Electrified Solar Zero Liquid Discharge: Exploring the Potential of PV-ZLD in the US.

Current brine management strategies are based on the disposal of brine in nearby aquifers, representing a loss in potential water and mineral resources. Zero liquid discharge (ZLD) is a possible strategy to reduce brine rejection while increasing the resource recovery from desalination plants. However, ZLD substantially increases the energy consumption and carbon footprint of a desalination plant. The predominant strategy to reduce the energy consumption and carbon footprint of ZLD is through the use of a hybrid desalination technology that integrates renewable energy. Here, we built a computational thermodynamic model of the most mature electrified hybrid technology for ZLD powered by photovoltaic (PV). We examine the potential size and cost of ZLD plants in the US. This work explores the variables (geospatial and design) that most influence the levelized cost of water and the second law efficiency. There is a negative correlation between minimizing the LCOW and maximizing the second-law. And maximizing the second-law, the states that more brine produces, Texas is the location where the studied system achieves the lowest LCOW and high second-law efficiency, while California is the state where the studied system is less favorable. A multiobjective optimization study assesses the impact of considering a carbon tax in the cost of produced water and determines the best potential size for the studied plant.

Achieving Decentralized, Electrified, and Decarbonized Ammonia Production.

The rapid reduction in the cost of renewable energy has motivated the transition from carbon-intensive chemical manufacturing to renewable, electrified, and decarbonized technologies. Although electrified chemical manufacturing technologies differ greatly, the feasibility of each electrified approach is largely related to the energy efficiency and capital cost of the system. Here, we examine the feasibility of ammonia production systems driven by wind and photovoltaic energy. We identify the optimal regions where wind and photovoltaic electricity production may be able to meet the local demand for ammonia-based fertilizers and set technology targets for electrified ammonia production. To compete with the methane-fed Haber-Bosch process, electrified ammonia production must reach energy efficiencies of above 20% for high natural gas prices and 70% for low natural gas prices. To account for growing concerns regarding access to water, geospatial optimization considers water stress caused by new ammonia facilities, and recommendations ensure that the identified regions do not experience an increase in water stress. Reducing water stress by 99% increases costs by only 1.4%. Furthermore, a movement toward a more decentralized ammonia supply chain driven by wind and photovoltaic electricity can reduce the transportation distance for ammonia by up to 76% while increasing production costs by 18%.

Formation of Carbon-Induced Nitrogen-Centered Radicals on Titanium Dioxide under Illumination.

Titanium dioxide is the most studied photocatalytic material and has been reported to be active for a wide range of reactions, including the oxidation of hydrocarbons and the reduction of nitrogen. However, the molecular-scale interactions between the titania photocatalyst and dinitrogen are still debated, particularly in the presence of hydrocarbons. Here, we used several spectroscopic and computational techniques to identify interactions among nitrogen, methanol, and titania under illumination. Electron paramagnetic resonance spectroscopy (EPR) allowed us to observe the formation of carbon radicals upon exposure to ultraviolet radiation. These carbon radicals are observed to transform into diazo- and nitrogen-centered radicals (e.g., CHxN2• and CHxNHy•) during photoreaction in nitrogen environment. In situ infrared (IR) spectroscopy under the same conditions revealed C-N stretching on titania. Furthermore, density functional theory (DFT) calculations revealed that nitrogen adsorption and the thermodynamic barrier to photocatalytic nitrogen fixation are significantly more favorable in the presence of hydroxymethyl or surface carbon. These results provide compelling evidence that carbon radicals formed from the photooxidation of hydrocarbons interact with dinitrogen and suggest that the role of carbon-based "hole scavengers" and the inertness of nitrogen atmospheres should be reevaluated in the field of photocatalysis.

Atomically Ordered PdCu Electrocatalysts for Selective and Stable Electrochemical Nitrate Reduction.

