J Mater Sci C H E M I C A L R routes O U T E S T Oto M Amaterials T E R I AL S Chemical

IrOx/CNxNTs as electrocatalysts for oxygen evolution reaction in a HCO32/CO2 system at neutral pH Weixin Lv1 , Suxian Liu1 and Wei Wang1,* 1 2

, Rui Zhang1,*

, Wenjuan Wang1

, Zhongxia Wang1

, Lei Wang2

,

School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, China Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, China

Received: 4 August 2017

ABSTRACT

Accepted: 19 December 2017

In the process of electrochemical reduction of CO2 in aqueous solution, the cathode mainly undergoes the CO2 reduction reaction, and the anode undergoes the oxygen evolution reaction (OER). Developing an efficient OER catalyst is very important for improving the energy efficiency of electrochemical reduction of CO2; however, only few reports are concerned with this problem for now. Herein, N-doped multiwalled carbon nanotubes surface modified IrOx nanoparticles (IrOx/CNxNTs) as a highly active catalyst for OER at neutral pH was prepared by solvothermal method. The obtained lowest overpotentials for OER in a HCO3-/CO2 system at neutral pH are 171 and 472 mV at the current densities of 1 and 10 mA cm-2 separately, which are better than the most catalysts those were reported for neutral media. No obvious current density decay was observed after 12-h electrolysis testing. The energy efficiency is a key factor which can reflect the real energy input for CO2 conversion. By using IrOx/ CNxNTs as the anode catalyst, the energy efficiency for electrochemical reduction of CO2 to formate on Sn cathode can achieve 44.7%.

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Introduction Electrochemical reduction of CO2 to useful products has received much attention recently [1–3]. However, this process has low energy conversion efficiency [4], which may discourage practical application in the very near future. The energy efficiency of CO2 conversion can be improved by lowering the overpotential of the CO2 reduction reaction on the cathode or the oxygen evolution reaction (OER) on the anode.

For now, some researchers had developed some efficient cathodic catalysts for lowering the overpotential of the CO2 reduction reaction; however, most of them overlooked the effect of the high overpotential of the anodic OER. It knows that the Pt electrode has low OER activity, whereas it was used as the anode in most studies on the CO2 reduction. Zhao et al. found that the energy efficiency for electrochemical reduction of CO2 to formate was 27% when using the SnOx/MWCNTs electrode as cathode and

Address correspondence to E-mail: [email protected]; [email protected]

https://doi.org/10.1007/s10853-017-1955-z

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Pt wire as anode [5]. Whipple et al. designed a microfluidic reactor with a continuous flowing electrolyte for electrochemical reduction of CO2, it can achieve a faradaic efficiency of 90% and an energy efficiency of 45% for formate production using Snbased gas diffusion electrode as cathode and Pt black electrode as anode. In order to further improve the energy efficiency of the CO2 conversion [6], it is essential to develop highly efficient anodic OER catalysts. Most of the OER studies were carried out under the conditions of strong acid or alkali for water electrolysis [7–9], whereas the strong acid or alkali environment has negative impacts on catalyst activity and electrolysis device and is not suitable for the CO2 reduction reaction. In view of the bicarbonate solution (pH value is neutral) commonly being used as the electrolyte in electrochemical reduction of CO2, it is necessary to study the OER performance of the catalyst in near-neutral pH aqueous solution. OER is a kinetically sluggish process through fourproton-coupled electron transfer, which requires an effective electrocatalyst to accelerate the reaction and reduce the overpotential. It was reported that the noble metal-based catalysts, such as Ir or Ru in the form of metal, oxide, or alloy, have excellent OER performances in acid solution [10–13]. However, their scarce nature and associated high-cost considerably limit large-scale implementation to industrial devices. It is necessary to improve the efficiency and reduce the cost for the practicality and commercialization of electrochemical CO2 conversion. For this purpose, many studies have focused on increasing the catalytic efficiency of the noble metal-based catalysts through tailoring their sizes and dispersing them by various methods [14, 15]. Among them, decorating the noble metal catalyst on stable, conductive support materials is a good approach for improving the dispersion and reducing the dosage of the noble metal. It was reported that the surface layer of N-doped multiwalled carbon nanotubes (CNxNTs) presents negative electricity due to the extra electron of the N atom [16–18]. Thus, CNxNTs can form the composite with functional materials without any pretreatment because of their surface activities. In this paper, CNxNTs surface modified iridium oxide nanoparticles (IrOx/CNxNTs) as an OER catalyst was prepared via solvothermal method and its OER performance was studied in KHCO3 aqueous solution saturated with CO2. It was also used as the

anode material for electrochemical reduction of CO2. The energy efficiency is a key factor which can reflect the real energy input of CO2 conversion; however, most researchers reported the study of the faradaic efficiency of electrochemical reduction of CO2, and only few of them were concerned about the energy efficiency of the process [19]. Herein, the energy efficiency for electrochemical reduction of CO2 to formate using the prepared IrOx/CNxNTs as anode catalyst was studied.

