Efficient Water Splitting Electrodes Based on Laser-Induced Graphene

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Efficient Water Splitting Electrodes Based on Laser-Induced Graphene Jibo Zhang, Chenhao Zhang, Junwei Sha, Huilong Fei, Yilun Li, and James M. Tour ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06727 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 29, 2017

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ACS Applied Materials & Interfaces

Efficient Water Splitting Electrodes Based on Laser-Induced Graphene Jibo Zhang,1,2 Chenhao Zhang,1 Junwei Sha,3,5,6 Huilong Fei,1 Yilun Li,1 and James M. Tour1,2,3,4* 1

Department of Chemistry, 2Nanosystems Engineering Research Center for Nanotechnology-

Enabled Water Treatment, 3Smalley-Curl Institute and NanoCarbon Center, 4Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, USA 5

School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China

6

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

*Email: [email protected]

Keywords laser-induced graphene, oxygen evolution reaction, hydrogen evolution reaction, OER, HER

Abstract Electrically splitting water to H2 and O2 is a preferred method for energy storage as long as no CO2 is emitted during the supplied electrical input. Here we report a laser induced graphene (LIG) process to fabricate efficient catalytic electrodes on opposing faces of a plastic sheet, for the generation of both H2 and O2. The high porosity and electrical conductivity of LIG facilitates the efficient contact and charge transfer with the requisite electrolyte. The LIG-based

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electrodes exhibit high performance for hydrogen evolution reaction and oxygen evolution reaction with excellent long term stability. The overpotential reaches 100 mA/cm2 for HER and OER is as low as 214 and 380 mV with relatively low Tafel slopes of 54 and 49 mV/dec, respectively. By serial connecting of the electrodes with a power source in an O-ring setup, H2 and O2 are simultaneously generated on either side of the plastic sheet at a current density of 10 mA/cm2 at 1.66 V, and can thereby be selectively captured. The demonstration provides a promising route to simple, efficient and complete water splitting.

Introduction The use of renewable energy sources, including wind and solar energies for the generation of electricity, is a topic of much research.1-4 This electricity can be supplied to users through power lines, if they exist. Oftentimes, the wind or solar-generated power is distant from users and building power lines is expensive; if the power lines exist, losses occur in transmission. The electricity is also irregular in its supply depending on the weather and the time of day. Storage of the electricity at the point of generation would have many benefits. However, the electricity generated is not easily stored in the large quantities needed to supply grid-level energy needs because of the limitations of present-day batteries. Hence, conversion of the electricity supply into a chemical fuel source is beneficial so that the energy can be stored in chemical bonds, and shipped as required.5-7 Among the clean-energy methods to accomplish that task is the splitting of water to H2 and O2.8-9 Not only can these products be combusted back into water, thereby affording energy with no subsequent CO2 emissions, but they can be used in fuel cells that are significantly more efficient than combustion engines.

However, the sluggish kinetics of

hydrogen and oxygen evolution reactions (HER and OER, respectively) in water splitting


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requires catalysts to increase the efficiencies.10-12 Apart from the noble metal materials such as Pt, RuO2 and IrO2, numerous noble-metal-free catalysts have been developed.10, 13-14 For example, transition-metal carbides,15-17 phosphides18-19 and sulfides20-24 have shown favorable performance in HER. The oxides/hydroxides of cobalt,25 nickel,26 manganese27 and iron28 also have proven to be efficient catalysts for OER. However, for the scalable production of H2 and O2, the catalytic electrodes should be easily fabricated to minimize their overall cost. It is therefore desirable to couple the HER and OER processes into a single simple package. Laser-induced graphene (LIG) is a 3D porous graphene material fused to a flexible substrate that is prepared by a one-step laser scribing process on commercial polyimide (PI, with a trade name Kapton) film.29 The LIG on PI has high porosity, and is easily formed in the air at room temperature in a scalable production route. It is low cost, has high electrical conductivity and good chemical stability. While LIG has been used for direct patterning applications in microsupercapacitors and related devices,30-33 in this study we exploit the LIG process to fabricate efficient catalytic electrodes for splitting water to H2 and O2 starting from a single sheet of plastic. A CO2 laser is used to convert PI into LIG with a predetermined pattern. The active species are either formed in situ in the LIG-forming process by having Pt nanoparticles impregnated in the starting PI film, or by subsequent electrodeposition of NixFey(OH)2x+3y or CoP/Co3(PO4)2 onto the LIG. 34-35 The high porosity and electrical conductivity of LIG facilitates efficient contact and charge transfer with the requisite electrolyte. The LIG-based electrodes exhibit high HER and OER performance with excellent long term stability. The overpotential reachs 100 mA/cm2 for HER and OER is as low as 214 and 380 mV with relatively low Tafel slopes of 54 and 49 mV/dec, respectively. Additionally, the catalytic electrodes are prepared on either side of the PI sheet, then the device is assembled with an O-ring setup. By serial



