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In the short term, quite some expectations are put worlwide on the contribution of green hydrogen, i.e. hydrogen produced with renewable electricity from water electrolysis, to reach the decarbonisation objectives. Although there is not a lack of ideas and initiatives on different power and volume scales, an often cited side-comment (or excuse) is that green hydrogen is still too expensive. In this contribution, we will demonstrate that this is not necessarily the case if we take as a fair point of comparison the principle of fossil parity. The latter means that the cost of green hydrogen is either equal to the cost (or price) of fossil SMR-based hydrogen (typically for industrial use), or that its cost as a fuel is similar to the cost of fossil-based fuels (e.g. in €/km for mobility applications). The most straightforward way to arrive at such fossil parity is to encourage H2 consumers also to become their own H2 producers. In that way, they can use the in-house produced hydrogen directly at its intrinsic production cost, rather than at a (volatile) market price. Indeed, one of the major advantages of green electrolytic hydrogen is that it can in may cases be produced inhouse, at a scale which corresponds to the consumer's own needs. The only additional requirement is that the green hydrogen consumer also becomes owner of its own renewable electricity production capacity. This then allows to guarantee a stable electricity cost (and hence also green H2 cost), directly related to the one-shot investment cost of the renewable electricity device, independent of any market flucuations. If these 2 conditions are met, it can be easily demonstrated that for specific market segments with an upfront guaranteed hydrogen demand, fossil parity is already reached today at the scale of a few MW.

Nanoindentation tests performed on nanocrystalline palladium films subjected to hydriding/dehydriding cycles demonstrate a significant softening when compared to the as-received material. The origin of this softening is unraveled by combining in situ TEM nanomechanical testing with automated crystal orientation mapping in TEM and high resolution TEM. The softening is attributed to the presence of a high density of stacking faults and of Shockley partial dislocations after hydrogen loading. The hydrogen induced defects affect the elementary plasticity mechanisms and the mechanical response by acting as preferential sites for twinning/detwinning during deformation. These results are analyzed and compared to previous experimental and simulation works in the literature. This study provides new insights into the effect of hydrogen on the atomistic deformation and cracking mechanisms as well as on the mechanical properties of nanocrystalline thin films and membranes.

Different magnetron sputtering‐based deposition methods of nickel oxide SiO₂‐passivated Si surfaces are compared. Results highlight that the presence of oxygen in the deposition chamber during reactive sputtering drastically affects the Si/SiO₂ interface. An alternative method for the preparation of NiO is the sputtering of metallic nickel in oxygen‐free atmosphere followed by a post oxidation of the deposited layer in an oxygen atmosphere without plasma exposition is proposed. This method is introduced as metal layer oxidation (MLO). Using this technique, the barrier height on n‐type silicon increases from ≈0.4 eV for reactively sputtered NiO to more than 0.6 eV for the MLO method. In situ photoelectron spectroscopy evidences the formation of an extra electronic state when NiO is reactively sputtered, which is assigned to the intense oxygen ion bombardment of the Si/SiO₂ surface during the process. This extra‐electronic state pins the silicon energy bands in an undesirable position. The extra‐electronic state is associated with oxygen interstitial in the SiO₂ implanted during reactive sputtering.

To further reduce the capital expenditure of alkaline water electrolyzers, an improvement in power density can still be achieved through process intensification. The change from traditional gap-cells to zero-gap cells has already been proven to be very promising in this respect. In the zero-gap design, macro-porous 3D structures are typically being used as electrodes. Here, pure nickel foams with different pore sizes in the range 450 – 3000 µm have been studied under well-controlled electrolyte flow conditions. To this end, a dedicated flow cell has first been constructed that allows for imposing the same relatively high flow rates on the order of 1 L/min that are typically encountered under industrial conditions. Our specific cell design was shown by computational fluid dynamic simulations to be able to homogenize the flow field before entering the electrodes. This is absolutely mandatory to reliably evaluate the intrinsic bubble removal efficiency of the differently sized foams. We then demonstrate that the effect of pore size and the electrochemical performance can be rationalized by considering the cell voltage as a function of electrochemical current density, i.e. the current divided by the theoretically available electrochemical surface area (ECSA) rather than by the projected electrode area. Based on this analysis, we were also able to quantify the bubble removal efficiency by estimating the effective fraction of the ECSA that is not impeded by bubble coverage.

Although largely studied, contradictory results on nickel oxide (NiO) properties can be found in the literature. We herein propose a comprehensive study that aims at leveling contradictions related to NiO materials with a focus on its conductivity, surface properties, and the intrinsic charge defects compensation mechanism with regards to the conditions preparation. The experiments were performed by in situ photoelectron spectroscopy, electron energy loss spectroscopy, and optical as well as electrical measurements on polycrystalline NiO thin films prepared under various preparation conditions by reactive sputtering. The results show that surface and bulk properties were strongly related to the deposition temperature with in particular the observation of Fermi level pinning, high work function, and unstable oxygen-rich grain boundaries for the thin films produced at room temperature but not at high temperature (>200 °C). Finally, this study provides substantial information about surface and bulk NiO properties enabling to unveil the origin of the high electrical conductivity of room temperature NiO thin films and also for supporting a general electronic charge compensation mechanism of intrinsic defects according to the deposition temperature.

