The Ensembles of Photosynthetic Nanoreactors (EPN) Energy Frontier Research Center is gaining new knowledge that will help bridge the gap in solar-to-hydrogen energy conversion efficiency between what is observed, i.e. <1%, and necessary, i.e. >10%, to substantially mitigate the effects of global climate change. A major focus is to couple correlative microscopic and spectroscopic measurements with numerical simulations. By doing so, we are overturning conventional wisdom in the understanding of the basic science and engineering that dictate several observations in the field of photocatalytic solar water splitting. Notably, charge separation in state-of-the-art Rh-doped SrTiO 3 and BiVO 4 nanoparticles is not driven by electric fields due to band bending, but instead by differences in mobility and/or lifetime of mobile electronic carriers. Moreover, we have observed that dopants in Rh,La-codoped SrTiO 3 nanoparticles sometimes reside in unexpected crystallographic locations. We have also observed extensive incorporation of Pt cocatalysts into the bulk of Rh-doped SrTiO 3 nanoparticles during Pt photodeposition, which coincides with the induction period for observation of H 2 . Also, using atomic layer deposition to deposit ultrathin permeable oxide coatings on Rh-doped SrTiO 3 nanoparticles, we have observed increased selectivity for photocatalytic H 2 evolution. Lastly, using thermodynamically rigorous detailed balance models, which support observations from experiments, we have shown that the solar-to-hydrogen energy conversion efficiency of an ensemble of optically thin light absorbers can exceed that of optically thick materials, providing new motivation for the study and advancement of photocatalytic, over photoelectrochemical, solar water splitting. Collectively, our discoveries support new approaches, and motivate additional research pathways, toward the development of technoeconomically promising artificial photosynthetic devices.

Electrocatalysts encapsulated by nanoscopic overlayers can catalyze redox reactions at the outer surface of the overlayer or at the buried interface between the overlayer and the active catalyst, leading to complex behavior in the presence of two competing electrochemical reactions. This study investigated oxide encapsulated electrocatalysts (OECs) comprised of iridium (Ir) thin films coated with an ultrathin (2-10 nm thick) silicon oxide (SiOx) or titanium oxide (TiOx) overlayer. The performance of SiOx|Ir and TiOx|Ir thin film electrodes towards the oxygen evolution reaction (OER) and Fe(II)/Fe(III) redox reactions were evaluated. An improvement in selectivity towards the OER was observed for all OECs. Overlayer properties, namely ionic and electronic conductivity, were assessed using a combination of electroanalytical methods and molecular dynamics simulations. SiOx and TiO¬x overlayers were found to be permeable to H2O and O2 such that the OER can occur at the MOx|Ir (M = Ti, Si) buried interface, which was further supported with molecular dynamics simulations. In contrast, Fe(II)/Fe(III) redox reactions occur to the same degree irrespective of whether electrocatalysts are bare, have TiOx overlayers with thicknesses less than 4 nm, or have SiOx overlayers with thicknesses less than 2 nm. This observation is attributed to facile electronic transport between the buried interface and outer surface of the overlayer, as measured with through-plane conductivity and ionic permeability measurements of wetted overlayer materials. These findings reveal the influence of oxide overlayer properties on the activity and selectivity of OECs and suggest opportunities to tune these properties for a wide range of electrochemical reactions.

Photocatalytic water splitting is a promising route to low-cost, green H2. However, this approach is currently limited in its solar-to-hydrogen conversion efficiency. One major source of efficiency loss is attributed to the high rates of undesired side and back reactions, which are exacerbated by the proximity of neighboring oxidation and reduction sites. Nanoscopic oxide coatings have previously been used to selectively block undesired reactants from reaching active sites; however, a coating encapsulating the entire photocatalyst particle limits activity as it cannot facilitate both half-reactions. In this work, area selective atomic layer deposition (AS-ALD) was used to selectively deposit semipermeable TiO2 films onto model metallic cocatalysts for enhancing reaction selectivity while maintaining a high overall activity. Pt and Au were used as exemplary reduction and oxidation cocatalyst sites, respectively, where Au was deactivated toward ALD growth through self-assembled thiol monolayers while TiO2 was coated onto Pt sites. Electroanalytical measurements of monometallic thin film electrodes showed that the TiO2-encapsulated Pt effectively suppressed undesired H2 oxidation and Fe(II)/Fe(III) redox reactions while still permitting the desired hydrogen evolution reaction (HER). A planar model photocatalyst platform containing patterned interdigitated arrays of Au and Pt microelectrodes was further assessed using scanning electrochemical microscopy (SECM), demonstrating the successful use of AS-ALD to enable local reaction selectivity in a dual-reaction-site (photo)electrocatalytic system. Finally, interdigitated microelectrodes having independent potential control were used to show that selectively deposited TiO2 coatings can suppress the rate of back reactions on neighboring active sites by an order of magnitude compared with uncoated control samples.

