<PAGE>


                                  Exhibit 10.18

                                 PROMISSORY NOTE

Principal Amount: $200,000                         Date of Note:  April 28, 2004

         FOR VALUE RECEIVED, the undersigned, SulphCo, Inc., a Nevada
corporation ("Borrower"), promises to pay in lawful money of the United States
of America to the order of Rudolf W. Gunnerman and Doris Gunnerman
(collectively, "Lender"), at such place as Lender may designate in writing from
time to time, the principal sum of Two Hundred Thousand and No/100 Dollars
($200,000.00), plus interest thereon as provided below.

1. INTEREST. The unpaid principal amount hereof from time to time outstanding
shall bear interest at a rate per annum equal to the Prime Rate (as hereinafter
defined) from time to time in effect. Accrued interest shall be payable on each
anniversary date of the making of this Note and at maturity, beginning with the
first interest payment on April 29, 2005. The term "Prime Rate" shall mean, as
of any point in time, the rate of interest then most recently announced by Bank
of America as its prime rate; and the applicable interest rate under this Note
shall change simultaneously with each change in the Prime Rate. Interest shall
be computed for the actual number of days elapsed on the basis of a year
consisting of 365, or when appropriate 366, days.

2. PAYMENTS AND TERM. The entire indebtedness of funded principal, if not sooner
paid, shall be due and payable upon demand made at any time after the first
anniversary date of the making of this Note (the "Maturity Date"). All payments
on account of the indebtedness evidenced by this Note shall be first applied to
any default interest and costs owed under this Note (if any) and then to
principal.

3. PREPAYMENT. The principal indebtedness evidenced by this Note may be prepaid,
in whole or in part, with or without interest, and without payment of a
prepayment fee.

4. DEFAULT. The occurrence of any one or more of the following shall constitute
an event of default under this Note:

         (i)      Failure to make any payment of principal or interest when due;
                  or
         (ii)     The occurrence of any other event of default under this Note
                  following ten (10) days written notice thereof to the
                  Borrower.

            Time is of the essence. If an event of default occurs under this
Note after the passage of applicable cure periods, (i) the entire principal
balance hereof shall, at the option of Lender, without notice, bear interest at
a rate equal to eighteen percent (18%) per annum (or the maximum rate permitted
by applicable law if that is less) from the date of the event of default until
such event of default is cured and (ii) the entire principal balance hereof and
all accrued interest shall immediately become due and payable (or issuable in
the case of stock) at the option of Lender, without notice. Lender's failure to
exercise any option hereunder shall not constitute a waiver of the right to
exercise the same for any subsequent event of default.


<PAGE>

5. COSTS AND ATTORNEYS' FEES. If an event of default occurs under this Note and
Lender consults an attorney regarding the enforcement of any of its rights under
this Note, or if this Note is placed in the hands of an attorney for collection,
or if suit be brought to enforce this Note, Borrower promises to pay all costs
thereof, including attorneys' fees. Said costs and attorneys' fees shall
include, without limitation, costs and attorneys' fees in any appeal or in a
proceeding under any present or future federal bankruptcy act or state
receivership.

6. SECURITY. This Note is unsecured.

7. WAIVER OF PRESENTMENT, ETC. Borrower hereby waives presentment and demand for
payment, notice of dishonor, protest and notice of protest.

8. GOVERNING LAW. This Note shall be construed, enforced and otherwise governed
by the laws of the State of Nevada.

Dated as of the 29th day of April, 2004.

SULPHCO, INC.



By:  /S/ KIRK S. SCHUMACHER
     ----------------------
         Kirk S. Schumacher
         President