## 2 Research Method
### Literature Reviews and Key Findings  
As an introduction to this subsection, it is worth noting that by 2050, the global energy demand is anticipated to be around 30 terawatts (TW), which is double that of 2011 [5]. Hydrogen is specifically being considered as an alternative fuel in the transportation sector to power fuel cell electric vehicles [6] and for electrifying the commercial aviation (viz., hybrid-electric and all-electric aircraft). The U.S. DOE projected that clean H\({}_{2}\) could be produced for USD 2/kg H\({}_{2}\) by 2025 (H\({}_{2}\)_New_, 2021), and in its 2021 \"Hydrogen Shot\" conference, the DOE drafted its long-term goal to reduce the cost of clean H\({}_{2}\) production to USD 1 per kg H\({}_{2}\) in one decade. Herein, a literature review of 75 published sources was conducted on various H\({}_{2}\) production technologies, specifically the most prominent conventional technologies and the most promising greener technologies. With a couple exceptions, only papers and reports published in the last decade (2013-2024) were consulted, with an effort to rely on publications from the past 5 years whenever possible, in order to ensure the comparative analysis contained the most updated information. From these sources, both quantitative and qualitative data were extracted to support the comparative assessment of H\({}_{2}\) production technologies. The conventional methods of H\({}_{2}\) production provide either \"gray\" or \"blue\" hydrogen. Steam-methane reforming (SMR) is widely acknowledged as the most well-established method [3], and it is often used as a point of reference to analyze and compare the performance and feasibility of non-conventional methods. Because SMR is a very mature technology, it is thoroughly studied and produces low-cost hydrogen, but it also has low potential for technical improvements that will improve efficiency or performance [7,8]. Several papers studying the conventional methods of producing blue H\({}_{2}\) discuss how CCS reduces emissions significantly but also increases production cost and decreases energy efficiency [8, 9, 10, 11, 12]. Waste valorization was evaluated as a more sustainable H\({}_{2}\) production technology. Utilizing waste streams eliminates both the polluting effects of waste build-up and the need for energy-intensive and high-emitting waste incineration. Biomass is a broad term and can be prepared as a feedstock from organic sources, such as grass, wood, agricultural products, animal waste, food scraps, municipal solid waste, and algae, and it can be valorized by methods such as gasification, pyrolysis, supercritical water gasification, and dark fermentation [11; 13]. One study noted that food waste is specifically suitable for H\({}_{2}\) production for a number of reasons, such as its carbohydrate richness, wide availability, and low cost [14]. Another study explained that converting food waste to H\({}_{2}\) via dark fermentation is energetically advantageous because it can be achieved under ambient temperature and pressure with low chemical energy requirements, but it must be performed onsite to be feasible due to the high water content and biodegradability of food waste [15]. Even more so than food waste fermentation, gasification of biomass is a highly studied process due to its convenient utilization of diverse waste streams and production of clean biohydrogen. Biomass gasification is currently faced with low conversion and thermal efficiencies, but they can be improved with the optimization of the process temperature, the catalyst used, and the biomass content [16]. Glycerol is another promising waste stream to utilize for H\({}_{2}\) generation, as it is produced abundantly as a byproduct of the transesterification of biomass to biodiesel. By strategically incorporating H\({}_{2}\) production plants into existing biodiesel production plants, glycerol could even become a free material stream.