Biotemplated Palladium Catalysts Can Be Stabilized on Different Support Materials

Catalytic materials used in many industrial and energy-conversion processes must be durable to maximize their useful lifetimes, especially when expensive precious metal catalysts are utilized. Conventional methods of catalyst formation typically use synthetic stabilizers and reductants to form nanostructures with high surface area and catalytic activity. Synthesized nanomaterials are then mixed with a chemical binder to form a catalyst ink that is applied to a support, often alumina or carbon, for various applications. The stability of the bond between the binder and support is important to keep the catalytic material in contact with the electrode support. Additionally, the durability of the ligand material is an important consideration, as ligands account for the bulk of electrode manufacturing costs, largely because they cannot be captured and recycled like the catalytic metal. Recently, there has been increased interest in developing sustainable, low-cost processes to fabricate nanoparticles and porous nanostructures with high surface areas by using biological materials as supports or biotemplates to replace synthetically formed nanomaterials. An electroactive biofilm of G. sulfurreducens has recently been used as a biotemplate to form a nanoporous catalytic palladium structure that was directly attached to an electrode surface using only materials naturally produced by the biofilm and avoiding the use of synthetic templating materials . The biotemplated palladium structure had a higher catalytic activity than an electrodeposited palladium control electrode or electrodes coated with a palladium/ Nafion catalyst ink. Electroactive bacteria offer a sustainable solution for the formation of a highly active nanoporous structure on an electrode, because the biofilm effectively disperses and reduces the catalytic material directly on the electrode surface without the use of any conventional synthetic binders, which increases the sustainability of the process. However, the stability of biotemplated palladium structures has not previously been tested. Although sustainable methods of catalyst formation are desirable, electrode durability must be similar to or better than that of electrodes produced using synthetic methods. Fuel cells and batteries are the primary applications for catalytically active electrodes. Two of the primary design considerations for electrodes in these systems are thermal and electrochemical stability. Electrode stability of conventional catalysts is often tested by applying a catalyst to an electrode support material before coating with a binder to ensure that the material remains on the surface during testing, or by applying a premixed binder/nanomaterial solution. The most common binders are proton-conducting polymers such as Nafion. More recently, electrically conductive polymers, primarily polyaniline (PANI), have been used as binders because of their ability to conduct electrons and be electropolymerized on an electrode. Pyrolyzed PANI has also been used as a starting material to form catalytic electrodes and corrosion-resistant coatings. Catalytic materials can also be electrodeposited into PANI films on support electrodes or deposited into freshly produced PANI films by soaking the film in a solution containing metal ions. The use of PANI as a conductive intermediate link for other purposes, such as the movement of artificial muscles, highlights its versatility as a binder. Electrochemical (oxidative) stability is one important factor for biotemplated electrodes, but another critical consideration is mechanical stability. Biotemplated catalyst layers do not require a chemical binder for attachment to the electrode surSustainably biotemplated palladium catalysts generated on different carbon-based support materials are examined for durability under electrochemical (oxidative) and mechanical-stress conditions. Biotemplated catalysts on carbon paper under both stresses retain 95% (at 0.6 V) of the initial catalytic activity as opposed to 70% for carbon cloth and 60% for graphite. Graphite electrodes retain 95% of initial catalytic activity under a single stress. Using electrodeposited polyaniline (PANI) and polydimethylsiloxane binder increases the current density after the stress tests by 22%, as opposed to a 30% decrease for Nafion. PANI-coated electrodes retain more activity than carbon-paper electrodes under elevated mechanical (94 versus 70%) or increased oxidative (175 versus 62%) stress. Biotemplated catalytic electrodes may be useful alternatives to synthetically produce catalysts for some electrochemical applications.