Supporting Information. Controlling the Interfacial Environment in the Electrosynthesis of MnOx Nanostructures

Transcription

1 Supporting Information Controlling the Interfacial Environment in the Electrosynthesis of MnOx Nanostructures for High Performance Oxygen Reduction/Evolution Electrocatalysis Pooya Hosseini-Benhangi a, Chun Haow Kung a, Akram Alfantazi b and Előd L. Gyenge a,* a Department of Chemical and Biological Engineering & Clean Energy Research Center (CERC), The University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T1Z3, b Department of Materials Engineering, The University of British Columbia, Vancouver, 6350 Stores Road, Vancouver, BC, Canada V6T 1Z4 * Corresponding author: egyenge@chbe.ubc.ca FT-IR analysis of samples before and after surfactant removal Before carrying out the factorial design of experiments and investigating the ORR/OER activity of the electrodeposited catalysts, the efficacy of post-deposition surfactant removal from the catalyst surface was evaluated. Several methods have been reported in the literature for surfactant removal from the electrodeposited samples, including heat treatment, UV/ozone treatment and acetone/isopropyl alcohol (IPA) washing methods 1 3. IPA washing for 15 min. at 343 K and 400 rpm rotation was chosen as a fast method without losing catalyst material and damaging the crystal structure of electrodeposited MnOx. FT-IR analysis was utilized to examine the effectiveness of the IPA washing method for surfactant removal from the electrodeposited MnOx surface in case of the highest surfactant concentration employed, i.e., 10 vol.% (Fig. S1). Since all of the surfactants contain a long hydrocarbon chain, traces of each surfactant on the electrodeposited MnO x can be detected using the C-H stretching and C-H deformation vibrations S-1

2 of these hydrocarbon chains between 2700 to 3100 cm -1 and at approximately 1490 cm -1, respectively 3. The black dotted line (curve I), which represents the FT-IR spectrum of 40 wt% PTFE treated carbon cloth after nitric acid pre-treatment, shows two characteristic peaks between 1100 and 1200 cm -1 which disappear after electrodeposition, confirming successful deposition of materials on the substrate in all cases (Fig. S1). For the electrodeposited and IPA cleaned samples there are no major peaks between 2700 and 3100 cm -1 indicating the efficiency of IPA washing method. In the case of MnO x electrodeposited from SDS containing solution, the two major peaks at 1200 and 1460 cm -1 are due to MnO 2 stretching and O-H bending vibrations of water, respectively 4,5. The broad absorption band starting from 2400 cm -1 and ending at 3600 cm -1 in the SDS case could also be related to the O-H stretching vibrations 6 9. S-2

3 Figure S1. FT-IR spectra of IPA washed MnOx samples electrodeposited on the carbon cloth in presence of SDS, Triton X-100 and CTAB. The electrodeposition factors for each sample are as follow: Carbon cloth (no electrodeposited material), SDS (C: 0.3 M, T: 295 K, S: 10 vol.%, E: 800 mvmoe), Triton (C: 0.1 M, T: 343 K, S: 10 vol.%, E: 800 mvmoe), CTAB (C: 0.3 M, T: 343 K, S: 10 vol.%, E: 1600 mvmoe). S-3

4 Representative XRD characterization of Triton electrodeposited manganese oxides Figure S2. XRD spectra of: A) 40 wt% PTFE treated carbon cloth used as the substrate for electrodeposition of manganese oxide, B) Triton run no. 1, C) Triton run no. 9 and D) Triton run no. 10. The electrodeposition factors for each sample are as follows: T1 (C: 0.3 M, T: 295 K, S: Triton, 10 vol.%, E: 800 mvmoe), T9 (C: 0.1 M, T: 343 K, S: Triton, vol.10 %, E: 800 mvmoe), T10 (C: 0.3 M, T: 295 K, S: Triton, 0 %, E: 1600 mvmoe). Red-colored symbols present major peaks corresponding to PTFE, graphite and manganese oxides where applicable. S-4

5 Table S1. XRD structural analysis of: 40 wt% PTFE treated carbon cloth, Triton run no. 1, Triton run no. 9 and Triton run no. 10. The XRD spectra of the samples are presented in Fig. S 2. Sample name 2θ: compound name (hkl) JCPDS reference code 18.4 : PTFE 25.7 : Graphite (100) 40 wt% PTFE treated 31.8 : Graphite (101) PTFE: carbon cloth 37.2 : Graphite (020) Graphite: (substrate) 41.7 : Graphite (003) Graphite: : Graphite (012) T : MnOOH (11-1) 30.3 : Mn3O4 (110) 34.3 : MnOOH (020) 36.6 : Mn3O4 (040) 39.5 : MnOOH (12-1) 49.5 : MnOOH (220) 52.3 : MnOOH (121) 66.4 : α-mno2 (412) MnOOH: MnOOH: MnOOH: Mn3O4: Mn3O4: T9 Unable to distinguish peaks for MnOx compared to the substrate which could be duo to the existence of amorphous phase for MnOx, nanostructured oxides, high intensity PTFE peak masking low intensity peaks and low signal-to-noise ratio. α-mno2: T10 Unable to distinguish peaks for MnOx compared to the substrate which could be duo to the existence of amorphous phase for MnOx, nanostructured oxides, high intensity PTFE peak masking low intensity peaks and low signal-to-noise ratio. N/A Pareto plots of estimates The JMP software employs Pareto plot of estimates to demonstrate the main and interaction effects of various factors on the responses that are being investigated in a factorial design study. In the factorial design investigations of the current work, i.e half-fraction 2 n factorial design S-5

