3D Printed Graphene Electrodes' Electrochemical Activation - ACS

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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3D Printed Graphene Electrodes’ Electrochemical Activation Michelle P. Browne, Filip Novotný, Zdeněk Sofer, and Martin Pumera* Center for the Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic

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S Supporting Information *

ABSTRACT: Three-dimensional (3D) printing technologies are emerging as an important tool for the manufacturing of electrodes for various electrochemistry applications. It has been previously shown that metal 3D electrodes, modified with metal oxides, are excellent catalysts for various electrochemical energy and sensing applications. However, the metal 3D printing process, also known as selective laser melting, is extremely costly. One alternative to metal-based electrodes for the aforementioned electrochemical applications is graphene-based electrodes. Nowadays, the printing of polymer-/graphene-based electrodes can be carried out in a matter of minutes using cheap and readily available 3D printers. Unfortunately, these polymer/graphene electrodes exhibit poor electrochemical activity in their native state. Herein, we report on a simple activation method for graphene/polymer 3D printed electrodes by a combined solvent and electrochemical route. The activated electrodes exhibit a dramatic increase in electrochemical activity with respect to the [Fe(CN)6]4−/3− redox couple and the hydrogen evolution reaction. Such in situ activation can be applied on-demand, thus providing a platform for the further widespread utilization of 3D printed graphene/ polymer electrodes for electrochemistry. KEYWORDS: 3D printing, graphene/polylactic acid electrodes, electron transfer, hydrogen evolution reaction, functionalized graphene

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the low-cost FDM 3D printers can be utilized.8,15,16 Foster et al. 3D printed electrodes for various electrochemical applications, including supercapacitors, Li-ion batteries, and water splitting catalysts, using the Black Magic filament.17 Foo and co-workers created a range of electrodes with the same commercial filament for the construction of a supercapacitor and a photoelectrochemical sensor for the detection of copper. However, Foo et al. also reported that the surface resistance of the 3D printed electrode was an issue most likely due to the insulating PLA and low graphene content (8 wt %) in the filament.18 Hence, Foo et al. electrodeposited expensive noble metals onto the 3D printed electrodes to improve the conductivity of the electrode assemblies for their application measurements. Our group has previously shown that the Black Magic 3D printed electrodes are nonresponsive to a range of electrochemical probes, including [Fe(CN)6]4−/3−. However, we demonstrated that a simple dimethylformamide (DMF) immersion treatment can modify these electrodes, enhancing their electrochemical activity. The DMF activation process was found to remove significant amounts of the PLA from the electrode surface with the organic solvent, exposing more active electrode sites.8 Herein, we demonstrate that a combined DMF and electrochemical activation process greatly improves the activity of the 3D printed graphene/polymer electrodes, significantly beyond that which we previously reported for DMF activation alone. Insight is provided into the

INTRODUCTION Three-dimensional (3D) printing, or additive manufacturing, is emerging as a technology that could revolutionize how research is conducted in numerous scientific research fields.1,2 3D printing allows the user to create a 3-dimensional structure through a layer-by-layer deposition process controlled by computer aided design (CAD) software, a 3D scanner, or photogrammetry.2 3D printing is a diverse manufacturing technology, as various precursor materials can be used, allowing for the fabrication of products with a variety of physical, chemical, mechanical, and electrical properties. As a result, 3D printing has become a popular technique in research laboratories for the fabrication of in-house and low-cost materials/devices.3−6 The last 5 years has seen a surge in literature reports utilizing 3D printing technologies for the fabrication of electrodes for electrochemical applications.7−10 The 3D printing of metal electrodes by selective laser melting produces highly conductive current collectors, but requires high-cost printers, and the resultant electrodes typically need further modification to function as active catalysts for practical applications.4,7,11−13 A widely reported alternative option is the 3D printing of a thermoplastic polymer (polylactic acid (PLA) or acrylonitrile butadiene styrene) with a conductive material.14 The 3D printing of a conductive material embedded into a polymer costs significantly less than metal 3D printing as a low-cost fused deposition modeling (FDM) printer can be utilized.2,14 The use of a commercial graphene/PLA filament called “Black Magic” has become very popular as of late in research laboratories for the fabrication of electrodes due to the fact that © XXXX American Chemical Society