Electrochemical nitrate reduction (NO3 RR) has attracted attention as an emerging approach to mitigate nitrate pollution in groundwater. Here, we report that a highly ordered PdCu alloy-based electrocatalyst exhibits selective (91% N2), stable (480 h), and near complete (94%) removal of nitrate without loss of catalyst. In situ and ex situ XAS provide evidence that structural ordering between Pd and Cu improves long-term catalyst stability during NO3RR. In contrast, we also report that a disordered PdCu alloy-based electrocatalyst exhibits non-selective (44% N2 and 49% NH4+), unstable, and incomplete removal of nitrate. The copper within disordered PdCu alloy is vulnerable to accepting electrons from hydrogenated neighboring Pd atoms. This resulted in copper catalyst losses which were 10× greater than that of the ordered catalyst. The design of stable catalysts is imperative for water treatment because loss of the catalyst adds to the system cost and environmental impacts.

Progress in Photochemical and Electrochemical C-N Bond Formation for Urea Synthesis.

ConspectusHere, we discuss recent advances and pressing challenges in achieving sustainable urea synthesis. Urea stands out as the most prevalent nitrogen-based fertilizer used across the globe, making up over 50% of all manufactured fertilizers. Historically, the Bosch-Meiser process has been the go-to chemical manufacturing method for urea production. This procedure, characterized by its high-temperature and high-pressure conditions, reacts ammonia with carbon dioxide to form ammonium carbamate. Subsequently, this ammonium carbamate undergoes dehydration, facilitated by heat, producing solid urea. A concerning aspect of this method is its dependency on fossil fuels, as nearly all the process heat comes from nonrenewable sources. Consequently, the Bosch-Meiser process leaves behind a considerable carbon footprint. Current estimates predict that unchecked, carbon emissions from urea production alone might skyrocket, reaching a staggering 286 MtCO2,eq/yr by 2050. Such projections paint a clear picture regarding the necessity for more eco-friendly, sustainable urea production methods. Recently, the scientific community has shown growing interest in forming C-N bonds using alternative methods. Shifting toward photochemical or electrochemical processes, as opposed to traditional thermal-based processes, promises the potential for complete electrification of urea synthesis. This shift toward process electrification is not just an incremental change; it represents a groundbreaking advancement, the first of many steps, toward achieving deep decarbonization in the chemical manufacturing sector. Since the turn of 2020, there has been a surge in research focusing on photochemical and electrochemical urea synthesis. These methods capitalize on co-reduction of carbon dioxide with nitrogenous reactants like NOx and N2. Despite the progress, there are significant challenges that hinder these processes from reaching their full potential. In this comprehensive review, we shed light on the advances made in electrified C-N bond formation. More importantly, we focus on the invaluable insights gathered over the years, especially concerning catalytic reaction mechanisms. We have dedicated a section to underline key focal areas for up-and-coming research, emphasizing catalyst, electrolyte, and reactor design. It is undeniable that catalyst design remains at the heart of the matter, as managing the co-reduction of two distinct reactants (CO2 and nitrogenous species) is complex. This process results in a myriad of intermediates, which must be adeptly managed to both maintain catalyst activity and avoid catalyst deactivation. Moreover, the electrolytes play a pivotal role, essentially dictating the creation of optimal microenvironments that drive reaction selectivity. Finally, reactor engineering stands out as crucial to ensure optimal mass transport for all involved reactants and subsequent products. We touch upon the broader environmental ramifications of urea production and bring to light potential obstacles for alternative synthesis routes. A notable mention is the urgency of accelerating the uptake and large-scale implementation of renewable energy sources.

Energy Management and Economic Considerations of Intermittent Photovoltaic-Driven Electrochemical Ammonia Production.

