Supporting Information Dechlorination of trichloroacetic acid using a noble metal-free graphene-cu foam electrode via direct cathodic reduction and atomic H* Ran Mao a, b, Ning Li a, b, Huachun Lan a, Xu Zhao a, Huijuan Liu a, Jiuhui Qu* a, Meng Sun a, b a Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, P.R. China b University of Chinese Academy of Sciences, Beijing, 100049, P.R. China *Corresponding author: Tel.: 86-010-62849151; fax: 86-010-62923558; e-mail: jhqu@rcees.ac.cn (J. Qu). Pages - 20 Tables - 4 Figures - 14 S1
Table S1. Observed reaction rate constants of chloroacetic acids with Cu foam and GR-Cu foam electrodes. Chloroacetic acid Cathode k obs (min -1 ) TCAA Cu foam 3.80 10-2 GR-Cu foam 1.37 10-1 DCAA Cu foam 1.64 10-2 GR-Cu foam 5.38 10-2 MCAA Cu foam 8.40 10-4 GR-Cu foam 5.10 10-3 Table S2. Fitting rate constants of the electrocatalytic dechlorination of TCAA on Cu-foam and GR-Cu foam electrodes. Cu foam electrode GR-Cu foam electrode k 1 (min -1 ) 0.0400 0.1336 k 2 (min -1 ) 0.0019 0.0060 k 3 (min -1 ) 0.0034 0.0288 k 4 (min -1 ) 0.0014 0.0100 k 5 (min -1 ) 0.0004 0.0119 k 6 (min -1 ) 0.0022 0.0028 S2
Table S3. Water quality of original Beijing tap water. Item TCAA DCAA MCAA DBAA MBAA Value 34.9 µg/l 15.7 µg/l N.D. a 4.2 µg/l N.D. a Ca 2+ 46.9 mg/l Mg 2+ 20.9 mg/l Br - - BrO 3 - ClO 4 NO - 3 -N NO - 2 -N 7.6 µg/l N.D. a 1.2 µg/l 1.7 mg/l N.D. a SO 4 2-68.7 mg/l Cl - 43.6 mg/l ph 7.7 a Not detected. S3
Table S4. Experimental results for the batch electrolysis experiments at different cathode potentials a Cathode Cu foam electrode GR-Cu foam potential U I t Conversion U I t Conversion (V) (V) (10-4 Ah) (%) (V) (10-4 Ah) (%) -0.7 2.99 8.8 11.8 3.18 9.5 34.9-1.0 4.27 11.5 26.5 4.65 12.5 75.2-1.2 5.23 13.0 34.2 5.47 13.8 89.5-1.5 6.81 17.5 58.6 7.23 18.7 82.5 a [TCAA] 0 = 2.5 mg/l, 2 mm Na 2 SO 4 (conductivity: 443 µs/cm), initial ph = 6.8, reaction time = 10 min. S4
Figure S1. Schematic diagram of the experimental set-up. (1) reactor; (2) cathode; (3) reference electrode (SCE); (4) anode: platinum wire; (5) proton-exchange membrane; (6) sampling port; (7) magnetic stirrer; (8) electrochemical workstation; (9) Computer. Figure S2. SEM image of the Pd-C electrode. Abundant sphere-like Pd nanoparticles are uniformly deposited on the carbon fibers with an average size of around 20 nm. S5
Figure S3. (a) SEM image of the GR-Cu foam; the corresponding EDX mapping images of (b) carbon and (c) copper elements. S6
0.0-0.5 k obs = 0.00084 min -1, R 2 =0.98 ln(c/c 0 ) -1.0-1.5-2.0-2.5-3.0 0.0-0.5-1.0-1.5 k obs = 0.038 min -1, R 2 =0.98 (a) 0 50 100 150 200 250 300 Time (min) TCAA DCAA MCAA k obs = 0.016 min -1, R 2 =0.99 k obs = 0.0051 min -1, R 2 =0.99 ln(c/c 0 ) -2.0-2.5-3.0 k obs = 0.137 min -1, R 2 =0.99-3.5-4.0-4.5-5.0 (b) k obs = 0.054 min -1, R 2 =0.99 TCAA DCAA MCAA 0 50 100 150 200 250 300 Time (min) Figure S4. Determination of the observed first-order reaction rate constants for electrocatalytic dechlorination of chloroacetic acids by (a) Cu foam electrode and (b) GR-Cu foam electrode ([TCAA] 0 = [DCAA] 0 = [MCAA] 0 = 3.06 µm, applied potential = -1.2 V vs. SCE, 2 mm Na 2 SO 4, ph = 6.8). Lines represent the fitting curves. S7
