20865
Despite the good progress in developing doped carbon catalysts for oxygen - reduction reaction ( ORR ) , the current metal - free carbon catalysts are still far from satisfactory for large - scale applications of fuel cell .
 Chemical: oxygen; Chemical: carbon
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: oxygen; Chemical: carbon