Global energy and environmental issues have stimulated tremendous ongoing research into developing sustainable and environmentally friendly energy conversion and storage systems [1
]. Creating highly active electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is crucial for the practical application of fuel cells, metal-air batteries, and electrolyzers. Currently, Pt-based catalysts are regarded as the most active ORR catalysts but are poor for the OER [2
], while IrO2
are the most efficient catalysts for the OER but are poor for the ORR [4
]. However, their commercialization has been greatly hindered by the scarcity of their materials, their consequent high cost, and their poor durability. As a result, extensive research efforts have been devoted to searching for low-cost alternatives with comparable catalytic activity to noble metal-based catalysts [5
]. Due to the sluggish kinetics of the ORR and OER [7
], it is highly challenging but imperative to develop a cheap and effective bifunctional electrocatalyst for the ORR and OER.
Extensive efforts have been made to develop non-noble-metal bifunctional electrocatalysts, such as transition metal oxides/sulfides [10
], carbon-based materials [9
], and perovskites [20
]. Among these promising bifunctional ORR/OER catalysts, metal-nitrogen-doped carbon materials have attracted much attention due to their remarkable activity, tunable surface chemistry, fast electron transfer capacity, and economic viability [21
]. However, substantially increasing the active sites of metal-nitrogen-doped carbon materials remains a great challenge. Recently, it was found that using nanosized carbon materials as supports, such as one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene, can further improve electrocatalytic performance [24
]. However, methods for the synthesis of metal-nitrogen-doped graphene/CNTs often requires the high temperature (600–1200 °C) which limits the practical application. And both CNTs and graphene nanosheets are inclined to aggregate with each other due to strong Van der Waals interactions, and this greatly hinders the full utilization of the active sites for catalytic reactions [21
Because of their three-dimensional network structure, extremely low density, high porosity, and large specific surface area, carbon-based aerogels have emerged as promising nanomaterials and are providing fascinating options for preparing new functional electrode materials [30
]. The incorporation of CNTs into graphene to produce a graphene–CNTs hybrid aerogel not only could favor the dispersion of graphene and CNTs while maintaining the original properties of both graphene and CNTs, but also could yield a 3D, interconnected, conductive network structure. This aerogel network structure, for ORR, favors the O2
transformation to the active sites; for OER, facilitates timely transfer evolved O2
molecules. It is expected that a graphene-carbon nanotube aerogel could result in better electrocatalytic performance. However, to the best of our knowledge, there is no report about M-N-C graphene-CNTs aerogel as an efficient oxygen electrode catalyst.
Herein, we report a one-pot hydrothermal method to prepare a cobalt and nitrogen co-doped graphene-CNTs aerogel (Co-N-GCA) as a bifunctional electrocatalyst for the ORR and OER. Benefiting from abundant catalytic active sites and a unique hierarchical structure that increases the exposure of active sites and favors electron transfer and the diffusion of O2 molecules, the obtained electrocatalyst exhibits superior electrocatalytic activity towards both the ORR and the OER in an alkaline medium.
2. Results and Discussions
The inset in Figure 1
a is a photo of the as-prepared Co-N-GCA aerogel after freeze drying, showing a black, low-weight, sponge-like material. The SEM images (Figure 1
a,b) reveal that the Co-N-GCA exhibits a well-defined and interconnected 3D porous network structure. The CNTs are randomly and uniformly distributed between the graphene sheets. The pore walls consist of thin layers of a network, which are cross-linked with graphene sheets and CNTs. TEM images (Figure 1
c,d) further confirm that the CNTs adhere tightly on the graphene substrates. No obvious CNT bundles or graphene agglomerates are observable. Few scattered metal nanoparticles could also be found by TEM inspection. The high-resolution TEM image (Figure S1
) reveals the lattice fringe space of 0.47 nm is consistent with the (111) plane of cubic Co3
spinel-phase. Photos of the prepared Co-GCA and N-GCA aerogels and the corresponding SEM images are also provided in Figure S2
adsorption–desorption isotherms and corresponding pore-size distribution curve of Co-N-GCA are shown in Figure 2
. According to the IUPAC classification, the N2
adsorption–desorption isotherms are of type IV, with the amount of absorbed N2
monotonically increasing/decreasing at high relative pressure (P
> 0.9); this is typically associated with capillary condensation, indicating the existence of mesopores (2−50 nm). Notably, the capillary condensation phenomenon occurs at a high relative pressure, indicating a large pore-size distribution. The hysteresis loop resembles type H3
, suggesting open slit-shaped capillaries between the parallel layers of graphene. The surface area of Co-N-GCA was calculated to be 456 m2
. The Barrett-Joyner-Halenda (BJH) method indicated that a hierarchical, meso- and macroporous material with a pore volume of 1.64 cm3
was formed by this method. This result is consistent with the SEM observations. The high surface area and hierarchically porous structure of Co-N-GCA would provide plenty of active sites and favor the mass transport of reactants and products, resulting in enhanced ORR/OER electrocatalytic activity.