Received: August 27, 2018 Accepted: October 25, 2018

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DOI: 10.1021/acsami.8b14701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

technologically relevant applications. This paper demonstrates that solvent/electrochemical activation goes well beyond the former for graphene/polymer electrodes and is in fact necessary to improve the activity of the 3D graphene/PLA electrodes for electrochemical reactions.

optimum activation conditions that can be used for 3D printed electrodes fabricated from the commercially available Black Magic filament. It has been well documented that the activation of carbonbased electrodes improves the performance toward electrochemical processes.19−24 For example, Sánchez et al. electrochemically activated a carbon nanotube (CNT)/polymer composite by chronoamperometry methods over various potentials/times and determined that the electrochemical activity can be tuned depending on the parameters utilized during the activation step.21 Using Raman spectroscopy, it was concluded that this enhancing effect is related to the increasing level of defects introduced to the CNT walls during activation.21 Pumera et al. also studied the heterogeneous electron transfer (HET) relationship between as-received and electrochemically activated CNTs on glassy carbon electrodes.24 It was determined that the increase in the HET rate for the electrochemically activated CNTs arose from the introduction of edge-like sites on the CNTs due to the high oxidative potential applied during electrochemical activation.24 Additionally, Wang et al. reported a short preanodization step that improved the HET rate of screen-printed carbon electrodes when compared to the same electrode prior to this activation step.19 Graphene-oxide-based electrodes have also been electrochemically activated by Moo et al. and their response in [Fe(CN)6]4−/3− was evaluated, which indicated that electrochemical reduction promoted the HET, while electrochemical oxidation hindered the HET.23 Owing to the already highly oxidized nature of graphene oxide, further electrochemical oxidation may create a highly oxygen-rich electrode material, hindering HET.23 This would indicate there is an optimum amount of oxygen surface moieties required to increase the HET of carbon-based materials; this lies in the region between that of lightly oxidized sp2 carbon and lower than that of graphene oxide.21,23,24 To date, there have been no reports on the electrochemical activation of 3D graphene/PLA electrodes manufactured from Black Magic filament. The activation of these 3D graphene/PLA electrodes may not be as straightforward when compared to pure carbon/graphene materials as they exhibit poor surface conductivity and sluggish electrochemical performance with respect to the surface-sensitive [Fe(CN)6]4−/3− redox couple, as previously reported by Foo et al. and Manzanares et al., respectively. Herein, we report on the electrochemical and DMF activation of 3D graphene/PLA electrodes fabricated from the commercially available Black Magic filament. Both solvent and electrochemical activation steps were conducted in unison and separately on the 3D electrodes. The electrochemical activation was carried out over a range of potentials and times. To evaluate the effect of the various activation parameters, the HET rates of the 3D electrodes were determined and the material properties of the electrodes were probed by various material characterization techniques: Raman spectroscopy, Xray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Finally, the 3D printed electrodes were investigated as hydrogen evolution reaction (HER) catalysts. We show that a combination of chemical and electrochemical activation of graphene/polymer electrodes outperforms activation by either process alone. We further demonstrate that such activated composite electrodes have potential uses in applications such as the HER, indicating that this approach may provide a platform for inexpensive electrodes for