As the energy sector shifts from fossil fuels to renewable energy, there is a need for long-duration energy storage solutions to handle the intermittency of renewable electricity. Electrofuels, or fuels synthesized from excess electricity, are an emerging medium poised to meet long-duration energy storage requirements. Ammonia as an electrofuel is potentially ideal because ammonia has a relatively low liquefaction pressure, indicating that ammonia can be easily stored and transported. Here, we develop a framework to optimize the electrochemical production of ammonia powered by intermittent photovoltaic power. We also explore various buyback policies to understand the impact that policy has on the cost of intermittent ammonia and optimal sizing ratios. The optimal ratio of the photovoltaic to the electrolyzer is ∼3.7 MWPV/MWELEC for a system that is completely powered by renewable photovoltaic power and operates intermittently. The optimal ratio of the photovoltaic to the electrolyzer is ∼3.3 MWPV/MWELEC for a system that uses photovoltaics in conjunction with grid electricity and operates continuously. For the purchase price at the avoided cost of electricity, the optimal ratio of the solar panel to the electrolyzer increases to ∼4 MWPV/MWELEC for a system that can only sell to the grid and ∼5 MWPV/MWELEC for a system that can buy and sell electricity to the grid at the avoided cost. Optimizing energy management by setting auxiliary battery size limits is essential to reducing ammonia costs, and the optimal battery size decreases as the buyback price of electricity increases. Finally, we find that systems connected to the grid and operating continuously have emissions comparable to the Haber-Bosch process because of the current emissions tied to the United States electricity generation. Thus, unless the grid is completely decarbonized, it is essential to create electrofuels that rely minimally on grid electricity.

A rotating ring disc electrode study of photo(electro)catalyst for nitrogen fixation.

There are numerous reports of photo(electro)catalysts demonstrating activity for nitrogen reduction to ammonia and a few reports of photo(electro)catalysts demonstrating activity for nitrogen oxidation to nitric acid. However, progress in advancing solar-to-fertilizer applications is slow, due in part to the pace of catalyst screening. Most evaluations of photo(electro)catalysts activity occur using batch reactors. This is because common product analyses require accumulation of ammonia or nitric acid in the reactor to overcome instrument detection limits. The primary aim here is to examine the use of an electroanalytical method, rotating ring disk electrode voltammetry (RRDE), to detect ammonia produced by a nitrogen fixing photo(electro)catalyst. To examine the potential for RRDE, we investigated a photo(electro)catalyst known to reduce nitrogen to ammonia (titania), while varying the applied electrochemical potential and degree of illumination on the disk. We show that the observed ammonia oxidation at the ring electrode corresponds strongly with ammonia measurements obtained from the bulk electrolyte. Indicating that RRDE may be effective for catalyst screening. The chief limitation of this approach is the need for an alkaline electrolyte. In addition, this approach does not rule out the presence of adventitious ammonia.

Prioritizing the Best Potential Regions for Brine Concentration Systems in the USA Using GIS and Multicriteria Decision Analysis.

We propose a methodology for identifying and prioritizing the best potential locations for brine concentration facilities in the contiguous United States. The methodology uses a geographic information system and multicriteria decision analysis (GIS-MCDA) to prioritize the potential locations for brine concentration facilities based on thermodynamic, economic, environmental, and social criteria. By integrating geospatial data with a computational simulation of a real brine concentration system, an objective weighting method identifies the weights for 13 subcriteria associated with the main criteria. When considering multiple dimensions for decision making, brine concentration facilities centered in Florida were consistently selected as the best location, due to the high second-law efficiency, low transportation cost, and high capacity for supplying municipal water needs to nearby populations. For inland locations, Southeast Texas outperforms all other locations for thermodynamic, economic, and environmental priority cases. A sensitivity analysis evaluates the consistency of the results as the priority of a main criterion varies relative to other decision-making criteria. Focusing on a single subcriterion misleads decision making when identifying the best location for brine concentration systems, identifying the importance of the multicriteria methodology.

In situ investigation of water on MXene interfaces.

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K+) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li+) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li+) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K+) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

Scalable Manufacturing of Hybrid Solid Electrolytes with Interface Control.

