The color changes in chemo- and photochromic MoO₃ used in sensors and in organic photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissociated at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a critical challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen reduction of MoO₃ thin films using atomic hydrogen in a water free environment. Hydrogen intercalates into MoO₃ forming H_xMoO₃, which slowly decomposes into MoO₂ +1/2 H₂O as evidenced by the fast reduction of Mo⁶⁺ into Mo⁵⁺ states and slow but simultaneous formation of Mo⁴⁺ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temperature hydrogen reduction of bulk molybdenum oxide. The specific mechanism is a result of the balance between the reduction by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.

The transformation from the fatuous consumption of fossil energy towards a sustainable energy circle is most easily marketable by not changing the underlying energy carrier but generating it from renewable energy. Hydrocarbons can be principally produced from renewable hydrogen and carbon dioxide collected by biomass. However, research is needed to increase the energetic and economic efficiency of the process. We demonstrate the enhancement of CO₂ methanation by sorption enhanced catalysis. The preparation and catalytic activity of sorption catalysts based on Ni particles in zeolites is reported. The functioning of the sorption catalysis is discussed together with the determination of the reaction mechanism, providing implications for new ways in catalysis.

Significance: Humankind must cease CO₂emissions from fossil fuel burning if dangerous climate change is to be avoided. However, liquid carbon-based energy carriers are often without practical alternatives for vital mobility applications. The recycling of atmospheric CO₂into synthetic fuels, using renewable energy, offers an energy concept with no net CO₂emission. We propose to implement, on a large scale, marine-based artificial islands, on which solar or wind energy powers the production of hydrogen and the extraction of CO₂from seawater and where these gases are catalytically reacted to yield liquid methanol fuel. The present work proposes specifications for such facilities and highlights essential challenges in physics, chemistry, and engineering which must be met to realize this ambitious proposal.

NaAlH₄ is the archetypical complex hydride for hydrogen storage. The extraordinary effect of dopants on the sorption kinetics triggered the investigation of this empirical finding. In this paper, a short review of the state of the art is given. To gain further understanding of the mechanisms involved we label the interacting species during the sorption process. This was experimentally realized by hydrogen–deuterium exchange measurements during the decomposition of NaAlH₄ followed by thermogravimetry, Raman spectroscopy and mass spectrometry. By these experiments we are able to obtain specific information on the diffusing species and formation of intermediates. The activation energy of tracer diffusion in NaAlH₄ is found to be 0.28 eV. The results are evidence for a vacancy-mediated desorption process of NaAlH₄.

Biofilms causing medical conditions or interfering with technical applications can prove undesirably resistant to silver nanoparticle (AgNP)-based antimicrobial treatment, whereas beneficial biofilms may be adversely affected by the released silver nanoparticles. Isolated biofilm matrices can induce reduction of silver ions and stabilization of the formed nanosilver, thus altering the exposure conditions. We thus study the reduction of silver nitrate solution in model experiments under chemically defined conditions as well as in stream biofilms. Formed silver nanoparticles are characterized by state-of-the art methods. We find that isolated biopolymer fractions of biofilm organic matrix are capable of reducing ionic Ag, whereas other isolated fractions are not, meaning that biopolymer fractions contain both reducing agent and nucleation seed sites. In all of the investigated systems, we find that silver nanoparticle–biopolymer interface is dominated by carboxylate functional groups. This suggests that the mechanism of nanoparticle formation is of general nature. Moreover, we find that glucose concentration within the biofilm organic matrix correlates strongly with the nanoparticle formation rate. We propose a simple mechanistic explanation based on earlier literature and the experimental findings. The observed generality of the extracellular polymeric substance/AgNP system could be used to improve the understanding of impact of Ag⁺ on aqueous ecosystems, and consequently, to develop biofilm-specific medicines and bio-inspired water decontaminants.