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EXPERIMENTAL SECTION

Reagents and Materials. Graphene/polylactic acid (PLA) filament (Black Magic 3D, New York), potassium chloride (SigmaAldrich, Czech Republic), dimethylformamide (DMF) (Fisher Scientific (Leics, U.K.)), absolute ethanol (Penta Prague, Czech Republic), potassium hexacyanoferrate(II) (Lach-Ner, Neratovice, Czech Republic), and potassium hexacyanoferrate(III) (Lach-Ner, Neratovice, Czech Republic) were used. The platinum wire counter electrode, Hg/HgO reference electrode, and Ag/AgCl reference electrode were purchased from CH Instruments (Texas) as well as phosphate buffer solution (PBS) Tablets from VWR chemical. Deionized water (16 MΩ) was used in all experiments. 3D Printing Parameters. The initial electrode design was carried out on open access software called Think-A-Cad. The file was spliced and converted to a .gcode file using Slic3r software. The 3D printing was performed on an FDM TRIBLAB printer (DelitiX, Czech Republic) using the commercially available Black Magic graphene/ PLA filament (BLACKMAGIC3D). For the 3D printing, the nozzle and bed temperatures were set to 220 and 55 °C, respectively. Electrochemical Parameters. All electrochemical measurements were carried out on an Autolab PGSTAT 204 potentiostat (Metrohm, Switzerland). Electrochemical activation of the 3D electrodes was performed in phosphate buffer solution (PBS) (pH 7.2) by applying a constant potential of 1.5, 2.0, or 2.5 V vs Ag/AgCl for different time periods (0−250 s). For the electrochemical activation step, a Ag/AgCl and a platinum wire were used as the reference and counter electrodes, respectively. DMF treatments were performed by soaking the 3D printed electrodes in DMF for 10 min.8 The 3D printed electrodes were then washed with ethanol and then water and allowed to dry for 24 h. When the DMF treatment was carried out for the electrochemical and DMF-treated electrodes, the DMF step was always performed prior to electrochemical activation. Cyclic voltammetry experiments were carried out at a scan rate of 100 mV s−1 in 1 mM [Fe(CN)6]4−/3− in 0.1 M KCl. The HET rate constants (k0obs) were calculated based on a method developed by Nicholson.25,26 This method relates the peak separation (ΔEp) to a dimensionless kinetic parameter in order to determine the k0obs. Electrochemical impedance spectroscopy was carried out in the frequency range of 100 000 to 0.1 Hz in a nonfaradic region in 1 M NaOH. Hydrogen evolution experiments were carried out at a scan rate of 5 mV s−1 in 1 M NaOH electrolyte. The reference and counter electrodes utilized were a Hg/HgO and a graphite rod, respectively. All HER potentials were converted from the Hg/HgO scale to the RHE scale using ERHE = EHg/HgO + 0.098 + (0.059*pH). Materials Characterization. An inVia Raman microscope (Renishaw, England) in backscattering geometry with a CCD detector was utilized for Raman spectroscopy analysis for the nontreated and treated 3D electrodes. A DPSS laser (532 nm, 50 mW) and a 50× magnification objective lens was used for the measurement. Instrument calibration was achieved with a silicon reference, which gives a peak position at 520 cm−1 and a resolution of less than 1 cm−1. High-resolution XPS was performed using a monochromated ESCAProbeP spectrometer (Omicron Nanotechnology Ltd., Germany) with an aluminum X-ray radiation source (1486.7 eV). The survey scans of all elements were performed with a pass energy of 100 eV with subsequent high-resolution scans with a pass energy of 20 eV for the C 1s and O 1s regions. An electron flood gun was used to eliminate sample charging during measurement (1−5 V). The morphology of the electrodes was investigated using a Tescan Maia3 Triglav high-resolution scanning electron microscope (SEM) with an field-emission gun electron source. SEM imaging was carried out using a 5 kV electron beam in high-resolution mode using an inlens secondary electron detector at a working distance of 5 mm. The B


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Figure 1. (a) Picture of the as-prepared/nontreated 3D printed graphene/PLA electrode. (b) Picture of the FDM 3D printer bed and nozzle used to fabricate the 3D printed electrodes from the commercially available Black Magic filament and (c) CV response of the nontreated 3D graphene/ PLA electrode in [Fe(CN)6]4−/3− redox couple.

