328 research outputs found
On the role of microkinetic network structure in the interplay between oxygen evolution reaction and catalyst dissolution
Transient hydrogen crossover in dynamically operated PEM water electrolysis cells - A model-based analysis
Electrochemical evaluation of the de-/re-activation of oxygen evolving Ir oxide
Understanding the influence of dynamic and stationary polarization on the deactivation of state-of-the-art IrOx catalysts is imperative for the design and operation of robust and efficient proton exchange membrane water electrolyzers. In this work, the deactivation and activity regeneration of a commercial IrOx catalyst investigated under potentiodynamic and potentiostatic conditions in acidic media by means of rotating disk electrode and electrogravimetry. Systematic electrochemical protocols were designed to decouple reversible from irreversible activity losses. Cyclic voltammetry provided a metric of the active surface area and traced the charge growth under different oxygen evolution reaction conditions. A direct logt dependent charge growth is observed, accompanied by the same fractional kinetic activity decay under potentiodynamic conditions. The loss is essentially recoverable after electrochemical reductive treatment, however at the expense of mild material dissolution. In contrast, extended potentiostatic operation induced irreversible intrinsic degradation after a critical time (0.5-1 h), accompanied by stability enhancement. This irreversible deactivation attributed to a gradual transformation of the hydrated IrOx to a dehydrated condensed oxide. Our results suggest that Ir dissolution during the regenerative treatment is not prohibitive, as long as the low potential modulations are not frequent
Optimal Inspection and Maintenance Planning for Deteriorating Structural Components through Dynamic Bayesian Networks and Markov Decision Processes
Civil and maritime engineering systems, among others, from bridges to
offshore platforms and wind turbines, must be efficiently managed as they are
exposed to deterioration mechanisms throughout their operational life, such as
fatigue or corrosion. Identifying optimal inspection and maintenance policies
demands the solution of a complex sequential decision-making problem under
uncertainty, with the main objective of efficiently controlling the risk
associated with structural failures. Addressing this complexity, risk-based
inspection planning methodologies, supported often by dynamic Bayesian
networks, evaluate a set of pre-defined heuristic decision rules to reasonably
simplify the decision problem. However, the resulting policies may be
compromised by the limited space considered in the definition of the decision
rules. Avoiding this limitation, Partially Observable Markov Decision Processes
(POMDPs) provide a principled mathematical methodology for stochastic optimal
control under uncertain action outcomes and observations, in which the optimal
actions are prescribed as a function of the entire, dynamically updated, state
probability distribution. In this paper, we combine dynamic Bayesian networks
with POMDPs in a joint framework for optimal inspection and maintenance
planning, and we provide the formulation for developing both infinite and
finite horizon POMDPs in a structural reliability context. The proposed
methodology is implemented and tested for the case of a structural component
subject to fatigue deterioration, demonstrating the capability of
state-of-the-art point-based POMDP solvers for solving the underlying planning
optimization problem. Within the numerical experiments, POMDP and
heuristic-based policies are thoroughly compared, and results showcase that
POMDPs achieve substantially lower costs as compared to their counterparts,
even for traditional problem settings
Multicritical Points and Crossover Mediating the Strong Violation of Universality: Wang-Landau Determinations in the Random-Bond Blume-Capel model
The effects of bond randomness on the phase diagram and critical behavior of
the square lattice ferromagnetic Blume-Capel model are discussed. The system is
studied in both the pure and disordered versions by the same efficient
two-stage Wang-Landau method for many values of the crystal field, restricted
here in the second-order phase transition regime of the pure model. For the
random-bond version several disorder strengths are considered. We present phase
diagram points of both pure and random versions and for a particular disorder
strength we locate the emergence of the enhancement of ferromagnetic order
observed in an earlier study in the ex-first-order regime. The critical
properties of the pure model are contrasted and compared to those of the random
model. Accepting, for the weak random version, the assumption of the double
logarithmic scenario for the specific heat we attempt to estimate the range of
universality between the pure and random-bond models. The behavior of the
strong disorder regime is also discussed and a rather complex and yet not fully
understood behavior is observed. It is pointed out that this complexity is
related to the ground-state structure of the random-bond version.Comment: 12 pages, 11 figures, submitted for publicatio
Universality of the Ising and the S=1 model on Archimedean lattices: A Monte Carlo determination
The Ising model S=1/2 and the S=1 model are studied by efficient Monte Carlo
schemes on the (3,4,6,4) and the (3,3,3,3,6) Archimedean lattices. The
algorithms used, a hybrid Metropolis-Wolff algorithm and a parallel tempering
protocol, are briefly described and compared with the simple Metropolis
algorithm. Accurate Monte Carlo data are produced at the exact critical
temperatures of the Ising model for these lattices. Their finite-size analysis
provide, with high accuracy, all critical exponents which, as expected, are the
same with the well known 2d Ising model exact values. A detailed finite-size
scaling analysis of our Monte Carlo data for the S=1 model on the same lattices
provides very clear evidence that this model obeys, also very well, the 2d
Ising model critical exponents. As a result, we find that recent Monte Carlo
simulations and attempts to define effective dimensionality for the S=1 model
on these lattices are misleading. Accurate estimates are obtained for the
critical amplitudes of the logarithmic expansions of the specific heat for both
models on the two Archimedean lattices.Comment: 9 pages, 11 figure
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