Normand Mousseau

Low-carbon hydrogen is increasingly considered a key option for decarbonizing hard-to-abate sectors, yet most transition analyses are conducted at national or global scales, with limited attention to metropolitan energy systems where climate policies are implemented. This paper examines how hydrogen can contribute to urban decarbonization under multi-level climate governance by extending the Energy Technology Environment Model (ETEM) to integrate full hydrogen supply chains and applying a structured downscaling approach to translate national and provincial data to the metropolitan scale. The framework is applied to the Montréal Metropolitan Community (CMM), a member of the C40 Cities network, to evaluate hydrogen deployment under different decarbonization scenarios. Results show that under full carbon neutrality constraints, hydrogen reaches approximately 7% of total final energy consumption by 2050, with total production exceeding 33 PJ, yet uptake remains concentrated in heavy industry and hard-to-electrify transport, underscoring hydrogen's role as a targeted complement to electrification rather than a dominant energy carrier. As climate constraints tighten, hydrogen production shifts toward electrolysis and CCS-enabled pathways, with deployment increasing significantly after 2040. Carbon neutrality further requires a minimum direct air capture capacity of 1 MtCO₂/year to offset residual emissions from sectors where abatement through fuel switching alone is technically infeasible. The proposed modeling framework is designed to be adaptable to other metropolitan regions and C40 cities with similar data infrastructure, offering a structured approach for assessing hydrogen integration in urban energy transitions.

Kinetic Monte Carlo (KMC) methods are commonly used to simulate the microstructure evolution of metals under irradiation due to their ability to generate the random walks underlying defect-mediated diffusion processes at the atomic scale. However, the range of applicability of KMC methods is severely limited by the kinetic trapping of the simulated trajectories within low energy basins presenting small intra-basin barriers. This results in dramatically reducing the efficiency of the classical KMC algorithm. Kinetic trapping can be alleviated by implementing non-local jumps relying on the theory of absorbing Markov chains. A factorization of an auxiliary absorbing transition matrix then allows to generate escaping paths and first-passage times out of trapping basins. Although the speed-up can be of several orders of magnitudes, this is sometimes not enough for very long-term prediction. We must then turn to homogenized rate-equation formulation of the problem. Usually solved deterministically, the corresponding large ordinary differential equation system often suffers from the curse of dimensionality. Dedicated Monte Carlo schemes can simulate the coarse-grained rate equations based on a chemical master equation. Finally, we show the relevance of relaxing the rigid-lattice assumption in the calculation of the free energy barriers and attempt frequencies to capture elastic effects that are important for certain systems, such as high entropy alloys or other concentrated alloys such as austenitic stainless steels. A new activation-relaxation technique combining barriers and prefactors on-the-fly calculations can be used for this purpose in kinetic Monte Carlo studies of slow diffusion processes.

Hydrogen embrittlement (HE) poses a significant challenge to the mechanical integrity of iron and its alloys. This study explores the influence of hydrogen atoms on two distinct grain boundaries (GBs), Σ 37 and Σ 3 , in body-centered-cubic (BCC) iron. Using the kinetic activation relaxation technique , an off-lattice kinetic Monte Carlo approach with an EAM-based potential, extensive catalogs of activated events for atoms in both H-free and H-saturated GBs were generated. Studying the diffusion of H, we find that, for these systems, while GB is energetically favorable for H, this element diffuses more slowly at the GBs than in the bulk. The results further indicate that the Σ 3 GB exhibits higher stability in its pure form compared to the Σ 37 GB, with notable differences in energy barriers and diffusion behaviors. Moreover, with detailed information about the evolution landscape around the GB, we find that the saturation of a GB with hydrogen both stabilizes the GB by shifting barriers associated with Fe diffusion to higher energies and reducing the number of diffusion events. For the Σ 37 GB, the presence of hydrogen causes elastic deformation, affecting the diffusion of Fe atoms both at the GB and in adjacent positions. This results in new diffusion pathways but with higher diffusion barriers, unlike for the Σ 3 GB. These results indicate that the presence of hydrogen rigidifies the direct GB interface layers while allowing more atoms to be active for the Σ 37 GB. This provides a microscopic basis to support the existence of competing mechanisms compatible with either plasticity (such as hydrogen enhanced localized plasticity—HELP) or energy-dominated (hydrogen enhanced decohesion mechanism—HEDE) embrittlement, with the relative importance of these mechanisms determined by the local geometry of the GBs.

Mass transport in bulk spinel ferrites NiFe2O4 is studied computationally using the kinetic activation–relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm. Diffusion mechanisms—difficult to observe with molecular dynamics—are described by k-ART. Point defects are assumed to be responsible for ionic diffusion; thus, both cation and anion defects are investigated. This work focuses on vacancies and interstitials by comparing their properties with two Buckingham potential parameterizations: one with nominal charges and the other with partial charges. Both potentials are corrected at short distances, thus allowing interstitial diffusion and avoiding the catastrophic infinite energies appearing with Buckingham at short distances. The energy landscape along different pathways is described in detail. Both potentials predict the same mechanisms but different migration energies. Mechanisms by which a normal spinel is transformed to an inverse spinel via cation diffusion are unveiled, and diffusion coefficients are predicted. We find that interstitial Ni diffusion involves the movement of two Ni ions and that O interstitials trigger a collective diffusion of O ions, while an O vacancy diffuses by an O ion moving to the center of a cuboctahedron.