Investigation of alternative materials as bifunctional catalysts for electrochemical applications 120 D O I: 1 0. 15 82 6/ ch im te ch .2 01 9. 6. 4. 01 Vayenas M., Vaitsis C., Sourkouni G., Pandis P. K., Argirusis C. Chimica Techno Acta. 2019. Vol. 6, No. 4. P. 120–129. ISSN 2409–5613 Vayenas M.a, Vaitsis C.a, Sourkouni G.b, Pandis P. K.a, Argirusis C.a,* a School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St, 15780 Zografou, Athens, Greece. b Clausthal University of Technology, Clausthaler Zentrum für Materialtechnik (CZM), 9 Leibnizstr., 38678 Clausthal-Zellerfeld, Germany *e-mail: amca@chemeng.ntua.gr Investigation of alternative materials as bifunctional catalysts for electrochemical applications A lab-scale custom made Zinc-Air battery cell was manufactured and tested with a variety of cathode catalysts. MnO2 has been examined both as an Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) catalyst, with more promising results as an ORR catalyst. MnO2 as well as a combination of MnO2 and MWCNTs (MOCN-10) has been examined in this work. In addition, two dif- ferent Metal Organic Frameworks (MOFs), specifically HKUST-1 and MOF-74, based on Cu and Ni, respectively, were investigated as an alternative and novel cathode catalyst directly on the battery cell. A power output of 20 mW · cm–2 was achieved by using MOCN-10, along with stability in prolonged discharge cycling at 5 mA · cm–2. Furthermore, MOF-loaded battery demonstrated astonishing performance in pulse cycling for more than 120 hours. Moreover, no dendrite formation was observed during long term pulse cycling. Keywords: Rechargeable Zinc-air Battery; ORR; OER; Polarization; Cycling; MOFs Received: 25.10.2019. Accepted: 18.11.2019. Published: 30.12.2019. © Vayenas M., Vaitsis C., Sourkouni G., Pandis P. K., Argirusis C., 2019 Introduction Advancement in  energy stor- age systems have recently been on high demand due to  the  vast development of  portable electronic devices, as  well as to the forthcoming outburst of electri- cal vehicles. Lithium-ion batteries have attracted a lot of attention and have been extensively developed [1, 2]. However, high cost of raw materials used in these batteries and public safety concerns have led to ex- plore alternative storage systems. Zinc-air batteries (ZABs) are seen as a promising candidate for next genera- tion energy storage devices. The needed ox- ygen is not stored within the ZAB and can be supplied from ambient air; therefore, they offer a high theoretical energy density of 1353 Wh/kg [3]. Zinc itself is a low-cost anode material and is also available in suffi- cient amounts in nature. During discharge, the zinc — air battery functions as a power generator, while electrochemical coupling of the zinc metal to the air electrode is oc- 121 curring in the presence of an alkaline me- dia. Metal zinc is oxidized at the negative electrode, producing zinc cations, and the liberated electrons leave the zinc elec- trode and travel through an external load to the air electrode. At the same time, oxy- gen from the surrounding air diffuses into the porous air electrode and is ready to be reduced to hydroxide ions via the oxygen reduction reaction (ORR). ORR is occur- ring at  the  three-phase (gas oxygen, liq- uid electrolyte, and solid electrocatalysts) boundary as a reaction site. Generated ani- ons OH– then migrate from the reaction site to the zinc electrode, forming zincate ions (Zn(OH)4 2−), which at supersaturated concentrations, further decompose into insoluble zinc oxide (ZnO) [2]. Upon charging, these reactions may be reversed. The zinc — air battery is capable of storing electric energy through the oxy- gen-evolution reaction (OER), which is oc- curring at the positive electrode-electrolyte interface. At the same time, zinc is depos- ited at the positive electrode surface. How- ever, the redox reactions of oxygen during the  charging and discharging cycles are kinetically hindered; thus, it