Experimental Preparation of catalysts CNxNTs were prepared from n-propylamine precursor with Co0.1Mg0.9MoO4 catalyst by using the method of chemical vapor deposition [20]. Thirty milligrams of CNxNTs powder was dispersed in alcohol/water (volume ratio, 9:1) solution and sonicated for 1 h. Then, different amounts of H2IrCl6 were added to the above solution and stirred for 2 h at room temperature. After that, the mixture was stirred for 6 h at 80 °C and transferred to an autoclave for solvothermal reaction at 150 °C for 4 h. The resulting product was centrifuged, washed with alcohol/water solution, and dried in vacuum oven at 80 °C for 10 h. The catalysts with different mass concentrations of Ir (8, 15, 25 and 35%) were obtained and denoted hereafter as 8-Ir/C, 15-Ir/C, 25-Ir/C and 35-Ir/C, respectively.

Modified electrode preparation Three milligrams of the as-prepared catalyst was dispersed in 0.2 mL of alcohol and 6 lL of 5 wt% Nafion solution by ultrasonication for 30 min. Then 5 lL of catalyst ink was spread on an L-style glassy carbon electrode (GCE, 3 mm in diameter) and dried in air. Prior to each modification, the GCE was polished with alumina powder, sonicated in alcohol and deionized water successively and dried under nitrogen.

Electrochemical studies All the electrochemical measurements were taken in an undivided three-electrode system on a CHI660E electrochemical workstation at room temperature.

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The electrolyte was 0.1 M KHCO3 aqueous solution bubbled with CO2 for 30 min (pH = 6.98). For the study of the performance of OER, the GCE modified with the as-prepared catalyst or a Pt electrode was used as the working electrode. The counter electrode was a Pt plate. The reference electrode was an Ag/AgCl electrode (sat. KCl). Linear sweep voltammetry (LSV) was tested under a scan rate of 2 mV s-1. The current density (j) was determined on the area of the working electrode. Potentials in the graphs were reported versus RHE. The conversion between Ag/AgCl and RHE was given from the following relation: ERHE = EAg/AgCl ? 0.199 V ? 0.0591 9 pH. Overpotentials were calculated by using the relation: g = ERHE - 1.23 V. For the study of the performance of CO2 reduction reaction, a Sn foil was used as the working electrode. A Pt plate (0.07 cm2) or IrOx/CNxNTs modified GCE (0.07 cm2) was chosen as the counter electrode, and an Ag/AgCl electrode (sat. KCl) was used as the reference electrode. LSV measurements were taken at a scan rate of 50 mV s-1. Electrolysis was performed at cathode potential in 0.1 M KHCO3 aqueous solution (saturated with CO2 before electrolysis) using a LAND CT2001C cell performance-testing instrument. CO2 was constantly aerated during the electrolysis process. The schematic of electrochemical reduction of CO2 using Sn foil as the cathode and the IrOx/ CNxNTs modified GCE as the anode is illustrated in Fig. 1.

Figure 1 Schematic of electrochemical reduction of CO2 on Sn cathode and IrOx/CNxNTs anode.

Analysis and calculations The morphologies of the samples were examined with transmission electron microscopy (TEM, JEOL2100). The elements were quantified by X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250). The liquid product (formate) was analyzed by ion chromatography (ICS-900 Dionex). The column was an IonPac AS11-HC anionic column using 0.02 M KOH as the mobile phase at the rate of 1 mL min-1. The faradaic efficiency for the formation of formate (f) was calculated as follows: f¼

2nF Q

ð1Þ

where n is the number of the formate molecules; F is Faraday’s constant; and Q is the total charge passed. The energy efficiency was calculated as follows: Energy efficiency ¼

E  f Applied cell potential

ð2Þ

where E° is - 1.43 V which represents the standard electromotive force of the cell.

