carbon composite catalysts for the alkaline oxygen evolution reaction

Accepted Manuscript Contribution of carbon support in cost-effective metal oxide/ carbon composite catalysts for the alkaline oxygen evolution reaction

Weiwei Cai, Xinlei Zhang, Jiawei Shi, Jing Li, Zhao Liu, Shunfa Zhou, Xiaomeng Jia, Jie Xiong, Konggang Qu, Yunjie Huang PII: DOI: Reference:

S1566-7367(19)30130-X https://doi.org/10.1016/j.catcom.2019.04.016 CATCOM 5684

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

28 October 2018 15 April 2019 20 April 2019

Please cite this article as: W. Cai, X. Zhang, J. Shi, et al., Contribution of carbon support in cost-effective metal oxide/carbon composite catalysts for the alkaline oxygen evolution reaction, Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.04.016

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ACCEPTED MANUSCRIPT Contribution

of

Carbon

Support

in

Cost-Effective

Metal

Oxide/Carbon Composite Catalysts for the Alkaline Oxygen Evolution Reaction

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Weiwei Cai, a Xinlei Zhang,a Jiawei Shi,a Jing Li,a* Zhao Liu,a Shunfa Zhou, a Xiaomeng Jia,a Jie

a

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Xiong,a Konggang Qub and Yunjie Huang,a*

Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of

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Geosciences Wuhan, 388 Lumo Road, Wuhan, 430074, China.

b

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E-mail: [email protected] (J. Li), [email protected] (Y. Huang). School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059,

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China.

ACCEPTED MANUSCRIPT Abstract In order to reveal the contribution of carbon support in metal oxide-based catalysts for the oxygen evolution reaction (OER), the active carbon is facilely functionalized with various active groups and composite with Ba0.5Sr0.5Co 0.8Fe 0.2O3-δ (BSCF). By

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systemically investigating the electrochemical catalytic properties of the composite

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catalysts, it can be found that the enhancement in the OER activity of BSCF is mainly

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attributed to the co-catalysis effect with the OER process altered. The effect of carbon support on the electron conductivity improvement, which is widely considered in

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electrochemical systems, can be ignored during the OER catalysis on the rotating disk

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electrode. Functionalization with proper groups of the carbon support is, therefore, considered to be an effective and facile strategy to further improve the OER activity

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of metal oxide-based catalysts.

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Keywords: Oxygen evolution reaction, Cost-effective-metal oxide, Carbon support,

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Co-catalysis, Electronic conduction, Specific surface area.

ACCEPTED MANUSCRIPT 1. Introduction Electrochemical evolution of oxygen from water has been widely researched as the cathode reaction for electrochemical water splitting technologies [1], despite the development of cost-effective hydrogen evolution reaction catalysts [2-4]. Oxides of

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Ir and Ru metals are considered the state-of-the-art oxygen evolution reaction (OER)

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catalysts, either under acid [5] or alkaline conditions [6, 7]. In spite of the high cost of

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Ir/Ru, wide application of water electrolysis technology is hindered by the unsatisfied OER catalytic activity of the Ir/Ru oxides [8]. Cost-effective metal oxides (CMOs)

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[9-13] or hydroxides [14-17] were extensively developed for superior OER catalytic

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performance and reduced cost. By engineering proper oxygen defects [18, 19] or regulating the lattice cations [12], OER activity of the metal oxide-based catalysts can

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be boosted by virtue of the altered surface electronic state. Moreover, oxygen defects

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in the ABO 3-type perovskite oxides can be facilely engineered by adjusting the metal

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cation ratios in either A or B sites [20, 21]. Ba0.5Sr0.5Co 0.8Fe 0.2O3-δ (BSCF) was experimentally proved to be one of the most effective perovskite oxide OER catalysts

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and has been recognized as the state-of-the-art noble metal-free OER catalyst [22-24]. Unfortunately, OER activity of the CMO catalysts is still underperformed after considering the great current loading in the practical electrolysis devices. Different strategies were taken to improve the OER activity of the CMO-based catalysts. Various nanostructures were developed [25-27] to expose the active sites embedded in bulky CMOs. Additionally, binary CMOs [28] were designed to produce extra defects or to utilize the synergistic effect of different transition metals.