is  common to use catalysts to accelerate the process [4]. Manganese oxide (MnOx) has been previously reported as  an  efficient, cost- effective catalyst for ORR and OER cataly- sis [5, 6]. It is considered as a potential can- didate for replacing traditional catalysts, such as noble metals (Pt, Pd, Ru, Ir), as well as their oxides and alloys. Metal-Organic Frameworks (MOFs) are also examined as alternative electro- catalysts [7, 8]. MOFs are hybrid crystalline materials, which consist of a metal-based centers and organic ligands. Their chemical versatility can lead to various morpholo- gies (cubes, spheres, rods, etc.) depend- ing on the target application. Their facile synthesis, high surface areas and open metal sites have made them highly desir- able in the field of catalysis, gas storage/ad- sorption and gas separation, while recently they have been shown to be very promising in biomedical and electrochemical appli- cations [9]. MOFs have been tradition- ally prepared via a solvothermal synthesis at high temperatures for a prolonged time (hours or even days). In  order to  avoid these harsh conditions, alternative meth- ods have emerged, such as  microwaves, electrochemistry, mechanochemistry and sonochemistry [10]. MOFs described in this work have been prepared by a high intensity sonicator. In this work, a custom-made, lab-scale zinc-air battery has been constructed in order to investigate two different types of materials as catalysts. More specifically, MnO2-containing materials and MOF structures were used as  ORR and OER electrocatalysts. Experimental Section Materials and Equipment All reagents were used as received with- out further purification. Potassium hydrox- ide (KOH, purity ≥ 98%), Copper chloride dihydrate (CuCl2·2H2O, purity ≥ 99%), 2,5-Dihydroxyterephthalic acid (dhtp, pu- rity 98%), Ethanol (purity ≥ 98%), Acetone (purity ≥ 99%), Hydrochloric acid (HCl, concentration 37%) were purchased from Sigma-Aldrich. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, purity ≥ 97%) was pur- chased from Honeywell. 1,3,5-Benzenetri- carboxylic acid (Trimesic Acid, purity 98%) was purchased from Alfa-Aesar. Dimethyl- formamide (DMF, purity > 99.5%), Metha- nol (purity > 99.8%) were purchased from 122 Chem-Lab. Manganese oxide (MnO2) was purchased from TOSOH Hellas. Nafion solution (5%) was purchased from Quin- tech. COOH Functionalized Multi-walled Carbon Nano-tubes (MWCNTs) were pur- chased from Hongwunematerial. Widely commercially available Zn sheets (purity >  99.9%) and Celgard-3401 membranes were also used without any modification, while carbon cloth was pretreated as de- scribed below. The sonicator used for MOF synthesis and carbon cloth pre-treatment was Vibra Cell VCX 750  W (20  KHz). The potentiostat used for electrochemical measurements was BioLogic SP-150. Catalyst Preparation Synthesis of Ni-MOF-74 The solution was prepared by dissolving a mixture of Ni(NO3)2 · 6H2O (3.14 mmol) and dhtp (0.949  mmol) in  a  15:1:1 mix- ture of DMF (75 mL), ethanol (5 mL) and deionized water (5  mL), while stirring. The mixture was transferred to a 3-neck round-bottom flask, and synthesis was carried out in  a  nitrogen environment via a  continuous flow under ultrasound irradiation for 1  hour at  a  power out- put of  65%. After letting the  vessel cool down to room temperature, the solid was recovered by  centrifugation and washed once with DMF, and then with methanol. The solid was kept immersed in methanol for 4 days; the solvent was refreshed once a day. Finally, Ni-MOF-74 was activated in vacuo for 12 hours at 100 °C. Synthesis of HKUST-1 The solution was prepared by dissolving a mixture of CuCl2 · 2H2O (3.75 mmol) and trimesic acid (3.75 mmol) in a 2:1:2 mix- ture of DMF (30 mL), ethanol (15 mL) and deionized water (30  mL), while stirring. The mixture was transferred to a 3-neck round-bottom flask and synthesis was carried out in  a  nitrogen environment via a  continuous flow