EX-10.3





 Exhibit 10.3 

ALTENERGY ACQUISITION CORP. 

600 Lexington Avenue 
 9th
Floor 
 New York, NY 10022 

March 25, 2021 
 AltEnergy Acquisition
Sponsor, LLC 
 600 Lexington Avenue 
 9th Floor 

New York, NY 10022 
 RE: Subscription
Agreement for Founder Shares 
 Ladies and Gentlemen: 

We are pleased to accept the offer AltEnergy Acquisition Sponsor, LLC (the “Subscriber” or “you”) has made to
purchase 5,750,000 shares (“Founder Shares”) of Class B common stock, $0.0001 par value per share (the “Class B Common Stock” together with all other classes of Company (as defined below)
common stock, the “Common Stock”), of AltEnergy Acquisition Corp., a Delaware corporation (the “Company”), up to 750,000 of which are subject to forfeiture by you if the underwriters of the proposed initial public
offering (“IPO”) of the Company pursuant to the registration statement on Form S-1 expected to be filed by the Company in connection with the IPO (the “Registration
Statement”) do not fully exercise their over-allotment option (the “Over-allotment Option”) as described below. The terms (this “Agreement”) on which the Company is willing to sell the Founder Shares to the
Subscriber, and the Company and the Subscriber’s agreements regarding such Founder Shares, are as follows: 
 1. Purchase of Founder
Shares. For the sum of $25,000.00 (the “Purchase Price”), which the Company acknowledges receiving in cash, the Company hereby sells and issues the Founder Shares to the Subscriber, and the Subscriber hereby purchases the
Founder Shares from the Company, subject to the forfeiture provisions of Section 3 below, on the terms and subject to the conditions set forth in this Agreement. Concurrently with the Subscriber’s execution of this Agreement, the Company
shall, at its option, deliver to the Subscriber a certificate registered in the Subscriber’s name representing the Founder Shares, or effect such delivery in book-entry form. 

2. Representations, Warranties and Agreements. 

2.1 The Subscriber’s Representations, Warranties and Agreements. To induce the Company to issue the Founder Shares to the
Subscriber, the Subscriber hereby represents and warrants to the Company and agrees with the Company as follows: 












 2.1.1 No Government Recommendation or Approval. The Subscriber understands that no
federal or state agency has passed upon or made any recommendation or endorsement of the offering of the Founder Shares. 
 2.1.2 No
Conflicts. The execution, delivery and performance of this Agreement and the consummation by the Subscriber of the transactions contemplated hereby do not violate, conflict with or constitute a default under (i) the formation and governing
documents of the Subscriber, (ii) any agreement, indenture or instrument to which the Subscriber is a party, (iii) any law, statute, rule or regulation to which the Subscriber is subject, or (iv) any agreement, order, judgment or
decree to which the Subscriber is subject. 
 2.1.3 Organization and Authority. The Subscriber is a Delaware limited liability
company, validly existing and in good standing under the laws of Delaware and possesses all requisite power and authority necessary to carry out the transactions contemplated by this Agreement. Upon execution and delivery by you, this Agreement will
be a legal, valid and binding agreement of the Subscriber, enforceable against the Subscriber in accordance with its terms, except as such enforceability may be limited by applicable bankruptcy, insolvency, fraudulent conveyance or similar laws
affecting the enforcement of creditors’ rights generally and subject to general principles of equity (regardless of whether enforcement is sought in a proceeding at law or in equity). 

