Equity incentive plans. In February 2011, the Company’s stockholders approved the Gevo, Inc. 2010 Stock Incentive Plan (as amended and restated to date, the "2010 Plan") and the Employee Stock Purchase Plan.

The 2010 Plan provides for the grant of non-qualified stock options, incentive stock options, stock appreciation rights, restricted stock awards, restricted stock units and other equity awards to employees and directors of the Company. In June 2021, upon approval of the shareholders at the 2021 Annual Meeting of Stockholders, the 2010 Plan was amended and restated, which increased the number of shares of common stock reserved for issuance under the 2010 Plan to 22,980,074 shares. At December 31, 2022, 4,221,225 shares were available for future issuance under the 2010 Plan.

Stock-based compensation expense. The Company records stock-based compensation expense during the requisite service period for share-based payment awards granted to employees and non-employees.

The following table sets forth the Company’s stock-based compensation expense for the periods indicated (in thousands):


	Year Ended December 31,
	2022		2021
Equity Classified Awards
Cost of production	$	(25)	$	43
General and administrative	14,342		5,631
Other	2,618		2,026
Total equity classified awards	16,935		7,700
Liability Classified Awards
General and administrative	—		2,165
Other	—		9
Total liability classified awards	—		2,174
Total stock-based compensation	$	16,935	$	9,874

Stock option award activity. Stock option activity under the Company’s stock incentive plans and changes during the year ended December 31, 2022 were as follows:


	Number of Options	Weighted Average Exercise Price (1)		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value
Options outstanding at December 31, 2021	4,746,368	$	5.11		$	—
Granted	1,627,683	$	3.45
Canceled or forfeited	(428,730)	$	5.49
Exercised	—	$	—
Options outstanding at December 31, 2022	5,945,321	$	4.65	9.1	$	—
Options vested and expected to vest at December 31, 2022	1,512,054	$	5.32	8.7	$	—

(1)Exercise price of options outstanding range from $2.42 to $11,340 as of December 31, 2022. The higher end of the range is due to the impact of several reverse stock splits during the years 2015 to 2018, subsequent to certain option grants, and relates to awards that expire during 2023.

During the year ended December 31, 2022, 1.5 million stock options vested. As of December 31, 2022, the total unrecognized compensation expense, net of estimated forfeitures, relating to stock options was $13.5 million, which is expected to be recognized over the remaining weighted-average period of approximately 2.4 years.

The following table sets forth the weighted average Black-Scholes option pricing model assumptions (no dividends were expected) and resulting grant date fair value for the stock options granted during the years ended December 31, 2022 and 2021:


	Year Ended December 31,
	2022			2021
Risk-free interest rate	2.90		%	0.92		%
Expected volatility factor	134		%	141		%
Expected option life (years)	6.0			5.9
Weighted-average fair value	$	2.18		$	4.59

There is a maximum contractual term of ten years for the share options. The Company settles stock option exercises with newly issued common shares. No tax benefits were realized by the Company in connection with these exercises as the Company maintains net operating loss carryforwards and has established a valuation allowance against the entire tax benefit.

Restricted Stock. The Company periodically grants restricted stock awards to employees and directors. The vesting period for restricted stock awards granted may be based upon a service period or based upon the attainment of performance objectives. The Company recognizes stock-based compensation over the vesting period, generally two to three years, for awards that vest based upon a service period.

Non-vested restricted stock awards at December 31, 2022 and changes during the year ended December 31, 2022 were as follows:


	Number of Shares	Weighted-Average Grant-Date Fair Value
Non-vested at December 31, 2021	6,882,502	$	3.77
Granted	2,360,605	$	2.57
Vested	(3,561,879)	$	2.86
Canceled or forfeited	(426,771)	$	4.83
Non-vested at December 31, 2022	5,254,457	$	3.94

The total fair value of restricted stock that vested during the years ended December 31, 2022 and 2021 was $10.2 million and $2.8 million, respectively. As of December 31, 2022, the total unrecognized compensation expense, net of estimated forfeitures, relating to restricted stock awards was $16.7 million, which is expected to be recognized over the remaining weighted-average period of approximately 2.3 years. As of December 31, 2022, there are no unvested liability-classified restricted stock awards.