Results and discussion Catalysts characterization Figure 2a shows the TEM image of CNxNTs. CNxNTs have typical bamboo-like structure, and their diameters are between 18 and 20 nm. Figure 2b reveals that the N element really exists in CNxNTs and its atomic concentration is 4.15%. The N 1s XPS spectrum of CNxNTs is shown in the inset of Fig. 2b, which is mainly composed of three peaks. The peaks at 398.5 and 400.7 eV can be assigned to the N atoms in the pyridinic-like N–C structure and the graphiticlike N–C structure, respectively. The peak at 402.8 eV can be assigned to nitrogen oxides of pyridinic-like N [21]. Figure 3 presents the TEM images of IrOx/CNxNTs. It can be found that ultrafine IrOx nanoparticles are uniformly dispersed on CNxNTs with a diameter of 1–2 nm. As the increase in the Ir loading amount, the reunion phenomenon of IrOx nanoparticles is more evident, especially for 25-Ir/C and 35-Ir/C. Figure 4a presents the XPS survey spectra of IrOx/ CNxNTs, indicating the presence of the elements C, N, Ir and O. With the increase in the Ir loading amount, the atomic concentrations of Ir and O

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Figure 2 TEM image (a) and XPS survey spectrum (b) of CNxNTs. The inset of b was the XPS spectrum of CNxNTs in the N 1s region.

increase gradually, whereas the C atomic concentration decreases gradually (as shown in Fig. 4b). This is due to that the surface of CNxNTs is covered by more IrOx nanoparticles. After the IrOx/CNxNTs catalysts were synthesized, their weights were measured, which are shown in Fig. S1. It can be seen that the weight of the IrOx/CNxNTs increases with the iridium content. The inset of Fig. 4a shows the XPS spectra of IrOx/CNxNTs in the Ir 4f region. The binding energies of Ir 4f7/2 for Ir0, Ir3?and Ir4? reported in the literature are around 60.7, 61.5 and 62.2 eV, respectively [22]. It can be seen from the inset of Fig. 4a that the binding energy of Ir-4f7/2 has a wider range which covers the binding energies of Ir0, Ir3?and Ir4?. The results indicate that the oxidation state of iridium may be composed of 0, ? 3 and ? 4. The XRD patterns of CNxNTs and IrOx/CNxNTs are shown in Fig. S2. No obviously diffraction peaks of IrOx nanoparticles can be observed, which is due to the low crystallinity of the IrOx nanoparticles. The above analysis results revealed that IrOx nanoparticles were successfully decorated on the surface of the CNxNTs. The small size of the nanoparticles may be beneficial to the enhancement of the catalytic activity.

OER performance of the composites For investigating the OER performances, the LSV measurements of Pt, CNxNTs and IrOx/CNxNTs were taken in CO2 saturated 0.1 M KHCO3 solution (Fig. 5a). As shown in Fig. 5a, IrOx/CNxNTs exhibit

excellent OER activities compared with Pt and CNxNTs. Although the CNxNTs show poor catalytic activity for OER, the CNxNTs support is necessary which can improve the electrical conductivity of IrOx/CNxNTs [23]. 15-Ir/C shows superior OER activity and only demands smaller overpotential of 171 and 472 mV to afford geometrical catalytic current densities of 1 and 10 mA cm-2, respectively. The OER activities of 25-Ir/C and 35-Ir/C are a little lower than that of 15-Ir/C. This strange phenomenon may due to the agglomeration of the IrOx nanoparticles on 25-Ir/C and 35-Ir/C, but the IrOx nanoparticles on 15-Ir/C exhibit high dispersion with small particle sizes (as shown in Fig. 3). Tafel plots of IrOx/CNxNTs in Fig. 5b indicate that 15-Ir/C possess a smallest Tafel slope at 170 mV dec-1. A small Tafel slope is very important for practical applications, because it could give a remarkable increase in OER rate. The Tafel slope for OER in neutral solution is rarely reported. Most Tafel slopes obtained in the acidic or basic solution are lower than 100 mV dec-1 [24, 25]. To our knowledge, Tafel slope is a crucial parameter to study the reaction mechanism and is related to the Butler–Volmer equation: bA ¼

2:303  R  T aA  n  F

ð3Þ

where bA is the Tafel slope, R is the universal gas constant, aA is the charge-transfer coefficient, n is the number of exchanged electrons in the reaction, and

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Figure 3 TEM images of 8-Ir/C (a, b), 15-Ir/C (c, d), 25-Ir/C (e, f) and 35-Ir/C (g, h).