2.1.4 Experience, Financial Capability and Suitability. The Subscriber is: (i) sophisticated in financial matters and is able to
evaluate the risks and benefits of the investment in the Founder Shares and (ii) able to bear the economic risk of its investment in the Founder Shares for an indefinite period of time because the Founder Shares have not been registered under
the Securities Act (as defined below) and therefore cannot be resold unless such transaction is registered under the Securities Act or an exemption from such registration is available. The Subscriber is capable of evaluating the merits and risks of
its investment in the Company and has the capacity to protect its own interests. The Subscriber must bear the economic risk of this investment until the Founder Shares are sold pursuant to: (x) an effective registration statement under the
Securities Act or (y) an exemption from registration available with respect to such sale. The Subscriber is able to bear the economic risks of an investment in the Founder Shares and to afford a complete loss of the Subscriber’s investment
in the Founder Shares. 
 2.1.5 Access to Information; Independent Investigation. Prior to the execution of this Agreement, the
Subscriber has had the opportunity to ask questions of and receive answers from representatives of the Company concerning an investment in the Company, as well as the finances, operations, business and prospects of the Company, and the opportunity
to obtain additional information to verify the accuracy of all information so obtained. In determining whether to make this investment, the Subscriber has relied solely on the Subscriber’s own knowledge and understanding of the Company and its
business based upon the Subscriber’s own due diligence investigation and the information furnished pursuant to this paragraph. The Subscriber understands that no person has been authorized to give any information or to make any representations
which were not furnished pursuant to this Section 2 and the Subscriber has not relied on any other representations or information in making its investment decision, whether written or oral, relating to the Company, its operations or its
prospects. 












 2.1.6 Regulation D Offering. The Subscriber represents that it is an
“accredited investor” as such term is defined in Rule 501(a) of Regulation D under the Securities Act of 1933, as amended (the “Securities Act”), and acknowledges the sale contemplated hereby is being made in reliance on a
private placement exemption applicable to “accredited investors” within the meaning of Section 501(a) of Regulation D under the Securities Act or similar exemptions under federal and state law. 

2.1.7 Investment Purposes. The Subscriber is purchasing the Founder Shares solely for investment purposes, for the Subscriber’s
own account and not for the account or benefit of any other person, and not with a view towards the distribution or dissemination thereof. The Subscriber did not enter into this Agreement as a result of any general solicitation or general
advertising within the meaning of Rule 502 of Regulation D under the Securities Act. 
 2.1.8 Restrictions on Transfer; Shell
Company. The Subscriber understands the Founder Shares are being offered in a transaction not involving a public offering within the meaning of the Securities Act. The Subscriber understands the Founder Shares will be “restricted
securities” as defined in Rule 144(a)(3) under the Securities Act and the Subscriber understands that any certificate or book entries representing the Founder Shares will contain a legend in respect of such restrictions. If in the future the
Subscriber decides to offer, resell, pledge or otherwise transfer the Founder Shares, such Founder Shares may be offered, resold, pledged or otherwise transferred only pursuant to registration under the Securities Act or an available exemption from
registration. The Subscriber agrees that if any transfer of its Founder Shares or any interest therein is proposed to be made, as a condition precedent to any such transfer, the Subscriber may be required to deliver to the Company an opinion of
counsel satisfactory to the Company. Absent registration under the Securities Act or an exemption therefrom, the Subscriber agrees not to resell the Founder Shares. The Subscriber further acknowledges that because the Company is a shell company,
Rule 144 may not be available to the Subscriber for the resale of the Founder Shares until at least one year following consummation of the initial business combination of the Company, despite technical compliance with the certain requirements of
Rule 144 and the release or waiver of any contractual transfer restrictions. 
 2.1.9 No Governmental Consents. No governmental,
administrative or other third-party consents or approvals are required, necessary or appropriate on the part of the Subscriber in connection with the transactions contemplated by this Agreement. 

2.2 Company’s Representations, Warranties and Agreements. To induce the Subscriber to purchase the Founder Shares, the Company
hereby represents and warrants to the Subscriber and agrees with the Subscriber as follows: 
 2.2.1 Organization and Corporate
Power. The Company is a Delaware corporation and is qualified to do business in every jurisdiction in which the failure to so qualify would reasonably be expected to have a material adverse effect on the financial condition, operating results or
assets of the Company. The Company possesses all requisite corporate power and authority necessary to carry out the transactions contemplated by this Agreement. 