ACCEPTED MANUSCRIPT The most facile method to boost the OER activity is to use functionalized materials [29] as substrates for the CMO catalysts. Among all the previously studied supporting materials, carbon materials attracted considerable attention due to the fact that their functionalization is facile [29, 30]. It is suggested that a carbon supporting

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material acts as both an electron conducting phase during the three-phase interface

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construction and a co-catalyst assisting the ORR/OER catalysis. By considering the

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co-catalysis effect independently, oxygen-containing intermediates, HO2˗ or H2O2 [31], were supposed to be generated on carbon active sites and which are further during the ORR/OER process [30].

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reduced/oxidized by the neighbored active sites

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By using X-ray absorption near edge structure spectroscopy (XANES), a significant electronic effect of carbon substrate on the metal oxide was also considered as an

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metal oxide catalyst [29].

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important factor for the OER/ORR activity improvement compared to the pristine

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In the present work, the contribution of carbon support in the BSCF/carbon composite catalysts by utilizing BSCF as a model CMO was investigated. Vulcan

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XC-72R active carbon, as the most widely applied carbon support in electrochemical catalytic systems, was facilely modified with different functional groups to combine with BSCF. The effect of various functional groups on OER performance was systematically studied and it can be concluded that the surface nitrogen-containing functional groups are of great benefit in co-catalyzing OER in alkaline conditions. As of the two other potential contributions of carbon support (Sche me 1), increase of specific surface area and that of electron conductivity can be ignored. By considering

ACCEPTED MANUSCRIPT the OER stability of the composite catalyst simultaneously, we believe that proper functionalization of carbon support is a facile and universal strategy to further

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improve the OER activity of (hydr)oxide-based catalysts.

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Sche me 1. Potential contributions of carbon support to OER catalysis on the CMO-based catalysts.

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2. Experimental 2.1. Synthesis of BSCF

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BSCF was synthesized via a traditional sol-gel method according to a previously

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reported study [32]. Appropriate amounts of metal (Ba, Sr, Co and Fe) nitrates, acting as metal precursors, were dissolved in deionized water. EDTA and citric acid were subsequently mixed with the metal nitrates to serve as complexing agents. The solution was then heated to evaporate extra water under stirring to generate a gel mixture. Before calcined at 800 oC for 2.5 h, the gel mixture was dried at 115 oC and the final BSCF product was grounded for further use. 2.2. Carbon support modification The pristine Vulcan XC-72R active carbon (denote as AC) was first treated with

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HNO3 (68 wt%). 100 mg of AC was then added in 20 mL of

concentrated HNO 3 and refluxed at 80 oC for 16 h under magnetic stirring. After being filtered, the acid-treated AC was subsequently rinsed with deionized water to pH=7 to remove any residual acid in the porous AC while the AC surface was

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modified with extra oxygen-containing groups (W-AC) [33]. Alternatively, the filtered

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acid-treated AC was stirred in NH3 solution (0.5 M) to adjust the pH value to 8. NH3

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removes the residual acid and react also with the surface -COOH groups to employ –NH2 groups on the AC surface. The NH 3-treated AC was then filtered and rinsed

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repeatedly with deionized water and the final solid was named N-AC [34]. Both

2.3. Catalysts characterization

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W-AC and N-AC were dried at 100 oC for 12 h before use.

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Fourier-transform infrared (FT-IR) spectra of the carbon supports and the

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composite catalysts were recorded on a Nicolet iS50 spectrophotometer (Thermo

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Fisher Scientific, USA) ranging from 4000 to 400 cm−1. The micromorphology of composite catalysts was observed by using a field emission scanning electron

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microscope (FESEM, Hitch SU8010, Japan). N2 adsorption/desorption isotherms (77 K) for surface texture characterization were recorded with a Micromeritics Tristar II 3020 instrument.

