3D-Printed Electrolyzer for the Conversion of Glycerol into Tartronate

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3D-printed electrolyzer for the conversion of glycerol into tartronate on Pd nanocubes Katia-Emiko Guima, Leticia Machado Alencar, Gabriel C da Silva, Magno Aparecido Gonçalves Trindade, and Cauê A. Martins ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03490 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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ACS Sustainable Chemistry & Engineering

3D-printed electrolyzer for the conversion of glycerol into tartronate on Pd nanocubes a

a

b

a

a*

Katia-Emiko Guima , Leticia M. Alencar , Gabriel C. da Silva , Magno A. G. Trindade , Cauê A. Martins a

Faculty of Exact Sciences and Technology, Federal University of Grande Dourados, Rodovia Dourados - Itahum, km 12,79804-970, Dourados, MS, Brazil b

Instituto de Química de São Carlos/Universidade de São Paulo, IQSC-USP, C.P. 780, Avenida Trabalhador Sãocarlense, 400, CEP 13566-590, São Carlos, SP, Brazil *[email protected]

ABSTRACT: Glycerol is a massive byproduct of biodiesel fabrication, which decreases its price and increases the risks of inadequate disposal. In this sense, more environmental friendly instruments and processes using glycerol are required to make this matrix more valuable. Here, a 3D-printed electrolyzer was developed and tested for long-period glycerol electrolysis in an alkaline medium. The new electrolyzer contains only three mobile parts and can be manufactured in less than 4 h using ~30 g of polylactic acid filament, with a total cost of less than US $5. This easily built and inexpensive reduced-scale electrolyzer has the advantage of using only a few milliliters of solution to perform tests for electrosynthesis. We synthesized Pd nanocubes to modify a glassy carbon working electrode, which was used for glycerol electrolysis. We found a remarkable selectivity of 99% towards tartronate production, which was induced by the extended (100) surface of Pd in the alkaline medium. Hence, we report a new 3D-printed platform for electrosynthesis and a new clean one-step method to produce tartronate.

KEYWORDS: 3D-printed electrolyzer; Pd nanocubes; glycerol; tartronate; electrocatalysis Introduction Electrochemical systems used to convert chemicals, such as electrolyzers, are beneficial compared to traditional reagent-based methods, since they (i) present mild conditions (diluted solution, low temperature and so on), (ii) allow reuse of the electrocatalysts if stable, (iii) are scalable and (iv) might be selective and even stereoselective.1 Glycerol, a byproduct of biodiesel fabrication, has been considered a substrate to electrochemically synthesize a myriad of carbonyl compounds 2–8 Glycerol might be converted to CO2 after total electrooxidation,9 but most of the chemical energy is extracted by consecutive reactions, producing partially oxidized compounds.10 These different pathways can be tuned towards a desired compound by changing the medium, applied potential and mainly by modifying the electrocatalysts. Kwon et al. modified Pt/C nanoparticles (NPs) with Bi, Sb, Pd, Sn and In ad-atoms for the glycerol electrooxidation reaction (GEOR).4 These authors used an online high performance liquid chromatography (HPLC) system coupled to an electrochemical cell and found that Sbmodified Pt/C is selective to produce dihydroxyacetone from the GEOR.4 The same group also proved how the crystallographic surface arrangement is important to tune

the selectivity by using Pt(111) to produce 1,3 dihydroxyacentone and glyceraldehyde, and Pt(100) to selectively obtain glyceraldehyde.7 Using long-period electrolysis, da Silva and coworkers found a high production of glycerate from the GEOR on Ru and Ni-decorated Pt dispersed on carbon nanotubes in an alkaline medium.3 In addition, using an ex-situ HPLC technique to identify the products of electrosynthesis, Wang et al. showed that a modified carbon support plays an important role in the selectivity of the GEOR.11 Pd has been also considered a metal-base for concomitant energy conversion and carbonyl compounds production in fuel cell-type configurations in alkaline media.12,13 Moreover, it has been reported that different shapes result in different electrochemical responses.14 The interest in using glycerol as a substrate for electrosynthesis is due to the high market price of several possible compounds. Considering the price to a final consumer, the increment from glycerol to derived compounds might reach almost nine thousand times, as found for hydroxypyruvic acid, which is used to produce amino acids. Another compound, tartronic acid, reached US $4.67/g compared to US $0.08 for glycerol. There is an industrial interest in tartronic acid since it can be used as