 2.2.2 No Conflicts. The execution, delivery and performance of this Agreement and
the consummation by the Company of the transactions contemplated hereby do not violate, conflict with or constitute a default under (i) the Certificate of Incorporation or Bylaws of the Company, (ii) any agreement, indenture or instrument
to which the Company is a party, (iii) any law, statute, rule or regulation to which the Company is subject, or (iv) any agreement, order, judgment or decree to which the Company is subject. 

2.2.3 Title to Securities. Upon issuance in accordance with, and payment pursuant to, the terms hereof, the Founder Shares will be duly
and validly issued, fully paid and nonassessable. Upon issuance in accordance with, and payment pursuant to, the terms hereof the Subscriber will have or receive good title to the Founder Shares, free and clear of all liens, claims and encumbrances
of any kind, other than (a) transfer restrictions hereunder and other agreements to which the Founder Shares may be subject which have been notified to the Subscriber in writing, (b) transfer restrictions under federal and state securities
laws, and (c) liens, claims or encumbrances imposed due to the actions of the Subscriber. 
 2.2.4 No Adverse Actions. There are
no actions, suits, investigations or proceedings pending, threatened against or affecting the Company which: (i) seek to restrain, enjoin, prevent the consummation of or otherwise affect the transactions contemplated by this Agreement or
(ii) question the validity or legality of any transactions or seek to recover damages or to obtain other relief in connection with any transactions. 

3. Forfeiture of Founder Shares. 

3.1 Partial or No Exercise of the Over-allotment Option. In the event the Over-allotment Option granted to the underwriters of the IPO
is not exercised in full, the Subscriber acknowledges and agrees that it (and, if applicable, any transferee of Shares) shall automatically forfeit at the time such Over-allotment Option expires (or earlier if the underwriters of the IPO waive their
ability to exercise such Over-allotment Option) any and all rights to such number of Founder Shares (up to an aggregate of 750,000 Founder Shares and pro rata based upon the percentage of the Over-allotment Option exercised) such that immediately
following such forfeiture, the Subscriber (and any such transferees), collectively with all other initial stockholders of the Company prior to the IPO, will own an aggregate number of shares of Class B Common Stock equal to 20% of the issued
and outstanding Common Stock immediately following the IPO. 
 3.2 Termination of Rights as Stockholder. If any of the Founder Shares
are forfeited in accordance with this Section 3, then after such time the Subscriber (or its successor in interest), shall no longer have any rights as a holder of such forfeited Founder Shares, and the Company shall take such action as is
appropriate to cancel such forfeited Founder Shares. 
  












 3.3 Share Certificates. In the event an adjustment to any certificate representing
the Founder Shares purchased pursuant hereto is required pursuant to this Section 3, then the Subscriber shall return such certificate to the Company or its designated agent as soon as practicable upon its receipt of notice from the Company
advising the Subscriber of such adjustment, following which a new certificate shall be issued in such amount representing the adjusted number of Founder Shares held by the Subscriber. Such new certificate, if any, shall be returned to the Subscriber
as soon as practicable. Any such adjustment for any uncertificated securities held by the Subscriber shall be made in book-entry form. 
 4.
Waiver of Liquidation Distributions; Redemption Rights. In connection with the Founder Shares purchased pursuant to this Agreement, the Subscriber hereby waives any and all right, title, interest or claim of any kind in or to any
distributions by the Company from the trust account which will be established for the benefit of the Company’s public stockholders and into which substantially all of the proceeds of the IPO will be deposited (the “Trust
Account”), in the event of a liquidation of the Company upon the Company’s failure to timely complete an initial business combination. For purposes of clarity, in the event the Subscriber purchases securities in the IPO or securities
of the Company issued in the IPO in the aftermarket, any additional Common Stock so purchased shall be eligible to receive any liquidating distributions from the Trust Account by the Company. However, in no event will the Subscriber have the right
to redeem any shares of Common Stock into funds held in the Trust Account upon the successful completion of an initial business combination. 

