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Pseudocapacitors offer a unique strategy to combine the rapid charging rates of capacitors with the high energy density of batteries, potentially offering a unique solution to energy storage challenges. Bending and twisting aromatic building blocks to form contorted aromatics have emerged as a new strategy to create organic materials with unique and tunable properties. This paper studies the union between these two concepts: molecular contortion and organic pseudocapacitors. The recent development of fully organic pseudocapacitors, including high-performing devices based on perylene diimide organic redox units, introduces the added benefit of low cost, synthetic tunability, and increased flexibility. We synthesize a series of polymers by joining perylene diimide with various linkers that incorporate a helical moiety from [4]helicene to [6]helicene into the molecular backbone. We prepare three new electroactive polymers that incorporate benzene, naphthalene, and anthracene linkers and study their pseudocapacitive performance to infer key design principles for organic pseudocapacitors. Our results show that the naphthalene linker results in the most strongly coupled redox centers and displays the highest pseudocapacitance of 292 ± 47 F/g at 0.5 A/g. To understand the pseudocapacitive behavior, we synthesized dimer model compounds to further probe the electronic structure of these materials through electronic absorption spectroscopy and first-principles calculations. Our results suggest that the identity of the aromatic linker influences the contortion between neighboring perylene diimide units, the coupling between redox centers, and their relative angles and distances. We find that competing molecular design factors must be carefully optimized to generate high-performance devices. Overall, this study provides key insights into molecular design strategies for generating high-performing organic pseudocapacitor materials. 

Despite asymmetric competition and a wide array of functional similarities, two ecologically important C₄perennial grasses,Andropogon gerardiiandSorghastrum nutans, frequently codominate areas of the mesic tallgrass prairie of the US Great Plains. A subtle difference in their vegetative reproduction strategies may play a role in preventing the exclusion ofS. nutans, the presumed weaker competitor in such regions.

WhileA. gerardiivegetative tiller densities peak in the early growing season and decline thereafter (determinate recruitment), those ofS. nutansmay continue to increase throughout the growing season (indeterminate recruitment), providing a potential avenue for recovery from more intensive early season competition. However, until now these patterns have only been informally observed in the field.

We examined the year‐to‐year consistency of growing season vegetative tiller dynamics (measured as seasonal change in tiller densities) of each grass species from an intact tallgrass prairie in Kansas – a site within the core of both species' distributions – over a period of 8 years. Then, to investigate environmental effects on these dynamics, we examined whether they differ across a Kansas landscape varying in topography, fire management regimes, and the abundances of the study species. Finally, we expanded the investigation of environmental effects on growing season tiller dynamics by observing them at the periphery of the species' distributions in central Colorado, where climatic conditions are dryer and the study species' abundances are reduced.

Synthesis. We found that the tiller densities ofA. gerardiidecline within seasons with striking consistency regardless of spatio‐temporal scale or environmental factors (topography and fire regimes). In contrast, we found the seasonal dynamics ofS. nutanstiller densities were dependent on environmental factors, with seasonal tiller density increases occurring only within the Kansas populations but not consistent between years. These observations lay the groundwork for establishing differences in tiller recruitment determinacy as a potentially important yet underappreciated mechanism for promoting coexistence and codominance among perennial plant species.

 

Redox-active two-dimensional polymers (RA-2DPs) are promising lithium battery organic cathode materials due to their regular porosities and high chemical stabilities. However, weak electrical conductivities inherent to the non-conjugated molecular motifs used thus far limit device performance and the practical relevance of these materials. We herein address this problem by developing a modular approach to construct π-conjugated RA-2DPs with a new polycyclic aromatic redox-active building block PDI-DA. Efficient imine-condensation between PDI-DA and two polyfunctional amine nodes followed by quantitative alkyl chain removal produced RA-2DPs TAPPy-PDI and TAPB-PDI as conjugated, porous, polycrystalline networks. In-plane conjugation and permanent porosity endow these materials with high electrical conductivity and high ion diffusion rates. As such, both RA-2DPs function as organic cathode materials with good rate performance and excellent cycling stability. Importantly, the improved design enables higher areal mass-loadings than were previously available, which drives a practical demonstration of TAPPy-PDI as the power source for a series of LED lights. Collectively, this investigation discloses viable synthetic methodologies and design principles for the realization of high-performance organic cathode materials. 

