Electroreduction of CO2 to CO on a Mesoporous Carbon Catalyst with

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Electroreduction of CO to CO on a Mesoporous Carbon Catalyst with Progressively Removed Nitrogen Moieties Rahman Daiyan, Xin Tan, Rui Chen, Wibawa Hendra Saputera, Hassan A Tahini, Emma Catherine Lovell, Yun Hau Ng, Sean C. Smith, Liming Dai, Xunyu Lu, and Rose Amal ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01409 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Energy Letters

Electroreduction of CO2 to CO on a Mesoporous Carbon Catalyst with Progressively Removed Nitrogen Moieties Rahman Daiyan#,1 Xin Tan#, 2 Rui Chen#, 1 Wibawa Hendra Saputera,1 Hassan A. Tahini,2 Emma Lovell, 1 Yun Hau Ng,1 Sean C Smith, 2 Liming Dai, 1,3 Xunyu Lu,1* Rose Amal1* 1

School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia.

2

Integrated Materials Design Laboratory, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia. 3

Center of Advanced Science and Engineering for Carbon (Case4carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA. Corresponding Author Rose Amal ([email protected]) Xunyu Lu ([email protected])

1 Environment ACS Paragon Plus

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Abstract In this study, we prepared nitrogen removed mesoporous carbon (NRMC) catalysts by applying various heat-treatments to nitrogen-doped mesoporous carbon (NMC), which were applied as novel electrocatalysts for CO2 reduction reaction (CO2RR). With the nitrogen moieties being progressively removed, the NRMC catalysts exhibited enhanced CO generation from CO2RR, whereas the competing hydrogen evolution reaction (HER) has been suppressed. Through suitable annealing treatment, the defect-rich NRMC catalyst is able to convert CO2 to CO with a Faradaic efficiency (FECO) of ~ 80% and a partial current density for CO (jCO) of -2.9 mA cm-2 at an applied overpotential of 490 mV. Density Functional Theory (DFT) calculations further revealed the active sites within NRMC catalysts were the defects generated by N removal, which lowered the energy barriers for CO2RR and will not be passivated by hydrogen. These findings provide design guidelines to develop efficient carbon-based catalysts that can display metal-like, and even better, performances for potential scalable CO2RR to fuels and chemicals.

ToC Graph.


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CO2RR is a versatile strategy in addressing global climate change by converting excessive CO2 in the atmosphere into useful feedstock such as CO, which can then be readily used as a building block for further chemical and fuel synthesis using current commercial technology such as the Fischer-Tropsch process.

1,2

Despite significant progress in this field, the current benchmarking

catalysts are still based on precious metals (e.g. Au, Ag and Pd)

3–5

that require high

overpotentials to generate desired products at appreciable rates (normally reflected as current density). In light of such limitations, nitrogen doped carbon catalysts have been extensively investigated as cost-effective alternatives for CO2RR.

6

These earth-abundant catalysts possess

some apparent benefits, notably a high surface area, high material conductivity and opportunity for tuning dopant concentration. For instance, nitrogen-doped carbon nanotubes and graphene are reported as active catalyst material for CO2RR to CO, where the pyridinic N sites are conjectured to play the major role in lowering the free energy for CO2 activation for CO2RR to CO.

7–9

However, other results indicated that quaternary N, graphitic N or even the C atoms

adjacent to N species may also act as the active sites for CO2RR.

10–12

Moreover, during the

preparation of N-doped carbon nanomaterials, thermal treatments at elevated temperature (e.g. 900oC) were normally adopted, which inevitably lead to the generation of defects caused by N dissipation during heating. 13 Specifically, the thermal treatment of N-doped carbon nanomaterial is reported to generate defects such as point (5-8-5) and Stone-Wales (5-7-7-5) defects (reported as being most common carbon defects), which can effectively catalyze hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). 13–16 As a consequence, the possible contribution of defects (generated by N removal) toward CO2RR must be properly evaluated in conjunction with previously reported N sites.