ph change during the process at different cathode potentials The ph value during the electrochemical process at various cathode potentials are measured, and the results are shown in Figure S5. The ph increases from 6.8 to more than 10.0 in the first 5 min and keeps nearly constant with the increase less than 0.3 afterwards in all the experiments. It can be seen from the inset that the ph rapidly increases from 6.8 to 10.09 within the first 1 min at -1.2 V. A similar trend is observed at the other potentials. The difference in the equilibrium ph value at the potential from -0.7 to -1.5 V is less than 0.5. The rapid increase in ph during the startup period could be ascribed to the hydroxyl ions generated from the cathode. And the protons produced at the anode would then migrate through the solution across the proton exchange membrane, which might explain the relatively constant ph. This result agrees with the previous observations of a decreased ph in the anode cell and an increased ph in the cathode cell. 1-3 11 ph 10 9 8 ph 10 9 8-1.2 V -0.7 V -1.0 V -1.2 V -1.5 V 7 7 0 50 100 150 200 250 300 Time (s) 0 10 20 30 40 Time (min) S8
Figure S5. Changes in ph value during the electrocatalytic dechlorination of TCAA at different cathode potentials ([TCAA] 0 = 3.06 µm, 2 mm Na 2 SO 4, initial ph = 6.8). Inset shows the change of ph in the first 5 min at -1.2 V. 0.20 0.15 k obs (min -1 ) 0.10 Cu foam GR-Cu foam 0.05 2 4 6 8 10 ph Figure S6. The kinetic constants (k obs ) of TCAA reduction with Cu foam and GR-Cu foam electrodes at different initial ph values ([TCAA] 0 = 3.06 µm, applied potential = -1.2 V vs. SCE). The initial conductivity of the catholyte at different ph values is kept constant. Hydrolysis test The effect of the ph on the stability of TCAA, DCAA, and MCAA is investigated by hydrolysis tests at different ph values. CAAs with the same initial concentration of 500 µg/l were spiked into the solution in the sampling containers, and the ph was adjusted to 2.1, 4.3, 6.8, and 11.2, respectively. The systems were allowed to remain undisturbed for 24 h protected from light at room temperature (22 ± 1 C) before the S9
detection. The measured concentrations of CAAs at various ph values are displayed in Figure S7. It can be seen that without applying any electrochemical potential, the measured concentration is almost identical to the initial concentration in the ph range from 2.1 to 11.2 for all the CAAs with the deviation less than 1.1%. Additionally, no increase in chloride ions is noted in both acidic and alkaline conditions. The obtained results suggest that TCAA, DCAA, and MCAA in the solution with various ph values in the tested conditions are stable, thus excluding the hydrolysis effect on their disappearance during the electrochemical process. 550 500 ph=2.1 ph=4.3 ph=6.8 ph=11.2 Concentration (µg/l) 450 400 50 0 TCAA DCAA MCAA Figure S7. The measured concentration of TCAA, DCAA, and MCAA at various ph values with the same initial concentration of 500 µg/l. S10
1.0 0.8 Blank N 2 TCAA/TCAA 0 0.6 0.4 0.2 k obs =0.137 min -1, R 2 =0.99 0.0 k obs =0.269 min -1, R 2 =0.99 0 10 20 30 40 Time (min) Figure S8. Effect of N 2 sparged electrolyte on TCAA reduction ([TCAA] 0 = 3.06 µm, applied potential = -1.2 V vs. SCE, 2 mm Na 2 SO 4, initial ph = 6.8). The solid lines represent the pseudo-first-order model fits. k 3 k 2 k 1 k 4 k 6 TCAA DCAA MCAA AA k 5 Figure S9. The reaction pathway of the electrocatalytic dechlorination of TCAA. S11