5. Restrictions on Transfer. 

5.1 Securities Law Restrictions. In addition to any restrictions to be contained in that certain letter agreement (commonly known as an
“Insider Letter”) to be entered into between the Company and the Subscriber in connection with the consummation of the IPO, the Subscriber agrees not to sell, transfer, pledge, hypothecate or otherwise dispose of all or any part of
the Founder Shares unless, prior thereto (a) a registration statement on the appropriate form under the Securities Act and applicable state securities laws with respect to the Founder Shares proposed to be transferred shall then be effective or
(b) the Company has received an opinion from counsel reasonably satisfactory to the Company, that such registration is not required because such transaction is exempt from registration under the Securities Act and the rules promulgated by the
Securities and Exchange Commission thereunder and with all applicable state securities laws. 
 5.2
Lock-up. The Subscriber acknowledges that the Founder Shares will be subject to lock-up provisions (the
“Lock-up”) contained in the Insider Letter. Pursuant to the Insider Letter, the Subscriber will agree (subject to certain customary exceptions) not to sell, transfer, pledge, hypothecate or
otherwise dispose of all or any part of the Founder Shares until the earlier to occur of: (A) one year after the completion of the Company’s initial business combination or (B) the date on which the Company completes a liquidation,
merger, stock exchange or other similar transaction after its initial business combination that results in all of its stockholders having the right to exchange their shares of Common Stock for cash, securities or other property. Notwithstanding the
foregoing, if the last sale price of the Common Stock equals or exceeds $12.00 per share (as adjusted for stock splits, stock dividends, reorganizations, recapitalizations and the like) for any 20 trading days within any 30-trading day period commencing at least 150 days after the Company’s initial business combination, the Founder Shares will be released from the Lock-up. 












 5.3 Restrictive Legends. All certificates representing the Founder Shares shall have
endorsed thereon legends substantially as follows: 
 “THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS AND NEITHER THE SECURITIES NOR ANY INTEREST THEREIN MAY BE OFFERED, SOLD, TRANSFERRED, PLEDGED OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER SUCH ACT OR
SUCH LAWS OR AN EXEMPTION FROM REGISTRATION UNDER SUCH ACT AND SUCH LAWS WHICH, IN THE OPINION OF COUNSEL (IF THE COMPANY SO REQUESTS), IS AVAILABLE.” 

“THE SECURITIES REPRESENTED HEREBY ARE SUBJECT TO LOCKUP PROVISIONS AND MAY NOT BE OFFERED, SOLD, TRANSFERRED, PLEDGED OR OTHERWISE
DISPOSED DURING THE TERM OF THE LOCKUP PERIOD.” 
 5.4 Additional Founder Shares or Substituted Securities. In the event of the
declaration of a stock dividend, the declaration of a special dividend payable in a form other than Common Stock, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar
transaction affecting the Company’s outstanding Common Stock without receipt of consideration, any new, substituted or additional securities or other property which are by reason of such transaction distributed with respect to any Founder
Shares subject to this Section 5 or into which such Founder Shares thereby become convertible shall immediately be subject to this Section 5 and Section 3. Appropriate adjustments to reflect the distribution of such securities or
property shall be made to the number or class of Founder Shares subject to this Section 5 and Section 3. 
 5.5 Registration
Rights. The Subscriber acknowledges that the Founder Shares are being purchased pursuant to an exemption from the registration requirements of the Securities Act and will become freely tradable only after certain conditions are met or they are
registered pursuant to a registration rights agreement to be entered into with the Company prior to the closing of the IPO (the “Registration Rights Agreement”). 