17

To this end, we

first prepared N-doped mesoporous carbon materials (NMC) and through precisely controlling the annealing treatments, we are capable of tuning the amounts of N species and defects; and



studying their respective roles in catalyzing CO2RR. It was found in this study that the defect rich, nitrogen removed mesoporous carbon (NRMC) is an efficient, metal-free electrocatalyst for CO2RR, which can convert CO2 to CO with a jCO of -2.9 mA cm-2 and a FECO of 80% at only 490 mV overpotentials, while the HER has been significantly suppressed. These results, along with the DFT calculations reveal the decisive role of defects rather than the previously invoked N species in catalyzing CO2RR. To carry out our systematic investigation, we first prepared NMC using a two-step fabrication technique (details refer to Experimental Methods in Supporting Information). NMC was found to display poor activity for CO2RR owing to the poor crystallinity and pore sintering (Figures S1-2). The NMC was then used as the precursor to produce a series of NRMC catalysts by subjecting it to an additional annealing treatment at 800oC, 900oC and 1000oC (NRMC-800, NRMC-900 and NRMC-1000), respectively. These catalysts were then tested for CO2RR. Only two gas phase products, H2 and CO were detected within all the catalysts and no liquid products were observed (as confirmed by NMR analysis). The detailed electrocatalytic results with the NRMC catalysts are displayed in Figure 1. The Faradaic efficiency towards CO (FECO) and H2 (FEH2) was shown to be dependent on the temperature applied to prepare the NRMC catalysts (Figure 1a & Figure S3a). Evaluating the catalysts on the basis of FE, it is clear that FECO increases with the raised annealing temperature while FEH2 faces an adverse trend. To be specific, NRMC-800 exhibited the lowest FECO at any given potentials among the three samples tested herein, with the maximum of merely 50% obtained at -0.6 V. In contrast, NRMC-900 and NRMC-1000 exhibited a FECO of 61% and 72%, respectively, at the same applied potential. This observation indicates the formation of additional active sites for CO2RR to incur as the annealing temperature elevated. Additionally, we determined the jCO with the three catalysts (Figure 1b) and discerned that the NRMC-900 demonstrated a higher jCO


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compared to NRMC-800 and NRMC-1000. Interestingly, even though these catalysts exhibited distinctive catalytic performances for CO2RR, their electro-kinetics are alike as confirmed by Tafel analysis (see details in Figure S3b). We confirm that the same rate determining step (single-electron transfer process) is applicable to all the NRMC catalysts and therefore the variation in CO selectivity as a result of increased annealing temperature can be attributed to material properties rather than the possible alteration in CO2RR pathway. 18

Figure. 1. Electrocatalytic performance of NRMC catalysts in CO2 saturated 0.1 M KHCO3. Dependence of (a) FECO and (b) jCO with applied potential for NRMC-800, NRMC-900 and NRMC-1000 in CO2 saturated 0.1 M KHCO3. Error bars represent the standard deviation from at least three independent measurements A series of characterization techniques are then applied to identify the physical and chemical properties of the NRMC samples. We first probed the surface morphology of the assynthesized catalysts using transmission electron microscopy (TEM). It can be observed from the TEM images (Figure S4) that even after the nitrogen removal step from NMC (to generate NRMC catalysts), the NRMC has largely retained the porous structure of NMC. A high magnification inspection of the TEM images obtained with NRMC-900 (Figures 2a-b) also indicated the porousness and high ordering of the catalyst where the individual segments were connected into a hierarchical porous structure. Additionally, Brunauer-Emmett-Teller (BET) characterizations (Figure S5-6) revealed that all the NRMC catalysts demonstrated a Type IV



hysteresis loop at relative pressure P/P0 > 0.5, indicating the presence of mesoporous structure. 19 The BET results were also in conformity with the TEM findings, with all NRMC catalysts displaying smaller surface areas compared to NMC (Supplementary Table S1). The X-ray diffraction (XRD) patterns with the NRMC catalysts are all alike (Figure S7), however a peak splitting at 2θ ~ 43o was observed within NRMC-1000, indicating the possible occurrence of significant structural reconfiguration as a result of the high temperature applied. The above results together with electrochemical surface area measurements (ECSA, Figure S8) reveal the fact that the physical changes of NRMC catalysts have no direct correlation with their varied capability towards CO2RR to CO. To further corroborate this, electrochemical impedance spectroscopy (EIS) measurements (Figure S9) were carried out with the NRMC catalysts and from the Nyquist plots, it is clear that the NRMC catalysts displayed similar impedance despite the varying annealing temperatures used during fabrication.


