Alternative fuels are essential to decarbonising maritime transport, provided they retain high energy and power density; hydrogen carriers fulfil these criteria and represent a promising fuel option. This research quantifies the power-energy trade-offs of four hydrogen carrier fuels: namely, sodium borohydride, ammonia borane, n-ethylcarbazole, and dibenzyltoluene. Each hydrogen carrier is combined with four energy converters: proton exchange membrane fuel cell (PEMFC), internal combustion engine (ICE), solid oxide fuel cell (SOFC), and gas turbine, respectively, resulting in 16 outcomes, which were assessed using Ragone plots. For each hydrogen carrier, one energy converter is selected as a case study. Four reference cases are also examined: marine diesel oil (MDO) and methanol combined with ICE, ammonia with SOFC, and liquid hydrogen with PEMFC. These 8 case studies (carrier and converter combinations) are evaluated for performance across several shipping applications. The results indicate that ammonia borane combined with ICE and sodium borohydride combined with a PEMFC offer energy and power density profiles suitable for various vessel types, including inland ships and ocean-going vessels. These combinations also show potential for specialised vessels such as fishing boats, dredgers, tugs, and ferries, which operate under unique load profiles and duty cycles. Compared with alternative fuels, such as ammonia, methanol, and liquid hydrogen, hydrogen carriers can demonstrate higher energy and power densities. Under specific circumstances, they may achieve performance comparable to MDO, highlighting their potential for zero-emission maritime propulsion.

The long-term operation of CO₂ electrolyzers using membrane electrode assemblies (MEAs) is limited by challenges related to water management. However, the water balance in CO₂ electrolyzer cells still has not been fully understood, and conflicting observations have been reported in the literature. In this study, a one-dimensional non-isothermal multiphysics model of an exchange MEA CO₂ electrolyzer with a Tokuyama A201 anion exchange membrane is developed to investigate the role of different physical and chemical phenomena on the water balance. The relative contributions of these processes vary with current density and membrane transport properties, which shift the dominant water transport mechanism in the cell. Our results highlight the significant contribution of homogeneous reactions, particularly OH⁻, to the water balance across the membrane. At low currents (i < 130 mA cm⁻²), homogeneous buffer reactions dominate the water balance and result in net water production near the catalyst layer. At higher currents (i > 130 mA cm⁻²), the flux is governed by electro-osmotic drag and a temperature gradient over the cathode gas diffusion electrode (GDE) with their relative contributions depending on membrane properties. Homogeneous buffering can re-emerge as the dominant mechanism at high currents if the hydroxide ion concentration in the membrane increases, for example under CO₂-limited cathode conditions, allowing hydroxide ions to react with depleted bicarbonate near the anode.

Accurate non-invasive estimation of trans-stenotic pressure drops remains a challenge. In clinical practice, pressure drops are often estimated from velocity measurements using Bernoulli-based formulas, but these simplified relations do not explicitly account for how pressure losses change with the flow regime. Here, we introduce a modified Bernoulli (MB) formulation that incorporates regime-dependent pressure losses through a Reynolds-number-dependent loss coefficient. Steady in-vitro experiments were performed in an idealized stenosis model over physiologically relevant flow rates (0.65–3.9 L/min), combining direct pressure measurements with ultrasound imaging velocimetry (UIV) and phase-contrast magnetic resonance imaging (PC-MRI) to measure velocities. The MB model was calibrated from the measured pressure drops and then evaluated against the simplified Bernoulli (SB) and extended Bernoulli (EB) formulations. Over the tested flow regime, MB agreed best with the measurements (typically within about ±10%). SB and EB showed larger biases, with errors of roughly 10–55% (SB) and -15 to 25%−(EB), and overestimated the pressure drop in the clinically relevant range. We additionally quantified the effect of PC-MRI in-plane pixel size on MRI-based pressure estimates. Coarse in-plane pixel size of 1.33 mm, corresponding to only approximately 3–4 pixels across the 3.3 mm throat radius of the 6.6 mm stenosis throat, led to systematic underestimation of flow rate and bulk velocity (about − 34 to − 44%) and, consequently, of the MB-predicted pressure drop (about − 52 to − 62%). In contrast, the peak throat velocity was substantially less sensitive to pixel size, resulting in smaller estimation errors (about − 13 to − 18.7%) when used as input for the MB. Overall, the results demonstrate that accounting for flow-regime-dependent loss mechanisms enhances pressure drop estimation, and that sufficient sampling of the stenotic throat is crucial for MRI-based flow rate and pressure drop estimation. In addition, peak-velocity-based MB pressure drop estimations are less sensitive to pixel size.

