Nonlimiting Hydrogen Electrosorption Properties of Asymmetric Palladium Nanoparticle-Modified Porous Carbon Electrodes Xiaoyin Xiao, a Susan M. Brozik, a Gabriel A. MontaÇo, b Cody M. Washburn, a David R. Wheeler, a D. Bruce Burckel,* a Ronen Polsky* a a Sandia National Laboratories, Department of Biosensors & Nanomaterials, PO Box 5800, Albuquerque, NM 87185, USA b Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 *e-mail: rpolsky@sandia.gov; dbburck@sandia.gov Received: August 26, 2011; & Accepted: October 27, 2011 Abstract Hydrogen electrosorption is studied on 3D porous carbon electrodes fabricated by interferometric lithography. Mixed acetonitrile/water solutions provided a route to electrodeposit Pd nanoparticles homogeneously throughout the structures. These monoliths exhibit hydrogen electrosorption profiles typical for limited volume electrodes and consequently are promising candidates for micro-hydrogen based fuel cells. In contrast, aqueous solutions resulted in asymmetric Pd depositions due to the hydrophobic nature of the carbon electrode and its unique 3D porous structure. These electrodes exhibited unusual nonlimiting hydrogen electrosorption profiles. Keywords: DOI: 10.1002/elan.201100471 1 Introduction Among the possible alternatives for fossil fuel based energy systems, hydrogen fuel remains attractive because of its great abundance and because it is considered a green technology. The storage and transportation of hy- drogen however still remains a great challenge [1]. The two main approaches for hydrogen storage are (1) physi- cal storage such as compression or liquefaction of molec- ular hydrogen, and (2) various sorption methods such as adsorption into porous materials, and chemisorption which relies on materials that can form alloys and com- pounds with hydrogen [2]. Pressurized hydrogen tanks and cryogenic liquefied hydrogen are effective, resulting in storage volumes three times greater than gasoline, but the low volumetric density of hydrogen requires high pressures or low temperatures which make these respec- tive techniques cost prohibitive [3]. Chemisorption onto metals such as magnesium, lithium, and aluminum, and dissolution into gases such as ammonia, can result in large gravimetric volumes of hydrogen stored but require energy-inefficient endothermic processes for release and are generally irreversible. [4] Solid state storage of hydro- gen is often desirable because of high energy efficiencies, low heats of adsorption, fast adsorption/desorption kinet- ics, and good reversibility. [5] For this approach porous materials have become the materials of choice due to their enhanced adsorption properties towards gaseous molecules and the ability to tune the pore sizes for func- tionality [6]. Among this class of materials aluminosili- cates and metal organic frameworks are widely studied as hydrogen storage materials because of their high porosity, low density, good crystallinity, and high surface area. Nanostructured carbon materials can also be created with high specific areas that make them good candidates for hydrogen storage [7]. Activated carbons in particular have been reported as good gas adsorbents that include hierarchical carbon structures with nano/microporosity. Nanopores are used for enhanced adsorption while mi- cropores can help promote quick transportation of hydro- gen to the bulk of the material [8]. However, pure carbon still has a drawback in that high amounts of hydrogen ad- sorption are not favored at room temperature and cryo- genic cooling is required. In order to overcome this, metal catalysts such as Pd are often used to cause catalyt- ic breaking of hydrogen–hydrogen bonds [9]. The result- ing hydrogen atoms can then diffuse into the palladium nanoparticle itself or, through a spillover mechanism, mi- grate to the carbon support and adsorb onto carbon active sites [10]. Recently, we reported on novel hierarchical porous carbon structures fabricated out of pyrolyzed photoresist films by interferometric lithography [11]. The highly or- dered carbon electrodes consist of five interconnected layers of hexagonal micropores with nanometer connect- ing spokes. These structures exhibit high mass transport profiles for certain redox analytes and hemispherical dif- fusion profiles over macroscopic volumetric areas [12]. We have also demonstrated that the unique properties of the pyrolyzed photoresist material can be used for the deposition of ultra small and highly dispersed Au, Pd, and Pt nanoparticles with narrow size distributions [11– SPECIAL ISSUE Electroanalysis 2012, 24, No. 1, 153 – 157  2012 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 153 Full Paper