2023 Roadmap on ammonia as a carbon-free fuel

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus

Preferred Experimental Practices for Photocatalytic Nitrogen Fixation

Ammonia is synthesized through the Haber-Bosch process, and has the highest carbon footprint of any synthetic chemical commodity. This prompts the need for green alternatives to meet net-zero goals in the chemical sector. Photocatalytic conversion of nitrogen to ammonia is one such alternative gaining attention. The current progress in photocatalytic nitrogen reduction suggests, however, that there exists a large gap in performance, before commercial use is viable. One of the major challenges is that highly active photocatalysts have not yet been developed. Furthermore, the development of photocatalysts is greatly hindered by false positive or non-reproducible data. This is because the current photocatalyst produce very low ammonia concentration, therefore the ammonia measurement can be easily affected by adventitious ammonia from the environment. Here, we will describe the current known causes of non-reproducible results in photocatalytic nitrogen fixation literature. We also will present the solution to mitigate these false positive results. Finally, we highlight the main challenges that remain to be overcome in this field. We aim to help researchers design more reliable experiments and inspire practical research in developing photocatalytic nitrogen fixation

Prioritizing the Best Potential Regions for Brine Concentration Systems in the USA using GIS and Multi-Criteria 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 Multi-Criteria Decision Analysis (GIS-MCDA) to prioritize the potential locations for brine concentration facilities based on thermodynamic, economic, environmental, and social criterion. By integrating geospatial data with a computational simulation of a real brine concentration system, an objective weighting method identifies the weights for thirteen sub-criteria associated with the main criterion. 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 sub-criterion misleads decision-making when identifying the best location for brine concentration systems, identifying the importance of the multi-criteria methodology

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 MW$_{PV}$/MW$_{ELEC}$ for an system that is completely powered by renewable photovoltaic power and operates intermittently. The optimal ratio of the photovoltaic to the electrolyzer is ~3.3 MW _PV/MW_Elec 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 MW _PV/MW_Elec for a system that can only sell to the grid and ~5 MW_PV/MW_Elect 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 reduce ammonia cost, 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 electro-fuels that rely minimally on grid electricity

A Combined Heat- and Power-Driven Membrane Capacitive Deionization System

Here, we experimentally investigate an alternative membrane capacitive deionization (MCDI) system cycle, which aims to reduce the required electrical energy demand for water treatment. The proposed heat and power combined MCDI system utilizes waste heat to control the electrostatic potential of the charged electrodes during the charging (desalination) and discharging (energy recovery) processes. The experimental findings suggest that with an increase in the temperature of the brine from 20 to 50 °C, the electrical energy consumed can be reduced by nearly 10%. We also show that the dependence of electrostatic potential on concentration may limit energy recovery performance (power), when moving toward higher water recoveries. Alternative desalination cycles can be further explored through evaluating non-isothermal and non-adiabatic system operation

Using Flow Electrodes in Multiple Reactors in Series for Continuous Energy Generation from Capacitive Mixing

Efficient conversion of “mixing energy” to electricity through capacitive mixing (CapMix) has been limited by low energy recoveries, low power densities, and noncontinuous energy production resulting from intermittent charging and discharging cycles. We show here that a CapMix system based on a four-reactor process with flow electrodes can generate constant and continuous energy, providing a more flexible platform for harvesting mixing energy. The power densities were dependent on the flow-electrode carbon loading, with 5.8 ± 0.2 mW m^–2 continuously produced in the charging reactor and 3.3 ± 0.4 mW m^–2 produced in the discharging reactor (9.2 ± 0.6 mW m^–2 for the whole system) when the flow-electrode carbon loading was 15%. Additionally, when the flow-electrode electrolyte ion concentration increased from 10 to 20 g L^–1, the total power density of the whole system (charging and discharging) increased to 50.9 ± 2.5 mW m^–2

Solar Energy Utilization and Photo(electro)catalysis for Sustainable Environment

This article is part of the Photo-Energy Utilization for a Sustainable Environment: Photo(electro)catalysis special issue

Influence of Feed-Electrode Concentration Differences in Flow-Electrode Systems for Capacitive Deionization

The common performance metrics ascribed to flow electrode capacitive deionization systems can vary significantly depending on the mode of operation and initial system conditions. Through varying the flow electrolyte ionic strength, performance values such as average salt adsorption rate and energy consumption can vary by as much as 51 and 55%. This variability from cycle to cycle is in part due to changes in the electrical conductivity (ohmic) but is also due to the introduction of competing transport processes. Diffusive transport can enhance or diminish the rate of desalination and energy recovery, creating a larger degree of error in reported values. Here, we propose a dimensionless ratio comparing diffusion and electromigration-based transport to measure performance stability. Unstable system performance is most prominent when the feed ionic strength does not match the flow electrode ionic strength. In this scenario, the ratio of the diffusion to electromigration transport reached a maximum. Conversely, stable operation occurs when the ionic strength of the feed matches the ionic strength in the flow electrode. With the growing interest to move flow electrode capacitive deionization into treatment regimens which operated with high concentration feedwater, characterizing and quantifying diffusive flux is important for assessing true electrochemical system performance