A zero-gap cell with porous electrodes is a promising configuration for alkaline water electrolysis. However, gas evacuation becomes a challenge in that case,as bubbles can get trapped within the electrode’s 3D structure. This work considers a number of 3D printed electrode geometries with so-called triply periodic minimal surfaces (TPMS). The latter is a mathematically defined structure that repeats itself in three dimensions with zero mean curvature, and can therefore be expected to be particularly well-suited to enhance gas evacuation. Another advantage as compared to other state-of-the-art 3D electrodes like foams or felts lies in the fact that their porosity, which determines the available surface area, and their pore size or flow channel dimensions, which determines the degree of bubble entrapment, can be varied independently. By a combined experimental and modeling approach, this work then identifies the structural parameters that direct the performance of such 3D printed TPMS geometries toward enhanced gas evacuation. It is demonstrated that an optimal combination of these parameters allows, under a forced electrolyte flow, for a reduction in cell overpotential of more than 20%. This indicates that efforts in optimizing the electrode’s geometry can give a similar electrochemical performance enhancement as optimizing its electro-catalytic composition.

Hydrogen and syngas production from natural gas is the fundamental process for ammonia production, methanol synthesis, and the petrochemical industry. However, the most widely deployed technology for hydrogen production technology, i.e., steam methane reforming (SMR), has several drawbacks including low energy efficiency and CO2 emission from its furnace that requires CO2 capture under the carbon-constrained scenario. Autothermal chemical looping reforming (aCLR) is proposed to address these two problems. The aCLR has been successfully tested from bench-scale semi-continuously operated fluidized beds to 140kWth continuously operated dual fluidized bed facilities. However, environmental concerns arising from the use of Ni-based oxygen carriers (OC). On the other hand, other investigated OCs, e.g., Cu- and Fe-based materials have lower reactivity towards methane reforming, which leads to low CH4 conversion. Modeling and simulation accounting for detailed reaction mechanisms and reactor hydrodynamics plays important role in designing, understanding, upscaling, and eventually optimizing the aCLR. Limited by the computational power, scale-bridging strategies are required to efficiently evaluate the reactor performance while understanding the relative importance of different phenomena, e.g., intrinsic reaction kinetics, gas-solid mass heat, mass transfer, and the residence time of solids in the bubbling fluidized bed (BFB). This thesis is aimed at investigating the feasibility of a more environmentally compatible aCLR. This process considers using the environmentally friendly Fe-based OC for transferring oxygen between the two reactors and PGM-based SMR catalyst for enhanced methane conversion, which is called catalyst-enhanced autothermal chemical looping reforming (CE-aCLR). CE-aCLR is studied by experimentally investigating the reaction kinetics of the potential Fe-based oxygen carrier and the PGM-based SMR catalysts, and performing modeling and simulation of the aCLR, (FSA - Sciences de l'ingénieur) -- UCL, 2022

The cement industry, which accounts for 7-8% of worldwide CO2 emissions, is essential to decarbonize but challenging especially due to its high process emissions, high temperature heat requirements and low maturity of decarbonization solutions. A new electrochemical path to produce calcium hydroxide, a precursor of cement, has been proposed recently. This technology is based on a new design of a water electrolyser working in tri-phase environment. It has the potential to electrify cement production while at the same time producing valuable gases, but is still at a very early stage of development (TRL-3). In this paper, lab-scale experiments in a 200mL cell were performed to have a better understanding of the electrochemical system. The development of a pH gradient between the anode and the cathode has been quantified for different electric charges applied. Due to acidification of the anode side, the dissolution of calcium carbonate is enhanced. A solubility ten times higher than at neutral pH has been observed. At the same time, the migration of Ca2+ and OH- ions to respectively the cathodic and anodic side has been demonstrated as well, thereby opening the way for an electrochemically-enhanced Ca(OH)2 precipitation.

The interaction of water with oriented NiO films is studied by using a combination of photoelectron spectroscopy with in situ sample preparation and electrochemical measurements in the stability window of water. In contrast to NiO(100), room temperature water exposure induces a downward band bending on the (110)- and (111)-oriented films, indicating a more positive surface charge. The surface charge is assigned to a dissociative adsorption of water accompanied by a removal of oxygen, which corresponds to a preferential adsorption of protons. Photoelectron spectroscopy suggests that the nonstoichiometric adsorption is related to the presence of adsorbed oxygen species prior to water exposure. For the NiO(110) surface, the stability of adsorbed bridging oxygen dimers is confirmed by density functional theory calculations. The preferential adsorption of protons, which requires interaction of two water molecules with the oxygen dimers, explains that water acts as an electron donor on many oxide surfaces. The different adsorption behavior is consistent with a lower electrochemical activity of the (110)- and (111)-oriented surfaces toward hydrogen adsorption due to an effective reduction of Lewis base sites.

