Elsevier

Electrochimica Acta

Volume 438, 10 January 2023, 141595
Electrochimica Acta

Interference effect of nitrogen-doped CQDs on tailoring nanostructure of CoMoP for improving high-effective water splitting

https://doi.org/10.1016/j.electacta.2022.141595Get rights and content

Abstract

The construction of bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is significant for developing water splitting technology. The carbon-based non-noble metal catalysts could be regarded as a preferred alternative to Pt-based materials in water splitting, but very rarely Pt-like activity has been obtained by non-noble metal catalysts. Herein, NCQDs/CoMoP/NF bifunctional catalysts were designed and synthesized by the coupling relationships between N-doped carbon quantum dots (NCQDs) and metal ions. The abundant functional groups on the surfaces of CQDs can effectively coordinate with metal ions with empty d orbitals to generate the stable composites. Meanwhile, the formation of metal nanoparticles can be restricted by the introduction of CQDs to form smaller crystals with stable nanostructures, thereby preventing the agglomeration and further growth of metal nanoparticles. The NCQDs-100/CoMoP/NF electrode shows a low overpotential of 65 mV (10 mA cm−2) for HER and 370 mV (100 mA cm−2) for OER in a 1.0 M KOH system. Furthermore, the constructed NCQDs-100/CoMoP/NF couple can reveal a small cell voltage of 1.75 V to reach the large current density of 50 mA cm−2.

Introduction

Hydrogen, as a promising alternative to fossil fuels, has been regarded as a clean and renewable energy source [1], [2], [3], [4]. Electrochemical water splitting is a promising pathway to achieve efficient hydrogen production, but water electrolysis usually requires high overpotentials for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to provide significant reaction rates [5], [6], [7], [8], [9]. Currently, Ir/Ru-based oxides [10] and Pt-based materials [11] are commonly used as benchmark electrocatalysts for HER and OER, respectively, but their high cost and scarcity restrict their application [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Along with the huge efforts in developing nonprecious metal catalysts, transition-metal phosphides [24], [25], [26], sulfides [27], [28], [29], selenides [30], and carbides [31,32] have been discovered and have received enormous interest in recent years. Generally, Transition metal phosphates (TMPs), such as Ni2P, CoP, FeP, and MoP nanoparticles, show great promise in electrocatalytic HER in part on the merit of their reversible dissociation of H2. However, their electrocatalytic performance is still restricted by low electronic conductivity, sluggish kinetics, and insufficient stability [33], [34], [35], [36]. A comprehensive strategy to tailor the compositions and redesign the micro-nanostructures of TMPs has become an attractive way to further enhance their water splitting performance. However, their electrocatalytic performance is still restricted by low electronic conductivity, sluggish kinetics and insufficient stability [37], [38], [39], [40]. A comprehensive strategy to tailor the compositions and the micro-nanostructures of TMPs has become an attractive way to further enhance their water splitting performance.

The electrocatalytic properties of TMPs can be further enhanced by introducing carbon-based support since it can prevent the aggregation of metal particles and speed up the reaction kinetics by improving the intrinsic activity and electronic structure of active sites thereon [41]. Carbon nanotube, porous carbon, graphene and carbon quantum dots have attracted increasing attention for applications in which they have been widely used in the construction of catalysts for HER. Among them, carbon quantum dots (CQDs) with sizes smaller than 10 nm have attracted broad attention because of their fast electron-transfer and large electron storage ability [42], [43], [44], [45]. In addition, CQDs can be doped with heteroatoms to form abundant surface functional groups (N, O, S, P, etc.). The electrocatalytic properties of composites can be enhanced by the heteroatom doped CQDs by boosting the electron transfer [46], [47], [48]. Besides, the functional groups thereon CQD surface are favorable for the preparation of multi-component and high-performance composite materials via anchoring sites and active sites. The N-CQDs supported flower-shaped MnO2 nanocatalysts were synthesized by Tian et al. using a microwave-assisted method [49]. The conductivity and the specific surface area of the catalyst was raised by splitting a large MnO2 nanoflower into several small nanoflowers with rough surface. The strongly coupled CQDs/MoP nanohybrids was developed by Zhang et al. through a charge-directed self-assembly strategy. The aggregation and the surface oxidation of MoP nanoparticles were effectively alleviated by the introduction of CQDs [50]. Although previously reported studies have demonstrated the excellent electrocatalytic properties of CQDs and metal composite matrix materials, the interaction mechanism of CQDs with metals is rarely studied. More importantly, the interference mechanism of CQDs on metal crystal growth has not yet been investigated.

In the present work, an innovative method was employed for preparing NCQDs/CoMoP/NF catalysts using N-doped CQDs from coal tar pitch. The evolution mechanism of coal tar pitch to CQDs and the effect of N atom doped on CQDs were deeply studied. The interaction effects of CQDs on metal crystal growth were also investigated. The NCQDs/CoMoP/NF catalyst exhibits excellent catalytic performance in HER, OER, and overall water splitting, and has great potential in the future.

Section snippets

Materials

Ni foam with a porosity of 98%, a areal density of 350 g m−2 and a pore diameter of 0.2 mm was purchased from Shenzhen Green and Creative Environmental Science and Technology Co. Ltd . All of the chemicals were used as received without any further purification. CTP was obtained from Taiyuan Coal Coking Co, Ltd. Cobalt chloride hexahydrate (Co((NO3)2•6H2O) (AR), melamine C3H6N6 (AR), ammonium molybdate ((NH4)2MoO4) (AR), hydrogen peroxide (H2O2) (AR), formic acid (HCOOH) (AR), ammonia (NH3•H2O)

Preparation and characterization of the NCQDs

During the calcination process, the coal tar pitch is dehydrated and condensed at high temperatures to form coke with a certain orientation through π-π stacking, and N atoms are doped into the coke [51]. During the oxidation process, the macromolecules of the coke are tailored into small molecules to become NCQDs (Fig. 1a) [52]. The changes in chemical structure among coal tar pitch, N-doped coke, and NCQDs are shown by infrared spectroscopy (Fig. 1b). The absorption band at 3100-2800 cm−1

Conclusions

In summary, the novel N-doped CQDs from coal tar pitch was designed and synthesized by calcination-oxidation method, which cross-linked with metal ions to form NCQDs/CoMoP/NF bifunctional catalysts. The NCQDs-100/CoMoP/NF electrode shows a low overpotential of 65 mV (10 mA cm−2) for HER and 370 mV (100 mA cm−2) for OER in a 1.0 M KOH system. Furthermore, the constructed NCQDs-100/CoMoP/NF couple can reveal a small cell voltage of 1.75 V to reach the large current density of 50 mA cm−2 at an

Declaration of Competing Interest

The authors declare no competing financial interest

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51902326), the Youth innovation promotion association CAS (No. 2021174) and the Natural Science Foundation of Shanxi Province for Young Scholars, China (Grant Nos. 201901D211588, 20210302124421).

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