Figure 2. Effect of electrochemical activation time on peak separation using the [Fe(CN)6]4−/3− redox probe. (a) Summary of peak separation values obtained for the different times utilized during the electrochemical activation for the DMF-treated 3D electrodes. The responses of the nontreated, electrochemical-only-treated, DMF-only-treated, and the DMF/electrochemical-treated 3D electrodes at various activation times can be observed in (b)−(f). Electrochemical activation took place at 2.5 V vs Ag/AgCl in PBS solution (pH: 7.2) for (b) 50 s, (c) 100 s, (d) 150 s, (e) 200 s, and (f) 250 s. Conditions: scan rate, 100 mV s−1; background electrolyte, 0.1 M KCl.

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ACS Applied Materials & Interfaces optical images of the electrodes were carried out using an optical profilometer (Sensofar, Spain) in confocal mode using a 50× objective.

electrochemical activation time was determined to be 150 s, observed from Figure 2a, as the smallest ΔEp value of 171 mV and the fastest HET constant of 2.43 × 10−3 cm s−1 were achieved. Interestingly, after 150 s, the peak separation values get subsequently larger, 183 mV for 200 s and 227 mV for 250 s, indicating slower electron transfer. The electrochemical activation times of 200 and 250 s exhibit HET values of 1.48 × 10−3 and 8.65 × 10−4 cm s−1, respectively. This decrease in activity could be due to the surface of the electrode becoming highly oxidized. It has been previously reported that a highly oxygenated surface will repel the [Fe(CN)6]4−/3− redox couple, leading to larger peak separation.23 These results reveal that both DMF and electrochemical activation are required to increase the electrochemical activity of the 3D graphene/PLA electrodes and induce faster electron transfer rates (Figure 2b−f). This is an important observation as with two simple activation steps these cheap 3D graphenebased electrodes manufactured from the commercial Black Magic filament could be more active for various electrochemical reactions. Further investigations were carried out to probe the effect of the applied potential during the electrochemical activation step, Figure 3. For these experiments, electrodes were polarized for 150 s, as this is the time period that produced the greatest enhancement in electron transfer kinetics in the previous section, Figure 2a.

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RESULTS AND DISCUSSION The 3D printed graphene/PLA electrodes in this study were fabricated by an FDM 3D printing printer, Figure 1a,b. After printing, the cyclic voltammetric (CV) response of the graphene/PLA electrode was investigated with respect to the surface-sensitive [Fe(CN)6]4−/3− redox probe; however, the response was extremely poor, Figure 1c. In a bid to improve the surface properties of the 3D graphene/PLA electrode, three activation procedures were carried out, and subsequently optimized, for the as-printed electrode. The three activation procedures employed in this study were (1) electrochemical activation; (2) solvent activation; and (3) DMF/electrochemical activation. Subsequently, the change in the electrochemical response of the treated 3D graphene/PLA-based electrodes was evaluated again using the [Fe(CN)6]4−/3− redox couple and compared to the untreated electrode. As previously reported by our group, the optimum time for solvent activation for the 3D graphene/PLA electrodes in DMF was reported to be 10 min.8 Owing to this previous observation, all solvent activation in this current study was carried out in DMF for 10 min. The electrochemical activation was carried out in PBS solution (pH 7.2) using a chronoamperometry method at high oxidizing potentials (1.5, 2.0, and 2.5 V vs Ag/AgCl) over a range of times (0− 250 s). Activation Studies and Optimization. The electron transfer kinetics of the [Fe(CN)6]4−/3− redox couple are known to be sensitive to changes on the surface of carbon electrodes, including the addition of charged groups, for example, oxygen-containing groups, as a result of electronic repulsion interactions between the negatively charged [Fe(CN)6]4−/3− complex.23,27−29 Hence, in this study, any surface modifications made to the 3D printed electrodes induced by the various activation regimes will result in a change in the CV response. The CV response for the [Fe(CN)6]4−/3− redox couple was carried out to determine the peak separation (ΔEp) and ultimately to calculate the HET (k0obs) constants of the nontreated and activated electrodes, Figure 2. Owing to the very slow electron transfer kinetic of the nontreated and electrochemically only treated electrodes, the ΔEp and k0obs values were not calculated, Figure 2a−f. The DMF-only-treated electrode exhibited a ΔEp value of 527 mV or a k0obs value of 2.13 × 10−5 cm s−1. The DMF-treated 3D electrodes were subjected to five various electrochemical activation times at a constant potential of 2.5 V in PBS solution and subsequently, the CV response in Fe(CN)4−/3− was determined, Figure 2b−f. As seen in Figure 2a, a significant difference in the ΔEp value can be observed for the various electrochemical activation times of the alreadyDMF-treated electrodes. After electrochemical activation for a period of 50 s, the ΔEp and k0obs values improved dramatically to 249 mV and 6.59 × 10−4 cm s−1, respectively, indicating the 3D electrode surface has been altered by the electrochemical activation. Further electrochemical activation time studies reveal that electrochemical activation for 100 s further enhances the electron transfer characteristics of the 3D graphene/PLA electrode as the ΔEp and k0obs values were determined to be 190 mV and 1.36 × 10−4 cm s−1, respectively. The optimum