The specific surface area (m2/g) of the samples was estimated

using the Brumauer-Emmett-Teller (BET) method, whereas the pore size distribution was obtained using the Barrett-Joyner-Halanda (BJH) method. 2.4. Electrochemical measurements All the electrochemical measurements, including linear sweep voltammetry (LSV),

ACCEPTED MANUSCRIPT cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) were performed on a Gamry (Interface 1000E, USA) electrochemical workstation with a typical three-electrode system configuration. Typically, The BSCF powder (4 mg) and carbon support (AC, W-AC or N-AC, 4 mg)

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were mixed with 150 μL isopropanol, 830 μL deionized water and 20 μL of 5 wt%

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Nafion solution. The mixture was dispersed under supersonic condition for 1 h to

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obtain homogeneous suspension for all four composite catalysts. Thereafter, 10 μL of the resultant ink (containing 40 μg of BSCF) was pipetted on a glassy carbon rotating

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disk electrode (RDE) surface (diameter of 4 mm, area of 0.1257 cm2) and then

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air-dried (BSCF mass loading was 0.318 mg/cm2). For comparison, pristine BSCF catalyst coated RDE without carbon support was also prepared with the same BSCF

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mass loading (0.318 mg/cm2). LSV at a scan rate of 10 mV/s was carried out under

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O2-saturated 0.1 M KOH electrolyte using RDE coated with varied catalysts as the

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working electrode, graphite rod as the counter electrode, and Hg/HgO electrode as the reference electrode, respectively. The RDE working electrode with casted catalyst

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underwent a cyclic voltammetry, cycled between -0.10 to 1.30 V for 5 cycles prior to LSV evaluation. CV was conducted between 0.16 and 0.26 V at various sweep rates ranging from 20 to 200 mV/s to estimate the electrochemical double-layer capacitances. CV was also conducted between 1.10 and 1.20 V at various sweep rates ranging from 20 to 200 mV/s to evaluate the electrochemical double-layer capacitances of the catalysts. EIS was conducted over the frequency range from 1 MHz to 0.1 Hz with an applied perturbation voltage of 5 mV as the excitation AC

ACCEPTED MANUSCRIPT amplitude and DC voltage biased at various cathodic overpotentials. For the CP stability investigation, the working electrode was biased at the current density at 10 A/gBSCF for 10 h. All of the potentials mentioned in this study were converted to values with reference to a reversible hydrogen electrode (RHE).

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3. Results and discussion

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Figure 1 shows FT-IR spectra of the pristine AC, W-AC and N-AC. No

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remarkable differences can be detected by comparing the FT-IR spectra of the W-AC and pristine AC since the bought XC-72R carbon support (AC) was pre-activated and

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characteristic peaks of –COOH groups can be therefore detected in both spectra.

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the other hand, the intensity of the characteristic IR bands in the W-AC spectrum is stronger than the corresponding one in the AC spectrum, indicating that more

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oxygen-containing functional groups were generated on the W-AC surface during the

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acid treatment, which can also be probed by the reduced BET surface area of the

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W-AC compared to AC (Fig.

S1, ESI). Alternatively, N-AC exhibited further

smaller specific surface area than the W-AC due to the amide groups generated

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through the reaction between –COOH and NH3 on the N-AC, as shown in Fig. 1.

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Figure 1. FT-IR spectra of pristine AC, W-AC and N-AC.