Hybrid solid electrolytes are promising alternatives for high energy density metallic lithium batteries. Scalable manufacturing of multi-material electrolytes with tailored transport pathways can provide an avenue toward controlling Li stripping and deposition mechanisms in all-solid-state devices. A novel roll-to-roll compatible coextrusion device is demonstrated to investigate mesostructural control during manufacturing. Solid electrolytes with 25 and 75 wt % PEO-LLZO compositions are investigated. The coextrusion head is demonstrated to effectively process multimaterial films with strict compositional gradients in a single pass. An average manufacturing variability of 5.75 ± 1.2 µm is observed in the thickness across all the electrolytes manufactured. Coextruded membranes with 1 mm stripes show the highest room temperature conductivity of 8.8 × 10-6 S cm-1 compared to the conductivity of single-material films (25 wt %, 1.2 × 10-6 S cm-1; 75 wt %, 1.8 × 10-6 S cm-1). Distribution of relaxation times and effective mean field theory calculations suggest that the interface generated between the two materials possesses high ion-conducting properties. Computational simulations are used to further substantiate the influence of macroscale interfaces on ion transport.

Constant chemical potential cycles for capacitive deionization.

The primary energy consuming operations which occur within a Capacitive Deionization (CDI) cell, are the ion removal (electrosorption), ion concentrating (electrodesorption), and solution switching processes. In theory the maximum system performance for a CDI system arises when solution switching occurs while maintaining a fixed number of ions (N), and when electrosorption/desorption occurs while maintaining a fixed chemical potential (µ). These fixed state variable based operations are analogous to the Carnot cycle, where heat transfer occurs at constant temperature and compression and expansion occur while maintaining constant entropy. In reality, maintaining a constant number of ions during switching is not practically feasible, thus here we investigate two alternative cycles where switching instead occurs while maintaining constant charge or voltage. Unlike constant number of ions, maintaining charge and voltage constant is feasible using a potentiostat. These theoretical cycles were chosen as they are analogues or ideal-like (Stirling and Ericsson) cycles, which are also practically feasible. The thermodynamic analysis reveals that these alternative cycles provide an avenue to approach the theoretical limit with low saline feed water; however, they are not capable of approximating ideal operations at elevated feed-water concentrations.

Influence of carbonaceous species on aqueous photo-catalytic nitrogen fixation by titania.

For decades, reports have suggested that photo-catalytic nitrogen fixation by titania in an aqueous environment is possible. Yet a consensus does not exist regarding how the reaction proceeds. Furthermore, the presence of an aqueous protonated solvent and the similarity between the redox potential for nitrogen and proton reduction suggest that ammonia production is unlikely. Here, we re-investigate photo-catalytic nitrogen fixation by titania in an aqueous environment through a series of photo-catalytic and electrocatalytic experiments. Photo-catalytic testing reveals that mineral phase and metal dopants play a marginal role in promoting nitrogen photofixation, with ammonia production increasing when the majority phase is rutile and with iron dopants. However, the presence of a trace amount of adsorbed carbonaceous species increased the rate of ammonia production by two times that observed without adsorbed carbon based species. This suggests that carbon species play a potential larger role in mediating the nitrogen fixation process over mineral phase and metal dopants. We also demonstrate an experimental approach aimed to detect low-level ammonia production from photo-catalysts using rotating ring disk electrode experiments conducted with and without illumination. Consistent with the photocatalysis, ammonia is only discernible at the ring with rutile phase titania, but not with mixed-phase titania. Rotating ring disk electrode experiments may also provide a new avenue to attain a higher degree of precision in detecting ammonia at low levels.

The Role of Adventitious Carbon in Photo-catalytic Nitrogen Fixation by Titania.

