Quarterly report pursuant to Section 13 or 15(d)

Note 12 - Stockholders' Equity - Stock-based Compensation Expense (Details)

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Note 12 - Stockholders' Equity - Stock-based Compensation Expense (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Stock-based compensation expense $ 687 $ 605
Plant Operation and Clean Up [Member]    
Stock-based compensation expense 29 25
Research and Development Expense [Member]    
Stock-based compensation expense 18 18
General and Administrative Expense [Member]    
Stock-based compensation expense $ 640 $ 562