Cobalt and Nitrogen Codoped Graphene with Inserted Carbon

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Research Article pubs.acs.org/journal/ascecg

Cobalt and Nitrogen Codoped Graphene with Inserted Carbon Nanospheres as an Efficient Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Xiaochang Qiao, Shijun Liao,* Ruiping Zheng, Yijie Deng, Huiyu Song, and Li Du The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510641, People’s Republic of China S Supporting Information *

ABSTRACT: It is highly desirable but remains challenging to develop efficient bifunctional electrocatalysts for both the oxygen reduction and oxygen evolution reactions (ORR/OER) in rechargeable metal−air batteries and unitized regenerative fuel cells. Herein, we developed a facile and cost-effective strategy to prepare a cobalt and nitrogen codoped three-dimensional (3D) graphene catalyst through inserting carbon nanospheres into the interlayers of graphene sheets. The catalyst exhibited not only excellent ORR performance but also excellent OER performance, and both were greatly enhanced by the insertion of carbon nanospheres. Its activities for the ORR/OER ranked among the best for doped carbon catalysts thus far reported. Its overall oxygen electrode activity parameter (ΔE) was as low as 0.807 V, which was much lower than that of Pt/C, and made it comparable with the best nonprecious metal based catalysts to date. Furthermore, the catalyst exhibited excellent stability toward ORR and OER, making it a new noble-metal-free bifunctional catalyst for future applications in the fields of alternative energy conversion and storage systems. KEYWORDS: Graphene, Carbon nanospheres, Insertion, Cobalt and nitrogen codoping, Oxygen reduction reaction, Oxygen evolution reaction

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INTRODUCTION The global energy crisis has aroused tremendous and continuous research interest in finding alternative energy conversion and storage systems that are inexpensive, highly efficient, and environmentally benign.1 Catalysts for the cathodic oxygen reduction reaction (ORR) and the anodic oxygen evolution reaction (OER) play a crucial role in various renewable energy technologies, such as rechargeable metal−air batteries and regenerative fuel cells.2−7 Currently, Pt and its alloys are the best ORR catalysts,8,9 while Ir and Ru are the most active catalysts for the OER. 10 However, their industrialization is severely hindered by their scarcity, consequently prohibitive cost, and poor durability. As a result, extensive studies have been focused on searching for alternative catalysts to replace noble metals and achieve comparable or even higher activities, combined with acceptable cost.11−25 Due to fact that the ORR and OER processes have completely different reaction mechanisms,26−28 designing an effective bifunctional electrocatalyst for both reactions is very challenging and has rarely been reported. Such a bifunctional catalyst would be of great importance for a rechargeable metal−air battery or a regenerative fuel cell.21,29−32 Recently, state-of-the-art nitrogen-doped carbon materials have aroused tremendous interest due to their competitive © XXXX American Chemical Society

catalytic activity, low cost, and significantly enhanced stability.33,34 In particular, N-doped graphene has drawn special attention in recent years, thanks to its high surface area, superior electrical conductivity, excellent mechanical properties, and chemical stability, making it a high-performance ORR or OER catalyst.35−40 However, because of the strong π−π stacking and van der Waals interactions between the graphene sheets, the sheets tend to form irreversible agglomerates or to restack.41 This effect (i) dramatically reduces the surface area, (ii) limits the permeation of the electrolyte between the graphene layers, and (iii) renders a substantial number of the catalytic sites inaccessible for O2 and ions, thereby reducing the efficiency of the catalytic reaction. These factors together constitute a serious barrier for graphene-based materials to be used as effective electrocatalysts. Herein, we report a nitrogen-doped graphene material containing a trace amount of cobalt and intercalated with conductive carbon nanospheres (Co-N-GCI) as a highperformance ORR and OER bifunctional catalyst. The carbon nanospheres between the graphene sheets act as “spacers,” Received: March 3, 2016 Revised: June 30, 2016

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DOI: 10.1021/acssuschemeng.6b00451 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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For the ORR, the electron transfer number (n) per oxygen molecule was calculated using the Koutecky−Levich (KL) equation as follows:14

increasing the graphene’s accessible surface area and providing abundant electrolyte channels, which are expected to facilitate the diffusion of reactive species to the catalytic active sites. In addition, the carbon nanospheres can serve as “shortcuts” for interplanar electron transport and thus guarantee the material’s good conductivity. The resulting Co-N-GCI catalyst shows excellent bifunctional catalytic activity and outstanding stability toward both ORR and OER in an alkaline medium.