under ultrasound irradiation for 1 hour at a power output of 65%. After letting the vessel cool down to room temperature, the solid was recov- ered by  centrifugation and washed once with DMF, then with water and then etha- nol within 4 days; the solid was kept im- mersed in water for 2 days and ethanol for 2 more days; the solvents were refreshed once a day. Finally, HKUST-1 was activated in vacuo for 12 hours at 100 °C. Air electrode preparation MOF catalysts were used as prepared and described above, MnO2 was used as purchased, while MnO2-MWCNTs com- bination was prepared by  mixing 18  mg MnO2 and 2 mg MWCNTs. In order for as-prepared catalysts to be loaded on cath- ode, 20 mg of each catalyst were dispersed in 1 ml ethanol and underwent sonication in an ultrasound bath for 10 min. 150 μl of Nafion solution (5%) were also added and the  final mixture was sonicated for 10 min. Finally, the dispersion was poured onto a pretreated carbon cloth (1.5 cm × 1.5 cm). Carbon cloth pre-treatment The catalyst support used for our bat- tery cells was sonicated in three different solutions before being loaded by the cata- lyst [11]. Carbon cloth (1.5 cm × 1.5 cm) was successively immersed into 20% HCl, acetone and deionized water. Each soni- cation lasted 15 min with a power output of 33%, while the carbon cloth was fully immersed. Results and discussion Different types of materials were pre- pared as  described above in  order to  be examined as Zn-air battery electrocatalysts. MnO2 and MOCN-10 (mixture of MnO2 123 and MWCNTs) were electrochemically compared with each other. Furthermore, MOF structures were also examined as electrocatalysts. As-prepared catalysts were directly loaded on carbon cloth in or- der to catalytically activate the three-phase boundaries. A  home-made rechargeable Zn-air battery was built (see Fig. 1) to  evaluate the  performance of  as-prepared electro- catalysts of  the  air electrode. In  Fig. 2a, a  stable Open Circuit Voltage (OCV) is  exhibited regardless using MnO2 or MnO2-CNTs catalysts. OCV for MnO2 catalyst cell is equal to 1.48 V, while 1.44 V OCV is monitored for MOCN-10 catalyst. Fig. 2b shows the  galvanodynamic polarization curves of  batteries loaded with MnO2 and MOCN-10 catalysts on the anode. The exhibited potential is moni- tored for increasing current density, while charging (positive current) or discharging (negative current) the cell with a scan rate of 0.5 mA · sec–1. Corresponding power curves for cells using different air catalysts are depicted in  Fig. 2c. MOCN-10 catalyst shows the highest power pick at 20 mW/cm2. En- ergy efficiency curve for the same battery was also calculated by dividing the values of charge polarization by the values of dis- charge polarization. Comparing to the cor- responding power curve, it is apparent that the energy efficiency is reduced to 29.1% while power peak is observed. That means that the battery’s energy efficiency has been reduced by 70.9% (see Fig. 2d). To  evaluate the  c ycling stabil- ity of  the  battery, a  low current density of 2 mA · cm–2 was first applied for pulsed and non-pulsed cycles. During non-pulsed cycling, 2 mA · cm–2 were applied for 2 h of charging and 4 mA · cm–2 were applied for 1 h of discharging while 13 cycles were obtained without any obvious deteriora- tion. Therefore, the  capacity output was equal for charge and discharge. The differ- ent currents applied are due to the better ORR activity of various manganese oxide forms compared to  the  OER [5]. With the utilization of MnO2 as a catalyst, cell potential during the  ORR process was 1.10 V, while the respective potential dur- ing OER process was 2.00 V (see Fig. 3 a, b). Surprisingly, both the ORR and OER potentials of the MOCN-10 battery were 1.20 V and 2.10 V, respectively (see Fig. 3 c, d). However, for both catalysts the voltage gap was equal. A  charge-discharge volt- age gap (Δη) of less than 900 mV has been Fig. 1. Photograph of zinc-air