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connection of the electrodes with a power source, H2 and O2 are simultaneously generated on either side of the plastic sheet at a current density of 10 mA/cm2 upon 1.66 V, and thereby selectively captured, providing a promising route to efficient and complete water splitting.

Result and Discussion Device fabrication and characterization Catalytic LIG electrodes are fabricated as illustrated in Figure 1 using a commercial laser system as found in most machine shops for cutting metal or wood patterns. The CO2 laser patterns the PI film to form LIG which is further assembled to devices with an active area of 5 × 5 mm2. The catalysts are either loaded by in situ doping such as LIG-Pt, or electrochemical deposition such as LIG-NiFe and LIG-Co-P.34-35 The procedures are summarized in the experimental section.


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Figure 1. The fabrication process of catalytic LIG electrodes. (a) Fabrication process of the LIGPt. Pt(II) acetylacetonate was mixed with polyamic acid at 5 wt%. The solution was vacuum dried and cast into film, then cured in an oven. Laser scribing was done in H2/Ar atmosphere to form Pt-doped LIG, followed by the device fabrication. (b) Fabrication process of the LIG-NiFe and the LIG-Co-P. LIG was directly patterned on a commercially available Kapton film with subsequent device fabrication. Then the catalysts were electrochemically deposited on the LIG to give LIG-NiFe and LIG-Co-P.

The LIG-Pt is prepared by direct laser writing on the Pt(II) acetylacetonate doped PI film in a H2/Ar atmosphere. The LIG-Pt possesses a 3D porous structure as illustrated in Figure 2a and Figure S1. The LIG structure is maintained through the laser process as demonstrated by the 5 ACS Paragon Plus Environment


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characteristic Raman peaks shown in Figure S2.29,

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The laser spot induces very high local

heating to carbonize the PI and simultaneously thermally reduce Pt(II) to Pt(0) under the protection of a H2/Ar atmosphere. The Pt nanoparticles are formed in situ and evenly distributed within the LIG domains with uniform size (Figure 2b). Figure S3 shows the XPS survey spectra of LIG-Pt and Figure 2c shows the X-ray photoelectron spectroscopy (XPS) spectrum of the LIG-Pt in Pt 4f region. The well separated spin-orbit components (∆ = 3.35 eV) at 71.13 and 74.48 eV from Pt(0) indicates the Pt(II) has been mostly reduced to Pt(0) that forms the nanoparticles and contributes to the HER (see below).36 The LIG-NiFe and LIG-Co-P are fabricated by laser writing on commercial Kapton film followed by the electrodeposition of catalysts. The potentiostat deposition of LIG-NiFe is done in a bath solution containing Ni2+, Fe3+ and NO3-. During the process, NO3- ions are reduced at the negative potential (-1.0 V vs Ag/AgCl) to form NH4+ and OH-. The bimetallic hydroxide of Ni and Fe (NixFey(OH)2x+3y) forms on the LIG surface.37 Due to the high catalytic activity of NixFey(OH)2x+3y, an optimized deposition time of 150 s is preferred. Since the loading of the catalyst is quite low, the morphology of LIG-NiFe resembles that of the pristine LIG as demonstrated by the porous surface with lined structure induced by laser writing (Figure S4). The thickness of the active layer is 60 µm. The XPS spectra of LIG-NiFe are shown in Figure S5. The Ni 2p peak at 855.9 eV and Fe 2p peak at 710.9 eV suggests the formation of hydroxide (NixFey(OH)2x+3y) by electrodeposition,34 and the atomic ratio of Ni2+ to Fe3+ is 3:4. The Co-P-derived electrode LIG-Co-P is prepared by potentiodynamic electrodeposition (see details in the Supporting Information).35 The transmission electron microscopy (TEM) images (Figure S6) of LIG-Co-P shows the Co3(PO4)2 and CoP nanoparticles are well-embedded in the LIG domains. The scanning electron microscopy (SEM) images in Figure S7 demonstrate