The development of dewatering methods driven by electric fields offers significant potential to reduce the energy demand for these processes. Electro-osmotic dewatering relies on the application of an electric field across a porous matrix possessing a surface charge, inducing liquid transport through electrokinetic mechanisms. While the feasibility and energy efficiency of electro-osmotic dewatering have been demonstrated, a fundamental understanding of how the irregular, porous microstructure affects the uniformity of dewatering is lacking. In this work, we investigate electro-osmotic flow in porous media micromodels and directly compare it with conventional pressure-driven flow under controlled experimental conditions. The micromodel is designed and microfabricated to exhibit a controlled, heterogeneous microstructure. Pore-scale velocity fields are measured by using particle tracking velocimetry and characterized through velocity probability density functions. A multiphysics model, coupling ion transport, electrostatics, and hydrodynamics, is validated against the experiments. The results show that, in contrast to pressure-driven flow, which exhibits broader velocity distributions with preferential flow paths and stagnant regions, electro-osmotic flow produces a narrower velocity distribution, even in heterogeneous porous structures. These findings demonstrate the potential of electro-osmotic dewatering to achieve more uniform water removal in low-permeability materials and provide a predictive framework for optimizing electrically driven dewatering.

Sodium borohydride (NaBH₄) is a promising hydrogen carrier for maritime applications due to its high gravimetric and volumetric energy densities compared to compressed or liquefied hydrogen. As a solid granular material, NaBH₄ can be stored under atmospheric pressure and room temperature, eliminating the need for extreme pressures, or cryogenic conditions. Although NaBH₄ is hygroscopic and can become cohesive when exposed to humid environments, when stored and handled sufficiently dry it remains free-flowing, which is essential for reliable conveying, storage, and discharge operations. Designing equipment for these processes requires an accurate understanding of NaBH₄’s free-flowing behaviour, motivating the need for modelling tools. To address this, we model granular, free-flowing NaBH₄ using the Discrete Element Method. Granular flow behaviour depends strongly on the flow regime—defined by the applied shear and confining pressure through the inertial number—and practical handling systems typically operate in the dense flow regime; consequently, this study focuses on dense-flow conditions. A standard calibration–verification–validation methodology is applied, using a ledge test and rotating drum for calibration and verification, and an inclined surface test for validation. In parallel, we evaluate the inertial number across all setups using a velocity-based approach to characterise both the global flow regime and locally occurring flow regimes within the flowing layer. Rather than assigning a single characteristic value per setup, we demonstrate that distinct segments exhibit different inertial numbers, indicating that the inertial number is spatially dependent.

The advancement of microbial electrosynthesis systems (MES) towards industrialization is currently hindered by a limited understanding of the fundamental constraints affecting selective production of high-value chemicals. To address this challenge, we develop a comprehensive computational model that integrates microbial, electrochemical, and acid–base reactions with pore-scale transport processes within a three-dimensionally resolved biofilm. This study investigates the H₂-mediated CO₂ fixation pathway to acetate, butyrate, and caproate. The effect of applied cathode potential and biofilm thickness on macroscopic parameters, such as efficiency and selectivity, is analyzed based on local concentrations and electrochemical and biochemical fluxes. Among the limiting factors, the availability of CO₂ emerges as the main limitation for biochemical reactions due to its low solubility and high half-saturation constant. Additionally, hydrogen – serving as the electron mediator – limits the reaction rate at low current densities and reduces electron transfer efficiency at higher current densities. A key insight from our study is the identification of an optimal electrode potential for each biofilm thickness, balancing both H₂ transfer and CO₂ consumption efficiencies. Furthermore, carbon selectivity shifts with increasing biofilm thickness: net acetate production declines while caproate production increases. This trend is attributed to the prolonged residence time of metabolic intermediates within thicker biofilms, promoting chain elongation pathways. Thus, our work takes an important step towards a fundamental understanding of caproate selectivity across different biofilms, which can be used to optimize the electrode structure and operating conditions to control the local biofilm thickness.

