Exhibit 1.1

Execution Version

AMENDMENT NO. 3 TO AT MARKET ISSUANCE SALES AGREEMENT

AUGUST 15, 2025

B. Riley Securities, Inc.

299 Park Avenue, 21st Floor

New York, NY 10171

Ladies and Gentlemen:

Plug Power Inc., a Delaware corporation (the “Company”), and B. Riley Securities, Inc. (the “Agent”), are parties to that certain At Market Issuance Sales Agreement dated January 17, 2024 (the “Original Agreement”), as amended by Amendment No. 1 to the At Market Issuance Sales Agreement, dated February 23, 2024 (“Amendment No. 1”), and as further amended by Amendment No 2 to the At Market Issuance Sales Agreement, dated November 7, 2024 (“Amendment No. 2” and, together with the Original Agreement and Amendment No. 1, the “Sales Agreement”). All capitalized terms not defined herein shall have the meanings ascribed to them in the Sales Agreement. The Company and Agent desire to amend the Sales Agreement as set forth in this Amendment No. 3 thereto (this “Amendment No. 3”) as follows:

1.             As of the date hereof, the references to “Prospectus Supplement” shall refer to the prospectus supplement relating to the offering and sale of the Shares filed by the Company with the Commission pursuant to Rule 424(b) on the date hereof, in the form furnished by the Company to the Agent in connection with the offering of the Placement Shares.

2.             All references to “January 17, 2024” set forth in Schedule 1, Schedule 2(b) and Exhibit 7(l) of the Original Agreement are revised to read “January 17, 2024 (as amended by Amendment No. 1, dated February 23, 2024, Amendment No. 2, dated November 7, 2024, and Amendment No. 3, dated August 15, 2025)”.

3.             Section 6 of Amendment No. 2 is amended and restated in its entirety as follows:

“Section 13(d) of the Original Agreement is amended and restated in its entirety as follows:

Unless earlier terminated pursuant to this Section 13, this Agreement shall automatically terminate upon the earlier to occur of (i) the 24-month anniversary of August 15, 2025 and (ii) the issuance and sale of all of the Placement Shares through or to B. Riley on the terms and subject to the conditions set forth herein, except that the provisions of Section 9 (Payment of Expenses), Section 11 (Indemnification and Contribution), Section 12 (Representations and Agreements to Survive Delivery), Section 18 (Governing Law and Time; Waiver of Jury Trial) and Section 19 (Consent to Jurisdiction) hereof shall remain in full force and effect notwithstanding such termination.”

4.             The Company will pay fees and disbursements of counsel to the Agent of up to $25,000 incurred in connection with this Amendment No. 3 and other related documents.

5.             Except as specifically set forth herein, all other provisions of the Sales Agreement shall remain in full force and effect.

6.             This Amendment No. 3, together with the Original Agreement, Amendment No. 1 and Amendment No. 2 (including all exhibits attached thereto), constitutes the entire agreement and supersedes all other prior and contemporaneous agreements and undertakings, both written and oral, among the parties hereto with regard to the subject matter hereof. Neither this Amendment No. 3 nor any term hereof may be amended except pursuant to a written instrument executed by the Company and the Agent. In the event that any one or more of the provisions contained herein, or the application thereof in any circumstance, is held invalid, illegal or unenforceable as written by a court of competent jurisdiction, then such provision shall be given full force and effect to the fullest possible extent that it is valid, legal and enforceable, and the remainder of the terms and provisions herein shall be construed as if such invalid, illegal or unenforceable term or provision was not contained herein, but only to the extent that giving effect to such provision and the remainder of the terms and provisions hereof shall be in accordance with the intent of the parties as reflected in this Amendment No. 3. All references in the Original Agreement to the “Agreement” shall mean the Original Agreement as amended by Amendment No. 1, Amendment No. 2 and this Amendment No. 3; provided, however, that all references to “date of this Agreement” in the Original Agreement shall continue to refer to the date of the Original Agreement.

7.             THIS AMENDMENT NO. 3 AND ANY CLAIM, CONTROVERSY OR DISPUTE ARISING UNDER OR RELATED TO THIS AMENDMENT NO. 3 SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF, THE STATE OF NEW YORK WITHOUT REGARD TO ITS CHOICE OF LAW PROVISIONS. THE COMPANY AND THE AGENT EACH HEREBY IRREVOCABLY WAIVE, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS AMENDMENT NO. 3 OR THE TRANSACTIONS CONTEMPLATED HEREBY.