Figure 3. Effect of electrochemical activation potential on peak separation using the [Fe(CN)6]4−/3− redox probe for an electrochemical activation time of 150 s for the DMF-treated electrodes. (a) Summary of the peak separation values obtained from the DMFtreated electrodes and the electrodes further electrochemically activated at 1.5, 2.0, and 2.5 V vs Ag/AgCl for 150 s and (b) CV response of the DMF-treated 3D electrodes in [Fe(CN)6]4−/3− reported in (a). Conditions: scan rate, 100 mV s−1; background electrolyte, 0.1 M KCl. D


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Raman spectroscopy provides an insight into the structural composition and the defect levels in the carbon lattice.28 Graphitic materials can be explored by investigating the D band and G band that are usually positioned at ca. 1350 and 1580 cm−1, respectively.21,29,30 The G band is attributed to the in-plane C−C stretching in sp2 carbon materials in the E2g vibration mode. The D band or the A1g vibrational mode represents forbidden/out-of-plane vibrations in the graphitic lattice. The intensity ratio of the D/G bands (ID/IG) can be used as an indication of the defect level in a material. As previously reported, the Raman spectrum of the nontreated 3D graphene electrode is not that of pristine monolayer graphene, but more typical of defective multilayer graphene, Figure S2.17 The ID/IG values for the nontreated, electrochemical-treated, DMF-treated, and DMF/electrochemical-treated 3D electrodes were determined as 0.83, 1.0, 0.58, and 0.71, respectively, Figure 5a,b. These results suggest that electrochemical

Prior to the electrochemical activation, DMF treatment was carried out, as before. From Figure 3a,b, the ΔEp, and hence the k0obs values, with respect to the [Fe(CN)6]4−/3− redox probe improved with increasing electrochemical activation potential up to 2.5 V. Additionally, even at the lowest electrochemical activation potential, of 1.5 V, improvements in the electrochemical activity are observed when compared to the DMFtreated 3D electrode. The ΔEp value decreases from 527 mV for the DMF-treated electrode to 298 mV for the DMF/ electrochemically treated electrodes at 1.5 V for 150 s. The electrode electrochemically activated at 2.0 V for 150 s exhibited ΔEp and k0obs values of 195 mV and 1.28 × 10−3 cm s−1, respectively. In this work, the optimum potential for the activation of these 3D graphene/PLA electrodes in the [Fe(CN)6]4−/3− redox probe was determined to be 2.5 V, with a calculated ΔEp value of 171 and a k0obs value of 2.43 × 10−3 cm s−1. To elucidate a possible explanation for this observed 0 values for the optimum increase in the ΔEp and kobs electrochemical activation time (150 s) and potential (2.5 V) in the [Fe(CN)6]4−/3− redox probe, multiple materials characterization tools were utilized. For all materials characterizations, four samples were used: nontreated, electrochemically activated at 2.5 V for 150 s, DMF-activated, and DMF/ electrochemically treated at 2.5 V for 150 s electrodes. Materials Characterization. SEM analysis was undertaken to determine the morphology of the nontreated and treated electrodes, Figure 4a. The morphology of the untreated electrode was difficult to determine due to the large amount of PLA present. From Figure 4a, all of the treated 3D graphene/ PLA electrodes exhibit a morphology resembling a network of wires. From the optical images, in Figure S1, the density of the wires appears to increase for the subsequent treatments.

