Tetramethylguanidine as an Aqueous Alkaline Electrolyte for Electrochemical Devices with Pt and Pd Daniel Konopka,* ,† Michael Errico, Poyan Bahrami, Michael Johnson, ‡ and Charles C. Hays § Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, United States * S Supporting Information ABSTRACT: 1,1,3,3-Tetramethylguanidine (TMG) is an organic superbase which has recently found use as a primary functional group in alkaline anion exchange membranes (AAEM). In this study we demonstrate the potential of aqueous TMG alkaline electrolyte as a substitute for NaOH (aq) /KOH (aq) with greater chemical similarity to AAEM functional groups. The water redox activities of Pt and Pd electrodes in 0.1 M TMG and 0.1 M KOH are compared, while hydrogen evolution/oxidation reactions (HER/HOR) and the oxygen reduction reaction (ORR) are studied in depth. Aqueous TMG supports comparable redox activity to KOH of equal concentration, but modifies surface processes through potential- dependent adsorptive behaviors. Rotating electrode, capacitance, and slow polarization measurements suggest that TMG and its decomposition species delay and participate in HER. HOR on Pt resembles that of a perfectly flat (111) surface in KOH, while hydrogen absorption on Pd is significantly hindered. ORR activity is dependent upon overpotential on both electrodes. But unlike most acid and alkaline electrolytes, TMG appears to alter the reaction pathway on Pt toward a 1.5e − route. We propose that native or modified analogues of TMG might be optimized for compatibility with various catalysts so as to be utilized in alkaline electrochemical devices in place of NaOH and KOH. ■ INTRODUCTION Organic superbases are an exceptional class of chemical species whose conjugate acid is stable and cannot be deprotonated by HO − in aqueous solutions. 1 1,1,3,3-Tetramethylguanidine (TMG) is one such material and can be utilized as an ionic solution with a pK a ∼ 15.2. 2 The imine of TMG is protonated by water to become tetramethylguanidinium, TMGH. Each of its three nitrogen groups are resonance-stabilized around a sterically centered carbocation. In aqueous environments, this behavior results in mobile [HO − ], which can be utilized for many chemical systems. Consequently, superbases are frequently studied for applications in synthesis processes. 3 Many important processes require an alkaline medium most often provided with aqueous metal hydroxides. Commercial electrodeposition processes employ NaOH aq to stabilize and diffuse metal precursors. 4 Industrial water electrolysis to produce hydrogen is more stable in aqueous NaOH or KOH. 5 Similarly, oxygen evolution in solar driven water splitting proceeds most efficiently in alkaline media. 6 Commercial alkaline fuel cells, such as those employed by NASA over the last several decades have utilized concentrated NaOH or KOH with Pt, Pd, Ag, Au, and Ni electrodes. 7 Enzymatic sensor systems often utilize small amounts of NaOH/KOH to carefully adjust pH in a buffered electrolyte. Metal-cation hydroxides are also important for base-mediated reactions in organic synthesis. Alkaline is often the preferred medium because it affords increased stability for many metal catalysts, as well as thermodynamically favored oxygen catalysis. 8 A significant challenge in advancing electrochemical devices is the development of a highly conductive alkaline anion exchange membrane (AAEM) that is stable above room temperature. Recently, TMG-based functional groups have been synthesized as a polymer ligand to facilitate ion transport in an alkaline membrane for potential application in fuel cells 9,10 (pK a ∼ 13.6). 11 Yet, these as well as more conventional AAEMs often use tetra-alkyl ammonium or similar groups that require heated treatment in, and continued exposure to, strong solutions of NaOH or KOH to become activated and remain ion-conductive. Conventional AAEMs typically achieve 1/3 or less of the performance and stability of analogous proton- exchange membranes such as Nafion. 12 The chemical dissimilarity of membrane- and liquid-base functional groups presents complications. During transient humidity in a device, metal-cations can become poorly dissolved and contribute to degradation of the membrane. 13 Additionally, alkaline systems often require separate scrubbing systems to remove bi/ carbonates which easily form from atmospheric impurities, CO and CO 2 . Otherwise, these species form precipitates that clog gas and liquid transport pathways. The independent behavior of guanidinium-type functional groups at the interface with common catalysts remains unclear because metal-cations are normally present during the operation of an AAEM. Received: July 9, 2014 Revised: September 24, 2014 Published: September 26, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 23768 dx.doi.org/10.1021/jp5068473 | J. Phys. Chem. C 2014, 118, 23768−23776