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reactant in the production of mesoxalic acid and other derivatives for medical purposes and for removing dissolved oxygen from alkaline water.15 Considering the industrial price, a kilogram of glycerol might be sold at US $0.11–0.99, while the product of GEOR dihydroxyacetone reaches US $2.00. Hence, electrochemical systems provide a clear conversion of a massive side product of biodiesel with a low market price into highly valuable compounds. Moreover, it opens up the possibility for the concomitant production of energy and compounds with market interest by using glycerol to feed fuel cell anodes.3,8 Regarding the methods to identify the products of the GEOR, although there are other techniques,16 HPLC seems the most appropriate, since it is well established, simple and ideal for soluble compounds. HPLC can be used online2 or ex situ, after electrolysis. Long-period electrolysis is interesting for industry due to the potential application at large scales. In the laboratory, the most reliable method is to use a H-type glass-made electrolyzer. This system is divided into two compartments separated by an ion-permeable membrane, one side contains a counter electrode and the other has the reference and working electrodes. The main disadvantage of such a system is its complexity. H-type glass-made electrolyzers might contain more than 15 mobile parts, considering the inlets, outlets, septum and so on. Among the measurement, all these parts need to be well cleaned and rearranged, increasing the risks of crashing the parts and delaying the speed of tests. The most critical point is the need to find a skillful professional to make the electrolyzer. Considering the significant interest in the electrolysis of glycerol, it is clear the need for a new platform of tests, which must be as reliable as the standard, but faster and easier to build and use. Here, we built a 3D-printed Htype electrolyzer containing only three mobile parts with a reduced scale. For the first time, we used glassy carbon modified Pd nanocubes as a working electrode to convert glycerol selectively to tartronate in an alkaline medium using the 3D-printed electrolyzer.

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lyzer, which are detailed in Figure S1. The best resolution for slicing and printing all pieces was 0.1 mm.

Figure 1. Illustrative scheme of the three-part 3D-printed electrolyzer. Part 2 and 3 are separated by an ionic exchange membrane. Samples of long-period electrolysis are withdrawn through part 2 by a glass syringe. The glassy carbon modified Pd nanocubes is used as working electrode.

Part 1 is a ~3 × 3 × 2 cm compartment designed to receive the working electrode, reference electrode, gas inlet and outlet, if necessary. Part 2 is a 3 × 3 × 3 cm chamber connected to a 1.5 cm cylinder in a single piece. This part receives the reference and working electrodes, where the chemical conversion takes place. Part 2 has a rectangular hole (0.2 × 0.6 cm) specially made to allow the withdrawal of aliquots close to the surface electrode, as showed in Figure 1. Part 1 is connected to part 2 through a simple juncture. Part 3 is a 2.2 × 2.0 × 2.0 cm chamber containing a rectangular hole specially designed to receive a Pt plate (or similar) as the counter electrode and two holes for gas inlet and outlet, if necessary. Part 3 has a doubled walled 1.8 cm cylinder designed to be connected to part 2 and to keep an ion exchange membrane of ~1 × 1 cm between the two pieces. More details are available in Figure S1. Synthesis of Pd nanocubes

Methods Building the 3D-printed electrolyzer 3D printing has emerged as a powerful tool to enable an unprecedented range of possibilities for rapid prototyping. It has attracted the attention of industry and research laboratories. 3D-printed devices have been widely used in electrochemistry, as detailed by Ambrosi and Pumera.17 In the field of microfluidics, the possible replacement of the traditional poly(dimethylsiloxane) by printed devices is now considered as a revolution.18 Here, we take advantage of such efforts for electrocatalysis applications. We used fused deposition modeling (FDM)17 to build the 3D-printed electrolyzer using thermoplastic polylactic acid (PLA). Software Autodesk® Inventor 2017 was used for modeling the device and Simplify 3D software was used for slicing. All parts were printed using a 3D printer, Sethi3D model S3. Figure 1 shows the parts of the electro-

