Catalyst-Selective Direct Liquid Fuel Cells without the Need for an Ion-Exchange Membrane to Separate the Anode Fuel and the Cathode Oxidant

Abstract

Among the several types of direct liquid fuel cells (DLFCs) under development with various liquid fuels, the direct methanol fuel cell (DMFC) is in the forefront. However, this technology is currently in a difficult situation. The challenges of the relatively well-developed DMFCs with the proton-exchange membrane (PEM) include (1) the high-cost of Nafion membrane, (2) the sluggish kinetics of electrode reactions (rely on the high-loading Pt-based catalysts), and (3) the crossover of the methanol fuel through the PEM.1 , 2 According to the above issues, two approaches have recently been proposed and attempted, which can individually overcome the challenges of (1) – (3). Operation of the DMFCs under alkaline condition is able to enhance the kinetics of the electrode reactions.3 Operation of the DMFCs under a laminar-flow principle can avoid the use of the expensive PEMs and without the fuel crossover concerns. However, each of these two approaches brings new unexpected problems. Operation of the DMFCs under alkaline conditions needs an OH--exchange membrane which is currently not quite available. The laminar-flow membraneless DMFCs can only be built to a miniature scale, which limits their practical applications.4 Herein we present a membraneless alkaline DLFC approach developed under a catalyst-selective philosophy. By employing a non-platinum, highly selective cathode catalyst (MnNiCoO4/N-MWCNT, MnNiCoO4 nano-particles on a nitrogen-doped multi-wall carbon nanotube)5 with favorable activity for the oxygen reduction reaction (ORR) but without any activity for the fuel oxidation reaction (FOR), membraneless-operation of the DLFCs is turned into reality without any concerns about the fuel crossover from the anode to the cathode. The first example of this presentation is an alkaline membraneless DMFC system with methanol as the anode fuel. The second example is an alkaline direct formate fuel cell (DFFC) with aqueous potassium formate solution as fuel. In comparison to the CH3OH-favorite catalyst PtRt/C and the HCOOK-favorite catalyst Pd/C, the MnNiCoO4/N-MWCNT does not show activities for oxidation of either CH3OH or HCOOK. The fuel cell performances of the membraneless DMFC (PtRu/C ǁ CH3OH/KOH ǁ MnNiCoO4/N-MWCNT) and the membraneless DFFC (Pd/C ǁ HCOOK/KOH ǁ MnNiCoO4/N-MWCNT) are superior to those of the traditional membrane-based DMFCs.