6. Other Agreements. 
 6.1
Further Assurances. The Subscriber agrees to execute such further instruments and to take such further action as may reasonably be necessary to carry out the intent of this Agreement. 

6.2 Notices. All notices, statements or other documents which are required or contemplated by this Agreement shall be in writing and
delivered: (i) personally or sent by first class registered or certified mail, overnight courier service or facsimile or electronic transmission to the address designated in writing, (ii) by facsimile to the number most recently provided
to such party or such other address or fax number as may be designated in writing by such party and (iii) by electronic mail, to the electronic mail address most recently provided to such party or such other electronic mail address as may be
designated in writing by such party. Any notice or other communication so transmitted shall be deemed to have been given on the day of delivery, if delivered personally, on the business day following receipt of written confirmation, if sent by
facsimile or electronic transmission, one (1) business day after delivery to an overnight courier service or five (5) days after mailing if sent by mail. 
  












 6.3 Entire Agreement. This Agreement, together with that certain Insider Letter to be
entered into between the Subscriber and the Company and the Registration Rights Agreement, each substantially in the form to be filed as an exhibit to the Registration Statement, embodies the entire agreement and understanding between the Subscriber
and the Company with respect to the subject matter hereof and supersedes all prior oral or written agreements and understandings relating to the subject matter hereof. No statement, representation, warranty, covenant or agreement of any kind not
expressly set forth in this Agreement shall affect, or be used to interpret, change or restrict, the express terms and provisions of this Agreement. 

6.4 Modifications and Amendments. The terms and provisions of this Agreement may be modified or amended only by written agreement
executed by all parties hereto. 
 6.5 Waivers and Consents. The terms and provisions of this Agreement may be waived, or consent for
the departure therefrom granted, only by written document executed by the party entitled to the benefits of such terms or provisions. No such waiver or consent shall be deemed to be or shall constitute a waiver or consent with respect to any other
terms or provisions of this Agreement, whether or not similar. Each such waiver or consent shall be effective only in the specific instance and for the purpose for which it was given, and shall not constitute a continuing waiver or consent. 

6.6 Assignment. The rights and obligations under this Agreement may not be assigned by either party hereto without the prior written
consent of the other party. 
 6.7 Benefit. All statements, representations, warranties, covenants and agreements in this Agreement
shall be binding on the parties hereto and shall inure to the benefit of the respective successors and permitted assigns of each party hereto. Nothing in this Agreement shall be construed to create any rights or obligations except among the parties
hereto, and no person or entity shall be regarded as a third-party beneficiary of this Agreement. 
 6.8 Governing Law. This Agreement
and the rights and obligations of the parties hereunder shall be construed in accordance with and governed by the laws of New York applicable to contracts wholly performed within the borders of such state, without giving effect to the conflict of
law principles thereof. 
 6.9 Severability. In the event that any court of competent jurisdiction shall determine that any provision,
or any portion thereof, contained in this Agreement shall be unreasonable or unenforceable in any respect, then such provision shall be deemed limited to the extent that such court deems it reasonable and enforceable, and as so limited shall remain
in full force and effect. In the event that such court shall deem any such provision, or portion thereof, wholly unenforceable, the remaining provisions of this Agreement shall nevertheless remain in full force and effect. 












 6.10 No Waiver of Rights, Powers and Remedies. No failure or delay by a party hereto
in exercising any right, power or remedy under this Agreement, and no course of dealing between the parties hereto, shall operate as a waiver of any such right, power or remedy of such party. No single or partial exercise of any right, power or
remedy under this Agreement by a party hereto, nor any abandonment or discontinuance of steps to enforce any such right, power or remedy, shall preclude such party from any other or further exercise thereof or the exercise of any other right, power
or remedy hereunder. The election of any remedy by a party hereto shall not constitute a waiver of the right of such party to pursue other available remedies. No notice to or demand on a party not expressly required under this Agreement shall
entitle the party receiving such notice or demand to any other or further notice or demand in similar or other circumstances or constitute a waiver of the rights of the party giving such notice or demand to any other or further action in any
circumstances without such notice or demand. 
 6.11 Survival of Representations and Warranties. All representations and warranties
made by the parties hereto in this Agreement or in any other agreement, certificate or instrument provided for or contemplated hereby, shall survive the execution and delivery hereof and any investigations made by or on behalf of the parties. 

