In recent years, three-dimensional (3D) graphene-based nanomaterials have been demonstrated to be efficient and promising electrocatalysts for oxygen reduction reaction (ORR) in fuel cells application. This review summarizes and categorizes the recent progress on the preparation and performance of these novel materials as ORR catalysts, including heteroatom-doped 3D graphene network, metal-free 3D graphene-based nanocomposites, nonprecious metal-containing 3D graphene-based nanocomposites, and precious metal-containing 3D graphene-based nanocomposites. The challenges and future perspective of this field are also discussed.
As a novel nanomaterial, graphene is a one-atom-thick carbon sheet with a hexagonal packed lattice structure. Since the first direct observation and characterization of a mechanically exfoliated graphene by Geim and Novoselov in 2004, this “miracle material” has attracted tremendous attention and research interest owing to its many unique properties, such as extraordinarily high in-plane electrical mobility, thermal conductivity, mechanical strength, and ultralarge specific surface area [
Although 2D graphene-based nanomaterials have been demonstrated to be promising in the above electrochemical applications, a huge challenge and need still remain for the efficient use of graphene’s large specific surface area and extraordinary electrical, chemical, and mechanical properties. For instance, for electrodes, large surface area and fast electrolyte transfer near electrode surface are required, in order to obtain high rate of electrochemical reaction. However, the graphene sheets on the electrode tend to form irreversible agglomerates or restack due to the strong
Recently, integrating nanomonolayer graphene into macroscopic 3D porous interconnected networks has attracted significant attention, since the nonagglomerated 3D structure can provide graphene-based nanomaterials with high specific surface areas, strong mechanical strengths, and fast mass and electron transport kinetics due to the combination of 3D porous structures and the excellent intrinsic properties of graphene [
On the other hand, the greenhouse gas emissions from the depletion of traditional fossil fuels become the primary cause for global warming and climate change. In this context, increasing demand for sustainable energy has stimulated intense research on energy conversion and storage systems that are highly efficient, of low cost, and environmentally friendly. Among the various promising energy conversion technologies, fuel cell, which converts chemical energy from fuels into electricity, is a clean and high-efficiency device with low or zero emissions from operation and has drawn a great deal of attention in terms of both fundamentals and applications [
In the past several years, tremendous efforts have been devoted to the development of synthetic methods for 3D graphene-based materials with various morphologies, structures, and properties, in order to satisfy the requirements arising from various applications. Considering the existence of several excellent reviews highlighting the classification and summarization of the recent progress in the preparation methods of these materials, the reader should be referred to these reviews to obtain in-depth coverage of the various preparation procedures [
Self-assembly, among the main strategies for the construction of 3D graphene architecture, is the most promising and widely used strategy to obtain 3D graphene-based materials as catalysts for ORR. As a typical example, 3D graphene structures can be fabricated through the gelation process of a graphene oxide (GO) suspension followed by reduction to convert GO to reduced graphene oxide (rGO). Both chemical and physical treatments, such as adding various kinds of cross-linkers, changing the pH value or temperature of the GO suspension, hydrothermal treatment, direct freeze-drying, and electrochemical reduction, could trigger the gelation of a GO suspension. Hydrogels, organogels, and aerogels are the main forms of 3D graphene usually as products of the self-assembly process, and aerogels are obtained from hydrogels and organogels via freeze-drying or supercritical CO2 drying. To create 3D graphene-based materials, including doped 3D graphene and 3D graphene-based composite, dopant-containing precursor and precursor ions are usually added during or after the GO gelation process followed by reduction for intrinsically doped 3D graphene and for