More than 90% of the world’s growing energy demand is satisfied by fossil fuels (BP Statistical Review …, 2015)¹. One consequence of the unrestrained use of this technology is the continuous increase of the CO₂ level of the atmosphere². There are also the challenges associated with the limitations of the corresponding resources (Hubbert, 1956; BP Statistical Review …, 2015). Climate change as a consequence of the growing CO₂ level (see text footnote 2, ESRL Global Monitoring Division, 2015) has been identified as one of the most critical challenges facing mankind and requires immediate action: “The Paris Agreement aims to strengthen the global response to the threat of climate change, ( … ) by low greenhouse gas emissions development, in a manner that does not threaten food production” (United Nations Framework …, 2015). How to reach the corresponding significant reduction of CO₂ emission by 2050 is not defined in this document, but it implies that mankind must transform its energy technology from a fossil to a renewable basis. Numerous studies and publications have indicated that the sun’s energy and its derivatives (wind, water) are by far sufficient to supply world’s energy demand (see, e.g., Smalley, 2005; Züttel et al., 2010); but the large daily and seasonal power variation of renewable energy is an additional complication for a wide spread replacement of fossil energy by renewable energy. This difficulty is drastically demonstrated by the economic crisis of some electricity companies (How to loose half …, 2013), which cannot cope with the growing but strongly varying supply of renewable energy while maintaining the public mandate of a safe and continuous electricity supply. Large scale daily and seasonal energy storage is thus urgently needed to make the transformation toward a renewable energy society possible. Various technologies are under discussion including mechanical storage (fly wheel, pumped hydroelectricity, compressed gas storage, etc.), and electrochemical storage (batteries). Here, we do not want to diminish the potential of these storage possibilities but highlight the one of energy storage by the production of chemical fuels such as hydrogen (Schlapbach, 2009). One argument is the sheer size: seasonal energy storage for northern countries means the storage of roughly 50% of their total annual demand: with an approximated consumption of 10 kW per capita, this corresponds to 158,000 MJ to be stored in 87,600 kg Li-batteries, or by 3285 kg fuel oil (diesel) per person and year. Apart from technical challenges, these numbers illustrate another important issue: many house holders in Europe are indeed used to store several 1000 l of fuel oil for heating. Changing the energy carrier hydrocarbons implies that the corresponding infrastructure supplying more than 50% of the world’s energy has to be replaced (pipelines for oil and gas, fuel stations, end user installations, the automotive and aviation industry, market models, etc.). Alternatively, the existing infrastructure could be used in accordance with the ambitious Paris agreement (United Nations Framework …, 2015), if the hydrocarbons are produced from renewable energy. This is the great challenge (Smalley, 2005), which may be partly overcome by tackling the Hydrogen Grand Challenge as described in the following. Light is the primary energy of the sun, and its conversion to chemical fuel has taken place on a Giga Joule scale by photosynthesis for millions of years. This imposing proof of concept of renewable energy storage may be taken as a guide to a renewable energy technology [“artificial photosynthesis” (Nocera, 2012)], although agriculture as a direct utilization of photosynthesis is not recommended (see statement of the Paris agreement quoted above). To store energy, plants split water releasing molecular oxygen, protons, and electrons at a high (negative) electrochemical potential, at which some algae can produced molecular hydrogen (Rupprecht et al., 2006). Under normal conditions, plants do not produce hydrogen, but “organic hydrides,”³ which are used to eventually produce carbon based chemical energy carriers by reduction of CO₂. Nature has thus solved energy storage problem by splitting water, which may be translated to technical needs as follows (Züttel et al., 2008): • Production of hydrogen: harvesting of solar light, conversion of photon energy into an electro-chemical potential, electro-catalytic conversion of water into oxygen, protons and electrons. • Hydrogen storage: directly as molecular hydrogen, and by chemical reaction with matter, in metal hydrides, by CO₂, carbon–carbon chain-linking, and reduction to hydrocarbons. • Hydrogen end use: fuel cells and combustion. These items needs define the technical part of the hydrogen