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Figure. 2. Physical and chemical properties of NMRC catalysts. (a, b) TEM images of NRMC-900 demonstrating the porous and ordered architecture of the catalyst. (c) Highresolution N1s XPS spectra, (d) Raman spectra, and relationship between (e) N content and (f) ID / IG with FECO (catalytic data obtained at an applied potential of -0.7 V) for NRMC-800, NRMC900 and NRMC-1000. Then, we turn our attention to the chemical changes of NRMC as a result of varied annealing temperatures. The chemical compositions of NRMC samples were firstly studied by X-ray photoelectron spectroscopy (XPS, Figures S10-11). As expected, only peaks for C, N and O can be observed with the catalysts (Figure S10) and the intensity of N species declined as the temperature was elevated. The high-resolution N 1s spectra (Figure 2c) of the NRMC catalysts



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were deconvoluted into four major peaks centred at ~ 398.5 eV, 399.7 eV, 401.2 eV and 402.6 eV, corresponding to pyridinic, pyrrolic, graphitic and oxidized pyridinic N, respectively.

8,20,21

From these spectra, it is clear that the higher temperature leads to more significant removal of N species, with pyridinic N (decreased from 2% to 0.6%), pyrrolic N (0.6% to 0.2%) and graphitic N (3.5% to 2.6%) decreasing as the temperature is raised from 800oC to 1000oC. (Table S2). The removal of N species will inevitably lead to the generation of defects, as indicated by the Raman studies. Displayed in Figure 2d, the ID / IG ratio (which represents the ratio of the intensity of the disorder with respect to the crystallinity of the related graphitic carbon material) increased from 0.926 for NRMC-800 to 0.959 for NRMC-900 and 0.969 for NRMC-1000 (Table S3), suggesting the formation of edge defects, such as the Thrower-Stones-Wales (5-7-7-5) and point defects (5-8-5 defects) which are among the most common carbon defects, as demonstrated by a symmetry breaking Raman D-band at 1350 cm-1.

22,23

It is known that the thermal treatments to

remove N from the carbon lattice causes the formation of vacant sites, which then re-arranges to form rings of various sizes such as pentagons, heptagons and octagons to minimize energy, thereby leading to the formation of 5-7-7-5 and 5-8-5 defects. 13,24 From these results, it becomes apparent that within NRMC catalysts, defects are playing an important role as the active sites for CO2RR in addition to the previously reported N,

7–9

since NRMC-800 exhibited lower FECO

compared to NRMC-900 and NRMC-1000 (Figure 2e). On the contrary, NRMC-1000 displayed the highest FECO. In fact, the trend of improvement in CO selectivity as a result of temperature variation can be directly correlated to the increasing defects (as indicated by Raman Spectroscopy, Figure 2f) and in light of such findings, defect sites emerge as the main contender for the major active sites for CO2RR to CO within our system. Our results established a substantially high temperature (e.g. 900oC and 1000oC) is required to treat the NMC to remove a significant amount of N moieties thereby endowing the


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resultant NRMC catalysts decent catalytic activity for CO2RR (Figure 1a). It has to be mentioned herein that even though NRMC-1000 exhibited the highest FECO among the NRMC catalysts, the extremely high temperature has caused pore sintering and shrinkage (Table S1), and diminished BET surface area. Thus, the jCO obtained with NRMC-1000 is smaller than NRMC-900, regardless of its higher FECO (Figure 1b). Additionally, it is known that high temperature annealing can induce edge recrystallization which would repair the defect sites present in graphene. 25 As a result, NRMC-1000 (despite removal of majority of N species) still presented low amounts of defects (indicated by the relatively small change in ID/IG as the temperature was increased from 900oC to 1000oC) and therefore it is imperative to prepare NRMC catalysts with greater defects to further improve CO2RR activity. In this regard, instead of annealing NMC precursors at extremely high temperatures, a novel method was designed to progressively remove N moieties from the NMC composites by annealing it at the same moderate temperature (900oC) for multiple times (details can be referred to Experimental Methods). Herein, NRMC-900-2 and NRMC-900-3 catalysts were obtained via annealing the NRMC-900 for a second and third time at 900oC. The activity of NRMC-900-2 and NRMC-9003 catalysts for CO2RR was tested and compared (with NRMC-900), as shown in Figures 3a & b. The CO2RR activity results indicated that the additional annealing treatments with NRMC-900 can lead to significantly improved CO2RR catalytic activity, reflected as higher FECO and larger jCO. All the catalysts prepared in this study demonstrated prominent stability towards long-term usage (as the example of NRMC-900-2 shown in Figure S12). Surprisingly, the NRMC-900-3 catalyst even exhibited a metal-like catalytic performance for reducing CO2 to CO. To be specific, the maximum FECO and jCO attained with NRMC-900-3 is 80% and -2.9 mA cm-2 at 0.6 V, respectively. These values are among the highest (Figure S13) for metal-free catalysts for CO2RR to CO in aqueous electrolyte at low applied overpotentials. Moreover, the NRMC-900-3



