Energy Materials are an integral part of our daily lives, with applications ranging from consumer electronics (battery life, size and power) to the drive to reduce our dependence on fossil fuels with alternatives such as solar- and fuel-cells to power vehicles and generators.
The properties of the neutron are ideally matched to investigate structural and dynamical processes, relating form with function. For example, the high penetration depth of the neutron can be used to probe complex systems and assemblies under working conditions. In many cases, the presence of light atoms such as oxygen, lithium and hydrogen require neutron-based techniques to elucidate structure and to determine mechanistic pathways. Fundamental research is also important to probe structure and dynamics in matter to understand and tailor physical properties so that next-generation materials can be designed to replace those used today.
Several representative examples are given below to highlight the current role of neutron scattering techniques.
Fuel cell research covers fundamental studies on new electrode materials, electrolytes and membranes for solid-oxide fuel-cells (SOFC), proton exchange membrane fuel-cells (PEMFC) and proton ceramic fuel-cells (PCFC) up to monitoring the in operando performance and lifetime of membrane-electrode assemblies (MEA) and operating fuel-cell stacks. Neutron scattering contributions include diffraction studies to locate hydrogen  or vacancies in candidate materials,  SANS to follow the kinetics of water uptake in PEMFC electrolyte materials,  tomography to show the accumulation of water in gas-flow channels in an operating fuel PEMFC (see Figure),  QENS to probe molecular motions in ionomer membranes  and NSE to investigate proton dynamics in proton conducting perovskites.  The impact of neutron scattering in proton conducting oxides for fuel-cell research is also the topic of a recent review. 
Storage materials are a broad class of materials that include hydrides, hydrates, clathrates, frameworks, metal-organic frameworks (MOF) and nanotubes, with an emphasis on reversible hydrogen storage, encapsulation of other small molecules, such as alkanes, and trapping of harmful gases, such as carbon dioxide. Storing hydrogen safely, efficiently and reversibly is one of the main technological challenges before hydrogen can be adopted as a widespread energy carrier. In-situ neutron diffraction has been widely used to monitor hydrogenation/dehydrogenation reactions and site preferences. [8-10] SANS and USANS are powerful techniques to study the effects of small molecules confined in gas reservoirs, such as shale gases in coal, with implications for CO2 sequestration. [11-12] Unique information can be obtained from INS [13-15], QENS  and NVS [17-18] to quantify the amount of hydrogen in a sample and show if it is present as H2, by measuring the characteristic rotational spectrum of the hydrogen molecule. Understanding how clathrates form, coexist, transform and decompose [19-20] under particular conditions of temperature and pressure are crucial to their uses as fuel sources and to investigate the origins of pipeline blocking due to the formation of clathrate plugs. Also of interest is CO2 sequestration and understanding the possible implications of climate change on uncontrolled release of methane and CO2 from undersea clathrate beds.
Battery materials containing lithium are extensively used in applications, from consumer electronics to communications devices, and are increasingly replacing technology based on metal hydrides. Batteries rely on ion exchange and/or ionic conductivity properties of lithium and hydrogen and their properties are ideal for investigations using neutron scattering methods. Diffraction methods are widely used to study new materials and, increasingly, to probe battery materials in operando. [21-22] The field is of such high technological importance that a dedicated instrument (SPICA) was funded for structural investigations on battery materials, currently under commissioning, at the J-PARC spallation neutron source in Japan. Radiography also offers the opportunity to visualise working batteries under charge and discharge conditions. [23-24]
Thermoelectrics have the potential to revolutionise waste-heat recovery and the refrigeration industry. The challenge is to combine low thermal conductivity with a high electrical conductivity – the so-called ‘phonon glass-electron crystal’ concept. In addition to the many diffraction studies, INS is a key technique used to study the interplay of the rattler modes of guest molecules in the host materials with the acoustic phonon branches [25-26] that are responsible for thermal conductivity. INS data and computational calculations reveal that local soft and strongly anharmonic modes of antimony dimers dominate and drive the dynamic response, leading to the required low thermal conductivity.
Other materials, such as solar cells, based on thin film ceramics or organic semiconductors offer the possibility of generating low-cost energy from sunlight. Diffraction is widely used in this field to characterise the bulk structural properties of materials and to study materials with enhanced efficiency.  Neutron reflectometry offers the possibility to study interfaces in thin film blends and to monitor time dependent composition.  Neutron scattering investigations may also have an indirect impact. INS has recently been used to study quantum oscillations of nitrogen atoms in uranium nitride,  which could have implications in the design of generation IV nuclear reactors using uranium nitride as a fuel.
Energy materials require the full breadth of neutron scattering techniques to understand and optimise their properties. The combination of advanced instrumentation with flexible data acquisition modes, sample environment, data visualisation and data analysis offered by the ESS will provide unparalleled opportunities and insights for researchers in this field. Studies of new materials and processes necessarily begin with small sample volumes, accentuating the need to improve the performance and efficiency of neutron scattering instruments.
 I. Ahmed, C.S. Knee, M. Karlsson, S.G. Eriksson, P.F. Henry, A. Matic, D. Engberg, L. Börjesson. J. Alloys Compounds 2008, 450, 103-110.