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j−1 = jk −1 + (0.62nFCD2/3r −1/6ω1/2)−1

(1)

where j and jk are the measured current density and the kinetic current density, respectively; n is the number of the electrons transferred per oxygen molecule in the ORR process; F is the Faraday constant (96485 C mol−1); C is the concentration of O2 in 0.1 M KOH (1.2 × 10−3 mol L−1); D is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10−5 cm2 s−1), γ is the kinetic viscosity of the electrolyte (0.01 cm2 s−1), and ω is the angular velocity of the rotating electrode. The Tafel plots were plotted after the measured current density was corrected by diffusion-limiting density to give the kinetic current density, calculated from

EXPERIMENTAL SECTION

Materials. Graphite powder was obtained from Sigma-Aldrich. Carbon nanospheres (CNS) (Vulcan XC-72) were obtained from Cabot Corporation. Cobalt(II) acetate tetrahydrate and melamine were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion (5 wt %) and commercial Pt/C (20 wt %) catalysts were purchased from DuPont and Johnson Matthey, respectively. IrO2/C (20 wt %) catalyst was prepared with a literature reported method.42 Carbon nanospheres were acid-treated with concentrated HNO3 at 110 °C for 3 h to remove metal impurities and enhance the nanospheres’ wettability. Other chemicals were all directly used as received without any further purification. Graphite oxide (GO) was prepared from the graphite powder by a modified Hummers’ method.43 Preparation of the Catalysts. A 1 g portion of melamine was dissolved in 200 mL deionized water, then 0.5 g GO was added and the mixture was sonicated for 1 h. Next, 0.02 g acid-treated carbon nanospheres and 1 mL cobalt(II) acetate tetrahydrate solution (0.5 mg mL−1 in terms of cobalt) were added into the above solution. This mixture was continuously stirred for 2 h, and the resulting solid was collected by rotary evaporation. This product was then transferred into a quartz boat for subsequent thermal treatment at 750 °C in a tube furnace for 30 min under an Ar flow. After cooling to room temperature under Ar protection, the Co-N-GCI material was obtained. For comparison, Co-N-G, N-GCI, and N-G hybrids were also prepared via similar procedures but in the absence of carbon nanospheres, cobalt reagent, or both. Physical Characterization. Scanning electron microscopy (SEM) images were generated on a Nova Nano 430 field emission scanning electron microscope (FEI, USA). Transmission electron microscopy (TEM) images were obtained on a JEM-2100HR transmission electron microscope (JEOL, Japan). The specific surface area and pore-size distribution were analyzed by the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods using the adsorption branch of isotherms obtained on a Tristar II 3020 gas adsorption analyzer (Micromeritics, USA). X-ray diffraction (XRD) patterns were performed on a TD-3500 powder diffractometer (Tongda, China). X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo VG Scientific, USA). Electrochemical Measurements. Electrochemical measurements were performed on an electrochemical workstation (Ivium, Netherlands) with a three-electrode cell. An Ag/AgCl electrode (3 M NaCl) was used as the reference electrode, and a Pt wire was used as the counter electrode. A 0.1 M KOH solution was employed as the electrolyte. To prepare the working electrode, 5 mg of the corresponding catalyst was dispersed into 1 mL Nafion ethanol solution (0.25 wt %) to form a homogeneous ink. Then 20 μL of the ink was dripped uniformly onto a polished glassy carbon electrode (5 mm diameter, 0.196 cm2 geometric area) and dried under an infrared lamp. The catalyst loading was 0.5 mg cm−2. Prior to the measurements being performed, the electrolyte solution was purged with high-purity N2 or O2 gas for at least 30 min. The Ag/AgCl (3 M NaCl) electrode was calibrated with respect to reversible hydrogen electrode (RHE), in 0.1 M KOH, E(RHE) = E (Ag/AgCl) + 0.982 V. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were performed at a potential scan rate of 10 mV s−1. All electrochemical impedance spectroscopy (EIS) tests were performed at 1 V (vs RHE), with frequency from 100 kHz to 1 Hz and the potential amplitude of 5 mV.

jkin =

jjdiff jdiff − j

(2)

Rotating ring-disk electrode (RRDE) measurements were performed using a glassy carbon disk with a polycrystalline Pt ring biased at 0.5 V (vs Ag/AgCl) at a rotating speed of 1600 rpm. The electron transfer number (n) and the H2O2 yield were calculated with the following equations:44

n=

4Id Ir /N + Id

H 2O2 % =

200Ir /N Ir /N + Id

(3)

(4)

where Ir and Id are the ring current and the disk current, respectively; N is the collection efficiency (0.37), as calibrated in deaerated 0.1 M NaOH with 0.01 M K3Fe(CN)6 at a rotation speed of 1600 rpm and 10 mV s−1 scan rate. The chronoamperometric response for the ORR was obtained at −0.3 V (vs Ag/AgCl) of in O2-saturated 0.1 M KOH solution at 900 rpm. For the OER, the OER potential was IR corrected by using the E− iR relation, where i is the current and R is the electrolyte ohmic resistance, which was the ionic resistance from the solution was measured to be ∼45 Ω via high-frequency AC impedance.