grand challenge. As the supply of energy to people is a fundamental necessity to people, the hydrogen grand challenge has also ecological, economic, and social boundaries. The hydrogen grand challenge may be divided into the three topical areas hydrogen production, storage, and end use, which are linked via the closed hydrogen cycle, by related scientific fundamentals, and through cross-cutting safety issues, and socio-economic boundary conditions (see Figure 1). The targets of these topical areas are well defined: high conversion efficiencies of hydrogen production (Carmo et al., 2013; Ebbesen et al., 2014) and hydrogen use (Kirubakaran et al., 2009; Fernández et al., 2016), and a high storage density of hydrogen storage (Graves et al., 2011; Lai et al., 2015), all at economically reasonable price (McFarland, 2014). However, specific challenges cannot be drawn from these topics, as the performance of the hydrogen cycle is based on the consecutive functioning of all steps as a whole. As an example, if hydrogen storage is realized by reaction of hydrogen with CO₂ producing hydrocarbons, low temperature fuel cells cannot be used. In this case, preferable end use of the fuel may be in internal and external combustion engines, high temperature fuel cells and heating, which usually have lower efficiencies than pure hydrogen conversion devices. Thus, although the challenge hydrogen storage is technically solved, the overall system performance is less than the one of the pure hydrogen system with hydrogen storage in pressurized cylinders (having other critical issues). The performance of the system is thus equally defined by the atomistic phenomena in the materials as well as at the system level. We consider the corresponding scientific bottlenecks as the specific challenges of the Hydrogen Grand Challenge (see also Figure 1): • Challenge 1: time-resolved surface science analytics under harsh conditions, • Challenge 2: materials science for hydrogen devices, • Challenge 3: systems integration. Figure 1. The three hydrogen grand challenges in a nutshell: although at the beginning, the (atomistic) phenomena as characterized by state-of-the art analytics (challenge 1) are utilized by advance materials (challenge 2), which are part of big systems (challenge 3) to store and distribute the energy of the sun. The hydrogen grand challenge comprises research and development from the nano-scale to the macroscopic world. Pictures are from NASA, and references (Züttel et al., 2008; Bielmann et al., 2011; Vogt et al., 2014). The overall hydrogen grand challenge is a true challenge of mankind (Nocera, 2012). Frontiers in hydrogen would like to stimulate research on scientific bottlenecks, in which breakthroughs will have significant impact on the hydrogen grand challenge as a whole. Harvesting of solar light and the conversion of the energy into an electrochemical potential is a prerequisite of hydrogen production. However, these tasks define the topic “photovoltaics,” and are as such too big to be implemented in this journal. The starting point of hydrogen production is thus electricity as produced by renewable energy, which may also include other sources than light (i.e., wind and hydroelectricity). As exceptions, Frontiers in Energy Research section Hydrogen Storage and Production would like to include photoelectrochemical and photocatalytic hydrogen production. Hydrogen production may be seen as the starting point in a closed circle using hydrogen as a renewable energy carrier. The fundamental processes of renewable hydrogen production by electrolysis are relatively well understood and pilot plants have demonstrated the feasibility on the industrial scale. Nevertheless, today’s hydrogen stems to 95% from fossil sources. Research and development of hydrogen production is needed to make renewable hydrogen cost competitive to fossil hydrogen. This generates demands for fundamental research on new concepts and materials in electrolysis. The corresponding scientific bottlenecks are relatively easy assigned, though not similarly easy overcome. Electrolysis is an electro-chemical process taking place at two spatially separated electrodes. The Faraday efficiency for water electrolysis is near one; this can be different in more complex reactions such as the electro-chemical CO₂ reduction (Kuhl et al., 2012). The voltage–current relation is thus sufficient to characterize the reaction and defines the overall efficiency (see Figure 2). Although additional losses may add up, the electron transfer reactions at the electrode–electrolyte interfaces are the rate-limiting step. Here, the main losses occur, and materials improvements will have highest impact, also because to minimize amount of the often precious materials [e.g., platinum, iridium