8.            EACH PARTY HEREBY IRREVOCABLY SUBMITS TO THE NON-EXCLUSIVE JURISDICTION OF THE STATE AND FEDERAL COURTS SITTING IN THE CITY OF NEW YORK, BOROUGH OF MANHATTAN, FOR THE ADJUDICATION OF ANY DISPUTE UNDER OR IN CONNECTION WITH THIS AMENDMENT NO. 3 OR ANY TRANSACTION CONTEMPLATED HEREBY, AND HEREBY IRREVOCABLY WAIVES, AND AGREES NOT TO ASSERT IN ANY SUIT, ACTION OR PROCEEDING, ANY CLAIM THAT IT IS NOT PERSONALLY SUBJECT TO THE JURISDICTION OF ANY SUCH COURT, THAT SUCH SUIT, ACTION OR PROCEEDING IS BROUGHT IN AN INCONVENIENT FORUM OR THAT THE VENUE OF SUCH SUIT, ACTION OR PROCEEDING IS IMPROPER. EACH PARTY HEREBY IRREVOCABLY WAIVES PERSONAL SERVICE OF PROCESS AND CONSENTS TO PROCESS BEING SERVED IN ANY SUCH SUIT, ACTION OR PROCEEDING BY MAILING A COPY THEREOF (CERTIFIED OR REGISTERED MAIL, RETURN RECEIPT REQUESTED) TO SUCH PARTY AT THE ADDRESS IN EFFECT FOR NOTICES TO IT UNDER THE SALES AGREEMENT AND AGREES THAT SUCH SERVICE SHALL CONSTITUTE GOOD AND SUFFICIENT SERVICE OF PROCESS AND NOTICE THEREOF. NOTHING CONTAINED HEREIN SHALL BE DEEMED TO LIMIT IN ANY WAY ANY RIGHT TO SERVE PROCESS IN ANY MANNER PERMITTED BY LAW.

9.             This Amendment No. 3 may be executed in two or more counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument. Delivery of an executed amendment by one party to the other may be made by facsimile transmission or electronic transmission (e.g., PDF).

[Remainder of Page Intentionally Blank]

If the foregoing correctly sets forth the understanding between the Company and the Agent, please so indicate in the space provided below for that purpose, whereupon this Amendment No. 3 shall constitute a binding amendment to the Sales Agreement between the Company and the Agent.

[Signature Page to Amendment No. 3 to Sales Agreement] 

Exhibit 5.1

August 15, 2025

Plug Power Inc.

125 Vista Boulevard

Slingerlands, NY 12159

Re:         Securities Registered under Registration Statement on Form S-3

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-3 (File No. 333-287577) (as amended or supplemented, the “Registration Statement”) filed on May 27, 2025 with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration by Plug Power Inc., a Delaware corporation (the “Company”), of any combination of securities of the types specified therein. The Registration Statement became effective upon filing with the Commission on May 27, 2025. Reference is made to our opinion letter dated May 27, 2025 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on August 15, 2025 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to $1,000,000,000 in aggregate offering price of shares (the “Shares”) of the Company’s common stock, par value $0.01 per share (“Common Stock”), covered by the Registration Statement. The Shares are being offered and sold by the sales agent named in, and pursuant to, the At Market Issuance Sales Agreement, dated as of January 17, 2024, as amended on February 23, 2024, November 7, 2024, and August 15, 2025, between the Company and such sales agent (the “Sales Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

For purposes of the opinion set forth below, we have assumed that the Shares are issued for a price per share equal to or greater than the minimum price authorized by the Company’s board of directors prior to the date hereof (the “Minimum Price”), that no event occurs that causes the number of authorized shares of Common Stock available for issuance by the Company to be less than the number of then unissued Shares that may be issued for the Minimum Price and that the total number of shares of Common Stock issued and outstanding, after giving effect to the issuance of Shares under the Sales Agreement, will not exceed the total number of shares of Common Stock that the Company has authority to issue under its Amended and Restated Certificate of Incorporation, as amended.

Plug Power Inc.

August 15, 2025

Page 2

For purposes of the opinion set forth below, we refer to the following as “Future Approval and Issuance”: (a) the approval by the Company’s board of directors (or a duly authorized committee of the board of directors) of the issuance of the Shares (the “Approval”) and (b) the issuance of the Shares in accordance with the Approval and the receipt by the Company of the consideration (which shall not be less than the par value of such Shares) to be paid in accordance with the Approval.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon Future Approval and Issuance, will be validly issued, fully paid and nonassessable

This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

This opinion letter is being furnished to you for submission to the Commission as an exhibit to the Company’s Current Report on Form 8-K relating to the future issuance of the Shares (the “Current Report”), which is incorporated by reference in the Registration Statement. We hereby consent to the filing of this opinion letter as an exhibit to the Current Report and its incorporation by reference and the reference to our firm in that report. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.