We developed a new method based on the previous reports of the synthesis of Pd nanocubes.19–21 We used PdCl2 as a metallic precursor, cetilmetilamonio bromide (CTAB, C19H42BrN) as a capping agent and ascorbic acid (C6H8O6) as a reducing agent. All reactants were purchased from Sigma-Aldrich. The synthesis was made to obtain 187 mL of Pd 0.002 mol L-1. Firstly, adequate amounts of PdCl2 and CTAB were mixed in 93.5 mL of deionized water (DI), sonicated for ~50 min and stored for 12 h. Next, the dispersion was stirred at 2500 rpm, while a 0.2 mol L-1 ascorbic acid solution was dripped in using a burette. The total time of ascorbic acid addition was 60 min. The bottom flask was then placed in a previously 90 °C heated oil bath and kept for 60 min under reflux to form Pd nanocubes. This dispersion was stored at room temperature. Aliquots were


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cleaned by 30 min of centrifugation at 6000 rpm, once by ethanol and twice by DI water. Then, the decanted material was dispersed in 1 mL of water to make an ink, which was used to prepare the electrodes. To reproduce this protocol, one must consider the proportionality of a 1 mL cleaned aliquot out of a 187 mL total of the batch of synthesis to prepare 1 mL of ink. Transmission electron microscopy (TEM, JEOL model JEM2100) with a LaB6 filament working at 200 kV was used to investigate the morphology of the NPs. The samples were collected directly from the ink, dispersed in 2propanol and deposited onto a 400-mesh copper grip. Preparation of electrodes and electrochemical measurements The half-cell measurements were performed in a classic three-electrode cell. An Ag/AgCl electrode was used as the reference and a high surface area Pt plate as the counter electrode. A modified 0.2 cm2 glassy carbon was used as the working electrode. All measurements were performed in a N2-saturated 0.1 mol L-1 KOH solution (with or without 0.2 mol L-1 glycerol) at 25 °C in a μAutolab potentiostat/galvanostat with current integration. All potentials were corrected to a reversible hydrogen electrode (RHE). To prepare the working electrode, 150 µL of the Pd dispersion was dropped onto the glassy carbon to form a theoretical loading of 160 µg cm-2. The current densities (j) were calculated as mA cm-2, considering 420 µC cm-2 as the charge released by the desorption of a Pd oxide monolayer to calculate the electrochemically active surface area. The electrochemical profiles of the Pd nanocubes were registered in a potential range of 0.0–1.3 V in an attempt to reach the hydrogen under potential deposition (HUPD) region.20,21 All cyclic voltammetry measurements were performed at 0.05 Vs-1. The same protocol, including the same electrodes, was followed for cyclic voltammetry using the 3D-printed electrolyzer. Electrolysis of glycerol in an alkaline medium was performed by applying 0.87 V for 9 h by chronoamperometry. This is the potential which provides 2/3 of the peak current density. Such potential was chosen to provide stable output current density. The potentiostatic experiments at 0.87 V displays stable pseudo-stationary current density, while some fluctuations were observed by applying the potential peak. The connection of parts 2 and 3 (Figure 1) was separated using a Nafion®424 membrane, reinforced with poly(tetrafluoethylene) fiber with a thickness 0.013 in. Chromatographic measurements The separation and quantification of the glycerol byproducts were performed using ultra-high performance liquid chromatography (Agilent Technologies, model 1220) with a Rezex ROA-Organic acid column with a parallel pre-column made of the same stationary phase cou-

