Phosphoric acid fuel cells (PAFC) use phosphoric acid (H3PO4) in silicon carbide as the electrolyte. This acid is a solid at room temperature but melts at 42°C and is stable at 200°C, allowing this cell to operate at higher temperatures and reducing the sensitivity to carbon monoxide poisoning 226 . The advantage of this high operational temperature is that these fuel cells can not only be used in stationary power applications, but the waste heat from operation can be captured and used for space heating and hot water temperature of operation is a disadvantage due to slow device startup and thermal shielding requirements, but it also removes the need for precious-metal catalyst, thereby reducing cost. SOFCs are also the most sulfur-resistant fuel cell type and are not poisoned by carbon monoxide. High operating temperatures also cause durability issues and strict material requirements. Therefore, the development of high temperature stable device materials or lowering the temperature of the cells to more intermediate levels are the main challenges facing this technology. These types of fuel cells are mostly used for stationary applications (auxiliary power, electric utility, distributed generation) and are highly used for production of electrical and useful thermal energy known as combined heat and power (CHP). 221, 227 In the area of hydrogen fuel cells, publication volume was flat with an almost equal distribution of documents between patents and journals up to 2018 (Figure ). It is in 2018 that one begins to see a rise in patents as well as documents in general with 2021 being the year of most overall interest and patent publications. Similar to water splitting, the fuel cell field seeks to improve performance and durability while lowering the costs of fuel cell components to make it more viable for market applications. This is reflected in the most commonly co-occurring concepts clustered network diagram (Figure ). The primary concept in the literature is as expected "Fuel Cells", but it is important to note that it is followed by "Solid Oxide Fuel Cells". SOFCs garnered more attention in the 2011-2021 decade than "Proton exchange membrane fuel cells" or "Polymer electrolyte fuel cells". One can also see that there is no mention of "Alkaline Fuel Cells" or "Phosphoric acid Fuel cells" on the top 125 concepts, demonstrating that these are losing popularity in modern times and that the future is moving towards SOFCs and PEMFCs.