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W-AC, N-AC and the pristine AC were subsequently mixed with BSCF to form

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composite catalysts for OER activity measurements. It can be seen from the SEM images (Fig. 2) that BSCF particles are uniformly covered by the carbon support for

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all the three composite catalysts. Compared with the pristine BSCF, the size of the

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BSCF particles in the composite

catalysts is significantly reduced from ca. 10 μm to

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2-3μm due to the isolation effect of the carbon support [30]. Among the three composite catalysts, carbon coverage on BSCF is higher for BSCF/W-AC and

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BSCF/N-AC compared with BSCF/ AC due to the interaction between the extra functional groups on the carbon support and the oxygen ions on the BSCF surface. The functionalized AC and BSCF can therefore synergistically affect the OER electrocatalytic process.

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Figure 2. SEM images of BSCF, AC, BSCF/AC, BSCF/W-AC and BSCF/N-AC (scale bar: 5 μm).

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LSV curves of the composite catalysts in 0.1 M KOH solution were collected and

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compared with that of the pristine BSCF as shown in Fig. 3. Surprisingly, OER

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activity on the BSCF/AC composite catalyst is slightly decayed compared with the pristine BSCF catalyst. The reason is that the overall ohmic resistance fitted from the

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EIS data at different potentials of the BSCF/AC electrode was enhanced (Fig. S2, ESI). The EIS data of the catalysts were collected in the potential range of 1.515-1.555 V, which is the active potential range for OER on BSCF based composite catalysts (Fig. S3, ESI). Overall ohmic resistances of the three composite catalysts are higher than that of the pristine BSCF due to the fact that the thickness of the catalyst layer on the RDE surface was significantly enhanced by the added AC. In the RDE system, the interface between the glassy carbon and the catalyst is most effective for

ACCEPTED MANUSCRIPT the studied electrochemical reaction [31]. It can be therefore concluded that the benefit of carbon support on electron conductivity improvement in the RDE system is not as great as desired, whereas

the co-catalyst effect of the un-modified AC on

OER can be ignored. As a result, poorer OER activity was achieved for the BSCF/AC

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catalyst compared with the pristine BSCF in the RDE system.

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Figure 3. LSV curves on BSCF and carbon supported BSCF composite catalysts

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measured in 0.1 M KOH solution (sweeping rate: 10 mV/s).

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However, when using W-AC and N-AC as supports of BSCF, OER activity was strongly improved compared with that of BSCF/AC catalyst. At the applied voltage of 1.7 V vs. RHE, OER current densities of the BSCF/W-AC and BSCF/N-AC were increased to 13.2 and 17.0 mA/cm2, which were almost doubled compared with the pristine BSCF catalyst. For the BSCF/N-AC catalyst, OER overpotential was reduced to 380 mV after iR correction (Fig. S4). Even though electrochemically active surface areas, which can be evaluated by double-layer capacity, of BSCF/W-AC and BSCF/N-AC are reduced compared with BSCF/AC (Fig. S5), the improved OER

ACCEPTED MANUSCRIPT activity of the BSCF/W-AC and BSCF/N-AC catalysts can be primarily attributed to the co-catalysis effect of the additionally employed functional groups during the acid treatment of the AC support. In order to clarify the roles of functional groups on the carbon support in the composite catalyst, the OER mechanism on the BSCF/carbon

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composite catalyst has to be studied. It is noted here that typical Tafel slope of

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alkaline OER on BSCF is ca.100 mV/dec due to the fact that lattice oxygen can

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participate in the O-O bond formation during the catalytic OER process. Tafel slope of the corresponding LSV curve of the present pristine BSCF catalyst

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was experimentally measured and found to be 95 mV/dec (Fig. 4a). For all the carbon

indicating a mechanism change

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supported BSCF composite catalysts, there is a significant change in the Tafel slope, during water oxidation under alkaline conditions

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[35]. Tafel slopes of the composite catalysts at high overpotential range are kept

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almost the same as that of the pristine BSCF catalyst. It can be therefore implied that

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BSCF dominates the OER reaction in the high overpotential range. At the relatively low overpotential range, Tafel slope values of the carbon supported BSCF composite

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catalysts are much greater than those at the high overpotential range. In fact, for the pristine BSCF catalyst, no OER current can be detected until the potential of 1.45 V vs. RHE is reached. With carbon materials, even with the pristine active carbon (XC-72 from Cabot) as support, OER on the composite catalysts occurs at much lower potentials, very close to the standard oxidation voltage of water under alkaline conditions. Since the three AC materials did not exhibit OER activity until 1.45 V (Fig.

