500 as anode and cathode exhibited good performance on overall water electrolysis with a cell voltage of only 1.675 V at current density of 10 mA cm−2. More interestingly, the obtained electrodes revealed excellent activity for photocatalytic CO2 reduction via the synergistic effect between α-Fe2O3 NAs and doped Cu2O nanogranules. The production rate for CH4 and CO was 1.18 µmol·gcatalyst−1·h−1 and 172.2 µmol·gcatalyst−1·h−1, respectively, and the total electron consumption yield was 884.6 µmol·gcatalyst−1. Based on the characterization and theory calculation, an enhanced OER kinetics and a plausible mechanism for CO2 photo-reduction were also proposed.
Section snippets
Materials
Ferric chloride hexahydrate (FeCl3·6H2O), copper sulfate pentahydrate (CuSO4·5H2O), sodium nitrate (NaNO3), lactic acid (C3H6O3), hydrochloric acid (HCl), nitric acid (HNO3), sodium hydroxide (NaOH), and carbon clothes were obtained from Shanghai Titan Scientific Co., ltd. All chemicals were of analytical grade and used as received without further purification.
Preparation of α-Fe2O3 nanorod arrays (α-Fe2O3 NAs)
Acid treatment of CC: Before modifying the CC, the CC was firstly cleaned by sequential sonication in acetone, ethanol, and deionized
In-Situ growth of Cu2O@Fe2O3@CC-x
The fabrication procedure of the flexible Cu2O@Fe2O3@CC electrode was illustrated in Scheme 1, involving in a three-step process. Firstly, the CC was pre-treated in the 4 M HNO3 solution. The binder-free fiber network of the activated carbon cloth was an advantage for the fabrication of continuous electrodes owing to its low electric resistance, high mass and electron transport. The activated carbon cloth served as a highly conductive substrate for the growth of hydrated phase of iron oxide
Conclusions
In summary, we demonstrated the facile synthesis of flexible self-supported multifunctional Cu2O@Fe2O3@CC-x electrode. The optimized Cu2O@Fe2O3@CC-500 electrode was highly active with low overpetential (296 mV for OER and 188 mV for HER) and Tafel slope (66 mV dec−1 for OER and 59 mV dec−1 for HER) at 10 mA cm−2 in alkaline medium. The electrolyzer using the bifunctional Cu2O@Fe2O3@CC-500 as both anode and cathode exhibited good performance on overall water splitting with a cell voltage of only
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 National Natural Science Foundation of China (51472162 and 21707093), the Foundation of Science and Technology Commission of Shanghai Municipality (18090503600).
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