Light‐Driven, Membraneless, Hydrogen Peroxide Based Fuel Cells

DOI: 10.1002/aenm.201400424 two-electron reduction of oxygen and the two-electron oxidation of water, and it is far less expensive than conventional oxidizers such as liquid oxygen. [ 8,9 ] In previous work, bilirubin oxidase (BOD) was observed to be perishable, resulting in poor stability. In order to better promote the performance of H 2 O 2 -based fuel cells, PB is used here as an excellent catalyst for H 2 O 2 reduction under acidic conditions because it can avoid the problems of high cost and poor stability that hinder BOD and Pt-based nanomaterials. [ 8,10 ] We fi nd that our H 2 O 2 fuel cell has a stable long-term operation in acidic media, which is much better than that reported previously. [ 10 ] Additionally, due to the utilization of light and chemical energy, a high OCP of 0.595 V and power density of 90 μW cm –2 could be achieved in our system, which are higher than those of state-of-art H 2 O 2 -based fuel cells with the use of low cost non-Pt materials; these were previosly the best ever reported. [ 9,12,13 ] Therefore, such a membraneless onecompartment fuel cell is more promising than two-compartment fuel cells with a membrane for the development of lowcost and miniaturization fuel cells. A general working principle of H 2 O 2 fuel cell in our system is shown in Figure 1 . It includesTNT and PB/ITO as the anode and cathode electrodes, respectively. We used a pure titanium foil to prepare the TNT photoanode through an electrochemical anodic oxidation (Experimental Section, Supporting Information). [ 14 ] The obtained TNTs were further calcined at 500 °C for 3 h to obtain highly crystalline TiO 2 with a mixture of anatase and rutile phases (Supporting Information, Figure S1 and Figure S2). PB was electropolymerized on the indium tin oxide (ITO) electrode in a freshly prepared solution containing 0.1 M KCl, 0.1 M HCl, 2.5 mM K 3 [Fe(CN) 6 ], and 2.5 mM FeCl 3 by applying a constant potential of 0.4 V for 300 s. [ 14,15 ] The successful preparation of the PB fi lm was confi rmed by its obvious color change (Supporting Information, Figure S3). Upon UV illumination, H 2 O 2 was oxidized by photogenerated holes in TNT while photogenerated electrons fl owed through an external circuit to the PB, which catalyzed the reduction of H 2 O 2 . We fi rst investigated the electrocatalytic activity of the as-prepared PB/ITO towards H 2 O 2 reduction by measuring the linear sweep voltammograms (LSVs) in 0.4 M HCl containing 0.4 M H 2 O 2 . As shown in Figure 2 A (traces a,b), there is no catalytic reduction current in the absence of H 2 O 2 , while a large cathodic wave appears in the presence of H 2 O 2 with an onset potential around 0.72 V vs. Ag/AgCl. This indicates that the PB possesses excellent electrocatalytic activity towards H 2 O 2 reduction. It should be noted that the catalytic reduction current towards H 2 O 2 in our system is higher than that of a state-of-art system (shown in Supporting Information Figure S4). [ 7 ] Figure 2 B shows the LSVs of PB/ITO in different concentrations of H 2 O 2 . We found that the corresponding reduction current densities As the awareness of environmental problems and the need for renewable sources of energy increases, the development of environmentally friendly technologies for energy supply, storage, and conversion has attracted increasing attention. [ 1–6 ] Among these technologies, fuel cells as a kind of electrochemical device have been widely studied because they can convert the chemical energy of fuels directly to electrical energy with high effi ciency. In particular, hydrogen peroxide (H 2 O 2 ) fuel cells have received increasing attention because H 2 O 2 as an oxidizer in fuel cells has high energy density and cell potential, and does not require a reduced over-potential relative to the oxygen. [ 7–9 ] Moreover, the entire H 2 O 2 -based fuel cell system can be designed more simply considering H 2 O 2 can be used as both a fuel and an oxidant in a fuel cell engine. Inspired by the above advantages, several new strategies for the preparation of H 2 O 2 -based fuel cells have been developed. Typical examples include the preparation of a two-compartment Nafi on-membrane H 2 O 2 fuel with Ni/C and Pt/C as catalysts at the anode and cathode, [ 8 ] and the construction of a hydrogen peroxide fuel cell, operated under acidic conditions using an iron–phthalocyanine complex as the cathode material. [ 9 ] Despite the great potential, H 2 O 2 -based fuel cells with low-cost non-Pt as the electrode material can only achieve an open-circuit potential (OCP) of 0.5 V with a maximum power density of 10 μW cm –2 , a limiting current density of 210 μA cm –2 , and they are not stable under the long-term operation. [ 9 ] Therefore, the design of a new concept for further enhancing the performance of H 2 O 2 -based fuel cells is a challenging topic. Here, we propose new, light-driven and membraneless, stable H 2 O 2 -based fuel cells (LFC) with TiO 2 nanotubes (TNT) and Prussian Blue (PB) as the photoanode and cathode, respectively, for enhancing their performance. We have recently constructed a new light-driven, membraneless, and mediatorless glucoseair biofuel cells with maximum power density of 47 μW cm –2