Solution-phase decoupled water electrolysis in a flow cell with a simple size exclusion membrane separator Obeten Mbang Eze,a,b Zeliha Ertekin,a Paula L. Lalaguna,a Malcolm Kadodwala,a and Mark D. Symes*a aSchool of Chemistry, University of Glasgow, Glasgow, G12 8QQ, United Kingdom bDepartment of Chemistry, University of Cross River State, Calabar, Cross River State, Nigeria *Email: mark.symes@glasgow.ac.uk About this dataset Files included in the dataset “Raw data for dialysis membrane paper” Software required to open the files in the dataset Microsoft Excel workbook Further instructions Data is organised into tabs relating to the data presented in the various figures and tables in the published paper. Abstract Perfluorosulfonic acid membranes are currently the state-of-the-art in terms of electrolytes for proton exchange membrane electrolysers for the production of green hydrogen using renewably-generated power. This is because such materials are chemically robust, have low resistance, and importantly because they greatly reduce the mixing of the hydrogen and oxygen products of electrolysis. However, these materials are also acknowledged to have a number of drawbacks for large-scale use, including high cost, supply shortages and the fact that perfluoro compounds are “forever chemicals” that persist in the environment and are difficult to recycle. In decoupled electrolysis, the hydrogen and oxygen products can be generated in entirely different spaces at entirely different times, and so (at least in theory), gas-impermeable perfluorosulfonic acid membranes are not required in order to prevent gas mixing. However, the use of alternative membranes in solution-phase decoupled electrolysis has received very little attention to date. Herein, we show that a (gas-permeable) simple cellulose-based membrane can be employed in a solution-phase decoupled electrolysis flow system across a range of current densities (25– 500 mA/cm2) for 5 h, without evidence for any significant gas mixing. Although optimisation of the membranes for more extended operation is required, this work serves to show that cheap and simple size exclusion membranes are viable for safe water electrolysis in a decoupled system, potentially allowing the replacement of perfluorosulfonic acid membranes in a number of electrolysis applications. Experimental procedure Electrochemical Flow Cell Configuration The detailed assembly of the electrochemical flow cells used in this study is provided in Fig. 1 and further illustrated in the Supporting Information (Fig. S1), which includes all the components used in the electrochemical cell setup. In brief, the oxygen- and hydrogen-generating flow cells featured a geometric electrode area of 13.7 cm2 (3.7 × 3.7 cm). Silicotungstic acid (H4SiW12O40·xH2O, Merck, CAS: 12027-43-9) was used as a redox mediator at a concentration of 0.5 M in ultrapure water (resistivity: 15.2 M?·cm). The system consisted of two separate electrochemical cells: one for oxygen generation and one for hydrogen generation, operated simultaneously and connected via two peristaltic pumps (MasterFlex) to circulate the redox mediator. The oxygen-generating cell was controlled using a BioLogic SP-150 potentiostat coupled to a BioLogic VMP-3B 20A/20 V booster, while the hydrogen-generating cell was operated using an Admiral SquidstatPlus potentiostat. Figure 1. Schematic of the flow cell system designed, constructed, and used in this study. Oxygen-Generating Cell Assembly: The anode consisted of a 3 mm-thick titanium serpentine flow plate with channel dimensions of 1 mm × 1 mm, and a 0.3 mm-thick titanium fibre felt (Fuel Cell Store) coated with titanium nanoparticles and loaded with 2.0 mg/cm2 of IrO2 catalyst (Fuel Cell Store). The preparation of the anode electrode was described previously [13]. The cathode comprised an identical titanium flow plate and a carbon cloth with a microporous layer, without additional catalyst. Both electrodes were sealed with 0.127 mm-thick Teflon gaskets (Fuel Cell Store). A regenerated cellulose dialysis membrane (SpectraPor® dialysis membrane, Repligen) with a 3.5 kDa molecular weight cut-off was used to separate the anode and cathode compartments. Hydrogen-Generating Cell Assembly: The hydrogen-generating cell followed the same design as the oxygen-generating cell, except for three key modifications: (i) the separator was a Nafion 117 membrane (Ion Power), (ii) the cathode electrode consisted of carbon cloth coated with 0.5 mg·cm?2 of Pt/C catalyst (Fuel Cell Store) and (iii) the anode consisted of Ti fiber felt without catalyst. All other components and sealing elements were identical to those used in the oxygen-generating cell. Bulk electrolysis Oxygen production in the first cell was achieved by applying a fixed current with the BioLogic SP-150. Before each experiment, argon was bubbled through the mediator solutions for 45 minutes to eliminate residual oxygen in the system. Meanwhile, hydrogen generation in the second cell was powered by an Admiral SquidstatPlus potentiostat, which supplied the current. Equal currents were maintained across both cells in order to place the system into steady-state operation. Gas chromatography analysis for oxygen and hydrogen To analyse the gas products produced during bulk electrolysis, 250 ?L gas samples were collected from the headspaces of the anolyte and catholyte reservoirs using a gas-tight syringe. Gas chromatography (Agilent 8860), equipped with a thermal conductivity detector (TCD), was used for analysis. The system was configured with a Porapak Q 80/100 Ultimetal column and a MoleSieve 13X 60/80 Ultimetal column. The oven temperature was initially held at 50?°C for 4 minutes, followed by a ramp of 10?°C·min?1 to a final temperature of 120?