for PEM electrolysers (Reier et al., 2015)]. This grand challenge of finding Advanced Materials for electro-catalysis may be simplified, if a sound scientific description of the process is available. Figure 2. Large panel: electrode potentials during alkaline water electrolysis. The required voltage to split water at a given current density is the sum of both potentials, and defines the efficiency of the electrolysis process. The potentials result from the electrochemical (half-) reactions at the electrode–electrolyte interface as sketched in the inset. An analysis of the dynamic process at this interface is an analytical challenge. Novel optical spectroscopy (green lines) may give further insights. Data from Miles et al. (1976). Theories for electron-transfer at electrode–electrolyte interfaces are still based on semi-empirical ideas, be it the Butler–Volmer theory (Erdey-Grúz and Volmer, 1930; Nørskov et al., 2004) or the Marcus theory (Marcus, 1993; Gao et al., 2000; Feldberg, 2010). Due to the polarity of the solvent water, the interface (“electrochemical double layer,” EDL) depends on the surface of the electrode, the dynamic properties of the water, impurities, and external parameters such as temperature. The classical description of corresponding electric field is obtained by solving the Poisson equation. However, the resulting potential barrier is of dynamic origin, i.e., the atomistic mechanism how such a barrier is overcome remains elusive. For example, an ion hopping from the neutral metal surface over this barrier into the electrolyte changes the local polarization drastically. The influence of the dynamic behavior is of highest importance for hydrogen production by homogenous solar water splitting due to the transient nature of the process. The grand challenge is to characterize such an interface – surface science under harshest conditions. Surface science has a long tradition in hydrogen based technologies: catalytic processes involving hydrogen such as Haber–Bosch synthesis (Ertl, 1990), steam reforming, methanation (Borgschulte et al., 2013, 2016), hydrogen combustion (Fernández et al., 2016), etc. The surface is the gateway of hydrogen entering (and leaving) bulk material (Schlapbach, 1992), which is relevant in hydrogen storage in metal hydrides (Borgschulte et al., 2008) and in hydrogen selective membranes (Delmelle et al., 2016) in addition to the electro-catalytic hydrogen production as discussed above. A multitude of experimental tools [photoemission (Salmeron and Schlögl, 2008; Delmelle et al., 2015); Infrared spectroscopy (Wandlowski et al., 2004; Yang et al., 2006); mass spectrometry (Johansson et al., 2008); STM (Mitsui et al., 2003; Wandlowski et al., 2004; Schlaup et al., 2015); tip-enhanced Raman spectroscopy (Pettinger et al., 2004); EXAFS (Ferri et al., 2011)] were developed in the past aiming at bridging the materials and pressure gap in surface science (Imbihl et al., 2007). Still, the operando characterization of the (electro-) catalytic active surface sites remains challenging. The investigation of real-time motions of (electro-) chemical reactions on the atomic level have recently become possible for model systems [H₂-desorption from Ru (Frischkorn, 2008), water dynamics (Tielrooij et al., 2010), electron-transfer reactions (Hsu et al., 2014), and solar water splitting (Zhang et al., 2014; Rodenberg et al., 2015)]. The implementation of femto-second time resolution with nanometer spatial resolution is the ultimate goal, and will open new perspectives for functional nanomaterials. Frontiers in Energy Research section Hydrogen Storage and Production would like to stimulate such investigations and their modeling with a focus on the applicability for hydrogen technologies. Solving the analytic challenge as described above is prerequisite to improve the performance of materials for hydrogen production, storage, and end use. With the rate-limiting steps taking place at the nm-scale (e.g., the EDL), materials functionalization is most efficient at the nanometer scale, too. For a long time, heterogeneous catalysis has been a play-ground for nanoscience (Bell, 2003); maximizing the surface to volume ratio is one of the most common strategies in applied catalysis, which asks for novel nanostructuring technologies (nanoparticle synthesis, surface functionalization, etc., see Figure 3). These ideas can be adopted one-to-one to all hydrogen-related applications, in which the gas–solid interaction at the surface plays an important role, i.e., catalytic CO₂ reduction (Borgschulte et al., 2015), catalytic hydrogen combustion (Fernández et al., 2016), catalysts for hydrogen sorption in metal hydrides (Schlapbach, 1992), and electro-catalysis at electrodes for hydrogen production (Carmo et al., 