catalyst demonstrated a CO2RR performance that is on par with the benchmarked metal-based catalysts (e.g. Au, Ag, Zn, Cu) for CO generation (Table S4), thereby advocating the suitability of the catalysts for potential large-scale applications.

Figure. 3. Improvement in CO selectivity for NRMC-900 catalysts as a result of additional annealing treatments at 900oC and defects present in NMC and NMRC catalysts. Dependence of (a) FECO and (b) jCO with applied potential for NRMC-900, NRMC-900-2 and NRMC-900-3 in CO2 saturated 0.1 M KHCO3. Error bars represent the standard deviation from at least three independent measurements. (c) EPR spectra at 120 K and (d) Bar graphs indicating double-integrated intensity of defects in EPR spectra for NMC and NMRC catalysts. The possible contributions from physical changes upon multiple annealing can also be excluded using a range of characterizations mentioned above (Figures S14-18). Moreover, EIS results (Figure S19) indicated that the overall change of physical properties (including conductivity) of the NRMC-900 catalysts are insignificant as the catalysts exhibited similar


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semi-circle radius for the Nyquist plots, therefore the contribution from reduced impedance can be ruled out. With the physical factors ruled out, we turn our attention to disparity in chemical composition and generated defects within NRMC-900 catalysts to explain the CO selectivity and improvement in jCO (Figures S20-21). With the progressive removal of N moieties, especially pyridinic N (from 1.6% to 0.9%) and pyrrolic N (0.5% to 0.37 %) species, more defects were produced within the NRMC catalysts. The Raman spectra (Figure S22 and Table S3) revealed that the ID/IG increased from 0.959 (NRMC-900), 0.979 (NRMC-900-2) to 1.008 (NRMC-9003). From these holistic catalytic results and material properties presented above, it is clear that defects (generated from the loss of N species), rather than the N moieties, are playing the role of active sites for CO2 reduction to CO within NRMC catalysts. We are also able to observe the zig-zag edges that are known to be correlated with defects within graphene structures by HRTEM (Figure S23). The visual determination of the exact types of defects in the NRMC catalysts by HRTEM is extremely difficult owing to their hierarchically porous structures, therefore, electron paramagnetic resonance (EPR) spectroscopy (Figure 3c & d) was employed to further confirm the presence of localized edge defects.

26–29

It is known that the presence of dangling

bonds and the existence of vacant atom sites within carbon materials induces magnetism in carbon samples, giving rise to strong EPR signals at g=2.004.

26,30

Similarly, the EPR spectra

with our NMC and NRMC catalysts (Figure 3c) also exhibited a similar distinct peak at g=2.001 and we detected that the intensity of the EPR signal (as measured by the double integrated intensity of EPR in Figure 3d) follows the same trend as all the ID/IG ratio measured using Raman Spectroscopy with the catalysts. Overall, we conclude that catalysts (e.g. NRMC-900-3) with greater defects (as indicated by Raman and EPR) demonstrate a higher activity for CO2RR to CO.



