Quarterly report pursuant to Section 13 or 15(d)

Condensed Consolidated Statements of Stockholders' Equity (Deficit)

Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	January 2019 Public Offering	May 2019 Public Offering	Over-Allotment Option	2018 Public Offering	Officers and Directors Purchase Plan	Common Stock	Common Stock January 2019 Public Offering	Common Stock May 2019 Public Offering	Common Stock Over-Allotment Option	Common Stock 2018 Public Offering	Common Stock Officers and Directors Purchase Plan	Common Stock Restricted Stock Units (RSUs)	Additional Paid-in Capital	Additional Paid-in Capital January 2019 Public Offering	Additional Paid-in Capital May 2019 Public Offering	Additional Paid-in Capital Over-Allotment Option	Additional Paid-in Capital 2018 Public Offering	Additional Paid-in Capital Officers and Directors Purchase Plan	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017							27,554,076
Beginning balance at Dec. 31, 2017	$ 58,965						$ 27							$ 113,780						$ (54,842)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	853													853
Common stock issued during period (in shares)										1,072,500	10,085,500	2,034	65,600
Common stock issued during period				$ 2,103	$ 26,647	$ 4				$ 2	$ 10						$ 2,101	$ 26,637	$ 4
Modification of Interstate Battery warrant 1	1,002													1,002
Net loss	(26,722)																			(26,722)
Ending balance (in shares) at Sep. 30, 2018							38,779,710
Ending Balance at Sep. 30, 2018	62,852						$ 39							144,377						(81,564)
Beginning balance (in shares) at Jun. 30, 2018							38,779,710
Beginning balance at Jun. 30, 2018	71,754						$ 39							144,016						(72,301)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	361													361
Net loss	(9,263)																			(9,263)
Ending balance (in shares) at Sep. 30, 2018							38,779,710
Ending Balance at Sep. 30, 2018	62,852						$ 39							144,377						(81,564)
Beginning balance (in shares) at Dec. 31, 2018							38,932,437
Beginning balance at Dec. 31, 2018	50,090						$ 39							145,147						(95,096)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	2,899													2,899
Warrants related to Veolia agreement	4,046													4,046
Common stock issued during period (in shares)								5,175,000	11,000,000				448,487
Common stock issued during period		$ 9,063	$ 20,317					$ 5	$ 11						$ 9,058	$ 20,306
Common stock issued for consulting services (in shares)							1,736,950
Common stock issued for consulting services	4,075						$ 2							4,073
Net loss	(33,525)																			(33,525)
Ending balance (in shares) at Sep. 30, 2019							57,292,874
Ending Balance at Sep. 30, 2019	56,965						$ 57							185,529						(128,621)
Beginning balance (in shares) at Jun. 30, 2019							56,889,876
Beginning balance at Jun. 30, 2019	64,615						$ 57							181,863						(117,305)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	901													901
Warrants related to Veolia agreement	1,734													1,734
Common stock issued during period (in shares)													91,319
Common stock issued for consulting services (in shares)							311,679
Common stock issued for consulting services	1,031						$ 0							1,031
Net loss	(11,316)																			(11,316)
Ending balance (in shares) at Sep. 30, 2019							57,292,874
Ending Balance at Sep. 30, 2019	$ 56,965						$ 57							$ 185,529						$ (128,621)