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g−1 at a rate of 0.5 A g−1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g−1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials. 

Species dominance and biodiversity in plant communities have received considerable attention and characterisation. However, species codominance, while often alleged, is seldom defined or quantified. Codominance is a common phenomenon and is likely to be an important driver of community structure, ecosystem function and the stability of both. Here we review the use of the term ‘codominance’ and find inconsistencies in its use, suggesting that the scientific community currently lacks a universal understanding of codominance. We address this issue by: (1) qualitatively defining codominance as mostly shared abundance that is distinctively isolated within a subset of a community, and (2) presenting a novel metric for quantifying the degree to which relative abundances are shared among a codominant subset of plant species, while also accounting for the remaining species within a plant community. Using both simulated and real‐world data, we then demonstrate the process of applying the codominance metric to compare communities and to generate a quantitatively defensible subset of species to consider codominant within a community. We show that our metric effectively distinguishes the degree of codominance between four types of grassland ecosystems as well as simulated ecosystems with varying degrees of abundance sharing among community members. Overall, we make the case that increased research focusses on the conditions under which codominance occurs and the consequences for species coexistence, community structure and ecosystem function that would considerably advance the fields of community and ecosystem ecology.

 

During the 1930s Dust Bowl drought in the central United States, species with the C₃photosynthetic pathway expanded throughout C₄-dominated grasslands. This widespread increase in C₃grasses during a decade of low rainfall and high temperatures is inconsistent with well-known traits of C₃vs. C₄pathways. Indeed, water use efficiency is generally lower, and photosynthesis is more sensitive to high temperatures in C₃than C₄species, consistent with the predominant distribution of C₃grasslands in cooler environments and at higher latitudes globally. We experimentally imposed extreme drought for 4 y in mixed C₃/C₄grasslands in Kansas and Wyoming and, similar to Dust Bowl observations, also documented three- to fivefold increases in C₃/C₄biomass ratios. To explain these paradoxical responses, we first analyzed long-term climate records to show that under nominal conditions in the central United States, C₄grasses dominate where precipitation and air temperature are strongly related (warmest months are wettest months). In contrast, C₃grasses flourish where precipitation inputs are less strongly coupled to warm temperatures. We then show that during extreme drought years, precipitation–temperature relationships weaken, and the proportion of precipitation falling during cooler months increases. This shift in precipitation seasonality provides a mechanism for C₃grasses to respond positively to multiyear drought, resolving the Dust Bowl paradox. Grasslands are globally important biomes and increasingly vulnerable to direct effects of climate extremes. Our findings highlight how extreme drought can indirectly alter precipitation seasonality and shift ecosystem phenology, affecting function in ways not predictable from key traits of C₃and C₄species.

 

Random species loss has been shown experimentally to reduce ecosystem function, sometimes more than other anthropogenic environmental changes. Yet, controversy surrounds the importance of this finding for natural systems where species loss is non‐random.

We compiled data from 16 multi‐year experiments located at a single native tallgrass prairie site. These experiments included responses to 11 anthropogenic environmental changes, as well as non‐random biodiversity loss either the removal of uncommon/rare plant species or the most common (dominant) species.

As predicted by the mass ratio hypothesis, loss of a dominant species had large impacts on productivity that were comparable to other anthropogenic drivers. In contrast, the loss of uncommon/rare species had small effects on productivity despite having the largest effects on species richness.

The anthropogenic drivers that had the largest effects on productivity nitrogen, irrigation, and fire experienced not only loss of species but also significant changes in the abundance and identity of dominant species.

Synthesis. These results suggest that mass ratio effects, rather than species loss per se, are an important determinant of ecosystem function with environmental change.