battery 124 observed while cycling regardless of the air catalyst. Fig. 4 shows a discharge profile plot for the cell where MOCN-10 acted as the cath- ode catalyst. After increasing the current density on 20 mA/cm2 discharge and suc- cessively reducing the  applied current density until 0.5 mA/cm2, only 49 mV re- duction has been observed, corresponding to 96.2% potential retention. Furthermore, MOF crystals were exam- ined as ORR and OER catalysts directly on Zn-air batteries. Due to their high porosity and large surface areas, MOFs can contrib- ute to the reaction area between the three- phase sites on cathode. Utilization of HKUST-1 as air catalyst shows increased battery performance, as the battery is able to remarkably perform for over 350 consecutive cycles on pulse charging of 20 min without any apparent deterioration (see Fig. 5). Battery cell us- ing Ni-MOF-74 has also been evaluated through galvanostatic cycling (see Fig. 6). However, the observed Δη between charg- ing and discharging is greater than 1 V. In Fig. 7, Zn sheets that have been used during cycling can be seen. The formation a b c d Fig. 2. Electrochemical performance of Zinc-air batteries using MnO2 (blue) and MnO2-CNTs (black) as air catalysts: a — OCV plots; b — Polarization curves and c — the corresponding power density; d — Energy efficiency curve for MOCN-10 cell (red) regarding its power density curve (black) vs current density 125 of ZnO structures that increase the polari- zation and lead to degradation can easily be observed [12, 13]. Although, much less ZnO formed during pulsed cycling than during non-pulsed cycling, even if the total time of pulsed cycling was longer. Conclusions In this work, a lab scale rechargeable zinc-air battery has been developed, and different material structures have been in- vestigated as ORR and OER catalysts. MnO2 has already been reviewed in the literature as an excellent ORR catalyst, but further testing needs to be done for the OER cata- lytic process. Hereby, we prepared MnO2 and MWCNTs mixture — loaded cathode and compare the respective battery’s per- formance with the MnO2 loaded battery. Relative to MnO2 — loaded cells, battery cells with MOCN-10 mixture show higher power output. In particular, we achieved 20  mW · cm–1 utilizing the  MnO2-CNTs mixture as air catalyst for ORR and OER. Additionally, MOF structures were evaluated as  catalysts. Cathode was pre- a b c d Fig. 3. a — Discharge and charge cycling at 2 mA · cm–2 for cell with MnO2 as electrocatalyst; b — Discharge (at 4 mA · cm–2) and charge (at 2 mA · cm–2) cycling for cell with MnO2 as electrocatalyst; c — Discharge (at 4 mA · cm–2) and charge (at 2 mA · cm–2) cycling for cell with MOCN-10 as electrocatalyst; d — Discharge and charge at 5 mA · cm–2 of MOCN-10 loaded cell 126 pared in the same way as with MnO2 cata- lysts and Nafion acted as binder between catalyst and Gas Diffusion Layer (GDL). Galvanodynamic polarization curves in the MOF-loaded cell, with current step of  0.5  mA · s–1, showed performance com- parative to  that of  the  MnO2 catalysts. In particular, battery cell with HKUST-1 on the cathode, performed at 7 mW · cm–1 and surpassed the  3.5  mW · cm–1 power output of  Ni-MOF-74 loaded cell. What is  more, great cycling stability has been achieved during short cycles (pulsed cy- cling). While utilizing HKUST-1 cell, more than 360 successive cycles (120 hrs) were Fig. 4. Discharge profile of the battery cell with MnO2-CNTs air catalyst for 0.5, 2, 5, 10 and 20 mA · cm — 2 and vice versa a b c Fig. 5. Electrochemical performance of Zinc-air batteries using HKUST-1 as air catalyst: a — Discharge polarization curve and the corresponding power curve; b — Charge polarization curve; c — Discharge and charge cycling at 2 mA · cm–2 127 performed at 2 mA · cm–1 without deterio- ration of the cell, indicating great stabil- ity during both ORR and OER operation. 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