## 2 Materials and Devices
Figure 3, retrieved from the report, \"1.5 \({}^{\circ}\)C pathways for the EU27: accelerating climate action to deliver the Paris Agreement\", by Climate Analytics in partnership with CAN (Climate Action Network) Europe and AirClim [37], shows that the EU27 could reach net-zero emissions by 2040, a decade earlier than planned. The report focused on two illustrative pathways--the HighRE scenario and the PAC scenario. The HighRE scenario is a downscaled version of an integrated assessment model pathway, which relies on low-cost renewables to limit warming to 1.5 \({}^{\circ}\)C, while the PAC scenario provides a bottom-up perspective of how the EU27 + UK can reach absolute zero CO\({}_{2}\) emissions by 2040. Henceforth, this study shows that, to be 1.5 \({}^{\circ}\)C compatible, the EU27 would need to cut its domestic emissions faster than currently planned. Therefore, the 1.5 \({}^{\circ}\)C-compatible pathways assessed in this report, and filtered to meet sustainability constraints, indicated that the EU27 can feasibly:  
* Reduce its GHG emissions by between 63 and 73% below 1990 levels, excluding land use, land use change, and forestry (LULUCF). Accounting for LULUCF, this corresponds to a 66-77% reduction by 2030, relative to 1990 levels;
* Reach net-zero GHG emissions between 2040 and 2045;
* Limit the EU27's total cumulative CO\({}_{2}\) emissions to 11-22 Gd CO\({}_{2}\) from 2020 until mid-century (including LULUCF). Figure 3: The EU pathways to meet its emissions-reduction targets by 2050. Reproduced with permission from [37]. Accordingly, in these pathways, electricity provides 66-70% of the final energy in 2050. There are also strong and sustained reductions in final energy demand, which means that, by 2050, total energy demand in the EU27 could be up to 58% lower than in 2019. Overall, renewables provide 48-54% of final energy demand in 2030, rising to 92-100% of the final energy by 2050. Thus, it is demonstrated how the faster emissions reductions, through a rapid transition to an efficient energy system powered by renewable energy sources--where green H\({}_{2}\) can also be included--are critical in order for the EU27 to keep the 1.5 \({}^{\circ}\)C compatible benchmark within reach [37]. In order to meet the present and future demand for green H\({}_{2}\), hydrogen fuel cells and water electrolyzers offer an alternative and improved solution for a decarbonized energy future [16; 22; 24; 38; 39; 40; 41]. Invented in 1839, the hydrogen fuel cells allow the generation of electrical energy with high efficiency through non-combustion, an electrochemical process, and more importantly, without the emission of CO\({}_{2}\), at their point of use. It is worth mentioning that the H\({}_{2}\) fuel cells' efficiency (currently \(>\)60%) can be a factor of two or more greater than that of internal combustion engines (with approximately 22% for gasoline or 45% for diesel internal combustion engines) [22]. This has dramatically contributed to the increasing large-scale use of hydrogen fuel cell power applications, particularly in large-scale power (combined electricity and heat) generation, the vehicle sector (including heavy transport applications, such as buses, forklift trucks, aviation, rail and maritime), and small-scale remote on-site energy generation [22; 38]. Basically, fuel cells are electrochemical conversion devices, which, in the presence of suitable catalysts, are capable of transforming the continuous supply of the fuel (hydrogen) to one electrode--the anode, where the hydrogen oxidation (loss of electrons) reaction occurs--and an oxidant (usually the oxygen in air) to the other electrode--the cathode, where the reduction (gain of electrons) reaction of the oxidant happens--directly into electricity (and heat), with only water (or steam) as the emission product. Connecting both electrodes, there is an external electric circuit that allows the flow of the electrons involved.