Section S1: Given that the electrocatalytic dechlorination reaction follows pseudo-first order kinetics, the concentrations of the substrate, intermediates and product as a function of reaction time can be obtained as follows. dtcaa RTCAA = = ( k1+ k2+ k3) C d t TCAA d R = = k C ( k + k ) C DCAA DCAA 1 TCAA 4 5 DCAA dt d R = = k C + k C k C MCAA MCAA 4 DCAA 2 TCAA 6 MCAA dt d R = = k C + k C + k C AA AA 3 TCAA 5 DCAA 6 MCAA dt (1) (2) (3) (4) 0 At t=0, C = C, C = C = C = 0. Integration of eqs (1)-(4) leads to: TCAA TCAA DCAA MCAA AA C C k k k t 0 TCAA = exp( TCAA ( ) ) 1+ 2+ 3 k C = C (exp( ( k + k + k ) t exp( ( k + k ) t)) C DCAA MCAA 0 1 TCAA k4+ k5 k1 k2 k3 1 2 3 4 5 k k k = C (( + ) ( )( ) 0 2 1 4 TCAA k6 k1 k2 k3 k6 k1 k2 k3 k4+ k5 k1 k2 k3 k1k4 (exp( ( k1+ k2+ k3) t) exp( k6) t) ( k + k k k k )( k k k ) (exp( ( k + k ) t) exp( k ) t)) 4 5 6 4 5 1 2 3 6 4 5 (5) (6) (7) S12
C AA k k k = C (( + ( )( ) 0 3 1 5 TCAA k1+ k2+ k3 k1+ k2+ k3 k1+ k2+ k3 k4 k5 k2k6 k1k4k6 + + ) ( k + k + k )( k + k + k k ) ( k + k + k )( k + k + k k )( k + k k k k ) 1 2 3 1 2 3 6 1 2 3 1 2 3 6 4 5 1 2 3 1 5 exp( ( k1+ k2+ k3) t) + ( ( k4+ k5 )( k4+ k5 k1 k2 k3 ) k k k + ( k + k )( k + k k k 4 5 4 5 1 1 4 6 k k 2 3 6 4 5 6 1 2 3 6 1 2 3 4 5 1 2 3 1 4 3 )exp( k6) t+ ( k4+ k5 k1 k2 k3)( k6 k4 k5) k1+ k2+ k3 1 2 3 1 2 3 4 5 4 5 4 5 1 2 3 1 )exp( ( k4+ k5) t) k )( k k k ) k2 k1k4 + ( + k k k k ( k k k k )( k + k k k k ) k1k5 k1k5 ( k + k + k )( k + k + k k k ) ( k + k )( k + k k k k ) ( k + k k k k2k6 k1k4k6 + k )( k + k + k k ) ( k + k )( k + k k k k )( k k k ) 2 3 1 2 3 6 4 5 4 5 1 2 3 6 4 5 k2 k1k4 k k k k ( k k k k )( k + k k k k ) 6 1 2 3 6 1 2 3 4 5 1 2 3 k1k4k6 ( k + k + k )( k + k + k k )( k + k k k k ) 1 2 3 1 2 3 6 4 5 1 2 3 k1k4 + ) ( k + k k k k )( k k k ) 4 5 1 2 3 6 4 5 (8) The above equations were fitted using MATLAB to obtain the value of k 1 to k 6. k S13
Figure S10. Fitting the kinetic data for TCAA dechlorination on (a) Cu foam electrode and (b) GR-Cu foam electrode ([TCAA] 0 = 2.0 mg/l, applied potential = -1.2 V vs. SCE, 2 mm Na 2 SO 4, ph = 6.8). S14
XPS, XRD and Raman analysis The chemical structure of the Cu foam and GR-Cu foam electrodes before and after nine-cycle TCAA dechlorination are first investigated by XPS. The high-resolution XPS spectrum of Cu (2p 3/2, 1/2 ) exhibits two pairs of spin-orbit components (Figure S11). The major spin-orbit component pair with binding energies of 932.36 and 952.21 ev can be assigned to Cu 0, suggesting that the fresh Cu foam and GR-Cu foam electrodes were predominated by the metallic Cu. 4 The minor spin-obit component pair at higher binding energies of 934.58 and 954.40 ev corresponding to Cu 2+ were obviously reduced on the used Cu foam and GR-Cu foam electrodes. The satellite structures observed on the binding energy of 943.98 and 941.90 ev of the core line 2p 3/2 are related to the presence of Cu 2+ species. 5 As can be seen from Figure S12, the C 1s XPS spectrum indicates that the variation in the chemical structure of GR on GR-Cu foam electrode was unnoticeable after reduction treatments. The results indicate that CuO are partly reduced to Cu 0 by cathodic current on both the Cu foam and GR-Cu foam electrodes during the process. From the XRD patterns in Figure S13a, the reflections (220) and (310) of metallic Cu are found to be intensified after nine cycles. And the Raman spectra in Figure S13b shows that the D and 2D bands in the used GR-Cu foam are broadened slightly, suggesting a small change of the in-plane sp 2 domains of the GR layers. However, this slight change seems not to induce any negative influence with respect to the stability and durability of the electrode. S15