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RESULTS AND DISCUSSION The special architecture of the nanosheet/nanosphere composite was confirmed by SEM (Figure 1a−d) and TEM (Figure 1e). The SEM and TEM images showed that the carbon nanospheres were well inserted into the graphene nanosheets as expected. In contrast, the N-doped graphene material containing a trace amount of cobalt in the absence of carbon nanospheres (Co-N-G) showed a closely restacked, multilayer graphene nanosheet structure with a crumpled morphology (Figure S1). Careful checking of the Co-N-GCI HRTEM image (Figure 1f) revealed scattered nanoparticles showing a clear lattice fringe, with a lattice spacing of about 0.245 nm, in good agreement with the (111) crystal face of CoO (PDF no. 65-2902). The XRD pattern of Co-N-GCI (Figure S2) also indicated that the cobalt in the Co-N-GCI is mainly presented as the state of CoO. The N2 adsorption−desorption isotherms and the corresponding pore-size distribution curves of the as-prepared Co-NGCI and Co-N-G are shown in Figure 2. The isotherm of CoN-GCI is a typical type IV with a distinct hysteresis loop in the medium- and high-pressure regions (P/P0 = 0.5−1), and its surface area determine from the isotherm is 365.2 m2 g−1, which was much higher than that of Co-N-G (195.6 m2 g−1). The average pore volume significantly increased from 0.55 cm3 g−1 for Co-N-G to 1.60 cm3 g−1 for Co-N-GCI. The increased B

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Figure 3. XPS high-resolution N 1s (a) and Co 2p (b) spectra of CoN-GCI.

Figure 3b presents a high-resolution Co 2p spectrum in which the Co 2p3/2 and Co 2p1/2 components are distinguishable, along with two shakeup (satellite) peaks. The binding energies of Co 2p3/2 (781.0 eV) and Co 2p1/2 (796.5 eV) are typical of Co2+ species.47 These peaks are attributable to the presence of CoO. This result is consistent with the TEM result. The ORR catalytic performance of Co-N-GCI was first measured by cyclic voltammetry (CV) in O2-saturated 0.1 M KOH solution. The CV curve of Co-N-G was also measured for comparison. As shown in Figure 4a, the CV curves for the CoN-GCI and Co-N-G electrodes displayed distinct oxygen reduction cathodic peaks. The ORR peak potential and peak current density of Co-N-GCI were more positive and much higher, respectively, than those of Co-N-G. The more positive ORR peak potential and the higher peak current density of Co-

Figure 1. SEM images (a−d) and TEM images (e and f) of Co-NGCI.

Figure 2. Nitrogen adsorption−desorption isotherms (a) and the corresponding pore-size distributions (b) of Co-N-GCI and Co-N-G.

pore volume should beneficial to the diffusion of the reactants during the ORR process. The elemental makeup of the Co-N-GCI composite was revealed by XPS. As shown in Figure S3, the XPS survey spectrum of Co-N-GCI exhibited a dominant C 1s peak (∼284.5 eV), a N 1s peak (∼401.0 eV), an O 1s peak (∼531.6 eV), and a Co 2p peak (∼780.0 eV),18 confirming the successful doping of Co and N. The atomic percentage of Co and N were calculated to be 0.59 and 5.66, respectively. Inductively coupled plasma mass spectroscopic (ICP-MS) analysis also showed a relatively low cobalt content of ∼1.65 wt % in the Co-N-GCI sample. For sample Co-N-G without insertion of carbon nanospheres, its Co and N percentages are 0.62 and 5.58 at% respectively, which were similar to those of sample Co-N-GCI (Figure S4). The high-resolution N 1s spectrum (Figure 3a) revealed three species of N in Co-N-GCI, corresponding to pyridinic N (397.6 eV), pyrrolic N (399.1 eV), and graphitic N (400.7 eV), with compositions of 50.5, 29.0, and 20.5 at %, respectively.45 Remarkably, pyridinic N which was critical for electrocatalysis was present in the greatest quantity (50.5 at %).46

