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Solar-to-fuel direct conversion devices are a key component to realize a full transition to a renewable-energy based chemistry and energy, but their limits and possibilities are still under large debate. In this review article, we focus on the current density as a fundamental figure of merit to analyse these aspects and to compare different device configurations and types of solar fuels produced from small molecules such as H$_2$O, CO$_2$ and N$_2$. Devices with physical separation of the anodic and cathodic zones, photoelectrochemical-type (PEC) or with a photovoltaic element integrated in an electrochemical cell (PV/EC), are analysed. The physico-chemical mechanisms involved in device operation that affect the current density and relations with device architecture are first discussed. Aspects relevant to device design in relation to practical use are also commented on. Then discussion is moved towards the relevance of these aspects to compare the behaviour in the state-of-the-art of the conversion of these small molecules, with focus on solar fuels from H$_2$O, CO$_2$ and N$_2$ conversion, highligthing the gaps and perspectives of such technologies. The still significant lack of crucial data, notwithstanding the extensive literature on the topic, has to be remarked on, particularly in terms of the need to operate these cells in conjunction with sun concentration (in the 50-100 sun range) which emerges as the necessary direction from this analysis, with consequent aspects in terms of cell and materials design to operate in these conditions. The work provides a guide for the optimisation of the investigated technology and the fixing of their practical limits for large-scale applications.

Nanocarbons are an important class of energy materials and one relevant application is for the nitrogen reduction reaction, i.e., the direct synthesis of NH3 from N2 and H2O via photo- and electrocatalytic approaches. Ammonia is also a valuable energy or hydrogen vector. This perspective paper analyses developments in the field, limiting discussion to nanocarbon-based electrodes. These aspects are discussed: i) active sites related to charge density differences on C atoms associated to defects/strains, ii) doping with heteroatoms, iii) introduction of isolated metal ions, iv) creation and in situ dynamics of metal oxide(hydroxide)/nanocarbon boundaries, and v) nanocarbon characteristics to control the interface. Discussion is focused on the performances and mechanistic aspects. Aim is not a systematic state-of-the-art report but to highlight the need to use a different perspective in studying this challenging reaction by using selected papers. Notwithstanding the large differences in the proposed nature of the active sites, fall all within a restricted range of performances, far from the targets. A holistic approach is emphasized to make a breakthrough advance.

The prospects, needs and limits in current approaches in catalysis to accelerate the transition to e-chemistry, where this term indicates a fossil fuel-free chemical production, are discussed. It is suggested that e-chemistry is a necessary element of the transformation to meet the targets of net zero emissions by year 2050 and that this conversion from the current petrochemistry is feasible. However, the acceleration of the development of catalytic technologies based on the use of renewable energy sources (indicated as reactive catalysis) is necessary, evidencing that these are part of a system of changes and thus should be assessed from this perspective. However, it is perceived that the current studies in the area are not properly addressing the needs to develop the catalytic technologies required for e-chemistry, presenting a series of relevant aspects and directions in which research should be focused to develop the framework system transformation necessary to implement e-chemistry.

This concept review paper analyses the possibility to develop a technical solution for the challenging reaction of CO$_2$ splitting to carbon and O$_2$ (CO$_2$tC). This is a dream reaction in the area of addressing climate change and greenhouse gas emissions. There are quite limited literature indications that were reviewed, with an analysis also of the limits and perspectives. Not-thermal plasma, in combination with catalysis, is one of the possibilities to promote this reaction. Current studies on plasma-assisted CO$_2$ splitting are limited mainly to the formation of CO. By combining data on the status of current studies on plasma CO$_2$ splitting with those on CO$_2$tC it is possible to propose a tentative novel approach. This is a conceptual idea presented and discussed as vehicle to stimulate research in the area, including proposing alternative possibilities. The challenges of this concept and reaction are discussed.