Figure 5. Raman spectroscopy for (a) nontreated and electrochemically only treated and (b) DMF-treated and DMF/electrochemically treated 3D printed electrodes.

activation increases the defect sites in the C−C structure of the graphene/PLA electrodes. This increase in defects can be observed between the nontreated and electrochemically treated electrodes, Figure 5a, and again between the DMFtreated and the DMF/electrochemically treated electrodes, Figure 5b. High-resolution XPS was used in this work to gain insight into the carbon functional groups on the surface of the 3Dbased electrodes before and after the various treatments, Figure 6a−d. The C 1s peak of the untreated 3D electrodes exhibits a similar XPS background to that of PLA but possesses a larger C−C peak due to the addition of the graphene into the filament, Figure 6a.31 After electrochemical activation, the C 1s peak shows significant differences; the C−O and CO peaks are suppressed. The loss in the C−O and CO peaks is presumably due to the loss of PLA from the electrode surface or the introduction of edge sites to the surface graphene, which then covers the PLA, Figure 6b. A comparison between the C 1s peak of the untreated, Figure 6a, and DMF 3D, Figure 6c, electrodes reveals the DMF process promotes a decrease in the CO, which is likely due to the physical spalling of the PLA polymer from the electrode, as DMF is known to swell/ break down PLA.32 Combination of the DMF and electrochemical treatment produces a C 1s spectrum indicative of function-

Figure 4. SEM images of the (a) nontreated, (b) electrochemically activated at 2.5 V for 150 s, (c) DMF-activated, and (d) DMF/ electrochemically treated at 2.5 V for 150 s electrodes. Scale bar = 10 μm. E


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Figure 6. (a) XPS C 1s core level for the nontreated electrode. (b) XPS C 1s core level for the electrochemically treated electrode at 2.5 V for 150 s. (c) XPS C 1s core level for the DMF-treated electrode. (d) XPS C 1s core level for the DMF/electrochemical-treated electrode at 2.5 V for 150 s.

alized graphene, which is known to be electrochemically active due to the increase in edge sites, Figure 6d.33,34 Proposed Mechanism of Activation. Based on the material characterization of the electrodes and previously reported work by others, the observed trend in the electron transfer kinetics, from Figure 2d, can be explained in terms of (1) the availability of graphene at the electrode surface; (2) the amount of PLA in the electrode; and (3) the different functionalized carbon groups present in the graphene at the surface of the electrode. The nontreated 3D graphene/PLA electrode contains high amounts of insulating polymer and has a high surface resistance, as previously reported by Foo et al.18 In this study, and in a previous study by our group, the nontreated electrode exhibits extremely poor HET characteristics in the [Fe(CN)6]4−/3− redox probe.8 After electrochemical activation, a slight increase in CV response can be observed in [Fe(CN)6]4−/3−; however, no HET rate or peak separation values could be determined. The slight increase in the [Fe(CN)6]4−/3 response may be rationalized from the XPS analysis. As XPS is a surface-sensitive technique, the data suggests that at the surface of the electrode, the availability of the sp2 carbon has increased (32−48%) and the CO functional groups have decreased compared to the nontreated 3D electrode, Table 1. This result indicates that there is a larger amount of graphene available and less PLA at the surface of the electrode. Unfortunately, the majority of the electrode still contains PLA, hindering the HET process. As previously reported, DMF treatment of the nontreated electrode would significantly reduce the amount of PLA in the overall electrode, in turn increasing the conductivity of the electrode.8 The reduction of the PLA in the electrode in DMF takes place through a physical process.8,32 The DMF causes the PLA to swell and ultimately causes the PLA to fall out of the electrode, resulting in an increase in the HET rate. To gain an insight into the relative resistive behavior of the electrodes after

