13. Employee Benefit Plans

2011 Stock Option and Incentive Plan

On May 12, 2011, the Company's stockholders approved the 2011 Stock Option and Incentive Plan (the 2011 Plan). The 2011 Plan provides for the issuance of up to a maximum number of shares of common stock equal to the sum of (i) 1,000,000, plus (ii) the number of shares of common stock underlying any grants pursuant to the 2011 Plan or the Plug Power Inc. 1999 Stock Option and Incentive Plan that are forfeited, canceled, repurchased or are terminated (other than by exercise). The shares may be issued pursuant to stock options, stock appreciation rights, restricted stock awards and certain other equity-based awards granted to employees, directors and consultants of the Company. No grants may be made under the 2011 Plan after May 12, 2021. On May 16, 2012, the stockholders approved an amendment to the 2011 Plan, to increase the number of shares of the Company's common stock authorized for issuance under the 2011 Plan from 1.0 million to 6.5 million. On July 23, 2014, the stockholders approved an amendment to the 2011 Plan, to increase the number of shares of the Company's common stock authorized for issuance under the 2011 Plan from 6.5 million to 17.0 million. For the years ended December 31, 2015, 2014, and 2013, the Company recorded expense of approximately $7.5 million, $3.6 million, and $1.8 million, respectively, in connection with the 2011 Stock Option and Incentive Plan.

At December 31, 2015, there were approximately 11.7 million options granted and outstanding and 4.3 million options available to be issued under the 2011 Plan, including adjustments for other types of share-based awards. Options for employees issued under this plan generally vest in equal annual installments over three years and expire ten years after issuance. Options granted to members of the Board generally vest one year after issuance. To date, options granted under the 2011 Plan have vesting provisions ranging from one to three years in duration and expire ten years after issuance.

Compensation cost associated with employee stock options represented approximately $7.5 million, $3.4 million and $1.4 million of the total share-based payment expense recorded for the years ended December 31, 2015, 2014, and 2013, respectively. The Company estimates the fair value of stock options using a Black-Scholes valuation model, and the resulting fair value is recorded as compensation cost on a straight-line basis over the option vesting period. Key inputs and assumptions used to estimate the fair value of stock options include the grant price of the award, the expected option term, volatility of the Company's stock, an appropriate risk-free rate, and the Company's dividend yield. Estimates of fair value are not intended to predict actual future events or the value ultimately realized by employees who receive equity awards, and subsequent events are not indicative of the reasonableness of the original estimates of fair value made by the Company. The assumptions made for purposes of estimating fair value under the Black-Scholes model for the 3,960,000, 4,246,000, and 3,090,900 options granted during the years ended December 31, 2015, 2014 and 2013, respectively, were as follows:

	2015	2014	2013
Expected term of options (years)	6	6	6
Risk free interest rate	1.52% - 1.87%	1.77% - 1.94%	0.93% - 1.70%
Volatility	104.03% - 105.29%	107.17% - 113.92%	92.46% - 107.01%

There was no expected dividend yield for the employee stock options granted.

The Company's estimate of an expected option term was calculated in accordance with the simplified method for calculating the expected term assumption. The estimated stock price volatility was derived from the Company's actual historic stock prices over the past six years, which represents the Company's best estimate of expected volatility.

A summary of stock option activity for the year December 31, 2015 is as follows (in thousands except share amounts):

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Terms		Aggregate Instrinsic Value
Options outstanding at December 31, 2014		8,367,271		$	3.94		8.7
Granted		3,960,000			2.46
Exercised		(173,334	)		0.96
Forfeited		(406,975	)		3.18
Expired		(46,176	)		59.35

Options outstanding at December 31, 2015		11,700,786		$	3.29		8.3	$	3,397


Options exercisable at December 31, 2015		4,273,425			3.66		7.2		3,159
Options unvested at December 31, 2015		7,427,361		$	3.08		9.0		815

The weighted average grant date fair value of options granted during the years ended December 31, 2015, 2014 and 2013 was $1.99, $4.05, and $0.32, respectively. As of December 31, 2015, there was approximately $14.9 million of unrecognized compensation cost related to stock option awards to be recognized over the next three years, all of this is expected to vest. The total fair value of stock options that vested during the years ended December 31, 2015 and 2014 was approximately $6.1 million and $3.4 million, respectively.

Restricted stock awards generally vest in equal installments over a period of one to three years. Restricted stock awards are valued based on the closing price of the Company's common stock on the date of grant, and compensation cost is recorded on a straight-line basis over the share vesting period. The Company recorded expense associated with its restricted stock awards of approximately $118 thousand, $84 thousand, and $33 thousand for the years ended December 31, 2015, 2014 and 2013, respectively. Additionally, for the years ended December 31, 2015, 2014 and 2013, there was $132 thousand, $265 thousand, and $207 thousand, respectively, of unrecognized compensation cost related to restricted stock awards to be recognized over the next three years.

A summary of restricted stock activity for the year ended December 31, 2015 is as follows (in thousands except share amounts):

	Shares			Aggregate Instrinsic Value
Unvested restricted stock at December 31, 2014		473,336
Granted		—
Vested		(191,114	)
Forfeited		(77,778	)

Unvested restricted stock at December 31, 2015		204,444		$	431

401(k) Savings & Retirement Plan

The Company offers a 401(k) Savings & Retirement Plan to eligible employees meeting certain age and service requirements. This plan permits participants to contribute 100% of their salary, up to the maximum allowable by the Internal Revenue Service regulations. Participants are immediately vested in their voluntary contributions plus actual earnings or less actual losses thereon. Participants are vested in the Company's matching contribution based on years of service completed. Participants are fully vested upon completion of three years of service. During 2002, the Company began funding its matching contribution in common stock. During 2015, the Company funded its matching contribution with cash. Accordingly, the Company has issued zero, 74,863, and 1,319,914 shares of common stock to the Plug Power Inc. 401(k) Savings & Retirement Plan during 2015, 2014 and 2013, respectively.

The Company's expense for this plan, including the issuance of shares, was approximately $889 thousand, $367 thousand and $371 thousand for years ended December 31, 2015, 2014 and 2013, respectively.

Non-Employee Director Compensation

Each non-employee director is paid an annual retainer for their services, in the form of either cash or stock compensation. The Company granted 89,490, 71,311, and 673,152 shares of stock to non-employee directors as compensation for the years ended December 31, 2015, 2014, and 2013, respectively. All common stock issued is fully vested at the time of issuance and is valued at fair value on the date of issuance. The Company's share-based compensation expense for this plan was approximately $267 thousand, $331 thousand and $332 thousand for the years ended December 31, 2015, 2014, and 2013 respectively.