The elemental compositions of Co-N-GCA and N-GCA were investigated by X-ray photoelectron spectroscopy (XPS) analysis. As shown in Figure S2
, the XPS survey spectrum of Co-N-GCA exhibited the signals of a C 1s peak (~284.5 eV), a N 1s peak (~398.1 eV), an O 1s peak (~531.1 eV), and a Co 2p peak (~780.1 eV) [22
]. The XPS results confirmed that Co and N elements were successfully doped into the carbon matrix, and the amounts of doped Co and N in Co-N-GCA were 0.44 and 8.92 at. %, respectively. In the case of N-GCA, the doped N content was 8.68 at. %, which was similar to that of Co-N-GCA (Table S1
). XPS analysis shows that a high amount of doped N can be achieved by this method.
The deconvoluted high-resolution N 1s spectrum (Figure 3
a) revealed four types of N species in Co-N-GCA: pyridinic N/Co–Nx
(398.9 eV), pyrrolic N (400.0 eV), graphitic N (401.2 eV), and oxidated N (405.1 eV) [31
]. Their corresponding contents are 47.5, 34.8, 13.5, and 4.2 at. %, respectively (Table S2
]. Notably, most of the doped N is pyridinic (47.5 at. %). It has been reported that the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N [33
]. We expected the as-prepared Co-N-GCA to exhibit outstanding electrocatalytic performance due to the high amount of pyridinic N. Meanwhile, M–Nx
structure have also been reported to contribute to ORR/OER active sites apart from the N–C active sites [34
]. The high-resolution N 1s spectrum and distribution of each N species of N-GCA are presented in Figure S3 and Table S2
b presents a high-resolution XPS spectrum of Co 2p. The peaks situated at 781.4 eV and 796.7 eV with two weak shake-up (satellite) peaks are assigned to the Co 2p 3/2 and Co 2p 1/2 atomic orbitals.
We have also performed inductively coupled plasma mass spectroscopic measurements to analyze the content of Co, the result showed a relatively low cobalt content of ~0.91 wt. % in the Co-N-GCA sample. Meanwhile, we also performed element analysis test, the result showed the N content in the Co-N-GCA is 17.5 wt. %.
a presents the XRD patterns of Co-N-GCA, N-GCA, and GCA. All three samples show two broad, weak diffraction peaks at 2θ ≈ 24° and 43.7°, which correspond to the (002) and (100) reflections of the graphitic peak (PDF#41-1487), respectively, confirming the graphitic crystal structure [35
]. For Co-N-GCA, no metal peaks or other than carbon are observed, which may be due to the low Co content.
b shows the Raman spectra of Co-N-GCA, N-GCA, and GCA. All of the samples exhibit two prominent peaks at ca. 1580 cm−1
and 1335 cm−1
, corresponding to the characteristic D and G bands, respectively. The D band belongs to the breathing modes of sp2
atoms in rings, whereas the G band is assigned to stretching in all pairs of sp2
atoms in both rings and chains [36
]. Generally, the intensity ratio of the D and G bands (ID
) is used to quantify the extent of defects in carbon materials. The ID
ratios for Co-N-GCA, N-GCA, and GCA were 1.25, 1.08, and 0.98, respectively, indicating that Co-N-GCA had more structural defects. In the Raman spectra of Co-N-GCA, there is a small peak at 672 cm−1
, which can be indexed to Co-O.
The electrocatalytic performance of Co-N-GCA for the ORR was initially evaluated separately by cyclic voltammetry (CV) in N2
- and O2
-saturated 0.1 M KOH solutions. As shown in Figure 5
a, the CV curve of Co-N-GCA was virtually featureless in the N2
-saturated electrolyte, while displayed well-defined oxygen reduction cathodic peaks in the O2
-saturated electrolyte. Notably, Co-N-GCA had the more positive ORR peak potential (0.837 V vs. RHE) than commercial 20 wt. % Pt/C (0.841 V vs. RHE) (Figure S4
), indicating the superior ORR catalytic activity of Co-N-GCA.