2013), only to mention a few. A particular challenge at electrode processes is the so-called triple-phase boundary. Different to purely catalytic processes, an electro-catalyst facilitates the interaction of electrons, ions, and neutral species. The complex nanostructure of the corresponding electrodes is specific to the type of device: Pt/carbon/Nafion PEM fuel cells (O’Hayre and Prinz, 2004) share only the general idea with Ni/YSZ solid electrolyte SOFC (Wilson et al., 2006), but the materials problems are different. Materials science is needed in both cases to improve properties and to prolong life times. In addition, hydrogen production and reaction with CO₂ may take place simultaneously in the same device (Kuhl et al., 2012), the so-called coelectrolysis of water and CO₂. Ultimately, harvesting of solar light and CO₂ from the atmosphere may be combined with the coelectrolysis of water and CO₂ representing the artificial leaf (Nocera, 2012). This approach may gather synergetic advantages, but it also combines all materials science challenges in one single device. Figure 3. Materials science is only limited by the creativity of the researchers: materials for hydrogen economy may be prepared by mass production methods such a wet chemistry and ball-milling (left side) as well as sophisticated thin film methodologies (right side). Many catalysts (here Ni-particles in a zeolite matrix, left top picture) are prepared by wet chemistry, while ball-milling (left bottom sketch) is a common method for hydrogen storage materials. Thin film preparation such as sputtering (right background picture) is perfectly suited for 2D-devices such as sensors (right picture) and facilitates the investigation of the growth mechanisms and atomistic phenomena (middle bottom sketch), which define the resulting materials properties both for wet chemistry as well as physical deposition. Thin films can stabilize systems suffering from hydrogen-induced mechanical stress (top middle figures: mechanical deformation of bulk Ti after hydrogenation, similar stress is induced in yttrium hydride films, but they are kept stable due to adhesion to the substrate). Membranes (middle photo) and electrodes are commonly prepared by wet chemistry methods as well as thin film deposition techniques. Hydrogen is the lightest of all elements, and thus the diffusion of hydrogen is exceptionally fast in many materials. This enables peculiar phenomena, in which hydrogen is readily exchanged in the bulk of materials, which may be exploited by applications in hydrogen storage (Züttel et al., 2008) and hydrogen selective membranes. However, hydrogen diffusion is not fast in all solid systems; complex hydrides as potential hydrogen storage materials suffer from slow sorption kinetics. To improve kinetics, one tries to shorten the diffusion paths. The latter requires diffusion path lengths on the order of nanometers, which might be possible by nanostructuring the hydrides (de Jongh and Adelhelm, 2010). For some hydrides, this strategy has been found to be successful: borohydrides (Gross et al., 2008) and alanates (Lohstroh et al., 2010) dissolved in nanoscaffolds show improved hydrogen sorption properties. Apart from shortening the diffusion path lengths, nanostructured hydrides possess a higher surface area than pure hydrides, which improves the surface reaction kinetics. There have been reports that nanostructuring can change the stability of the hydride (Lohstroh et al., 2010; Baldi et al., 2014; Griessen et al., 2016) and even its decomposition reaction path (Mueller and Ceder, 2010). Both effects might originate from an increased contribution from the surface energy relative to the volume energy (Mueller and Ceder, 2010; Fichtner, 2011; Griessen et al., 2016). It is telling that most of the research on hydrogen in nanoparticles relies on nanostructuring of palladium (Baldi et al., 2014; Griessen et al., 2016): Pd is a hydrogen absorbing, and chemically inert material, and nanostructuring recipes exist since long (Adams and Chen, 2011). The true challenge for materials science lies in finding and functionalization of non-precious metals with improved properties [e.g., as demonstrated by Lohstroh et al. (2010), see also Figure 3]. Hydrogen storage by adsorption suffers from too weak gas–solid interactions (Hirscher, 2010), much hope is laid on novel high-surface area materials with tailored (internal) surface properties such as metal-organic frameworks (Paik Suh et al., 2012). In addition, materials science will help to improve auxiliary equipment, which makes the hydrogen technology practically possible. These are hydrogen impermeable tubes and tanks (Schlapbach, 1992), and their counterparts: hydrogen permeable materials for membranes (Delmelle et al., 2016); and...