pled to a diode array detector. The best condition was found for 0.005 mol L-1 sulfuric acid as an eluent, 75 °C as the column temperature and a 200 nm wavelength (λ). The flow rate was performed in gradient mode with the elution starting at 0.24 and finishing at 0.4 mL/min for a total time of 20 min. To identify (using the retention time) and quantify (using the peak area) the target compounds, an external calibration curve at a concentration range between 2.0×105 and 1.0×10-4 mol L-1 was built with the following SigmaAldrich standard reactants: oxalic acid, tatronic acid, formic acid, glyoxylic acid, dihydroxyacetone, glycolic acid, glyceric acid and glyceraldehyde. These standards were chosen based on previous reports.3,11 Illustrative chromatograms of the standard reactants are shown in Figure S2. After the long-period electrolysis, an aliquot of 100 µL was withdrawn (Figure 1) from a region close to the electrode surface and diluted to 1 mL 0.5 mol L-1 H2SO4. At this point, the anionic compounds were protonated and filtered through a 0.22 µm Nylon® membrane. Finally, 30 µL was automatic injected in the HPLC system for analysis. Results and discussion The novelty of this work lies on the new 3D-printed electrolyzer, new synthesis of Pd nanocubes and the selective conversion of glycerol towards tartronate, as discussed in the next three sections. 3D-printed electrolyzer The printing parameters per piece, including building time (BT), used filament length (FL), plastic weight (PW) and material cost (MC), are shown in Table 1. Although part 3 exceeds 90 min of BT, the total time is ~4 h. It is worth noting that we used 0.1 mm as slicing and printing resolution to guarantee the quality of pieces, which increases the BT. The total FL was 10.04 m, corresponding to ~30 g. The total MC of the 3D-printed electrolyzer is less than US $5. Table 1. Printing parameters of the electrolyzer Piece

BT/min

FL/cm

PW/g

MC/US$

Part 1

54

220.7

6.58

0.97

Part 2

86

518.5

15.47

2.29

Part 3

97

264.8

7.90

1.17

Total

237

1004

29.95

4.43

Only a 3D printer allows such fast manufacturing, which is much faster than the time to build an ordinary H-type glass-made electrolyzer. Other important advantages are the very low price and easy handling, since a 3D printer might be controlled even by an untrained user, replacing the need for a hyalotechnical (for this exact



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procedure). Finally, the reduced scale represents a reduction of high pure reactants used in electrocatalysis, since 5–10 mL of solution might be used in the chambers (parts 2 and 3 of Figure 1). Hence, the high speed and low cost to manufacture and use allows a researcher to produce (and modify) electrolyzers for a myriad of applications. Glycerol electrooxidation on Pd nanocubes and 3D-printed electrolyzer testing Prior to use, the Pd nanocubes were investigated by TEM, as showed by the representative images in Figure 2. The new method successfully induces the synthesis of 15– 40 nm Pd nanocubes. The main difference between the developed protocol and those reported in the literature19 is the absence of a step to produce H2PdCl4. Here, we prove that it is possible to obtain nanocubes by skipping this step, which decreases the time of synthesis.

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surface processes on Pd are the same by using both systems, we are allowed to use the 3D-printed electrolyzer to perform the GEOR. The glycerol electrooxidation shows an anodic current in a wide range of potential during the positive potential going scan, starting at ~0.66 V. During the reverse scan, after the reduction of surface oxide species at 0.76 V, there is a well-defined oxidation peak, which is characteristic of glycerol electrooxidation on well-ordered surfaces.23 Figure 3 shows five consecutive cyclic voltammograms, the cycles are stable from the second cycle onwards. The first cyclic voltammogram in the presence of glycerol (the only one showing smaller currents in Figure 3c) has been proven to show random features.23 This response assures the stability of the system for consecutive cycles, similar to classic half-cell measurements.

The glassy carbon modified Pd nanocubes was submitted to an electrochemical characterization using cyclic voltammetry in a conventional electrochemical cell and using the 3D-printed electrolyzer, as shown in Figure 3. This measurement is imperative to show the extent of reliability for half-cell measurements and to the electrolysis, accordingly. Figures 3a and 3b show characteristic profiles of Pd in the alkaline medium22 for a conventional electrochemical cell and for the 3D-printed electrolyzer working as half-cell. Both systems provide equivalent electrochemical characteristics of Pd, as surface oxide formation, starting at ~0.7 V during the positive potential going scan, reduction of surface oxides centered at ~0.7 V during the reverse scan and HUPD region between 0.005 and 0.5 V.