Electronic and ionic conductivity of an oxide overlayer can dictate the active site location, which can increase OER selectivity over competing reactions.

Photocatalytic water splitting is one promising route to reducing the cost of H2 production to industrially relevant levels. However, these systems are currently limited in their solar-to-hydrogen efficiency requiring further development in designing highly active and selective reaction sites. Reverse reactions, such as hydrogen oxidation or oxygen reduction, can greatly reduce overall system efficiency and are exacerbated by the close proximity of neighboring particles, which creates locally high concentrations of reactants. Metal oxide coatings have previously been used to selectively block undesired reactants from reaching the active sites, but can be limited in their ability to allow two different desired half reactions to occur as required for photocatalysts. In this work, we target selectively-deposited ALD of oxide films (e.g., TiO2 and SiO2) onto metallic co-catalysts for enhancing reaction selectivity while maintaining a high overall activity. Pt and Au were used an exemplary reduction and oxidation co-catalysts sites, where Au was deactivated towards ALD growth through self-assembled thiol monolayers while TiOx coated Pt suppressed Fe(II) redox reactions while maintaining the hydrogen evolution reaction (HER). The efficacy of the selective ALD growth was assessed through ellipsometry and X-ray photoelectron spectroscopy (XPS), while reaction selectivity was assessed through cyclic voltammetry. A planar photocatalyst model of patterned interdigitated arrays of Au and Pt was developed to further assess the selective ALD on the micron scale. Patterned electrodes were evaluated through cross-sectional scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS) and XPS, while reaction selectivity towards the HER over Fe(II) reduction was estimated using scanning electrochemical microscopy (SECM). Finally, interdigitated electrodes were used to simulate the effect of neighboring particle photocatalysts, where selectively deposited TiOx showed a significant reduction in reverse reactions over the uncoated samples.

The direct current bias for photovoltaic (PV) modules interconnected in series‐strings may include both high voltage negative (“HV−”) and positive (“HV+”) polarity with respect to the electrical ground. Multiple degradation modes, resulting in quantifiable optical loss, were found to occur during HV−/HV+ sequential stress, including corrosion of the external glass surface, encapsulant delamination (at its interfaces with the glass and the PV cell), internal haze formation (resulting from a chemical interaction between the glass and the encapsulant), corrosion and migration of the gridlines, and corrosion of the silicon nitride (Si x N y ) antireflective coating on the cell. The effects of these separate modes were examined using monolithic (e.g., glass or PV cell) and laminated‐coupon (glass/encapsulant/glass or glass/encapsulant/cell/encapsulant/backsheet) specimens. Characterizations during and after unbiased accelerated testing at 85°C/85% relative humidity included spectrophotometry, optical microscopy, electron microscopy, and ellipsometry. For some module components (i.e., the glass and the Si x N y coating), the optical performance was determined through iterative analysis of empirical measurements. Concentrating on just their spectral effect, a novel model was then developed to estimate the transfer of light to the PV cell and the return of light from the PV module with simultaneous degradation mechanisms, which was compared with a mini‐module previously subjected to HV−/HV+ stress. The model suggests that one third of the current loss observed for the mini‐module can be attributed to the optical degradation of the packaging materials. The dominant degradation modes include encapsulant delamination and corrosion of the Si x N y coating. Recommendations are given so that the optical model may be improved relative to accelerated testing and validated relative to field aging.

