This concept has been proposed by Dwork and Naor in 1992 to fight email spam. [0]
[0] Dwork, Cynthia, and Moni Naor. "Pricing via processing or combatting junk mail." Annual International Cryptology Conference. Springer, Berlin, Heidelberg, 1992.
[0] Dwork, Cynthia, and Moni Naor. "Pricing via processing or combatting junk mail." Annual International Cryptology Conference. Springer, Berlin, Heidelberg, 1992.
https://www.semanticscholar.org/paper/%E2%80%9C-Proof-of-Wor...