A Novel Fluorine Production Process in a Proton （一种新型氟生产过程在一个质子）.pdf

All rights reservpotential to produce fluorine as an oxidation product. Results are pressure was determined to be approximately 2.5 mol/kg (KF•xHF; xcompared in this paper from cells employing these anhydrous sol-vents and cells with water as the membrane solvent, which are adirect analogy to the anhydrous HCl recycle process. To the best ofour knowledge, this is the first time that fluorine production has beenattempted in a polymer-membrane-separated reactor.ExperimentalThe PEM reactor consisted of flow-through gas-diffusion elec-trodes separated by a Nafion 115 membrane. A schematic of the reac-tor is displayed in Fig. 1. The effective area of the reactors used in thiswork was 1 cm2unless otherwise noted. Monel and platinum currentcollector plates were machined with serpentine flow fields for deliv-ery of reactants and removal of products. Gas-diffusion electrodes(GDE) were either carbon cloth (E-TEK, Inc.) or Monel mesh(Cleveland Wire Cloth & Mfg. Co.). Catalyst layers were either coat-ed on the GDE in ELAT form or laminated directly on the membranesurface in the form of a membrane electrode assembly (MEA).7= 20). The resulting liquid was drained into a reservoir which facili-tated gravitational feed to the reactor. Anhydrous HF gas was period-ically bubbled through the reservoir to maintain the melt concentra-tion at ambient conditions.A scrubber sequence was installed for separation and neutraliza-tion of the product streams. A process flow diagram is shown in Fig.2. NaF•xHF (Engelhard Co., x = 1 as received) scrubbers absorbedHF. The scrubbers were activated by heating to 200°C with a contin-uous nitrogen purge for 24 h. To quantify fluorine content, the anodeproduct stream bubbled through a 5.0 M KOH scrubber followed by a0.2 M KI scrubber with 0.1 M KH2PO4buffer. An alumina scrubberwas used to remove fluorine by conversion to oxygen when the ana-lytical scrubbers were bypassed. Product samples were periodicallyinjected into a gas chromatograph with a mass selective detector(Hewlett Packard 6890 series) for analysis of CF4and other fluorinat-ed gases. Any OF2formed as a side product by electrolysis of adven-Electrochemical and Solid-State LetterS1099-0062(99)06-042-3 CCC: $7.00 ©A Novel Fluorine Production Process in a PrRobert Lowrey,a,*,zMarc Doyle,aDepartment of Chemical Engineering, School of Engineering, UniverbDuPont Central Research and Development, Experimental Station,Fluorine production was demonstrated in a Nafion 115 membrane-separated electrochemical reactorHF from the gas-phase did not occur. Instead, the bifluoride ion (HFfor sustained fluorine evolution. Transport of bifluoride ion to catalyst-coated ELAwas hindered by electrostatic exclusion from the perfluorinated ionomer solution catalyst binder and membrane. flooded with KF•xHF molten salt to circumvent transport restrictions by proGalvanostatic operation at 0.5 kA/m2yielded fluorine at 7.2 V with 62% current ef© 1999 The Electrochemical Society. S1099-0062(99)06-042-3. Manuscript submitted June 7, 1999; revised manuscript received June 21,A proton exchange membrane (PEM) reactor for electrolytic pro-duction of fluorine was evaluated as a potential alternative to theelectrolyzers used for commercial fluorine production. This processis similar in concept to the anhydrous HCl recycle process which uti-lizes a PEM reactor to electrolyze anhydrous HCl forming chlorineand hydrogen gas.1,2Fluorine is produced commercially by bulkelectrolysis of KF•xHF molten salt (1 < x < 4).3,4The lifetime of acommercial fluorine cell is limited by corrosion of the carbon anodeand other components such as the electrode-current collector con-nections. The anticipated advantages of the PEM reactor are contin-uous operation, higher current densities per unit cell area, lower oper-ating voltages due to the reduction of the interelectrode gap distance,and better product gas separations leading to improved safety. HF electrolysis presents new challenges compared with HCl elec-trolysis. The thermodynamic potential for fluorine evolution isapproximately 1.8 V more positive than the potential for chlorineevolution. Materials of construction for an HF PEM reactor must bestable to voltages higher than those used in an HCl PEM reactor. Inaqueous media such as the water-swollen Nafion membrane used inthe HCl process, there is an inherent oxidative stability limit due tothe oxygen evolution reaction. This limit may preclude an aqueousprocess for fluorine production as oxidation of water to form oxygenoccurs well below the thermodynamic fluorine evolution potential(1.23 V H2O/O2vs. H+/H2, 2.83 V HF/F2vs. H+/H2).5When water is removed from a perfluorinated ionomer mem-brane, the ionic conductivity of the membrane decreases to less than10-7S/cm. Fortunately, perfluorinated ionomer membranes conductions when swollen with a variety of nonaqueous solvents.6The chal-lenge is to find such a solvent which does not oxidize at high voltageor which generates fluorine as an oxi。