Degradation from ultraviolet (UV) radiation has become prevalent in the front of solar cells due to the introduction of UV-transmitting encapsulants in photovoltaic (PV) module construction. Here, we examine UV-induced degradation (UVID) in various commercial, unencapsulated crystalline silicon cell technologies, including bifacial silicon heterojunction (HJ), interdigitated back contact (IBC), passivated emitter and rear contact (PERC), and passivated emitter rear totally diffused (PERT) solar cells. We performed UV exposure tests using UVA-340 fluorescent lamps at 1.24 W.m (at 340 nm) and 45 degrees C through 4.02 MJ.m(-2) (2000 h). Our results showed that modern cell architectures are more vulnerable to UVID, leading to a significant power decrease (-3.6% on average; 11.8% maximum) compared with the conventional aluminum back surface field (Al-BSF) cells (< 1% on average). The power degradation is largely caused by the decrease in short-circuit current and open-circuit voltage. A greater power decrease is observed in bifacial cells with rear-side exposure compared with those with front-side exposure, indicating that the rear side is more susceptible to UV damage. Secondary ion mass spectroscopy (SIMS) confirmed an increase in hydrogen concentration near the Si/passivation interface in Hi and IBC cells after UV exposure; the excess of hydrogen could result in hydrogen-induced degradation and subsequently cause higher recombination losses. Additionally, surface oxidation and hot-carrier damage were identified in PERT cells. Using a spectral-based analysis, we obtained an acceleration factor of 5x between unpackaged cells (containing a silicon nitride antireflective coating on the front) in the UV test and an encapsulated module (with the front glass and encapsulant blocking 90% of the UV at 294 nm and 353 nm, respectively) in outdoor conditions. From the analytical calculations, we show that a UV-blocking encapsulant can reduce UV transmission in the module by an additional factor of similar to 50.

This study verifies the reliability of lumped thermodynamic fuel cell electric vehicle (FCEV) tank model under considerably slow-fill conditions. Many countries put efforts into research and development of FCEVs. As part of the efforts, thermodynamic hydrogen fueling models have been developed to understand the hydrogen temperature in the onboard tanks of FCEVs during the fueling process. Most of the models treat the hydrogen inside the tanks to be a lumped system and assume the hydrogen temperature to be uniform throughout the tanks. In other words, the hydrogen temperature is treated as average bulk temperature. This study certifies whether treating the hydrogen temperature in a tank as the bulk gas temperature is suitable by evaluating the temperature distributions inside the tank by a three-dimensional computational fluid dynamics (3D CFD) model.

Similar to natural photosynthesis, Z-scheme photocatalytic water splitting relies on two different light absorbing components that are coupled by a redox active mediator that shuttles charge between them. Such a two-absorber system possesses several advantages over single absorber photocatalytic system, including higher theoretical solar-to-hydrogen conversion efficiency, relaxed band alignment requirements, and the potential for inherently safe operation whereby H2 and O2 evolution occur in separated compartments. However, a major disadvantage and challenge for Z-scheme photocatalysis is that the presence of a redox mediator introduces two undesirable back-reactions on top of parasitic H2 oxidation and O2 reduction reactions that can occur in a single absorber photocatalytic system. Previous research efforts have identified the use of semi-permeable oxide coatings as an attractive approach to suppress these thermodynamically favored redox reactions while still permitting the desired water splitting and mediator redox reactions to occur. Here, we present a combined experimental and computational approach based on model thin films that is used to (i) probe the performance limits of oxide-encapsulated photocatalysts, (ii) quantify the effects of coating defects on performance, and (iii) guide the rational design of coatings aimed at maximizing the solar-to-hydrogen conversion efficiency of a target photocatalytic system. This work specifically focusses on the development of silicon and titanium oxide coatings for Z-scheme water splitting based on a Fe(II)/Fe(III) mediator, showing that the best coatings can achieve selectivities > 90 % towards the H2 and O2 evolution reactions over undesired Fe(II)/Fe(III) back reactions. Another key finding from this work is that coating defects can have a significant influence on the performance of encapsulated electrodes, as revealed by scanning electrochemical microscopy (SECM) measurements that were used to locally quantify the parasitic back reaction rates around individual defects to determine their impact on the global selectivity of an encapsulated electrode.