Table 1. C 1s Contributions for the Untreated and Optimum-Treated 3D Electrodes C 1s contribution nontreated sp2 sp3 C−O CO O-CO residual STD error

32.18 19.4 24.45 23.94 n/a 1.08

electrochemically treated

DMF treated

DMF and electrochemically treated

48.10 26.69 15.34 9.87 n/a 1.06

32.8 21.5 28.5 11.6 n/a 1.25

40.17 29.87 10.56 9.88 9.52 1.07

the various activation treatments, electrochemical impedance spectroscopy was carried out on all the samples in this study, Figure S3. The Nyquist plots verify our theory, as after DMF treatment the 3D printed electrodes exhibit less resistive behavior. At the surface of the DMF-treated electrode, XPS analysis also reveals a decrease in the CO functional groups, further indicating less PLA than the untreated sample. Hence, the DMF treatment enhances the rate of HET by reducing the amount of PLA in the electrode and, thus, increasing overall electrode material conductivity. Furthermore, from the optical images, Figure S1, after DMF treatment, the availability of wirelike structures (i.e., the graphene) appears to have increased at the surface, that is, the loss of the PLA has resulted in the ‘freeing’ of the graphene at the surface of the electrode. Further electrochemical treatment of these more conductive DMF electrodes results in the oxidation of the exposed graphitic sites, introducing more functionalized moieties (O− CO), Table 1, which, in turn, enhances the electron transfer kinetics, Figure 2.21,23,35 Hydrogen Evolution Reaction. The HER is an important reaction in the overall water splitting process as H2 gas is produced. The H2 can then be utilized to make electricity for F


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ACS Applied Materials & Interfaces countries’ national grids or as a fuel for H2 cars.36 Unfortunately, this process is currently regarded as uneconomical as the optimum materials for the HER are based on the expensive and rare platinum group metals.37 Therefore, a huge amount of research is under way to find the next efficient, cheap HER catalysts.37−39 The linear sweep voltammetry (LSV) responses of the nontreated and treated 3D electrodes were evaluated for their electrochemical activity in the HER region in 1 M NaOH. It is evident from Figure 7 that the DMF/electrochemical activity

study, it is evident that sequential DMF and electrochemical activation steps are required to produce graphene/PLA electrodes with the fastest HET rate and the most electrocatalytic HER activity. This DMF/electrochemical activation approach of 3D printed graphene/PLA electrodes fabricated from the commercial Black Magic filament opens the door toward the fabrication of highly active 3D printed graphenebased electrodes, which may have use in a plethora of electrochemical and electronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14701. Optical images, Raman spectra of the nontreated 3D printed electrode, and electrochemical impedance measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951

Figure 7. HER LSV curves of the nontreated, electrochemically treated at 2.5 V for 150 s, DMF-treated, and DMF/electrochemicaltreated at 2.5 V for 150 s electrodes. Conditions: scan rate, 5 mV s−1; electrolyte, 1 M NaOH.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work emanated from financial support from the Advanced Functional Nanorobots project (Reg. no. CZ.02.1.01/0.0/0.0/ 15_003/0000444 financed by the EFRR). M.P.B. would also like to acknowledge the European Structural and Investment Funds, OP RDE-funded project “ChemJets” (No. CZ.02.2.69/ 0.0/0.0/16_027/0008351). Z.S. was supported by Czech Science Foundation (GACR no. 16-05167S) and by the financial support of the Neuron Foundation for science support.

