For the three months ended September 30, 2023 and 2022, cost of revenue was $1,703,553 and $932,364, respectively, representing an increase of $771,189, or 83%. The increase corresponds to the increase in our revenue during the period. The gross margin percentage was 44% and 33% for the three months ended September 30, 2023 and 2022, respectively. The gross margin percentage increased due to the increase in higher margin revenue from contract services.
For the nine months ended September 30, 2023 and 2022, cost of revenue was $4,513,285 and $1,478,954, respectively, representing an increase of $3,034,331, or 205%. The increase corresponds to the increase in our revenue during the period. The gross margin percentage was 40% and 32% for the nine months ended September 30, 2023 and 2022, respectively. The gross margin percentage increased due to the increase in higher margin revenue from contract services.
Research and Development
Research and development (“R&D”) includes expenses incurred in connection with the R&D of our CFV thermal management solution, high-areal-capacity battery electrodes, 3D engineering for a rechargeable battery and related non-cash stock-based compensation expenses. Research and development expenses are charged to operations as incurred.
For the three months ended September 30, 2023 and 2022, R&D expenses were $1,640,959 and $1,069,852, respectively, representing an increase of $571,107, or 53%. The increase during 2023 was comprised primarily of $515,634 related to planned increases in headcount in order to build future capacity, and $28,380 related to an increase in employee stock-based compensation for option awards.
For the nine months ended September 30, 2023 and 2022, R&D expenses were $4,873,841 and $2,790,683, respectively, representing an increase of $2,083,158 or 75%. The increase during 2023 was comprised primarily of $1,604,843 related to planned increases in headcount in order to build future capacity, amortization of prepaid cash consideration for Vibetech asset purchase agreement of $375,000, equity compensation for Vibetech asset purchase agreement of $128,650, rent expense of $42,948 for a new facility for R&D initiatives designed to build future revenue growth and $40,482 for depreciation expense, partially offset by a reduction in outsourced R&D costs.
We expect that our R&D expenses will increase as we expand our future operations, presuming that we raise adequate capital to do so.
Selling, General and Administrative
Selling, general and administrative expenses consisted primarily of stock-based compensation, payroll taxes and other benefits, consulting fees, registration fees, office expenses, rent expense, directors’ and officers’ insurance, travel and entertainment, marketing and advertising, and filing fees.
For the three months ended September 30, 2023 and 2022, selling, general and administrative expenses were $4,793,523 and $4,349,373, respectively, an increase of $444,150, or 10%. The increase is primarily due to increases in depreciation and amortization expense of $486,724 primarily due to enhancements to our primary facility, labor costs of $240,766, various legal and professional fees of $107,912, software license and utility fees of $78,703, board compensation fees of $61,250, insurance fees of $50,044, franchise tax of $32,342, and office related expenses due to the increase in headcount of $21,997, partially offset by a decrease in marketing and advertising expenses of $416,140, and a decrease of $252,557 in stock-based compensation.
For the nine months ended September 30, 2023 and 2022, selling, general and administrative expenses were $15,546,915 and $12,210,458, respectively, an increase of $3,336,457, or 27%. The increase is primarily due to increases in labor costs of $1,279,768, depreciation and amortization expense of $1,086,541, consulting fees of $837,758, travel and entertainment costs to build future revenue growth of $218,348, software license and utility fees of $265,030, and costs to attend conferences and seminars of $152,888, partially offset by decreases in stock-based compensation of $793,866.
Other Expense
For the three months ended September 30, 2023 and 2022, net other expense was $465,246 and $628,181, respectively, representing a decrease of $162,935, or 26%. The change is primarily attributable to a $383,276 decline in interest expense primarily related to a note payable that was paid in 2022 and $158,675 decline in PPP loan forgiveness.
For the nine months ended September 30, 2023 and 2022, net other expense was $1,062,401 and $678,960, respectively, representing an increase of $383,441, or 56%. The change is primarily attributable to a $383,441 increase in interest expense primarily related to the prepaid advance liability that was entered into in September 2022, partially offset by a $158,675 decline in PPP loan forgiveness.