Coherence of interacting oscillating entities has importance in biological, chemical, and physical systems. We report experiments on populations of chemical oscillators and verify a 25-year-old theory of Kuramoto that predicts that global coupling in a set of smooth limit-cycle oscillators with different frequencies produces a phase transition in which some of the elements synchronize. Both the critical point and the predicted dependence of order on coupling are seen in the experiments. We extend the studies both to relaxation and to chaotic oscillators and characterize the quantitative similarities and differences among the types of systems.

The engineering and economic feasibility of large-scale electrochemical reduction of carbon dioxide to formate salts and formic acid is the focus of this Full Paper. In our study we investigated the long-term performance of tin and other proprietary catalysts in the reduction of carbon dioxide to formate/formic acid at a gas/solid/liquid interface, using a flow-through reactor. The overall economics and energy consumption of the process are evaluated through a value chain analysis. The sensitivity of the net present value of the process to various process parameters is examined.

We show that mutual entrainment in interacting oscillators can be characterized using phase models that are developed from direct experiments with a single oscillator. The models are used to predict order-disorder transitions in populations and the dependence of order on system parameters; the description is verified in independent experiments in sets of chemical oscillators. The experiment-based model properly describes in-phase and antiphase mutual entrainment with positive and negative interactions in small sets as well as dynamical clustering in populations of oscillators.

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The ternary semiconductors Cu2SnX3 (X=S, Se) are found frequently as secondary phases in synthesized Cu2ZnSnS4 and Cu2ZnSnSe4 samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their structural and electronic properties as calculated using a first-principles approach. We find that: (i) the diverse range of crystal structures such as the monoclinic, cubic and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coordination. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu3Sn and Cu2Sn2 clusters around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low energy compounds Cu2SnS3 and Cu2SnSe3 should be in the range of 0.8-0.9 eV and 0.4 eV respectively, and are weakly dependent on the long-range structural order. A direct analogy is drawn with the ordered vacancy compounds found in the Cu(In, Ga)Se2 (CIGS) solar cell absorbers.

Experiments on the nontrivial collective dynamics and phase synchronization of populations of nonidentical chaotic electrochemical oscillators are presented. Without added coupling no deviation from the law of large numbers is observed. Deviations do arise with weak global or short-range coupling; large, irregular, and periodic mean field oscillations occur along with (partial) phase synchronization.

Amplitude death was observed in experiments with two coupled periodic electrochemical oscillators without time delay. Simulation results confirmed that the observed amplitude death was obtained via a Hopf bifurcation. The two oscillators must have a minimum discrepancy and both be near their individual Hopf bifurcations for amplitude death to occur. Phase drift (coexisting with unstable asymmetric phase-locked solutions), amplitude death, and in-phase synchronization were observed in both the experiments and simulations as coupling strength was increased. In addition, the simulations showed that a stable asymmetric phase-locked solution exists between the phase drift and amplitude death regions.

The application of electrocatalysts used at high cathodic overpotentials for the electrochemical reduction of pollutant species such as CO 2 has revealed a lack of understanding of the cathodic degradation mechanisms of those materials. Pure Sn is one of the most relevant candidate materials mainly because of its high selectivity for the reduction of CO 2 to formic acid and formate salts. Degradation of the electrocatalyst can arise from a combination of cathodic polarization and induced changes to the surface by CO 2 reduction products. In this study, the cathodic degradation mechanisms of pure Sn were studied as a function of rotation rate, time, current density, electrolyte concentration, grain size, and orientation in a nitrogen-saturated atmosphere using a rotating disk electrode. Several degradation morphologies were observed, but three were dominant. In the first type, electrochemical alterations of grains with specific orientations produced substantial weight changes, both losses and gains. The second type resulted in an alkali-rich deposit that had a high coverage but produced small weight changes. The third type consisted of carbon-rich stains that typically had a small coverage.