Figure 4. CV curves of Co-N-GCI and Co-N-G in O2-saturated 0.1 M KOH solutions (a); LSV curves of Co-N-GCI, Co-N-G, and commercial Pt/C (b); Tafel plots of Co-N-GCI, Co-N-G, and commercial Pt/C (c); RRDE disk and ring currents recorded for CoN-GCI, Co-N-G, and commercial Pt/C (d); H2O2 yields and corresponding electron transfer numbers (n) vs potential of the catalysts calculated from the RRDE data (e); and current−time chronoamperometric response of Co-N-GCI, Co-N-G, and commercial Pt/C (f). C

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conditions, indicating the excellent stability of the Co-N-GCI catalyst. Furthermore, we also recorded the LSV plots of Co-N-GCI and Co-N-G in O2-saturated 0.1 M KOH at 1600 rpm before and after 1000 continuous CV cycles. As shown in Figure S10, after 1000 continuous CV cycles, the half wave potential loss of Co-N-GCI catalyst is only ca. 8 mV, which is obviously smaller than that of Co-N-G catalyst (∼12 mV), further demonstrating its remarkable electrochemical stability. We also investigated the OER performance of our Co-N-GCI catalyst and found it to be an excellent OER catalyst. To study the OER performances of the various catalysts, LSV measurements were conducted by loading each catalyst in O2-saturated 0.1 M KOH at 1600 rpm; the ohmic potential drop (iR) losses caused by the solution resistance were all corrected. In Figure 5a, the OER LSV curve of Co-N-G has a more negative onset

N-GCI suggest that the insertion of carbon nanospheres greatly enhanced the ORR catalytic activity of the graphene. In order to gain further insight into what role the inserted carbon nanospheres played in the ORR, the LSV curves of Co-N-GCI, Co-N-G, and commercial 20 wt % Pt/C were recorded in an O2-saturated 0.1 M KOH solution. As shown in Figure 4b, CoN-GCI exhibited improved ORR catalytic performance, superior to that of Co-N-G, with a half-wave potential of 0.857 V (vs RHE), which was about 10 mV more positive than that of Co-N-Gindicating that it had greater intrinsic catalytic activity than Co-N-Gand a diffusion-limiting current density (at 0.6 V) about 34% higher than Co-N-G, showing that it achieved more efficient mass transport of reactants and products. The LSV curves of other catalysts also can be seen in Figure S5. The ORR performance of Co-N-GCI was also superior to that of commercial Pt/C, making it an excellent noble-metal-free catalyst for the ORR in an alkaline medium. Figure S6 shows the EIS results; clearly, Co-N-GCI catalyst exhibited lower internal resistance and superior transport property (as indicated by more precipitous slope at a low frequency) as compared to Co-N-G catalyst.40 The Tafel plots of Co-N-GCI, Co-N-G, and 20 wt % Pt/C (Figure 4c) were derived from the corresponding LSV data. The Tafel slope of Co-N-GCI (58 mV dec−1) was smaller than that of Co-N-G (72 mV dec−1) and commercial 20 wt % Pt/C catalyst (68 mV dec−1), further demonstrating the better catalytic activity for the ORR after the insertion of the carbon nanospheres. The low Tafel slope indicates the high intrinsic catalytic activity of Co-N-GCI. To gain more information on the electrochemical kinetics of Co-N-GCI in the ORR, we performed LSV tests at different rotating speeds in an O2-saturated 0.1 M KOH solution (Figure S7a). Well-defined steady-state diffusion-limiting currents, following mixed kinetic-diffusion regions, were observed on the LSV curves of Co-N-GCI at different rotating speeds from 900 to 2500 rpm. The diffusion-limiting currents increased with the increase of rotational speed. The corresponding KL plots at various electrode potentials are shown in Figure S7b. The good linearity and parallelism of KL plots indicate the first-order reaction kinetics with respect to the concentration of dissolved oxygen. On the basis of the slopes of the KL plots, the electron transfer number (n) of Co-N-GCI was calculated to be ∼4.0 at 0.6820.482 V, indicating Co-N-GCI favors a dominant fourelectron ORR pathway. For comparison, we also calculated the electron transfer number of Co-N-G, obtaining a smaller average value of 3.79 (Figures S8 and S9). RRDE measurements were further carried out to monitor the formation of peroxide (H2O2) byproducts during the ORR process. As illustrated in Figure 4d, on the basis of the ring and disk currents, the H2O2 yields were below 5% and 11% for CoN-GCI and Co-N-G, respectively, over a potential range of 0.3−0.6 V (vs RHE), giving corresponding electron transfer numbers of ∼3.9 and ∼3.8, the H2O2 yield and electron transfer numbers of Co-N-GCI were also very close to those of the reference Pt/C catalyst. It is noted that the electron transfer numbers obtained from the RRDE experiments were consistent with the results obtained by the KL analysis. The stability of Co-N-GCI was tested by chronoamperometric measurement. As shown in Figure 4f, Co-N-GCI has an excellent stability, after 20 000 s of operation at 0.682 V (vs RHE), the Co-N-GCI electrode retained >91.4% of its initial current density, whereas the retentions for Co-N-G electrode and Pt/C electrode are only 85 and 68% under the same