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the rough surface of the LIG-Co-P with a thickness of 55 µm fused to the underlying PI film. The LIG-Co-P retains the striped pattern induced by the laser raster scanning, with a large number of clusters on the surface as seen at higher magnification (Figure S7a and S7b). The elemental mapping of the as prepared electrode reveals the elemental composition of the clusters to be C, Co, O and P (Figure S8). The Co 2p and P 2p XPS spectra (Figure S9) show the details of the anticipated components. The Co 2p3/2 core level region has two main peaks at 778.4 and 781.9 eV with two satellites at 785.2 and 786.6 eV, respectively.38 The 778.4 eV peak is very close to that of metallic Co. The high P 2p spectrum shows three main peaks at 129.7 and 133.3 eV. The 129.7 eV peak of P and 778.4 eV peak of Co 2p3/2 are associated with the typical binding energies of Co 2p and P 2p in CoP.38 The P 2p peak at 133.3 eV originates from the PO43- species,39 indicating the existence of Co3(PO4)2 in consideration of the Co 2p peak at 781.9 eV. The performance of the catalytic LIG electrodes should benefit from the well distributed catalysts on the porous conductive material. For each type of catalyst the loading amount was optimized. For LIG-Pt, the 5 wt% in PAA is desirable since Pt is expensive. For the LIG-NiFe, the catalytic performance improved as we increased the deposition from 50 to 150 s. However, further extension of the deposition time did not affect the performance (onset potential or Tafel slope). For the LIG-Co-P, the performance of devices deposited by 10 cycles shows the best performance (onset potential or Tafel slope). Further increase of deposition cycles had little effect.

Electrochemical characterization



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Figure 2. (a) SEM and (b) TEM images of LIG-Pt. (c) XPS spectrum of Pt 4f region of LIG-Pt. (d) LSV of LIG-Pt for HER at a scan rate of 5 mV/s. Inset: corresponding Tafel plots of LIG-Pt. All tests are done in 1 M KOH with 95% iR compensation.

The as-prepared LIG electrodes can be directly used for HER or OER in accordance with the activity of the catalysts using a standard three-electrode electrochemical cell. The HER catalytic activity of the LIG-Pt is evaluated as shown in Figure 2d by the linear-sweep voltammograms (LSV) at a scan rate of 5 mV/s in 1 M KOH with 95% iR compensation. The Pt nanoparticles in LIG-Pt contribute to efficient activity as demonstrated by the nearly zero onset potential (η) vs RHE, after which the current density increases rapidly. The overpotentials 8 ACS Paragon Plus Environment

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needed to reach 10 and 100 mA/cm2 are 107 and 256 mV, respectively. These values are comparable to the recent published catalysts (see Table S1, Supporting Information). Conversely, the pristine LIG shows very poor activity for HER with η higher than 500 mV. It should be noted that the Tafel analysis of LIG-Pt reveals a slope of 83 mV/dec, which is slightly higher than conventional Pt-based electrodes such as Pt/C and Pt wires. The reason may be that the Pt nanoparticles formed in situ are partially covered by the LIG as indicated in Figure S1, and consequently the active sites are not fully exposed to the solution during the catalytic reaction. To make a comparison with commercial Pt/C, those catalysts are usually prepared as powders. Then they are mixed with binders, and sometimes also with conductive fillers such as carbon black, in solvents to form an ink that is eventually dropped on a glassy carbon for testing. In our case the catalysts are directly loaded on LIG. The performance of LIG-Pt is slightly worse than that of Pt/C in terms of the Tafel slope. However, the introduction of LIG significantly shortened the typical preparation process. Therefore, it is difficult to do control experiments without LIG. There would be no base upon which to attach the materials.



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Figure 3. Electrocatalytic performance of the LIG-NiFe for OER. (a) CV curve of the LIG-NiFe at a scan rate of 10 mV/s. (b) LSV at a scan rate of 5 mV/s and (c) corresponding Tafel plots of the LIG-NiFe. Inset: enlarged region from 1.3 to 1.5 V (vs RHE). The dashed circle shows the overlap between OER onset and the oxidation of Ni(II). (d) Stability test: LSV of the LIG-NiFe after 1000 OER cycles compared to the initial curve. All tests are done in 1 M KOH with 95% iR compensation.