Industrial and public interest in hydrogen technologies has risen strongly recently, as hydrogen is the ideal means for medium to long term energy storage, transport and usage in combination with renewable and green energy supply. In a future energy system, the production, storage and usage of green hydrogen is a key technology. Hydrogen is and will in future be even more used for industrial production processes as a reduction agent or for the production of synthetic hydrocarbons, especially in the chemical industry and in refineries. Under certain conditions material based systems for hydrogen storage and compression offer advantages over the classical systems based on gaseous or liquid hydrogen. This includes in particular lower maintenance costs, higher reliability and safety. Hydrogen storage is possible at pressures and temperatures much closer to ambient conditions. Hydrogen compression is possible without any moving parts and only by using waste heat. In this paper, we summarize the newest developments of hydrogen carriers for storage and compression and in addition, give an overview of the different research activities in this field.

Physisorption of hydrogen in nanoporous materials offers an efficient and competitive alternative for hydrogen storage. At low temperatures (e.g. 77 K) and moderate pressures (below 100 bar) molecular H 2 adsorbs reversibly, with very fast kinetics, at high density on the inner surfaces of materials such as zeolites, activated carbons and metal–organic frameworks (MOFs). This review, by experts of Task 40 ‘Energy Storage and Conversion based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency, covers the fundamentals of H 2 adsorption in nanoporous materials and assessment of their storage performance. The discussion includes recent work on H 2 adsorption at both low temperature and high pressure, new findings on the assessment of the hydrogen storage performance of materials, the correlation of volumetric and gravimetric H 2 storage capacities, usable capacity, and optimum operating temperature. The application of neutron scattering as an ideal tool for characterising H 2 adsorption is summarised and state-of-the-art computational methods, such as machine learning, are considered for the discovery of new MOFs for H 2 storage applications, as well as the modelling of flexible porous networks for optimised H 2 delivery. The discussion focuses moreover on additional important issues, such as sustainable materials synthesis and improved reproducibility of experimental H 2 adsorption isotherm data by interlaboratory exercises and reference materials.

We are currently witnessing the dawn of the hydrogen (H2) economy, where H2 will become a primary fuel for heating, transportation, and long-distance and long-term energy storage. Among the diverse possibilities, H2 can be stored as a pressurized gas, cryogenic liquid, or solid fuel via adsorption onto porous materials. Metal-organic frameworks (MOFs) have emerged as the adsorbent materials with the theoretical highest H2 storage densities on both a volumetric and gravimetric basis. However, a critical bottleneck for the use of H2 as a transportation fuel has been the lack of densification methods capable of shaping MOFs into practical formulations whilst maintaining their adsorptive performance. Here, we report a high-throughput screening and deep analysis of a database of MOFs to find optimal materials, followed by the synthesis, characterisation, and performance evaluation of an optimal monolithic MOF (monoMOF) for H2 storage. After densification, this monoMOF stores 46 g L-1 H2 at 50 bar, 77 K, and delivers 41 and 42 g L-1 H2 at operating pressures of 25 and 50 bar, respectively, when deployed in a combined temperature–pressure (25-50 bar/77 K → 5 bar/160 K) swing gas delivery system. This performance represents up to an 80% reduction in the operating pressure requirements for delivering H2 gas when compared with benchmark materials, and an 83% reduction compared to compressed H2 gas. Our findings represent a substantial step forward in the application of high-density materials for volumetric H2 storage applications.