Figure 3. Representative cyclic voltammogram of Pd nanocubes (a) in a conventional glass-made three-electrode electrochemical cell and in a 3D-printed H-type prototyped -1 electrolyzer in (b) absence and (c) presence of 0.2 mol L -1 glycerol. All measurements performed at 0.05 V s in O2-free -1 0.1 mol L KOH electrolyte. Figure 2. Representative images of Pd nanocubes obtained by transmission electron microscopy.

It is noteworthy that low potentials were reached, with the intention of achieving a full hydrogen region to characterize the nanocubes. As the potential domains of the

Although we used the 3D-printed cell with the aim of electrosynthesis, Figure 3 evidences the reliability of this new system, which opens up a wide range of possibilities for electrocatalysis investigation. It is worth noting that the challenge about using 3D-printed cells for fundamental investigations is the cleaning process. One must be aware of the filament material in use to avoid damaging


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the system. For instance, the use of highly concentrated alcoholic KOH washing solution might partially damage the PLA. Hence, it is necessary to clean it using diluted washing solutions, followed by intense sonication in DI water, whether the researcher intends to reuse the system. The use of diluted acid and alkaline media, as those usually used for half-cell measurements and fundamental research, do not damage the device, which guarantees reliable measurements. Conversion of glycerol to tartronate on Pd nanocubes using 3D-printed electrolyzer The long-period electrolysis using the new 3D-printed electrolyzer, containing the synthesized Pd nanocubes as

working electrode, selectively converted glycerol into 99% tartronate after 9 h at 0.87 V in 0.1 mol L-1 KOH and 0.2 mol L-1 glycerol (chromatogram shown in Figure S3). We also detected 1% of a mixture of glyceraldehyde, dihydroxyacetone, glycolate and glycerate. This impressive selectivity in mild conditions is a sum of diluted electrolyte and potential application for 9 h to a high extent of the preferentially Pd(100) surface, since the surface of cubes are doubtless built by (100) structures.19–21 The improved activity of the modified and non-modified (100) surface of Pd nanocubes has been previously reported for the electrooxidation of organic molecules.14,24–26 To rationalize the reaction pathway, we need to revisit the suggested paths found in the literature.

-1

-1

Figure 4. Paths of glycerol electrooxidation reaction on Pd nanocubes in 0.1 mol L KOH + 0.2 mol L glycerol for 9 h electrolysis at 0.87 V at 25 °C.

To the best of our knowledge, the high production of tartronate (or tartronic acid) on a Pd surface has never been reported. The first identification of glycerol products after long-period electrolysis was reported by Roquet et al.27 These authors followed 25 h electrolysis in an alkaline medium of 0.01 mol L-1 glycerol at 0.79 V (vs. RHE) and found ions of glyceraldehyde, glycolic acid, formic acid, tartronic acid and glyceric acid on a Pt electrode, without major selectivity.27 Using Pd as a catalyst, Zalineeva et al. found that nanocubes are more active than nanooctahedrons for the GEOR in an alkaline medium.12 The authors suggested that glycerol electrooxidizes to carbonate, passing through glyceraldehyde, glycerate and tatronate/mesoxalate/hydroxypyruvate.12 Wang et al.11 and Holade et al.8 suggested the production of ion tartronate going through a glycerate intermediate. However, Wang et al. proposed a direct conversion of glycerol to glycerate (or glyceric acid if protonated, as indicated in11), while Holade et. al.8 suggested that glyceraldehyde is converted to glycerate ions before tartronate production. Most of the literature reports that selectivity is achieved by modifying a base metal to build bimetallic surfaces.6,16,28 However, in a pioneering investigation, Garcia et al. showed that dehydrogenated glycerol binds the (100) surface by only one primary carbon atom to form glycer-

aldehyde.7 Therefore, considering the products found here and the previous reports regarding the GEOR, we suggest the pathway of the GEOR on Pd nanocubes in an alkaline medium as shown in Figure 4. In the two main starting reactions, glycerol electrooxidizes to dihydroxyacetone and/or glyceraldehyde. The extended (100) surface induces glyceraldehyde,7 which undergoes a reaction to glycerate. Glycerate might be converted to glycolate and other small molecule products as formate (not found here in detectable concentrations), which might lead to carbonate. The experimental conditions used here led glycerate to tartronate, as shown in Figure 4. It is worth noting that the stationary electrochemical configuration used here only allows the conversion of the glycerol molecules close to the electrode surface. Further investigations are needed to scale up this configuration to a hydrodynamic controlled masstransport in attempt to convert glycerol from bulk solution. Therefore, we showed a new method based on FDM to easily 3D print an electrolyzer. This electrochemical device showed reliable half-cell measurements, containing a few mobile parts, besides being re-usable and inexpensive compared to regular H-type glassy-made systems. We also found a new way of rapidly synthesizing Pd



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nanocubes, which were used in long-period electrolysis. For the first time, we showed how the preferential orientation at the nanoscale influences the selectivity of the GEOR. By using Pd nanocubes in the 3D-printed electrolyzer, we found remarkable selectivity to produce tartronate. Our results enable a wide range of possibilities to develop 3D-printed electrolyzers. We prove that it is possible to work with controlled nanostructured surface to find important fundamental and applied information by using inexpensive printed electrochemical devices. Moreover, the catalysis reported here might help future works concerning the developing of new nanocatalysts. Conclusions A simple electrolyzer was prototyped and 3D-printed using fused deposition modeling with polylactic acid filament with a very low material cost and building time. The optimized configuration provided an inexpensive three part electrolyzer in less than 4 h. The H-typed 3Dprinted electrolyzer containing a proton exchange membrane displays no detectable charge transfer resistance and can be used for half-cell measurements if necessary. To test the electrolyzer, we synthesized 15–40 nm Pd nanocubes through a new protocol based on ascorbic acid method assisted by cetilmetilamonio. Glassy carbon modified Pd nanocubes was used as working electrode in alkaline medium in long-time electrolysis at 0.87 V. The glycerol electrooxidation was selectively led to 99% tartronate, at the region close to the electrode surface. This remarkable selectivity was rationalized as a consequence of the extent of (100) surface of the nanocubes, which induces the glyceraldehyde path to be converted to tartronate. The main product electrosynthesized here, tatronic acid, an acyclic carboxylic acid, is usually synthesized through several steps from a starting material, namely, reduction, oxidation, precipitation, filtration and redissolution, where each step is critical. This multiple steps increase its end consumer price, which is more expensive than the price of glycerol. Here, we synthesize tartronate (tartronic acid if protonated) from glycerol with high efficiency in only one step in a clean process, without the need for additional reactants. This work demonstrates the combination of electrode potential, electrolyte medium and mainly the controlled nanocatalyst surface of a single metal to provide selective glycerol electrooxidation. Moreover, we showed that it is possible to use inexpensive 3D-printed devices to investigate new materials for electrosynthesis.

[email protected] [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

ACKNOWLEDGMENT The authors acknowledge financial assistance from CNPq (Grant # 454516/2014-2), FUNDECT (Grants # 026/2015 and #099/2016), CAPES and FINEP. The authors also thank Gabriella L. Caneppele for helping with the synthesis of the nanocubes.

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ASSOCIATED CONTENT Supporting Information contains details of the 3D-printed electrolyzer and representative chromatograms.

AUTHOR INFORMATION Corresponding Author *Cauê Alves Martins

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A low-cost, reusable and easily manufactured 3-D printed electrolyzer is introduced. Pd nanocubes were used to electrooxidize glycerol into tartronate.


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3D-Printed Electrolyzer for the Conversion of Glycerol into Tartronate

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