Although Pt-based electrocatalysts are the most active and popular catalysts for ORR at the cathode in fuel cells, large-scale commercialization is still restricted due to high cost, poor durability, and sluggish electron transfer kinetics. Therefore, the development of alternative catalysts, including metal-free catalysts and nonprecious metal catalysts (NPMC), has drawn tremendous attention in the past decades. In 3D graphene-based electrocatalysts for ORR, 3D graphene may play the role of either inherent and single catalysts such as heteroatom-doped 3D graphene network (discussed in Section
More recently, heteroatom-doped carbon nanomaterials (e.g., carbon nanotubes, nanotube cups, ordered mesoporous graphitic arrays, and graphene) have been emerged as a new class of metal-free catalysts or supporting materials for NPMC for ORR [
In 2012, Zhao and coworkers developed a method to prepare versatile, N-doped, ultralight 3D graphene framework (GF), which is demonstrated to be a promising metal-free catalyst for ORR in an alkaline solution (Figures
(a, b) Photographs of an as-prepared superlight GF and one with a piece of GF size of 1.8 cm × 1.1 cm × 1.2 cm standing on a dandelion. (c, d) SEM images of the sample in (a). (e, f) Typical TEM images of the walls of GF and the corresponding electron diffraction patterns consistent with 1–4 crystalline graphene layers (reprinted from [
In 2013, Su and coworkers developed 3D nitrogen and sulfur codoped graphene frameworks (N/S-GFs) as efficient electrocatalysts under alkaline conditions [
Although the aforementioned nonmetallic heteroatom-doped 3D graphene networks are claimed as inherent “metal-free” electrocatalysts for ORR, it should be pointed out that the specific catalytic sites as well as the catalytic mechanisms are still unclear. Moreover, because the starting material GO contains metallic impurities brought by its preparation method and it has been demonstrated that even a slight trace of Mn metallic impurities in graphene materials is sufficient to alter or dominate their electrocatalytic properties towards ORR, Wang and coworkers argued that the “metal-free” electrocatalysis of the ORR on heteroatom-doped graphene is caused by trace levels (ppm) of metallic impurities (manganese oxide in particular) present within the graphene materials [
As mentioned above, N-doped carbon materials have been reported to be efficient metal-free electrocatalysts for ORR; however, the low level of nitrogen concentration limited by their preparation methods may affect further improvements in their catalytic performances. Graphitic carbon nitride (g-C3N4), which possesses high nitrogen content including both pyridinic and graphitic nitrogen moieties which could be potential active sites for ORR, is thus considered to be a potential catalyst or an effective component in composite catalyst for ORR.
In 2014, Tian and coworkers constructed a novel 3D porous supramolecular architecture of ultrathin g-C3N4 nanosheets and rGO (g-C3N4/rGO) by solution self-assembly of g-C3N4 and GO followed by photoreduction catalyzed by g-C3N4 (Figure
Schematic diagram to illustrate the fabrication process of 3D porous supramolecular architecture of g-C3N4 nanosheets and rGO (reprinted from [
UV-vis absorption spectra and corresponding photographs (inset) of aqueous dispersions of g-C3N4 nanosheets (a), GO (b), g-C3N4/GO (c), and g-C3N4/rGO (reprinted from [
Besides the aforementioned metal-free 3D graphene/inorganic composite as ORR catalyst, Zhuang and coworkers reported a graphene/biofilm composite with a 3D structure as biocathode in microbial fuel cell (MFC) [
As alternative ORR catalysts for Pt-based materials, nonprecious metals (Fe, Co, etc.) or metal oxides (Fe3O4, Co3O4, etc.) have been actively studied; however, these catalysts tend to degrade during operation of the fuel cell due to dissolution, sintering, and agglomeration. Supporting these nonprecious metal/metal oxide nanoparticles (NPs) on 3D graphene networks would be an attractive way to solve these problems, as a geometric confinement of these NPs within graphene layers would enhance their interface contact and suppress the dissolution and agglomeration of NPs, thereby promoting the electrochemical activity and stability of the hybrids.