The LIG-NiFe exhibits significant OER activity as illustrated in Figure 3. Cyclic voltammetry (CV) in Figure 3a demonstrates the catalytic behavior of the LIG-NiFe. A pair of redox peaks are observed at ~1.35 to 1.5 V (vs RHE), corresponding to the formation of Ni with higher oxidation states such as Ni(III) and Ni(IV) species that catalyze OER.40 It can be


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observed from the LSV curve shown in Figure 3b that the anodic current of the LIG-NiFe increases sharply at ~220 mV whereas there is an overlap from the redox of Ni. A more detailed LSV of the LIG-NiFe is shown in Figure S10, where the scan is done in reverse to minimize the disturbance from the redox peaks. The ηOER is determined to be 230 mV and the LIG-NiFe reaches 10 and 100 mA/cm2 at low overpotentials of 292 and 380 mV, respectively. The pristine LIG, by contrast, does not show significant current until very high η (more than 400 mV) is applied. The OER process is generally initiated by OH- electrosorption on the electrode surface. Therefore, a high electrochemical active surface area (EASA) and a high afﬁnity for adsorbed OH- intermediates are the basis for an efficient OER catalyst.41 We estimate the EASA of LIG-NiFe using the CV curves measured in a non-Faradaic region at scan rates ranging from 10 to 100 mV/s as summarized in Figure S11.42 By plotting the current density at 0 V (vs Hg/HgO) as a function of scan rate with subsequent fitting, we calculate the double layer capacitance (Cdl) to be 1.76 mF/cm2 (an average of 1.79 and 1.73 mF/cm2 from charge and discharge currents, respectively). Remarkably, the Cdl value suggests that the LIG-NiFe possesses a high EASA of 11 cm2, which is 44 times higher than its geometric area of 0.25 cm2. The affinity for OH- intermediates also needs to be considered. Since the equilibrium coverage of OH- intermediates on the catalytic surface is achieved quickly, the subsequent O2 generation is the rate determining process and results in a lower Tafel slope.41 As calculated from Figure 3c, the Tafel slope of LIG-NiFe is as low as 49 mV/dec, indicating the rapid reaction kinetics on the LIG-NiFe. This value is superior to the noble metal based OER catalyst RuO2, and is also competitive to the state-of-the-art catalysts, which is particularly noteworthy in consideration of the facile preparation process (detailed comparison



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summarized in Table S2, Supporting Information). Additionally, LIG-NiFe shows good durability as shown in Figure 3d. After 1000 cycles of water oxidation, the overpotential to deliver a current density of 10 mA/cm2 slightly increases from 292 to 302 mV.

Figure 4. Electrocatalytic performance of the bifunctional LIG-Co-P. (a) Water electrolysis of the LIG-Co-P illustrated by the CV curves. The LSV of the LIG-Co-P for (b) HER and (c) OER. Inset: corresponding Tafel plots. (d) Stability test; LSV of the LIG-Co-P after 1000 cycles of HER and OER comparing with the initial curves, respectively. All tests are done in 1 M KOH with 95% iR compensation.

The electrochemically deposited LIG-Co-P works as a bifunctional catalytic electrode for either HER or OER in accordance with the applied potential as shown in Figure 4a. We first 12 ACS Paragon Plus Environment

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evaluated the HER activity of the LIG-Co-P in 1 M KOH by the LSV (Figure 4b). The data shows that pristine LIG is relatively inert so that the catalytic activity is almost completely from the Co-P composites. The ηHER of HER is determined to be 75 mV from Figure S12a, and the current density reaches 10 and 100 mA/cm2 at 141 and 214 mV, respectively. The corresponding Tafel slope for HER calculated from the LSV (Figure 4b) is 54 mV/dec. The HER process in alkaline solution is expressed by either the Volmer-Heyrovsky process or the Volmer-Tafel pathway.43-44 Both pathways involve the adsorption of a H2O molecule and subsequent reduction of H2O into adsorbed OH- and H atom.41 The detachment of OH- to refresh the catalyst surface and the formation of adsorbed H intermediates to generate H2 are essential for efficient HER. CoP is considered to facilitate such processes because of the charge inbalance between Co and P through the transfer of electron density from Co to P.45 The locally positively charged Co centers have strong affinity to adsorb the OH- from H2O decomposition whereas the P sites favor H adsorption. Therefore, the possible occupation of OH- on H sites that blocks the active sites is eliminated and H2 is efficiently generated.26 Indeed, the high Cdl of LIG-Co-P calculated in Figure S11 may be caused by the high adsorption affinity to OH-. The OER activity of LIG-Co-P is evaluated in the same solution as HER (1 M KOH, Figure 4c). The anodic current increases rapidly after approximately 1.50 V (vs RHE). The ηOER is 270 mV from Figure S12b, and the corresponding Tafel slope is 84 mV/dec. The overpotential to deliver 10 and 100 mA/cm2 are 364 and 530 mV, which are slightly higher than the LIGNiFe. It is worth noting that the OER activity is likely from the higher oxidation states of the CoP under anodic conditions, where the Co-oxo/hydroxo site and PO43- are formed.38 As shown in Figure S9 and S13, the XPS spectra of a post-OER LIG-Co-P is significantly different from an as-prepared electrode. The Co 2p peak at ~778 eV lessens compared to the