Cu foam before dechlorination Cu 0 2p 3/2 CuO 2p 1/2 Cu 0 2p 1/2 CuO 2p 3/2, sat CuO 2p 3/2 Cu foam after dechlorination Intensity (a. u.) GR-Cu foam before dechlorination CuO 2p 1/2 Cu 0 2p 1/2 CuO 2p 3/2, sat CuO 2p 3/2 Cu 0 2p 3/2 GR-Cu foam after dechlorination 955 950 945 940 935 930 Binding energy (ev) Figure S11. High-resolution XPS spectra of Cu 2p on Cu foam and GR-Cu foam electrodes before and after nine-cycle dechlorination reaction. S16
(a) C=C Intensity (a. u.) C=O C-OH 292 290 288 286 284 282 280 Binding energy (ev) (b) C=C Intensity (a. u.) C=O C-OH 292 290 288 286 284 282 280 Binding energy (ev) Figure S12. XPS C 1s spectra of GR-Cu foam electrode (a) before and (b) after nine- cycle dechlorination reaction. S17
(a) Cu (111) Intensity (a. u.) Cu (200) Cu (220) Used Cu foam Cu (310) Cu (222) Cu (400) Used GR-Cu foam 40 50 60 70 80 90 100 110 120 2θ (degree) (b) Intensity (a. u.) D G Used Cu foam 2D Used GR-Cu foam 500 1000 1500 2000 2500 3000 3500 Raman shift (cm -1 ) Figure S13. (a) XRD patterns and (b) Raman spectra of the Cu foam and GR-Cu foam electrodes after the nine- cycle dechlorination reaction. S18
Energy cost analysis The unit energy consumptions E, expressed in kwh necessary to removal of 1 g TCAA, is calculated by the formula: E = 10-3 UItm -1, where U = total cell potential (V), I t is the integral area under the curve I vs t (Ah), and m = mass of the removed target (g). The consumption is calculated on the basis of the first 10 min reaction, since most of TCAA could be removed within the first 10 min by electroreduction at the GR-Cu foam electrode. The details such as total cell potential, I t (Ah), and time with removal rates are summarized in the Table S4. The results are shown in Figure S14. 0.20 Cu foam GR-Cu foam Energy cost (KWh/g) 0.15 0.10 0.05 0.00-0.7-1 -1.2-1.5 Cathode potential (V) Figure S14. Energy cost for TCAA removal at various cathode potentials for electrochemical process with the Cu foam and GR-Cu foam electrodes under the same conditions with Table S4. S19
References (1) Chae, K. J.; Choi, M.; Ajayi, F. F.; Park, W.; Chang, I. S.; Kim, I. S. Mass Transport though a Proton Exchange Membrane (Nafion) in Microbial Fuel Cells. Energy Fuels 2008, 22, 169-176. (2) Gil, G. C.; Chang, I. S.; Kim, B. H.; Kim, M.; Jang, J. K.; Park, H. S.; Kim, H. J. Operational parameters affecting the performance of a mediator-less microbial fuel cell. Biosens. Bioelectron. 2003, 18, 327-334. (3) Rozendal, R. A.; Hamelers, H. V. M.; Buisman, C. J. N. Effects of membrane cation transport on ph and microbial fuel cell performance. Environ. Sci. Technol. 2006, 40, 5206-5211. (4) Abdulla-Al-Mamun, M.; Kusumoto, Y. New simple synthesis of Cu-TiO 2 nanocomposite: Highly enhanced photocatalytic killing of epithelia carcinoma (HeLa) cells. Chem. Lett. 2009, 38, 826-827. (5) Casella, I. G.; Gatta, M. Anodic electrodeposition of copper oxide/hydroxide films by alkaline solutions containing cuprous cyanide ions. J. Electroanal. Chem. 2000, 494, 12-20. S20