6.12 No Broker or Finder. Each of the parties hereto represents and warrants to the other that no broker, finder or other financial
consultant has acted on its behalf in connection with this Agreement or the transactions contemplated hereby in such a way as to create any liability on the other. Each of the parties hereto agrees to indemnify and hold the other harmless from any
claim or demand for commission or other compensation by any broker, finder, financial consultant or similar agent claiming to have been employed by or on behalf of such party and to bear the cost of legal expenses incurred in defending against any
such claim. 
 6.13 Headings and Captions. The headings and captions of the various sections of this Agreement are for convenience of
reference only and shall in no way modify or affect the meaning or construction of any of the terms or provisions hereof. 
 6.14
Counterparts. This Agreement may be executed in one or more counterparts, all of which when taken together shall be considered one and the same agreement and shall become effective when counterparts have been signed by each party and
delivered to the other party, it being understood that both parties need not sign the same counterpart. In the event that any signature is delivered by facsimile transmission or any other form of electronic delivery, such signature shall create a
valid and binding obligation of the party executing (or on whose behalf such signature is executed) with the same force and effect as if such signature page were an original thereof. 

6.15 Construction. The words “include,” “includes,” and “including” will be deemed to
be followed by “without limitation.” Pronouns in masculine, feminine, and neuter genders will be construed to include any other gender, and words in the singular form will be construed to include the plural and vice versa, unless
the context otherwise requires. The words “this Agreement,” “herein,” “hereof,” “hereby,” “hereunder,” and words of similar import refer to this Agreement as a
whole and not to any particular section unless expressly so limited. The parties hereto intend that each representation, warranty, and covenant contained herein will have independent significance. If any party hereto has breached any representation,
warranty, or covenant contained herein in any respect, the fact that there exists another representation, warranty or covenant relating to the same subject matter (regardless of the relative levels of specificity) which such party hereto has not
breached will not detract from or mitigate the fact that such party hereto is in breach of the first representation, warranty, or covenant. 












 6.16 Mutual Drafting. This Agreement is the joint product of the Subscriber and the
Company and each provision hereof has been subject to the mutual consultation, negotiation and agreement of such parties and shall not be construed for or against any party hereto. 

7. Voting and Redemption of Founder Shares. The Subscriber agrees to vote the Founder Shares in favor of an initial business combination
that the Company negotiates and submits for approval to the Company’s stockholders and shall not seek redemption with respect to such Founder Shares. Additionally, the Subscriber agrees not to redeem any Founder Shares in connection with a
redemption or tender offer presented to the Company’s stockholders in connection with an initial business combination negotiated by the Company. 

8. Indemnification. Each party shall indemnify the other against any loss, cost or damages (including reasonable attorneys’ fees
and expenses) incurred as a result of such party’s breach of any representation, warranty, covenant or agreement in this Agreement. 

[Signature Page Follows] 












 If the foregoing accurately sets forth our understanding and agreement, please sign the
enclosed copy of this Agreement and return it to us. 
  

			
	Very truly yours,
	
	ALTENERGY ACQUISITION CORP.
		
	By:	 	 /s/ Russell Stidolph

		 	Name: Russell Stidolph
		 	Title: Chief Executive Officer


 Accepted and agreed this 25th day of March, 2021. 

ALTENERGY ACQUISITION SPONSOR, LLC 
  

			
	By:	 	 /s/ Russell Stidolph

		 	Name: Russell Stidolph
		 	Title: Manager

 [Signature Page to Subscription Agreement]