Figure 5. OER LSV curves at 1600 rpm in O2-saturated 0.1 M KOH solution (a); OER Tafel plots (b); OER LSV plots of Co-N-GCI in the beginning and after 200 cycles at a scanning rate of 10 mV s−1 (c); and the overall LSV curves of Co-N-GCI, Co-N-G, and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH solution (d).

potential and a much higher current density than that of Pt/C, showing the former’s excellent catalytic activity toward the OER. Notably, the current density for Co-N-GCI was much higher than for Co-N-G, implying that the former’s high catalytic activity toward the OER due to the carbon nanospheres embedded in the graphene sheets. The OER catalytic activities are commonly evaluated by the potential corresponded to the current density of 10 mA cm−2.29 Noticeably, the Co-N-GCI composite exhibits a low overpotential (η) of 426 mV at 10 mA cm−2, which was much lower than the values for Co-N-G (472 mV) and commercial Pt/C (621 mV) and close to the value for IrO2/C (370 mV). The Tafel plots were also provided to indicate the OER kinetics of the catalysts. As shown in Figure 5b, the Tafel slopes were ∼69, ∼78, ∼168, and ∼83 mV dec−1 for Co-N-GCI, CoN-G, commercial Pt/C and IrO2/C, respectively. The Co-NGCI composite exhibited the smallest Tafel slope, suggesting the outstanding intrinsic OER kinetics of Co-N-GCI. To investigate the catalyst’s durability for the OER, we performed accelerated stability tests of Co-N-GCI in O2saturated 0.1 M KOH at room temperature. As shown in Figure D

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2015A030312007), and Educational Commission of Guangdong Province (Project No. 2013CXZDA003).

5c, after 200 continuous potential cycles, the Co-N-GCI composite displayed a only 12 mV increase in η to achieve a current density of 10 mA cm−2, whereas the Co-N-G composite exhibited a 30 mV increase in η under the same conditions (Figure S11), demonstrating the remarkable electrochemical stability of Co-N-GCI. Generally, the overall oxygen electrode activity of a bifunctional electrocatalyst could be estimated by the difference (ΔE) of OER potential corresponding to OER current density of 10 mA cm−2 to the ORR potential corresponding to the ORR current density of −3 mA cm−2.48 The smaller the difference, the better is the material suitable as a bifunctinal catalyst. As shown in Figure 5d, The ΔE values are 0.807, 0.892, and 1.040 V for Co-N-GCI, Co-N-G, and commercial Pt/C, respectively, indicating that the bifunctional oxygen electrode activities of the samples followed the order Co-NGCI > Co-N-G > Pt/C. Noticeably, the ΔE value of Co-N-GCI is, if not smaller than but, comparable to most of the nonprecious metal based bifunctional catalysts reported in the literature previously (Table S1), demonstrating the superior bifunctional activities of our Co-N-CIG catalyst.

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CONCLUSIONS In summary, surprisingly high ORR and OER activities were achieved using codoped graphene nanosheets with carbon nanospheres intercalated between the layers (Co-N-GCI). The insertion of the carbon nanospheres considerably enhanced the catalyst’s performance. This outcome is attributable to the carbon nanospheres (i) acting as “spacers” between the graphene sheets and thereby increasing the graphene’s accessible surface area, (ii) providing abundant electrolyte channels, which one would expect to facilitate the diffusion of reactive species to catalytic active sites, and (iii) serving as “shortcuts” for interplanar electron transport, thus guaranteeing the material’s good conductivity. The high bifunctional catalytic performance and excellent long-term durability of Co-N-GCI make it a promising noble-metal-free catalyst for oxygen electrochemistry in regenerative fuel cells and rechargeable metal-air batteries, and hence important for a variety of energy technologies and applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00451. Additional figures and table as mentioned in the text (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax: +86 20 87113586. E-mail: [email protected] (S.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Project Nos. 21276098, 21476088, 51302091, U1301245), Department of Science and Technology of Guangdong Province (Project Nos. 2014A010105041, 2015B010106012), Natural Science Foundation of Guangdong Province (Project No. E

DOI: 10.1021/acssuschemeng.6b00451 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.6b00451 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX