energies Article A Paper-Based Microfluidic Fuel Cell Using Soft Drinks as a Renewable Energy Source Jaime Hernández Rivera 1,2, David Ortega Díaz 3 , Diana María Amaya Cruz 4 , Juvenal Rodríguez-Reséndiz 5,* , Juan Manuel Olivares Ramírez 2,* , Andrés Dector 6,* , Diana Dector 1 , Rosario Galindo 7 and Hilda Esperanza Esparza Ponce 1 1 Centro de Investigación en Materiales Avanzados, Chihuahua 31136, Mexico; [email protected] (J.H.R.); [email protected] (D.D.); [email protected] (H.E.E.P.) 2 Renewable Energy Department, Universidad Tecnológica de San Juan del Río, Querétaro 76800, Mexico 3 Instituto Tecnológico de San Juan del Río, Querétaro 76800, Mexico; [email protected] 4 Facultad de Ingeniería, Universidad Autónoma de Querétaro, Campus Amealco, Querétaro 76010, Mexico; [email protected] 5 Facultad de Ingeniería, Universidad Autónoma de Querétaro, Querétaro 76010, Mexico 6 Renewable Energy Department, CONACYT–Universidad Tecnológica de San Juan del Río, Querétaro 76800, Mexico 7 CONACYT–División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Guanajuato 36050, Mexico; [email protected] * Correspondence: [email protected] (J.R.-R.); [email protected] (J.M.O.R.); [email protected] (A.D.); Tel.: +52-427-122-4463 (J.M.O.R.) Received: 9 April 2020; Accepted: 11 May 2020; Published: 13 May 2020 Abstract: The research aims were to construct an air-breathing paper-based microfluidic fuel cell (paper-based mFC) and to evaluated it with different soft drinks to provide energy for their prospective use in portable devices as an emergency power source. First, in a half-cell configuration, cyclic voltammetry showed that glucose, maltose, and fructose had specific oxidation zones in the presence of platinum-ruthenium on carbon (PtRu/C) when they were individual. Still, when they were mixed, glucose was observed to be oxidized to a greater extent than fructose and maltose. After, when a paper-based mFC was constructed, PtRu/C and platinum on carbon (Pt/C) were used as anode and cathode, the performance of this mFC was mostly influenced by the concentration of glucose present in each soft drink, obtaining maximum power densities at room temperature of 0.061, 0.063, 0.060, and 0.073 mW cm−2 for Coca Cola R , Pepsi R , Dr. Pepper R , and 7up R , respectively. Interestingly, when the soft drinks were cooled, the performance was increased up to 85%. Furthermore, a four-cell stack mFC was constructed to demonstrate its usefulness as a possible power supply, obtaining a power density of 0.4 mW cm−2, using Coca Cola R as fuel and air as oxidant. Together, the results of the present study indicate an alternative application of an mFC using soft drinks as a backup source of energy in emergencies. Keywords: fuel cell application; microfluidic fuel cell; power supply; soft drinks 1. Introduction Actually, the performance of low-consumption electronic devices such as wireless sensors, digital clocks, and small medical devices, among others, is limited, due to the use a rechargeable lithium-ion battery [1]. In this way, the intensive research to obtain small energy sources as a possible application for low-consumption electronic devices has been directed to the use of micro fuel cells [2–6]. Energies 2020, 13, 2443; doi:10.3390/en13102443 www.mdpi.com/journal/energies Energies 2020, 13, 2443 2 of 13 The paper based microfluidic fuel cells (paper based mFC) have been proposed as an external power source for low-consumption electronic devices [7–9]. The paper-based mFCs have the following advantages: • The paper used in the paper-based mFCs construction is a very low-cost material, and consequently, the fuel cells are disposable. • Due to the movement of reactants through capillary action, the use of external pumps is unnecessary [7]. • The ion transport is carried out through the electrolyte contained in the reactants, and as a consequence, the use of a proton or anion exchange membrane is unnecessary [10]. • These paper-based mFCs may use different liquid organic compounds as fuel, being reported to date ethanol [11], methanol [7,11], formate[12], urea [13,14], and glucose [8,15]. Research into paper-based mFCs using glucose as fuel has experienced a growing interest in the past few years and showing potential in electricity generation for portable devices [8,15,16]. The main advantages of using glucose had been non-toxicity and handling since it had been obtained from reactive grade glucose to body fluids such as human blood [9,17]. The above assumes that the use of other economic sources of glucose that does not require a previous treatment would constitute a significant achievement. In this context, soft drinks are an excellent alternative, since they are a cheap and highly available source; in addition, they contain other saccharides such as fructose, galactose, sucrose, lactose, and maltose, which could be used as fuel in a paper-based mFC. Some relevant works evaluating paper-based mFCs under soft drinks are listed in the following lines: The first research has evaluated different kinds of soft drinks (iced red tea, vegetable juice, fruit juice, and aerated water) in a miniature biofuel cell using glucose dehydrogenase and bilirubin oxidase as anode and cathode, respectively [18]. The mFCs in the best performance recollected 140 mW cm−2, and 0.51 V of power density and open circuit potential, using glucose dehydrogenase as the biocatalyst on single-walled carbon nanohorn. The present investigation collocated the perspective of using the biofuel cell as a portable power source. Other researchers used soft drinks such as Nutri-Express, Coca Cola, and Minute Maid grape juice, as fuels in a miniature origami biofuel cell employed glucose dehydrogenase as bio-anode, obtaining power in the order of microwatts [19]. The system was described as a new approach for effective green energy systems. In a third work [20], was reported a 3-cell stack paper-based biofuel cell powered by Gatorade R + NAD+1 mM biofuel solutions at pH 7.3 (physiological pH). The anode (4.5 cm2) and the cathode (2.5 cm2) employed in this 3-cell stack were constructed employed Glucose Dehydrogenase (GDH) and Bilirubin oxidase (Box), respectively. The performance obtained through the stack system was from 1.8 V and 0.18 mWmg−1 GDH (0.216 mW cm−2, calculated in this work from 3 mg GDH reported by the authors and cathode area). Finally, in more recent research a miniature self-pumping paper-based enzymatic biofuel cell was constructed using glucose oxidase and laccase as anode and cathode, respectively, and soft drinks as fuels. The authors reported that power density obtained was attributed to the glucose contained in the soft drinks, which was fresh watermelon juice > 7up > Mountain Dew > Pepsi (14.5, 13.5, 12, and 6.15 µWcm−2)[21]. The above reports have used enzymatic electrodes for the oxidation of saccharides present in the soft drinks. As another alternative, the use of inorganic catalysts that oxidize saccharides in ideal and real conditions has been directed at the use of gold-based materials in many jobs [17,22–25]. In this sense, another material that has shown activity in the electro-oxidation of saccharides in fuel cells but has not had much application in real conditions has been the PtRu. This work has proposed a paper-based mFCs that use a catalyst anode such as PtRu/C, which is used due to a minimal poisoning rate and lower cost than Pt/C [26]. In addition, to assess their behavior, the individual Energies 2020, 13, 2443 3 of 13 voltammetry assessment of glucose, fructose, and maltose is reported, as well as the simulated Coca Cola R , Pepsi R , Dr. Pepper R , and 7up R , evaluation, this means the mixture of these saccharides according to their reported content. Finally, the real soft drinks were employed, for the first time, like fuel in a non-enzymatic paper-based mFC; and the effect of two temperatures (4 ◦C, as consumption temperature and 25 ◦C, as room temperature) on the performance of the fuel cell was evaluated. The novelty of this work is the use of inorganic catalyst material in paper-based mFC for power generation using soft drinks as fuels. This approach has not been widely studied in this type of material. In addition, a new concept of emergency energy through this paper-based mFC was created. Furthermore, due to the easy access to sugary soft drinks that we have today, these paper microfluidic fuel cells could actually be applied as possible backup energy sources for low-consumption electronic devices. This article is outlined as follows: Section2 depicts methodology, begins with the catalytic material used; Pt/C and Pt-Ru/C. After describing the electrochemical characterization to saccharides oxidation, in this section, describe so the design of the paper microfluidic fuel cell. Section3 displays the results, and discussion about the characterization of Ru distribution on carbon paper, a cyclic voltammogram of PtRu/C in the presence of fructose, glucose, and maltose, after in presence on the 7up R , Coca Cola R , Dr. Pepper R , and Pepsi R . Finally presents the polarization and power density curve. 2. Development of mFC 2.1. Materials All chemicals were reagent grade and were used without further modification. PtRu/C (20 wt.%) and Pt/C (30%) were from E-TEK. Nafion 5%, glucose, fructose, and maltose acquired from Sigma-Aldrich. Carbon paper Toray R was obtained from Technoquip Co Inc TGPH-120. Soft drinks were purchased from a local retail market. Isopropyl alcohol and KOH acquired from J.T. Baker. Whatman filter paper, grade Fusion 5 was used for paper-based mFC construction.