Photo-catalytic fixation of nitrogen by titania catalysts at ambient conditions has been reported for decades, yet the active site capable of adsorbing an inert N2 molecule at ambient pressure and the mechanism of dissociating the strong dinitrogen triple bond at room temperature remain unknown. In this work in situ near-ambient-pressure X-ray photo-electron spectroscopy and density functional theory calculations are used to probe the active state of the rutile (110) surface. The experimental results indicate that photon-driven interaction of N2 and TiO2 is observed only if adventitious surface carbon is present, and computational results show a remarkably strong interaction between N2 and carbon substitution (C*) sites that act as surface-bound carbon radicals. A carbon-assisted nitrogen reduction mechanism is proposed and shown to be thermodynamically feasible. The findings provide a molecular-scale explanation for the long-standing mystery of photo-catalytic nitrogen fixation on titania. The results suggest that controlling and characterizing carbon-based active sites may provide a route to engineering more efficient photo(electro)-catalysts and improving experimental reproducibility.

A physical catalyst for the electrolysis of nitrogen to ammonia.

Ammonia synthesis consumes 3 to 5% of the world's natural gas, making it a significant contributor to greenhouse gas emissions. Strategies for synthesizing ammonia that are not dependent on the energy-intensive and methane-based Haber-Bosch process are critically important for reducing global energy consumption and minimizing climate change. Motivated by a need to investigate novel nitrogen fixation mechanisms, we herein describe a highly textured physical catalyst, composed of N-doped carbon nanospikes, that electrochemically reduces dissolved N2 gas to ammonia in an aqueous electrolyte under ambient conditions. The Faradaic efficiency (FE) achieves 11.56 ± 0.85% at -1.19 V versus the reversible hydrogen electrode, and the maximum production rate is 97.18 ± 7.13 µg hour-1 cm-2. The catalyst contains no noble or rare metals but rather has a surface composed of sharp spikes, which concentrates the electric field at the tips, thereby promoting the electroreduction of dissolved N2 molecules near the electrode. The choice of electrolyte is also critically important because the reaction rate is dependent on the counterion type, suggesting a role in enhancing the electric field at the sharp spikes and increasing N2 concentration within the Stern layer. The energy efficiency of the reaction is estimated to be 5.25% at the current FE of 11.56%.

Integration of colloids into a semi-flexible network of fibrin.

Typical colloid-polymer composites have particle diameters much larger than the polymer mesh size, but successful integration of smaller colloids into a large-mesh network could allow for the realization of new colloidal states of spatial organization and faster colloid motion which can allow the possibility of switchable re-configuration of colloids or more dramatic stimuli-responsive property changes. Experimental realization of such composites requires solving non-trivial materials selection and fabrication challenges; key questions include composition regime maps of successful composites, the resulting structure and colloidal contact network, and the mechanical properties, in particular the ability to form a network and retain strain stiffening in the presence of colloids. Here, we study these fundamental questions by formulating composites with fluorescent (though not stimuli-responsive) carboxylate modified polystyrene/latex (CML) colloidal particles (diameters 200 nm and 1000 nm) in bovine fibrin networks (a semi-flexible biopolymer network with mesh size 1-5 µm). We describe and characterize two methods of composite preparation: adding colloids before fibrinogen polymerization (Method I), and electrophoretically driving colloids into a network already formed by fibrinogen polymerization (Method II). We directly image the morphology of colloidal and fibrous components with two-color fluorescent confocal microscopy under wet conditions and SEM of fixed dry samples. Mechanical properties are studied with shear and extensional rheology. Both fabrication methods are successful, though with trade-offs. Method I retains the nonlinear strain-stiffening and extensibility of the native fibrin network, but some colloid clustering is observed and fibrin network integrity is lost above a critical colloid concentration that depends on fibrinogen and thrombin concentration. Larger colloids can be included at higher volume fractions before massive aggregation occurs, indicating surface interactions as a limiting factor. Method II results in a loss of measurable strain-stiffening, but colloids are well dispersed and template along the fibrous scaffold. The results here, with insight into both structure and rheology, form a foundational understanding for the integration of other colloids, e.g. with stimuli-responsive functionalities, into semi-flexible networks.