F is the Faraday constant. On the basis of the equation mentioned above, it can be concluded that the electron-transfer rate of OER in the neutral solution is lower than that in the acidic or basic solution. Figure 5c further compares the mass activities of the catalysts per unit Ir loading amount of IrOx/ CNxNTs at a fixed overpotential of 472 mV. The results indicate a much higher mass activity for 8-Ir/ C (0.097 mA lg-1 Ir). As the increase in Ir loading amount, the mass activity decreases. It also can be seen at the fixed overpotential of 472 mV that the 15-Ir/C (0.063 mA lg-1 Ir) has big current density

compared with other three Ir-loaded samples. The stability of IrOx/CNxNTs measured by bulk electrolysis at the fixed overpotential of 472 mV demonstrates that the OER current densities of IrOx/ CNxNTs could be maintained for 12 h without significant degradation, also suggesting the good stability of IrOx/CNxNTs for neutral OER electrocatalytic process (Fig. 5d). It is very important to compare the performance of catalysts with related literature results. So far, few OER studies in neutral electrolytes have been reported. Table 1 shows the overpotentials of the catalysts

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Figure 4 a XPS survey spectra of IrOx/CNxNTs and b contents of C, N, O and Ir in CNxNTs and IrOx/CNxNTs obtained by XPS analysis. The inset of a is the XPS spectra of IrOx/CNxNT in the Ir 4f region.

used in the neutral electrolytes at different electrolytic conditions. It can be seen that the 15-Ir/C catalyst has better OER performance than most of the reported catalysts. The OER overpotential on RuO2/ Ti mesh (10 mA cm-2) is better than that on 15-Ir/C, whereas the noble metal loading on the RuO2/Ti mesh (2.4 mg cm-2) is higher than that on 15-Ir/C (0.16 mg cm-2). The overpotential of the 8-Ir/C catalyst is only 36 mV higher than that of the 15-Ir/C catalyst at the current densities of 10 mA cm-2, which shows that the 8-Ir/C catalyst will be a promising material because of its low noble metal loading. A non-precious metal catalyst Co–Pi nanoarray/Ti shows better OER performance than 15-Ir/C [29], which indicates that the cost of the high performance OER catalysts can be reduced in the future.

Study of the energy efficiency for CO2 conversion We are very interested in the practical result of the 15-Ir/C catalyst (robust OER catalyst) for CO2 reduction. Therefore, the electrochemical reduction of CO2 was carried out by using the commonly used Sn foil as the cathode and the 15-Ir/C modified GCE or the commonly used Pt electrode as the anode. According to the results of the literature and our previous work, the optimal cathode electrolysis potential for electrochemical reduction of CO2 was

- 1.8 V versus Ag/AgCl [34, 35]. However, for calculating the energy efficiency, only knowing the cathode potential is not enough, it is necessary to know the applied cell potential (the potential between the cathode and anode). Here, we designed a simple experiment for obtaining the optimal applied cell potential. Firstly, the value of the current density (- 3.2 mA cm-2) for the reduction of CO2 on the Sn cathode and the Pt anode is obtained at - 1.8 V versus Ag/AgCl from curve a in Fig. 6. Secondly, curves b and c in Fig. 6 are recorded in two-electrode mode (Sn cathode, Pt anode or 15-Ir/C anode, without using reference electrode) for obtaining the cell potentials. When their current densities reach - 3.2 mA cm-2, the corresponding potentials are the optimal electrolysis cell potentials. The values are 3.50 V (Pt anode) and 2.75 V (15-Ir/C anode), respectively. The electrolysis experiments were carried out for electrochemical reduction of CO2 on Sn cathode at the cell potentials of 3.50 and 2.75 V when the Pt electrode and the 15-Ir/C modified GCE are used as the anodes, respectively, and the results are shown in Fig. 7. It is clearly seen that the cell potential for the 15-Ir/C anode is 0.75 V lower than that of the Pt anode, and the current density curves of the 15-Ir/C anode and the Pt anode are nearly the same. After the electrolysis process, the obtained faradaic efficiencies for producing formate by using the two anodes are both around 86%. Although the faradaic efficiencies

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Figure 5 a LSV curves of Pt, CNxNTs and IrOx/CNxNTs for OER in CO2 saturated 0.1 M KHCO3 solutions. b Tafel plots of IrOx/CNxNTs. c Mass activities per unit Ir mass loading

(histogram) and current densities (filled square) of IrOx/CNxNTs at a fixed overpotential of 472 mV. d Time-dependent current density curves of IrOx/CNxNTs at a fixed overpotential of 472 mV.