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~781 eV peak, suggesting a transition of the original Co valence to higher oxidation states.46 Meanwhile, the P 2p peak at ~133.2 eV arising from PO43-becomes dominant in the post-OER LIG-Co-P with the significant decreased intensity of P species that have a binding energy of ~130 eV.47 The PO43- might be responsible for the effective OER kinetics, since the surface PO43- group has been known to assist the proton-coupled electron transfer during OER catalysis.48-49 The synergetic effect between CoOx and Co3(PO4)2 was also suggested to improve the OER performance.46, 50 The LIG-Co-P shows excellent long-term stability for both HER and OER as illustrated in Figure 4d. The performance was nearly unchanged after 1000 cycles of HER or OER, respectively.

Full water splitting The accessibility of catalytic LIG electrodes enables full water splitting from one piece of PI film with HER and OER catalysts formed on either side as shown in Figure 5a. In the prototype electrolyzer, LIG is patterned on both sides of a PI film and then fabricated into LIG-Co-P and LIG-NiFe on respective sides. The migration of OH- is enabled by a small pinhole located at the end of the film, which might later be coated by an ion exchange membrane for large scale applications.


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Figure 5. Illustration of the integration of catalytic LIG electrodes as a full water electrolyzer. (a) Schematic drawing of an integrated LIG device. (b) LSV curve of the electrolyzer in 1 M KOH with 95% iR compensation. (c) A photograph of the electrolyzer working at 50 mA/cm2. Bubbles are the generated H2 (left side) and O2 (right side) as depicted.

A preliminary investigation on the LIG electrolyzer for full water splitting was carried out in 1 M KOH. As expected, the LIG electrolyzer exhibits both efﬁcient HER and OER activity. The LIG-Co-P works for HER and LIG-NiFe for OER, simultaneously, to deliver a current density of 10 mA/cm2 at 1.66 V as illustrated in Figure 5b. The current density can be further increased to 400 mA/cm2 at 1.9 V without degradation of the electrolyzer. The full water splitting performance is comparable and even better than many recently reported systems.51-53



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More importantly, the PI film serves as an inherent separator for water splitting, wherein the H2 comes out of one side at the same time as O2 from the other side of the same device. The device is rigged in a simple O-ring system as shown in Figure 5c, such that the gasses that evolve are separated: H2 into one tube and O2 into the other, and can therefore can be stored separately. Figure S15 and S16 illustrate the long-term stability of the LIG electrolyzer. Supplementary videos (Video S1-S3, Supporting Information) show the real-time water splitting by the LIG electrolyzer working at 50 mA/cm2. Large amount of H2 and O2 bubbles form immediately when a potential is applied, demonstrating the high catalytic activity of LIG electrolyzer. The facile fabrication and easy tunability of LIG electrolyzer also favors other catalysts that can be simply loaded onto LIG.

Conclusion In summary, catalytic LIG electrodes are fabricated on inexpensive plastic films with catalysts loaded by either in situ doping or electrochemical deposition. The LIG electrodes, LIGPt, LIG-NiFe and LIG-Co-P, show superior activity to state-of-the-art catalysts for HER and OER in alkaline medium. The integration of LIG electrodes further permits full splitting of water into H2 and O2 using just one piece of plastic film. The straightforward and convenient fabrication of catalytic LIG electrodes demonstrated here not only provide a practical solution for H2 and O2 production that can drive a fuel cell. But it might also serve as a harbinger for constructing efficient devices for other electrocatalytic processes such as CO2 reduction and O2 reduction.