Hydrogen production vs. demand Hydrogen demand is expected to increase significantly over the coming decades. According to a recent study from the International Energy Agency (IEA) [1], a 4-5 fold increase up to 300 Mt/yr is projected for reaching climate neutrality by 2050, further increasing to more than 500 Mt/yr by 2070. By then, the major use will shift from industry to transport, with 30% being dedicated to direct use in fuel-cell driven trucks, busses and trains, 20% to liquid hydro-carbon production as a synthetic fuel for aviation, and 10% to the production of ammonia as a shipping fuel. From 2050 onwards, a small fraction of hydrogen gas will be used in buildings and in the power sector as well. By then, its direct use in refineries will have completely disappeared.

Green hydrogen production by water electrolysis constitutes a fundamental process for the transition to the hydrogen economy. While alkaline water electrolysis is considered a mature technology to this end, recent work has shown that we can achieve further process intensification with forced electrolyte flow through macro-porous 3D electrodes. The objective of this work is to simulate and analyze electrolyte flow through an industrial-scale pilot and through lab-scale 3D printed electrodes using Computational Fluid Dynamics (CFD). Initial results show that our current pilot cell design is inadequate for forced flow and offer insights in order to design a cell structure that is more suited for gas removal.

The current ammonia production plants - Steam Methane Reforming (SMR) and Haber-Bosch (HB) - are responsible for 1.2% of global CO2 emissions. As more sectors are shifting to electrification, so does the NH3 industry as shown by the amount of studies focusing on electrolysers and Pressure Swing Adsorbers (PSA) to replace SMR. Haber-Bosch itself is being questioned as a new alternative technology is emerging: Chemical Looping (CL). Such transformations imply changes to the current process. This work assesses such implications by comparing 2 alternative scenarios with the conventional case. In terms of emissions, the electrification is clearly benefic with a cut of 74-95% when using wind energy. The replacement of SMR by electrolysers and PSA also improves the energy efficiency, as >60% of the energy consumption in conventional plants originates from the compressors and heat integration. Finally, the combination of Alkaline Electrolysers (AE) and CL is determined as the most suitable for small-scall flexible and economical installations.

Initial laboratory results using 3-D electrodes with forced electrolyte flow indicate a significant process intensification potential. This work aims to validate this process intensification on a larger scale, using a pilot-scale alkaline water electrolyser with a 6 kW cell stack working in the ranges 5-10 bar, 20-70°C and with 30% KOH. This pilot plant is also representative of industrial electrolysers in terms of gas collection, cell stacking and current distribution. In addition, it has been modified to allow for forced electrolyte flow using KOH compatible pumps. The pilot system was first validated using standard 2-D electrodes and showed state-of-the-art results. Various cell stacks with 3-D electrodes were then tested under conditions of forced flow, showing interesting intensification potential. However, obstacles in terms of stack design still limit the flow effect. The latter are currently being tackled using CFD simulation tools.

Hydrogen produced from renewable electricity through Power-to-Hydrogen can facilitate the integration of high levels of variable renewable electricity into the energy system. An electrolyser is a device that splits water into hydrogen and oxygen using electricity. When electricity is produced from renewable energy sources, electrolytic hydrogen can be considered to be green. Key green hydrogen production technologies, mostly PEM and alkaline electrolysers, are still further maturing, both in technical (efficiency), economical (CAPEX) and durability (lifetime) performance. Nonetheless, we will show in this contribution how fossil parity for green hydrogen, i.e. a Total Cost of Ownership (TCO) similar to grey H2 coming from CO2 intensive SMR processes, can already be achieved today. Moreover, this can be realised at a scale which corresponds to the basic units of renewable electricity generation, i.e. a few MW.

Water electrolysis plays a key role in the energy transition as it can be used to store energy by the conversion of electrical energy from renewable and intermittent sources to chemical energy in the form of hydrogen gas. Currently, the most mature technology is alkaline water electrolysis. The benchmark electrolyzer configuration is a zero-gap cell with porous electrodes. Gas evacuation is a challenge when using this kind of electrode, as bubbles can get trapped within the 3-D structure. This work aims to identify parameters that can help the construction of enhanced gas evacuation periodic electrodes. It was found that electrodes that direct the flow and present a lower pressure drop are better in terms of gas removal.

The application of pulses is a promising method to enhance the efficiency of water electrolysis. In this review, we first present a historical overview with several seminal references on the topic. Then, we give an overview of the last researches. Possible reasons for the efficiency improvement are outlined, including an increase in reactant concentration at the electrode surface, the improvement of bubble detachment from the electrode, and the perturbation of the electrical double layer.