To reduce global emissions, hydrogen is increasingly considered as an energy carrier for renewable energy storage. However, traditional storage methods for hydrogen such as compression or liquefaction require high pressures, extremely low temperatures, and still result in a low volumetric energy density. As a solution, sodium borohydride (NaBH₄) is proposed as an alternative method to store hydrogen. NaBH₄ is a granular material that can be stored using ambient temperature and pressure, and has a relatively high volumetric and gravimetric energy density compared to traditional hydrogen storage. This paper explores the application of NaBH₄ as a fuel in the maritime industry, and elaborates on how the use of NaBH₄ leads to a circular bunkering (refuelling) process. By using hydrolysis to extract hydrogen from NaBH₄ during vessel operation, a so called spent fuel remains and needs to be stored on the vessel until next port call. Additionally, examples of various bunkering equipment that can be used to design the circular bunkering process of NaBH₄ are presented. Moreover, it explains how design of bunkering equipment depends on the mechanical characteristics of the fuel and spent fuel. The main finding of this work is that NaBH₄ is a promising solution for a sustainable future. Before NaBH₄ can be used as a fuel, vessels and ports need to be adapted to facilitate circular bunkering with such a novel solid-state energy carrier.

In this study we investigate the mechanochemical regeneration of sodium borohydride (NaBH₄) from a system comprising hydrated sodium metaborate ( [Formula presented] ) and magnesium hydride (MgH₂). We explore the individual and joint impact of key operational parameters (rotational speed, milling time, ball-to-powder ratio (BPR), and molar ratio) on the regeneration yield. Furthermore, a method for quantifying chemical conversion is introduced relying only on water and thus, offering environmental benefits. This approach additionally facilitates the production and storage of a “ready-to-use” NaBH₄ solution with minimal losses at room temperature. Notably, a yield of 90% is achieved, with a 20% reduction in rotational speed compared to prior literature. This research contributes to sustainable hydrogen storage and presents practical advancements in mechanochemical processes.

Electrochemical conversion of CO₂ to hydrocarbons is limited by the low solubility and slow transport of CO₂ in aqueous systems. We demonstrate that we can reach partial current densities for CO₂-to-CO of 40 mA/cm² in fully aqueous systems, without the use of gas diffusion electrodes. We alleviate the mass transfer limitation by combining a suspension of catalytically active silver nanoparticles (Ag NPs) with a flow-through current collector. This extends the reactive area into the electrolyzer channel and improves the accessibility of dissolved CO₂ in a larger volume of electrolyte. The flow-through electrode system also outperforms a fully suspended electrode (based on carbon black particles), due to enhanced electric conductivity and smaller carbon area to minimize parasitic side-reactions. Additionally, we show that the distribution of the Ag NPs is pivotal for high CO₂ conversion rates, demonstrated by the highest CO current density obtained when a suspension of Ag NPs and SDS as surfactant is flowing through the 3D electrodes as pre-treatment. A stable CO current density can be sustained for more than 4 h. Although the conversion rate is still moderate compared to gas-fed CO₂ electrolzyers, the partial current density for flow-through electrodes is more than an order of magnitude larger than for planar flow systems. This work shows that CO₂ conversion in aqueous systems can be enhanced considerably by exploiting larger electrolyte volumes via smart electrode designs, such as a flow-through principle.

Electrochemical reactors, such as water electrolyzers, CO₂ electrolyzers, fuel cells, and flow batteries, will be essential in electrifying industry as part of the global transition towards a defossilized and sustainable economy. These technologies require further optimization to enhance efficiency and reduce costs for widespread adoption. Hydrodynamics and mass transfer at electrode–electrolyte interfaces significantly affect electrochemical conversion reactions by influencing the reactant availability and pH in the local reaction environment. 3D electrodes, such as flow-through foams and suspension electrodes, hold a great advantage over 2D electrodes as they moderate pH changes and reactant depletion by spreading the current over a larger electrode area and electrolyte volume. We study the diffusion boundary layer in operando around a single mm-sized particle, representing an element of a 3D electrode. We visualize the local and transient pH with Fluorescence Lifetime Imaging Microscopy (FLIM) during H₂O reduction at various current densities and electrolyte flow velocities at a resolution down to 9 μm and 2 Hz. In addition, we apply an intermittent current to investigate how long the capacitive electric double layer of a suspension electrode particle can maintain an electrochemical reaction during their time of non-contact with a current collector, mimicking applications with Faradaic charge transfer (i.e. flow batteries, microbial fuel cells, capacitance-based electrolyzers). We demonstrate that the diffusion boundary layer is not symmetrical, but depend on the direction of the electric field, the current density and the flow conditions. The substantial pH gradients and boundary layer formation at the scale of hundreds of micrometers underline the importance of controlling flow in or around electrodes, making 3D electrodes an important asset for creating suitable reaction conditions in mass transport-limited electrochemical conversions.