Figure. 4. Free energy diagrams of CO2RR and HER. Calculated free energy diagram for CO2RR and HER at (a, b) N-doped sites and (c, d) 5-8-5 and 5-7-7-5 (Stone Wales Defect, SW) edge defect sites, respectively. To gain further theoretical insights, DFT calculations (Figure 4) were carried out where we considered the reaction mechanism of CO2RR to CO through the well accepted COOH* adsorbed intermediate

7,31–33

on all possible active sites that includes the different N-doped sites

and edge defects on NRMC (Figures S24 & 25). It is clear from the free energy diagram for the lowest energy pathways of CO2RR to CO (Figure 4a) that the pristine graphene has the largest free energy barrier (~2.52 eV) for the first proton-coupled electron transfer step to form COOH*, thereby hindering CO2RR. Moreover, calculations demonstrated that the introduction of N atoms in the graphitic lattice (specifically triple-pyridinic, single-pyridinic, pyrrolic, and edge-pyrrolic N) would lower energy barriers for CO2RR to CO (< 0.71 eV). However, to fully understand the


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role of N-doped sites for CO2RR, we must also consider the competing HER, which can be evaluated by calculating the free energy barrier for the formation of intermediate H* (ΔG∗ ), which was reported as a key descriptor for HER activity on electrocatalysts.

34

The free energy

diagram (Figure 4b) for the lowest energy pathways of HER at all N-doped sites revealed very negative values for ΔG∗ ( 1.23 eV), which hinders CO2RR on H-passivated N-doped sites (Figure 4a), indicating N-doped sites are not the main active sites for CO2RR with NRMC catalysts. With theoretical calculations ruling out the role of N sites for CO2RR, we then turn our attention to the various edge defects on NRMC catalysts. From the calculated free energy diagram for the lowest energy pathways for CO2RR to CO on edge defect sites (Figure 4c), we determine three defect atomic sites, namely 585_1, 585_2 and SW_1 (5-7-7-5) exhibit small barriers for CO2RR to CO (< 0.68 eV). Additionally, the hydrogen binding energy on edge defect atomic sites are not very strong (| ∗ | < 0.45 eV), indicating the edge defect atomic sites are not to be passivated by strongly bounded H* (Figure 4d). These collective experimental and theoretical findings confirm the ability of defect sites in catalyzing CO2RR, providing the first insights and designing guidelines to develop cheap, scalable carbon catalysts for large scale applications.



In summary, we demonstrated that defects generated from the removal of nitrogen species assists in tuning the selectivity of nitrogen doped mesoporous carbon catalysts in producing CO during electrochemical reduction of CO2. Through investigative studies, we revealed the role of defect sites as the main active sites for CO2RR in our NRMC catalysts. Furthermore, through multiple annealing treatment at 900oC, the metal-free NRMC catalyst can exhibit metal-like activity towards CO2RR, with a FECO of 80% and a jCO of -2.9 mA cm-2 at a very low applied overpotentials of 490 mV. ASSOCIATED CONTENT Supporting Information Experimental Methods, electrocatalytic performances, TEM, XPS, XRD, Raman, BET, BJH, ECSA, EIS, DFT methods, DFT models and tables of comparison of high performing carbon and CO generating catalysts. This material is available free of charge via the Internet at www.acs.org AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Author Contributions R.A. directed the project. X.L. conceived the experiments. R.D. and R.C. performed materials synthesis and catalytic measurements. R.D., R.C., W.H.S. and E.L. performed material characterizations. X.T. and H. A. T. performed DFT calculations. R.D., X.L., X.T. and R.A. cowrote the manuscript. R.D., X.L., X.T., R.C., W.H.S., E.L., Y.H.N., S.C.S., L.D. and R.A. analyzed and discussed the results. Notes The authors declare no competing financial interest. Acknowledgments. All material and surface characterizations were carried out at Mark Wainwright Analytical Centre (MWAC), UNSW. We thank Dr. Bill Gong from MWAC for the XPS measurements and Dr. Kuang-Hsu Wu for discussion on carbon nanomaterials. The authors also thank Dr. David Mitchell from University of Wollongong Electron Microscopy Centre for his assistance in HAADF–STEM measurements. Computational studies were undertaken through the Australian National Computational Infrastructure (NCI) Merit Allocation Scheme, as well as the UNSW Partner Share at the NCI. The work was supported by the Australian Research Council (ARC)


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under the Laurate Fellowship Scheme FL-140100081 and Discovery Early Career Researcher Award DE170100375.

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Electroreduction of CO2 to CO on a Mesoporous Carbon Catalyst with

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