Experimental Section:


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Materials and Methods Preparation of LIG-Pt. Poly(pyromellitic dianhydride-co-4,4’-oxidianiline amic acid) (7.8 g) solution (12.8 wt % in N-methyl-2-pyrrolidone (NMP)/aromatic solvent, 80:20, 25038-81-7, Sigma-Aldrich) was used as a precursor solution for formation of a PI sheet. Pt(II) acetylacetonate (15170-57-7, Sigma-Aldrich) (50 mg, 0.13 mmol) was dissolved in 1 mL of NMP and then added to the PAA solution with bath sonication (Cole Parmer, model 08849-00) for 2 h to form a homogeneous mixture. The solution was poured into an aluminum dish (~5 cm in diameter) and then placed in a vacuum oven (~120 mm Hg) at 60 °C 3 d to remove the solvent. The film-imidization process was completed in an oven at 240 °C for 30 min and then 300 °C for another 30 min. LIG was generated by a CO2 laser cutter system (Universal XLS10MWH laser cutter platform) on the Pt doped PI film using 3% of the full power (75 W) and 5% of the full speed. The gas flow of H2 (3%)/Ar was maintained during the laser scribing process. LIG was patterned into a rectangle of 5 × 6 mm2. After that, Pellco colloidal Ag paint (No. 16034, Ted Pella) was applied on one side of the LIG pattern (5 × 1 mm2) to improve the conductivity. The electrodes were then connected with conductive Cu tape. A Kapton PI tape was employed followed by an epoxy (Machineable-fast set, Reorder # 04002, Hardman) sealing to protect the common areas of the electrodes. The geometrical area of the LIG electrode is 5 × 5 mm2. The fabricated LIG-Pt device was stored in a vacuum desiccator (~120 mm Hg) at room temperature for future test. An electrochemical workstation was employed for further deposition or testing. Preparation of LIG-NiFe. All samples were prepared under room temperature and ambient air. Kapton® PI films (McMaster-Carr, Cat. No. 2271K3, thickness: 0.005") were used as received. LIG was generated by a CO2 laser cutter system (Universal XLS10MWH laser cutter platform)



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on the Kapton® polyimide film using 3% of full power (75 W) and 5% of full speed in air. The fabrication of LIG electrode was prepared as described above. Then the deposition of the active material on the LIG was carried out in a 3-three electrode configuration, with Ag/AgCl as the reference electrode and a Pt plate as the counter electrode. The electrolyte bath contained 3 mM Ni(NO3)2·6H2O and 3 mM Fe(NO3)3·9H2O. The deposition was done at a constant potential of 1.0 V (vs Ag/AgCl) at room temperature. The optimized deposition time was 150 s. The LIGNiFe device was stored in a vacuum desiccator (~120 mm Hg) at room temperature for future test. Preparation of LIG-Co-P. The LIG electrodes were prepared in the same way as the LIG-NiFe electrodes. The deposition of active material on LIG was carried out in a 3-three electrode configuration, with Ag/AgCl as the reference electrode and a Pt plate as the counter electrode. The electrolyte bath contained 50 mM CoCl2, 0.5 M NaH2PO2, and 0.1 M NaOAc. The deposition was done by consecutive linear scans between -0.3 and -1.0 V (vs Ag/AgCl) at a scan rate of 5 mV/s for 10 cycles. The LIG-Co-P device was stored in a vacuum desiccator (~ 120 mm Hg) at room temperature for future testing. Characterization. SEM images were obtained on a FEI Quanta 400 high-resolution field emission SEM. TEM images were obtained by a JEOL 2100F field emission gun transmission electron microscope. The electrochemical measurements were carried out in a three-electrode configuration using the same workstation as LIG MSCs. Pt plate and Hg/HgO (in 1M KOH) were used as the counter and reference electrode, respectively. All tests were done in 1M KOH with 95% iR compensation. For the Hg/HgO electrode, E (vs RHE) = E (vs Hg/HgO) +0.930 V in 1M KOH.


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Acknowledgements The Air Force Office of Scientific Research (FA9550-14-1-0111), the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500) and the Chinese Scholarship Council provided funding. Rice University gratefully acknowledges the support of Universal Laser Systems, and for their generously providing the XLS10MWH laser system with Multiwave Hybrid™ technology that was used for this research. Mr. J. Hillman of Universal Laser Systems kindly provided regular helpful advice.

Supporting Information TEM and SEM images, Raman and XPS spectra and additional experimental details can be found free via the Internet at http://pubs.acs.org.

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Efficient Water Splitting Electrodes Based on Laser-Induced Graphene

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