One of the most promising energy carriers for transport applications are hydrogen-based energy carriers. NaBH₄ is a hydrogen energy carrier and produces hydrogen bubbles when it is dissolved in water. The formation of hydrogen bubbles hinders experimental measurements of the thermodynamic and transport properties of aqueous NaBH₄ solutions at elevated temperatures. Accurate knowledge of these properties is essential for the NaBH₄ hydrolysis reactor modeling and design. Molecular dynamics (MD) simulations provide the option to study the thermodynamic and transport properties of NaBH₄ aqueous solutions without hindering hydrogen bubble formation. In this work, a new force field is developed for BH₄^-, namely, the Delft force field of BH₄^- (DFF/BH₄^-), which, combined with additional force fields, can accurately describe experimental densities and viscosities of 0 to 5 m (mol salt/kg water) NaBH₄, 0 to 3 m NaB(OH)₄, and 1 m NaOH aqueous solutions at 295 K within 1.8% and 10.8% maximum deviation, respectively. Empirical fitting correlations are created for densities, viscosities, and self-diffusivities obtained from the MD simulations of 0 to 5 m NaBH₄, 0 to 5 m NaB(OH)₄, and 0 to 1 m NaOH aqueous solutions at 295-363 K for NaBH₄ hydrolysis reactor modeling and design purposes.

We compare the influence of tangential (shear) and normal (compressive) stress events on the mechanochemical regeneration of sodium borohydride NaBH₄ from hydrated sodium metaborate [Figure presented] and magnesium hydride MgH₂. Discrete element method (DEM) mechanical descriptors are used to design experiments that either maintain the mill at a constant rotational speed or maintain a constant total dissipation power, thereby separating stress distribution from net power input. Under constant power operation, a tangential rich regime achieves a record 94% conversion yield with 37.5% shorter milling time, 40% lower ball-to-powder ratio, and 34% reduced speed. However, this high yield requires such a substantial power consumption that the converted mass per Watt drops to only 0.090 gW⁻¹, below both balanced (0.113 gW⁻¹) and normal-bias (0.108 gW⁻¹) cases. By contrast, a tangential bias at half the power (3 W) still delivers 84% yield and peaks at 0.185 gW⁻¹, illustrating the often disregarded trade-off between absolute conversion and energetic productivity in mechanochemistry. Specific yield (conversion per Watt) likewise peaks at 0.28 W⁻¹ and declines linearly with fill ratio (R²>0.99). Mechanochemical energy leverage analysis reveals that, at most, 1.7–3.7% of input mechanical work is theoretically recoverable on an enthalpy basis, 2.1–4.4% on a Gibbs free energy basis, and 4–8.7% when considering the fuel value of all available hydrogen. Our mill-agnostic framework provides a transferable blueprint for cross-platform optimization of mechanochemical processes.

Hydrogen carriers are attractive alternative fuels for the shipping sector. They are zero-emission, have high energy densities, and are safe, available, and easy to handle. Sodium borohydride, potassium borohydride, dibenzyltoluene, n-ethylcarbazole, and ammoniaborane are hydrogen carriers with high theoretical energy densities. The energy density is paramount to implementing hydrogen carriers as a high energy density enables compact and lightweight storage. The effective energy density depends on integrating heat and masses with energy converters. This combination defines the energy efficiency and, thus, the energy density of the system. This paper addresses the effective energy density of the hydrogen carriers, including the dehydrogenation process. Using a 0D model, we combined the five carriers with two types of fuel cells, namely proton exchange membrane (PEM) and solid oxide fuel cells (SOFC), an internal combustion engine and a gas turbine. N-ethylcarbazole and dibenzyltoluene offer medium energy densities, reaching almost 4 MJ/kg. However, the effective energy density of sodium borohydride and ammoniaborane is very high, up to 15 MJ/kg, including the energy converter. This is similar to the energy density of marine diesel oil combined with an internal combustion engine. Thus, we conclude hydrogen carriers are alternative fuels that deserve more attention because of their strong potential to make shipping zero-emission.