are close, the energy efficiency of the 15-Ir/C anode (44.7%) is higher than that of the Pt anode (35.1%). This result can be due to the excellent OER performance of the 15-Ir/C catalyst. Zhao et al. [6] reported that the obtained faradaic efficiency was 64%, and the energy efficiency was only 27% when using SnOx as the cathode catalyst for electrochemical reduction of CO2 to formate. Hori reported that the energy efficiency of electrochemical reduction of CO2 to useful products (formate, CO or methanol) would be roughly 30–40% under appropriate conditions [36]. Our previous work reported that the energy efficiency for electrochemical reduction of CO2 to formate on Sn plate cathode was 35.6% when a Pt electrode was used as the anode, and it increased to 42.1% when an IrxSnyRuzO2/Ti electrode was used as the anode [37]. Apparently, the energy

efficiency obtained in this work is a high value compared to the values reported in the literature, because 15-Ir/C is an efficient catalyst which can lower the overpotential of OER. In practical application, 15-Ir/C as the anode catalyst in electrochemical reduction of CO2 can be used not only for producing formate, but also for producing CO, methanol or hydrocarbons, and so on.

Conclusions By using the inherent surface activity of CNxNTs, IrOx nanoparticles have been successfully anchored on the surface of CNxNTs by solvothermal method. The obtained IrOx/CNxNTs catalyst shows excellent catalytic activity for OER in CO2 saturated 0.1 M

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Table 1 Comparison of OER performances of the electrocatalysts in neutral electrolytes Catalyst

Electrolyte

Overpotential (mV)

Current density (mA cm-2)

Reference

15-Ir/C

0.1 M KHCO3 saturated with CO2 (pH = 6.98) 0.1 M KHCO3 saturated with CO2 (pH = 6.98) 0.1 M KHCO3 saturated with CO2 (pH = 6.98) 0.1 M NaHCO3 saturated with CO2 (pH = 6.7) 0.5 M Na2SO4 (pH = 6.6) 0.1 M Na2SO4 (pH = 7.0) 0.1 M PBS (pH = 7.0) 0.1 M PBS (pH = 7.0)

472

10

171

1

508

10

* 710

10

This work This work This work [26]

540 606 * 200 620

10 1 1 3

[27] [28] [29] [30]

0.1 0.1 0.1 0.1

460 * 440 590 400

10 10 10 1

[31] [31] [32] [33]

15-Ir/C 8-Ir/C NiOx film Bi2WO6 nanoplates Co3O4 nanorods CoPi/N-graphene Ultrathin Co3O4 nanosheets Co–Pi nanoarray/Ti RuO2/Ti mesh Co(PO3)2 nanoparticles Co3O4/SWNTs

M M M M

PBS PBS PBS PBS

(pH (pH (pH (pH

= = = =

7.0) 7.0) 7.0) 7.0)

Figure 6 a LSV curve on a Sn electrode with a Pt counter electrode and an Ag/AgCl reference electrode for electrochemical reduction of CO2. LSV curves on a Sn electrode with a 15-Ir/C modified GCE (b) or a Pt electrode (c) as counter electrode for electrochemical reduction of CO2 recorded in two-electrode mode. The measurements were taken in CO2 saturated 0.1 M KHCO3 solutions.

KHCO3 solution. 15-Ir/C is the most suitable catalyst which shows the lowest overpotential of 171 and 472 mV at the current densities of 1 and 10 mA cm-2, respectively. 8-Ir/C shows highest mass activity (0.097 mA lg-1 Ir). Overall, less load of noble metal

Figure 7 Variations in the current density and cell potential with the electrolysis time for electrochemical reduction of CO2 on the Sn cathode using the Pt electrode (open square) or the 15-Ir/C modified GCE (filled circle) as the anode in 0.1 M KHCO3 solution.

Ir will greatly improve the OER performance of the CNxNTs catalysts. The obtained faradaic efficiencies for producing formate on Sn cathode are both around 86% by using the Pt anode and the 15-Ir/C anode, whereas the energy efficiency of the 15-Ir/C anode (44.7%) is higher than that of the Pt anode (35.1%). Through this work, it can provide effective reference for the further researches on OER catalysts and the

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energy efficiency for CO2 conversion under neutral conditions.

[7]

[8]

Acknowledgements The work was supported by the National Natural Science Foundation of China (21603184, 21575123, 21705140), the Natural Science Foundation of Jiangsu Province (BK20170474), and the joint research fund between Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (GX2015105).

[9]

[10]

[11]

Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.

[12]

Electronic supplementary material: The online version of this article (https://doi.org/10.1007/ s10853-017-1955-z) contains supplementary material, which is available to authorized users.

[13]

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