°C. The total analysis time was 11 minutes. The gas chromatograph was calibrated using certified hydrogen gas standards (5%, 3%, 2%, and 1% H2 in argon), obtained from CK Gas Products Limited, UK. The resulting calibration curves are shown in Fig. S2, along with additional details on calculating the decoupling efficiency (%) of the mediator reduction process. Gas crossover measurements were performed at various current densities in both the cathode and anode loops. Gas samples were collected using a gas-tight syringe, and all sampling and analysis was repeated three times. Hydrogen peak areas obtained from gas chromatography analyses were converted to the percentage of hydrogen (%H2 in the headspace) using the calibration curve. Error bars represent the standard deviation of the three replicate measurements. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy was conducted using a BioLogic SP-150 potentiostat paired with a BioLogic VMP-3B 20A/20V booster to determine the cell resistance. Electrochemical impedance spectroscopy data were obtained in galvanostatic mode using a BioLogic SP-150 potentiostat. The obtained electrochemical impedance spectroscopy data were fitted to an equivalent electrical circuit using BioLogic EC-Lab software, selecting the model (see Fig. 2) that yielded the lowest chi-squared (?²) value and was consistent with literature reports [14-18]. The flow cell was constructed and tested using parameters presented in Table 1, which utilises a least-squares optimisation routine to determine the flow cell resistance. The frequency range was from 10 kHz to 50 mHz with 6 measurement points per decade. Each 6-point per decade scan takes approximately 11 minutes and 20 seconds. The amplitude value is set to 10% of the applied current density. The flow cell voltage was monitored during scans to verify that no noticeable drift occurred. This determination was qualitative since the signal slightly affected the cell output potential, most noticeably at low frequencies. Figure 2. The equivalent circuit model used in the electrochemical impedance spectroscopy analysis. Circuit components are defined in the main text. Table 1. The experimental parameters used in the electrochemical impedance measurements. Applied Current Density (mA/cm2) 500 Amplitude (mA/cm2) 50 Initial Frequency (kHz) 10 Final Frequency (mHz) 50 Points/ Decade 6 In this model, R1 represents the ohmic resistance of the flow cell components. The charge transfer resistances at the cathode and anode are denoted by R2 and R3, respectively. Additionally, C2 and C3 correspond to the constant phase elements associated with the cathodic and anodic processes. W represents a Warburg element, which models diffusion. It has a phase of 45° and its magnitude decreases with the square root of frequency. A finite-length Warburg adjusts for limited diffusion distance [14]. Membrane Characterisation The morphological structure of the membrane after electrolysis was characterised using scanning electron microscopy. The scanning diffraction angle 2? ranged from 10° to 90° at a rate of 5 minutes per data point, with a scanning rate of 1° min?1. A scanning electron microscopy (Tescan Clara) equipped with Energy-Dispersive X-ray Spectroscopy (Oxford Instruments Ultim Max) analysis was used to probe surface morphology. Atomic force microscopy (AFM) measurements were performed using a Bruker Dimension Icon Atomic Force Microscope System with ScanAsyst and PeakForce tapping mode using a silicon tip (ScanAsyst-Air-HPI). Atomic force microscopy was performed on the dialysis membrane (1 cm × 1 cm) sample to detect the membrane’s mean roughness parameter (Ra) both before and after electrolysis. To provide insights into the molecular structure and composition of the membrane, as well as identify the functional groups and chemical bonds, Fourier Transform Infrared (FTIR) spectroscopy was performed using a Nicolet™ Summit™ FTIR Spectrometer. Thermogravimetric analyses were performed using a TA Instruments thermogravimetric analyser (Discovery TGA-5500), and a DSC–TGA instrument (SDT Q600) was used to evaluate the membrane’s thermal stability and degradation behaviour before and after electrolysis. The analysis was conducted at a heating rate of 100 °C/min up to 600 °C in an argon flow of 100 mL/min. A four-point probe, Lucas Pro4 4000 sheet and bulk resistivity measurement system, combined with a Keithley 2450 source meter, was used to measure and determine membrane conductivity. Water Uptake and Ion Exchange Capacity Measurement The water uptake was determined by measuring the weight differences between the thoroughly dried and the fully hydrated membranes. The membranes were dried in a desiccator for 24 hours and then soaked in deionised water for another 24 hours. Afterwards, the membranes were removed, wiped with tissue paper, and quickly weighed using a microbalance. The water uptake (Wup) of the membrane was calculated using equation 1: W_up= W_w/W_d ×100%, [1] where W_w and W_d are the weights of the wet and dry membranes, respectively. Ion exchange capacity allows the determination of the accessible number of functional groups within a membrane, which directly or indirectly determines the thermal stability, water uptake, and conductivity of the membrane [19]. A regenerated cellulose dialysis membrane (SpectraPor® RC Membranes 3) comprises natural cellulose derived from cotton linters. The polymer structure of the membrane is based on cellulose fibres, which consist of repeating glucose units connected by ?(1?4) glycosidic linkages [19, 20] . These glucose units, in turn, contain hydroxyl (?OH) groups [19]. For the ion exchange capacity measurements, the regenerated cellulose dialysis membrane was immersed in a 0.01 M HCl solution for 24 h at room temperature before titration. 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