Nanotechnology makes biomass electrolysis more energy efficient than water electrolysis

ARTICLE Received 15 Feb 2014 | Accepted 5 May 2014 | Published 3 Jun 2014 DOI: 10.1038/ncomms5036 Nanotechnology makes biomass electrolysis more energy efficient than water electrolysis Y.X. Chen1,2, A. Lavacchi1, H.A. Miller1, M. Bevilacqua1, J. Filippi1, M. Innocenti1,3, A. Marchionni1, W. Oberhauser1, L. Wang1,2 & F. Vizza1 The energetic convenience of electrolytic water splitting is limited by thermodynamics. Consequently, significant levels of hydrogen production can only be obtained with an electrical energy consumption exceeding 45 kWh kg  1H2. Electrochemical reforming allows the overcoming of such thermodynamic limitations by replacing oxygen evolution with the oxidation of biomass-derived alcohols. Here we show that the use of an original anode material consisting of palladium nanoparticles deposited on to a three-dimensional architecture of titania nanotubes allows electrical energy savings up to 26.5 kWh kg  1H2 as compared with proton electrolyte membrane water electrolysis. A net energy analysis shows that for bio-ethanol with energy return of the invested energy larger than 5.1 (for example, cellulose), the electrochemical reforming energy balance is advantageous over proton electrolyte membrane water electrolysis. 1 ICCOM-CNR, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy. 2 Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy. 3 Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy. Correspondence and requests for materials should be addressed to A.L. (email: ) or to F.V. (email: ). NATURE COMMUNICATIONS | 5:4036 | DOI: 10.1038/ncomms5036 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5036 T he exploitation of hydrogen in fuel cells has been widely advocated as the ideal solution for an energetically and environmentally sustainable future for transportation1,2. An essential part of such a solution will be the use of renewable energy resources (photovoltaics, wind, wave power and so on) in conjunction with water electrolysis to provide a ‘zero emission’ source of hydrogen3. Water electrolysis is a well-established commercial technology; nevertheless, its practical application for the mass production of hydrogen is still limited as it suffers from several major obstacles. The costly electrical energy consumption is certainly one of the major factors hampering the diffusion of electrolysis. This has been highlighted by the US Department of Energy (DOE). Indeed, in 2011, the DOE has pointed out that the electrical energy input to the electrolyser stack should drop from 45 to 43 kWh kg  1H2 by 2020 (ref. 4), while keeping constant the hydrogen production rate. Proton electrolyte membrane (PEM) electrolysers are at present the best-performing technology for the electrolytic production of hydrogen. This is because of the smaller ohmic (iR) losses as compared with the more traditional alkaline electrolysers5,6. Low iR losses allow PEM electrolysers to operate at current densities larger than 1 A cm  2, thus producing useful quantities of hydrogen. Unfortunately, an inherent limitation to the diffusion of PEM electrolysers arises from the fact these devices employ acidic PEMs that require the use of expensive and rare platinum, ruthenium and iridium catalysts. Both alkaline and PEM electrolysers exploit water splitting, evolving hydrogen at the cathode and oxygen at the anode. The standard reaction potential for this process is 1.23 V, meaning that water splitting is a strongly uphill reaction. In practice, to get current densities in the range of 1 A cm  2 the cell potential usually ranges between 1.6 and 2 V (ref. 6). Considering 1.8 V as a reasonable average, we conclude that 68.3% of the energy input is consumed by thermodynamics, while kinetic factors account for only 31.7%. Replacing anodic oxygen evolution with the oxidation of much more readily oxidizable species leads to a significant reduction of the potential required to produce hydrogen. Following this strategy, compounds such as ammonia7,8, methanol9, ethanol10–13, glycerol14,15 and urea16 have been recently tested. These electrolytic processes that lead to the concomitant generation of chemicals at the anode and hydrogen at the cathode are often indicated as ‘electrochemical reforming’. Among these compounds, ethanol is certainly promising due to the possibility of production from the fermentation of biomass or steam reforming of cellulosic materials with reasonably low energy cost. The use of such compounds allows electrolysis at potentials o1 V, leading to electrical power savings as compared with conventional electrolytic water splitting. Figure 1 shows schematically the Power supply e– e– Cathode Anode H2 O2 e– Cathode H2 Power supply OH– e– Anode CH3COONa + H H2O H 2O PEM Pt Ir C2H5OH + H2O + NaOH AEM Pt/C Pd/TNTA-web Figure 1 | Electrochemical reforming and water electrolysis comparison. Schematic of (a) PEM water electrolysis and (b) alkaline electrolyte membrane electrochemical reforming. 2 difference between a state-of-the-art PEM electrolyser and an alcohol electrolyser. Equations (1–3) describe the anode, cathode and overall electrochemical-reforming reactions, respectively. CH3 CH2 OH þ 5OH  ! CH3 COO  þ 4e  þ 4H2 O ð1Þ 4H2 O þ 4e  ! 2H2 þ 4OH  ð2Þ CH3 CH2 OH þ OH  ! CH3 COO  þ 2H2 ð3Þ While promising, these approaches suffer the limitation of delivering current densities well below 1 A cm  2, and as such are unpractical for technological exploitation. Here we propose to overcome such limitations using threedimensional (3D) nanostructured TiO2 nanotube arrays (TNTAweb) as a support for Pd nanoparticles (NPs) (Pd/TNTA-web). The combination of Pd and TiO2 nanotubes on flat surfaces has been recently shown to be effective for alcohol electrooxidation17. The novelty of the approach consists in the fact that the TNTAs are prepared by the anodization of a Ti fibre web support rather than a titanium foil17. The system therefore does not rely on high surface area carbon supports, as previously reported for TiO2 nanotube electrodes18, to obtain high Pd dispersion. In addition, no binder and/or ionomer are required to assemble the electrodes, allowing good mechanical stability and high utilization of the active phase. Here we demonstrate that electrochemical reforming can occur at current densities comparable to that of state-of-the-art PEM water electrolysis. Furthermore, we report for the first time a net energy analysis of hydrogen production by bio-ethanol electrochemical reforming. Particularly, we define the energy cost break-even point, expressed in terms of the energy return of the invested energy (EROI)19 of bio-ethanol production, at which electrochemical reforming becomes more energy efficient than water electrolysis. Resu (...truncated)