 A. Magraso, J. M. Polfus, C. Frontera, J. Canales-Vasquez, L-E. Kalland, C.H. Hervoches, S. Erdal, R. Hancke, T. Norby, R. Haugsrud. J. Mater. Chem. 2012, 22(5), 1762-1764.
 G. Gebel, S. Lyonnard, H. Mendil-Jakani, A. Morin. J. Phys. Cond. Matt. 2011, 23(23), 234107.
 M. Strobl, I. Manke, N. Kardjilov, A. Hilger, M. Dawson, J. Banhart. J. Phys. D: Appl. Phys. 2009, 42, 243001.
 J-C. Perrin, S. Lyonnard, F. Volino. J. Phys. Chem. C 2007, 111, 3393.
 M.Karlsson, D. Engberg, M.E. Björketun, A. Matic, G.Wahnström, P.G.Sundell, P. Berastegui, I. Ahmed, P. Falus, B. Farago, L. Börjesson, S. Eriksson. Chem Mater. 2010, 22(3), 740-742.
 M. Karlsson. Dalton Trans. 2012, (in press).
 V.K. Peterson, Y. Liu, C.M. Brown, C.J. Kepert. J. Am. Chem. Soc. 2006, 128(49), 15578-9.
 T. Yildirim, M.R. Hartman. Phys. Rev. Lett. 2005, 95(21), 215504.
 Y. Yan, I. Telepeni, S.H. yang, X. Lin, W. Kockelmann, A. Dailly, A.J. Blake, W. Lewis, G.S. Walker, D.R. Allan, S.A. Barnett, N.R. Champness, M. Schroder. J. Am. Chem. Soc. 2010, 132(12), 4092.
 C. R. Clarkson, M. Freeman, L. He, M. Agamalian, Y. B. Melnichenko, M. Mastalerz, R. M. Bustin, A. P. Radli?A ski, T. P. Blach. Fuel 2012, 95(0), 371–385.
 Y. B. Melnichenko, L. He, R. Sakurovs, A. L. Kholodenko, T. Blach, M. Mastalerz, A. P. Radli?A ski, G. Cheng, D. F. R. Mildner. Fuel 2012, 91(1), 200–208.
 P.A. Georgiev, D.K. Ross, A. De Monte, U. Montaretto-Marullo, R.A.H. Edwards, A.J. Ramirez-Cuesta, M.A. Adams, D. Colognesi, Carbon 2005, 43(5), 895-906.
 A.J. Ramirez-Cuesta, P.C.H. Mitchell. Catalysis today 2007, 120(3-4), 368-373.
 C.M. Brown, Y. Liu, T. Yildirim, V.K. Peterson, C.J. Kepert. Nanotechnology 2009, 20(20), 204025.
 F. Salles, D.I. Kolokolov, H. Jobic, G. Maurin, P.L. Llewellyn, T. Devic, C. Serre, G. Ferey. J. Phys. Chem. C 2009, 113(18), 7802-7812.
 F. M. Mulder, B. Assfour, J. Huot, T. J. Dingemans, M. Wagemaker, A. J. Ramirez-Cuesta. J. Phys. Chem C 2010, 114(23), 10648-10655.
 L. Ulivi, M. Celli, A. Giannasi, A.J. Ramirez-Cuesta, M. Zoppi. J. Phys Cond. Matt. 2008, 20(10), 104242.
 M.M. Murshed, W.F. Kuhs. J. Phys. Chem. B 2009, 113(15), 5172-5180.
 D.K. Staykova, W.F. Kuhs, A.N. Salamatin, T. Hansen. J. Phys. Chem. B 2003, 107(37), 10299-10311.
 N. Sharma, V.K. Peterson, M.M. Elcombe, M. Avdeev, A.J. Studer, N. Blagojevic, R. Jusoff, N. Kamrulzaman. J. Power Sources 2010, 195, 8258-8266.
 S-I. Nishimura, G. Kobayashi, K. Ohoyama, R. Kanno, M. Yashima, A. Yamada. Nature Mat. 2008, 7, 707-711.
 N. Kardjilov, A. Hilger, I. Manke, M. Strobl, W. Treimer, J. Banhart. Nucl. Inst. Meth. A 2005, 542, 16-21.
 A. Senyshyn, M.J. Muehlbauer, K. Nikolowski, T. Pirling, H. Ehrenberg. J. Power Sources 2012, 203, 126-129.
 M. Christensen, A. B. Abrahansen, N.B. Chrsitensen, F. Juranyi, N. H. Andersen, K. Lefmann, J. Andreasson, C. R. H. Bahl, B. B. Iversen. Nature Mat. 2008. 7, 811-815.
 W. Schweika, R.P. Hermann, M. Prager, J. Persson, V. Keppens. Phys. Rev. Lett. 2007, 99, 125501.
 S. Schorr. Solar Energy Materials \& Solar Cells 2011, 95, 1482-1488.
 A. J. Parnell, A. D. F. Dunbar, A. J. Pearson, P. A. Staniec, A. J. C. Dennison, H. Hamamatsu, M. W. A. Skoda, D. G. Lidzey, R. A. L. Jones. Adv. Mater. 2010, 22, 2444-2447.
 A.A. Aczel, G.E. Granroth, G.J. MacDougall, W.J.L. Buyers, D.L. Abernathy, G.D. Samolyuk, G.M. Stocks, S.E. Nagler (submitted).