To further investigate the ORR activity, the linear sweep voltammetry (LSV) curve of Co-N-GCA was recorded at 1600 rpm in O2
-saturated 0.1 M KOH solution. For comparison, we present the LSV curves of Co-N-GCA, N-GCA, GCA, and commercial 20 wt. % Pt/C in Figure 5
b. In terms of the onset potentials and diffusion-limiting current densities of the ORR, Figure 5
b show that the ORR activity follows the order GCA < N-GCA < Co-N-GCA. The Co-N-GCA is more electrocatalytically active toward the ORR than Pt/C, with an onset potential of 0.975 V (vs. RHE), which is 15 mV more positive than that of commercial Pt/C. The diffusion-limiting current density of Co-N-GCA (at 0.6 V) is about 29% higher than that of commercial Pt/C, which further indicates that the mass transport on Co-N-GCA is more efficient than on Pt/C.
To gain more information on the ORR kinetics of the Co-N-GCA catalyst, LSV curves were recorded in an O2
-saturated 0.1 M KOH solution at various rotation rates, increasing from 1600 to 3600 rpm (Figure 5
c). The diffusion current density of oxygen reduction increased with the rotation rate, owing to enhanced mass transport. In addition, the K–L plots at different electrode potentials displayed good linearity. The K–L equation was adopted to calculate the electron transfer number (n) of Co-N-GCA in the potential range from 0.482 V to 0.682 V, and an average n value of 3.96 was obtained, indicating that the ORR proceeded via a four-electron pathway.
The stability of the Co-N-GCA catalyst was further evaluated by LSV in O2
-saturated 0.1 M KOH at 1600 rpm for 1000 continuous cycles. As shown in Figure 5
d, there wasn’t an obvious change in the half-wave potential of Co-N-GCA after 1000 continuous cycles, suggesting that Co-N-GCA has excellent ORR stability in an alkaline medium.
The electrocatalytic OER activity of Co-N-GCA was investigated by LSV measurements, which were conducted in O2
-saturated 0.1 M KOH at 1600 rpm. For comparison, the OER with N-GCA, GCA, and IrO2
/C was also performed under the same conditions (Figure 6
a). OER catalytic activities are commonly judged by the potential required to oxidize water at a current density of 10 mA cm−2
. Significantly, compared with the standard reaction potential (1.23 V), the Co-N-GCA composite can reach 10 mA cm−2
with a small overpotential (η) of 408 mV, while N-GCA, GCA, and IrO2
/C reach the same current density with overpotentials (η) of 515, 685, and 377 mV, respectively.
The Tafel plots were also generated to study the OER kinetics of these catalysts. As shown in Figure 6
b, the resulting Tafel slopes of Co-N-GCA, N-GCA, and GCA were found to be ~65, ~98, and ~160 mV dec−1
, respectively. The Co-N-GCA composite exhibited the smallest Tafel slope, suggesting its extremely favorable reaction kinetics.
Accelerated stability tests in O2
-saturated 0.1 M KOH at room temperature for Co-N-GCA were also carried out to investigate its durability for the OER. As shown in Figure 6
c, after 1000 continuous potential cycles, the overpotential of Co-N-GCA increased by only 20 mV at a current density of 10 mA cm−2
, indicating Co-N-GCA has superior electrocatalytic stability for the OER.
The overall electrocatalytic activity of a bifunctional electrocatalyst as an oxygen electrode can be evaluated by taking the difference in potential between the OER current density at 10 mA cm−2
and the ORR current density at −3 mA cm−2
(ΔE). If the difference ΔE is smaller, this usually indicates the material is more suitable for practical applications. The overall electrocatalytic activities for the above catalysts are shown in Figure 6
d. The ΔE values for Co-N-GCA, N-GCA, and GCA are 0.821, 0.983, and 1.389 V, respectively, showing that the bifunctional catalytic activities of the samples followed the order Co-N-GCA > N-GCA > GCA. More importantly, the ΔE value for Co-N-GCA is comparable to or even much smaller than many nonprecious metal-based bifunctional oxygen electrode catalysts reported previously (Table S3
), suggesting the as-prepared Co-N-GCA is an effective bifunctional catalyst for the ORR and OER.