6 studies in presence of various surfactants, these plots are powerful tools for judging the significance of the effect that each factor or two-factor interaction can have on the final responses, hence help remove the insignificant two-factor interaction in each aliased pair as explained earlier in the manuscript. To calculate the main (X i ) or two-factor interaction (X i X j ) effects in each surfactant category, the following equations have been employed: Main effect of X i = (Response at high X i ) (Response at low X i) (Half the number of factorial runs) Interaction effect of X i X j = (Response at high X ix j ) (Response at low X i X j ) (Half the number of factorial runs) (1) (2) The levels of two-factor interactions (X i X j ) for each factorial run in eq. 2 are determined based on the levels of each individual factor, i.e. X i and X j. For example, high level of factor X i ( + ) and low level of factor X j ( - ) lead to low level of their two-factor interaction X i X j ( - ). Fig. S3 shows the estimates of effects that each main factor and aliased two-factor interaction pair have on the ORR mass activity of the electrodeposited MnOx in presence of Triton X-100 as a response for the factorial design study. The graph shows clearly that between the four main factors (see Table 1), surfactant concentration (S), Temperature (T) and applied anodic potential (E) have the most significant effect on the response while Mn concentration (C) is the most insignificant factor in this case for the defined response (Fig. S3). Employing the Ockham s razor principle, one can assume the most significant two-factor interactions are S T, S E and E T among the C E+S T, C T+S E and C S+E T aliased pairs, respectively, since Mn concentration is shown to have the least effect on the ORR mass activity of the electrodeposited manganese oxides. Hence, the surface plots can be easily constructed using the most significant three main factors and three two-factor S-6

7 interactions. This is similar to the case that Mn concentration is discarded from the beginning and a full factorial design with three factors and three center-points leading to 11 runs in total is being constructed for the Triton X-100 samples. Figure S3. Pareto plot of estimates for the effects of four main factors (i.e. surfactant concentration (S), temperature (T), Mn concentration (C) and applied anodic potential (E)) and three aliased two-factor interactions on the ORR mass activity in the factorial design study on the anodic electrodeposition of MnO x in presence of Triton X-100. Curvature effect Traditionally, the curvature effect is estimated as the difference between the average of the center-point responses and the average of the factorial points. A strong curvature effect is then a reflection of a non-linear system behavior. JMP demonstrates the curvature effect in terms of RSquare when center-points are added to the factorial design study. RSquare estimates the S-7

8 proportion of variation in the response that can be attributed to the model rather than to random error. RSquare (also called the coefficient of multiple determination in the JMP software) is calculated as: RSquare = Sum of squares (C. total) Sum of squares (Error) Sum of squares (C. total) (3) where sum of squares (C. total) is the sum of the squared differences between the response values and the sample mean (representing the total variation in the response values) and sum of squares (Error) is the sum of the squared differences between the fitted values and the actual values (representing the variability that remains unexplained by the fitted model). A RSquare closer to 1 indicates a better fit to the data than does a RSquare closer to 0, meaning the curvature effect is negligible. A RSquare near 0 indicates that the model is not a much better predictor of the response than is the response mean, meaning a degree of non-linearity in the behavior of variables. Triton X-100 The Rsquare for the responses in case of Triton X-100 (Fig. 4 and Table 2), i.e. ORR mass activity, OER mass activity and ORR/OER potential window, is 0.987, and 0.778, respectively, indicating negligible curvature effect for the first two responses (Rsquare close to 1) while a degree of non-linearity is observed in the behavior of the factors on the third response, i.e. ORR/OER potential window. SDS In case of the SDS samples, the Rsquare for the ORR mass activity, OER mass activity and ORR/OER potential window is 0.997, and 0.833, respectively, showing negligible curvature S-8

9 effect for the first two responses (Rsquare close to 1) while a degree of non-linearity is observed in the behavior of the factors on the third response, i.e. ORR/OER potential window. CTAB For the CTAB samples, the Rsquare is found to be 0.906, and for the ORR mass activity, the OER mass activity and the ORR/OER potential window, respectively. While a degree of non-linearity is observed in the behavior of the factors, i.e. C, T, S and E, on the ORR mass activity as well as the ORR/OER potential window, negligible curvature effect (Rsquare close to 1) is shown for the OER mass activity. S-9

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