## 1 Introduction  
Affordable and clean energy represents one of the seventeen sustainable development goals (SDGs) adopted by the 193 members of the United Nations General Assembly five years ago [1]. To ensure the achievement of the SDGs by 2030, countries have introduced their implementation in the own national development programs. These joint actions should allow facing the global energy crisis through a worldwide approach, simultaneously reaching other strictly related SDGs, e.g., the \"climate action\" (the thirteenth SDG).  
Although a recent report has revealed that the \"affordable and clean energy\" goal (seventh SDG) will not be met by the agreed date [2], there are encouraging signs that energy is becoming more sustainable and available to the global population. Actually, thanks to the impressive advances of renewable energy sources (RESs) in the electricity sector [3], access to electricity in poorer countries is accelerating.  
The sustainability of energy production systems represents a challenge in many contexts, including policy and scientific research. The large variety of initiatives undertaken in the latest years to improve efficiency in resource management has greatly boosted the field of energy production from renewable sources, gathering growing attention on electrochemical energy storage (EES) technologies and strongly promoting research in this area [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The development of cost-effective materials allowing improving power density, cyclability and round-trip efficiency is a key point both for more mature EES devices, such as redox flow batteries [15; 16; 17; 18], and for the newer, developing ones, such as post-lithium batteries [19; 20; 21; 22; 23]. Presently, research on the latter topic is greatly focused on nanostructured materials [18; 24; 25; 26; 27; 28]. Among them, electrospun nanomaterials, suitable for production at the large-scale and able to meet requirements of a wide range of EES applications [18; 27; 28], have been extensively evaluated as active components in post-lithium batteries [18; 27; 28; 29; 30; 31].  
In the scenery of post-lithium rechargeable batteries, the ones exploiting sodium-ion storage, even if not commercialized at the large scale yet, probably represent the technology with the highest level of maturity and sustainability [19; 32]. The development of highly performing anode materials is one of the barriers to still be overcome [33]. Transition metal oxides, exhibiting high theoretical reversible capacities and low cost, have broadly been evaluated as anode materials.  
Iron (III) oxide that stores sodium ions via the widely agreed Fe\({}_{2}\)O\({}_{3}\) + 6 Na\({}^{+}\) + 6 e\({}^{-}\rightleftharpoons\) 3 Na\({}_{2}\)O + 2 Fe\({}^{0}\) conversion mechanism [34; 35] has received noticeable attention due to its relevant theoretical specific capacity (1005 and 926 mAhg\({}^{-1}\) for its \(\alpha\)- and \(\gamma\)-phase, respectively [35]), large abundance, environmental friendliness and non-toxicity [35; 36; 37]. Nanostructure engineering has been proved to be a viable strategy to cope with detrimental effects of the large volume change experienced during Na\({}^{+}\) intercalation/de-intercalation [35; 38]. Incorporating tetravalent impurities in the Fe\({}_{2}\)O\({}_{3}\) lattice, a commonly adopted approach to enhance the conductivity of hematite photo-anodes [39; 40; 41], has been demonstrated to improve the electrochemical performance of nanostructured Fe\({}_{2}\)O\({}_{3}\)-based anode materials for sodium-ion batteries (SIBs) [36; 37].  
This work presents the results of a comprehensive investigation of the effects produced by germanium incorporation in the nanostructure and crystalline phase of electrospun Fe\({}_{2}\)O\({}_{3}\) nanofibers (NFs). The electrochemical activity of Fe\({}_{2}\)O\({}_{3}\):Ge NFs as active anode material in SIBs is evaluated.