In 2012, Wu and coworkers reported 3D N-doped graphene aerogel- (N-GA-) supported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient cathode catalysts for ORR [
Fabrication process for the 3D Fe3O4/N-GAs catalyst. (a) Stable suspension of GO, iron ions, and PPy dispersed in a vial. (b) Fe- and PPy-supporting graphene hybrid hydrogel prepared by hydrothermal self-assembly and floating on water in a vial and its ideal assembled model. (c) Monolithic Fe3O4/N-GAs hybrid aerogel obtained after freeze-drying and thermal treatment (reprinted from [
Iron porphyrin plays a vital role in oxygen transport and reduction reactions in biological system. Thus, it would be attractive to consider if supporting iron porphyrin on graphene could function as an alternative to Pt-based electrode in fuel cells for ORR. In 2012, Jahan and coworkers synthesized an electrocatalytically active 3D graphene-iron porphyrin metal organic framework (MOF) composite for ORR by reacting the pyridine-functionalized graphene with iron porphyrin [
Recently, Mao and coworkers reported a 3D N-doped crumpled graphene- (N-CG-) cobalt oxide nanohybrids (N-CG-CoO) with a stable structure for use as bifunctional catalysts for both ORR and oxygen evolution reaction (OER) in alkaline solutions [
Although great contribution has been made to devise various nonnoble metal electrocatalysts, the demands of the high catalytic activity for the four-electron ORR and the acidic environment of the fuel cell electrode still make noble metal-based nanostructures the most promising for practical application. In this context, it is a realistic strategy to further maximize the activity of noble metal-based nanostructures and minimize the use of precious noble metal by loading these highly active nanomaterials on the surface of supporting materials with low cost, high surface area, and good electrical conductivity, which not only maximizes the availability of surface area for electron transfer and decreases the aggregation of these electrocatalysts, but also provides better mass transport of reactants to the electrocatalysts. As expected, 3D graphene network, among the catalyst supports for ORR, is believed to be the most promising candidate owing to its unique properties such as high electrical and thermal conductivities, great mechanical strength, inherent flexibility, and huge specific surface area.
In 2010, Zhu and coworkers constructed a hybrid 3D nanocomposite film by alternatively assembling the graphene nanosheets modified by imidazolium salt-based ionic liquid (IS-IL) and Pt NPs through electrostatic interaction (Figure
Schematic representation of the assembling process of the IS-IL modified graphene/Pt NPs multilayer films (reprinted from [
In 2013, Sattayasamitsathit and coworkers reported for the first time on the use of lithographically defined 3D graphene microstructure as support for catalytic metal NPs (Figure
SEM images of (a) bare 3D graphene substrate before modification with metal nanoparticles and (b) 3D graphene after modification with Pt NPs using electrodeposition at 0.4 V for 750 s (reproduced from [
In fuel cells, an effective electrocatalyst for ORR at the cathode plays a crucial role in improving their energy conversion efficiency and enhancing the high energy and power density. In recent years, the newly developed 3D graphene-based materials have been demonstrated to be promising as efficient electrocatalysts for ORR. In this paper, we have reviewed the primary preparation method of these novel materials and their performances as catalysts for ORR. From the viewpoint of whether containing precious metals or not, Sections
In spite of significant progress made in this area within only several years, the research of 3D graphene-based materials still remains at its initial stage and at least two aspects of challenges for practical application of these materials in fuel cells should be addressed. First, the systematic exploration and deeper understanding of the different catalytic mechanisms of various 3D graphene-based materials for ORR are required for optimum material design and device performance optimization. For example, for N-doped 3D graphene as inherent ORR catalyst, though the critical role of N dopant for the catalytic enhancement is proved, the precise relationship between catalytic activity and N species is still controversial, while, for 3D-graphene-based nanocomposites, more experimental and theoretical studies are needed to reveal the interactions between the 3D graphene and loaded active nanomaterials. Second, the development of facile, green, cost-effective, and controllable preparation method is still a significant issue, because the current assembly process usually needs to be carried out in rigorous conditions (such as high temperature or pressure) or involves tedious procedures, which increases the degree of preparation difficulty. Thus, much further research is needed to realize the aim of final industrial implementation, large scale, low cost, and simple production of 3D graphene-based materials with high catalytic activity and practical durability for ORR. This review is anticipated to encourage more future works to achieve this aim and to promote sustainability.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Natural Science Foundation of Shanxi (no. 2015021081), the Project for Construction of Science and Technology Innovation Team of Shanxi (no. 2015013001-04), and the Provincial Science and Technology Major Project of Shanxi (no. 0111101059).