exhibits the optimum HER activity when compared to the remaining electrodes. At a current density of −1.5 mA cm−2, the improvement in the HER activity is ca. 600 mV when compared to the nontreated electrode, 300 mV compared to the electrochemical-only-treated electrode, and ca. 80 mV when compared to the DMF-treated electrode. The optimum HER activity for the DMF/electrochemically treated 3D electrode can be attributed to both the degradation of the insulating PLA polymer by the DMF treatment and the increase in graphene defects induced by the electrochemical activation indicated by the Raman analysis and XPS analysis, Figures 5b and 6, respectively. It is evident from this study that both the DMF and electrochemical treatments of the 3D graphene/PLA electrodes are necessary for the improved electrochemical activity. This is an encouraging result as the further modification of these bare carbon-based DMF/electrochemically treated electrodes with active HER catalysts, such as MoS2 and WS2, could significantly improve the HER activity to rival that of the state-of-the-art Pt catalysts, Figure 7. In turn, the overall water splitting process could become a more efficient, clean, and cheap process for generating H2 and subsequently electricity.

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REFERENCES

(1) Manzanares Palenzuela, C. L.; Pumera, M. (Bio)Analytical Chemistry Enabled by 3D Printing: Sensors and Biosensors. TrAC, Trends Anal. Chem. 2018, 103, 110−118. (2) Ambrosi, A.; Pumera, M. 3D-printing Technologies for Electrochemical Applications. Chem. Soc. Rev. 2016, 45, 2740−2755. (3) Symes, M. D.; Kitson, P. J.; Yan, J.; Richmond, C. J.; Cooper, G. J. T.; Bowman, R. W.; Vilbrandt, T.; Cronin, L. Integrated 3D-printed Reactionware for Chemical Synthesis and Analysis. Nat. Chem. 2012, 4, 349−354. (4) Ambrosi, A.; Pumera, M. Self-Contained Polymer/Metal 3D Printed Electrochemical Platform for Tailored Water Splitting. Adv. Funct. Mater. 2018, 28, No. 1700655. (5) Meloni, G. N.; Bertotti, M. 3D printing Scanning Electron Microscopy Sample Holders: A Quick and Cost Effective Alternative for Custom Holder Fabrication. PLoS One 2017, 12, No. e0182000. (6) Hensleigh, R. M.; Cui, H.; Oakdale, J. S.; Ye, J. C.; Campbell, P. G.; Duoss, E. B.; Spadaccini, C. M.; Zheng, X.; Worsley, M. A. Additive Manufacturing of Complex Micro-architected Graphene Aerogels. Mater. Horiz. 2018, 5, 1035−1041. (7) Tan, C.; Nasir, M. Z. M.; Ambrosi, A.; Pumera, M. 3D Printed Electrodes for Detection of Nitroaromatic Explosives and Nerve Agents. Anal. Chem. 2017, 89, 8995−9001. (8) Manzanares Palenzuela, C. L.; Novotný, F.; Krupička, P.; Sofer, Z.; Pumera, M. 3D-Printed Graphene/Polylactic Acid Electrodes

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CONCLUSIONS We have demonstrated, for the first time, the effects induced by various activation routes on 3D printed graphene/PLA electrodes. The results show that electrochemical-only treatment somewhat decreases the amount of the insulating PLA polymer at the surface of the electrode, making the graphene more accessible. The DMF step causes a significant increase in conductivity of the electrodes due to the physical spalling of the PLA from the entire electrode. After DMF treatment, the subsequent electrochemical step produces an electrode with characteristics similar to functionalized graphene. From this G


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3D Printed Graphene Electrodes' Electrochemical Activation - ACS

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