4b), the improved OER activity of the composite catalysts can be attributed to

ACCEPTED MANUSCRIPT the fact that the AC materials can alter the OER process on BSCF (see Eqs. 1-4). Eqs. 2-3 requiring

two surface active sites are considered as rate-determining steps (RDS)

of OER on BSCF [22], and can be therefore accelerated by the functionalized AC

(2)

Cat-OOH-+Cat-OH-→Cat-OO-+H2O+e-

(3)

Cat-OO-+→O2↑+e-

(4)

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Cat-O2-+ Cat-OH-→Cat-OOH-+OH-+e-

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(1)

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Cat-OH-+OH-→Cat-O2-+H2O+e-

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support.

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Figure 4. (a) Tafel curves of OER on the pristine BSCF catalyst and carbon supported BSCF composite catalysts measured in 0.1 M KOH solution. (b) LSV curves of AC,

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W-AC and N-AC in O2-strated 0.1 M KOH solution (sweeping rate: 10 mV/s). Although N-AC performed slightly poorer than W-AC (Fig. 4b), the BSCF/N-AC performed much better than the BSCF/W-AC since the N-doped carbon active sites were reported to be effective to catalyze the O-O bond formation in the OER process [20]. At the same time, charge transfer resistance of OER on the BSCF/N-AC catalyst is smaller than that on BSCF/W-AC catalyst (Fig. S6). The BSCF/AC catalyst also displayed a lower charge transfer resistance than the pristine BSCF catalyst, in good

ACCEPTED MANUSCRIPT agreement with the OER performance comparison at relatively low voltages. Above all, it can be concluded that carbon support can enhance the OER activity of BSCF catalyst in alkaline conditions mainly via the co-catalysis effect of the functional groups present on the carbon surface.

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To confirm the application potential of the performance improving strategy for

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the BSCF catalyst, it is necessary to evaluate its catalytic stability toward OER on the

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carbon-supported BSCF composite catalysts. The OER stability of the BSCF/N-AC catalyst was therefore measured at 10 A/gBSCF in O 2-strated 0.1 M KOH solution, and

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results are displayed in Fig. 5. It can be seen that BSCF/N-AC catalyst can stably

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catalyze OER during the 10-h test with no increment of the overpotential, which can be confirmed by the remaining of the amide group in the BSCF/N-AC catalyst after

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10 h of OER (Fig. S7, ESI).

Figure 5. Time dependence of OER overpotential on the BSCF/N-AC catalyst at 10 A/gBSCF in O2-strated 0.1 M KOH solution. 4. Conclusions The contribution of carbon support in the BSCF-based composite catalyst during the OER electrocatalytic process was systematically studied by combining physical

ACCEPTED MANUSCRIPT characterization and electrochemical measurements. The carbon support was differently functionalized via a facile acid-treatment and the following conclusions were obtained: (i) OER activity of BSCF catalyst can be remarkably improved by properly

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functionalized the carbon support.

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(ii) The improved OER activity is primarily attributed to the co-catalysis effect of the

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functional groups.

(iii) Effect of carbon support on electron conductivity can be ignored in the RDE

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electrochemical system.

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(iv) OER activity performance of the metal oxide catalyst was not decayed with operational time by the functionalization of the carbon support.

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Acknowledgements

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We are grateful for financial support from the National Natural Science Foundation of

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights

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Carbon support was differently functionalized via facile acid treatment

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OER activity can be remarkably improved by properly functionalized carbon support

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The improved OER activity is primary resulted from the co-catalysis effect

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