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Aqueous zinc (Zn) batteries have attracted global attention for energy storage. Despite significant progress in advancing Zn anode materials, there has been little progress in cathodes. The predominant cathodes working with Zn2+/H+ intercalation, however, exhibit drawbacks, including a high Zn2+ diffusion energy barrier, pH fluctuation(s) and limited reproducibility. Beyond Zn2+ intercalation, alternative working principles have been reported that broaden cathode options, including conversion, hybrid, anion insertion and deposition/dissolution. In this review, we report a critical assessment of non-intercalation-type cathode materials in aqueous Zn batteries, and identify strengths and weaknesses of these cathodes in small-scale batteries, together with current strategies to boost material performance. We assess the technical gap(s) in transitioning these cathodes from laboratory-scale research to industrial-scale battery applications. We conclude that S, I2 and Br2 electrodes exhibit practically promising commercial prospects, and future research is directed to optimizing cathodes. Findings will be useful for researchers and manufacturers in advancing cathodes for aqueous Zn batteries beyond Zn2+ intercalation. [ABSTRACT FROM AUTHOR]

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials. [ABSTRACT FROM AUTHOR]

A looming global scarcity of the nutrient phosphorus (P) requires solutions to prevent losses via recycling and recovery. One approach is to address this practically in lunar space. Here we propose for the first time a 'circular' solution that uses P-fertiliser fed to lettuce-hydroponics to grow at scale to feed six (6) astronauts. The P-feed is created via minerals leaching from P-containing moon crust. Phosphorus in the non-edible part of the lettuce is recycled via mineralisation of the roots to ash. The ash permits the recovery also of potassium (K). Judicious experimentation to determine mineralisation at various temperatures (250 to 750 °C) confirmed that a 'low' treatment at 250 °C was optimal for P recovery supported via leaching with an organic acid. The leached solution contained, respectively, 117 mg P L−1 and 1083 mg K L−1. Significantly, for the P-bioprocess, a Material Circular Indicator (MCI) score was computed to be 0.59, a value similar to processes in 'Circular Mars Biofactories', and a factor of 2.7 with respect to conventional terrestrial extraction. A holistic sustainability assessment via Life Cycle Assessment (LCA) demonstrated a global warming potential (GWP) of 0.3 kg CO2-eq. g−1 and favourable E-factor (mass ratio of waste to product) 0.02. We reliably estimate N nutrient productivity via non-thermal plasma N2-fixation in a N–P–K fertiliser and production in a moon colony, and conclude the hydroponic proposal will provide an N–P–K fertiliser of, respectively, 248, 47, and 350 mg L−1, in lunar space to grow lettuce for six astronauts. Findings will be of benefit and interest to a wide range of engineers and researchers for growing food in circular fashion and under extreme conditions. [ABSTRACT FROM AUTHOR]

Understanding the late stages of the electrochemical CO2 reduction reaction (CRR) in the formation of various C2 products provides great opportunities for fully achieving a selective CRR to target products. Here, we report, for the first time, that C2 product (including ethylene, ethane and ethanol) selectivity can be directed by the active site's oxygen affinity on a range of non-metal doped copper surfaces (Cu–X). Through theoretical evaluation of adsorption energetics of key post-C2-coupling intermediates, we demonstrate how branching reaction pathways are competing with each other and how selectivity for different products is achieved. The oxygen affinities of different active sites on Cu–X catalysts, including the dopant atom and Cu sites, are identified as descriptors for C2 product selectivity. The ethylene pathway is shown to be favored on Cu–X surfaces doped with strong oxygen affinity atoms, such as boron. Ethane selectivity is generally boosted by Cu–X, whereas ethanol generation is suppressed by non-metal doping on Cu surfaces. Bader charge analyses of electron distribution on intermediates and electronegativity analyses of non-metal dopant atoms are integrated to identify the impact of intrinsic electronic properties on adsorption behaviour, together with the resultant C2 product selectivity on Cu–X catalysts. Our findings shed light on controlling product selectivity by modulating oxygen affinity in complex electrochemical reactions. [ABSTRACT FROM AUTHOR]

The electrochemical hydrogen evolution reaction (HER) is regarded as a practical means for the production of high-purity hydrogen from abundant water. Understanding the HER mechanism is therefore crucial to the design of high performance HER catalysts. Although approaches based on activity descriptors for the acidic HER have proven to be successful in revealing the origin of activity, and in directing the synthesis of electrocatalysts, the alkaline HER mechanisms and the activity descriptors remain largely unexplained. This impedes the overall design of electrocatalysts. Here, we systematically investigate the HER mechanism and the underlying origin of catalytic activity for the HER using Pt and Ru dimer structures as model catalysts. For the acidic HER, we found that PtRu dimer on nitrogen-doped graphene displayed a hydrogen adsorption free energy of −0.07 eV with higher atomic utilization. For the alkaline HER, we propose the dissociative chemisorption energy of water (ΔEdiss) as a singular activity descriptor following the analysis of several potential activity descriptors. ΔEdiss is proposed both because it has the capacity to identify the smallest theoretical thermodynamic overpotential and because it scales linearly with the kinetic barrier. We present new insight into the mechanism route from the electronic structure (d-band center) viaΔEdiss to alkaline HER activity. We conclude that this work will be of immediate benefit to guide rational development of electrocatalysts via electronic structural engineering to regulate ΔEdiss for the alkaline HER. [ABSTRACT FROM AUTHOR]

Photocatalytic hydrogen production from water is a green and renewable path for solar fuel production. Hydrogen can be advantageously stored directly and burned without emission of deleterious CO and NOx gases. Photocatalysis therefore shows significant promise as a part solution to a sustainable and affordable energy supply in an era post-fossil fuels. Influenced by the Fujishima–Honda effect, significant advances in photocatalytic hydrogen production have occurred at the laboratory-scale. For wide adoption however, the photocatalysts will need to be made from earth-abundant materials, be stable and scalable from laboratory-to-large-scale, and have high conversion efficiency. In this regard, metal-free photocatalysts show practical promise in meeting these requirements. To foster research in materials design, here we critically review recent significant developments in metal-free photocatalysts consisting of carbon, nitrogen and phosphorus, and discuss how future large-scale hydrogen production via overall water-splitting could be accomplished economically. [ABSTRACT FROM AUTHOR]

The electroreduction of carbon dioxide (CO 2 ) and carbon monoxide (CO) to liquid alcohol is of significant research interest. This is because of a high mass-energy density, readiness for transportation and established utilization infrastructure. Current success is mainly around monohydric alcohols, such as methanol and ethanol. There exist few reports on converting CO 2 or CO to higher-valued diols such as ethylene glycol (EG; (CH 2 OH) 2 ). The challenge to producing diols lies in the requirement to retain two oxygen atoms in the compound. Here for the first time, we demonstrate that densely-arrayed Cu nanopyramids (Cu-DAN) are able to retain two oxygen atoms for hydroxyl formation. This results in selective electroreduction of CO 2 or CO to diols. Density Functional Theory (DFT) computations highlight that the unique spatial-confinement induced by Cu-DAN is crucial to selectively generating EG through a new reaction pathway. This structure promotes C-C coupling with a decreased reaction barrier. Following C-C coupling the structure facilitates EG production by (1) retaining oxygen and promoting the *COH-CHO pathway, which is a newly identified pathway toward ethylene glycol production; and, (2) suppressing the carbon-oxygen bond breaking in intermediate *CH 2 OH-CH 2 O and boosting hydrogenation to EG. Our findings will be of immediate interest to researchers in the design of highly active and selective CO 2 and CO electroreduction to diols., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)