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, like those we recently invented. Systems using low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently operates at low voltages through use of an ensemble of optically thin photoabsorbers each operating at a low rate. Collectively the ensemble exhibits larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 and BiVO4 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively, our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

Successful salt (NaCl) reduction strategies are required to reduce the salt content of snacks while maintaining saltiness perception and consumer acceptance. Previous research suggests that particle physicochemical design rules (small particle size, low density, low hydrophobicity, optimised particle shape) can be leveraged to produce salt particles that enhance saltiness perception. This study aimed to validate these design rules by applying optimised model salts to unsalted potato crisps at a 30% reduced salt content to produce prototype products. A selection of commercial products were also chosen to represent the salt content and crisp style of the broader market, with the aim to investigate the potential of other salt reduction strategies including; direct salt removal without compensation for loss of salt content and increasing time in mouth, while exploring the impact of consumer mouth behaviour type on consumer product preference. Nine products varying in salt content (6 standard, 1 crinkle-cut, 1 thick-cut batch-fried, 1 baked reconstituted potato) were subject to descriptive sensory analysis with a trained panel (n = 11). A subset (seven products) were assessed for consumer acceptance (n = 93). A salt reduction of 30% was achieved while maintaining saltiness perception and consumer acceptance using model salts, while direct removal of salt without perceptual impact was only achievable by 15%. To investigate key drivers of liking, consumers were segmented based on product liking and mouth behaviour. Results suggested that whilst salt content was the primary driver, specific texture profiles were polarising. However, mouth behaviour had minimal influence on preference. These results validate previously described physicochemical design rules for developing novel salt particles for salt reduction and inform ingredient design for the food and flavour industries.

Many older adults fail to meet their daily protein requirements, potentially due to social, physical and medical factors, including sensory and appetite changes. Additionally, our previous research has identified potential sulfurous off-flavours, originating from heat-treatment of protein ingredients, which could play a role in consumer acceptance. This study aims to determine the hedonic impact of these potential off-flavours when added to a dairy beverage, identify the specific off-flavour concentrations which cause rejection by consumers, and lastly investigate difference in acceptance between older and younger consumers. A rejection threshold (RjT) protocol was used, in combination with Best Estimate Thresholds (BET), whereby sulfurous flavours (dimethyl sulfide, dimethyl disulfide and dimethyl trisulfide), and diacetyl were added to create a range of concentrations. 95 participants (younger n = 49, 18-38 years; older n = 46, 60-79 years) tasted 7 pairs of samples (one blank and one with ascending off-flavour concentration) and selected their preferred samples. Sulfurous flavours negatively impacted consumer acceptance, however, the extent to which they impart a negative effect differs between age groups. Younger adults rejected samples containing low concentrations of sulfurous off-flavours (1.55 ppb), however, older adults rejected samples with concentrations over 3 times higher (5.08 ppb). When combined with sulfurous flavours, diacetyl increased the rejection threshold for both groups. In conclusion, these observations imply that a greater quantity of off-flavour may be present before acceptance is reduced in the older consumer group. Moreover, diacetyl demonstrates partial masking abilities of sulfurous off-flavours, and BET gave a more conservative estimate of acceptability. This knowledge will help guide sensory innovation of high-protein beverages for older consumers to support product acceptance and optimal intake.

Photovoltaic module degradation from a high system voltage is a prevalent degradation mode in the field, where the enabling degradation mechanisms are inherently dependent on the voltage bias polarity of the installed system. Here, the effects of positive bias on module performance are confirmed and the underlying chemical degradation processes are more thoroughly investigated to reveal different degradation pathways from those previously reported in negative bias studies. When cells are under +1000 V stress, crystalline silicon mini-modules with poly(ethylene-co-vinyl acetate) (EVA) encapsulant demonstrated a significant photocurrent loss due to EVA discoloration and delamination from increased chemical reactivity at the front-side EVA/cell metallization interface. Brown discoloration of the EVA encapsulant near the cell gridlines is linked to an electrochemical reaction at the Ag gridlines under hot and humid conditions (85 degrees C, 85% relative humidity). Chemical compositional analysis using X-ray photoelectron spectroscopy (XPS) confirmed that the discoloration is attributed to the formation of silver sulfide (Ag2S) and/or silver oxide (Ag2O) species at the EVA/Ag gridline interface. The subsequent migration of Ag ions from the cell gridlines into the bulk of the EVA was evident from XPS depth profiling and optical microscopy. However, the Ag signal was not detected at the EVA/glass interface, inferring limited ionic transport through the nominally 0.45 mm thick encapsulant. For the samples studied herein, the sulfur is believed by the process of elimination to come from the ambient air, diffusing into the module through the permeable polymer backsheet.